Do any of you guys know of a way to add an overdrive to an old GM manual tranny (saginaw or muncie)? I turn like 3000rpm going 70 on the highway and not only do I not think that's good for the engine but i'm pretty sure that doesn't help gas mileage either. I'm going to be going to college soon and I really wouldn't like to risk problems with running relatively high rpms on long trips too and from when I visit back home. Do you guys have any ideas? and no, I don't want to just stick a tremec in there. I want to use my original column shifter so it has to have the linkages on the side for an external shifter (like in a saginaw or muncie). I just want 1-4 on the column and I can make shifters on the floor for reverse and OD.

This transmission would be absolutely perfect for what I need but they go for about $2500 by themselves (same price I actually paid FOR MY WHOLE CAR) so it's a little bit out of my price range. But it has the Overdrives like I need and it is a "rod transmission" I think is how it's called? (it has the linkages for it to be shifted via rods...

So any ideas on how to not turn serious rpms while keeping the 3 on the tree setup would be great... Thanks guys...

If you want to retain the original column shift linkage, then you will want to look for a three-speed Saginaw transmission with overdrive, used in Chevrolets from 1966-1970. Be sure that you get the solenoid with the overdrive, since that solenoid is unique, and those that fit other overdrives do not fit the Saginaw.

God's Peace to you.

d
Inliner #1450

For now, you can install taller tires.

A 28-29" tall should fit pretty easy, as long as they are not too wide.
It might look a little funny, but hey, your RPMs will drop down & tires do not cost that much.


MBHD

You might want to consider an New Process 440/RPO MY6, aka A833. It is a 4spd and 4th is an over drive. It was originally designed for Chrysler, used in everything from the Duster to the 3/4 ton trucks, and behind everything from the slant six to the Hemi. Here is a link that shows where they were used;

http://www.drivetrain.com/parts_catalog/...erhaul_kit.html

All the trans under Chevrolet and GMC would be a direct bolt in. You may have to shorten your drive shaft. You would also end up with a floor shift.

Larry

You don't want one of those richmonds. I have one and it is a big, heavy, angry, clunky, moany, whiny, brutal piece of equipment. The 3 speed with o/d is a way better option for what you are looking for.

Putting the OD from a Saganaw 3 speed on a Saganaw 4 speed is an option but as Don stated those solenoids are one of a kind, rare, and expensive now. Also shifting a 4 speed from the column is a whole other set of problems. Just the possible driver error issue of shifting into reverse while still in OD could wreck it all. A good option would be a Borg T-85 3 speed OD. The one I have has the R-11 OD with 4 plantaries. I got it cheap on eBay. The T-85 is as close as you get to bullet proof. They were used in some late '50s to early '60s Studes and lots of Ford pickups, vans and Broncos I think. All I have seen have the tall Ford bolt pattern. You may have to drill holes or build an adapter. Mine is going in my '53 pickup but I'm not mentioning the engine here.

My GM 833 tranny is shifted from a column shifter - took a little creative engineering but when sitting behind the wheel you cannot tell it from stock by looking at it.



Here is where I mocked everything up - you have to switch the direction of one of the levers - I made my own bellcranks with bronze bushings and used the stock truck linkages from the column to the bellcranks and then the standard floor shift linkages modified (heat and bend) from the tranny to the bellcranks.

Reverse is handled with a push-pull morse cable and linkage with the handle under the dash.

That is a nice piece of work. So first was where reverse would have been? I'd like to see more about this conversion. Beater

Yes - first is where reverse was. The linkage required a little bit of guess and check with the clutch linkage, brake lines and exhaust all crowding for the same area, but it works out fine and I am pretty happy with it.

Here is a shot after I got it all working but before things got too crowded with the rest of the stuff that goes here.



One of the shifter arms needed to be cut to make room for the intake manifold - which required that I then weld up one of the holes in the newly made bellcranks and redrill to make the throws for 1-2 match the throws for 3-4



Here is what it looks like with everything in place . . .


The reverse linkage was done with a bit of a rube-goldberg approach, but it works well. I used the same linkage design that is used in a lot of automatics with a jack shaft carried between the tranny and the frame. I just turned the threads off of a couple of grade 8 bolts in the lathe and fitted a piece of pipe over them with nylon bushings - the short piece of red rubber hose absorbs any compression from the tranny moving around due to torque.

Thanks Ken. That is a neat job. I had forgotten how crowded that area is until I had to replace a motor mount a while back. What master cylinder are you using?

I am using a Nova from ~70 I think, don't remember which year exactly. I picked one that worked for drum/drum that had the same form factor as a same year car that had disc/drum just in case I wanted to upgrade in the future.

Rock Auto had it for under $35 . . .

I adapted the original mount to carry this MC - I wanted to be able to fill it from the same hole in the cab floor as stock.

I used a bandsaw to section the original mount to pull the stock cylinder mounting surface in to make room for the wider Nova cylinder - then I used a piece of 3/8" thick flat bar to create an ear for the Nova cylinder to mount to and then welded it all up after making sure the push rod was in proper alignment. Only took about 4 hours to make the mount using scrap that I had laying around.

You do nice work. Thanks for showing us. That is a clean conversion. Beater

Bigger tires will affect you in all gears and take away some of your get up. Whereas an o/d only affects your engine speed in o/d. O/D lets you keep the good off the line chacteristics and then keep your rpms calmer on the highway.

Old '52 chevys with their stock 216s (90 hp) ran 4.11s with 2.94 low gears, you could barely get thru an intersection before you had to shift outta low gear (but at least you could get thru the intersection without having to open the door and push it off!)

Frank mcGurk always recommended the PowerGlide rear end ratio (my favorite) 3.55 for them when their engines were modified. My experiances with early chevy lls and the later motors with 3.55 gears/early Nova size tires were all good. I could buzz along the highway at about 70 taching little over 3000 rpm with a 1.00:1.00 high gear (it certainly would get the condensation outta the oil pan!) On short 100 mile jogs it was ok, but I will admit I wouldn't want to drive coast to coast that way. It is surprising how doggy a low numerical axle ratio will make a small engine feel. 396 chevelles, 409/340 hp, and the big V8s can pull them well-but thats because they have some 200 more foot/lbs torque.

A good rule of thumb to compare stuff like that, is assuming the engine size to be your torque spec (not entirely accurate-but reasonable) and multiply it times your rear axle ratio, then compare it to other engine/axle configurations--the more the better, now multiply in the low gear ratio also and you can get a grasp on how it will come off the line when compared to others. Now you can see how 283 '55 chevys and 4.56s can run with '66 396 Chevelles w/ 3.08s (a 4.56 on a summertime Texas highway is not a pretty thing with most any motor- there is where the worth of an o/d lies). Just rule of thumb mind you.

First off, snowman does not have much money to buy anything let alone a different trans.
At least that's my impression of our PMs to eachother seem to reveal. Maybe he can reiterate his finacial situation.

I know I dont have money to finish my projects.

I suggested a cheap way to lower the high RPMs on the freeway while he drives to college & back home etc,freeway driving.
Sure, it will look funky w/the tall tires, but installing 355's in the rear is going to raise his RPMs on the freeway,don't think he is looking to do that.
Also, him switching to 355's gears now will drop his ET down to what w/a stock car?

IIRC, snowman has 323's & he is turning too high rpm's going 70 MPH,, yes, that is too high & yes, will get poor mileage.

Running 3.55's & short tires are a bad choice for the freaway , & going 100 miles is a short stint, not @ 3000 RPM's in my book.

With todays gas prices, & snowman is trying to save money ,not blow it out the tailpipe w/those 3.55's.

Snowman needs to drop those rpm's, he is not drag racing his car to college.

He is going to turbo his car & the lowest gears I would suggest him use is 3.42's & a 27-28" tall tire.
He still would need an OD trans w/those gears,on the freeway, but that is all that is needed to run quick.

Running a PG & 355's? PG's are horrible for small engines as far as get up & go,, how bout a TH350 @ least?
Frank mcGurk ,,,,where OD trans around for the old 6 cyl's,pre 62?, did he turbocharge cars?

Snowman is not building an all out drag car that is only ok to drive on the street.
He is building a low RPM torque engine,he does need to gear it low in the rear.His engine is going to think it's a mild 350 SBC,,, for now.

Snowman needs his car to get up to the more modern age.Eventually an OD trans would be nice,EFI,,,, for sure.

My moms 1973 stock 250, PG, 308, 4 dr ran 19.8 seconds in the 1/4 mile @ 3000+ ft altitude track. Yes slow,but very very nice on the freeway. A TH350 would have dropped the ET quite a bit over a PG. But overall,it was not a looser of every race on the street, & mind you, that was w/a horrible tall first gear of a PG.

Just think when you were 16 years old, ,, did you have money to change tranmission, change your rear gears or rear end?
Did you have excess $$$$ for much of anything? I know I did not.

Buying used or new tires are a cheap suggestion to lower snowmans rpms on the freeway,,BTW, is too high, even if he had a large camshaft,it still would not be in the efficient RPM zone,3000 RPM, paleaze!

MBHD

"Frank mcGurk ,,,,where OD trans around then, did he turbocharge cars?"
Borg ODs were being used by most US manufacturers in McGurk's day. Snowman is the one who asked about tranny options. When my son went to Texas to college his Stude wagon had 4.26 gears. That old Borg T-86 OD was not strong but it was a life saver. We once lost the OD in Vegas. The drive to Kingman was less than pleasant. We were lucky to find a guy there who fixed the solenoid. There are lots of late '50s Chevy OD transmissions laying around. The ones pre Saganaw will work till the turbo is installed. At this point he would be better off with an OD than a turbo. Beater

I thought snowman wants to lower the RPMs, he is also all over the place thinking about tranny changes, change this change that, it's OK to think about it, but when you do not have the funds to do any major chages you have to realize, he could probably only afford larger tires @ this point, he is trying to save 1500 or less dollars or so for a turbo install, & even getting 1500 or less will take a long time to save.

I could tell him to put a 200R4 fat stuff3 EFI,etc,etc,etc & blow up his mind & buy all kinds of nice stuff, but in actuallity, I know he cannot afford that type of stuff, @ least not @ this time in his life.

Forgive me for knowing what it's like not having money to buy the stuff you want been there done that & am still there LOL.

I am just being practicle for him, maybe he wants to keep getting suggestions from you guys & make him change his mind over & over & over & never buy one item.

Look how long it took for him just to get a used 4 bbl carb & intake.

Did McGurk recommend using OD trans & run 355 gears? I have no idea.

He could use a saginaw 4 speed & adapt the 3 speed OD unit. Not sure how the 4 speed will shift w/a collum shift? Anyone?

That's just more way money being spent (that he does not have) on a tranny that I personally would not recommend to use for street/strip racing.

Who knows, maybe snowman has a large amount of $$$$ somewhere & he is just acting like he does not have money?

MBHD

Whats a "PM"

Many times lower gearing will help keep one out of the power stage of the carb and result in better milage

Driving slower is also a solution to "excessive" rpm

3.55s are not drag racing ratios in Texas

Why not suggest some 33" tires and a 2.56 axle, this, with a steady light trottle may approach say 87 mpg. and allow you plenty of time (and load) to spool up the old hairdryer and not need an o/d thus saving money

Nobody said anything about using a PowerGlide, only the ratio for a PowerGlide car

Frank could have adapted a Packard (floor shifted or column shifted) o/d tranny (w/ synchro low as early as 1939) to the old torque tube cars, while never turboing any cars he did drive in an Indy 500 and provide us with products/info to place many flathead V8 in our old stovebolts' rear view mirrors

Chevy began offering o/ds as options in 1955 (which is pre '62)

Ivory soap is mild too

When I was 16 years old I was replacing the first of 5 standard trannys that my ol' 261 began to eat, I had a paper route at that time, when I got my Chevy ll I was going to college and working part time as a blueprint boy for 25 dollars a week, I know all about wealth

Paleaze-what is the efficent rpm zone if not 3000 rpm

Bert Monro (the fastest indian) used to FILE himself connecting rods outta old catipillar treads - four speeds and a reverse on the column is not impossible - guys in high school used to make their own floor shifters outta old 40s and early 50s Ford column shifter units, just need to lengthen the shaft and add another lever and linkage (gimme a Hurst any day)... where there's a will... etc.,etc.,etc.

A 3.42 axle with a .70 od will bring the o/a ratio to 2.39 and I doubt if the carb would EVER get out of power circuit short of going down a hill.

In the nonmodern years o/ds usually were complimented with 4.11 gearing resulting in a final drive of 2.88 and buddy you needed flat land and a lot of throttle to maintain 70 mph. In 1963 chevy used 3.70 as their o/d rear gear (o/a gearing 2.59)in their big cars, sixes would then need constant downhill to maintain 70 mph

Going from a suspected 26" tire to a 28" tire would result in an approximation of a 3.18 rear gear with the 26" tire,or about a 7%
rpm reduction at any given speed. Of course with those tires on the rear, the car would think its going downhill all the time.

It aint mine, so do what you want to...as Rick said "You cant please everyone, you got to please yourself"

There were OD options for Model Ts. Chevy had a Truckstell OD that was fitted into the closed drive line in the '40s or early '50s. Borg Warner made the R-10 & R-11 ODs for everyone Packard, Ford, GM, Studebaker, etc. There were always options.

My McGurk book doesn't mention transmissions at all but he does say that 3.55 gears were the best choice for the highway. Tire sizes were 6.00x16 or 6.70x15. Powerful engines could run 700x16s. That produced 115mph ay 4,600 rpms and needed 180hp.

We all know what money i$, one of the reasons I've been driving the same beat up old pickup for over 35 years. Another reason is because I love it. 3.55s, T-5, 28" tires. It'll run 70 to 80 all day under 3000 and get 17 to 18 mpg while ignoring mountains. 4,000 + miles in one week just to attend the Lone Star Roundup in Austin. You can take the boy out of Texas but you can't take Texas out of the boy!

I am of the opinion that the op should be the one to decide what he can and cannot afford. It is not our job to be practical FOR him. He DID ask about transmissions and overdrive units. If he changes his mind over & over & over AND is happy with the final decision/results due to information that he gleaned from our posts, then isn't that all any of us can ask for? I would like to think so. Besides, maybe he is inquiring so he knows what to ask for as a birthday gift. Who knows. Even if he dosen't actually buy any thing we are suggesting, the knowledge alone can help in making educated decisions and/or priortizing. With the economy the way that it is, knowledge is at a premium.

Thank you very much. That is EXACTLY why I ask so many questions about different things so I can make an educated purchase when I finally get the money.



I've been haranguing the guy to FINALLY send me the holley 390 and offy intake and I'm pretty sure he'll FINALLY send it on saturday...

As it comes to what I can afford guys, I got a job about 2 months ago at a body shop and I make $8 an hour and I work 6-10 hours a week and it's increasing recently so I'll hope to work a considerably large amount of my summer. I make about $50-70 a week as it is and as it will be until summer (but I also have random expenses so not ALL of it is car money).

As for transmissions, I've been looking at old used tremecs like the 3450, 3550, and the TKOs on craigslist. I've found a few for $400 or $500 so that should be affordable past halfway through the summer. I'm not very faithful in the power of only having 3 gears (because the 4th converted to overdrive). I just know that in my current situation, my engine is a dog all the way through 3rd gear. I get great power in first, good power in second, and then NOTHING in 3rd. I don't know if that will be gapped once I get my turbo setup (moar torque!) but as it is, I REALLY feel I should have a 4th gear to bridge the gap between my 2nd and 3rd. I already planned putting 1-4 on my column and then reverse (or reverse and OD) on a separate lever through the floor so fabbing up that and cutting the holes for that isn't a big deal.

Yep 2nd to 3rd is a big time drop in power on those trannys.

I had the same in my Camaro & hated it, had to fan the clutch a few times in 3rd to get the car to move.


MBHD

A PM is a private message.

If you think 3000 RPM on the freeway (or driving on the street) w/a stock 250 engine is the efficent RPM,, getting the best mileage, I dont know what else to say.

MBHD

Amazing

Snowman, Congrats on your job. You have to start somewhere and you'll learn a lot there. In these times any job is good. You obviously have a family that cares about you. You sound like a game young guy who wants to learn. Our group is shy on those, but we have a glut of old farts who are willing to pass on their experiences. Take what is offered here and use what fits your needs. What is the deal with someone not sending your carb & manifold? Is he trying to stiff you? How long has it been? Where is he? Beater

Thanks! I appreciate your response.

The deal is that I found him on craigslist and called him around my birthday (end of september of last year). I asked him about it and he has a rebuilt holley 390 and a offenhauser intake for the 250 for $250 and we made a deal for $225. He couldn't use it because his old headers he was planning on using didn't work and he decided to sell it. He lives in atlanta, GA and is a trucker so he's out most of the time and the reason why it WAS going to be convenient is because he said he got trips to Memphis pretty often (I live in a memphis suburb). It turns out that he got like 2 trips right before I called him and then didn't get any trips anywhere near here for like 4 or 5 months so he hasn't been able to get it to me. I just finally got fed up near a month ago and asked him to ship it and asked if I could pay through paypal. The reason I was reluctant to pay for a big deal like this online is because, around a year ago, I was stiffed $400 out of a laptop through a scam (and that money WAS going to go towards my car). ANYWAY... he's been in and out of his home for the past month (so he says) trying to get shipping together. He said he's getting back and finally getting shipping done and getting it sent tomorrow but we'll see...

So that's the intake/carb situation...

Snowman,I remember back in the late `70`s Hot Rod or Car Craft had an article on how to make a 4 speed saginaw ( it was the one to use because reverse was on the main case,not on the tailhousing like borg-warners or muncies.) into a 5 speed using the then plentiful saginaw 3 speed w/OD tailhousing. It was a poorman`s version of the then new-out Doug Nash 5 speeds. The OD was actuated by cable as I remember and a 4 speed shifter was modified to operate the concoction. Maybe you can access this info on their website? Also when I was in the Army in Germany in the mid to late `70`s 4 on the tree was the norm for the average German car. I don`t remember the makes but it is something to go on... Neccessity is the mother of invention... Speaking of poor, I`ve been without a paycheck for so long that when I take any money to the bank, the tellers all band together and sing a chorus of "Me and My Poverty".

Somehow, I missed Lgriffin's post about the A-833. Looked at it and it's damn-near exactly what I need. Can anyone vouch for it and does anyone know anything about it? Would a chrysler tranny work in a buick/chevy engine/car? Seems like I could drop highway rpm to 2000-2200 going 70mph. Would this be reasonable?

Overdrives work great as long as the ground is flat and the load is light (ie. not towing or empty PU bed) otherwise the mechanical effiency drops and can cost you fuel mileage. If you are in favor of better fuel mileage raise your right foot and keep the RPM's in the 1800-2200 range.

It seems he (snowman)is turning 3000 RPM keeping up w/traffic.

OD, gears have more uses than just going flat level ground or needing to be light on your load to use OD.


MBHD

Just applying 3,000,000 plus driven miles using direct and overdrive transmissions and recording fuel mileages along with a lot of other drivers doing the same and the results back up my statement. Overall a vehicle with a direct drive transmission and a 3.20:1 rear gear will return better fuel mileage than an overdrive transmission with a .76 overdrive and a 3.90:1 rear gear running the same speeds. Most engines have a sweet spot where they will consistantly return better mileage and my experience with the GM family of inlines is the 1800-2150 range.

So, in general , it seems you do not recommend a OD trans?

MBHD

Well if the A-833 OD would put me at about 2000rpm-2200rpm going 70, it seems like that would give me the best mileage since it's in your mentioned range

Here is a old post with info on the MY6 tranny's. We ran one in my Dads 66 chevelle with a BB Chevy in it. Will bolt in to anything that had a Saginaw or Muncie. The column shift would take some work.
https://www.inliners.org/ubbthreads/ubbthreads.php?ubb=showflat&Number=40579&fpart=1

after all the kidding aside the biggest enemy of highway gas milage is the BTU's of energy required to push the car through the atmosphere at speed. Example: From an old Scientific American Magazine --Half of the energy required to make a bicycle go 30 mph is used to simply to overcome wind resistance.

70 mph takes a certain amount of hosses, BTU's, or Gasoline due to frontal area of vehicle. Each type vehicle is different, what engine speeds/gearing you select to produce that horsepower is a way less important factor-just so you are using the carb's main metering system only and staying out of the power stage and the idle circuits. The absolutely cheapest change (and most effective) you can make is to drive a little slower.

As far as "excessive rpm" wearing out the engine, I've personally seen a '54 chevy 235 have only a half thousanth wear on its rod journals after some 83,000 miles of running highways at 60-70 mph WITHOUT an oil filter ('cause they didn't come with 'em) but did have regular change intervals, non detergent oil. The mains had less wear than that. This was a stovebolt with insert type bearings and the factory full pressure lubrication system, 26" dia.(6.70 x 15) tires and factory stick shift with 3.70 gears (thats 2800 to 3200 rpm range). Dont be afraid of a little rpm, it wont hurt ya. It will help keep the condensation outta the oil pan.

I have to agree w/jalopy45. RPM for fuel mileage is about 1900-2200.
3000 RPM will hurt snowman in his quest for better mileage.

Running 3000 RPM will use more fuel as opposed to 2200 RPM, nobody stated excessive RPM is wearing out the bearings,but it is wearing out the bores & rings.

It sounds like snowman is keeping up w/traffic & turning 3000 RPM's, if he brings the rpm's to the more efficient range of 2200 or a bit less, he would be impeding traffic.

So all kidding aside, he does need a OD trans.

MBHD

Not sure if Snowman has a tach yet.

Don't think he knows what rear gear he has either.
Both would be really good ideas before the O/D quest.

Going from a tire that is 24" tall to a tire 27" tall can change rpm's at 70 mph by atleast 300 rpm with a 3.36 gear. That would be a cheap start.

Running 3000 rpm will not use any more fuel to produce "X" horsepower than at 2000 rpm to produce the same "X" horsepower!!! Maybe in neutral, but not when trying to push against the 70 mph headwind.

The throttle setting required to produce the required horsepower against the 70 mph headwind will be greater at 2000 rpm than at 3000 rpm--that is part of the horsepower forumla PLAN.

P is pressure produced by the explosions; L and A is the length of stroke and the area of the bore ( or together the Size of the engine); and N is the NUMBER of times these explosions happen per mimute or RPM!!

Increase ANY of these letters and you increase horsepower, decrease Any of the letters and you decrease horsepower.


The throttle setting is determinimg the PRESSURE portion of the formula in the opening sentence. How about this-- 3000 rpm will not use more fuel when in neutral than 2000 rpm will use when in any driven gear! RPM aint got hardly anything to do with fuel consumpton. Its the load or demand placed on the engine that determines fuel consumption.

This is why all F.I systems must have manifold vacumm inputs to tell the computer the load on the motor-to determine the correct a/f mixture for that load at the amount of air flow being let in by the particular throttle setting.

If you would or could find a dyno chart that also graphed the mean brake specific (whatever the engineering term) curve along with the hp and torque curves, this would be easily shown. On a dyno chart, all the numbers are achieved with wide open throttle settings, because the whole reason for the dyno run is to see what the MAXIMUM HP/TORQUE is for the test engine, many times this fuel consumption graph is included. If I remember correctly there are some shown on a few of Tom Lowe's tests.

The total amount of hp needed to maintain 70 mph IS PRODUCED only at that the particular throttle opening needed to keep the vehicle at 70 mph, no more, no less. And unless said vehicle is powered by Cushman Eagle, then it will never be a full throttle setting.

Since nobody stated that excessive rpm is wearing out the bearings and somebody did state that excessive rpm IS wearing out the bores & rings, let me state that the before mentioned 83,000 mile 235 engine went back together with STANDARD rings and original pistons- they were also oiled by the same unfiltered non detergent oil. Since the cylinder bores are primarily oiled by the oil slung off of the rod bearings, they recieve better oiling at the higher rpms.

Just makes me wonder.

http://www.chevyhiperformance.com/techarticles/46140_chevrolet_transmission_swap_guide/index.html

Why would anyone need an OD trans,not mileage increase, must have to do w/emissions only thing.

http://www.answerbag.com/q_view/1129090

http://en.wikipedia.org/wiki/Overdrive_(mechanics)

http://answers.yahoo.com/question/index?qid=20070409055131AAASInh

http://www.fordification.com/tech/overdrive.htm

http://www.answers.com/topic/overdrive

MBHD

physics is physics

Well, I do know my rear end is a 3.23, I do have a tach, and I know my tires are 25.8" tall. I guess I should just fill up my car and take it on the highway for like 40-50 miles and then fill up again and see what my mileage is. From what I understand, one of the theories is that my milage should be worse than city mpg because I'm at a constantly high rpm. The other theory is that it should be better because I edski's explanation. I'll just give em a try.

Glad to hear it is a 3.23 and you have a tach. A 27" tire will help lower rpms and not be out of sorts because it is too big. Mine are that big.
Your highway mileage will certainly be better than city driving.

http://www.mechadyne-int.com/vva-reference/part-load-pumping-losses-si-engine

Look at the P-V diagram in Figure 2. The difference between the power loop area and pumping loop area is the engine power output. Greater throttle open at 2200 rpm compared to smaller throttle opening at 3000 will result in less pumping loss (smaller pumping loop on the diagram). Granted, there is more to the story (fuel and spark curves and mechanical losses for example) but this is the major reason OD helps mileage. It's also the major reason diesel engines get better mileage. Diesels have very small pumping loops at all operating conditions because they are not throttled. It is also the major reason small engines get better mileage than large ones. The small engine will require greater throttle opening and therefore have less pumping loss.

If you are having difficulty with the pumping loss concept, consider that pressure in the crankcase is 15psi or 1 atmosphere while pressure in your intake manifold is much lower at part throttle, let's say it's 5psi. The piston has to push against a 10 psi differential on the intake stroke. This is pumping loss.

Have you ever noticed how letting off the gas doesn't slow a diesel (without a jake brake) vehicle as much as it does a gasoline? That's becasue the pumping loss is much greater in the gasoline engine.

I feel the 16 to 22 :1 compression ratios (P in the formula)is the diesel's greatest forte.

- now are we speaking of four cycle engines or two cycle types? All the loop talk reminds me of two stroke engine talk where you have no exhaust or intake valves just ports or slots in the cylinder bores to allow intake charge from the somewhat pressurized crankcase and exhaust passage from the upper cylinder area after firing. I think this is what a two stroke detroit diesel does or is.



I can see atmosphere pressure in the crankcase , I am at a loss as to why there is 5 psi (should that be 5" of mercury vacumm?) in the intake. How can you have 5 PSI in the intake manifold and still be naturally aspirated (non-blown) on a four stroke gasoline engine?

Its over my head--duck here it comes again!

Four stroke. True, higher compression helps the diesel but probably less than reduced pumping loss.

Intake manifold vacuum is relative to atmosphere or "gage" pressure. Intake manifold pressure is relative to zero or "absolute" pressure. Really the same thing, just less confusing to do calculations if you use absolute pressure instead of manifold vacuum.

Maybe it's helpful to think of the Fig2 P-V diagram this way: A diesel engine is unthrottled and always has a small pumping loop area. Load control is achieved by underfueling which makes the power loop area smaller to match the load.

A gasoline engine at WOT is unthrottled and also has a small pumping loop. But when throttled for load control the pumping loop gets bigger and the power loop gets smaller to match the load. This bigger pumping loop for a throttled gasoline engine is a major power loss at cruise. Really nothing you can do about it except to get a smaller engine or overdrive.

I don't agree with you panic.

Pressure on top of piston on intake stroke is manifold pressure while pressure below piston is crankcase pressure. The difference results in pumping loss at the crankshaft. You are thinking too deep. Go back to the basic theory. P-V diagram.

Greetings . . .

I am with strokersix on this one. An engine is first and foremost an air pump. Where there is pressure differential there is work to be done. Part throttle conditions create vacuum because of atmospheric pressure difference between the intake plenum and the crankcase. This creates pumping loss which uses fuel when cruising.

The crankcase doesn't need to be sealed in order to be under pressure. The weight of the earth's atmosphere (which varies a little depending on where you are with respect to sea level) is always there. Moreover, a vacuum in an intake manifold is still a positive pressure when compared to the true vacuum of outer space! What matters is the difference.

An overdrive unit reduces RPM and therefore the amount of air passing through the pump over a give period of time. The objective is to cruise by applying just enough fuel to do the work of maintaining speed.

Jets and circuits in a carburetor 'weep' fuel into a vacuum and abate as pressure equalizes. With a carburetor the throttle plate position is controlled by the drivers foot. In a modern engine the plate is controlled by a computer with input from a mass airflow sensor.

Cruising at low throttle openings is to be in too low of a gear and therefore at to high RPM.

I vote for the OD.

regards,
stock49

Have a nice day.

While an engine might be described as an air pump, it's source for power is gasoline. Before gasoline can be used as fuel for an engine, it must be atomized so that it can be mixed with air to form a combustable mixture. Contrary to popular belief, gasoline in its liquid state is not combustable; only gasoline vapor will burn. Vaporization is the act of changing from a liquid state to a gasseous state and only occurs when the liquid absorbs enough heat to boil. The temperature-pressure relationship dictates that as pressure is reduced, the boiling point is also reduced.

Since the pressure in the intake manifold is far less than atmosphere, the boiling point of the gasoline is lowered considerably. At this reduced pressure, latent heat absorbed from the many air particles surrounding each fuel particle causes some vaporization, which is further aided by heat on the intake manifold floor.

Because complete fuel vaporization is the result of many factors (ambient temperature, fuel temperature, manifold vacuum, and intake manifold temperature), it is easy to see that anything which reduces any one of these factors will adversely affect vaporization and thus reduce fuel economy (and increase exhaust emissions). Some examples would be cold weather, an inoperative exhaust heat control valve, and high overlap camshafts and/or heavy throttle demands.

While the effects from lower temperatures are obvious, reduction of manifold vacuum either by valve timing or heavy throttle operation are highly detrimental to fuel economy due to the higher pressures (and boiling points) resulting in the intake manifold which reduces the amount of fuel vaporization which will occur by the time the charge enters the combustion chamber. Fuel not vaporized at the time of induction, is to a large extent, exhausted unburned from the combustion chamber ( and can cause high hydro-carbon exhaust emissions).

I am having a hard time with this crankcase pressure stuff...isn't the crankcase pressure dealt with by either a road draft tube or better yet the Positive Crankcase Ventilation system? A properly working PCV system "reclaims" unburned blow-by gasses by returning them to the intake manifold and, this being a combustable mixture, is added to the new air-fuel charge. Wouldn't the increased gasoline economy of the PCV system at least counteract the crankcase pumping losses?

Fasteddie, Crankcase pressure is assumed equal to atmospheric pressure, nominally 15psi. Sure, the deeper you look it's more complicated as panic suggests but the basic assumption is valid.

PCV won't change the situation much because it is a mixture of air and fumes but EGR (exhaust gas recirculation) will because it's inert (composition nitrogen and combustion products). Including inert exhaust gas with the fresh charge increases the inlet manifold pressure and reduces pumping loss. There are some other benefits to EGR as well. It's not a bad thing, contrary to popular belief. EGR increases mixture motion in the combustion chamber because there is more of it and provides heat for vaporization. On the negative side EGR reduces peak temperatures which reduces nitrogen oxides which is the reason for EGR. Properly functioning EGR will help fuel economy and won't hurt power because at WOT EGR shuts off.

Why wouldn't PCV change the situation much? You are removing some of the pressure from under the piston. It seems it would be at least as effective as EGR because EGR is putting UNCOMBUSTABLE exhaust into the combustion chamber taking up space where COMBUSTABLE air and fuel could reside thus effectively diluting the intake charge. PCV is displacing combustable intake charge with COMBUSTABLE mixture.

On a more basic level, wouldn't the atmospheric pressure under descending piston A be inversely proportional to the atmospheric pressure under ascending piston B in the same engine?? The pressure trying to compress under piston A due to its downward travel would simply fill the void left by piston B's upward travel thus netting no pressure increase in the crankcase?

PCV won't change the pumping loss because it is a combustible mixture. PCV flow must be accompanied by reduced flow through the carb. EGR does change the pumping work loss because it is inert. Adding inert exhaust gas does not change the amount of new mixture through the carb so you get a net increase in intake manifold pressure. Increased intake pressure translates directly to reduced pumping loss.

Evacuating the crankcase doesn't help because you pay an offsetting penalty on the exhaust stroke. We've been talking about intake stroke because the intake is throttled but the pumping work argument applies to the exhaust stroke as well. Pumping work on the intake stroke is a loss proportional to the difference between crankcase pressure and intake manifold pressure. Likewise exhaust pumping work is proportional to the difference between exhaust manifold pressure and crankcase pressure. Fortunately the exhaust manifold isn't throttled so this loss is small. If you evacuate the crankcase it helps intake pumping but hurts exhaust pumping in an equal trade off.

Fig2 on this link can be used to understand all this discussion and more:

http://www.mechadyne-int.com/vva-reference/part-load-pumping-losses-si-engine

Supercharging, turbocharging, valve event timing, pretty much every aspect of engine operation affects the shape of this P-V diagram. It's just a tool to understand what's going on inside the engine.

For the descending piston A versus B yes, you are correct. This is why the crankcase pressure is roughly constant at 15psi or one atmosphere absolute pressure. Crankcase gases pushing from under piston A to piston B and spinning crankshaft drag are additional losses above and beyond the pumping loss I'm talking about.

O.K. we are getting nowhere....obviously what I am calling "pumping losses" and what you are calling "pumping losses" are not the same thing. Define "pumping losses".

OK let's try this:

We all can agree that the pressure above the piston on the power stroke is greater than the pressure above the piston on the compression stroke and is a net positive. This is what we're after, positive work.

Now think about the intake and exhaust strokes. The pressure above the piston on the exhaust stroke is greater than the pressure above the piston on the intake stroke. This is what we have to pay to transfer gasses out and in the cylinder, negative work. The term used is pumping work. Pumping work is quite large when the pressure above the piston on the intake stroke is low in a throttled condition.

The power loop of fig2 is compression and power strokes while the pumping loop is the exhaust and intake strokes. Areas inside loops are work. Power loop is positive work, pumping loop is negative.

Crankcase pressure cancels out of all four strokes because it is assumed constant.

Help any?

Greetings . . .

Here's a link that I found really useful in getting one's head around pumping loss:
StraightDope
I am big fan of practical stories.

And here's some links showing empirical data that lower RPM is the place to be:
TDIclub
MetroMPG

This is where I am coming from when I vote for lowest possible RPM and a decent throttle position equals best economy. As the desired cruising speed rises the need for OD becomes evident.

regards,
stock49

You guys are a long ways from Louisiana to be practicing such VOODOO!

Remember Diet Smith in the Dick Tracy comics, a few decades ago? He used to say "He who controls magnetism, controls the world"

Snowman are you getting all this? I just took the '53 for a drive after the long winter. I was afraid to shift into 5th gear for fear of loosing power, wasting fuel, or something worse. Beater