Inliners International Forums General Discussion MOPAR max substained RPM for highway driving

Somewhere I read that the max substained RPM for long stroke engines was 85% of rated horsepower RPM. The 218 cu flathead Plymouth engine falls into the long stroke engine catagory. The horse power is rated at 3600 RPM which limits the top driving RPM to 3060 RPM. Does anyone know if the 85% rule is a true statement for long stroke engines and where does it come from? Somewhere someone has a manual for engine design which probably has this information and I would like to know more about the subject as I have been cruising mine up to 3400 RPM and just replaced all of the piston pin bushings and all of the bearings. The bearings all showed signs of destress and two cylinders had the top ring broken. It was suggested that I had run the engine at too high of an RPM range for long periods of time and maybe to much advance. The top half of all four main bearings showed more wear than the bottom halfs did, what gives here? The engine had less,than 8600 miles from total rebuild, but I did a 4600 mile trip in August which probably did the most damage as we cruised between 65 and 75 MPH for hours on end (3200 to 3400 RPM).

In 1951 my father and i made a trip from Iowa City to Davenport, then 55 miles on two-lane, in a recently acquired 1933 Plymouth two-door. Exactly one hour after we left Iowa City the Plymouth was on a lube rack in Davenport, for its scheduled oil change. On the way home, later that day, that little six threw at least one rod through the pan.

A friend towed us home on a towbar, and it was then that we discovered that the speedometer in the Plymouth was 20 mph slow.

Education is always expensive.

God's Peace to you.

d
Inliner #1450

Jim,

I think you're running your engine WAY too hard! That sucker was never expected to maintain those kinds of constant speeds! If you look at the Torque specifications chart, you'll see that the 217.8 (218) engine's peak torque is at about 1200 rpm for '49 thru '53 and 1600 in '54.

You could really put an overdrive to good use, if you want to maintain those kinds of speeds. I believe if you attempt to use the torque ratings as a guide for gearing for highway driving, you'd be closer to the optimum drivability. Your ability to take off from traffic lights and climb hills may be hampered a little, but highway driving should be a lot less stressful.

I'd like to see other's comments on this subject.

Dennis;

I think your right, an OD trans would help.

This topic's being discussed under 'engines' right now.

Happy trails.

Hello, assuming you have 27 inch tires, a 3.90 diff, at 65 mph you are cranking at 3100 rpm. For sustained use, that is probably to much. If you had even a 3.54 diff, your rpm would still be 2863 at 65 mph.
With the R10 OD trans, you could no doubt travel at those speeds ie 65 to 70 mph at 2200 to 22500 rpm, which is a lot more relaxed for this engine.
Your present set-up was designed to travel comfortably at 55 to 60 mph.
I know someone who back in the 60s had an early 50s Plymouth with a 4.10 rearend, he was heading to Grand Beach, 70 mile north of the City, he was doing 80 mph most of the way, he blew the engine unfortunately that day and had to be towed home.
You could always convert to a T5 trans, from an 80s S 10 truck, with some mods to your bell housing this trans will work, in 5 th gear you should be able to cruise nicely at 70 mph.
My car, a 1947 Chrysler Coupe has a 4.30 diff, which was transplanted way back when, I have a 3.54 waiting to go in it's place. With this present diff, I would not venture over 50 mph, so no highway cruisng right now, even at 45 mph she is getting up there.........Fred

I installed a 90 Dakota rear end with 3.55:1 ratio, with my tires I am turning 3300 RPM at 75. I was running 10% advance at idle which I have been advised by George Asche to be too much. Is this Fred from the P15-D24 fourm with the Chrysler?

Yes That'd be me. George will steer you in the right direction, or you could go with a Don C. T-5 trans set-up, or bigger tires and slow down to 60 mph. As you know Greg G. has a 4.10 rear end , with P225 75 15 tires, he was cruisn to NC at 60 to 65 a majority of the way, no probs either.........Fred

Overdrive is the ticket here. In 1957 i rode many miles in a '53 Plymouth Belvedere with a 265 cid Chrysler ("long block") and overdrive at more 100 mph. That lovely piece would bury the speedometer needle, and with five of us aboard once made the trip from Abilene, Texas, to San Angelo--89 miles on two-lane--in 62 minutes. Don't try that with a stock Plymouth--even with an overdrive!--but overdrive will surely increase the cruising speed.

God's Peace to you.

d
Inliner #1450

I have driven those roads many times, I retired 4 years ago after 32 years with the state of Texas. I worked in the Aviation Division of what became TXDOT. I inspected all of the open to public airports in the state of Texas for compliance with FAA safety standards. Probably drove almost every back road in the state of Texas. Sounds like you might have been the Airforce, both towns had an active base then, now just Abilene.

'Overdrive' is one of the most under-used words with these cars...

I wouldn't contemplate having one without it.

Jim;
As has been mentioned my T-5 is the ticket. With the T-5 and a 3.55/1 differential my long block Desoto engine runs my P-15 down the road at 70 MPH turning 2200 RPM's.The speed is measured with a GPS and the RPM's with a tachometer. I have ran at 70-75 MPH for hours on end with no engine problems.

Have you decited what way to go with repairs? Hope you stick with the flathead but I will understand if you do not.

Hi Guys
Does anybody have a handle on what the weak link is that causes this problem? I know there have been some moderately successful guys running Mopar flatheads at Bonneville, so there must be a fix..... (not that you would want drone down the freeway at 3300 anyway, you should by one of those rice rocket motorcycles for that)

Does it need more oil volume and pressure?

I have concluded along with Geoege Asche that 10 degrees initial advance at 500 rpms might be a little much. The centrifugal advance is 16 to 20 degrees at 2,050 and 18 to 22 degrees at 2,850 and 14 to 18 degrees on the vacuum at 2,850. since I was turning 3200 rpm I will take the high side of 22 degrees advance from centrifugal and 18 degrees from the vacuum plus the 10 degrees initial and the results are 50 degrees before TDC. If I take the low side of the 2,850 numbers which would be 18+14+10=42 degrees before TDC. Either of the two numbers is between 5 degrees and 10 degrees too much advance. In answer to Dons question yes I am going to keep the flathead. As of this moment the crank out of my original engine is being turned and will replace the one now in the engine. I have a T-5 from a S10 already and have started modifing the bell housing but might go with Paul Curtis's adapter setup. The cost will probably be about the same if I get the pilot shaft puck machined and buy a clutch disc and also have the hole in the bell housing opened up. The fellow who has the bridge port machine and lathe and was going to help me has taken his wife and gone to Alaska for the summer and has not returned. I have not pulled the #1 main bearing cap to see if it is also damaged. I rolled the old top main bearing shells out and rolled the new top shells in. The bearing can only go in one way for the stop to fit in the grove so I do not think that is the problem. On first fire up the #1 rod knocked so I shut the engine off and thought maybe the oil had just not reached the rod yet. On restart the engine did not knock and ran at 50 mph with 50 psi oil pressure. The bearing must have spun on initial start up and fused to the crank and turned in the rod. May have something in the oil passage in the crank or block, will find out on teardown.

Jim-

Sounds like your flatty is on the road to recovery once again. Happy to hear you've decided to keep the six, and go with a T-5. I think the T-5 will make a huge difference in drivability for you.

Hope to meet you again one day, and take you for a ride in my '50...

Pete

The poor old B-W overdrive... underrated again!

Much more original, rewired it's much more fun, and it's a taller gearing anyway!

Ray:
The only ones that I have been able to find are in the hands of retailers and they are asking $1200 to $1600 complete. I have three B-W overdrives for Chevrolets which I have bought at swap meets over the years. Ones needs new bearings in the front gearbox and on one the overdrive engages and disengages by its self. One is in my 55 Chev pick up and .70% overdrive is to high with the 3.9:1 rear and P 235 75 15 tires. My 48 Plymouth has a 90 Dakota rear end installed in it with 3.55:1 gears so the .70% overdrive might be too high for it. To buy and install from a retailer with shipping would be about one third the resale value of the car. My car is a Special Delux 4 door sedan and "Old Car Price" book lists a #4 car at $3080. Now I have had this car for 20 years and love it but it still needs new rubber all around and glass plus upholstery and paint. Now that same car as a #2 is valued just a little over $6500. I cannot do the things to make it a #2 car for $3500.

For Leif in Oakland:

Except for late model cars with overdrive automatics, I have never had a standard shift car that did much over 20 mph per 1000 rpms. If it did it had a 5th gear overdrive. If you will read any of the old road tests before 1972 20 mph per 1000 rpm was pretty much standard practice with all manufactors. That being the case then you drone down the highway at 3750 rpm at 70 mph, granted that would be too much for a flat head six but not for most modern engines. Now I think that you will find that the "Rice Rocket Motorcycles" drone down the highway at a much higher RPM than 3300 at 70 mph, as for as oil pressure my engine ran with 50 psi at 50 mph which is more than adaquate.

Dear Jim;

The 3,750 mentioned is/was to high for long periods etc.

Remember; in the 50s the max speed was 55 MPH and long before the Interstate System too.

It's difficult to drive 55 today, so the T-5 is a 'good move' if your going to travel the Interstate a lot.

Happy trails.

John:

I installed a 90 Dakota rearend with 3.55:1 gears and the engine is turning 3184 at 70 mph. The horsepower on these engines was taken at 3600 rpm and should be able to be maintained without hurting the engine. With the stock 3.9:1 rear end 55 mph was 2750 rpms. 3184 rpms gives you 63.75 mph with the 3.9 rearend. I would not think that 435 more substained rpm would damage the engine but I could be wrong.

J;

You should be okay with 3,184 if all else is good. The 'original' Mfg. design though was in the 2.500 RPM range though.

Try to slow down if It's safe etc.

Happy trails.

Well today I finally pulled the front main bearing cap to see if what I thought had happened did happen. My "dumb" mechanic messed up, he installed the botton front main shell in the top saddle in the block. For those of you who do not know all four of the main bearings have an oil hole in both the upper and lower bearing shell except for the number one main which only has the oil hole in the upper shell that goes in the block and not the cap. Now when you install the shell without a hole in it no oil goes to the main or the number one rod bearing. The damage is not as great as I first thought. The number one bearing surface on the crank looks good and the number one rod journal polished out but is .0015 out of round as best as I can read the mic. I have to set the mic and then remove it so that I can read the scale. The mic will open up while removing from the journal. The bad part of all of this is that the "dumb" mechanic is me.

I already have my other crank turn .010/.010 and new rod and main bearings on the way just in case. I am going to try to buy just one rod bearing in -.020 size for the damaged one and reassemble the engine. The crank mics .001 to .0015 out of round and I think it will run a long time if the oil pressure holds up, going to try it anways, only cost the cost of the rod bearings and 5 quarts of oil. If it doen't work then I still have the option of pulling the engine and replacing the crank and bearings.

Jim, glad you discovered the problems, and for hi-speed driving you may have to go with an OD trans or a T5.
But your original question on the max allowable sustained rpm, I have read articles, where a 51 Desoto p passenger with a 4.10 rearend was cruising at 60 to 70 mph, on long trips and also pulling a camper, through the Mojave Desert and in the Rocky Mountains. Now the owner would routinely run the engine in the 3000 to 3200 rpm range, he never cooked his engine. This article is in www.allpar.com.Is this for real, do ya think.
Right now I have either a 4.3 or 4.1 rearend on my 47 Chrysler, 3 spd trans. Until I can get my 3.54 installed I want to take it easy, as this engine has miles on her. I hope to at least cruise at 50 to 55 mph, which is in the neighbourhood of 2700 to 2900 rpm. Should I run the engine like this or not. I am fortunate, beacuse there are a lot of small highways around here, and I can avoid the Interstate type highways.Thanx 47

I see no problem running in the 2700 to 2900 RPM range. The horse power on these engines is rated at 3600 RPM and 85% of that is 3060 RPM. I have concluded that I was running way too much ignition advance with the 10 degrees initial at 500 RPM. The book calls for 0 degrees at TC. I timed it like I do my 235 I/6 Chevorlet. At 3300 RPM it could have been firing 50 degrees before TC if both centrifugal and vacuum advances were at their maxium. The centrifugal and vacuum advances more on MOPAR cars than it does on old Chevrolets.

I know this post is old but I,m new here and looking into the same swap. My quick question to you all is are there 6volt tachs? Stupid I know, I,m used to seeing the 4/6-8 "cyl" designation but not voltage....

I found some at a place called the tach shack.com
they are not cheap. Hope this helps.

Try these guys: http://www.westach.com/

Hmmm... I wonder?



This is in a 1935 Auburn, which was originally 6V. But, of course, it may be mechanical.

Not an answer, really, but maybe the basis for further questions?

Here is the Allpar article, it concerns a 1951 DeSoto Suburban limousine which the owner bought new.

http://www.allpar.com/cars/desoto/suburban-1951.html

This is the best report I have seen on the potential long term life of a Chrysler/DeSoto flathead 6.

At the time of this writing (1975) he had put around 180,000 miles on it.

Much of it towing a trailer at speeds of 70 MPH.

He says:"Operation above 3600 RPM is avoided in the interest of long engine life, especially with such a long stroke. But continuous full throttle operation on upgrades at 3200 presents no problems".

75MPH in a DeSoto Suburban with 4:11 gears and 8.20X15 tires = 3600 RPM

3200RPM = 66 MPH

He also reports at 10,000 feet elevation, towing a trailer in the Rocky Mountains "at 70MPH the engine smooths out like a perfectly balanced turbine"

This is with a 4 3/4 stroke crankshaft, the longest stroke Chrysler made for this engine.

I have a vintage Sun Tachometer in my '41 Plymouth. The key is the sending unit. Mine is for 6 cylinders and 6 volts. I found both the tach and the sending unit on eBay.
Jim Yergin

Round number for maximum sustained RPM: 2,500 feet per minute mean piston speed. With a 4.25" stroke this is 3,529 RPM. To calculate: divide 15,000 by stroke length.