Inliners International Forums General Discussion Engines Exhaust size effect - is bigger always better?

I recently changed the exhaust on our 250 from dual 2.25" pipes with glass packs and short exits behind the cab to a homemade 'Y' pipe into a 2.25 I/O 'turbo' muffler that still exits near the rear of the cab. The muffler is a local muffler shop's store brand. The results were more than surprising. The amount of low end power (sub 2000 rpms) is outrageously better. The top end did seem to suffer a bit, but most of the driving is not done over 4000.

For some background, we took on the exhaust changeover due to the old system rusted and broke after 4 Upstate NY salt covered road winters (surprise!), and we were looking something a little less noisy, but still 'throaty'. We did the project at home without the aid of a pipe bender. Also, we are running Langdon headers.

Right after the change I thought it seemed like it ran better, but after driving it for several days I found that I can pull hills that I never used to be able to without shifting down and with much less throttle. It also lost a hesitation that it used to have around 18-1900 when cruising and then applying more throttle. The only thing I assume is that the additional back pressure and/or less scavenging effect of the smaller exhaust is beneficial. And now I can hear the radio when driving and I don't have to shut it off when ordering at the drive through:)

I wish I had put it on a chassis dyno prior to changing the exhaust, so as to have a better, real number comparison. But I never really thought that the duals were hurting the driveability that much.


Haven't posted in a while, but log in occasionally and check things out. FYI - we've put almost 40K on the 250 since the rebuild and it still runs the same as when we first put it together.

My understanding is that a smaller exhaust size will increase velocity, which will help with "pulling" your spent combustion fumes out of the chamber so the "new" intake charge isn't diluted by the old fumes. If you have too large of an exhaust, you don't get the scavenging effect and the exhaust, at low rpms, stays where it is, diluting the new intake charge.

That's my guess as to why you are feeling the better low end torque, you had a previously oversized exhaust. There is a chart somewhere that has a rough estimate of exhaust size vs power output. Since, generally, a 4 cylinder with 500 hp needs the same exhaust size as a 8 cylinder, it's a pretty accurate way of figuring it out. The v8 just gets to split the exhaust volume with 2 smaller pipes!

2 2.25 pipes is pretty large for a 250 cubic inch engine, unless you have some serious cam and head work. My opinion anyways!

Well area of a circle (cross-section of exhaust pipe) is pie (3.14etc) times radius squared. So:

2" pre-'75 exhaust = 3.14 sqin
2.25" 1975 on = 3.98 sqin
2.5" HD 292 truck = 4.91 sqin
dual 2" = 6.28 sqin
dual 2.25" = 7.96 sqin

The single 2.25" pipe at the Y is half the cross-section width of the 2.25" duals (obviously). I think for street application for the 250 it should be at least single 2.5" up to dual 2" pipes, but I'd like to see the chart on that too. I'm planning a HD 292 manifold for 250 with a 2.5" single exhaust hooked to a Z28 post-cat split setup (gets progressively larger from 2.5" to dual 2" ending with dual 2.25").

Found a Vizard (intelligent car guy) reference

Vizard's rules of thumb: A pipe will flow 115cfm per square inch of cross-sectional area. You need 2.2cfm of flow capacity per horsepower.

A chart, not sure if it's the one I originally found

http://www.thirdgen.org/techboard/attach...pe-diameter.jpg

Ah that's interesting. For a 250cid on that chart:

150hp with single 2.25" or dual 2"
200hp with single 2.5" or dual 2"
250hp with single 2.5" or dual 2.25"
300hp with single 3" or dual 2.5"

Looking at that chart, cross-section is only one factor as the recommended dual is not one half of the cross-section of the recommended single. I'm betting that probably has to do with laminar flow of air, with the air contacting the pipe moving the slowest due to friction. A dual 2" has a total circumference of 12.56" while the single 2.5" has 7.85", which is a lot less surface area of the inside of the exhaust pipe that the air is dragging against, even tho the cross-sectional area is a little over 1 square inch less with the 2.5" pipe over the dual 2"

My 250 engine ran great with dual exhaust 2" pipe.

I later switched to a single 2.5".

The duals sounded like a V-8 idling.

MBHD

Is that duals with or without the crossover pipe?

I did not use a crossover pipe.
I would suggest using one though to smooth out the idle if you have a rough idling camshaft.

MBHD

For what it's worth my 79 p/u had dual pipes out of the manifold that were 2" for about 18-20" long into a single 2.5 pipe to the cat then 2.5 to the muffler then a 2" tailpipe. For some reason they crossed over the pipe after the cat before the muffler. 6cyl trucks had right side tailpipe v8 left. I have always wondered why that was done,after all it cost more to crossover the exhaust. I think that I read on here that Duece Coupe did some expermentaion with pipes. Perhaps he will comment. Jay 6155

I've wondered why they dropped down from 2.25" on integrated head cars and 292 SD trucks exhaust, and from 2.5" on the HD 292 and integrated 250 trucks, down to a 2" single tailpipe? The V8 cars had dual 2" tailpipes out of the muffler, but in the L6s they kept a single 2" tailpipe. Seems restrictive.

A SINGLE 2 1/2" PIPE WITH A REGULAR MUFFLER IS THE SINGLE BEST EXHAUST SYSTEM TO A 250/292 CHEVY EXCEPT FOR HI-COMPRESSION PERF. ENGINES. THIS IS FROM ALMOST 40 YRS OF ME RUNNING CHEVY 6'S. 230 AND 194 CU.IN. ENGINES NEEDS 2 1/4 OR 2" RESPECTIVLY. JMO
BOB

Regarding the small 2" tailpipe:

I'll bet using the same part as v8 plus quieter exhaust are two reasons for the small diameter tailpipe.

Restriction in an exhaust is necessary to an extent, unless your vehicle is a race-only rig or turbocharged. If you have no restriction, you will not get the scavenging effect. Think of dropping a marble in a glass of water. The marble "pushes" the water it hits downward, but the water around is "pulled" in after the marble because of the vacuum created by the pressure of the marble falling in. The pressure of the exiting combustion cycle will create vacuum behind it that will pull in your next intake charge. If you have an exhaust with no restriction, the exiting combustion pressure will dissipate before it creates enough vacuum to pull in your intake charge. This will cause a low rpm rough idle and or stall. You will have to increase idle speed to avoid this, or put a more restrictive exhaust to create more of a vacuum effect. The less restrictive exhaust will create a scavenging effect at high RPMS, but low RPM performance will suffer. The opposite is true for a more restrictive exhaust. However, too much restriction will cause everything to suffer.

I think I confused myself...

So how come airplane engines have straight out exhausts - sometimes no exhaust? One would think they would be seeking efficiency? I understand - not.

regards.Rod.

airplanes are hi-compression and run very hi octane fuel plus there tuned to run with exhaust that way. there like a race engine with open headers.

Airplane engines in general are low compression air-cooled engines.

Cessna 150,172 ,182 Piper lances etc. all air cooled

They have to use a fuel that should not detonate, hence a lot of lead & 100 Octane, plus be able to flow @ altitude.

200 4 cly CI to 540 6 cyl CI. they all use 100 octane low leaded fuel, not a great fuel to race with, but it does have a lot more lead in it than any automotive fuel ever had.
About 10 times more lead.

They are not a high RPM engines either, 2200-2600 RPM max IIRC.

The cylinder heads are the worst design as far as getting the most power out of them, 90 degree turns, no short turn radius to speak of.

The engines that had to run 130 octane were the supercharged military & higher HP engines.

The 130 octane airplane fuel has not be available since I don't know when, probably 30 years ago minimum?

MBHD

I respectfully disagree.

Probably the number 1 reason for a 2" tail pipe is---cost. 2" pipe is cheaper than 2.25 or 2.5". You can get away with a smaller tailpipe because the exhaust gases are cooler and have contracted somewhat compaired to right after the manifold. This is especilly true in a non-cat car or truck. Look at new trucks and suv's the tailpipes are much bigger then in the old days. I believe that volicity is the key to a good system. A too big ex pipe will let the gases slow down and hurt performance. This is most noticible at low speeds on smaller engines. This is why IMHO people think that they need "backpressure" to make it work. When the smaller system was put on volicity went up at low engine speeds and helped scavage out the ex gases. After all why doesn't a high end car like a new Corvette have a 5" exhaust system? Jay 6155

I think J78 and FTF said it well.
Every car I've ever had went as fast or faster with no backpressure. You do typically need to jet richer though.
An engine that's too lean WILL be slower uncorked - until you re-jet.

The thing about velocity is key too, as long as that length of pipe is part of the tuning of total length vs pulses at a given RPM. That said, better too big than too small.

Primary header tubes that are too big can really hurt an engine, but beyond that, exhaust pipes that are "too big" don't hurt very much - but pipes that are "too small" will really hurt, in a v8 they can kill off ~100hp on a really hot engine.

I suspect JimW's old rusted out system was acting kind of like "open exhaust", and leaned things out too much so it was slower - and also shortened the overall length which took low end away.

Well, the exhaust wasn't always rusted out and the the only leak that developed was when the glass pack flange broke. The rest of the piping and the other glass pack was too rusted to salvage.

What I was driving at in the original post that the 2.25" duals were creating poor situation for daily driving. The exact reason why? I can't be sure, but I would safely say there is a major difference between tuning an exhaust system for performance vs. daily driving. This was only the second time that I've modified an exhaust system and the effects are similar - more restrictive helps bottom end torque and open helps WOT driving.

Obviously, too restrictive would just be silly, but 'the word' on the street (and in most forums) is that bigger is almost always considered better - I say not. People such as David Vizard have done extensive research determining the effects of exhaust sizing, and I would not consider contradicting him. All I'm driving at is : in my nonextensive research, my original 2.25" duals were too big for 225ish HP 250 inline to use as a daily driver.

Regarding what GM was thinking/doing when they built the exhaust systems for inlines, I can't tell if they were using what was convenient because they assumed no one would be performance minded if they were buying an inline, or if they put alot of research into attempting drag every last fl/lb of torque out the 'little' inline. Who knows? Certainly not me, but it would seem logical that there wasn't a ton of concern at GM for how to build the best inline exhaust. The inlines were more of utililitarian engine, I could see a group of engineers sitting around in the 60's saying 'if someone wants more go, then buy a 366 or 402'.

Just my opinion and I wanted pass along my experience to others who might be building an inline for a daily driver.

p.s. - has there been recent issues with this site? This the first time since I started the thread that I could log back onto this site???

Having a difficult time responding, this is the third try.

The exhaust system wasn't always rusted out and the I've determined in my non-extensive research that dual 2.25" pipes were too big for my 225ish HP 250.

bcowanwheels says:
"airplanes are hi-compression and run very hi octane fuel plus there tuned to run with exhaust that way. there like a race engine with open headers."
Then Mean buzzen half dozen A.K.A. Hank says:
"Airplane engines in general are low compression air-cooled engines."
Now it is obvious even to my limited intellect, that you cannot both be right.
I am more confused than ever. Perhaps it is time for me to give Google a workout and see what HE says.

regards,Rod.

Frenchtown Flyer, what is it you disagree with? I'm always open to seeing another point of view. If you disagree, please explain why so I and others can understand.

Also, my post was about overall drive quality, not achieving the fastest quarter mile. The original post was about his ability to pull hills, generally a lower RPM torque situation, not a high RPM race situation. When you are only considering speed or overall acceleration, like 1/4 mile times, low RPM scavenging isn't even at play. You never spend time in the rpm range where it's important. I'm not saying you will have faster 1/4 mile times with a more restrictive exhaust, it'll never be the case. I'm talking about overall driveability, being able to pull hills and have a nice idle and leave stop signs without reving to 3000 rpms.

Everyone should understand the difference between driveability and overall performance. Once generally detracts from the other, especially when fuel injection is not present.

I also think that Deuce Coupe is onto something. Notice in the first post by JW that the heasition at 18-1900 rpms when cruising are gone. At that engine speed you are on the main metering system instead of idle or off idle system. At cruise with just a slight push on the pedal he may not of opened up the power enrichment system or it heasitated until the power system opened. This could be a too lean condition that was helped with his new exhaust. Maybe a combination of things are at play here. Wapolla, my father was a private pilot and he and I flew in a lot of those planes that Hank mentions. Hank is right about the rpm range and I think he is right about compression also. In the 1960's and early 70's there was 80 octane avagas that was sold at airports and we used. Yes I said 80 octane. Some time in the 70's I think they did away with 80 gas and 100 high lead gas and everyone had to run 100 octane low lead. I remember that this was considered a bad idea for 80 engines because of the higher lead content. I don't know if any engine specs were changed or not to work with 100LL. Probably not because of certification costs. IMHO lead was the best thing ever to happen to gasoline and every effort should be made to get it back in. Jay 6155

The pressure differential when the exhaust valve opens doesn't depend on a few lbs. per square inch of mercury from "backpressure", since the initial exhaust pressure is perhaps 50 psi.

There is a long-standing tendency to conflate all the effects of an exhaust system. There are at least 3:
1. Systemic pressure (measured in the pipe before the muffler). This increases pumping loss, but not necessarily the conditions at any individual exhaust valve.
2. Local port pressure (directly after the valve seat). This is where backpressure is always detrimental, since it frustrates overlap flow and chamber purging. It's frequently the result of 3. (below), rather than 1. (above).
3. Multiple wave reflections from all valves opening, and changes in cross-sectional area including the muffler and tailpipe.

Factory exhaust installs are carefully designed and arranged so that any accidental positive wave arriving at an exhaust port during overlap is damped as much as possible. Something as small as juggling the pipe lengths before and after a muffler, or changes in muffler volume will make this better or worse - it's not backpressure.

I had only thought that the wave would only have any real effect in a open exhaust situtaion like with open headers. This is why the length and diameter are tuned i.e. primary pipes in a header. Is there a way to make the waves help out in a street system? Some forumla to use? Jay 6155

I think Panic "hit" on part of the tuning thing - changes in x-section area. "Open headers" really just means the x-section area ratio becomes infinity (or zero). But if you had a huge muffler, say a 55-gallon drum, the exhaust would "see" pretty much the same thing.

Often a dyno room will just use huge sewer pipes (6" diam or so) to route exhaust out of the room - hoping the effect is the same as open headers. Its not, but its pretty close.

There ARE formulas around for pipe length vs tuning RPM, but they always depend on valve opening / closing. Measured HOW? Seat-to-seat? I usually don't like the answers I get from those.

Usually you end up with primary tubes determined by the fit in the car, unequal length and to me that's fine for street or drag since each cylinder will have a different "peak HP" rpm.

The collector length (including ideally the exhaust pipe that's the same diameter) then gives a lower RPM peak, say the peak torq RPM. This length we can tune, but that has long been done at the strip using open headers, literally cut & try to get the best ET's. But its not too hard to do. And of course in any tuning, you do have the compromise that the pipe DIAMETER has to be big enough to avoid backpressure, which is always bad, but not so big that you lose velocity and the pulse signal, at that point your tuning effect is all done.

Then, that's where you want the 55-gallon drum muffler.
Of course that's usually right under the floor pan, the worst possible place for ground clearance, let alone a 55-gallon drum.

So a compromise is to put an H-pipe or X-pipe at that spot, to fake the exhaust into thinking that's "open" to the air.

Of course it wont be fooled so you will get some pulse effect but not all of it.

From there, the best you can do is put the biggest mufflers you can fit as close up as you can, typically under the back seat.

After that, the pipe size doesn't matter so much, you could use a 3" or a 20" pipe or even a smaller eg 2.5" because the exhaust has cooled somewhat - you just need enough pipe to get the exhaust out without backpressure.

This is kinda what the factory tended to do, they just made more compromises for cost than we can.

Remember, above all, the six was meant to be CHEEEEP - not just good on gas, but cheep to make, cheep to repair, cheep to own.

Thank you for explining to me why people put in x or h pipes. For those of us that are running manifolds the only thing avaible for "tuning" would be pre-muffler pipe diameter, length to a limited degree(changing muffler location usually not much choice here) and type of muffler(s). I don't know about others but there is not quite enough room for me to run a 55 gallon drum. It would probably drone on the freeway anyway. I currently have a 2.25 pipe all the way from the engine to the outlet of the single glasspack then it drops down to 2" for the 15" tailpipe. I think there would be a small gain by going to a 2.5 pipe if it will fit. Jay 6155

Now THAT I agree with.

You can do a Google search on exhaust size vs hp and come up with some dyno charts that will show the effects of small to large exhaust pipe swaps. Pretty interesting, although almost all of the sacrifice some bottom end and gain top end. Although most are on V8s, the concept is the same.

Just wondering what google has said?

I am an A&P mechanic, I have worked on the smaller air cooled low performance engines in school & my first job was fueling airplanes, then later was working on them, so I know first hand.

I started working on aircraft in 1988,even then, there was only 100 low lead fuel available , never seen 130 octane even available to this day.

A Cessna 150 has a Continental 0-200, 200 cubic inch air cooled opposed 4 cylinder engine, nothing high performance with that engine.
The cylinder head design is horrendous, the exhaust looks just as bad.
That is just one example, there are plenty more.

MBHD

I think that is another "urban legend" about the avgas being ultra high octane. I have always heard that also even though I had never seen it more than a 100 or so octane.

Mr Google really didn't answer my question. Perhaps I just couldn't find the info, but I did try. Still would like to know.
Engines like the V12 RR Merlin, Packard,Allison etc all had a little stub exhaust about 3 or 4 inches long. You could see the port inside. They went REAL GOOD.
Surely someone will come up with a detailed answer.

regards,Rod.

That goes with what I stated earlier, military planes, P-51 Mustangs are high performance, they tried the best they could so they could outrun the enemy and carry heavy loads.
The short stacks on the Merlins P-51 Mustangs are made as not to create any back pressure, those engines you mentioned had superchargers available IIRC, but they all did not have them.

I just know Continental, Lycoming, opposed 4 & 6 cylinder air cooled engines, had terrible cylinder head designs.
Turbocharged, they ran low boost.

TSIO 540= turbocharged injected opposed , something like that.
540 cubic inch 6 cylinder I dont know if you think 230 to 350 horsepower out of 540 cubic inches is high performance, but to me, that is not high performance.

More info here. http://en.wikipedia.org/wiki/Lycoming_O-540

continental 0-200 100 hp from 200 cubic inches.

http://en.wikipedia.org/wiki/Continental_O-200

Fuel

http://en.wikipedia.org/wiki/Avgas

http://www.shell.com/global/products-ser...ypes/avgas.html

MBHD

Found these engine specs for the Rolls-Royce Merlin V-12's used in the P51 Mustangs. They were supercharged and had 6:1 compression and made 1735 HP@3000 RPM. Here's the link below.
Rolls-Royce Merlin V12

A couple comments for consideration:

Remember that aircraft piston engines need to operate at widely variable ambient pressure, and are often supercharged.

Also, aircraft and automotive fuels are rated differently. Automotive octane rating is R+M/2 while aircraft fuel is something else. See the wiki avgas link Hank posted above.

Not to change the subject from airplane engines, but for poops and giggles I took off the muffler and ran it for a day.

4 things : 1st , it was loud, 2nd, the hesitation between 18-1900 was back, 3rd, a lot more top end, 4th, it was really loud.
Keep in mind the homemade 'y' pipe ends under the driver seat outside of the frame rail and under the cab - so, pretty short.

I would love to have the money and time to make an exhaust system with mandrel bent pipes to see what I could gain and maybe try a a 2 1/2" in and out muffler, but that is what bench racing is for I guess. Thought I would pass it along, and I'm standing by that large exhaust is for top end only - driveability is compromised - FWIW.

Still want to put on a turbo and MS1 or MS2 with mutliport - no funds, but lots of fun thinking about it. Maybe someday.

Here is mine from the 65 Chevelle wagon.
Hooker long tube headers, 2 1/4 out the Y pipe, 2.5 muffler input and 3.0 tailpipe. It pulls at all rpms with a mild cam and 250, 5 speed and 2.56 gears. No overdrive.
Duals to a single is the way to go.

Once you get the turbo on, you'll find the less exhaust the better. Turbos are quite a restriction themselves, so any exhaust afterwards is necessary only for noise reduction. Many people run no muffler because the turbo has somewhat of a muffling effect itself.

Nice, a lot prettier than mine. I'll try to post a pic tomorrow if I can. Have you seen what the effect of dropping the muffler or 'Y' pipe off on a chassis dyno?

Not to change the subject Tom, but are you still looking for cylinder head cores? If yes, then I found a JY within 20 miles that is ready to scrap a bunch of truck 250 engines. PM me if interested.