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Old 02-21-2011, 01:07 PM
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some exhaust restriction, good or bad

This is a theoretical question.

I have heard that you can go too big on the exhaust pipes. Not the headers, coming off the heads, but the rest of the piping aft of the headers. Some people telling me this say the engine needs a slight bit of back pressure.

I believe you can have some benefits for having the headers tuned to a certain length, long headers favor lower RPM running, shorter headers favor higher RPM.

I realize that there is a point of diminishing returns, but would a small block engine in a pickup for example, make more horse power. with pipes that are "too big"?

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Old 02-21-2011, 02:49 PM
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You certainly can go too big on both exhaust pipes and headers depending on what RPM range you want your torque to peak. However it is not restriction (back pressure) that changes the torque curve rather it is the exhaust velocity. With smaller pipes the exhaust is forced to increase in velocity so when the piston reaches near TDC there will be a scavenging effect from the higher velocity exhaust creating a slight vacuum effect at the exhaust port. If the pipes are too big then the exhaust will be slower so when the piston reaches TDC there will be little or no scavenging effect at lower RPM. The smaller pipes produce more torque at a lower RPM and depending on other performance perimeters this may be quite desirable but it is a trade-off in higher RPM power. This trade-off occurs because as RPM, and thus exhaust gas volume, increases the low RPM scavenging effect will be overcome and offset by the added restriction of the smaller pipes. Trading the smaller pipes to bigger diameter less restrictive pipes can result in power increases in upper RPM ranges due to less exhaust restriction but these too have to be small enough to create enough exhaust gas velocity to create the necessary scavenging effect.

Bigger is not always better and that is why sometimes it can be counter productive to go overboard on valve sizes, intake runner size and exhaust pipe size. By the same token smaller is not necessarily better either so the trick is to have a balance of parts and don't go "hog-wild" with the biggest of everything. When someone says that an engine needs a bit of back pressure they are confusing back pressure or exhaust restriction with scavenging, back pressure will rob an engine of performance but at lower RPM the scavenging effect created by smaller pipes will make more torque until the exhaust volume reaches the point that enough back pressure is created to cancel the scavenging effect. Back pressure is a power killer and in itself is unwanted so the trick is to balance the pipe size to put the torque where you need it, a smaller pipe will generally give high velocity with good scavenging at lower RPM but at the cost of power robbing back pressure at higher RPM, a bigger pipe will generally give less torque at lower RPM due to lack of scavenging effect but will make upper RPM power as exhaust velocity increases and scavenging is created but without the added restriction of the smaller pipes to create as much back pressure.

The bottom line is that back pressure itself is unwanted and the noticeable increase in bottom-end power that often occurs with a change to smaller pipes/header tubes is due to better lower RPM exhaust port scavenging and not because of added exhaust back pressure these pipes will cause at higher RPM.
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Old 02-21-2011, 03:03 PM
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Oldred, Thanks for the reply.
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Old 02-21-2011, 04:58 PM
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Some things I have learned from testing header design with EGT....Ideal back pressure is very dependent on the cam and it's timing. Not enough back pressure and the combustion is not completed before exiting into the header. With less back pressure you will note a higher exhaust header temps. This can also be seen by a sooted up header from the fuel burning in the header instead of the cylinder. Loss of power and fuel economy is just some of the problems when you reach this point.
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Old 02-21-2011, 06:49 PM
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Quote:
Originally Posted by gow589
Not enough back pressure and the combustion is not completed before exiting into the header.

I am a bit puzzled about how that would work could you explain?


I am NOT trying to disagree or be argumentative I honestly just would like to understand the mechanics of what you are saying.
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Old 02-21-2011, 07:14 PM
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Well learning this started with building some software and circuitry to build measure things; Things such as O2, EGT, RPM, Timing, Timing advance, etc.... I didn't have too many goals with the project; it was more of a discovery to see what I could capture and what I could do with it. This is not a dyno nor am I trying to pass it off as.

I built a special set of headers for a car. In hooking it up and running it EGT's was one of the first things I got working....but I ran into a snag. If you look at these measurements from a steady 2000rpm run, look at the difference in temps. The bottom number is a reference and the top number is a calibrated temp.







4 cyl running in the 1000 degree range and 4 running in the 4-500 range.

At first I thought I had a problem with my circuits so I swapped thermal couples. Turned out readings were reversed and measurements were spot on.

So I thought what the hey? (or something like that). Then I looked at the lack of thought that went into the exhaust collected in the headers.

Fire order 15426378. Cylinder 1 and 4 are collected 2 into one. When one fires it has NO back pressure. Look at the temp. 1008. 5 fires then 4 fires. #4 still has back pressure and the burn is taken place in the cylinder nice and clean. You fire 5 more cylinders before getting back to cyl #1 and it has no back pressure again.

Cylinder 1 header is sooted up. Cylinder 4 is clean as a whistle. There are 4 pairings doing the exact same thing.

So if the fuel is burning in the exhaust instead of the cylinder, there goes fuel mileage and power.

I am actually building a second set of headers and muffler where I can control and test the back pressure.
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Old 02-21-2011, 07:44 PM
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my thoughts.
value = -0.01 cents.

plug fires.
cyl psi is hi.
hi dur ex vlv opens early during down stoke of ign cycle.
fuel mix still burning.
large pipes offers lower back pressure than cyl press.
burning fuel mix escapes into headers due to press imbalance.

Last edited by 001mustang; 02-21-2011 at 07:57 PM.
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Old 02-21-2011, 08:00 PM
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Quote:
Originally Posted by oldred
I am a bit puzzled about how that would work could you explain?


I am NOT trying to disagree or be argumentative I honestly just would like to understand the mechanics of what you are saying.
"not enough backpressure and the combustion is not completed before exiting into the header"

That's because the statement is a bunch of baloney. GOW, you need to do some more research on exhaust theory and practice, and brush up on your internal combustion engine knowledge.

Backpressure does not "hold the mixture in the cylinder for a complete burn".

I'm sorry but you have really missed it on this "theory" of yours.
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Old 02-21-2011, 08:33 PM
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Quote:
Originally Posted by ericnova72
"not enough backpressure and the combustion is not completed before exiting into the header"

That's because the statement is a bunch of baloney. GOW, you need to do some more research on exhaust theory and practice, and brush up on your internal combustion engine knowledge.

Backpressure does not "hold the mixture in the cylinder for a complete burn".

I'm sorry but you have really missed it on this "theory" of yours.


Then explain what is happening.
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Old 02-21-2011, 08:52 PM
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Header pipe #1 is not dirty because it opened to no backpressure, it is dirty because it was overscavenged by the inertial wave pulse from being paired with the #4 cylinder. That long combined scavenge pulse that resulted from #4 following #1 pulled some of the fresh incoming mixture from cylinder #1 out into the #1 header pipe during valve overlap.

Backpressure is never a desirable thing to have. If adding backpressure helps the engine you are working on make better power then some other/several other parameters(valve timing, cam timing, intake tract length, header primary pipe length, mixture density, carb venturi size or jetting, etc) is incorrect.

There is no benefit to trying to hold the exhaust into the engine. You can't burn it again, so it doesn't help in any way except emissions compliance(EGR valve).
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Old 02-21-2011, 09:02 PM
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1st I don't appreciate the assumption that I don't know anything. That was a poor assumption on your part.

2nd, if a scavenged pulse can pull more fuel/air through a chamber, back-pressure can stop it.

3rd. With the difference obtained in this scenario and with the long time between cyl firings I don't buy it. The scavenge pulse is gone. None the less I will take your thoughts, measure some velocities and see if it is even possible...but I don't buy it.

4. If your remember long a go when Kassie brought the wrong headers to the engine masters. The set he brought were more restrictive and he thought he blew his shot. What happened was he made more power then he ever did across the board and ended up winning the event.

5. I personally believe people get caught up in scavenge and ignore a proper balanced system. Without a crossover pipe your an odd man out anyway.

I am open to your thoughts.
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Old 02-21-2011, 10:06 PM
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gow589, here is a thought. In my dim memory, I seem to remember about an article in a Hot Rod magazine about somebody experimenting with "360 degree" headers. On your engine, Cylinder 1 would be paired with 6, cylinder 5 with 3, cylinder 4 with 7, and cylinder 2 with 8. Yes, it involved a bunch of wacky under engine crossover pipes, and would not be practical in most cars or trucks. But the idea was to have each cylinder scavenge the same. Or it might have been "180 degree headers with Cylinder 1, 4, 6, and 7, on one side of the engine, and 5, 2, 3, and 8 on the other side. The idea was to have even out the time between each cylinder firing. Have fun getting all the pipes equal length.

But my question was really not about headers. It was about what is best after the headers,or manifold. A little restriction, "to keep the gas velocity up" or as free flowing as is possible.
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Old 02-21-2011, 11:21 PM
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restrictio
Quote:
Originally Posted by DanielC
A little n, "to keep the gas velocity up" or as free flowing as is possible.

I think maybe there is a bit of terminology confusion here, "restriction" in the exhaust would indeed create undesirable back pressure. The smaller pipe would "constrict" the gases into a smaller stream forcing them to increase in velocity and actually aid in removing exhaust. A simple restriction like a muffler or catalytic converter of course would do nothing to help increase power.

The explanation I tried to give did take into consideration that you were talking about the exhaust pipe after the header and in that case pipe size is important to exhaust velocity, too little from a pipe too big is counter productive.



Dang these guys have some nice tools! Back in "the day" all we had was what we could read and trial and error.
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Old 02-22-2011, 12:09 AM
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pipes

when a bend,T,Y etc. is thrown in the mix...the nice high velocity laminar flowing gases become turbulent. turbulent gases increase pressure drop. takes some serious calcs or experimenting to get tuning perfect.
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Old 02-22-2011, 05:13 AM
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Quote:
Originally Posted by DanielC
gow589, here is a thought. In my dim memory, I seem to remember about an article in a Hot Rod magazine about somebody experimenting with "360 degree" headers. On your engine, Cylinder 1 would be paired with 6, cylinder 5 with 3, cylinder 4 with 7, and cylinder 2 with 8. Yes, it involved a bunch of wacky under engine crossover pipes, and would not be practical in most cars or trucks. But the idea was to have each cylinder scavenge the same. Or it might have been "180 degree headers with Cylinder 1, 4, 6, and 7, on one side of the engine, and 5, 2, 3, and 8 on the other side. The idea was to have even out the time between each cylinder firing. Have fun getting all the pipes equal length.

But my question was really not about headers. It was about what is best after the headers,or manifold. A little restriction, "to keep the gas velocity up" or as free flowing as is possible.
The 180's on this car take up the trunk. I was not willing to lose the trunk:







But were talking two different things here.

The appropriate back pressure through and engine is not the same as a timing pulse which can pull an extra boost of air in. They are two different but related events.

Take any engine whether it has headers, a basic intake manifold. If the back pressure is in range the exhaust is burned and the manifold is clean, or grey. Have an exhaust leak and what happened? The exhaust turns black and that cyl is sooted up.

If the back pressure is not at least in the appropriate range to keep the intake charger in long enough, timing of headers for an additional pulse or charge won't help.

If you pull the exhaust manifolds off a car you suddenly have a car that won't run worth a crap, won't idle, spews fire and black smoke (fuel) out the chambers. This has nothing to do with cyl timing.
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