Manifolds, headers, back pressure and valve adjustments

Hi everyone. I have gotten some conflicting information concerning exhaust back pressure. Some say reduce it as much as possible, you don't need that back pressure for anything. It's a myth. Others say if you get rid of too much you will burn your valves out unless you do some kind of adjustment to the valves.

So... can you replace those manifolds with long tube headers and just leave the valves alone or not?

Reply to
SgtSilicon
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Well, years ago, Hot Rod magazine did a dyno test of various exhaust set-ups using only one vehicle and discovered that some back pressure is actually beneficial. This may vary from engine to engine but I've heard many people say that total elimination of back pressure actually hurt engine performance.

Group: alt.autos.gm Date: Sun, Mar 19, 2006, 10:54pm (EST+5) From: snipped-for-privacy@ihatespam.net (SgtSilicon)

Hi everyone. I have gotten some conflicting information concerning exhaust back pressure. Some say reduce it as much as possible, you don't need that back pressure for anything. It's a myth. Others say if you get rid of too much you will burn your valves out unless you do some kind of adjustment to the valves. So... can you replace those manifolds with long tube headers and just leave the valves alone or not?

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% "So why do I drive a big SUV? It's because I have to haul numerous people and things to places." ~ R. Lee Baxton ~

Reply to
Rich B

Thanks for the response. It looks like it is still up in the air so to speak though. :)

Reply to
SgtSilicon

I think it is indeed up in the air, but I would like to see dynamometer data on it.

The Goodheart Wilcox Automotive Encyclopedia has graphs that show that at 60 miles per hour, the effect is as follows Loss of horsepower Backpressure at muffler

2% 2 psi 4.5% 4 psi 7% 6 psi 10% 8 psi 13% 10 psi

They also publish a fuel consumption chart, at 75 mph.

Gallons/BHP/Hour Backpressure at muffler

0.108 0 psi 0.114 2 psi 0.118 4 psi 0.125 6 psi 0.127 8 psi 0.129 10 psi

These are test data only, and are intended for demonstration, not for direct engineering application.

Reply to
<HLS

Understood. Less back pressure is good for performance. Fuel economy, torque and HP benefit. Of that there is no doubt. What is in question is, do valves or other things need to be adjusted or modified to prevent damage if back pressure is reduced passed a certain threshold?

Somebody here has to know if valves need adjust>

Reply to
SgtSilicon

No valve adjustment needed. The damage is supposed to be caused by the engine running lean. A lean air/fuel mixture causes an increase in heat. This increase in heat is claimed to burn the exhuast valves. Don't know if this is possible in the real world.

Reply to
Mike

Apples and oranges.

Valves don't need adjusting.

If you have a complete exhaust system, catalytic converter, crossover pipe, muffler, and tailpipe; there will be a certain amount of backpressure period. You never clarified that.

Even a simple exhaust with a crossover and 3" muffler pipe will create some backpressure. In this case, you want some mild backpressure. This creates a resonation that aids exhaust flow. This is needed when the intake and exhaust valves are open, however minutely, at the same time. The intake valve opening gases cool the exhaust valve slightly. With no exhaust resonance, the exhaust gases don't lend to cooling from the intake.

If you're running just straight header, you may check out using a high temp tolerant exhaust valve.

Reply to
Jonny

Reply to
SgtSilicon

Depends on engine size and rpm.

Wrong. For maximum performance you don't want any back pressure. Back pressure equals restricton. You want a free flowing exhaust for performance.

This creates

Wrong. The diameter and length of the exhaust pipe aid exhaust flow not back pressure.

This is needed when the intake and

Wrong again. The fuel mixture determines how hot the valves will get. A rich fuel mixture burns cooler compared to a lean mixture. A richer air/fuel mixture makes more power. Valve overlap ( when the intake and exhaust valves are open, however minutely, at the same time ) relies on a properly tuned intake and (unrestricted) tuned exhaust to remove all the exhaust gases from the cylinder AND completely fill the cylinder with a fresh air/fuel mixture. The more exhaust you can remove from the cylinder the more room you have for a fresh air/fuel mixture. The more fuel you can cram into the cylinder the more power you can make. Most of the exhaust valve cooling occurs when the valve is closed. The exhaust valve transfers heat to the valve when closed, which colls the exhaust valve. This is why you don't want too narrow valve seat on the exhaust side.

Probably not need on an engine designed for unleaded fuel unless used for long periods of wide open throttle, such as nascar.

Some interesting reading on the subject:

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Reply to
Mike

Back pressure is bad all the time. Back pressure equals restriction. You ALWAYS want a free flowing exhaust. You use exhaust pipe length and diameter to tune an exhaust NOT backpressure.

Some interesting reading:

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Reply to
Mike

Sorry...The answer is no...you do not need to adjust the valves differently. As in every case, the ignition timing should be adjusted properly.

Reply to
<HLS

*********Technically correct
*********Technically incorrect. Old wives tale.
*********A resonant condition can aid exhaust flow, just like a resonant stack on the intake can improve the induction. But it is not a function of backpressure. Consider the stack or the exhaust to be like an trombone...at a certain length, it becomes resonant to sound waves. An engine has pulses, due to the pistons pulling in air and pushing out exhaust, which have a frequency component and are in fact sound waves. Tune the intake and exhaust and you can improve the throughput.

Some racers used to put extensions on their headers and run the engine at specified RPM for a few minutes. You can, more or less, accurately tell where to cut the extension off, due to the heat discoloration point at that RPM:

Reply to
<HLS

Reply to
SgtSilicon

I will try to check that out a bit later tonight if I have time. Thanks.

Reply to
SgtSilicon

To answer your question, no, the valves won't need adjusting. To answer the questions you didn't ask:

Yes, you will likely need to re-jet to allow for a richer mixture. Yes, you may need to alter your air intake system to allow for more air flow. No, you shouldn't run headers only. Yes, if you run headers and pipes all the way back, you'll need an X pipe or H pipe behind the headers for maximum flow. After these changes, you may want to check your plugs occasionally to see how they look.

Dave

Reply to
Hairy

Thank you Dave. I think I will go with a cat back setup to avoid any engine issues and any EPA type issues. I do appreciate everyone's comments and suggestions. Hopefully they are helpful to others as well.

Reply to
SgtSilicon

It goes a bit farther than "some back pressure". Has to due with exhaust tuning so that the exhaust pressure pulse wave is used to reduce back pressure under some conditions, and increase it for others. Sound damping, vibration damping, and fuel usage are considerations, along with power.

Reply to
Chuck

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