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Car Forum / GMC Cars / April 2006Manifolds, 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? 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: secretspam@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 ~ Thanks for the response. It looks like it is still up in the air so to speak though. :) >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 [quoted text clipped - 17 lines] >people and things to places." >~ R. Lee Baxton ~ > Thanks for the response. It looks like it is still up in the air so > to speak though. :) 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. 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 adjusting for this! C'mon any hot rodders in the crowd? >> Thanks for the response. It looks like it is still up in the air so >> to speak though. :) [quoted text clipped - 23 lines] >These are test data only, and are intended for demonstration, not for >direct engineering application. > Understood. Less back pressure is good for performance. Fuel > economy, torque and HP benefit. Of that there is no doubt. What is [quoted text clipped - 4 lines] > Somebody here has to know if valves need adjusting for this! C'mon > any hot rodders in the crowd? 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. >>> Thanks for the response. It looks like it is still up in the air so >>> to speak though. :) [quoted text clipped - 24 lines] >>These are test data only, and are intended for demonstration, not for >>direct engineering application. > Understood. Less back pressure is good for performance. Fuel > economy, torque and HP benefit. Of that there is no doubt. What is [quoted text clipped - 4 lines] > Somebody here has to know if valves need adjusting for this! C'mon > any hot rodders in the crowd? 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. Thanks. >> Understood. Less back pressure is good for performance. Fuel >> economy, torque and HP benefit. Of that there is no doubt. What is [quoted text clipped - 7 lines] >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. > Understood. Less back pressure is good for performance. Fuel > economy, torque and HP benefit. Of that there is no doubt. What is [quoted text clipped - 4 lines] > Somebody here has to know if valves need adjusting for this! C'mon > any hot rodders in the crowd? 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 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. >> Understood. Less back pressure is good for performance. Fuel >> economy, torque and HP benefit. Of that there is no doubt. What is [quoted text clipped - 18 lines] > >Dave 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. > 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 [quoted text clipped - 17 lines] > people and things to places." > ~ R. Lee Baxton ~ > Hi everyone. I have gotten some conflicting information concerning > exhaust back pressure. Some say reduce it as much as possible, you [quoted text clipped - 4 lines] > So... can you replace those manifolds with long tube headers and just > leave the valves alone or not? 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.  SignatureJonny Thanks for the info. It gets more and more interesting. As far as clarification, well I posed my question such is there a certain threshold below which can lead to problems. It sounds like there is. Somewhere between open headers and full exhaust system there comes a point where there is not enough back pressure to prevent problems unless high temp valves are used. Or the mixture gets richer. Or something. >> Hi everyone. I have gotten some conflicting information concerning >> exhaust back pressure. Some say reduce it as much as possible, you [quoted text clipped - 22 lines] >If you're running just straight header, you may check out using a high temp >tolerant exhaust valve. > Thanks for the info. It gets more and more interesting. As far as > clarification, well I posed my question such is there a certain [quoted text clipped - 3 lines] > unless high temp valves are used. Or the mixture gets richer. Or > something. 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: http://www.popularhotrodding.com/enginemasters/articles/hardcore/0505em_exh/ >>> Hi everyone. I have gotten some conflicting information concerning >>> exhaust back pressure. Some say reduce it as much as possible, you [quoted text clipped - 26 lines] >>temp >>tolerant exhaust valve. I will try to check that out a bit later tonight if I have time. Thanks. >> Thanks for the info. It gets more and more interesting. As far as >> clarification, well I posed my question such is there a certain [quoted text clipped - 41 lines] >>>temp >>>tolerant exhaust valve. >> Hi everyone. I have gotten some conflicting information concerning >> exhaust back pressure. Some say reduce it as much as possible, you [quoted text clipped - 15 lines] > Even a simple exhaust with a crossover and 3" muffler pipe will create > some backpressure. Depends on engine size and rpm. > In this case, you want some mild backpressure. 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 > a resonation that aids exhaust flow. Wrong. The diameter and length of the exhaust pipe aid exhaust flow not back pressure. 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. 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. > If you're running just straight header, you may check out using a high > temp tolerant exhaust valve. 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: http://www.popularhotrodding.com/enginemasters/articles/hardcore/0505em_exh/ "Jonny" <spamyourself@blackworm.net> wrote in message news:viKTf.8103 > Even a simple exhaust with a crossover and 3" muffler pipe will create some > backpressure. *********Technically correct >In this case, you want some mild backpressure. *********Technically incorrect. Old wives tale. > This creates a resonation that aids exhaust flow. *********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: |
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