First off i am building a set of headers at work for a mildly built 440 in a 37 plymouth pu but when i was sitting there welding on them the question poped in my head i wonder about back pressure does it need it to run right or not?! ive always been told show me a race car with back pressure and i will show you a loser. i know this is no race car but i would like to hear your opion on this issue.
If you understand step headers and how they do the magic they do for horsepower gains you will not be a fan of back pressure. If you are the build torq down low at all cost camps you might want some. If you are from the "you'll suck a valve camp" there is no hope stick to stock.
As a general rule any 'back pressure' is undesirable, 'Marks engineering handbook' quotes a figure of 4% loss for every psi above atmospheric, however it is true that many OEM exhaust systems are designed in a way that does create some pressure. Typically improvements in tractablity and engine braking are the claimed reasoning. If you look at Yamaha's EXUP system which uses a servo control valve in the exhuast to introduce a restriction at small throttle openings and low engine rpm it shows that there are some benefits to being able to do this. However as I see it a fixed restriction results in higher pressures at high gas flow rates so any benefits come at the expence of poor performance further up the rev range.
I believe this is more to do with moving/eliminating the exhaust pulse at low engine speeds than creating back pressure. Hence why it only happens at low speed because this is the only time you can put a restriction in the exhaust without it creating a restriction leading to loss of power.
Unless you want a bigger HP number at all costs and don't are about drivability you'll want some. If you understand low to midrange torque and you want it, you'll want some. If you understand scavenging and want an efficient running engine, you'll want some. Now to much is a bad thing, as is not enough. Proper engineering and design will present you with a couple of things to think about. Are the headers you are building equal length or tri-Y? Long tube or short? There's a lot more that goes into it than people think.
That would be my point. If anything I would be aiming towards a lower than atmospheric pressure rather than higher as being desireable within the exhaust. As I said before Marks quotes a loss of 4% for every psi above atmospheric, that's 4% off the overall efficency, not just less peak power, that's not helping gas milage or anything else ether. If we want to talk about scavenging, then the requirement for the exhaust to be at lower pressure than the incoming air at the point when the inlet starts to open is fairly obvious, and I would find it difficult to see a case for wanting the pressure in the exhaust to be higher than the pressure of the air you are trying to drag in to purge the cylinder. I would stick to just calcing the lengths off the exhaust valve timing and a vague assumption of the temp and hence the velocity of the pressure wave, and a total volume of between four and six times swept and call it a day at that.
Backpressure and torque is one of the oldest myths in Hot Rodding. The fact is that ANY back pressure means that the exhaust gases must Push the positive (back) pressure before they can leave the combustion chamber. Energy is wasted overcoming the initial back pressure before any exhaust is pushed out of the combustion chamber. It is generally accepted by the SAE (society of Automotive Engineers) that for every inch of Hg Backpressure 1 HP is lost. Many stock exhaust systems can create 18-20 "of Hg of back pressure. What motors really need is a low Backpressure, high velocity system. A fast moving, free flowing exhaust gas column creates a rarefaction (negative pressure wave or vacuum) behind the exhaust valve as it opens. This vacuum scavenges the cylinder of the exhaust gases and reduces pumping loses that happen with high back pressure. The negative wave during overlap with high velocity also helps increase the incoming intake charge to more effectively fill the cylinders. This is achieved by the correct diameter and length of the tubing diameter along with an effective collector design that has each cylinder increase the negative wave in the tubes for the other cylinders. Too large of diameter tubing will have low Backpressure BUT will also have low velocity which will reduce scavenging and effect low rpm torque. Part of the myth about back pressure comes from installing to large of header pipe diameter for the motor and then trying to use back pressure to try and help with the slow velocity from the large tubing. Ideally exhaust tubing diameter should change with rpm but of course that isn't truly possible or practical. Motorcycles manufacturers have made systems with elaborate valves and chambers to increase internal volume as rpm increases and this does help. Mercedes Benz, BMW, and VW have been doing this on upscale cars as well. A test was done a number of years back. The motor was a 351" V8 with headers and an elaborate exhaust system that was measured to create 0 Backpressure. The motor made 423 ft-lbs at 4700 and 441 HP at 6300. A restrictor was then fitted to the exhaust that made only 1.5 psi of back pressure - torque dropped by 4% and HP by 5%. Next 2.5 psi was dialed in and both Torque and HP dropped by 7%. I believe that the only time you would see an increase in power by adding back pressure would be due to an overly lean mixture or timing that is not optimized. We also have seen on the Dyno that increased back pressure can cause detonation due to the diluted intake charge. if you don't believe that exhaust gases going back into the combustion chamber will cause a decrease in power, the next time you are doing a Dyno run, take a flexible hose and run it from the exhaust pipe up next to the air cleaner - you will see a noticeable drop in power. As Z man says, there is a lot that goes into efficient header design including type of header, diameter, length, collector size etc.
I don't think that's correct, spec for many auto manufactures say anything in the 3 to 5 PSI range for backpressure indicates a plugged or restricted exhaust. I'd wonder if the engine would even start with so much (18-20 PSI) restriction?
One of the things to remember is that the backpressure is not constant. It's a pulse, and it doesn't take much. And it really doesn't do more than slow down the exhaust. If timed correctly it helps keep raw fuel from being sucked through on overlap and makes the engine more efficient. All you guys raging about about free flowing exhaust should do some real world testing. Take a Harley for instance. Straight pipes cost you power in the low to mid range and a lot of times on top. Fact is a properly tuned 2 into 1 with a muffler on it makes more power all the way across. Why is that? hmmm...
Great info! Can anyone in this discussion post the math required to build a good set of headers for any given application? Maybe just point in the right direction for a good resource.
I think I know what you're trying to say, but it's still misleading. The vacuum (in/hg) is measured at the intake. The higher the intake vacuum is, the lower the back pressure will be and vice-versa (as I know for stock to mild cammed engines), but I would assume that that two like-kind engines would show the same results.
It seems that you are confusing Backpressure and Reversion which are two very different things. Put a pressure gauge on an exhaust pipe and you will see that the pressure actually is fairly constant. The pressure doesn't really change based on the direction of flow. I have done real world testing, on both cars and Harley's, Hundreds of hours in fact - it's part of my job. And yes I will agree that a 2 into 1 makes better all around power than drag pipes, just as 4 into 1 or Tri Y's make better power on unblown motors than Zoomies and my Real World Dyno and Drag testing, along with many miles of road testing for a couple of major manufacturers tells me that it is due to the extra scavenging created by the collector allowing the pulses from one cylinder helping to create a vacuum behind the valve for the other cylinder. Not Backpressure. Controlling the timing of the reversion is where we tune the pipe to keep from over-scavenging while not diluting the incoming charge with exhaust as well. Harley's create another real challenge due to the odd firing order which means for the 2 into 1 to be as effective as possible the tube length of the rear cylinder needs to be a different length than the front to time the events properly. Just saying a muffler improved an exhausts performance due to increased back pressure is naive. There are many mufflers that don't increase back pressure an appreciable amount. What is really happening is further tuning of the volume of the system and controlling the velocity and the reversion in a properly designed system. I did a Dyno test where we took an over sized set of Drag Pipes on a Harley (2 1/4") and ran them Open, with a typical baffle, and with a hole drilled through them and a 16 penny nail dropped through. The one with the nail made the highest average power, and the best Torque under the curve. It had to do with breaking up the reversion pulse and nothing more. Can back pressure be used to compensate for a poorly designed header or exhaust system? Yes - but it is merely a band aid that is still throwing ultimate power capabilities away. And yes it's done everyday if not by design, certainly in practice.
using trywhy headers 327hi rpm sbc....best perfomance with 13/4 foot long tqoue tube installed on pass side...21/2 foot tube bolted on drivers side no mufflers tunnel ram intake
Depends on the gauge. Most pressure gauges will not read the pulses. Not sensitive enough. I'm sorry but I was taught that a wave traveling back towards the engine was backpressure, i.e. reversion. No one ever said cork it up. I guess what we are talking about is the same thing. Never said bolting on a muffler would cure the problems of an ill designed exhaust. Again I never said cork it up. And I also said "properly tuned". An older gentleman showed me on a dyno in the early 80's just about the same thing as you're talking with the Harley with some screwdrivers. He too referred to it as backpressure. Talked about controlling the velocity of the exhaust and creating a wave in proper timing with the cam timing. I also many long conversations about this over the dyno when I was racing and tuning. But then again I didn't mess much with Harley's, to slow and ill handling for my taste. and by a lot of major exhaust companies. But then I guess since the majority of me early education in this was with 2 strokes, expansion chambers, and motorcycle road racing maybe that's where the terminology issue comes from.
Well ZMan it looks like we are talking about basically the same thing, just the wrong terminology. Backpressure in an exhaust is defined as positive pressure in the exhaust compared to atmospheric pressure at the outlet. Or, Pressure due to a force that is operating in a direction opposite to that being considered, such as that of a gas flow. In other words something restricting the flow of the gases. Reversion is what happens during overlap when the intake valve opens and creates a negative pressure and tries to suck exhaust gases back in to the intake system. And since exhaust gases cannot be burned a second time, this is a bad thing. So the lower the back pressure and the higher the velocity the less exhaust charge can be sucked back in. AR cones, Torque Cones, even baffles help to keep the exhaust from flowing back wards helping with controlling reversion. So in my Nail example what was happening is was the too large diameter pipe was killing velocity, exhaust was staying in the pipes. When the intake valve opened this exhaust was sucking back in killing power. The nail both help increase velocity by crating two chambers for flow and by creating a resistance to reversing that flow.
But sometimes restricting the flow of exhaust if timed correctly can be a good thing. I've also experienced issues during overlap where the charge is pulled into the exhaust. So controlling the velocity can be beneficial to control that.
I use degrees before bottom dead centre that exhaust valve opens plus 180 multiplied by 125 divided by RPM to give me a base length in feet for the pipe. It assumes that the pressure wave travels at 1500ft per sec in the hot gas rather than 1100ft/sec in normal air. The source was Phil Irving's Tuning for Speed, But he quotes J. G. Morrison's paper on exhaust systems published in volume 34 of Institution of automotive engineers proceedings.
The Jalopy Journal
