I was reading some earlier post regarding exhaust system backpressure and wanted to share what we have found. I have been building engines for Reno Racing for several years. MOST street racers are using the stock camshaft/cylinder head/piston combinations! Are engines are used in oval track racing and depending on the engine RPM and torque needed will determine what size and length header tubes we will use. I read an opinion that you dont want backpressure, backpressure has more to do with torque and to an extent, yes horsepower. I do like the sound of a good street car with a nice sounding exhaust. But I DO KNOW you take away to much backpressure and your torque will suffer. Remember, engines I build turn an average from 4500 to 8500rpm's. Don't confuse a racing engine with a street engine, when the street engine has exterior bolted on horsepower, THEY ARE 2 DIFFERENT ANIMALS!!!!!!!!!!Read on about scavenging and it might help folks with the wonder of WHY they do it! Exhaust headers are among the best means to boost power on most engines. They can make a profound performance difference. How do they do this? The answer is more involved and interesting than just back pressure reduction.
Unlike exhaust manifolds, headers have individual tubes for each cylinder that join together into a collector. The tubes are of equal length and tuned. "Tuned" means the tubes are of the proper diameter and length to perform optimally for a given engine displacement and RPM range.
Exhaust gases don't flow continuously, they flow in pulses from a given cylinder with a new pulse coming with each exhaust valve opening. Each pulse creates a point of high pressure traveling through a header tube. Between each pulse the pressure is relatively low.
Back Pressure - Header tubes are equal length. Each exhaust valve opens at a different time - 120 degrees of crankshaft rotation apart for a 6 cylinder engine. Because of equal tube length and non-concurrent exhaust valve opening, exhaust pulses from different cylinders don't collide in the collector. The high pressure pulse from one cylinder arrives at the collector in the low pressure zone between pulses from other cylinders. This reduces overall back pressure of the system. If the tubes where not equal length the pulses would collide in the collector and increase back pressure.
That explains the reduction in back pressure. But why not eliminate the exhaust system entirely and let the valve vent directly to the atmosphere? That would surely deliver even less back pressure. Yet ALL race cars use headers. It turns out that headers do more than just reduce back pressure. Read on.
Scavenging - Each pulse of gas has mass, and as it moves down the tube it develops momentum. If you suddenly try to stop the flow of that pulse by closing the exhaust valve, it will attempt to keep moving (a body set in motion will remain in motion). The result is that something of a vacuum is created behind the pulse. If the exhaust valve is still partially open, that vacuum draws the residual exhaust gases out of the chamber, improving evacuation. This is called "scavenging" and is one of the key benefits of a header system. If the cam has a bit of overlap (intake valve and exhaust valve open at the same time) the intake charge is sucked into the cylinder, delivering a dense uncontaminated intake charge.
Sizing - The diameter and length of the header tube are critical. For a given engine displacement, a smaller tube will cause the exhaust pulses to flow faster down the tube, thus increasing the momentum and the scavenging effect. Too small a tube and back pressure increases. Long header tubes provide superior low RPM performance while shorter tubes work best at high RPM. Optimal tube length and diameter depends on displacement and the desired RPM for the power band. Hence big race engines - big tubes, small street engines - small tubes.
Bonus information for reading to the end - intake runners have a similar tuning effect! Engine components need to be balanced - exhaust, cam, intake, RPM range and displacement all have to work together.
Posts: 270
Joined: 4/26/2003 From: United States Status: offline
Thanks enginebuilder. My point in the original post was that stock heads and intake with the usual bolt on's will suffer the loss of low end torque with long tube headers. You're getting rid of the air faster than it can be produced. Short tube un-equal lengths will meet in the collector and create backpressure. As your heads and intake (after market, racing, etc.) increase the flow, it becomes necessary for freer flow to go to a larger equal length header, thus increasing power. Am I on the right track? I originally thought that all engines need some backpressure, but thru research (you mention scavenging, sizing, pulses), I have changed my tune somewhat. But I totally agree with you that it depends on what the engine was built to accomplish, stock vs racing, displacement, circle track drag race, etc. Hope you stay on this site, very informative to learn from a pro.
First of all, thanks for the welcome Makarovboy. I will try to stay in contact with the forum, when time permits. I've been a tech for many years and have seen some strange things happen to street cars. You ever wonder why sometimes you can build an engine whether for street use or strictly racing, go to race the vehicle and get smoked by a stock type engine? I've seen it on many occasions. I remember when folks use to buy the biggest camshaft they can fit in either a small block or big block engine. Engine sound good, hit a mean lick, shake the ground with all the noise, and won't fall out of a tree when it comes down to ET times. Telling my age here but alot of the folks back in the seventies (and at the present) didn't put the rest of what they needed to make the engine perform, you could buy rockets straight from the showroom. I've seen some street stangs go to the track and run heads up. There are definitely alot of fast mustangs on the road. What's really killer about street racing is when you think you have a bad machine, win quite a few races, next thing you know a few more newbies hit the road and you get smoked, like bigtime. Now the thinking cap goes on and you wonder what you can do to improve your performance and beat the guy who just smoked you. Haha.............Same thing with stock car racing, kinda a funny thing, but it's in our blood, and all you guys know it........A guy gave me this advice one time, you don't buy a 2,000 watt stereo amp and hook it up to $10.00 speakers, and vice verse. The same thing with an engine, its the combination that you use. Another mystery to me, you can take 10 stock 5.0 engines, 350 small blocks or whatever you want to use, and there will always be about 3 of them that will just scream. I have an idea about that, but its my secret.......Have a great evening!
Posts: 495
Joined: 6/4/2003 From: United States Status: offline
Enginebuilder.....I'm a more knowledgeable man today because of you, thank you. --
I've been planning to build a really nice street car and I know that creating a harmonic engine (input/process/output or intake/engine/exhaust)(and of course suspension to apply it physically) is the way to go, but reading articles like you just posted sets me sharper focus on my objectives for my build.
....wow, that hurt my head.... hahahaha
Thanks a million....hope you stick around the forum...
Posts: 270
Joined: 4/26/2003 From: United States Status: offline
I built a 400M motor for my street stock dirt car. I had to run a 2bbl carb and stock exhaust manifolds because of the class. I used my stock 351 heads shaved .030, and everybody would come by and ask me "why did you order an RV cam?" I had the local muffler shop run a Y pipe with no mufflers. When the folks that asked the cam question saw me race, they knew. Due the the restrictive heads and stock exhaust manifolds, I ran that cam for the scads of torque it offered. That car was out of the turns in a heart beat and surprised a lot of chevelles.
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Exhaust system backpressure - 
