Heckeng Posted February 21, 2009 Share Posted February 21, 2009 Good conversation everybody, I love this stuff! Open headers hurt because they don't keep the velocity up high enough and long enough after the collecter to get maximum scavenging. They dump to the air which kills any scavenging that could have helped. Many drag racers will tune their open header systems to add 10" to 24" bolt-on extensions onto the collectors to keep the velocity high and get all the scavenging possible. Everybody gets backpressure and velocity confused when talking about exhaust, you don't need backpressure to keep velocity high and you don't get scavenging from backpressure, but you will keep velocity high by using a more conservative pipe diameter compared to a larger one. That is where people got the though that "you need backpressure". If backpressure was important, Sam's straight through slightly perforated mufflers would hurt his performance. Instead, you will see improvements because the open design of your mufflers will allow the exhaust pulses to travel through the exhaust freely and retain their full, non muffled impulses at max velocity which will keep scavenging at a maximum. When you add backpressure, you kill the exhaust pulses and hurt your scavenging. I did some googling and found this Carcraft article that backs me up--I got lucky Carcraft article on scavenging with a collector extension Quote Link to comment Share on other sites More sharing options...
mc71454 Posted February 28, 2009 Share Posted February 28, 2009 Back Pressure = Bad Any back pressure is bad...The old belief about needing back pressure for the street is just an old tale that is unfortunately not true. Low grunt low RPM motors will typically have a short duration of valve opening overlap so that the intake charge is not contaminated with exhaust charge. When the RPM's go up, there is not enough time to get enough fresh charge in while the old is getting out....which brings upon the saying "I ran out of camshaft" That is also why they can get away with lower static compression ratios because a relatively small amount of cylinder presure is lost with short overlap periods. On the Converse, the high RPM designed camshafts will have a lot of overlap, and that is why you have a Pig when you put in too much cam in our heavy cars without the gear and compression and converter required. The overlap is also why cylinder pressure is lost and one of the reasons big cams need big static compression ratios assumming we leave Lobe Separation Angles out of this. As the topic was exhaust size I will tip my 2 cents in. The exhaust system will act as a big heat sink and depending on the type of mufflers you have will be an important part of the equation. The small block you are contemplating will not like the 3 1/2" exhaust. So no matter what kind of a deal you get, you may always wonder if it would run better with a smaller exhaust, maybe you won't care and I don;t want to pretend I know what you prefer, so no disrespect intended. I do know that many that have done this and gone smaller afterwards. Someone discussed velocity, and they were right on. Remember the flow rate in Cubic Feet per Minute (CFM) will be the same throughout the system, but restrictions in the flow whether it is rough edges, kinks in the pipe, the mufflers, a shifted gasket will all effect the CFM. Look at two extremes.........Use a straw as a tailpipe, imagine the backpressure. Now consider open headers....you get the idea.. But the right sized exhaust system can make the same average power (area under the curve) as an open header. The difference will be that the open header will win up top in the RPM range and the properly sized exhaust will win in the lower range, but the overall average will be almost identical. The difference is that for a street/strip vehicle you have to weigh your preferences and look at your total combination to see what is best. if you want the best driveability and great performance at the track, you need to be very picky and informed about your exhaust system selection and be ready to experiment. If all you care about is to be loud around town and get the best MPH at the track with a cam that lives above 4500 RPM, put in sewer pipes, they will act as an open header. From my experience with my own vehicles and buddies that have raced for more than 25 years, I offer the following for what it is worth. For a typical BB 400 to 450 HP low to mid RPM motor, a 2 1/2" system will work very well with free flowing mufflers. 500 HP - A nice setup is a 3" front pipe to quiet mufflers with 2 1/2" tails. 550 to 600 HP will also like 3" all the way with free flowing mufflers 650 to 750 HP 3 1/2" with 3" tails Quote Link to comment Share on other sites More sharing options...
sspdfreek Posted March 1, 2009 Share Posted March 1, 2009 Love the license plate on that car. Quote Link to comment Share on other sites More sharing options...
wallaby Posted March 1, 2009 Share Posted March 1, 2009 Really there is a lot of discussion about header size and tailpipe size, and it all seems to be viewed from the perspective of the engine...what I mean is that the whole system is more tailored towards "getting the exhaust out as freely as possible". At first glance this would seem logical. The goal then might be to try to replicate open headers while still using tailpipes and mufflers: If there was room for 12" system under the car, people would buy them. I find it easier to understand if I think of the system from the other direction. Instead of trying to design a system that makes it easy for the engine to push exhaust through....I imagine the exhaust system as a siphon hose where the flow of heat will pull exhaust from the engine. If it's easier to visualize, you can think of it as a fireplace in your house and you are tinkering with different size chimneys: If the chimney is too big smoke goes anywhere/everywhere including into the house and there is no draft to further excite your fire. You get the same result from too small a chimney. But if you have the right size chimney it will draw fresh air into the fire as the smoke exits, and can even create lower pressure (vacuum) inside the house. No smoke spills out into the house, it's determined to go up the chimney. Quote Link to comment Share on other sites More sharing options...
Heckeng Posted March 2, 2009 Share Posted March 2, 2009 That's a good way to think about it Mark, I like the chimney analogy. People believe both extremes of the argument which are both wrong! Some think you NEED restriction to make power, and others think you need NO restriction to make power. They just need to make sure velocity is high enough to help scavenge (suck) air out of the headers efficiently--just like finding your perfect chimney size! Quote Link to comment Share on other sites More sharing options...
kc8oye Posted March 2, 2009 Share Posted March 2, 2009 isn't that what I Said? Quote Link to comment Share on other sites More sharing options...
wallaby Posted March 3, 2009 Share Posted March 3, 2009 yea, but you forgot to mention the chimney. Quote Link to comment Share on other sites More sharing options...
Heckeng Posted March 3, 2009 Share Posted March 3, 2009 isn't that what I Said? No, you said Originally Posted By: kc8oye you need the back pressure to generate the velocity which creates the scavenging effect. Backpressure is bad and doesn't create velocity. Small cross sectional area creates high velocity and depending on your system can also create high backpressure. I'm probably nit-picking though Quote Link to comment Share on other sites More sharing options...
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