i just cant accept that completely. i was a smog tech for a while and all the schooling i had to take lightly touched on this. for raw full horsepower perfromace you still want to tune the exhaust. this does factor in some back pressure and allot of other factors. a quick example is top fuel drag cars. if they didnt need any back pressure they would not have the headers. a burnt exhaust valve happens from cold air in the exhaust touching the hot valve. usually from an exhaust gasket leak. its the rapid hot then cold exposure to the valve that will result in the damage. now a lean air fuel mixture is part of the problem. a lean mixture will have higher exhaust temps, thus making the problem worse. my final reason for not believing this is my experience with my fox mustang. if you take a stock fox mustang with its small shorty headers 2 1/4" exhaust pipe size and 4 cats and raced it then put a big tube header 2.5 exhaust with no cats on it, the stock system will be faster. when i was playing with the stock motor i tried lots of combos. the best was the stock headers with a 2.5" high flow cat h-pipe. so the exhaust system does need back pressure for optimum performance. other wise every hot roder out there would run huge pipes no mater what there motors size and power was. oh and ive never heard of that the air goes through the carb twice to get a double dose of fuel. i know that there is a reverberation factor going on in the intake runners but it stops in the plenum below the carb. it doesnt go back out then back into the carb. but that link is to a bmw site and those germans do stuff differently.
The stuff it's saying is that it matters less after headers/manifolds, we all know you can tune header to make more HP/TQ. If having a specific pipe afterwards did make more HP/TQ than all those dragsters would be running them instead of just open collectors. At least that's what I got from it.
I'm with Bryant and after building a set of headers for my sons Comet with a 347 12.5:1 compression and stepping up from the boss heads to a Yates head and larger cam thought that I should step up the tube diameter for the primary's to 2 inch but ended up having to call Roush Motorsports on another problem on intake side and they reminded me of the fact that NASCAR runs only a 1 3/4 primary tube and was reschooled in exhaust pressure and flow. Then after being way over educated and slightly lost I put a call into Yates and was advised that with my particular set up I should step the primarys down to 1 3/4 primary with a 3.5 collector and 3 exhaust pipes and 2.5 tail pipes. Would help out torque substantially so after having several hours into the first set of headers I scraped them blew out a new set of flanges on the water jet and just finished up the driverside header all tubes are equal length and have full turning radius clutch operation and able to go in and out in 1 piece. now if u have put headers on a 302 in a maverick or comet you know it can be a chore now imagine making them from scratch then scrapping the first set just because you wanted the most performance. I may reinstall them at a latter date and run it on the dyno just to check but, I personally think that the guys at Yates And Roush have a pretty good idea as to what they are talking about when it comes to Ford motors over some article about BMWs on the Internet
Maybe I'm a little naive, but I've never heard the "double dosing" effect on a carb due to backpressure. Everything I have been taught and learned the hard way has told me that you tune an exhaust to your needs. A dragster or top fuel car runs at wide open throttle, full power for 3-6 seconds. You don't need much exhaust tuning. A pro-stock car needs a little more tuning for a wider power band but a NASCAR vehicle or especially a road racer needs all the power he can use across a wide RPM range, making exhaust tuning an art. I think scavenging is a much better term to use than backpressure since it more appropriately describes what someone designing an exhaust is trying to accomplish. Back pressure in the late 70's and 80's was a necessity for proper operation of EGR systems more than anything else. I believe if you design an exhaust for your engine's capability instead of bragging rights or straight up sound, you will find better performance across a much wider RPM range than just sticking whatever you want under the car. To each his own. I use a local exhaust fabricator that said the easiest way to estimate your needs without a ton of math is to take your projected horsepower and figure an inch in diameter per hundred horsepower, approximately of course for the best combination of horsepower and torque. Another test is to build your exhaust and use a test port (fitting that you can tap) just beyond the headers and check your backpressure at the engines torque peak (most street engines have maximum airflow potential at this point) and prove your theory or check your existing system. 1-2 PSI would be ok and still allow some scavenging effect but more or less may hurt performance depending on what you are tuning for. CarCraft actually did a test on a 350hp(I think, it's been a while) GTO and the best performance was achieved with a 2.5" dual system with 2.25" tailpipes. Dr Gas has some cool products and information as well. http://www.spintechmufflers.com/ http://insideracingtechnology.com/drgas.html
From Borla's website: Q. Is a bigger pipe and muffler system better? No, there has to be a balanced design to enhance the maximum engine output, exhaust gas velocity, and sound. If the diameter of the tubing is too large, the exhaust gas velocity will be reduced and rob the exhaust of thermal efficiency. Unfortunately we often disprove accepted racer mentalities like bigger is better. We must spend a great deal of energy explaining to someone how a 4" exhaust pipe will not work as efficiently as a 3" pipe. That is hard sometimes, but it is why we have magazine editors. Even with our excellent "track record" winning more professional car races than all the other muffler makers combined, we still often have to prove our theories to very well know top racers. One of these issues is volume versus velocity. The late John Lingenfelter helped us prove this over and over with his legendary Corvette exploits. People were often amazed how he would run 600 plus horsepower Vettes with 2 1/2" exhaust. If 4" would have made John go faster, he would have run it. We need to have a pipe large enough to overcome the boundary layer restriction in a dynamic flow situation while maintaining exhaust speed and evacuation. This isn't a simple axiom to understand, but the second best explanation is to say in race classes where let us say the budgets are seldom limited and the rules are not restrictive regarding exhaust, you will observe much smaller exhaust header tubing and exhaust pipes than some kids run on their 190-horsepower hand-me-down Honda sports compacts on the street.
Sometimes I do know what I am talking about. I haven't lived this long without learning something. Thanks for posting those articles and confirming my info.
