Quote:
|
Originally Posted by denis gow
No Backpressure in Exhaust systems is good,
In doing some googling , I found some information on this subject .
They say that you want no backpressure as this will effect engine power .
There are certain mufflers that can reduce backpressure.
No muffler will completely reduce backpressure .
Look at the exhaust system on the Nascar race cars , they have crossover pipes that come down to huge tail pipes and zero mufflers and no backpressure which is what they want .
http://auto.howstuffworks.com/muffler5.htm
Den Downunder |
Well, your half right!
Let’s start here. You "cannot" compare a NASCAR engine to a street automobile engine. A NASCAR engine runs at 8500RPM and pretty much stays there for the duration of the race. A road going car is designed for stop and go driving thus needs backpressure for good performance.
Why??
At low RPM, let's say 1000-3000 RPM your street engine is running in its normal driving power band. In this range, the back pressure actually keeps some of the exhaust stroke pressure behind the valve during valve overlap. (When both intake and exhaust valves are open at the same time.) This back pressure actually keeps quite a bit of your fuel/air mixture in the cylinder where you want it, not going out your exhaust.
Okay, Then why have valve overlap???
Now, Lets look at the 3000-10,000+ RPM range. Now the valve overlap helps. Since the 4 stroke combustion process is happening much faster, this overlap is now creating vacuum with the exhaust gasses and is sucking the mixture in the cylinder as fast as it can. At this point there is no need for back pressure. NASCAR engines are designed with lots of valve overlap and fine tuned, free flowing exhausts because they run in a high RPM power band.
In conclusion, If your building a high RPM Mercedes racing engine “not for street use”, run straight pipes. If you want to be a loud street car and don’t care about performance, run straight pipes. If you want peak performance from your exhaust on a street going auto, stick as close as you can to your well engineered stock exhaust and be happy. If you want higher performance the first thing to think about is how to get more air to your intake, not out of your exhaust.
PS: None of this applies to turbocharging, it’s a whole different scenario there as well.
Adam
__________________
Current Stable:
01 ML55 AMG
92 500E (a few mods)
87 300E (lots of mods)
00 Chevy 3500HD Diesel Box Truck
68 18' Donzi Marine
06 GT i-Drive7 1.0 Mountain Bike (with GPS!
)
PREVIOUSLY OWNED:83 300SD, 87 420SEL, 88 420SEL, 90 420SEL, 86 560SEL, 86 190E 2.3-16V AMG, 94 E320
