Mine too.
FlowMaster back pressure?
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,712
Likes: 31
From: Shreveport LA
LA Events Coordinator
FlowMaster back pressure?
I have a GM Performance 350/300hp with 2 1/2 duals to FlowMaster 40 series. Runs FANTASTIC and love the sound....but was wondering....when I run down the Interstate at 70-80mph (3000-3500rpms) and let off the gas (taking an exit) there seems to be some "back pressure" sounds coming from the exhaust. It's not missing or back-firing...just a slight "pup, pup, pup, pup,pup...." sound until I began to accelerate.
Is this normal for the FlowMasters I have installed?
Is this normal for the FlowMasters I have installed?
Burning Brakes
Joined: Apr 2007
Posts: 1,028
Likes: 1
From: SOUTH ELGIN IL
So you guys all get the "pup-pup"s with the exhaust running all the way out the back? I had some on a car and figured I was getting it because I used 2 1/2" turn downs right after the mufflers...
I liked the agressivness too by the way, but I like the 3" side pipes a whole lot better - aside from the frequent leg burns...
I liked the agressivness too by the way, but I like the 3" side pipes a whole lot better - aside from the frequent leg burns...
Race Director
Joined: Nov 2000
Posts: 16,416
Likes: 1
From: NC,USA
That comes from the leaning when the throttle snaps shut. Unburned fuel slips through the combustion process and ends up in the exhaust system. Enough oxygen is present in the exhaust to light it off.
Systems with more back pressure do it less... Its the same thing going on when you watch a night race and someone downshifts before a turn and gets off the throttle. Pop and crackle with fireballs. Porsche in particular do this... same deal with other short exhaust systems like the ones that dump right behind the front wheel. The BOOM on a downshift though, is when they rev match... keep the foot in and the engine runs free during the shift and loads up the exhaust with a super rich mix... and then get out. (Same thing is visible on real cold, humid days days with a puff of real moist (rich) air!)
A backfire as the books say is a lean condition, but the one the rusted out pickup heard round the world is running super rich and with a poorly tuned exhaust system. As it misses it loads up the exhaust until a roman candle fireworks display erupts inside the rusted out muffler...
Systems with more back pressure do it less... Its the same thing going on when you watch a night race and someone downshifts before a turn and gets off the throttle. Pop and crackle with fireballs. Porsche in particular do this... same deal with other short exhaust systems like the ones that dump right behind the front wheel. The BOOM on a downshift though, is when they rev match... keep the foot in and the engine runs free during the shift and loads up the exhaust with a super rich mix... and then get out. (Same thing is visible on real cold, humid days days with a puff of real moist (rich) air!)
A backfire as the books say is a lean condition, but the one the rusted out pickup heard round the world is running super rich and with a poorly tuned exhaust system. As it misses it loads up the exhaust until a roman candle fireworks display erupts inside the rusted out muffler...
Last edited by ZD75blue; Dec 8, 2007 at 10:22 PM.
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