Exhaust Piping too big??
#17
08-07-2008 | 10:43 AM
Originally Posted by clockwo12k
the right amount of back pressure is a good thing. Zero back pressure(especially on a n/a motor, or most motors making <600hp A.K.A. not all out fully-built dragsters) causes an actual loss in HP.
Please get ASE certified or at least do a little research before responding plz.
Please get ASE certified or at least do a little research before responding plz.
The ideal exhaust system has absolutely no back pressure and has the highest exhaust velocity possible. This setup is basically impossible to achieve on an NA motor, so the focus is on exhaust velocity and keeping the exhaust gases hot. Back pressure does not create power at any point along the power curve. None, zilch, nada. On the exhaust stroke of the engine, if there is any restriction in the pipe (ie back pressure), power is lost. Period. It's as simple as that. When people add exhaust systems that are too large and they loose low end power and throttle response, the common answer is "oh, you need back pressure to restore the power". BS. The problem is the piping is too large for the exhaust gas coming out of the head at a low rpm. The exhaust gas cools too quickly in the pipe (which is often thinner walled and as much worse heat retention than OEM) plus the exhaust gas looses some intended direction when being dumped into the larger pipe. These two main factors (there are some other minor factors that I won't go into) cause turbulence in the pipe and what you get is soft low end power and poor low rpm throttle response. As the rpms increase, the exhaust gas flow starts to increase and improve. As the rpms get relatively high, the larger diameter piping does start to help power, but by that time, it's about time to shift. Basically what you get is a motor that's soft below 3500rpms and ramps up in power quickly at around 5000rpms and then takes off. You might gain some topend power, but may also loose low and midrange power which is detrimental to overall acceleration because power under the curve is what really accelerates the car, not the last 1000rpms of the power band.
#19
08-11-2008 | 12:30 PM
Registered User
Joined: Dec 2003
Posts: 2,514
Likes: 4
From: Marietta, Georgia
Before changing anything MEASURE the pressure average and peaks at different locations along the exhaust from rear of cat to exhaust tip at different rpms.
Easy to calculate the restriction vs power loss for any device in system.
1 inch Hg [1/2 PSI] backpressure = ~~1 HP lost
Don't confuse an old 2 valve V8, the reason is that the exhaust overlap factor on is vastly lower on a 4-valve engine as compared to a 2-valve of the same specific output.
http://www.se-r.net/header_exhaust/general.html
http://www.team-integra.net/sections...p?ArticleID=48
http://www.team-integra.net/sections...?ArticleID=355
Easy to calculate the restriction vs power loss for any device in system.
1 inch Hg [1/2 PSI] backpressure = ~~1 HP lost
Don't confuse an old 2 valve V8, the reason is that the exhaust overlap factor on is vastly lower on a 4-valve engine as compared to a 2-valve of the same specific output.
http://www.se-r.net/header_exhaust/general.html
http://www.team-integra.net/sections...p?ArticleID=48
http://www.team-integra.net/sections...?ArticleID=355
#20
08-11-2008 | 01:21 PM
Former G35driver Vendor
iTrader: (23)
Joined: Jul 2004
Posts: 3,054
Likes: 85
From: Los Angeles California
Back Pressure Vs HP and TQ on the REVUP engine.
Back Pressure Vs HP and TQ on the REVUP engine.
As backpressure increases, Power and TQ only go down. There is absolutely no benefit to backpressure on this engine.
Could backpressure be good on a different engine? Possibly... but none I am aware of. And certaintly not on this engine.
Below is a series of dyno runs showing the effect of back pressure.
Analysis shows that as pressure increases, power and torque only go down.
Please copy and paste this plot everywhere as objective dyno proof that backpressure decreases performance.
Put this horrible myth to bed.
As backpressure increases, Power and TQ only go down. There is absolutely no benefit to backpressure on this engine.
Could backpressure be good on a different engine? Possibly... but none I am aware of. And certaintly not on this engine.
Below is a series of dyno runs showing the effect of back pressure.
Analysis shows that as pressure increases, power and torque only go down.
Please copy and paste this plot everywhere as objective dyno proof that backpressure decreases performance.
Put this horrible myth to bed.
#21
08-11-2008 | 04:11 PM
Originally Posted by DaveB
This has nothing to do with back pressure. Back pressure is never a good thing and it does not produce power, torque, etc. The key to the correct exhaust design is keeping up exhaust velocity. If you make the exhaust piping too large, you kill exhaust velocity because the large piping allows the exhaust gases to cool and become turbulent in the pipes. Keep the heat up and keep the velocity high and you'll make power. IMO, 2.5 dual piping is way more pipe then these NA motors need. The OEM exhaust is nearly 2.5" and that's pretty large for NA seeing that a 3" single exhaust flow enough air for a modded 350whp LS1 motor. I think you're on the right track with going with smaller piping. I'd even suggest considering 2" piping.
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