Hey - I'm still adjusting to the real world of cars (Used to Cavaliers, Civics, etc) - and back on that planet, there is such a thing of not having enough back pressure... hurts performance, and makes some ugly noises coming off of revving...

In the big engine world, namely our LS1's - is there such a thing of having too little backpressure, that it HURTS performance?

Also, is that 'choking' or 'gurgling' noise I hear after revving a good noise? Cant explain it... REALLY noticable early in the morning when I wake up to go to work...



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-2002 Red Camaro SS
Chesapeake, Virginia
http://www.HeadRushPerformance.Com

 

Old wive's tale.
No, you don't need back pressure.
It takes hp to overcome exhaust pressure.
The only way to do away with it anyway is to run the exhaust straight out of the cylinder head, not a good idea.
If back pressure was good, folks wouldn't run open headers or cut-outs either.
Dave

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Exactly. Ratio is correct.

 

When you lower backpressure it may and will affect the way 02 sensors are reading. Thus a/f gets wrong but you get over it by tuning and eventually will net hp.

Too low backpressure in civic world probably is condition where 02 sensors can't read right.

 

All your oxygen sensors need is to be so many inches from the end of your exhaust. Assuming of course a "sane" pipe diameter. This is so they aren't reading outside air.
There is also the fact that as long as your engine is pushing exhaust through pipe, there is back pressure. It just is more or less depending on the exhaust components.

 

I agree that backpressure cant be too low, but I will say that if you stray too far from what the factory tuning is designed for, you may lose power (particularly just off idle). But this is due to the tuning not being correct, not the low backpressure.

a freer flowing exaust can change A/F, for example, headers usualy mess with A/F ratios. that would be where any power loss comes from, though usualy, you will make more power on the dyno, it seems most of the power loss is below the RPM capability of a dyno.

Proper tuning can restore the lower end, and usualy add some power throughout the rest of the RPM band as well

 

Less backpressure due to larger pipes or headers causes a shift in the torque band, or a loss of low end torque in favor of high end torque. That can be misconstrued as a power loss, especially when enough torque is moved up the power band that it can be felt seat of the pants. Most street driving requires low end torque, a car will feel slower if that torque is moved to higher rpms. This is due to exhaust gases moving slower through the larger pipe. With smaller pipe, exhaust velocity and low rpm cylinder scavenging are up and so is low end power. Sounds great, but small pipe limits upper rpms more than large pipe limits lower rpm. Like was stated, you (or even the pcm) can tune away problems caused by large pipes, headers, or low back-pressure... but you can't tune away restrictive exhaust.
Dave

 

The big misconception is people think of backpressure when the real problem is velocity. There is such a thing as going with two big of an exhaust for your powerplant. It's not the lack of backpressure, but it is the lack of velocity that is caused. Your car needs to be able to push out the exhaust and when you have too large of a system, you lose velocity and can slow down the efficiency of your exhasut. That is the major misconception, backpressure is totally unnecesarry, but velocity is.

 

Umm, dunno about the velocity thing with near stock cams and setups when there's not too much cam lobe overlap and the header is not really right length. The piston has to push the gases out and easier that is the better.
If you have mean h/c setup then I believe velocity needs to be ok and matched with primary length. Though, if you use off the shelf components you never really get matched setup and it again is pretty much about backpressure. If you are lucky and components are matched together the header can create somekind of a vacuum to help scavenging and in the end give a pulse back to stop fuel mixture from flowing to exhaust.