opinion on backpressure??
TECH Enthusiast
Joined: Apr 2004
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From: Chandler, Arizona
This is an ongoing debate. It's also not any easy question to answer. Do a search, there was one website someone linked that talked about keeping the velocity up to increase the scavenging affect of the exhaust. However, most people will argue that saying our engines need backpressure to run properly is an urban legend.
TECH Addict
Joined: Jun 2005
Posts: 2,660
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From: England
Originally Posted by rbrooks
do you think that to much exhaust flow is bad. i currently run no muffler and no cats - is that bad?
Whats your complete setup (all engine mods and current exhaust setup - lts, tubing size and so on).
Also what are your goals with the car. On the street no mufflers is probably way too loud and it's just a matter of time before you get a ticket for it.
As for back pressure, well it depends. No you don't neccicarily want back pressure, although it can be good for low end torque. So in a truck that does a lot of towing it could be benifical. But too much flow can be just as bad, as it will have excessive noise, and you'll not be able to make the most of the available flow - it would be wasted potential. Plus you would loose under the curve torque in the low rpms.
It's a fine balance between to little and too much.
If you took a STOCK Ls1 and simply removed the mufflers, cats and upper the tubing bore to 3" it would flow great. But the PCM is tuned to give optimum performance with some backpressure. Without re-mapping your tuning for air/fuel it would be pretty pointless increasing the exhaust flow, an potentially may even loose you BHP. Although due to the noise many would beleive it was quicker even if it where not.
you don't want backpressure at all. what you want is exhaust velocity (speed). backpressure means a restriction. it's a balance of pipe size to rpm.
for example: all stock engine 6000rpms max, low, moderate hp output. you wouldn't need a dual 3" or single 4" pipe. as the pipe size is too large to maintain low rpm speed of the exhaust.
example 2: large cubic inch, n2o, supercharger, 3500-7000 rpms race use, high hp output. you couldn't use a dual 2" or single 2.5" exhaust. as the pipe would be too small to maintain high rpm speed. hence this is where backpressure comes into play. it's a restriction.
so, where is the balance? it just depends on intended use.
it's like the water hose trick. you turn the water on barely. water just trickles out of a hose thats 5/8" diameter. now, you use pinch close the end. same volume of flow, the water comes out faster and father away from the end. it's not a matter of restriction as the thinking is by pinching it off, it's been restricted. no, the hose diameter has been made smaller to match the volume of water flowing.
now, do the opposite. turn the water on full blast. now, use a 1/4" hose. comes out and sprays pretty far. just the hose can't handle the volume being sent to it. now you have backpressure. now you trade in the 5/8". water still flows at the same speed as the smaller hose, you just have more volume coming out.
i've talked with darrell, the owner of bassani exhaust. he has cars using his products in the 9s with a dual 2.5" exhaust. it's just a matter of matching the pipe size to the intended output/use the vehicle needs/has.
for example: all stock engine 6000rpms max, low, moderate hp output. you wouldn't need a dual 3" or single 4" pipe. as the pipe size is too large to maintain low rpm speed of the exhaust.
example 2: large cubic inch, n2o, supercharger, 3500-7000 rpms race use, high hp output. you couldn't use a dual 2" or single 2.5" exhaust. as the pipe would be too small to maintain high rpm speed. hence this is where backpressure comes into play. it's a restriction.
so, where is the balance? it just depends on intended use.
it's like the water hose trick. you turn the water on barely. water just trickles out of a hose thats 5/8" diameter. now, you use pinch close the end. same volume of flow, the water comes out faster and father away from the end. it's not a matter of restriction as the thinking is by pinching it off, it's been restricted. no, the hose diameter has been made smaller to match the volume of water flowing.
now, do the opposite. turn the water on full blast. now, use a 1/4" hose. comes out and sprays pretty far. just the hose can't handle the volume being sent to it. now you have backpressure. now you trade in the 5/8". water still flows at the same speed as the smaller hose, you just have more volume coming out.
i've talked with darrell, the owner of bassani exhaust. he has cars using his products in the 9s with a dual 2.5" exhaust. it's just a matter of matching the pipe size to the intended output/use the vehicle needs/has.
ive dealt with this PERSONALLY on my last two camaros and i can tell you this for sure, when i free up my exhaust too much (ie worst with opne headers) my cars lost ALL low end power, spining the tires was out of the question.
sounds mean as hell though!
sounds mean as hell though!
Staging Lane
Joined: Oct 2005
Posts: 92
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IMO, what would be the best would be to have a full exhaust with prolly one or 2 x's for ballance, for the low end, and have electronicly controled exhaust cutouts that open up arround 4500 and close below that. But, thats a farfetched idea. But, itd give ya the backpressure u need at low rpm's, and the unrestricted flow at higher rpms. lol.
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Don't look at it from a backpressure perspective - look at it from this point of view:
How much flow do I need for my setup?
jrp's exhaust guide does a great job of explaining what kind of pipe size you need for a given power level i do believe. check that out to figure out what you need. Go from there. I personally think to worry about backpressure is a backwards way of thinking about it. Its like worrying about having too much power.
True, you can lose some power if you don't design your exhaust correctly, but you're not going to damage anything unless you run your car w/o any exhaust at all...just straight off the heads or straight off the headers and no extensions(fresh air washing over the o2's all the time, bad readings very possible).
Just my 2 cents on the subject.
How much flow do I need for my setup?
jrp's exhaust guide does a great job of explaining what kind of pipe size you need for a given power level i do believe. check that out to figure out what you need. Go from there. I personally think to worry about backpressure is a backwards way of thinking about it. Its like worrying about having too much power.
True, you can lose some power if you don't design your exhaust correctly, but you're not going to damage anything unless you run your car w/o any exhaust at all...just straight off the heads or straight off the headers and no extensions(fresh air washing over the o2's all the time, bad readings very possible).
Just my 2 cents on the subject.
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TECH Regular
Joined: Oct 2005
Posts: 430
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Forget anything you've ever read, heard or dreamed re exhaust systems in general and back pressure in particular that contradicts this article by one of the true masters of engine technology (Link swiped from the Advanced Tech Forum): http://www.popularhotrodding.com/en..._exh/index.html
12 Second Club
Joined: Jun 2004
Posts: 345
Likes: 0
From: Brazoria
Originally Posted by 300bhp/ton
yes, no and maybe.
Whats your complete setup (all engine mods and current exhaust setup - lts, tubing size and so on).
Also what are your goals with the car. On the street no mufflers is probably way too loud and it's just a matter of time before you get a ticket for it.
As for back pressure, well it depends. No you don't neccicarily want back pressure, although it can be good for low end torque. So in a truck that does a lot of towing it could be benifical. But too much flow can be just as bad, as it will have excessive noise, and you'll not be able to make the most of the available flow - it would be wasted potential. Plus you would loose under the curve torque in the low rpms.
It's a fine balance between to little and too much.
If you took a STOCK Ls1 and simply removed the mufflers, cats and upper the tubing bore to 3" it would flow great. But the PCM is tuned to give optimum performance with some backpressure. Without re-mapping your tuning for air/fuel it would be pretty pointless increasing the exhaust flow, an potentially may even loose you BHP. Although due to the noise many would beleive it was quicker even if it where not.
Whats your complete setup (all engine mods and current exhaust setup - lts, tubing size and so on).
Also what are your goals with the car. On the street no mufflers is probably way too loud and it's just a matter of time before you get a ticket for it.
As for back pressure, well it depends. No you don't neccicarily want back pressure, although it can be good for low end torque. So in a truck that does a lot of towing it could be benifical. But too much flow can be just as bad, as it will have excessive noise, and you'll not be able to make the most of the available flow - it would be wasted potential. Plus you would loose under the curve torque in the low rpms.
It's a fine balance between to little and too much.
If you took a STOCK Ls1 and simply removed the mufflers, cats and upper the tubing bore to 3" it would flow great. But the PCM is tuned to give optimum performance with some backpressure. Without re-mapping your tuning for air/fuel it would be pretty pointless increasing the exhaust flow, an potentially may even loose you BHP. Although due to the noise many would beleive it was quicker even if it where not.
12 Second Club
Joined: Jun 2004
Posts: 345
Likes: 0
From: Brazoria
Originally Posted by Mrwilson
IMO, what would be the best would be to have a full exhaust with prolly one or 2 x's for ballance, for the low end, and have electronicly controled exhaust cutouts that open up arround 4500 and close below that. But, thats a farfetched idea. But, itd give ya the backpressure u need at low rpm's, and the unrestricted flow at higher rpms. lol.
NO back pressure has more power but all up top
people say you need some for driving around town (torque at red lights needed)
but if all you do is race launching at 5000 rpms and up you don't need it
people say you need some for driving around town (torque at red lights needed)
but if all you do is race launching at 5000 rpms and up you don't need it
TECH Addict
Joined: Jun 2005
Posts: 2,660
Likes: 14
From: England
Originally Posted by electricz28
All I can say is that when I open my c/o which is 12" after the ory my car pulls so much better than going thru the muffler.
If the catback and muffler flow in excess of what is needed the only affect a cutout will have is on the exhaust pressure wave. This 'may' yield power, but again if you design the rest of the exhaust with it in mind, then there's no reason that it would have any decissive affect.
And remember the extra noise can be quite deceptive in terms of actual performance, louder things often feel faster, even when they arn't.
I think it has been said that the stock tuning favors a little back pressure. I immediately noticed a bit more low end torque when I went to a restricted plated borla from a free flowing LM (no cats/LT's). I really need to take it to the track to see how the #'s look.
Fredzep
Fredzep
