backpressure question
09-20-2005, 08:20 PM
#16
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Re: backpressure question
I've seen cars pick up 100 rwhp by removing the seemingly good exhaust system they have.
Some LS1's don't gain any hp by removing the exhaust. This is what spawned the whole reverse-split duration cam craze for the LS1 folks. If the cylinder head is over-exhausted, then it can benefit from having more intake than exhaust duration.
That's neither here nor there, though. . . I've heard of certain engines losing power due to lack of backpressure, but in every case, it's been an extreme application with too much overlap. I'm talking about Hemi's with 400+ cfm heads and cams in the 280@.050 range. There's so much overlap that the exhaust pulls the air/fuel right through the chamber into the exhaust. Backpressure helps, but reducing overlap helps more.
That said. . . running open headers with an LT1 causes some other problems, namely O2 sensor readings. The O2 sensors are too close to the collector, so fresh air eddy's and pulses into them, thus giving them a false lean reading. The computer will quickly start pouring fuel in. After about 3 miles, the car will barely run. I know this from experience. All you really need to fix this problem is about 18" of pipe extending off the collector.
Mike
Some LS1's don't gain any hp by removing the exhaust. This is what spawned the whole reverse-split duration cam craze for the LS1 folks. If the cylinder head is over-exhausted, then it can benefit from having more intake than exhaust duration.
That's neither here nor there, though. . . I've heard of certain engines losing power due to lack of backpressure, but in every case, it's been an extreme application with too much overlap. I'm talking about Hemi's with 400+ cfm heads and cams in the 280@.050 range. There's so much overlap that the exhaust pulls the air/fuel right through the chamber into the exhaust. Backpressure helps, but reducing overlap helps more.
That said. . . running open headers with an LT1 causes some other problems, namely O2 sensor readings. The O2 sensors are too close to the collector, so fresh air eddy's and pulses into them, thus giving them a false lean reading. The computer will quickly start pouring fuel in. After about 3 miles, the car will barely run. I know this from experience. All you really need to fix this problem is about 18" of pipe extending off the collector.
Mike
09-20-2005, 08:24 PM
#17
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Re: backpressure question
Originally Posted by engineermike
I've seen cars pick up 100 rwhp by removing the seemingly good exhaust system they have.
Some LS1's don't gain any hp by removing the exhaust. This is what spawned the whole reverse-split duration cam craze for the LS1 folks. If the cylinder head is over-exhausted, then it can benefit from having more intake than exhaust duration.
That's neither here nor there, though. . . I've heard of certain engines losing power due to lack of backpressure, but in every case, it's been an extreme application with too much overlap. I'm talking about Hemi's with 400+ cfm heads and cams in the 280@.050 range. There's so much overlap that the exhaust pulls the air/fuel right through the chamber into the exhaust. Backpressure helps, but reducing overlap helps more.
That said. . . running open headers with an LT1 causes some other problems, namely O2 sensor readings. The O2 sensors are too close to the collector, so fresh air eddy's and pulses into them, thus giving them a false lean reading. The computer will quickly start pouring fuel in. After about 3 miles, the car will barely run. I know this from experience. All you really need to fix this problem is about 18" of pipe extending off the collector.
Mike
Some LS1's don't gain any hp by removing the exhaust. This is what spawned the whole reverse-split duration cam craze for the LS1 folks. If the cylinder head is over-exhausted, then it can benefit from having more intake than exhaust duration.
That's neither here nor there, though. . . I've heard of certain engines losing power due to lack of backpressure, but in every case, it's been an extreme application with too much overlap. I'm talking about Hemi's with 400+ cfm heads and cams in the 280@.050 range. There's so much overlap that the exhaust pulls the air/fuel right through the chamber into the exhaust. Backpressure helps, but reducing overlap helps more.
That said. . . running open headers with an LT1 causes some other problems, namely O2 sensor readings. The O2 sensors are too close to the collector, so fresh air eddy's and pulses into them, thus giving them a false lean reading. The computer will quickly start pouring fuel in. After about 3 miles, the car will barely run. I know this from experience. All you really need to fix this problem is about 18" of pipe extending off the collector.
Mike
09-20-2005, 08:26 PM
#18
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Re: backpressure question
Originally Posted by mrr23
collector extensions is what it's called that you are wanting to do. this helps to promote more midrange power. the pipe will be the same size as your collector diameter. how long will be a guessing game. make a long piece. spray a paint line on them. make a run. look for the point where it burned the paint off. cut there.
Last edited by 93camaroLT1; 09-20-2005 at 08:31 PM.
09-20-2005, 08:29 PM
#19
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Re: backpressure question
Originally Posted by mrr23
spray a paint line on them. make a run. look for the point where it burned the paint off. cut there.
What, exactly, does the exhaust temperature, conductivity of exhaust tubing, and paint decomposition temperature have to do with the power curve???
09-20-2005, 09:27 PM
#26
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Re: backpressure question
Originally Posted by GreenZLT1
My car ran like crap with open headers. No power anywhere. Put on the y-pipe and catback, and now it runs like a mother....
Where are your O2 sensors?
09-21-2005, 01:35 AM
#27
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Re: backpressure question
There are countless numbers of people on here that say that their cars run terrible on open headers, which was the reason for this post. But looking at everything I suspect that a lot of these people are attempting to run their 02s with open headers which would in terms cause their cars to run like crap. (that and possibly something with tuning, or lack of it.)
09-21-2005, 01:45 AM
#28
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Re: backpressure question
only 2 things I can really think of towards why a car would run bad on open headers, but good with a exhaust.
1. Trying to run the O2s in the collectors with open headers causing false readings and therefore the car runs poorly.
2. Their actually is a lack of enough backpressure causing a performance loss, meaning that you do need some kind of backpressure on LT1's N/A without lots of compression. (I do question this though.. I attempted to run my civic si which is a 1.6 liter with 10:1 compression without a exhaust and flooring it on the highway at 60 the car litterally would not move, proving that backpressure (or velocity, whatever you want to call it) is needed in some applications. I just don't know if that is the case with a LT1 being that it has over three times the displacement of the honda.)
1. Trying to run the O2s in the collectors with open headers causing false readings and therefore the car runs poorly.
2. Their actually is a lack of enough backpressure causing a performance loss, meaning that you do need some kind of backpressure on LT1's N/A without lots of compression. (I do question this though.. I attempted to run my civic si which is a 1.6 liter with 10:1 compression without a exhaust and flooring it on the highway at 60 the car litterally would not move, proving that backpressure (or velocity, whatever you want to call it) is needed in some applications. I just don't know if that is the case with a LT1 being that it has over three times the displacement of the honda.)
09-21-2005, 01:48 AM
#29
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Re: backpressure question
Originally Posted by 93camaroLT1
stock cars/ race cars are also running much, much, much higher compression than I am and nitro-methane. I am only running 10.7:1 compression, i'm sure open headers will be awesome for power with the nitrous, but I question if the car will make as it could N/A with open headers, but then again I don't know since I havent experimented, which is why I ask.
Pro stock don't run nitro.
If ya aren't running 02's just get it tuned.
09-21-2005, 08:51 AM
#30
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Re: backpressure question
I run a Borla XR-1 on each collector. The local track required that all cars have mufflers, and then the next year they added the requirement that the exhaust dumps at least past the rear edge of the door. I added 3" pipes with turndowns, and could see no difference in rwHP with and without the pipes/turndowns. And that's based on running the dyno pulls (NA and N2O) one right after the other.
http://cjcfo.fbody.com/members/injun.../DCP03675a.jpg
I would be careful with running a pipe the same diameter as the collector, with no muffler. In effect, your are extending the collector, and that might not be good. It sometimes helps to "decouple" the collector from the rest of the exhaust with a "chamber".
http://cjcfo.fbody.com/members/injun.../DCP03675a.jpg
I would be careful with running a pipe the same diameter as the collector, with no muffler. In effect, your are extending the collector, and that might not be good. It sometimes helps to "decouple" the collector from the rest of the exhaust with a "chamber".