2.5" or 3" True Duals??
Registered User
Joined: Nov 2000
Posts: 164
From: Plano, TX USA
3" FOR SURE! For all those that say that too big you will lose HP and torque is simply BS. I have a CC306 in my Impy and gain 9 RWHP and 20 RWTQ EVERYWHERE with open headers (no losses anywhere). Explain that? And that is dyno proven. And when you finally go with a stroker or power adder you won't have to re-do stuff later down the road. Go 3"!
Registered User
Joined: Mar 2002
Posts: 565
From: mt. holly, nc , usa
Originally posted by 4DRSS
3" FOR SURE! For all those that say that too big you will lose HP and torque is simply BS. I have a CC306 in my Impy and gain 9 RWHP and 20 RWTQ EVERYWHERE with open headers (no losses anywhere). Explain that? And that is dyno proven. And when you finally go with a stroker or power adder you won't have to re-do stuff later down the road. Go 3"!
3" FOR SURE! For all those that say that too big you will lose HP and torque is simply BS. I have a CC306 in my Impy and gain 9 RWHP and 20 RWTQ EVERYWHERE with open headers (no losses anywhere). Explain that? And that is dyno proven. And when you finally go with a stroker or power adder you won't have to re-do stuff later down the road. Go 3"!
Registered User
Joined: Jul 1999
Posts: 3,647
From: Winnipeg, Manitoba, Canada
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Originally posted by Mr. Horsepower:
Ok guys, as promised in the other thread I have the dyno results from the dual swap. First off, I'm not making any claims as to which exhaust you should run, just giving you the numbers.... so do with them as you see fit.
I would also prefer not getting into the combination as this is not my car. It belongs to a friend. I know a little about the setup but I don't want to turn this into a "What cam is he running" questionathon (my own word there ).
I will tell you this much... the heads are AFR 200's and I did about an hours worth of cleanup/blending on them last year. The cam is a custom ground Isky hydraulic roller and the other mods are as follows...
1.6:1 CC Magnum rockers, TPIS 1-3/4 headers, ported intake, K&N CAI, Granatelli MAF, AS&M 52mm TB, T56 w/LT4 clutch assembly. Oh yeah, it's a 383 running 11.3:1 compression. That's really about all I know other than the fact that I built the exhaust for it.
Ok, this car was running a 4" Mufflex catback and Rich replaced the Flowmaster with a Borla XR-1. Here are the dyno results from last weekend. I had to transfer these from the dyno graph so they aren't 100% accurate... good enough for this comparison though.
2000 rpm 139 rwhp/356 rwtq
2500 rpm 177/365
3000 rpm 226/395
3500 rpm 288/443
4000 rpm 341/459
4500 rpm 388/443
5000 rpm 408/428
5500 rpm 416/405
5600 rpm 418/396 (rwhp peak)
Tonight we tested the duals. This setup uses 3" to 2.5" transitions right off the collector. It then goes into the Dr. Gas x-pipe and then through a chambered muffler similiar to that used in the CMMG catback system. The system then exits at the side ahead of the rear wheel. It's a 2.5" system. There is a noticable tone difference with this setup. The x-pipe also creates a higher pitch exhaust note in comparison to the 'Y' or a straight dual setup.
Anyways, here are the results...
2000 rpm 150/370
2500 rpm 189/383
3000 rpm 236/416
3500 rpm 292/455
4000 rpm 339/458
4500 rpm 385/442
5000 rpm 413/430
5500 rpm 422/412
5600 rpm 426/401
Obviously the x-pipe is making some good torque over the Mufflex up til about 4000 rpm. From there til just past 4500 rpm the "Y" is winning, then the edge goes back to the x-pipe. This is usually typical of an x-pipe dual exhaust unless the situation is compensated for through valve timing. A little less exhaust duration would probably help gain back some of that torque loss in the 3500-4500 rpm range. AFR's generally have good exhaust ports so that only compounds the problem, which I'd conclude is a loss of volumetric efficiency in that rpm range. Maybe a swap to 1.5's on the exhaust would help. Either way, it warrants a little experimentation.
That's a wrap on the test so, considering this combination is pretty much inline with what most of you are doing, you can draw your own conclusions and go from there.
Any questions?
Take care,
Chuck Riddeck
Progressive Race Engine Development
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Originally posted by Mr. Horsepower:
Ok guys, as promised in the other thread I have the dyno results from the dual swap. First off, I'm not making any claims as to which exhaust you should run, just giving you the numbers.... so do with them as you see fit.
I would also prefer not getting into the combination as this is not my car. It belongs to a friend. I know a little about the setup but I don't want to turn this into a "What cam is he running" questionathon (my own word there ).
I will tell you this much... the heads are AFR 200's and I did about an hours worth of cleanup/blending on them last year. The cam is a custom ground Isky hydraulic roller and the other mods are as follows...
1.6:1 CC Magnum rockers, TPIS 1-3/4 headers, ported intake, K&N CAI, Granatelli MAF, AS&M 52mm TB, T56 w/LT4 clutch assembly. Oh yeah, it's a 383 running 11.3:1 compression. That's really about all I know other than the fact that I built the exhaust for it.
Ok, this car was running a 4" Mufflex catback and Rich replaced the Flowmaster with a Borla XR-1. Here are the dyno results from last weekend. I had to transfer these from the dyno graph so they aren't 100% accurate... good enough for this comparison though.
2000 rpm 139 rwhp/356 rwtq
2500 rpm 177/365
3000 rpm 226/395
3500 rpm 288/443
4000 rpm 341/459
4500 rpm 388/443
5000 rpm 408/428
5500 rpm 416/405
5600 rpm 418/396 (rwhp peak)
Tonight we tested the duals. This setup uses 3" to 2.5" transitions right off the collector. It then goes into the Dr. Gas x-pipe and then through a chambered muffler similiar to that used in the CMMG catback system. The system then exits at the side ahead of the rear wheel. It's a 2.5" system. There is a noticable tone difference with this setup. The x-pipe also creates a higher pitch exhaust note in comparison to the 'Y' or a straight dual setup.
Anyways, here are the results...
2000 rpm 150/370
2500 rpm 189/383
3000 rpm 236/416
3500 rpm 292/455
4000 rpm 339/458
4500 rpm 385/442
5000 rpm 413/430
5500 rpm 422/412
5600 rpm 426/401
Obviously the x-pipe is making some good torque over the Mufflex up til about 4000 rpm. From there til just past 4500 rpm the "Y" is winning, then the edge goes back to the x-pipe. This is usually typical of an x-pipe dual exhaust unless the situation is compensated for through valve timing. A little less exhaust duration would probably help gain back some of that torque loss in the 3500-4500 rpm range. AFR's generally have good exhaust ports so that only compounds the problem, which I'd conclude is a loss of volumetric efficiency in that rpm range. Maybe a swap to 1.5's on the exhaust would help. Either way, it warrants a little experimentation.
That's a wrap on the test so, considering this combination is pretty much inline with what most of you are doing, you can draw your own conclusions and go from there.
Any questions?
Take care,
Chuck Riddeck
Progressive Race Engine Development
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Registered User
Joined: Nov 2001
Posts: 2,985
From: In a house by the bay
"Torque"?
Torque where? At an rpm range you don't use at the race track? I like torque as much as any other guy but I tend to ignore torque that falls out of the rev range I race in, especially if I can pick up a bit of hp up top with the same or slightly better "usable rev-band" torque. The more hp you can add above your torque peak, the longer the engine will hang on (making power) past that peak.
wrt the velocity thing. I think it matters alot less the further you get from the engine. Velocity through the header primaries isn't going to change.... they're still the same diameter. So you go from a 1&3/4" primary to a 3" collector, everything is the same and there is still the velocity drop when it gets to the 3" collector. Now you take the dropped velocity exhaust and route it to the back through a 3" exhaust. Would it gain any velocity back by going through a 2.5" pipe? Not from what I understand of energy loss from so many years ago.
If you're running a 396 then you need as much exhaust as you can get. The bigger you get the less the low unusable rpm range torque loss is going to matter... you get the torque in the cubes. JMO.
-Mindgame
Torque where? At an rpm range you don't use at the race track? I like torque as much as any other guy but I tend to ignore torque that falls out of the rev range I race in, especially if I can pick up a bit of hp up top with the same or slightly better "usable rev-band" torque. The more hp you can add above your torque peak, the longer the engine will hang on (making power) past that peak.
wrt the velocity thing. I think it matters alot less the further you get from the engine. Velocity through the header primaries isn't going to change.... they're still the same diameter. So you go from a 1&3/4" primary to a 3" collector, everything is the same and there is still the velocity drop when it gets to the 3" collector. Now you take the dropped velocity exhaust and route it to the back through a 3" exhaust. Would it gain any velocity back by going through a 2.5" pipe? Not from what I understand of energy loss from so many years ago.
If you're running a 396 then you need as much exhaust as you can get. The bigger you get the less the low unusable rpm range torque loss is going to matter... you get the torque in the cubes. JMO.
-Mindgame
Registered User
Joined: Nov 2000
Posts: 164
From: Plano, TX USA
Here is a good thread comparing a 2.5" duals to 3" duals. 409 LT1 with hot cam, picked up 10 HP on motor and 40 HP on spray. Believe me the bigger exhaust works.
And honestly 95% of a 1/4 mile run is over 5,000 RPM, how is low end torque gonna help you much?
I'll take the top end power thank you. Like a cammed 350 or stroker motor is short on torque anyway, we're not talking Honda motors here guys!
http://www.impalassforum.com/cgi-bin...c&f=1&t=004375
And honestly 95% of a 1/4 mile run is over 5,000 RPM, how is low end torque gonna help you much?
I'll take the top end power thank you. Like a cammed 350 or stroker motor is short on torque anyway, we're not talking Honda motors here guys! http://www.impalassforum.com/cgi-bin...c&f=1&t=004375
Registered User
Joined: Jan 2002
Posts: 274
From: Decatur
He's got a built 409 for crying out loud. You have to match your exhaust to your motor. Just to say that becaue his large motor benifited from a larger exhaust is not to say that a 350 will. It depends on the motor.
Ben
Ben
Registered User
Joined: Nov 2000
Posts: 164
From: Plano, TX USA
Originally posted by Momar
He's got a built 409 for crying out loud. You have to match your exhaust to your motor. Just to say that becaue his large motor benifited from a larger exhaust is not to say that a 350 will. It depends on the motor.
Ben
He's got a built 409 for crying out loud. You have to match your exhaust to your motor. Just to say that becaue his large motor benifited from a larger exhaust is not to say that a 350 will. It depends on the motor.
Ben
Originally posted by 94droptopz
i personally dont see the need to go bigger with a stroker motor. theres plenty of 383 and 396 guys runnin 2.5" once again just my opinion
i personally dont see the need to go bigger with a stroker motor. theres plenty of 383 and 396 guys runnin 2.5" once again just my opinion
Registered User
Joined: Feb 2002
Posts: 146
From: Amelia, OH
For Cryin out loud, if LIKE THE SOUND OF DUAL EXHAUST, AND Have the $$$ then do it!!!!!! If you got a guy to do it that is very skilled, THEN DO 3" WHY NOT?!?!?!?!?!?! If you have a guy doing it that isnt very skilled at it and will have a hard time with the clearance, then go with 2.5's!!!! I dont see any worthwhile gains going with 2.5's except clearance....... but its obvious if you know what your doing, you can make 3" ers work!
ALSO, you guys crying over 10 TQ at idle lol WTF WE DONT HAVE 4 BANGERS!!!!!!!!!!!!!!!!!
ALSO, you guys crying over 10 TQ at idle lol WTF WE DONT HAVE 4 BANGERS!!!!!!!!!!!!!!!!!
Registered User
Joined: Oct 1999
Posts: 738
From: Tulsa, OK, USA
Originally posted by 4DRSS
3" FOR SURE! For all those that say that too big you will lose HP and torque is simply BS. I have a CC306 in my Impy and gain 9 RWHP and 20 RWTQ EVERYWHERE with open headers (no losses anywhere). Explain that? And that is dyno proven. And when you finally go with a stroker or power adder you won't have to re-do stuff later down the road. Go 3"!
3" FOR SURE! For all those that say that too big you will lose HP and torque is simply BS. I have a CC306 in my Impy and gain 9 RWHP and 20 RWTQ EVERYWHERE with open headers (no losses anywhere). Explain that? And that is dyno proven. And when you finally go with a stroker or power adder you won't have to re-do stuff later down the road. Go 3"!
Open headers I assume?
Registered User
Joined: Nov 2000
Posts: 164
From: Plano, TX USA
Originally posted by Ken95Z28
From what type of setup did you have before?
Open headers I assume?
From what type of setup did you have before?
Open headers I assume?
You can see the cut outs I have at my header collectors on my webpage. With them opened I make 364 RWHP and 410 FT LBS at the rear wheels. Well worth it at the track
Registered User
Joined: Nov 2001
Posts: 2,985
From: In a house by the bay
I still don't see the point some of you are making.
Do you want power from "lugging-rpm" so you can pull from a 4th gear roll (M6 guys) at 30 mph or do you want the power in the rpm range you like to race in?
You can't have your cake and eat it too. You will gain power up top and in the rev range you want it in with a bigger exhaust..... it's been proven for years now. As a matter of fact, I'd guarantee that some of you running strokers would see some gains in a header change if it was available. 1-7/8 vs the 1-3/4 everyone builds for the LT.
-Mindgame
Do you want power from "lugging-rpm" so you can pull from a 4th gear roll (M6 guys) at 30 mph or do you want the power in the rpm range you like to race in?
You can't have your cake and eat it too. You will gain power up top and in the rev range you want it in with a bigger exhaust..... it's been proven for years now. As a matter of fact, I'd guarantee that some of you running strokers would see some gains in a header change if it was available. 1-7/8 vs the 1-3/4 everyone builds for the LT.
-Mindgame
Registered User
Joined: Jan 2002
Posts: 274
From: Decatur
Have you ever heard of scavenging? With the right size exhaust for your motor whether large or small you will see a benifit. Scavenging is when one exhaust pulse kind of pulls the next one along. It creates a vacuum/void and when the next exhaust valve opens the air rushes from the high pressure area into the void/low pressure area. Since it is heading into a low pressure area it moves faster than it would have otherwize. Then the same happens with the air out of the cylinder following it. If you have enough overlap on your cam it will actually suck fresh air in through the intake valve when it is pulling the air out of the exhaust valve. This will cause more power. If your exhaust is to large for the motor the exhaust hits too big of a mass of air and cant move it. Then the exhaust after it runs into it and slows down. This doesnt allow as much room in the cylinder for the fresh air to come in the intake. This effect is more of a problem with oversized pipe than open headers because with the oversized pipe there air doesnt move out of the way as quickly. In the same manner if you run to small of a pipe you get backpressure which is bad. It is when there is to much exhaust flow for the size pipe it is trying to enter. This causes "backpressure" not letting the cylinder completely "exhale" if you will. This is kind of a simple way of explaining it but it should get the message across. A crossover pipe such as an x or h help this process because there is a point at which 2 cylinders fire real close to the same time and it tries to over fill the exhaust pipe. The crossover pipe allows some of the pressure to be bled off to the other side in order to keep velocity up.
Try reading one of David Vizards books that he talks about exhaust in. He has some real good information.
Ben
Try reading one of David Vizards books that he talks about exhaust in. He has some real good information.
Ben
Registered User
Joined: Nov 2001
Posts: 2,985
From: In a house by the bay
Originally posted by Momar
Have you ever heard of scavenging? With the right size exhaust for your motor whether large or small you will see a benifit. Scavenging is when one exhaust pulse kind of pulls the next one along. It creates a vacuum/void and when the next exhaust valve opens the air rushes from the high pressure area into the void/low pressure area. Since it is heading into a low pressure area it moves faster than it would have otherwize. Then the same happens with the air out of the cylinder following it. If you have enough overlap on your cam it will actually suck fresh air in through the intake valve when it is pulling the air out of the exhaust valve. This will cause more power. If your exhaust is to large for the motor the exhaust hits too big of a mass of air and cant move it. Then the exhaust after it runs into it and slows down. This doesnt allow as much room in the cylinder for the fresh air to come in the intake. This effect is more of a problem with oversized pipe than open headers because with the oversized pipe there air doesnt move out of the way as quickly. In the same manner if you run to small of a pipe you get backpressure which is bad. It is when there is to much exhaust flow for the size pipe it is trying to enter. This causes "backpressure" not letting the cylinder completely "exhale" if you will. This is kind of a simple way of explaining it but it should get the message across. A crossover pipe such as an x or h help this process because there is a point at which 2 cylinders fire real close to the same time and it tries to over fill the exhaust pipe. The crossover pipe allows some of the pressure to be bled off to the other side in order to keep velocity up.
Try reading one of David Vizards books that he talks about exhaust in. He has some real good information.
Ben
Have you ever heard of scavenging? With the right size exhaust for your motor whether large or small you will see a benifit. Scavenging is when one exhaust pulse kind of pulls the next one along. It creates a vacuum/void and when the next exhaust valve opens the air rushes from the high pressure area into the void/low pressure area. Since it is heading into a low pressure area it moves faster than it would have otherwize. Then the same happens with the air out of the cylinder following it. If you have enough overlap on your cam it will actually suck fresh air in through the intake valve when it is pulling the air out of the exhaust valve. This will cause more power. If your exhaust is to large for the motor the exhaust hits too big of a mass of air and cant move it. Then the exhaust after it runs into it and slows down. This doesnt allow as much room in the cylinder for the fresh air to come in the intake. This effect is more of a problem with oversized pipe than open headers because with the oversized pipe there air doesnt move out of the way as quickly. In the same manner if you run to small of a pipe you get backpressure which is bad. It is when there is to much exhaust flow for the size pipe it is trying to enter. This causes "backpressure" not letting the cylinder completely "exhale" if you will. This is kind of a simple way of explaining it but it should get the message across. A crossover pipe such as an x or h help this process because there is a point at which 2 cylinders fire real close to the same time and it tries to over fill the exhaust pipe. The crossover pipe allows some of the pressure to be bled off to the other side in order to keep velocity up.
Try reading one of David Vizards books that he talks about exhaust in. He has some real good information.
Ben
What I think most of this is missing is in the fact that most exhaust theory stuff I've read only talks about it up to the collector. Most collectors are 3-3-1/2" or bigger for the race stuff and range from 4" in length to much longer. The "low pressure area" you're referring to is the collector is it not? When that "velocity" slows down in the collector what is it going to do from there? Speed up with a smaller pipe (3.0-3.5" collector to 2.5" exhaust) or is it going to lose energy/heat and slow down?
My thought is that the least amount of restriction from that collector to the exhaust the better. The collector outlet is ~3", why not just go 3" on the exhaust?
If backpressure is not our friend, and it's not, then why not go with the larger exhaust? If you start to pump more power through the engine later on at least you'll have an exhaust that can keep up.
If you get too much backpressure, you increase the pumping losses of the engine and may even pull some of the exhaust back into the combustion chamber. All of that only costs you power.
If you have enough overlap on your cam it will actually suck fresh air in through the intake valve when it is pulling the air out of the exhaust valve. This will cause more power. If your exhaust is to large for the motor the exhaust hits too big of a mass of air and cant move it. Then the exhaust after it runs into it and slows down. This doesnt allow as much room in the cylinder for the fresh air to come in the intake. This effect is more of a problem with oversized pipe than open headers because with the oversized pipe there air doesnt move out of the way as quickly.
Why would a "too large" exhaust have a hard time moving a greater mass of air? That seems just the opposite doesn't it? Or maybe I'm missing something.
As for "slowing down", didn't it already make a slow down at the collector? So now the collector is out of the equation and the pipe size is all that matters? Even more
Anyways....
-Mindgame