2.5" vs 3" exhaust
Drifting
Joined: Oct 2005
Posts: 1,920
Likes: 51
From: Angier nc
If you did 1 5/8 headers you should have seen a gain(albeit small).
Have you retuned the computer for the change in air/fuel ratio?
Did you chassis dyno it before and after or do you not "feel" a difference?The butt-meter can lie.
Drifting
Joined: Oct 2005
Posts: 1,920
Likes: 51
From: Angier nc
To address the OP the factory 2.75 tubing is more than adequate for 400+HP.Addressing the resonator would be my first choice.Aftermarket one or x-pipe should be an improvement.The stock mufflers arent a big restriction at stock/mild power levels either IMO.
Instructor
Joined: Feb 2009
Posts: 191
Likes: 0
From: Woodstock IL
I would have expected an increase in low end and mid range torque if nothing else.I can see 1 3/4 primaries and less restriction from the cat removal actually hurting low end some.
If you did 1 5/8 headers you should have seen a gain(albeit small).
Have you retuned the computer for the change in air/fuel ratio?
Did you chassis dyno it before and after or do you not "feel" a difference?The butt-meter can lie.
If you did 1 5/8 headers you should have seen a gain(albeit small).
Have you retuned the computer for the change in air/fuel ratio?
Did you chassis dyno it before and after or do you not "feel" a difference?The butt-meter can lie.
Race Director
Joined: Nov 2003
Posts: 12,221
Likes: 446
From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
From David Vizzard's "Auto Exhaust Science".....
Secondaries -- Diameters and Lengths
Well, so much for primary pipe dimensions and their effect on output. Let us now consider the collector/secondary pipe dimensions and configurations. The first point to make here is that the secondary diameter is as critical as the primary. A good starting point for the collector/secondary pipe size of a simple 4-into-1 header is to multiple the primary diameter by 1.75. Fortunately, the collector can be changed relatively easily and it is often best optimized at the track rather than the dyno.
As for the secondary length-that is from about the middle of the collector to the end of the secondary (or the first large change in cross-sectional area), we find a great deal more sensitivity than is seen with the primary. Ironically, few racers pay heed to collector length even though it is easy to adjust. In practice, collector length and diameter can have more effect on the power curve than the primary length. A basic rule on collectors is that shorter, larger diameters favor top end while longer, smaller diameters favor the low end. Except for the most highly developed engines, many collectors I see at the track are too large in diameter and either too short, or of excessive length.
------------------------------------------------------------------------
What! There's such a thing as TOO large? NO! It can't be. We have someone who told us that's not true!
In the context above, collector means secondary. On a standard 4-1 race header setup, the collector (size and length) is the secondary. I wonder how many people think Vizzard doesn't know what he's talking about. Better yet,,,think he's confusing primaries with secondaries.
Since secondaries can be up to 50" (or more), you also have to infer that exhaust pipe can/is part of the secondary "system". Especially in Vizzards' design where he uses a terminator box to simulate open atmosphere.
Me too.
Less restriction is never gonna hurt (which isn't the same thing as increasing velocity with the correct pipe size.)
Secondaries -- Diameters and Lengths
Well, so much for primary pipe dimensions and their effect on output. Let us now consider the collector/secondary pipe dimensions and configurations. The first point to make here is that the secondary diameter is as critical as the primary. A good starting point for the collector/secondary pipe size of a simple 4-into-1 header is to multiple the primary diameter by 1.75. Fortunately, the collector can be changed relatively easily and it is often best optimized at the track rather than the dyno.
As for the secondary length-that is from about the middle of the collector to the end of the secondary (or the first large change in cross-sectional area), we find a great deal more sensitivity than is seen with the primary. Ironically, few racers pay heed to collector length even though it is easy to adjust. In practice, collector length and diameter can have more effect on the power curve than the primary length. A basic rule on collectors is that shorter, larger diameters favor top end while longer, smaller diameters favor the low end. Except for the most highly developed engines, many collectors I see at the track are too large in diameter and either too short, or of excessive length.
------------------------------------------------------------------------
What! There's such a thing as TOO large? NO! It can't be. We have someone who told us that's not true!
In the context above, collector means secondary. On a standard 4-1 race header setup, the collector (size and length) is the secondary. I wonder how many people think Vizzard doesn't know what he's talking about. Better yet,,,think he's confusing primaries with secondaries.
Since secondaries can be up to 50" (or more), you also have to infer that exhaust pipe can/is part of the secondary "system". Especially in Vizzards' design where he uses a terminator box to simulate open atmosphere.
Less restriction is never gonna hurt (which isn't the same thing as increasing velocity with the correct pipe size.)
Last edited by GREGGPENN; Jun 30, 2010 at 12:07 AM.
Safety Car
Joined: Dec 2007
Posts: 3,616
Likes: 5
From: Sandhills of North Carolina
Here's a comparison for you guys to think about. And I have dyno graphs to prove it. My mods at the time these dyno pulls were done are as follows.
Bullet 226/230.565.565/111lsa cam
LS7 lifters
guide plates
7/16 valve spring studs
NSA 1.6 Magnum Comp Cams roller rockers
GM hardened guide plates
ARP hardened push rods
Patriot valve springs
Stainless Works 1 5/8 LT headers
High flow bullet cats
Cloyes double roller timing chain
EWP
K&N air filter
48 mm stock throttle body
Custom Tune
On my '96 from the headers with high flow cats back I have the stock exhaust system except for the resonator which was replaced by an X pipe. Right at the bend under the rear end I have electric cutouts (see avatar). I try really hard to make dyno comparisons with any major mod I make on the car. Below are the dyno graph links for a exhaust flow comparison I did a few months ago.
For those to lazy to write down the numbers here they are:
Stock system with X pipe...................333/317
Same system with cutouts open..........336/320
Same system with cutouts half open....341/222
Stock exhaust system minus resonator with X pipe
http://i274.photobucket.com/albums/j...sresonator.jpg
Stock exhaust system with cutouts open
http://i274.photobucket.com/albums/j...utoutsopen.jpg
Stock exhaust system with cutouts half open.
http://i274.photobucket.com/albums/j...tshalfopen.jpg
Granted the rwhp increase was not that significant with the cutouts half open but it shows that the system needed a certain amount of back pressure to flow at it's maximum. A straight shot out the back was not the answer.
The point here is fooling around with the flow of the stock system for a increase in rwhp is pointless on a stock or slightly bolton modded motor. Making the diameter of the entire system larger will most likely hurt performance. I would bet that if that cam wasn't in there these results would show very little if any increase across all three graphs. If you want more hp/torque do mods to the engine itself not the exhaust. If you want more noise so everyone will think it has a lot of power have at it and enjoy your drone!
Mike
Bullet 226/230.565.565/111lsa cam
LS7 lifters
guide plates
7/16 valve spring studs
NSA 1.6 Magnum Comp Cams roller rockers
GM hardened guide plates
ARP hardened push rods
Patriot valve springs
Stainless Works 1 5/8 LT headers
High flow bullet cats
Cloyes double roller timing chain
EWP
K&N air filter
48 mm stock throttle body
Custom Tune
On my '96 from the headers with high flow cats back I have the stock exhaust system except for the resonator which was replaced by an X pipe. Right at the bend under the rear end I have electric cutouts (see avatar). I try really hard to make dyno comparisons with any major mod I make on the car. Below are the dyno graph links for a exhaust flow comparison I did a few months ago.
For those to lazy to write down the numbers here they are:
Stock system with X pipe...................333/317
Same system with cutouts open..........336/320
Same system with cutouts half open....341/222
Stock exhaust system minus resonator with X pipe
http://i274.photobucket.com/albums/j...sresonator.jpg
Stock exhaust system with cutouts open
http://i274.photobucket.com/albums/j...utoutsopen.jpg
Stock exhaust system with cutouts half open.
http://i274.photobucket.com/albums/j...tshalfopen.jpg
Granted the rwhp increase was not that significant with the cutouts half open but it shows that the system needed a certain amount of back pressure to flow at it's maximum. A straight shot out the back was not the answer.
The point here is fooling around with the flow of the stock system for a increase in rwhp is pointless on a stock or slightly bolton modded motor. Making the diameter of the entire system larger will most likely hurt performance. I would bet that if that cam wasn't in there these results would show very little if any increase across all three graphs. If you want more hp/torque do mods to the engine itself not the exhaust. If you want more noise so everyone will think it has a lot of power have at it and enjoy your drone!
Mike
Last edited by Weav's Vet; Jun 30, 2010 at 06:19 AM.
Burning Brakes
Joined: May 2010
Posts: 1,167
Likes: 2
From: Pittsburgh Pa
I don't think I can agree with your findings. I opened all of your dyno plots, and the first immediate issue is that the scaling is all over the place. It's difficult to really make a comparison unless you rescale this or do a number chart. Just doing a quick replot scaling them all the same, I think the difference between open and mid opened is too small to justify as a difference. It's very much within the error of a typical dyno, and unless I saw 10+ runs of each I'd dout there's much difference if any.
I think the difference between a partially opened cutout and a fully opened cutout would only be realized when the system itself is pushed to the edge of the flow it can support efficiently. With the cutout opened at all you have a pressure differential, rarefaction waves within the system are influenced in timing due to the change in system length whether the cutout is opened all the way or not. The only true difference is that your imparting turbulence at the outlet leading to some small level of restriction, which like I said, would only likely come in to play if you were pushing the limits of the system.
The only solid conclusion I see is that the stock system didn't work nearly as well...that's it. Weave...if you have your original runfiles I can replot them in WinPep accurately.
Red-stock
Blue-open
Green-mid
I think the difference between a partially opened cutout and a fully opened cutout would only be realized when the system itself is pushed to the edge of the flow it can support efficiently. With the cutout opened at all you have a pressure differential, rarefaction waves within the system are influenced in timing due to the change in system length whether the cutout is opened all the way or not. The only true difference is that your imparting turbulence at the outlet leading to some small level of restriction, which like I said, would only likely come in to play if you were pushing the limits of the system.
The only solid conclusion I see is that the stock system didn't work nearly as well...that's it. Weave...if you have your original runfiles I can replot them in WinPep accurately.
Red-stock
Blue-open
Green-mid
Last edited by Z06X; Jun 30, 2010 at 01:55 PM.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
69427 is right; once you get past the cats, there is no meaningful gain from "velocity" derived from smaller piping. The exhaust system "tuning" is DONE. All the system is doing at that point, is getting the gasses and fumes out to the ***-end of your car.
Race Director
Joined: Nov 2003
Posts: 12,221
Likes: 446
From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
That, I'd probably agree with -- if the OP is going to reinstall cats up front. I don't remember cats mentioned up to now.
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Race Director
Joined: Nov 2003
Posts: 12,221
Likes: 446
From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
That, I'll agree with. Larger pipes will let out more sound.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
I HATE the phrase "back pressure" in attempting to describe better results! The only issue is velocity -- which can help overcome intake reversion (which also tries to hold gases in the cylinder). Pressure, schmesser.
That, I'll agree with. Larger pipes will let out more sound.
That, I'll agree with. Larger pipes will let out more sound.
Tech Contributor
Joined: Jun 2004
Posts: 20,931
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
From David Vizzard's "Auto Exhaust Science".....
Secondaries -- Diameters and Lengths
Well, so much for primary pipe dimensions and their effect on output. Let us now consider the collector/secondary pipe dimensions and configurations. The first point to make here is that the secondary diameter is as critical as the primary. A good starting point for the collector/secondary pipe size of a simple 4-into-1 header is to multiple the primary diameter by 1.75. Fortunately, the collector can be changed relatively easily and it is often best optimized at the track rather than the dyno.
As for the secondary length-that is from about the middle of the collector to the end of the secondary (or the first large change in cross-sectional area), we find a great deal more sensitivity than is seen with the primary. Ironically, few racers pay heed to collector length even though it is easy to adjust. In practice, collector length and diameter can have more effect on the power curve than the primary length. A basic rule on collectors is that shorter, larger diameters favor top end while longer, smaller diameters favor the low end. Except for the most highly developed engines, many collectors I see at the track are too large in diameter and either too short, or of excessive length.
------------------------------------------------------------------------
What! There's such a thing as TOO large? NO! It can't be. We have someone who told us that's not true!
In the context above, collector means secondary. On a standard 4-1 race header setup, the collector (size and length) is the secondary. I wonder how many people think Vizzard doesn't know what he's talking about. Better yet,,,think he's confusing primaries with secondaries.
Since secondaries can be up to 50" (or more), you also have to infer that exhaust pipe can/is part of the secondary "system". Especially in Vizzards' design where he uses a terminator box to simulate open atmosphere.
Me too.
Less restriction is never gonna hurt (which isn't the same thing as increasing velocity with the correct pipe size.)
Secondaries -- Diameters and Lengths
Well, so much for primary pipe dimensions and their effect on output. Let us now consider the collector/secondary pipe dimensions and configurations. The first point to make here is that the secondary diameter is as critical as the primary. A good starting point for the collector/secondary pipe size of a simple 4-into-1 header is to multiple the primary diameter by 1.75. Fortunately, the collector can be changed relatively easily and it is often best optimized at the track rather than the dyno.
As for the secondary length-that is from about the middle of the collector to the end of the secondary (or the first large change in cross-sectional area), we find a great deal more sensitivity than is seen with the primary. Ironically, few racers pay heed to collector length even though it is easy to adjust. In practice, collector length and diameter can have more effect on the power curve than the primary length. A basic rule on collectors is that shorter, larger diameters favor top end while longer, smaller diameters favor the low end. Except for the most highly developed engines, many collectors I see at the track are too large in diameter and either too short, or of excessive length.
------------------------------------------------------------------------
What! There's such a thing as TOO large? NO! It can't be. We have someone who told us that's not true!
In the context above, collector means secondary. On a standard 4-1 race header setup, the collector (size and length) is the secondary. I wonder how many people think Vizzard doesn't know what he's talking about. Better yet,,,think he's confusing primaries with secondaries.
Since secondaries can be up to 50" (or more), you also have to infer that exhaust pipe can/is part of the secondary "system". Especially in Vizzards' design where he uses a terminator box to simulate open atmosphere.
Me too.
Less restriction is never gonna hurt (which isn't the same thing as increasing velocity with the correct pipe size.)
What is this, the third or fourth time I've had to correct you. The Op asked about the exhaust system size, not the header/collector size, and yet you can't quite figure out the difference. Are you ever going to post anything of substance regarding the actual undercar exhaust?
FWIW, I think Vizzard is a sharp guy. I've read several of his papers, and so far I haven't seen anything of his I've disagreed with.
But, you seem to not understand a few things. If the OP has headers (I don't believe headers were mentioned), everything is already fixed. Note that Vizzard mentioned the collector length being from the center of the collector until the point of cross-sectional area change. Question for you: What is always hooked to the end of the collector on a street vehicle? A reducer! Meaning that smaller pipes are now attached to the headers. The collector length was not increased due to the exhaust piping. So, for the fifth time, can you try to grasp the concept of exhaust pipes (as the OP asked about), not header pipes???
I HATE the phrase "back pressure" in attempting to describe better results! The only issue is velocity -- which can help overcome intake reversion (which also tries to hold gases in the cylinder). Pressure, schmesser.
More nonsense.
Can I ask you, how do you determine the efficiency of a medium (in this case, exhaust tubing) that is charged with containing the flow of a gas? Answer: You measure the pressure drop required to get the gas to flow through the tube. You may not like the term backpressure, but we engineers are completely comfortable with descriptive technical terms like this.
But, hey, I am glad you do agree that "less restriction is never gonna hurt"!
Race Director
Joined: Nov 2003
Posts: 12,221
Likes: 446
From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
If the OP has headers (I don't believe headers were mentioned), everything is already fixed. Note that Vizzard mentioned the collector length being from the center of the collector until the point of cross-sectional area change. Question for you: What is always hooked to the end of the collector on a street vehicle? A reducer! Meaning that smaller pipes are now attached to the headers. The collector length was not increased due to the exhaust piping. So, for the fifth time, can you try to grasp the concept of exhaust pipes (as the OP asked about), not header pipes???
You assume headers. I assume no cats. Both of us could be wrong. I assume in Vizzards point than piping can be an extension of the system. Anything/everthing before a terminator box contributes to that "system". (Or, end of the exhaust pipe in our case). It sounds like you think everything about scavenging is only in the headers. I haven't interpreted my readings that way. And, it sounds like others haven't either. I guess we'll have to disagree.
Last edited by GREGGPENN; Jun 30, 2010 at 10:25 PM.
Safety Car
Joined: Feb 1999
Posts: 4,724
Likes: 16
From: Little Elm TX
I did a before and after dragstrip test years ago on exhaust pipe diameter.....
My first set-up was Hooker 1 3/4" headers through the stock front Y pipe, through a straight through tube that replaced the cat and out the stock Y pipe and through some Muffler eliminator resonators.
I then swapped out everything behind the headers for true dual 3" pipe with an H-pipe..... drove from the muffler shop to the dragstrip..... no et gain or mph change at all..... I was disappointed.
I will say, there was probably a small HP gain, simply because my et and mph wasn't hurt, yet the true dual 3" pipe was seriously heavier than the original pipe.... probably along the lines of 40-50 lbs or so..... thus, maybe 5-8 HP gain..... but just guessing.
This was a 6000 rpm 383 Superram motor that made in the neighborhood of 400-450 flywheel HP to begin with.....
I'll only add, in my opinion, the larger diameter pipe is more important with rpm motors that spin 7000+ rpm that have moderate to big cubes.....I'll also add, I regretted the change to 3" dual pipe for two reasons 1) no et gain 2) The exhaust was significantly louder with additional resonance as well..... not my taste.... I prefer as quiet as possible, without sacrificing power.
My first set-up was Hooker 1 3/4" headers through the stock front Y pipe, through a straight through tube that replaced the cat and out the stock Y pipe and through some Muffler eliminator resonators.
I then swapped out everything behind the headers for true dual 3" pipe with an H-pipe..... drove from the muffler shop to the dragstrip..... no et gain or mph change at all..... I was disappointed.
I will say, there was probably a small HP gain, simply because my et and mph wasn't hurt, yet the true dual 3" pipe was seriously heavier than the original pipe.... probably along the lines of 40-50 lbs or so..... thus, maybe 5-8 HP gain..... but just guessing.
This was a 6000 rpm 383 Superram motor that made in the neighborhood of 400-450 flywheel HP to begin with.....
I'll only add, in my opinion, the larger diameter pipe is more important with rpm motors that spin 7000+ rpm that have moderate to big cubes.....I'll also add, I regretted the change to 3" dual pipe for two reasons 1) no et gain 2) The exhaust was significantly louder with additional resonance as well..... not my taste.... I prefer as quiet as possible, without sacrificing power.
Race Director
Joined: Nov 2003
Posts: 12,221
Likes: 446
From: Overland Park Kansas
2020 Corvette of the Year Finalist (appearance mods)
C4 of Year Winner (appearance mods) 2019
I did a before and after dragstrip test years ago on exhaust pipe diameter.....
My first set-up was Hooker 1 3/4" headers through the stock front Y pipe, through a straight through tube that replaced the cat and out the stock Y pipe and through some Muffler eliminator resonators.
I then swapped out everything behind the headers for true dual 3" pipe with an H-pipe..... drove from the muffler shop to the dragstrip..... no et gain or mph change at all..... I was disappointed.
I will say, there was probably a small HP gain, simply because my et and mph wasn't hurt, yet the true dual 3" pipe was seriously heavier than the original pipe.... probably along the lines of 40-50 lbs or so..... thus, maybe 5-8 HP gain..... but just guessing.
This was a 6000 rpm 383 Superram motor that made in the neighborhood of 400-450 flywheel HP to begin with.....
I'll only add, in my opinion, the larger diameter pipe is more important with rpm motors that spin 7000+ rpm that have moderate to big cubes.....I'll also add, I regretted the change to 3" dual pipe for two reasons 1) no et gain 2) The exhaust was significantly louder with additional resonance as well..... not my taste.... I prefer as quiet as possible, without sacrificing power.
My first set-up was Hooker 1 3/4" headers through the stock front Y pipe, through a straight through tube that replaced the cat and out the stock Y pipe and through some Muffler eliminator resonators.
I then swapped out everything behind the headers for true dual 3" pipe with an H-pipe..... drove from the muffler shop to the dragstrip..... no et gain or mph change at all..... I was disappointed.
I will say, there was probably a small HP gain, simply because my et and mph wasn't hurt, yet the true dual 3" pipe was seriously heavier than the original pipe.... probably along the lines of 40-50 lbs or so..... thus, maybe 5-8 HP gain..... but just guessing.
This was a 6000 rpm 383 Superram motor that made in the neighborhood of 400-450 flywheel HP to begin with.....
I'll only add, in my opinion, the larger diameter pipe is more important with rpm motors that spin 7000+ rpm that have moderate to big cubes.....I'll also add, I regretted the change to 3" dual pipe for two reasons 1) no et gain 2) The exhaust was significantly louder with additional resonance as well..... not my taste.... I prefer as quiet as possible, without sacrificing power.
I have a 2.5" front "Y" waiting to go back in my new 383. I was considering dual 2.5" all the way back or even 3". After all the reading and talking to forum members (plus my exhaust guy), I'm starting to believe that I should put the "Y" pipe back in. At least try it anyway.
