Back pressure and torque
08-15-2007 | 11:25 PM
#41
Originally Posted by 2002_Z28_Six_Speed
From my frame of mind, building at mass manufacture, I was thinking why overdo it. If the car has a little backpressure at peak power and no where else then I didn't waste money. Yes, on my personal car I put a free flowing exhaust. I was just thinking about money.
08-16-2007 | 07:45 AM
#42
Originally Posted by mrr23
again, backpressure is not needed. not in normal driving or hotrodding it.
a properly designed exhaust system will have very little backpressure throughout the entire rpm range, without being too large to make the low speed exhaust gasses move too slow.
when someone puts too large of an exhaust on their car, it slows the low rpm exhaust pulses. this is where the magical 'needs more backpressure' myth comes into play.
another thing that slows exhaust down is cold pipes. with the pipes being cold, the heat starts exchanging into the pipe thus slowing the pulse down. with the pipe being at least the same temp as the exhaust, the heat has nowhere to go. this keeps the velocity up, until it gets to a lower temp spot and/or exits the system. this is where ceramic coating the exhaust can make a little more power.
a properly designed exhaust system will have very little backpressure throughout the entire rpm range, without being too large to make the low speed exhaust gasses move too slow.
when someone puts too large of an exhaust on their car, it slows the low rpm exhaust pulses. this is where the magical 'needs more backpressure' myth comes into play.
another thing that slows exhaust down is cold pipes. with the pipes being cold, the heat starts exchanging into the pipe thus slowing the pulse down. with the pipe being at least the same temp as the exhaust, the heat has nowhere to go. this keeps the velocity up, until it gets to a lower temp spot and/or exits the system. this is where ceramic coating the exhaust can make a little more power.
08-16-2007 | 07:48 AM
#43
Originally Posted by mrr23
no, it creates an acceleration of the gasses with the neck down. with it suddenly opening back up, it creates a scavenging effect. what scavenging does is speed up the exhaust pulse and create a vacuum behind it. this vacuum helps to pull the next exhaust pulse coming out and speed it up as well. and so on and so on.
if it stayed necked down, this would create a restriction because of the distance needing to travel.
if it stayed necked down, this would create a restriction because of the distance needing to travel.
08-16-2007 | 08:29 PM
#45
Originally Posted by edcmat-l1
Its basically based on the venturi effect. Same as a carburetor. As the gases pass thru the venturi, they speed up, or increase velocity, which in a carb pulls fuel (from the vacuum, or pressure differential) and in the exhaust helps scavenge.
08-18-2007 | 10:03 PM
#46
Joined: Nov 2003
Posts: 6,170
Likes: 219
From: Wichita KS / Rancho San Diego
After installing LT's, most noticeable thing about the exhaust note is its "pulsating" tone. Left untreated, this becomes the dreaded "rasp." It seems to me that the scavenging effect discussed above is what's responsible for the pulsating sound.
Does a true merge in the Y (like the Flowmaster) which brings each of the pipes of the Y in side by side, and gradually blends them, make for better velocity than a pipe which slams the two down to one?
Does using a 3/2.5" reducer cone off the collectors into the Y pipes help or hurt velocity and flow?
Does is it matter if the pipes are 2.5" or 3" if they neck down into a single 3"?
It would seem to me that the initial reduction from 3" to 2.5" into a FM merge and 3" I would be better for velocity than two 3" pipes slammed together into one 3". This is because the reduction in flow capacity would be gradual rather than sudden, maintaining velocity rather than creating a bottleneck.
Does a true merge in the Y (like the Flowmaster) which brings each of the pipes of the Y in side by side, and gradually blends them, make for better velocity than a pipe which slams the two down to one?
Does using a 3/2.5" reducer cone off the collectors into the Y pipes help or hurt velocity and flow?
Does is it matter if the pipes are 2.5" or 3" if they neck down into a single 3"?
It would seem to me that the initial reduction from 3" to 2.5" into a FM merge and 3" I would be better for velocity than two 3" pipes slammed together into one 3". This is because the reduction in flow capacity would be gradual rather than sudden, maintaining velocity rather than creating a bottleneck.
Last edited by RevGTO; 08-19-2007 at 07:06 PM.
08-19-2007 | 08:45 AM
#47
I think the rasp is more due to what kind of muffler. Some drone. Some are raspy.
I dont see any point in necking the exhaust down before stepping it back up. The benefit of the venturi effect is most beneficial right after the 4 primaries merge. Necking it down, further down the stream, I dont think would have much beneficial effect.
I dont see any point in necking the exhaust down before stepping it back up. The benefit of the venturi effect is most beneficial right after the 4 primaries merge. Necking it down, further down the stream, I dont think would have much beneficial effect.
08-19-2007 | 12:11 PM
#48
Originally Posted by edcmat-l1
I think the rasp is more due to what kind of muffler. Some drone. Some are raspy.
I dont see any point in necking the exhaust down before stepping it back up. The benefit of the venturi effect is most beneficial right after the 4 primaries merge. Necking it down, further down the stream, I dont think would have much beneficial effect.
I dont see any point in necking the exhaust down before stepping it back up. The benefit of the venturi effect is most beneficial right after the 4 primaries merge. Necking it down, further down the stream, I dont think would have much beneficial effect.
Last edited by DrkPhx; 08-19-2007 at 12:20 PM.
08-19-2007 | 12:18 PM
#49
Originally Posted by DrkPhx
I do think the muffler has a noticeable effect on raspiness, but the Y-pipe merge point does has some effect as well. I use the Hooker catback with the Aerochamber muffler which is not considered a raspy setup. Anyway, years ago when I first installed some MAC headers and Y (no cats), I noticed the raspiness right away. So I bought a Flowmaster merge collector and had it welded in and the raspiness went away. Now I run the QTP's which comes with the merge collector.
08-19-2007 | 07:02 PM
#51
Joined: Nov 2003
Posts: 6,170
Likes: 219
From: Wichita KS / Rancho San Diego
I've been involved in exhaust experiments (some might call it exhaust insanity) ever since getting my LT's installed: 3" Y/2.5" Y, slammed/FM merge, no cats/catted, GMMG/Hooker ...
The pulsations caused by the scavenging effect of the LT's are there on my car even with cats, FM merge, and Hooker. Each of those plays some role in muting them so there is in effect, no rasp, but you still hear the pulsations. Switched back to my GMMG on Friday and it added a raspy edge to the pulses.
As for the Y, if you are going into a single 3" pipe, does it matter if the main pipes are 2.5" or 3"?
The pulsations caused by the scavenging effect of the LT's are there on my car even with cats, FM merge, and Hooker. Each of those plays some role in muting them so there is in effect, no rasp, but you still hear the pulsations. Switched back to my GMMG on Friday and it added a raspy edge to the pulses.
As for the Y, if you are going into a single 3" pipe, does it matter if the main pipes are 2.5" or 3"?
Last edited by RevGTO; 08-19-2007 at 09:21 PM.
08-19-2007 | 07:16 PM
#52
As an interesting note to the above discussion about Y piped exhausts, I just switched from an ORY, LM1 setup, to the TSP true duals with the X pipe and 3" tubing. Not only did I get rid of the white trash rasp, but the car also feels much stronger in the bottom RPM range. A noticable low to mid range torque gain, as well as a slightly better top end.
08-19-2007 | 08:55 PM
#53
I have kind've an odd setup for exhaust and was waundering how much power I'm leaving on the table.I have LS7 heads that are slightly ported on the exhaust side.The thing is, I had bought the LS1 version Kooks 1 7/8 LT's long ago for my LS1 and now I'm waundering if I'm really hitting a wall coming out of my ported D-shaped heads into cathedral port headers.I called Kook's on changing this and it wouldn't be a problem......question is, would it be worth it?Any gains? 
Traver
Traver
08-19-2007 | 09:05 PM
#54
Originally Posted by ramairws6
I have kind've an odd setup for exhaust and was waundering how much power I'm leaving on the table.I have LS7 heads that are slightly ported on the exhaust side.The thing is, I had bought the LS1 version Kooks 1 7/8 LT's long ago for my LS1 and now I'm waundering if I'm really hitting a wall coming out of my ported D-shaped heads into cathedral port headers.I called Kook's on changing this and it wouldn't be a problem......question is, would it be worth it?Any gains? Traver
Traver
08-19-2007 | 09:09 PM
#55
Originally Posted by LivingCanvas
As an interesting note to the above discussion about Y piped exhausts, I just switched from an ORY, LM1 setup, to the TSP true duals with the X pipe and 3" tubing. Not only did I get rid of the white trash rasp, but the car also feels much stronger in the bottom RPM range. A noticable low to mid range torque gain, as well as a slightly better top end.
08-20-2007 | 05:13 AM
#56
On The Tree
Joined: Dec 2006
Posts: 121
Likes: 1
From: Aloha, OR
08-24-2007 | 01:23 AM
#57
Wow!!! I'm glad that I read this, cause I've been thinking about shoving something into my exhaust to make more backpressure and therefore make more power. Now what a minute, that don't make any sense does it?
I hate it when people try to talk this backpressure BS, it is probably the single most annoying topic about engines. Anybody that says backpressure makes more power doesn't know anything about cars and they are just repeating what they have been told in a pathetic attempt to look smart.
I hate it when people try to talk this backpressure BS, it is probably the single most annoying topic about engines. Anybody that says backpressure makes more power doesn't know anything about cars and they are just repeating what they have been told in a pathetic attempt to look smart.
