Sandman
03-21-2007, 09:50 PM
I found this post on NLOC and found it to be very informative. Thought I would post it on here for reference.
http://www.nloc.net/vbforum/showthread.php?t=34370
Locally, Andre (1BADTK, myself, and other HALO club members have been investigating and documenting the effects of exhaust back pressure on power. Changes have ranged from partial removal of cats to replacement of cats and stock exhaust manifolds. One of the things we discovered is that exhaust back pressure @ WOT is controlled by the cats: Stock= 10 psi, Bassani= 5 psi, and Magnaflow= 3 psi, regardless of the headers and mufflers installed.
Some speculated that reducing back pressure might reduce low r[m torque, others felt there was not a significant improvement with exhaust changes.
Steve's MyRedToy saw ~0.3 et (12.51 to 12.21, Hoosiers and better air later gave another 0.1) improvement when going from stock to Bassani cats. Steve already had Bassani headers and catback, but stock cats.
Reducing exhaust back pressure provided NO measured torque loss for me. Dyno'd today after long tube and cat installation. This is a BEFORE and AFTER Comparison with the BEFORE being after removing the second (rear) set of stock cats and lowering back pressure from 10 psi (stock) to 7 psi. The modified cats had been worth a 0.1 et improvement. The AFTER is with JDM/Kooks header and cat installation and a back pressure of 3 psi. I used a Dynomax 2 in-2 out muffler for both tests.
RESULT: 12-15 hp increase in peak hp at ~ 5000 rpm. While that does not sound like much, at 3800 (lowest measured rpm) torque was up 50-60 lb-ft and hp 40-50! This level of gain slowly tapered to the 12 -15 at 5000 rpm and stayed at that level or greater until 5700+. A lot of new area under the curve and very noticible at seat-of-pants. The same level of gain was seen with two different chip programs and and one program was run twice in both BEFORE and AFTER configurations to verify consistency of the numbers. Gains were likely higher at still lower rpm and more so had I started with a full set of stock cats.
We just completed a driveway install of Hedman short tubes and Bassani cats in front of Silver Bullet cat-back on Andre's truck and saw a big increase in sound and will do his AFTER dyno soon. Analysis of those tests will allow us to compare characteristics (magnitude and distribution of gains) of long vs short tubes. I am sure he will add his results to this string at that time.
Did I mention the incredible sound? We also discovered that the cats do more to determine sound level at idle and WOT that mufflers of headers, while the headers add a distinct cadence and pitch/growl.
End result: Very glad I made did the install. I had heard complaints of too little power gain before and that would be the case if you looked only at peak numbers, but the improvements at less than 5000 rpm is huge! Guess where our engines are at most of the time? New peak numbers: 430 SAE (corrected) hp and 527 lb-ft. More to come as soon as new cams arrive.
Will post graph overlays and track improvements later; racing this Sunday and planning/hoping for first eleven second run. Last run before headers was a 12.09 @ 111 mph. @ 4700# & 60F
Thanks again to Keith and Chris for help with my driveway install.
http://www.nloc.net/vbforum/showthread.php?t=34370
Locally, Andre (1BADTK, myself, and other HALO club members have been investigating and documenting the effects of exhaust back pressure on power. Changes have ranged from partial removal of cats to replacement of cats and stock exhaust manifolds. One of the things we discovered is that exhaust back pressure @ WOT is controlled by the cats: Stock= 10 psi, Bassani= 5 psi, and Magnaflow= 3 psi, regardless of the headers and mufflers installed.
Some speculated that reducing back pressure might reduce low r[m torque, others felt there was not a significant improvement with exhaust changes.
Steve's MyRedToy saw ~0.3 et (12.51 to 12.21, Hoosiers and better air later gave another 0.1) improvement when going from stock to Bassani cats. Steve already had Bassani headers and catback, but stock cats.
Reducing exhaust back pressure provided NO measured torque loss for me. Dyno'd today after long tube and cat installation. This is a BEFORE and AFTER Comparison with the BEFORE being after removing the second (rear) set of stock cats and lowering back pressure from 10 psi (stock) to 7 psi. The modified cats had been worth a 0.1 et improvement. The AFTER is with JDM/Kooks header and cat installation and a back pressure of 3 psi. I used a Dynomax 2 in-2 out muffler for both tests.
RESULT: 12-15 hp increase in peak hp at ~ 5000 rpm. While that does not sound like much, at 3800 (lowest measured rpm) torque was up 50-60 lb-ft and hp 40-50! This level of gain slowly tapered to the 12 -15 at 5000 rpm and stayed at that level or greater until 5700+. A lot of new area under the curve and very noticible at seat-of-pants. The same level of gain was seen with two different chip programs and and one program was run twice in both BEFORE and AFTER configurations to verify consistency of the numbers. Gains were likely higher at still lower rpm and more so had I started with a full set of stock cats.
We just completed a driveway install of Hedman short tubes and Bassani cats in front of Silver Bullet cat-back on Andre's truck and saw a big increase in sound and will do his AFTER dyno soon. Analysis of those tests will allow us to compare characteristics (magnitude and distribution of gains) of long vs short tubes. I am sure he will add his results to this string at that time.
Did I mention the incredible sound? We also discovered that the cats do more to determine sound level at idle and WOT that mufflers of headers, while the headers add a distinct cadence and pitch/growl.
End result: Very glad I made did the install. I had heard complaints of too little power gain before and that would be the case if you looked only at peak numbers, but the improvements at less than 5000 rpm is huge! Guess where our engines are at most of the time? New peak numbers: 430 SAE (corrected) hp and 527 lb-ft. More to come as soon as new cams arrive.
Will post graph overlays and track improvements later; racing this Sunday and planning/hoping for first eleven second run. Last run before headers was a 12.09 @ 111 mph. @ 4700# & 60F
Thanks again to Keith and Chris for help with my driveway install.