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 destinct 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.

 

Awesome info. Keep us updated!

 

Great Post ...

Keep the info coming

Doug

 

Thanks for the info.

Any plans on testing with no cats? Maybe with a cut-out or with an offroad pipe?

 

No plans for no cats or cut-outs mainly due to inspection and code issues. While they are surmountable, current 3 psi back pressure at WOT seems likely to have reached the point of deminishing returns. Sound is quieter, but better than open headers, did take a short drive before hooking up muffler. Dynomax claims no significant losses with my 2 in/out muffler at hp levels up to 750 hp, 800 hp for twin bullets. I had tried the Magnaflow 2 in/out earlier, but removed it because I personally did not like the sound. However, I did not see a performance problem with the single Magnaflow although I suspected one before the change.

 

Thanks Bob. That had to be a lot of work and time. I especially appreciate the detailed approach/post you provided.

Dan

 

Good info. I have two dynomax race magnums and like the sound also with the high flow cats.

How much boost are you running. What pullies?


Thanks,

Jeff

 

Great info, thanks!

 

Thanks this is a great post..........

PEaCE................

 

Good Info,

My question is: Has anyone Blown or Melted a Plug with Long Tubes and without NOS?

The only Blown or Melted Plugs I have heard of are either with Nitrous or with the Stock/Shorty Headers. correct me if I am wrong.

Jim

 

WOW, awesome post. Long tubes and cats are in my future...

 

The work was a club effort with many participating to help us work the idea. That is the great thing about being a member of such an active club (HALO).

My opinion is that blown plugs are from the plugs working loose; I lost one when my truck was stock. Melted electrodes are from running lean and shattered plugs or generally from severe detonation. Melted or shattered plugs are not a matter of what headers or set-up up have, but of too aggressive tune (timing and/or A/FR or too hot a plug if rest of tune is correct. A cold plug will not fix a bad tune, but a hot plug that is too hot can make for a bad tune.

 

Very cool info.

Can't wait to see the dyno sheets posted so we can all see the new curves and results

 

The before and after sheet:

 

WOW
This is a fine example of what this board used to be, before all the petty mud-slingin he said/she said began. I can only hope, as ever other member hopes, the new year is bringing back this type of info.
Thanks to bobcmt, 1BADTK, Andre and the fine group known as HALO