SYSTEM-OH-SS Posted October 31, 2007 Report Share Posted October 31, 2007 ok so i have been tryin to decide how im goin to go about doin my exhaust system... the plan is to do a 3" true dual setup w/ longtubes high flow cats and borla mufflers...... my problem is w/ the longtubes, ive been researchin and everything so im not askin question bout which headers etc... i think im gonna go w/ pacesetters.... my question is regarding back pressure... i realize that right now w/ out any head or cam mods and no forced induction 3" is not worth doin b/c i will have less back pressure than what is recomended... but later on i want to get a k.b. twin screw... so 3" seems feasible for down the road.... so heres the main question... if i do my 3" setup now and put a "restricter" plate in between two flanges w/ say a 2.25" whole would this creat enough back pressure to make it worth while for now, and then simply remove the plate for down the road... here is a sketch in case i didnt describe it right Quote Link to comment Share on other sites More sharing options...
RacerJJ Posted October 31, 2007 Report Share Posted October 31, 2007 ok so i have been tryin to decide how im goin to go about doin my exhaust system... the plan is to do a 3" true dual setup w/ longtubes high flow cats and borla mufflers...... my problem is w/ the longtubes, ive been researchin and everything so im not askin question bout which headers etc... i think im gonna go w/ pacesetters.... my question is regarding back pressure... i realize that right now w/ out any head or cam mods and no forced induction 3" is not worth doin b/c i will have less back pressure than what is recomended... but later on i want to get a k.b. twin screw... so 3" seems feasible for down the road.... so heres the main question... if i do my 3" setup now and put a "restricter" plate in between two flanges w/ say a 2.25" whole would this creat enough back pressure to make it worth while for now, and then simply remove the plate for down the road... here is a sketch in case i didnt describe it right I'm not sure about your plate idea. We are only talking about 1/2 inch here. You are not going to notice it that much. I were you, I would scrap the plate idea and add an H or X pipe. Either one of those will help with your back pressure and torque now and it certainly won't hurt anything once you get the charger on there. Good luck! Quote Link to comment Share on other sites More sharing options...
Professiona1 m0d Posted October 31, 2007 Report Share Posted October 31, 2007 Yes, back pressure helps an engine run richer than an exhaust system that is free flowing. This is accomplished because more back pressure means more work the motor has to perform in order for it to evacuate exhaust gases from a particular cylinder, which in turn affects the cylinder that is on its intake stroke. In short, less work on a cylinder's exhaust stroke, means more air is able to enter a different cylinder on that same revolution. However, with modern computer controlled systems and monitoring (and of course as long as the fuel system can support it), the computer will adjust for such changes and adjust for stochiometric operation. Or simple tuning can help the computer along before the computer automatically learns and adjusts. Obviously dramatic changes in back pressure the computer will most likely not be able to compensate for. A free flowing exhaust may also affect fuel mileage in a positive or negative way. With this said, please read one of my favorite articles that may help you better understand this than I was able to explain. http://www.uucmotorwerks.com/html_product/...etorquemyth.htm Quote Link to comment Share on other sites More sharing options...
kelleyperformance Posted October 31, 2007 Report Share Posted October 31, 2007 id say just do the exhuast and forget the plate. IMO a flat plate that close to the engine would not be a good thing.... the restriction would be signifigant due to the plate being flat, and might cause problems. id agree that dual 3" is too much exhaust for an engine not moving much more air than stock. id say just do 2 1/2" true dual. you would have to move a hell of alot of air to create any power-robing backpressure w/ longtubes and 2 1/2" professiona1-what your saying is correct in a sense.... out of PE mode (PE=power enrichment, active during WOT) the PCM will try and compensate for the airflow changes, but like you said can only do that to an extent. however, in PE the PCM has a fixed table and will not relearn according to modification, it will need a tune. Quote Link to comment Share on other sites More sharing options...
Professiona1 m0d Posted October 31, 2007 Report Share Posted October 31, 2007 A tune of course will be needed when in PE mode, otherwise it would have adverse affects on AFR, fuel mileage, and such. Quote Link to comment Share on other sites More sharing options...
kelleyperformance Posted October 31, 2007 Report Share Posted October 31, 2007 exactly Quote Link to comment Share on other sites More sharing options...
SYSTEM-OH-SS Posted October 31, 2007 Author Report Share Posted October 31, 2007 thanx for the help guys.... i have some things to think about now... racre j - i was thinkin bout an x-pipe but i just didnt think it would make a big difference in back pressure... good to know prof..1 - good article bookmarked and printed prof..1 and kelley perf..- so u think 2 1/2" true dual and a good tune, even w/ a future supercharger, should be sufficient... thanx again guys i love this site, learnin alot... Quote Link to comment Share on other sites More sharing options...
RacerJJ Posted October 31, 2007 Report Share Posted October 31, 2007 thanx for the help guys.... i have some things to think about now... racre j - i was thinkin bout an x-pipe but i just didnt think it would make a big difference in back pressure... good to know prof..1 - good article bookmarked and printed prof..1 and kelley perf..- so u think 2 1/2" true dual and a good tune, even w/ a future supercharger, should be sufficient... thanx again guys i love this site, learnin alot... Glad to help! I am actually thinking of adding a an X or H pipe to mine because I think I lost some low end with my new exhaust. Good luck. Quote Link to comment Share on other sites More sharing options...
Krambo Posted October 31, 2007 Report Share Posted October 31, 2007 A tune of course will be needed when in PE mode, otherwise it would have adverse affects on AFR, fuel mileage, and such. I dyno tuned my set-up with 2.5" dual no cats and then went to a full dual 3" setup after and my AFR leaned out roughly .2. I advise checking your actual AFR vs commanded in PE mode anytime you do a signifigant change. Otherwise your PCM will adjust your fueling non PE. BTW, I would do your 3" set-up, no plate and leave it. Quote Link to comment Share on other sites More sharing options...
Professiona1 m0d Posted October 31, 2007 Report Share Posted October 31, 2007 System, I would have to absolutely agree with Krambo and the others. Stay with the 3" headers and 3" full dual there after, get it done and out of mind. Also make sure you get some sort of cross pipe. Then get it tuned! Just as an FYI, I have my blown 408 go to 3" headers which both go to a Y collector (3" as well), then 3" pipe with a 3" cutout runs to the muffler which then has two outputs at 2.5". On the dyno I saw no gain when the cutout was open or when it was closed. You should be just fine. I am glad we all could help you! Quote Link to comment Share on other sites More sharing options...
kelleyperformance Posted October 31, 2007 Report Share Posted October 31, 2007 there really wont be any power gains worth the extra noise from the 3" and you wont loose that low end torque while your waiting for your boost Quote Link to comment Share on other sites More sharing options...
kelleyperformance Posted October 31, 2007 Report Share Posted October 31, 2007 here, for an even better comparison: i made roughly 503whp average of three runs (with the auto and yank2600, no runs on the MT yet) with 1 3/4" LTs (dynatech) no cats and a corsa touring cat-back. this was @ 8 lbs of boost. i then added true dual 2.5" exhaust with an X pipe and a straight-thru muffler and removed the corsa. eveyone was telling me that i had alot of backpressure with the corsa. unfortunately i listened, and now i spent $500 more and its louder. when i dynoed the true dual/x pipe setup, i made 507 whp (average of 3 runs). the a/f and timing was confirmed to be the same for all 6 runs BTW. then i dynoed the truck with open headers (same place as cutout), tuning it of course. it made about 20 wtq less peak, and lost even more tq in the bottom end. the peak power gain was.... nothing lol. all i did was loose torque. SO as long as your running some LTs and a straight thru muffler(s) you will be fine. Quote Link to comment Share on other sites More sharing options...
Professiona1 m0d Posted November 1, 2007 Report Share Posted November 1, 2007 Exactly as Kelley said! You WILL be fine! Quote Link to comment Share on other sites More sharing options...
SYSTEM-OH-SS Posted November 1, 2007 Author Report Share Posted November 1, 2007 aight more great info... i owe all u guys a round ... looks like i will have to start gettin everything together so i can start fabricatin... dual 3" here i come thanx again Quote Link to comment Share on other sites More sharing options...
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