I just installed my race style QTP longtubes and had my exhaust done at a local shop... I had 3'' off the headers without cats into an H and then into some strait through Kooks Racing Mufflers and then dumped before the axle. I plan to have it extended out to the bumper soon, probably under the axle. But what I was wondering is does this exhaust rob my engine the way it is now because of the loss of back-pressure? I plan on Heads/Cam next but until then it this exhaust too big? I also upgraded the Intake from a stock ls1 intake to a ported ls6 intake and polished throttle body(I believe it needs a tune). The car is running a little rough but i think the computer needs some figuring for the new mods...

Sorry for the long story, i'm just trying to be precise...

 

You're fine. It's not hurting anything now, and it's perfect for your future plans. Drive it and enjoy it!!

 

You did kill your low end torque.
Take a look at this for a better idea of correct sizing...http://www.mk5cortinaestate.co.uk/calculator5.php
There is a substantial difference between mandrel and compression bent diameters.

 

Yeah, but I plan on going for big power so it should be ok... and by the way, that calculator is pretty cool. It sounds a little funny now, I'm hoping a nice cam and some better heads will fill out the exhaust note nicely.

 

I run true DUAL 3" all the way back.. My car is nearly a stock ls1 and it put out numbers on the dyno close to what stock LS6s put down on the same dyno..

Exhaust too big is bull in almost all cases. It may not be optimal but this isnt an F1 car...other factors are way more important.

 

its called exhaust scavenging. the bigger diameter pipe you use takes more exhaust volume to equal the same velocity as a smaller diameter pipe. exhaust velocity=low end torque. but really all that will happen is you will see a loss in low end torque and probably an increase in high end hp. if you go to a cam later on the greater lift and duration will help increase that velocity. you can google exhaust scavenging and find lots of write ups on it.

 

Yes!! I was counting on that fact to help me later on down the road. Damn I love Physics...

 

i noticed no loss in low end power going from a single 3" catback to a 3" TDs setup with no mufflers.

so if reduced backpressure kills low end torque...its too small to ever notice. you REALLY think you can feel 5ftlbs?

one thing i think that makes it FEEL like you lost power down low is that the louder exhaust tricks your brain.
i noticed with straight pipes i'd subconsciously drive around at lower rpms than when i put mufflers on. moving through traffic at 1200rpm sucks..but straight pipes at 1600rpm gets old quick.
so to deal with the noise i inadvertantly dropped my rpms...which makes the car sluggish.

without thinking about it you'd say..."i lost my low end"...but in reality its still there!

 

A stated above, it's not backpressure, that's bull - it's velocity and you want the right size exhaust to lower restriction but maintain velocity.

 

In most cases you're not losing low end power. The low end power stays basically the same with the gains coming higher in the rpm range, therefore giving people the impression that low end power has been lost.

 

I have ARH LTs with catless dual 3" Y'd into single 4" and into a hooker maxflow (strait through) muffler and dumped right after and I feel no loss in power anywhere. That's in a 5200lb AWD SSS too...take those things with a grain of salt.

 

I hope for a dyno soon to see what all has been gained. So in summary I probably wont GAIN much with this exhuast set up until I upgrade her cam and get those heads swapped. Sound about right?

 

probably

might gain a little in the very top end

 

take a look at my dyno sheet...let me know if u think i'm lacking in the low end tq department....

 

before doing any major modifications to you ride, you should do your homework first. yes even after asking questions or asking for opinion's on this site. a good shop or even reading books will help. backpressure or exhaust scavenging, the wording has changed but people reference them as the same. it is how some people reference a engine as a motor, especially sins we all know a motor is driven by electric.

 

I've always interpreted back pressure as the resistance to exhaust flow. The scavenging effect can be visualized by imagining the high-pressure pulse with a trailing low-pressure area behind. The faster the high-pressure pulse moves the stronger the draw on the low-pressure gasses and the gasses behind that.

 

yes back pressure would be like what a cat or a chambered muffler creates(compared to straight pipe). aka: restriction. back pressure will ultimately rob hp/tq because that exhaust velocity/scavenging is being interrupted. therefore back pressure is not scavenging. and reducing it will always net a gain.

 

First of all, I was only asking for other people's opinions because I was arguing with a guy at work. I wouldn't have done an exhaust set up like this if i didn't think it would benefit me later on down the road. Rather than get the pipe set up applicable for the stock ls1, I went ahead and oversized the piping so that the increased exhaust flow would have room to escape from my engine once the cam and heads are upgraded.