exhaust vs backpressure

**Madcity T/A** · 08-02-2007, 07:57 AM

OK this started in the Show your Bird thread and thought it might be better out here. So what do you think on a stock engine is open headers bad for your car because of the back pressure.

**SteelmanLS1** · 08-02-2007, 08:08 AM

I was told here by a local "LS1" only shop that your STOCK motor Does need back pressure to perform to its best.. I have a stock motor and I told him that I was going to run 3" pipe straight off my long tubes and dump it before the rear axle... and he said since your a stock motor your really going to be losing low end torque and your power band is really only going to be from 4,500-5,800... Which being a 6 spd he said might not be to bad if you can row the gears and keep it in a high RPM.. But he told me whatever exhaust I get that I should defintally make it go all the way back.. He said now once I have the heads ported and put a cam in it, if I wanted to dump it, Go Ahead! He said once you have a cam and headword it helps instead of hurts... Just what I was told, Hope this helps.. I trust this guy he is the only guy I let do work on my LS1.. He had 2 camaro's and 3 trans ams in the shop and atleast another 5 ls1 cars setting outside

**nhraformula** · 08-02-2007, 03:37 PM

backpressure = myth

**Rikki_SeVeN** · 08-02-2007, 04:23 PM

I'm just here to see where this is going...

**Mr. Luos** · 08-02-2007, 04:27 PM

Open against capped...
Red is open, blue is capped.
Large cammed 402ci LSx.

I can feel a huge diffence in the lower (under 2,500) RPM's.

**mrr23** · 08-02-2007, 04:31 PM

this debate is older than dirt.

a motor doesn't need backpressure. backpressure is where you have a restriction. what a motor needs is correct pipe sizing for correct velocity of the exhaust gases throughout the rpm range for the intended use of the motor.

just like picking a camshaft for your motor, you need to pick the correct pipe size.

steelman LS1 - your local LS1 shop is correct. not because of a loss of backpressure, though. it's for the loss of low rpm velocity of the exhaust gases. you need to keep the speed of the exhaust going. having dual 3" pipes will cause a loss of low rpm torque.

i like to use the water hose scenario.

take a fire hose and turn the water on the hydrant very low. the water comes out very slowly and not far. now turn the water on full blast. you get alot of water and it shoot far.

now, put a 3/8" hose on the fire hydrant. do the same thing. at the low volume, the water comes out a little faster and shoots a little further than the 3/4" hose. when you turn it on full blast, the water comes out quickly and shoots far, but you'll notice the hose starts bulging. it can't handle the volume of water coming out.

**whiteLS1TA** · 08-03-2007, 04:33 PM

ok tim trying to understand what you guys are saying. dumping means??? does that mean the location of the muffler? because im running the loud mouth 2 i dunno recall if its 2.5 or 3 inch diameter, with a 2.5 slp y pipe. i didnt really feel a difference. what did i do wrong?

**mrr23** · 08-04-2007, 09:03 AM

dumping means where the exhaust will exit at.

**secondgearscratch** · 08-04-2007, 09:20 AM

you wouldnt want to put a set of headers with 1 7/8 primaries on a motor that isnt making above a certain power level because you lose low end torque like mrr23 said.

if you really broke it down, its actually an exact science where primary size, collector volume, intermediate piping, and even exhaust tip diameter affect flow velocity depending upon the size pipe you use which is why so many cars go 2.5 to 3 in to blah blah blah....

i took my stock catback off, so where the y merge is is where my stock exhaust dumps. immediately i noticed i had more on the top end above say 3k rpms.

you just need to think about it in a broader spectrum. almost all track driven cars that run low e.t.s use no mufflers or are equipped with some sort of cut out in order to dump the exhaust as close as possible after it exits the merge collectors. running open headers will be louder than shit, but will afford you more top end power with alittle less torque under 3k. but if you build a torquey motor as a daily driver and a weekend track car you wont have a problem with under curve torque and still have the power up top when going down the strip.

at the track, how long do you spend around 3k?

not long hopefully.

**Z ROADSTER** · 08-04-2007, 09:23 AM

Dynomax has a muffler under development that has vacuum operated baffles that open up the exhaust similar to the Vette Z06 system. Its basically the same theory as Variable Engine Timing or "VET" as Honda calls it. As the engine approches its torque range the internal baffles gradually open up the exhaust to maintain maximun torque as it transitions to peak horsepower.

**secondgearscratch** · 08-04-2007, 09:26 AM

Originally Posted by Z ROADSTER

Dynomax has a muffler under development that has vacuum operated baffles that open up the exhaust similar to the Vette Z06 system. Its basically the same theory as Variable Engine Timing or "VET" as Honda calls it. As the engine approches its torque range the internal baffles gradually open up the exhaust to maintain maximun torque as it transitions to peak horsepower.

variable valve timing and electronic lift contol you mean? VTEC?

gm is starting to use vvt i heard.

and that z06 exhaust is wicked!!!!