Is 3" Overkill? Yeah, maybe.
Thread Starter
Staging Lane
Joined: Jan 2003
Posts: 50
Likes: 0
From: West Palm Beach, FL
Is 3" Overkill? Yeah, maybe. Quick backstory, then the tech stuff. My buddy, who owns a very fast rx7, and I were discussing my exhaust setup. I told him that I was planning on running dual 3" pipes and he called me crazy. So, after exchanging insults for a few minutes we got to doing some math. Here's what we came up with.
Assuming that an engine (regardless of piston or rotary) is just a big airpump, we calculated how much fuel his car needs to make his 425 RWHP. Backing up the figures, his injectors, at 80% duty cycle, are using 316 lbs/hr. The rotary engine is around an 80% volumetric efficiency, so it needs a little more fuel to make equal power. If he's making 425 RWHP at an 11.0 a/f ratio, and is only using 316 lbs/hr of fuel, why is his single 3" exhaust more than enough??
Figuring a stock injector 01 LS1 has 8 x 28.6 lbs/hr injectors, at the same 80% duty cycle, is using 183 lbs/hr of fuel. NOWHERE near as much as his car. Maxing out the stock injectors "should" land you somewhere near 400 RWHP on the LS1 so we should be close. Add in the better volumetric efficiency, and we should be around the same.
So, with all that, why wouldn't a single 3" exhaust be MORE than enough exhaust for the LS1 making ~425 RWHP? Isn't the dual 3" a bit of an overkill considering?
Anyone who can find fault with this, please explain.
Assuming that an engine (regardless of piston or rotary) is just a big airpump, we calculated how much fuel his car needs to make his 425 RWHP. Backing up the figures, his injectors, at 80% duty cycle, are using 316 lbs/hr. The rotary engine is around an 80% volumetric efficiency, so it needs a little more fuel to make equal power. If he's making 425 RWHP at an 11.0 a/f ratio, and is only using 316 lbs/hr of fuel, why is his single 3" exhaust more than enough??
Figuring a stock injector 01 LS1 has 8 x 28.6 lbs/hr injectors, at the same 80% duty cycle, is using 183 lbs/hr of fuel. NOWHERE near as much as his car. Maxing out the stock injectors "should" land you somewhere near 400 RWHP on the LS1 so we should be close. Add in the better volumetric efficiency, and we should be around the same.
So, with all that, why wouldn't a single 3" exhaust be MORE than enough exhaust for the LS1 making ~425 RWHP? Isn't the dual 3" a bit of an overkill considering?
Anyone who can find fault with this, please explain.
Re: Is 3" Overkill? Yeah, maybe. you basically figured out how much fuel it takes to run an ls1 at 425 horsepower. you didn't take into account the fact that when burned it expands and therefore takes up more space than it would leaving the engine. a single 3" won't hurt you too much at 425 but dual 3" is better at that much power. also keep in mind that the rx-7 has less displacement than a ls1 which means it won't need nearly as much exhuast flow at 400-plus horsepower that a ls1 would.
Thread Starter
Staging Lane
Joined: Jan 2003
Posts: 50
Likes: 0
From: West Palm Beach, FL
Re: Is 3" Overkill? Yeah, maybe. Regardless of the displacement of the motor, the amount of air/fuel it requires to make a certain amount of power is fixed. Whether it be a 6.0 liter motor, or a 1.5 liter motor, they both need the same amount of fuel and air to make the same power (taking into account v/e and a/f ratio).
I'm looking strictly at airflow here. If his single 3" exhaust is MORE than, not just enough, but more than enough to produce that much power, then dual 3" pipes is shear overkill.
<small>[ February 17, 2003, 07:59 PM: Message edited by: Lost Time ]</small>
I'm looking strictly at airflow here. If his single 3" exhaust is MORE than, not just enough, but more than enough to produce that much power, then dual 3" pipes is shear overkill.
<small>[ February 17, 2003, 07:59 PM: Message edited by: Lost Time ]</small>
Re: Is 3" Overkill? Yeah, maybe. </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Lost Time:
<strong> Regardless of the displacement of the motor, the amount of air/fuel it requires to make a certain amount of power is fixed. Whether it be a 6.0 liter motor, or a 1.5 liter motor, they both need the same amount of fuel and air to make the same power (taking into account v/e and a/f ratio). </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">hold on you don't understand i'm not talking about the amount of fuel and air being different i'm talkin about the amount of exhaust flow needed being different on different displacement engines.
</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Lost Time:
<strong>I'm looking strictly at airflow here. If his single 3" exhaust is MORE than, not just enough, but more than enough to produce that much power, then dual 3" pipes is shear overkill. </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">yeah a single 3" will flow enough to make 500 horsepower. but you still have not proved to me why dual 3" is overkill. have you dynoed a single 3" against an exhaust that flows alot more (dual 3" or single 4") to see the difference. i've seen dyno sheets showing guys around 425 horsepower with single 3" exhaust make some power by switching to 3" duals explain that if you can.
<small>[ February 19, 2003, 05:49 PM: Message edited by: Cody Brooks ]</small>
<strong> Regardless of the displacement of the motor, the amount of air/fuel it requires to make a certain amount of power is fixed. Whether it be a 6.0 liter motor, or a 1.5 liter motor, they both need the same amount of fuel and air to make the same power (taking into account v/e and a/f ratio). </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">hold on you don't understand i'm not talking about the amount of fuel and air being different i'm talkin about the amount of exhaust flow needed being different on different displacement engines.
</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Lost Time:
<strong>I'm looking strictly at airflow here. If his single 3" exhaust is MORE than, not just enough, but more than enough to produce that much power, then dual 3" pipes is shear overkill. </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">yeah a single 3" will flow enough to make 500 horsepower. but you still have not proved to me why dual 3" is overkill. have you dynoed a single 3" against an exhaust that flows alot more (dual 3" or single 4") to see the difference. i've seen dyno sheets showing guys around 425 horsepower with single 3" exhaust make some power by switching to 3" duals explain that if you can.
<small>[ February 19, 2003, 05:49 PM: Message edited by: Cody Brooks ]</small>
