Random Tech Flow Results - Q?
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Launching!
Joined: May 2004
Posts: 203
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From: Stranger in a Strange Land
Random Tech Flow Results - Q? I posted this one a while ago on the dyno section but no usefull responses.
I dynoed my car cutout opened vs closed.
Did three pulls only the last two is on the sheet.
2nd pull blue cutout opened, and the 3rd one red cutout closed.
I really got suprised, hardly any gains over the closed cutout only like 0.7 rwhp gain and 0.02 rwtq but up to 3500 rpm there's about 10 rwtq and 6-7 rwhp increase with the cutout closed.
ASA (226/236 0.525"/0.525" 110 LSA) cam, Random Technology 3" catback with the cutout on the Y pipe right before the rear axle, the rest of the mods in my sig.
The tuner made the tune for opened cutout. (I like making noise, ASA sounds so much meaner with and opened cutout.)
SO IF SOMEBODY COULD EXPLAIN THIS...I EXPECTED MORE POWER WITH AN OPENED CUTOUT OVER THE CLOSED EXHAUST.
Or I should just explain it for myself: Random Tech flows very-very well.
Well I have a sleeper at least now.
I dynoed my car cutout opened vs closed.
Did three pulls only the last two is on the sheet.
2nd pull blue cutout opened, and the 3rd one red cutout closed.
I really got suprised, hardly any gains over the closed cutout only like 0.7 rwhp gain and 0.02 rwtq but up to 3500 rpm there's about 10 rwtq and 6-7 rwhp increase with the cutout closed.
ASA (226/236 0.525"/0.525" 110 LSA) cam, Random Technology 3" catback with the cutout on the Y pipe right before the rear axle, the rest of the mods in my sig.
The tuner made the tune for opened cutout. (I like making noise, ASA sounds so much meaner with and opened cutout.)
SO IF SOMEBODY COULD EXPLAIN THIS...I EXPECTED MORE POWER WITH AN OPENED CUTOUT OVER THE CLOSED EXHAUST.
Or I should just explain it for myself: Random Tech flows very-very well.
Well I have a sleeper at least now.
TECH Enthusiast
Joined: Apr 2004
Posts: 749
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From: Chandler, Arizona
I had my car tuned after the cam and headers and made 381 hp with an open cutout. I made one more run with the cutout closed and it killed over 25 hp with the stock exhaust. Can't explain your results.
Thread Starter
Launching!
Joined: May 2004
Posts: 203
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From: Stranger in a Strange Land
Originally Posted by glennhl
I had my car tuned after the cam and headers and made 381 hp with an open cutout. I made one more run with the cutout closed and it killed over 25 hp with the stock exhaust. Can't explain your results.
Originally Posted by TheHun
Yes I would blame that 25 hp on your stock catback. It would explain the difference.
I pick up 24 hp with the cutout over my Edelbrock cat back. When I put this cat back on ,my car was stock. I gained no mph at the track over stock but did not lose any either. 420/405 cutout- 396/392 through muffler.
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Launching!
Joined: May 2003
Posts: 290
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From: Canadian West Coast
Not sure how to really explain it, other than not all cars react the same way to changes. SOme gain a lot, some gain a little, some gain none. I agree that you should have gained something, but you never know.
I only gained 2 rwhp and 2 rwtq on my set up with the cutout open, so they may not always give you a lot.
Could also be just normal dyno variance. A dyno can vary from one run to the next by 1-2 % easy, and on a 400 rwhp car, thats 4-8 rwhp. So, it may be that when you opened the cutout, you DID gain, but a slight variance in the dyno cancelled out the increase. That happens.
I only gained 2 rwhp and 2 rwtq on my set up with the cutout open, so they may not always give you a lot.
Could also be just normal dyno variance. A dyno can vary from one run to the next by 1-2 % easy, and on a 400 rwhp car, thats 4-8 rwhp. So, it may be that when you opened the cutout, you DID gain, but a slight variance in the dyno cancelled out the increase. That happens.
On The Tree
Joined: Mar 2005
Posts: 131
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From: L.A
Its better for the motor without the cut out, your needs a certain amount of back pressure to run at its best and with the cut out open your taking away the back pressure your car needs and leaving a small risk for motor damage, unless your running nitrous, thats mainly what a cut out should be used for.
Originally Posted by TheHun
SO IF SOMEBODY COULD EXPLAIN THIS...I EXPECTED MORE POWER WITH AN OPENED CUTOUT OVER THE CLOSED EXHAUST.
Exhaust is kind of a grey area because of exhaust pulses and too many other things to list, but I think your unimpressive gains could be due to exhaust velocity. Dumping exhaust too early can hurt performance because it kills velocity. The exhaust slows down near the cut out, and naturally once it hits the atmosphere.
I'm guessing the cat back flows very well and keeps the velocity up. The cat back may not flow as well as a cut out on a flow bench, but a full length exhaust keeps the velocity up compare to an open headers or a cut out.
I've personally seen two head/cam cars lose HP with cut outs. An LT1 actually lost 15 rwhp believe it or not. Not because of back pressure, but because of velocity. Velocity is the key.
Well, the comment about back pressure is an old wives tale/superstition. Like avoiding black cats, or not walking under ladders. Engines do not "need" back pressure.
Exhaust is kind of a grey area because of exhaust pulses and too many other things to list, but I think your unimpressive gains could be due to exhaust velocity. Dumping exhaust too early can hurt performance because it kills velocity. The exhaust slows down near the cut out, and naturally once it hits the atmosphere.
I'm guessing the cat back flows very well and keeps the velocity up. The cat back may not flow as well as a cut out on a flow bench, but a full length exhaust keeps the velocity up compare to an open headers or a cut out.
I've personally seen two head/cam cars lose HP with cut outs. An LT1 actually lost 15 rwhp believe it or not. Not because of back pressure, but because of velocity. Velocity is the key.
Exhaust is kind of a grey area because of exhaust pulses and too many other things to list, but I think your unimpressive gains could be due to exhaust velocity. Dumping exhaust too early can hurt performance because it kills velocity. The exhaust slows down near the cut out, and naturally once it hits the atmosphere.
I'm guessing the cat back flows very well and keeps the velocity up. The cat back may not flow as well as a cut out on a flow bench, but a full length exhaust keeps the velocity up compare to an open headers or a cut out.
I've personally seen two head/cam cars lose HP with cut outs. An LT1 actually lost 15 rwhp believe it or not. Not because of back pressure, but because of velocity. Velocity is the key.
