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Old 06-05-2011, 11:57 PM   #1
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Exhaust backpressure

Right now my setup up on my 94 gt is stock headers to gutted cats to stock h pipe and cutoff about 3 inches after the h pipe .. Ive heard it increases hp but then I heard I'm losing hp .. Which is it?
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Old 06-05-2011, 11:59 PM   #2
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Re: Exhaust backpressure

you might gain a little but you cant really loose hp unless you plug your exhaust up. your not going to get a noticeable hp gain from exhaust.
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Old 06-06-2011, 12:08 AM   #3
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How can I stop the popping when I go up to about 4000 rpm
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Old 06-06-2011, 12:14 AM   #4
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Re: Exhaust backpressure

does the car loose power around that rpm? when it pops?
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Old 06-06-2011, 12:20 AM   #5
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Well when it shifts to second it bogs . When I want to shift fast I let off the gas at about 5 and does decent but when I don't it hits rev limiter and kinda stops until you let off the gas
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Old 06-06-2011, 12:22 AM   #6
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Re: Exhaust backpressure

since your car is older i would suggest cleaning the maf. it will probably help you out
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Old 06-06-2011, 07:27 AM   #7
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I cleaned it about a month ago
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Old 06-06-2011, 08:09 AM   #8
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Re: Exhaust backpressure

the more back pressure you have the more low end you tend to keep the less the more top end they say
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Old 06-06-2011, 06:00 PM   #9
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Re: Exhaust back pressure

Gutted cats make your car sound bad and they may be making it pop. Without the internals they form an expansion chamber, similar to what they use on two stroke engines.

While they have a positive effect on two stroke engines, the hot exhaust gasses hitting that chamber and expanding quickly on a 4 stroke engine will cause your exhaust to make strange sounds and most likely it has a negative effect on your sound and power, because the gas has to re-compress to get out of the other side of the chamber.

If you ever have the chance to see some high performance exhaust set ups, you will notice that they usually increase the pipe size slowly and sometimes in steps to allow for a smooth flow and expansion of the exhaust gasses.

If you must keep the cats on for appearance at inspection time, I would weld a piece of the appropriate size exhaust pipe completely through the shell of the cats.

Exhaust leaks will also cause a vehicle to backfire or pop. Mostly when you let off the gas and it sucks cold air into the hot exhaust system.
You are probably losing a bit of horsepower with what you have, I would put the full exhaust back on it.

Since you gutted the cats and have no mufflers you are probably not getting the O2 sensors hot enough to work properly, which affects your air/fuel ratio and will make your car run poorly.
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Old 06-07-2011, 10:37 AM   #10
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Can you give me a system to make it sound good but keep the 3-4 hp? Thanks
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Old 06-07-2011, 12:34 PM   #11
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Put the cats back on and a noise canceling muffler, the exhaust was designed to the job, unless you get heads with high flow rate then you actually lose power...
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Old 06-07-2011, 12:35 PM   #12
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Quote:
Originally Posted by 254S197
Put the cats back on and a noise canceling muffler, the exhaust was designed to the job, unless you get heads with high flow rate then you actually lose power...
Before I argue this point. Why would you loose power?
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Old 06-07-2011, 12:55 PM   #13
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The back pressure from the exhaust also helps to scavenge the exhaust gases from the chamber, if you don't get all of that out you end up with less A/F.

It boils down to the head being the king. It gets air in, MORE AIR = MORE POWER. without the back pressure pulsations to get the **** out you can't get **** in.

Same principal as you eating all the time and not taking a dump.
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Old 06-07-2011, 01:00 PM   #14
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Quote:
Originally Posted by 254S197
The back pressure from the exhaust also helps to scavenge the exhaust gases from the chamber, if you don't get all of that out you end up with less A/F.

It boils down to the head being the king. It gets air in, MORE AIR = MORE POWER. without the back pressure pulsations to get the **** out you can't get **** in.

Same principal as you eating all the time and not taking a dump.
How does having backpressure pull air out of the head. Wouldnt it be keeping exhaust in?
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Old 06-07-2011, 01:07 PM   #15
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YouTube Or google AIR FOILS and aerodynamics. Every power stroke push gases out but between these strokes the crankshaft slows down causing low pressure in the exhaust system, that's when barometric pressure pushes it back in the exhaust system until the next power stroke. The back pressure pulsations help keep the barometric pressure out. That's why headers are "tuned" and the real race cars have exhaust systems about the same length as the intake runners it's all about balance.
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Old 06-07-2011, 01:12 PM   #16
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Quote:
Originally Posted by 254S197
YouTube Or google AIR FOILS and aerodynamics. Every power stroke push gases out but between these strokes the crankshaft slows down causing low pressure in the exhaust system, that's when barometric pressure pushes it back in the exhaust system until the next power stroke. The back pressure pulsations help keep the barometric pressure out. That's why headers are "tuned" and the real race cars have exhaust systems about the same length as the intake runners it's all about balance.
Well I've read it on paper but never seen it proven. Not on street cars anyways. We took a 02 gt and dyno'ed it. Then we made it open headers. Dynoed again. No gain no loss. Maybe on high performance racecars but we didn't get any results for an 02 gt
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Old 06-07-2011, 01:19 PM   #17
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You just answered your question. Open headers. Shorter exhaust which is closer to intake runner length.
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Old 06-07-2011, 01:26 PM   #18
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Originally Posted by 254S197
You just answered your question. Open headers. Shorter exhaust which is closer to intake runner length.
So why would he be losing power by dumping after the xpipe. An if he did lose any I guessing it would be like 3hp. Wich isnt much
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Old 06-07-2011, 01:50 PM   #19
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It's more than 3. Just do some solid research. You've got a lot to learn. It's more than "if I cut my exhaust off I'll get power AND sound like a badass." Don't you think if that were the case no one would have an exhaust system.
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Old 06-07-2011, 01:54 PM   #20
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Quote:
Originally Posted by 254S197
It's more than 3. Just do some solid research. You've got a lot to learn. It's more than "if I cut my exhaust off I'll get power AND sound like a badass." Don't you think if that were the case no one would have an exhaust system.
I've done research. On paper the theory of backpressure makes sence. But I can't seam to prove it on the dyno. I took my cats off and put dynomax on and I gained hp. According to the dyno
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Old 06-07-2011, 02:04 PM   #21
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Okay so I've learned to put mufflers back on . But instead of the twin chamber flows what's better?
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Old 06-07-2011, 02:04 PM   #22
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Oh and 3" piping would be better right?
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Old 06-07-2011, 02:09 PM   #23
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2.5" would be fine. If you don't have stock cats use high flowing ones. And get some good mufflers. I'll find the dyno sheets to show you the losses. We did a dyno pull with a Bola exhaust kit and we lost 43hp that was a 3" kit.
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Old 06-07-2011, 02:13 PM   #24
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Quote:
Originally Posted by 254S197
2.5" would be fine. If you don't have stock cats use high flowing ones. And get some good mufflers. I'll find the dyno sheets to show you the losses. We did a dyno pull with a Bola exhaust kit and we lost 43hp that was a 3" kit.
I'm sorry. But that sounds like bs to me. Exhaust unless it's crushed closed won't take that kinda power away.
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Old 06-07-2011, 02:17 PM   #25
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I'm not going to argue with you, you're going to bring me down to your stupidity and beat me with experience.
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Old 06-07-2011, 02:26 PM   #26
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Originally Posted by 254S197
I'm not going to argue with you, you're going to bring me down to your stupidity and beat me with experience.
My stupidity? I'm being logical. It's a pipe that removes the exhaust gases away from the car. Changing the size or shape of the pipe isn't going to make you lose 43 hp. Unless you go to such a small pipe that the engine can't relieve itself. And last I checked experience means alot. I don't argue with doctors because they know more than me. I don't argue with politicians. Because they know more than me. But I know a good deal about cars. I'm not saying your not smart or that I'm smarter. I'm saying that losing 43hp through a borla exhaust is bs. You can make anything sound true on paper. But it don't mean anything if it don't hold true in the streets. Like I said. Backpressure makes sence on paper. Just not on the street.
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Old 06-07-2011, 02:37 PM   #27
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Which is better tho? 3" or 2.5" and preferably which mufflers?
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Old 06-07-2011, 02:38 PM   #28
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Originally Posted by 94mustanggt5.0
Which is better tho? 3" or 2.5" and preferably which mufflers?
In my honest opinion I say go 3 inch. And for the 5.0 flowmasters sound pretty good
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Old 06-07-2011, 02:43 PM   #29
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I've been working with/at speed shops for 7 years now, I've been through automotive performance schools, I've been through tuning schools, but obviously you know more than me. I'm sorry to insult your intelligence and I wish you the best of luck with the mods you have an plan on doing. Maybe when I get back to Texas we can go to the strip?
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Old 06-07-2011, 02:48 PM   #30
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Quote:
Originally Posted by 254S197
I've been working with/at speed shops for 7 years now, I've been through automotive performance schools, I've been through tuning schools, but obviously you know more than me. I'm sorry to insult your intelligence and I wish you the best of luck with the mods you have an plan on doing. Maybe when I get back to Texas we can go to the strip?
I'm not saying I know more. I just said that the numbers you through out were too crazy. Now if this was done on a 1500hp racecar than it sounds more logical. But if your talking about a 02 gt or something, borla exhaust ain't gonna bring it down to v6 status. I'm catching some smart elec in your post. Why? I'm simply trying to figure this out for myself. Your performance background sounds good. But out of all the cars I've put mufflers on you never felt a difference in hp. And I would defiantly feel 43 hp. So stop being smart and trying to make me sound like a know it all I'm just trying to get the facts straight.
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Old 06-07-2011, 02:49 PM   #31
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Quote:
Originally Posted by 254S197
I've been working with/at speed shops for 7 years now, I've been through automotive performance schools, I've been through tuning schools, but obviously you know more than me. I'm sorry to insult your intelligence and I wish you the best of luck with the mods you have an plan on doing. Maybe when I get back to Texas we can go to the strip?
Well what's your input for my new exhaust? Thanks
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Old 06-07-2011, 03:07 PM   #32
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Unless you have forced induction or a big block under the hood a 2.5" system is plenty enough for your car.
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Old 06-07-2011, 03:17 PM   #33
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If you have a high pressure reflected wave arriving a few milliseconds before exhaust valve closure, you prevent the loss of intake air:fuel out the exhaust valve at cam overlap. The exhaust backpressure at this crankshaft degree in the exhaust stroke prevents leaking out or bleeding out of you intake charge into the header and ensures all of it goes into the chamber for combustion ,when the intake valve is opening further, and the piston has reached TDC and starts downward for the intake stroke. Combustion chamber and intake port pressures are higher than the pressure in the exhaust just before exhaust valve closure . So the burnt gases are scavenged out of the combustion chamber.

If you take mustang "X" and remove it's back pressure, you lose power.
Take mustang "X" and retune it with some back pressure you gain bottom end torque and top end speed.
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Old 06-07-2011, 03:23 PM   #34
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Quote:
Originally Posted by 254S197
If you have a high pressure reflected wave arriving a few milliseconds before exhaust valve closure, you prevent the loss of intake air:fuel out the exhaust valve at cam overlap. The exhaust backpressure at this crankshaft degree in the exhaust stroke prevents leaking out or bleeding out of you intake charge into the header and ensures all of it goes into the chamber for combustion ,when the intake valve is opening further, and the piston has reached TDC and starts downward for the intake stroke. Combustion chamber and intake port pressures are higher than the pressure in the exhaust just before exhaust valve closure . So the burnt gases are scavenged out of the combustion chamber.

If you take mustang "X" and remove it's back pressure, you lose power.
Take mustang "X" and retune it with some back pressure you gain bottom end torque and top end speed.
Like I've said the theory sounds good. And I agree with it. What I don't agree with is someone losing 43hp because they put a borla exhaust on their mustang. I could see the loss of some power just not 43hp. Not disagreeing with the theory though
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Old 06-07-2011, 03:32 PM   #35
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Are you questioning my integrity?
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