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Old 03-11-2007, 07:01 PM   #1 (permalink)
Lethal5Oh
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Default Not enough back pressure?

Well i got my stang out of storage the other day and put the cats back on for my emissions test. It feels to me like that car has better power with the converters on then off. Am i nuts? the car has 100k miles on it. Any one have a idea on this? Thanks
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Old 03-11-2007, 08:33 PM   #2 (permalink)
goodyear1984
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Default Re: Not enough back pressure?

whats up man...what kind of exhaust do u have
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Old 03-12-2007, 11:47 AM   #3 (permalink)
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Default Re: Not enough back pressure?

Hmm, good post, cuz ever since i cut my tailpipes off, i havent been able to run as fast in the 1/4, but pretty close.
I wonder if it hurts the torque that much?
Im pretty sure an n/a 5.0 will feel more torque with Some backpressure, anyone have any dyno test to confirm this?
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12.69 @ 107.35, 1.71 60' 26x8.5 drag's 3.90 gear

13.20 @ 106.91 - 235/60/15 firestones 2.3 60' 3.27 gear
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Old 03-12-2007, 12:53 PM   #4 (permalink)
Lethal5Oh
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Default Re: Not enough back pressure?

Well i did some research and found that you can infact have too little backpressure. Without the back pressure you get too much scaveging and actuallly suck fuel and air out the exhaust causing a lean condition, killing torque. To conteract you have to have a fuel injection system that can correct for this problem. Im getting a adjustable fuel pressure regulator and see if i can richen it up. If that doesnt work i guess ill get the upr cat xpipe.
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Old 03-12-2007, 01:28 PM   #5 (permalink)
Coupe50h
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Default Re: Not enough back pressure?

i seem to have a rich condition, but of course it may seem that way because i smell more exhaust, and with no cat's, i almost want cat's so it wont stink as much, do you have the smog pump?
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X-Texas highway patrol ssp 1990 coupe - exploder Gt-40 iron heads, Explorer intake, 19 lber's. E-cam. crane 1.7 rollers. 190fp. 75mm maf. 65mm tb, tubular subframe connectors, mac cai, Asp crank pulley, T-5, king cobra clutch, flowtech 1-5/8 unequals, mac X-pipe Frpp driveshaft, lakewood Lca's.
race weight 3,160

12.69 @ 107.35, 1.71 60' 26x8.5 drag's 3.90 gear

13.20 @ 106.91 - 235/60/15 firestones 2.3 60' 3.27 gear
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Old 03-12-2007, 01:38 PM   #6 (permalink)
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Default Re: Not enough back pressure?

I picked up a tenth and .5mph with dumps.....
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Old 03-12-2007, 03:41 PM   #7 (permalink)
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Default Re: Not enough back pressure?

Absolutely. There is more to your problem than a rich/lean fuel ratio. Lean engines actually develop more horsepower to a point. When you have a higher flowing exhaust system paired up to a 5.0 requiring much less flow, you can create too much scavenging as you're indicating.

Since exhaust is like any other gas, it will flow from areas of high pressure to areas of low pressure. Obviously, the upstroke of the piston helps force exhaust out of the cylinder; however, a large diameter, straight through exhaust system will actually create a sort of vacuum for gases inside the cylinder at the top or near the top of the stroke. The reason for this is two fold. Heat creates pressure and so does confined spaces. Having a large exhaust space opening up from the exhaust port creates an area of low pressure the exhaust gas will accelerate into. In addition, the temperature further down the exhaust is cooler, which will tend to draw the gases out of the cylinder.

You’ll see large chambers on 2 stroke engine exhaust systems in order to dramatically increase scavenging and get the burned exhaust mixture out of the engine quicker, thereby increasing the clean air for the next combustion cycle. At the same time, the exhaust is shaped to create back pressure waves as the chamber is suddenly filled with high pressure exhaust because a two stroke engine does not generally have any sort of valve to close off the flow of air through the engine and into the exhaust. If there were no constrictions, the engine would run very inefficiently because air and fuel would be blown directly through the engine into the exhaust without stopping for compression. As it is, the 2 stroke engine is inherently inefficient because a backpressure wave is not nearly as efficient as a valve sealed cylinder like 4 stroke engines have.

Onto your 4 stroke engine. Scavenging works the same way on 4 stroke engines. It gets the old burned exhaust that will decrease burn performance out of the engine while simultaneously promoting a negative pressure environment to pull new fuel air mixture inside the cylinder on the intake stroke. The problem on a 4 stroke engine is mainly overlap. During overlap, both the exhaust and intake valves of the engine are open so an exhaust system designed to flow far more than the engine needs can create a vacuum effect inside the cylinder and pull unburned, fresh intake air/fuel out of the cylinder before combustion takes place. If this happens, cylinder pressure on combustion will be reduced thereby directly decreasing the torque generated, especially at lower rpms. At higher rpms, the effect is somewhat nullified or reversed because the scavenging improves the efficiency of the engine regarding the removal of waste gases and pulling fresh air/fuel into the engine becomes more important than the amount of cylinder pressure lost in the thousandths of seconds the overlap is occurring.

In summary, exhaust, just like everything else on your engine is best matched to your combination for maximum performance.
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Old 03-12-2007, 08:57 PM   #8 (permalink)
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Default Re: Not enough back pressure?

Dang unit 5302! That explains everything. Guess ill have to trade in the offroad h pipe for the UPR catted x pipe. The recyclers should give me one heck of a good price for the stock cats. Thanks again for the post, YOUR THE MAN UNIT 5302.
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Old 03-12-2007, 10:33 PM   #9 (permalink)
Unit 5302
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Default Re: Not enough back pressure?

I'm not necessarily telling you to swap the setup out. You'll need a dyno to tell you which system is actually delivering better performance throughout your powerband. The "seat of the pants" meter is unlikely to be able to accurately analyze your cars true performance between the two.

Of course, if you're happier with one than the other, that's the ultimate test, isn't it?
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Old 03-17-2007, 11:33 PM   #10 (permalink)
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Default Re: Not enough back pressure?

I did the same changed out the factory cats for the offroad h-pipe mated to 2.5 flowmaster. THe car runs like a champ but is running real rich from inside the car I can smell gas fumes. Do I need make adjustments to my fuel regulator( AmFlo ).

Has anybody heard of AmFlo fuel regulators?
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Old 03-18-2007, 02:15 PM   #11 (permalink)
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Default Re: Not enough back pressure?

You would have to have a air fuel gauge to tell that. My car smelled rich too, your smelling the unburned gas thats being sucked out of the cylinder.
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Old 03-18-2007, 08:58 PM   #12 (permalink)
warlock 92
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Default Re: Not enough back pressure?

Quote:
Originally Posted by Lethal5Oh View Post
You would have to have a air fuel gauge to tell that. My car smelled rich too, your smelling the unburned gas thats being sucked out of the cylinder.

have a Accel guage coming in the mail. Sounds like both our cars have about the same mods what PSI should I be running?
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Old 03-19-2007, 09:49 PM   #13 (permalink)
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Default Re: Not enough back pressure?

about a fuel pressure reg....whats a good one and what do i have to do to install it...i'm thinking i have the same problem to...
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Old 03-19-2007, 09:58 PM   #14 (permalink)
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Default Re: Not enough back pressure?

Quote:
Originally Posted by DaLe five OH! View Post
about a fuel pressure reg....whats a good one and what do i have to do to install it...i'm thinking i have the same problem to...
BBK makes a good one. Summit Racing has instructions how to install one. I wound spend extra $55 buck and buy the fuel pressure gauge
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Old 03-21-2007, 11:30 AM   #15 (permalink)
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Default Re: Not enough back pressure?

BBK has a high rate for rebuild requirements on their regulators, just my .02 cents. Aeromotive sells a good one IMO
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Old 03-21-2007, 10:38 PM   #16 (permalink)
Lethal5Oh
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Default Re: Not enough back pressure?

As for the AFPRs, i have a aeromotive. Had one about two years ago and the diaphram shrunk up on me, aeromotive claimed to have redesigned them since. Anyway i bought another one and i set it at 39-40 psi with the vacuum line attached. The instuctions say to set it to 48 psi static (with the vacuum line dettached). even with my stock cats back on i noticed an improvement at higher rpms. On another subject anyone have the UPR catted x pipe? just got mine today and installed it. It didnt fit! It pushed my exhaust back to the axle! Any comments.
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Old 03-27-2007, 10:39 AM   #17 (permalink)
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Default Re: Not enough back pressure?

Just FYI, with an off road pipe, it will smell rich. That's normal with them.
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