Remove catback for drag racing on s/c car?

[Cobra912]

Can anyone tell me - would removing exhaust from the h-pipe back hurt or help my car as it is a 331 with a Paxton 1220SL?

I'm running 2.5 mac shorties, h-pipe and cat back with tail pipes. I don't think I would loose to much low end but hope to gain on the high side = mph. But would it hurt me to loose back pressure?

 

[kck6894]

It's worth a try just to see. One of those deal where you'd need to make back to back passes. One with the cat back and one without.

I think it's gonna depend a lot on how you launch the car and the quality of the mufflers. I know when I was running 2.5 magnaflows on my car I saw a big difference but with the 3" ones I'm running now I don't gain or loose anything from having the cat-back there.

 

[Cobra912]

But do you think I will loose power? My launches at 4000 on stock suspension are violent to say the least, I just swapped out my rear control arms so that should help out quite a bit. The car dynoed at 448 / 419 so if I loose a few down low to gain up top it may help my e.t.

 

[90lxcoupe]

you may loose boost from the loss of backpressure, when i switched to a 3" exhaust with the blower the car slowed down cause i went from 10psi to 8psi

 

[NIKwoaC]

It might confuse the computer because the O2s won't get as good a reading.

 

[90mustangGT]

you may loose boost from the loss of backpressure, when i switched to a 3" exhaust with the blower the car slowed down cause i went from 10psi to 8psi

That doesn't make sense. Sounds like a tuning issue. Your 10psi has to do with restriction, it's all about CFM. It flowed better so it couldn't build as much pressure. Are you running a turbo? I just don't understand how backpressure in the exhaust coud effect a belt driven supercharger.

 

[Cobra912]

That doesn't make sense. Sounds like a tuning issue. Your 10psi has to do with restriction, it's all about CFM. It flowed better so it couldn't build as much pressure. Are you running a turbo? I just don't understand how backpressure in the exhaust coud effect a belt driven supercharger.

Hmmm.......Now I'm confussed. I was always under the impression that even N/A cars benefited from a bit of back pressure, depending on the build of course. So should I see the same 6-7 lbs of boost, loose a little low end torque but gain up top?

I don't really wanna rip my exhaust off at the track tomorrow, but would be willing to try, just don't wanna waste my time if it makes no sense to do. It will probably be my last time this year and I'm aching for a low 11 time slip!!

 

[kck6894]

Hmmm.......Now I'm confussed. I was always under the impression that even N/A cars benefited from a bit of back pressure, depending on the build of course. So should I see the same 6-7 lbs of boost, loose a little low end torque but gain up top?

I don't really wanna rip my exhaust off at the track tomorrow, but would be willing to try, just don't wanna waste my time if it makes no sense to do. It will probably be my last time this year and I'm aching for a low 11 time slip!!

my 3" mufflers on my stock 94 Cobra killed my low end. I put the 2.5's on it and it was way better. My 347 in it was much happier with the 3's than the 2.5's and it made quite a bit of difference at the track.

Doing the exhaust at the track on back to back passes is the only way you'll really know if it does you any good. Otherwise it could be as simple as good or bad air on that night making the difference and not the exhaust.

 

[LaserSVT]

That doesn't make sense. Sounds like a tuning issue. Your 10psi has to do with restriction, it's all about CFM. It flowed better so it couldn't build as much pressure. Are you running a turbo? I just don't understand how backpressure in the exhaust coud effect a belt driven supercharger.

It does and it doesent. The car should have run the same with slightly lower boost due to overall efficency. It prolly slowed down because now it was running too rich since I assume it was tuned for 10psi.

As for the pressure loss, it will happen on a turbo (with no EBC) or a supercharger. The cylinders can evacuate more exhaust with a less restrictive exhaust system. In return there is more volume to fill on the intake stroke causing there to be less pressure.
On a turbo that just has a spring on its waste gate it will be similar in PSI drop. Now if the turbos boost was controlled by an electronic boost controller then it would remain the same boost pressure cause the controller would keep the wastegate closed enough to reach the commanded boost level. If the turbo was at its limits though then that would also change the outcome.



Anyway, OP. Since you launch so high in the RPMs I would bet the car may be a tenth quicker with no exhaust. You may want to put a smaller pully on to keep the same boost level though so your fuel map stays inline and it does not run overly rich.

 

[kck6894]

It does and it doesent. The car should have run the same with slightly lower boost due to overall efficency. It prolly slowed down because now it was running too rich since I assume it was tuned for 10psi.

As for the pressure loss, it will happen on a turbo (with no EBC) or a supercharger. The cylinders can evacuate more exhaust with a less restrictive exhaust system. In return there is more volume to fill on the intake stroke causing there to be less pressure.
On a turbo that just has a spring on its waste gate it will be similar in PSI drop. Now if the turbos boost was controlled by an electronic boost controller then it would remain the same boost pressure cause the controller would keep the wastegate closed enough to reach the commanded boost level. If the turbo was at its limits though then that would also change the outcome.



Anyway, OP. Since you launch so high in the RPMs I would bet the car may be a tenth quicker with no exhaust. You may want to put a smaller pully on to keep the same boost level though so your fuel map stays inline and it does not run overly rich.

What are we gonna do with you Laser? 4K high? You're not getting high till it tops 6500....and approaches 7K.

 

[LaserSVT]

I ment as opposed to some people that launch at 2000 RPM.
Sorry, been hanging with the Mark VIII crowd too much and they all launch at idle-2200 RPM.

 

[kck6894]

I ment as opposed to some people that launch at 2000 RPM.
Sorry, been hanging with the Mark VIII crowd too much and they all launch at idle-2200 RPM.

oh I understand now. Apparently I've been hanging around the guys at the drag strip too much.

It's never a good feeling when you notice your didn't remember to turn the launch limiter on when you're in the return road. It's then that you realize you just got done launching off the 7500 top end limiter. oops

 

[LaserSVT]

I have never had anything with the ability to launch like that. I had an S4 but the most I ever pushed it at launch was 5000 RPM. It was under warrenty though.

I am envious of you guys and teh bad ass cars. Even when I do gears and Trac Lok and drag radials on my Mark the best i could ever do with a Stall would be 2500 on launch.

 

[BK_CAULEY]

It might confuse the computer because the O2s won't get as good a reading.

the o2's are at the front of the H pipe. they will read the same. he is talking about undoing the mufflers from the h pipe

 

[Cobra912]

the o2's are at the front of the H pipe. they will read the same. he is talking about undoing the mufflers from the h pipe

Yep. Loading the car right away. Gonna see if I can make some runs later today with it off......see how it works.

 

[bentley429isBAC]

Maybe I missed it but whats your catback? If its decent you wont be gaining anything, longtubes and a x pipe will deff help.

 

[Cobra912]

it's all mac, seems to work nice but want the advantage.

I ran today, it was freakin hot out, and don't know the humidity but it was high. I was hopin for some sweet low 11 runs but no luck. I just installed tubular uppers, moroso drag springs, adjsutable weight jacker lowers and an air bag. THe car seemed to launch more civilized instead of the usual one tire hang and I was able to up the launch rpm to 4200. I made 5 runs as follws:

11.76 @ 119.76 with exhaust, 1.73 60ft
11.78 @ 118.02 with exhaust, 1.73 60ft
12.13 @ 113.36 with exhaust, missed 4th gear, 1.68 60ft
11.93 @ 115.94 without catback + tailpipes, 1.69 60ft
11.86 @ 119.36 without catback + tailpipes, 1.74 60ft

It appears that removing the exhaust made little difference. This is to my previous best of 11.54 @ 121, 1.57 60ft. (first run on cool morning) I thought for sure I was headed towards some faster times.

There is a member "The Bock" I believe who has the same blower on a notch with a 302 running 10.90's!

 

[BK_CAULEY]

There is a member "The Bock" I believe who has the same blower on a notch with a 302 running 10.90's!

what kind of suspension is he running? same engine?

 

[Cobra912]

1993 black coupe : 302, F cam, TW, GT-40 intake, 30 lbs inj, PMAS 95mm velocity maf, 70mm tb, Paxton novi 1220SL, UPR boost pipe, G-force T5, 31 splines cobra diff.
10.93@ 124.3 mph, 1.49 60 feet, 3200 lbs

 

[Cobra912]

He is a member on here too I believe, very friendly

YouTube - Mustang drag Napierville, [email protected] mph .