Remove catback for drag racing on s/c car?

[90mustangGT]

People dont relize that larger exhaust pipe diameter can lead to more restriction, or backpressure. Bigger isn't better. If the exhaust is too big, the gasses will lose velosity, and will then lose the energy to push the rest of the gasses out, and then lose the pulse that actually pulls exhaust out of the combustion chamber. It's like trying to push into a lake vs. pushing downstream.

Think about it, when the exhaust valve opens the gasses come rushing out at a high rate of speed, then the valve closes and the pulse of gasses (remember that even gasses have mass) keep moving and effectivly cause a negative pressure in the runner and depending on runner length, that pulse will pull gasses out of the next runner at the collector. If the runners or system is too big, it will lose this pulse. Same goes with intake.

 

[LaserSVT]

People dont relize that larger exhaust pipe diameter can lead to more restriction, or backpressure. Bigger isn't better. If the exhaust is too big, the gasses will lose velosity, and will then lose the energy to push the rest of the gasses out, and then lose the pulse that actually pulls exhaust out of the combustion chamber. It's like trying to push into a lake vs. pushing downstream.

Think about it, when the exhaust valve opens the gasses come rushing out at a high rate of speed, then the valve closes and the pulse of gasses (remember that even gasses have mass) keep moving and effectivly cause a negative pressure in the runner and depending on runner length, that pulse will pull gasses out of the next runner at the collector. If the runners or system is too big, it will lose this pulse. Same goes with intake.

Huh, that actually made allot of sense.

 

[kck6894]

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.

yeah well I have tons of pics of the carnage that happens when you do things like that so I don't know how envious you should be!

It sure if fun though.....until you hear that BANG as the starting line!

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!

do you know what the corrected altitude was for each run? Sometimes the time slips will have it printed on the bottom. Was it getting hotter as you were making the runs without the cat-back or was it getting cooler?

People dont relize that larger exhaust pipe diameter can lead to more restriction, or backpressure. Bigger isn't better. If the exhaust is too big, the gasses will lose velosity, and will then lose the energy to push the rest of the gasses out, and then lose the pulse that actually pulls exhaust out of the combustion chamber. It's like trying to push into a lake vs. pushing downstream.

Think about it, when the exhaust valve opens the gasses come rushing out at a high rate of speed, then the valve closes and the pulse of gasses (remember that even gasses have mass) keep moving and effectivly cause a negative pressure in the runner and depending on runner length, that pulse will pull gasses out of the next runner at the collector. If the runners or system is too big, it will lose this pulse. Same goes with intake.

When I had my headers built by kooks they told me to they wanted me to use 1 3/4 long tubes, 3" x-pipe and then reduce it and run a 2 1/2 muffler. The told me to do this for the exact same reason that you were describing.

It didn't work, but the theory was good. I picked up 2mph in the 1/4 by removing my cat back. I've since switched and run a full 3" system and don't see much if any difference with or without the cat back there.

 

[NIKwoaC]

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

I realize that. I've heard in a few different places that O2 sensors like a little bit of back pressure to get a good reading. I'm not a tuner, so I was just throwing that out there.

 

[90lxcoupe]

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 wasnt a tuning issue, when you put a larger exhust on a blower car it will make less boost, im not gonna throw around a bunch of engineering words like volumetric efficiency or anything like that, but it is common knowledge that when you open up the exhaust you loose boost. The shop that i used to go to told me that with the 3" stuff i would loose about 2psi, and they were on the money