Anybody ran an underdrive crank pulley to lower boost?

VIPERBLUELX · Apr 28, 2004

I'm thinking about running a steeda underdrive crank pulley on my 99 Cobra with an S-trim to keep the boost down till I do a forged shortblock. Is there any downsides to this?

GinoGT · Apr 28, 2004

Why? Cobra engines are good for about 425-450RWHP with a good tune.

VIPERBLUELX · May 1, 2004

GinoGT said:
Why? Cobra engines are good for about 425-450RWHP with a good tune.

Because an S-trim will make up near 500 and that's too much for a stock bottom end.

Aaron 4.6 · May 1, 2004

No reason why you couldn't although most people would simply run a larger blower pulley.

DBMSTNG · May 1, 2004

you really shouldn't mess with the crank pulley unless you absolutely have too. also, you'd have to have a program specifically made for running less boost. just get a bigger pulley if you insist on lowering the boost. or just shift early.

VIPERBLUELX · May 1, 2004

This is a brand new tuner kit, and if I change pullies it will void the warranty. The car will be dyno tuned after the S-trim is installed, i'm not using the supplied Vortech chip.

Monkie · May 1, 2004

HUH!? How much boost is the S-trim pulley making? Sounds like you don't even have the supercharger on yet so how do you know if you need or don't need to lower the boost? And what warranty are you talking about? Why don't you wait until it goes on before you start lowering boost. And just because you make X amount of power doesn't always mean you are going to break something. Yes, if you make 500rwhp and go to the track every week and pull 1.6 60' you will break something quite quickly. Honestly your thread makes no sense to me. I don't understand your reasoning.

2002BLGT · May 1, 2004

Yeah i dont think the S trim is going to quite hit 500 rwhp atleast not by itself .

VIPERBLUELX · May 1, 2004

Monkie said:
HUH!? How much boost is the S-trim pulley making? Sounds like you don't even have the supercharger on yet so how do you know if you need or don't need to lower the boost? And what warranty are you talking about? Why don't you wait until it goes on before you start lowering boost. And just because you make X amount of power doesn't always mean you are going to break something. Yes, if you make 500rwhp and go to the track every week and pull 1.6 60' you will break something quite quickly. Honestly your thread makes no sense to me. I don't understand your reasoning.

An S-trim head unit on a 99 Cobra will make 13lbs of boost at 7,000 RPM's right out of the box. This is not the E-trim or SC-trim that Vortech sells in the complete kit. I ordered an S-trim head unit specifically so that I would have plenty of room to grow when I build a forged shortblock.

13lbs of boost is not going to last long on a stock bottom end and if I change the blower pulley itself to lower the boost I will void the 2 or 3 year warranty that Vortech gives on all their new headunits.

Is there any part of that you don't understand?

2002BLGT · May 1, 2004

I think you need to call vortech , I dont think they will void your warranty for underdriving the blower . I am sure if you explain what your plans are they will give you the pulley you need and still cover the warranty on the unit .

VIPERBLUELX · May 1, 2004

I doubt Vortech makes larger than the stock 3.6 pulley

GinoGT · May 1, 2004

What do people with the Mongoose kit put down? I know one guy here has/had a 99 Cobra and was making mid 400's RWHP with it. Those are the same blowers you're going to use. It might be an SQ, but I've heard they're practically identical.

If that S trim has the same size pulley on it that would come on a stock S trim for a GT, I can't see it hitting 13lbs of boost with how much more flow the 4V heads have. For reference again, I belive the S/SQ trims in the GT Mongoose kits do about 8-10lbs with the base setup. If that's the same pulley, 13lbs doesn't make sense, even with the higher redline.

todd4566 · May 2, 2004

You'd be surprised how much boost you will lose with an underdrive pulley. I had a Kenne Bell on my old 5.0. A 10psi pulley matched with an underdrive crank pulley netted about 3 psi. The car felt stock with such little boost. In my case, the small crank pulley costed about 7psi.

2002BLGT · May 2, 2004

GinoGT said:
What do people with the Mongoose kit put down? I know one guy here has/had a 99 Cobra and was making mid 400's RWHP with it. Those are the same blowers you're going to use. It might be an SQ, but I've heard they're practically identical.

If that S trim has the same size pulley on it that would come on a stock S trim for a GT, I can't see it hitting 13lbs of boost with how much more flow the 4V heads have. For reference again, I belive the S/SQ trims in the GT Mongoose kits do about 8-10lbs with the base setup. If that's the same pulley, 13lbs doesn't make sense, even with the higher redline.

my thoughts exactly the better flowing heads with less restriction should net less boost regardless of the blower .

Anybody ran an underdrive crank pulley to lower boost?

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