Choosing gear ratios for a weekend racer/street car

[ManHunter]

I'm currently building a street-legal racing car with the following specs:

Motor: SBC V8 - 640 hp - 6500 rpm redline
Gearbox: Porsche G50/01 5 speed manual
Tires: 18" (26" diameter) stree tires

The car weight 2200 pounds.

I'm wondering if these gear ratios are acceptable:

1st - 2.43
2nd - 1.58
3rd - 1.29
4th - 0.97
5th - 0.76
Final Drive - 3.44

Is 1st gear too tall, given a 2200 pounds car and a 640 hp motor? Any gear too far apart?

 

[Tmoose]

Are BSFC numbers under .4 attainable, or desireable, in a high performance engine? That 0.5 lb/hr at max power has been around quite a while.

 

[ManHunter]

That's an interesting comment. The car's pedal is connected to a spring on the carburator by a metal cable. I don't think there's much progession going on there. I'll have to look at that.

I don't know what are the impact of low BSFC numbers on the engine?

Eric

 

[evelrod]

That's an interesting comment. The car's pedal is connected to a spring on the carburator by a metal cable. I don't think there's much progession going on there. I'll have to look at that.

I don't know what are the impact of low BSFC numbers on the engine?

Eric

Sorry, ManHunter---I understand that "free" advice is often worth it's cost...Never the less, I'll go out on a limb and advise you to look up some local 'car guys' and make them your 'best' friends. As I see it, you are in for one heck of a learning experience in building this car. Best of luck, sir, you'll need all you can get, IMO!

Rod

 

[patprimmer]

Like Rod says.

The throttle cable being connected to a spring would make the car totally uncontrollable. I think it will be connected to an arm on the carby. A return spring will also be attached to that arm to pull it back to idle as the pressure on the pedal is reduced.

Low BSFC causes the engine to burn holes in pistons. That can be very inconvenient and expensive.

Regards

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[GregLocock]

Back in the good old days we would spend a lot of time developing the shape of the cam on the butterfly, to improve drivability. Nowadys we just bash in a curve of butterfly angle vs TPS

Cheers

Greg Locock

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