diff ratio, much of a difference?

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Valvebounce

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My dad kindly let me run has valiant at the Drags, Saturday just been.
Aside from the huge wad of cash he's sunk into the motor, Trans,and converter, the car is little changed from how it was when he first bought it back in '81, or '82. (I must have been about 5 when he bought it) currently I'm having issues with getting a clean shift between gears. (Full manual valvebody) so sooner or later the original split back bench is going to have to go, (dad used to be a heavy guy, so the seat is pretty well past it anyway) and some kind of drag friendly floor shifter will have to go in.
Aside from that, my best time to date is 12.578@107. I'm fairly sure with the addition of a proper tacho with shift light (really only been guessing the shift points thus far) and without the cumbersome shifter arrangement it should run south of 12.50.
The thing is currently it's still sporting a 2.9:1 rear axle. And I fit a pair of M&H 235/60/14 drag radials for strip duty.
Car is an a body, and I ForgET what the stall speed is. But it's matched to the two eighty something/242@50 cam.
Long story short, how much quicker might it run with say a 3.55 back end?
There's so much knowledge here. And despite not being complete newbies, the F.A.B.O hive mind is a formidable source of experience.

Picture for thread.

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I would go 3.91, i went from a 2.45 gear running13.40's to a 3.23 running 13.20's.
I would concentrate on your 60ft.
 
Mathematically,You might be trapping in second gear at 6400, which while a little high,ain't all that crazy,for that cam. Or in Drive at 4400, choked pretty good.
4.10s would get you down to 6200 now in direct,and 3.91s to 5900, more on target
I'm guessing 2/10s
there can be 5/10s in the 60ft
At 107mph the bulletin book is saying 336hp@3400pounds. And SS ET for that is a tic over 12.4.....................So you're pretty on target already; ET to mph speaking. That's surprising to see.
 
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Not sure of tire diameter so I guessed 27 inches.

That means if you are in high gear you are crossing at about 4K.

A 4.10 puts you at about 5500. You need to know how many RPM you want to trap at, exact tire height and do the math. If it's a drag only thing, you'll need at least a 4.56 for that, if not a bit more.


How much converter?
 
Is this a drag only car? Will it be street driven? If so, some compromise is in order. With stock tires, 3.91 could be a good compromise favoring drag racing, while 3.55 would favor street driving.
 
I don't recall exactly what the stall speed is. But I think it's around 3400.
60ft is currently 1.950. I have no idea what the car weighs. Unlike with the street tires (which it can cremate at will) the m&H tires seem to bite quite hard. The car still runs a BW75 diff with 25 spline axles too. We e got one with 3.55's that we could slot under there.
 
To a certain extent the MPH tells the real power, the ET tells how well the car uses that power. The flatter the engine's power curves, the less effect a gear ratio change will have. You have a fairly stout engine there, but you will need a lot more info to choose the right gear for your car. That being said - in a general sense, assuming you can hook it, any drop in gear ratio (meaning higher numerically) will make it easier for that engine to accelerate the car. So any lower axle ratio should help the ET, that is until you go too far. 3.55 isn't anywhere near "too far".
 
I'm actually quite suprised how nicely it hooks up. An old car of mine I used to race was terrible to launch. It took.me a long time to.learn to balance what little traction it had. Then when I fitted drag radials I broke an axle.before I could unlearn that to suit the improved grip.
Given this car has a lot more power the tyres really seem to hold their own.
 
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