gear change....possible gain?

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360duster

360duster

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Hi Guys,

as always i´m searching for ways to lower the E.T. for my 68 Valiant. PB for now is 11.61 with a 1.67 60 ft. and 115 mph, engine is a relatively mild 408SB around 450 HP / 727 trans 3500 stall / 3.73 gears and 26x8" tires.

I do have a 4.10 gear set, and think i have to buy new tires after the season.....so i thought why not put in the 4.10s, step up to 27 or 28 inch tires with 9" tread width.

Do you think there are any gains? or should i better keep the 3.73 gears?

Michael
 
putting on a taller tire will require a lower gear to keep the rpm at the big end the same. If it hooks now with what you have, going to a bigger tire and lower gear isn't likely to help any. And remember, you have to pull those bigger tires all the way down the track, it is possible that you may actually slow down. Only way to know for sure is to test.
 
more gear and a taller tire is not going to help, going to a wider tire will help if u have some wheelspin now. more gear can help if u are crossing the finish line at less than ideal rpm
 
You didn't say what your RPM is as you cross the line; from the MPH you posted, and working backwards, you're turning about 5550rpm. Am I close?

Using that calculated number for rpm, switching to 28 inch tall tire, and 4.10 gears I crunched the numbers and you slowed down to 113mph, not sure what the gear change would do to your 60ft; it may drop it and then that would drop you ET.

You stated that you already have 4.10 gears, put them into the car, if you don't like them, take them back out. It's only 34 nuts each way and bench pressing a center section under the car.

Good luck!
 
I want to say; You cant just throw gears at it and hope something sticks. But that sounds a little condescending.So how about this; I wouldnt ju......
I would Gear for the big-end.But thats not saying much either. So how about this; The standard reply is gear it to put the car on the line,about 300 to 400 rpm past peak horsepower.
But,you say, I dont know where my engine peaks! Well youre screwed. Take it out, put it on the dyno and find out. Not. Kidding again!
Theres at least two ways of figuring that out, or at least getting close.
#1), if its a streeter, and you have a common combo, a simple guestimate will do. Afterall the gear ratio jumps are about 5 or 6 %, and 6% of 6000rpm is 360rpm,so close is good.But..
#2) if you want to be competitive, you'll need to be right on. So get yourself one of those windshield mounted accelerometers, and learn how to use it.They can be a real eyeopener.Dont get caught up in the numbers,right off. Just find the peak and the rate of decay after the peak, then gear the car accordingly.
Theres other ways,too, if you need to research it.
Lastly, I agree with whats already been posted; that is,throw those 4.10s at and see if they stick. Hah
 
i run 4.10 with 28x9 and my tacho shows 6000 over the line
my 60's though are a bit less than yours, @1.65 but its nearly the same, but i have the low first gear in my trans,
personally i think where you are right now is right, and as others have said altering the gear and height could slow things down a bit
i would put the 4.10 in :)
 
thanks guys for the information. Unfortunately the rear end is not a 8.75, it´s a Ford 8.8 so gear change is not this easy....but it seems the only way to find out is to test.

At the moment i´m crossing the stripe around 5900. Don´t know if the converter is slipping....could be. The engine is pulling hard to 6500, so i think the car could benefit from the lower gears. I made some calculations, there is around 6 % speed difference between the old combo 26" / 3.73 and the possible new combo 27" / 4.10, so i´d expect a 6% gain in torque at the starting line - is this correct? I think with the wider tire there´s no problem getting it to hook (car is equipped with SS Springs and drag shocks all around).

Any more info / suggestions welcome!

Michael
 
This was my set up and it worked in my 67 Dart with a healthy slant 6.
Using a 5000 stall 8 inch converter with 26'' tire and Sure Grip 3.73's ran 13 teens in 1/4 mile.Only change was to 4.30's and a spool car ran 12.90's.Approx 2 tenths or so gain.
 
Rule of tumb you picked up a 10th for every change...4.10-4.30-4.56...
 
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