rear end ratio

[asloan5]

I have read and read and the general answer is stay with a 3.42 but....
I looked at my datalog at the 1/4 mile line I was at 5450 rpm with a 1.0 trans ratio 129 mph with 28" tall tire and a 3.42 axle ratio @24lbs of boost When I plug all the #'s into a drag racing calc it's telling me convertor slip is 2.9% I believe my cam and engine combo has it's power range around 5-6250 rpm. my shift light comes on at 6k and shift occurs around 6150-6200 rpm I want the car to mph about 137-138 to put me into a high 9 with boost about 30lbs. using the info from the drag racing calc I plug the desired #'s in with desired rpm at finish line 6100 and I'm getting about 3.63 ratio.... I just purchased a 9" ford rear end for my car used and it just so happens to have a 3.7 ratio already in it. Before doing the above stated math I was going to put a 3.5 ratio in it. Are all of my ideas in the correct line to stay with the 3.70 ?

 

[Sam Colalillo]

Should you decide to try the 3.70 rear end gear, I think that converter you have will probably tighten up even more. You will notice the amount of rpm drop will slightly increase inbetween shifts when the converter gets tighter.

Since you already have the gear I would try it before you change it. I think you will learn the 3.42 seems to be the magic number for our cars though. Once you start making even more power you may switch to a 3.23 gear.

 

[fubar2.5]

I have read and read and the general answer is stay with a 3.42 but....
I looked at my datalog at the 1/4 mile line I was at 5450 rpm with a 1.0 trans ratio 129 mph with 28" tall tire and a 3.42 axle ratio @24lbs of boost When I plug all the #'s into a drag racing calc it's telling me convertor slip is 2.9% I believe my cam and engine combo has it's power range around 5-6250 rpm. my shift light comes on at 6k and shift occurs around 6150-6200 rpm I want the car to mph about 137-138 to put me into a high 9 with boost about 30lbs. using the info from the drag racing calc I plug the desired #'s in with desired rpm at finish line 6100 and I'm getting about 3.63 ratio.... I just purchased a 9" ford rear end for my car used and it just so happens to have a 3.7 ratio already in it. Before doing the above stated math I was going to put a 3.5 ratio in it. Are all of my ideas in the correct line to stay with the 3.70 ?

I think you are looking at this from a N/A point of view.

What will your car MPH with 30 lbs of boost.That will drive up the RPM.

AN over simplification but------

HP= mph

GR=torque

JMHO

 

[Sam Colalillo]

I think you are looking at this from a N/A point of view.

What will your car MPH with 30 lbs of boost.That will drive up the RPM.



JMHO

Creating good exhaust velocity is an important part of the strategy when running a turbo car. In my opinion this why the 3.42 gears works so well on several cars. They load the engine just about perfect in every Turbo Buick application.

 

[fubar2.5]

Creating good exhaust velocity is an important part of the strategy when running a turbo car. In my opinion this why the 3.42 gears works so well on several cars. They load the engine just about perfect in every Turbo Buick application.[/QUOTE

I don't disagree with that. However it seems to me that the OP is comparing RPM at 24 lbs with 3.42s of boost to 30lbs of boost with 3.70 gears and what I'm thinking is if he runs 30lbs of boost with the 3.42s he should be right at his target RPM at the end of 1/4.
Eg 6 lbs of boost at 15hp per =90hp or9 mph =approx 900rpm

Try the 3.42s at 30 lbs of boost before specing gears is all I'm suggesting.

 

[Sam Colalillo]

i don't disagree with that. However it seems to me that the op is comparing rpm at 24 lbs with 3.42s of boost to 30lbs of boost with 3.70 gears and what i'm thinking is if he runs 30lbs of boost with the 3.42s he should be right at his target rpm at the end of 1/4.
Eg 6 lbs of boost at 15hp per =90hp or9 mph =approx 900rpm

try the 3.42s at 30 lbs of boost before specing gears is all i'm suggesting.

The way i understand his first post is: he purchased a ford 9 inch with a 3.7 gear already in it. That is why I recomend him to run that gear. Simply because he already owns it.

I do agree with your statement. Everything you said I agree with. I think the 3.42 will take him into the 9's if he can support the boost level he is targeting. Then he may need to switch gears to get deaper into the 9's and keep the RPM's down where he wants them.

Testing proves everything though.

in my opinion a 3.23 gear will allow a car to hook easier than a 3.42 or 3.50 gear when coming out of the gate at a higher RPM and boost level because with that gear it is harder to spin the tires. This allows a powerfull car to rocket out of the gate by launching the car in its upper power zone and hanging there through out the 1/4 mile. It also loads the motor harder creating more exhaust velocity in my opinion. When it all coms togethor the turbo is working at creating boost as fast as possible. A car with less power may not be able to handle that gear ratio and bog coming out of the gate. Which obviously would ruin everything in the quest for a nine second pass.

Testing...testing .....testing proves what works for every combination. Running the 3.70 which he already has gives him more data to come up with a good decission.

 

[toofastforyou]

I tried some 3.73's and 4.10's and when I returned to the stock ratio (3.42), my car went quicker...I guess they're the best for these cars. I always wondered how the car would perform with 3.23's though...

Claude.