depends on tranny your running and the last gear ratio.
www.smokemup.com has auto math where you can check your gear ratio with tire size and tranny that you want to use.
if you need a login - pm me
www.smokemup.com has auto math where you can check your gear ratio with tire size and tranny that you want to use.
if you need a login - pm me
TOM B said:
Give me what trans you have and your rear tire size and I'll make you a chart like this one
http://www.2mfffabshop.com/mycar.htm
Since you ask, I'll give you a little math. It is not exact because it sounds like you have a C4 automatic which does not lock up when cruising. Sorry if you are already aware of the following.
Your rpm will change by the ratio of the ratios - 3.8/2.8 = 1.375 so your rpm's will increase by 37.5%. If you're pulling 2000 rpm at 60 now, with the 3.8 you'll turn 2000*1.375=2700 rpm. That is quite a jump and fuel mileage and engine wear will increase. Now if you put in an over drive transmission with about a 0.67 final drive gear, your rpm's will go 0.67*2700=1800 rpm and you will still have the acceleration advantage of the 3.8 gear!
With the mods you have done to the engine, I'm suprised you want to stay with the 8" rear. Remember, Ford only used the 8" for up to about 220 hp with the C code engine, a car ordered with the K code, 271 hp, got the 9". So Ford apparently thought the 8" was only good for something less that 271 hp, and that was with skinny tires that did not have the grip of today's rubber. I know many feel the 8" is OK for up to 400 hp, but it shoulds like you have way more than that. I agree with brianj5600 - go to the 9".
Your rpm will change by the ratio of the ratios - 3.8/2.8 = 1.375 so your rpm's will increase by 37.5%. If you're pulling 2000 rpm at 60 now, with the 3.8 you'll turn 2000*1.375=2700 rpm. That is quite a jump and fuel mileage and engine wear will increase. Now if you put in an over drive transmission with about a 0.67 final drive gear, your rpm's will go 0.67*2700=1800 rpm and you will still have the acceleration advantage of the 3.8 gear!
With the mods you have done to the engine, I'm suprised you want to stay with the 8" rear. Remember, Ford only used the 8" for up to about 220 hp with the C code engine, a car ordered with the K code, 271 hp, got the 9". So Ford apparently thought the 8" was only good for something less that 271 hp, and that was with skinny tires that did not have the grip of today's rubber. I know many feel the 8" is OK for up to 400 hp, but it shoulds like you have way more than that. I agree with brianj5600 - go to the 9".
How do you like the holley comander system. I was going to get that or the F.A.S.T. but ended up with the Tweecer. I haven't checked pricing on the commander what are they running at?
I'd stick with a 3.25 rear with the blower you'll get up too high and too quick in the rpms with anything more.
But if your engine can scream till 7500 rpm then go for higher.
I'm going with 3.55 with a wide ratio tranny because I know the engine can got 7k no problem and still make power with the 2.08 valves.
I'd stick with a 3.25 rear with the blower you'll get up too high and too quick in the rpms with anything more.
But if your engine can scream till 7500 rpm then go for higher.
I'm going with 3.55 with a wide ratio tranny because I know the engine can got 7k no problem and still make power with the 2.08 valves.
TOM B said:
Here some charts I already had
http://www.2mfffabshop.com/gr/c4_280_2356014.htm
http://www.2mfffabshop.com/gr/c4_325_2356014.htm
http://www.2mfffabshop.com/gr/c4_355_2356014.htm