Need help with the math for a setup...

[rch10007]

Ok guys, I don't know how to ask for exactly what I need so I will lay it all out there and hopefully you can help.

 

I own a 1998 Ford F150 (4.6). It currently has a 8.8" 3.08 diff. The rear axle uses drum brakes. I want to go rear disc. I know there are conversion kits out there, but I thought I could replace the rear axle for about the same price.

 

If I replace the rear axle to get disc brakes, I should probably go ahead and upgrade the gear ratio? What do you think?

 

What size should I upgrade to? Would a 9.75" be better than the 8.8"? Should I go as high as 4.10 versus a 3.73 or even 3.55?

 

I do plan on future HP and TQ upgrades later but I want to get the suspension and drive train ready to take it.

 

Am I on the right track considering a 4.10 or even a 9.75? How much is too much?

 

I run at about 2K RPM at 78 MPH. I don't want to lose too much highway speed but having more TQ at higway speed will help with hauling a boat this summer.

 

Thanks. I will have more questions later but this is a good start.

 

p.s. What math is involved when calculating what's the best ratio?

[Tinman]

http://www.csgnetwork.com/gearratcalc.html

[rch10007]

Thanks for the calculator, Tinman. Got any advice about whether you think the 9.75 would be better than the 8.8?

 

Also, my truck isn't a 4X4.

 

Thanks.

[Tinman]

I doubt it would be worth the added expense. What are your plans for the truck?

 

BTW, when converting to rear discs you will need a different proportioning valve and possibly a different master cylinder.

[rch10007]

The truck will be a daily driver mainly, but I would like to do some hauling with it also. Once I kill the 351 in my farm truck I want to build it and put it in the truck. I should be able to easily squeeze a mild 400 RWHP out of it and I want to know that my rear end can handle it. This would be nice to play around on track days.

 

The 4R70w tranny should be good enough to handle it.

 

I do understand I will need a new master cylinder - I need one anyways!

 

Since the rear drum is anti-lock now, how would I convert the front (disc) to anti-lock to match the new rear disc. Should I try to pull a whole brake setup out of a junkyard to go with the axle swap?

 

Thanks.

[Mensan]

The 4R70w tranny should be good enough to handle it.

 

 

No sir.

 

The 4r70w is weak, it will break. The 351 will never bolt up to it, you would need an older AODE. If an 8.8 is strong enough for high HP drag cars, your truck probably won't hurt it. 4.10 is a great gear for the 4.6.

[rch10007]

Thanks for pointing that out Mensan. I can use the tranny out of the farm truck. It's an AODE.

 

I have read that many Mustangs use the 8.8. I wasn't sure if there was benefit to using the 9.75 since it is a truck and a bit heavy, not to mention I do pull a trailer from time to time.

 

I think I will certainly consider the disc conversion and new 4.10 gears. Ever done one of those conversions?

 

Thanks.

[Dave1647545494]

keep the rear drums and just put a set of gears in it

 

stay with a factory option gear for your truck and the dealer can flash the pcm to correct the speedo

[Guest Removed]

a hand held programer will take care of the speedo being off after the gear swap..

 

and why do you fell you need disc brakes in the rear?

[rch10007]

and why do you fell you need disc brakes in the rear?

 

I plan on doing an engine swap later when my farm truck that holds a 351W dies. In the mean time, I am starting at the back on this build up. That means the rear diff.

 

Right now, I don't think I will get the performance from a 8.8 3.08 as I would from say a 9" 31 spline T/L 3.90 once I put the built 351 in there.

 

There are some other factors in my decision also. I need to do a rear brake job and replace the seals right now. Then I have to add that freakin $30 a bottle fluid to the diff after I change the seals.

 

Instead of repairing the rear and since I need to consider gear change for the future, I am trying to decide the best course of action now. I am trying to minimize my expenses and increase the value of my decisions.

 

So, a future power increase will require a gear change. Instead of spending money on new drum brakes, why not swap the whole rear end, since I need taller gears anyways and while I'm swapping, why not upgrade to disc?

 

Does that make sense? I'm planning for the future while addressing today's needs.

[Guest Removed]

No sir.

 

The 4r70w is weak, it will break. .

I can say the same thing about aod, aode, 200r4, and the 700r4 but lentech is building them too be badass.johny lightings new ride has a lentech 4r70w in it with his GT 5.4 stroked and poked motor, with 74mm TT on it, plans on high 8's, and has already went 9.30 on a lazy tune.

[Guest Removed]

I plan on doing an engine swap later when my farm truck that holds a 351W dies. In the mean time, I am starting at the back on this build up. That means the rear diff.

 

Right now, I don't think I will get the performance from a 8.8 3.08 as I would from say a 9" 31 spline T/L 3.90 once I put the built 351 in there.

 

There are some other factors in my decision also. I need to do a rear brake job and replace the seals right now. Then I have to add that freakin $30 a bottle fluid to the diff after I change the seals.

 

Instead of repairing the rear and since I need to consider gear change for the future, I am trying to decide the best course of action now. I am trying to minimize my expenses and increase the value of my decisions.

 

So, a future power increase will require a gear change. Instead of spending money on new drum brakes, why not swap the whole rear end, since I need taller gears anyways and while I'm swapping, why not upgrade to disc?

 

Does that make sense? I'm planning for the future while addressing today's needs.

true but the junk yard rearend, may need all new seals and brakes aswell, and the 8.8 is strong as shit, just like the 9", you just cant swap gears in 30 mins like you can with a 9".8.8 some years had stock dics brakes, most older 9"s wont fit (too wide) then will need cut down, and finding a set of old lincoln rear discs is about the cheapest you can get on a drum to disc swap, on a 9". and normally people swap gears first to help apply the power too the ground better(going from 3.08 gears to a 3.9 or 4.11 is night and day difference)and it may be cheaper too stick with the drum brakes(way cheaper on parts)and the 8.8, now if you plan on tryying to putt down around 1000 or so HP then you might warrant the 9". the stone stock 8.8 in my car was putting up with all kinds of abuse from my BB., plus swaping too disc you'll need a new master and prop valve , and park brake cables etc.

p.s you dont nee 30 gear oil, just additive, (about 7 bucks or so) and the weak link in the truck 8.8 is the diff itself, not the whole rearend.

[rl]

No sir.

 

The 4r70w is weak, it will break. The 351 will never bolt up to it, you would need an older AODE. If an 8.8 is strong enough for high HP drag cars, your truck probably won't hurt it. 4.10 is a great gear for the 4.6.

 

4r70w's are not weak, the older ones need some updating, but they can be very strong (rediculously strong). He would need a different case with the 351 (a late model v6 case).

[rl]

I can say the same thing about aod, aode, 200r4, and the 700r4 but lentech is building them too be badass.johny lightings new ride has a lentech 4r70w in it with his GT 5.4 stroked and poked motor, with 74mm TT on it, plans on high 8's, and has already went 9.30 on a lazy tune.

 

 

It doesnt take a lentech 4r70w to be strong, a fully updated 4r70w with a jerry modded valve body and the right torque converter will handle 700+rwhp with ease.

[Tractor]

There are plenty of calculators out there for gears. Tire size and main function of the truck are also important in figuring gears. If your wanting to keep the RPMs stock then just follow the charts. As for the ford 9" rear end, in the 4x4 offroad world its not a very strong rear end and the bigger one is much better. especially once you start getting really low gear sets in them. For your application the rear end should be fine since your not putting 400HP to use trying to turn over 40" tires. Not sure on the tranny since I"m not really a ford kinda guy, but I know the AOD seems to be popular.

 

As for the disk brakes I don't know if I'd try to get anti lock set up on the rear on my own. I'd have to do alot of research on that one because you could do something that would cause a weird condition in your brakes and could be potentially dangerous. In the offroad world we usually disconnect abs systems as one of the first mods we do to our rigs.

 

Evan

[Guest Removed]

It doesnt take a lentech 4r70w to be strong, a fully updated 4r70w with a jerry modded valve body and the right torque converter will handle 700+rwhp with ease.

thats the point i was tryying to make, IMO the 4r70w is way better then any of the aods.

 

 

 

and heres a calculator http://wallaceracing.com/Calculators.htm

 

should still have the tires size ,gear ratio, engine rpm calculator, then all you need are the gear ratios from your trans so you can figure out where every gear would shift, or what rpm range you can get it at etc.

[Guest Removed]

The truck will be a daily driver mainly, but I would like to do some hauling with it also. Once I kill the 351 in my farm truck I want to build it and put it in the truck. I should be able to easily squeeze a mild 400 RWHP out of it and I want to know that my rear end can handle it. This would be nice to play around on track days.

 

The 4R70w tranny should be good enough to handle it.

 

I do understand I will need a new master cylinder - I need one anyways!

 

Since the rear drum is anti-lock now, how would I convert the front (disc) to anti-lock to match the new rear disc. Should I try to pull a whole brake setup out of a junkyard to go with the axle swap?

 

Thanks.

 

why?those systems arent designed for what you are tryying to do.

[verse]

My buddy here at work was reading this and said that the 8.8" rear out of an Explorer would bolt up perfect and for what you specificied you wanted to be able to do a 3.73 would be a better ratio for you to run.

[RangerTurbo]

My buddy here at work was reading this and said that the 8.8" rear out of an Explorer would bolt up perfect and for what you specificied you wanted to be able to do a 3.73 would be a better ratio for you to run.

 

Except the bolt pattern is different, its 5X4.5.

[copperhead]

My buddy here at work was reading this and said that the 8.8" rear out of an Explorer would bolt up perfect and for what you specificied you wanted to be able to do a 3.73 would be a better ratio for you to run.

 

Wouldn't that rear be more narrow than a fullsize truck rear?

 

To the threadstarter;

 

8.8 is plenty strong

 

trucks get most of their braking from the front, except when under load. You would be far better off with good trailer brakes that are properly set up

 

don't downgrade your truck by pulling the 4.6 out, except to replace it with a 5.4