I have a 1988 Iroc 305 TPI and am debating switching the rear axle gearing for more torque. If I do this will I need to get my ecm changed bc the transmission is an automatic? Is this a good way to get more torque or does it not work out as a win in the $=performance area?

 

ANYONE OUT THERE?????????

 

No, the ECM has nothing to do with shifting the transmission.


Can't think of an easier way to drastically improve rear-wheel torque.

 

If you're keeping the TPI, then no more than a 3.45:1 gear.
First determine what gear you're starting with. If you already have a 3.23:1 or 3.27:1, then the swap to a 3.42:1 or 3.45:1 isn't worth it, and you won't feel it. If you have 2.73:1 or 2.77:1, then definitely do the gear swap.

 

I am 99% sure it has the 2.73 however I have not opened the rear end just yet. Also I was thinking more along the lines of 4.10 gearing... so my next question is why would you not go over a 3.45:1 ?

PS.. Thanks for replies this is helping me a lot!!!

 

If you change the gearing you'll need to change the speed sensor gear to keep the speedo accurate and the ecm fed with the correct speed. You can figure out your current ratio by jacking up the rear and turning one wheel and counting the prop shaft revolutions.

 

If you go 4.10, you'll constantly be in and out of overdrive while city driving.
I have my TPI/700R4 in my Mazda pickup with a 3.909 gear... I can't wait to put my 3.23 rear in it.
BTW, 3000 rpm's at 80 miles per hour is where the 3.909's put me on the freeway... Waaaay out of torque range of the engine.

 

4.10 is a little much for a 700R4 and 26" tall tires. 3.73 is a better option. If you put some 28" tall tires on the car then 4.10 would be better.

 

Like stated by Apeiron the computer does not shift 700-R4s.

Do you have a 10 bolt or a 9 bolt? You can count the bolts on the cover of the rear end to find out or look for a large rubber plug on the cover. If you see a plug you more than likely have a 9 bolt.

9 bolts are 7.75" ring gear while 10 bolts are 7.5". They don't use the same gear sets or internal parts.

10 bolt gear sets are all over. 9 bolts are slightly harder to come by.

If it's a 2.77 9 bolt you have a series 2 carrier and you'll need a different carrier (thing the ring gear bolts onto and the axles fit into) to upgrade gears.

I can keep on rambling but find out if you have a 10 bolt or 9 bolt and which gears you have before you jump in buying stuff else you may have some returns in your future.

 

It is a 10 bolt rearend..... the only way I know how to check the gear ratio is by pullling the cover and reading it on the ring gear..... how do you do the tire/driveshaft rotation thing to find out the ratio.... please explain step by step.
According to the tech listings on this site it either has the stock 2.77 gears or the g92 optio n 3.23 gears

 

I meant 2.73gears not 2.77 sorry

 

Jack up the rear and put a chalk mark on the prop shaft and one tire so you can see when each rotates one revolution. Line up the chalk marks with an easy reference point. Turn the wheel one full rev or until the prop shaft has turned a full number of revs. I find it easier to keep turning until the wheel and prop shaft marks both end up exactly where they started. eg mine does 21 prop shaft revs for 13 wheel revs. Divide 13 by two as it's an open diff then divide 21 by 6.5 to get 3.23 gear ratio. If it's posi you don't need the first div by 2.

 

With TPI, the length of the intake runners will limit your useful RPM range, whatever cam you choose. TPI just doesn't have the RPM capabilities for 4.10:1 gears. A 4.10:1 is something you might use with an engine that pulls past 6000 RPM. You might get your TPI to happily spin 5500 in first, but rally, it's a 5000 RPM proposal. I know, I did lots of mods to my '86 IROC-Z before swapping to a carbureted 350.