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.................-- GM 7.5 10 Bolt vs. GM 8.75 10 Bolt --..

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If you are looking for taller gears (lower numerical gear ratio) for a land speed car there are a few choices. With a Ford 9 inch 2.47 is as low as you will be able to go. There has been talk of a few lower ratios being made, but I've never found them after extensive searches and phone calls.

In the GM line 2.41 is it for most of the popular rearends that can handle more HP. The 10 bolt 7.5 rear though does have 2.28 and 2.14 gears and these rearends have been used with success. One example is Sparky used one in his low 900 HP na BBC to set records in the high 200's and low 300's. The car weighed a little over 3000 lb.. You can also find Torsen type posi's from late 90's Camaro's for these rearends at a resonable price new on ebay. The Torsen has a positive aspect to it in that it can transfer power from the wheel without traction to the one with traction if different ratios. I'm using one of these rearends in the lakester with the Torsen and 2.14 gears but doubt this car will ever have more than 900 HP if that.

Hooley's Stude has had a 2.47 nine inch Ford rearend and runs a G-Force 101A transmission that lets him run an overdrive 3rd gear as 4th gear with the old 4th (1 to 1) now 3rd. To keep the jump between 3rd and 4th in the 15% or less range it would be nice to put in a rear with the 2.14 or 2.28 ratio. Problem is the car is heavy, over 5000 lb., and with the twin turbos on the 572 BBC now has the potential of 1500+ HP levels. It is doubtful if the GM 7.5 is up to the HP and the weight.

One option we have been pursuing is a kind of odd-ball GM rearend that was made for the heavier GM cars in the 70's. It is the GM 8.75 that is in a 10 bolt housing but there again not the housing you normally see as a 10 bolt.

It looks like the one in the picture above with the inward scallops on the sides of the cover by the axle tubes. It came with 28 and 31 spline axles and the ring gear is held on with 12 bolts vs. the more normal 10 bolts for ring gears in other 10 Bolt rearends. It has a 1 1/4 inch pinion nut with 3 inch axle tubes.

The lowest ratio for this rearend is a 2.28 (no 2.14) but the good news is that it is a very strong rearend compared to the 7.5 10 bolt. The ring and pinion gears and the pinion shaft are all much larger than the 7.5 inch rearend. It looks like a very good choice for the Studebaker and probably a number of other cars that run over 275 mph but for a couple facts about it.

So what is the 'bad'? The rearend was never popular with the aftermarket, probably mainly because most are looking for a rearend with much lower gears for the dragstrip just the opposite the gearing requirements for our needs and there were other popular rearends, such as the 9 inch Ford, with those ratios. You can find some with a posi, but Eaton only made the posi for GM (not as an aftermarket) and talking to Eaton on the phone they no longer support the rearend so posi parts are going to have to probably be found used or NOS. Also I can not find anyone who made a spool for the rearend so unless you want to run it open or with a posi that might not be serviceable you are out of luck.

I've made a couple calls to see if anyone is interested in making a spool for the rearend. Mark Williams said they might be interested and said they could make axles and I have a lead with another manufacture that I haven't heard back from that might be able to make a spool. Ford axle ends can be welded on to get around the C-clip issue.

So, the reason I've posted all of this is to see if anyone else might be interested in this rearend and in a spool for one. I'm figuring that if someone was to make more than one spool the price would come down and they might be more interested in taking this on.

If you are interested please post here on landracing.com or send me a PM or if you are reading this on my site send me an email at:

( contact20 (at) purplesagetradingpost (dot) com

Also if anyone has any other info on this rearend I'd like to hear about it.

Below are pictures comparing 7.5 ring and pinion gears vs. the 8.75 and more info about the 8.75 can be found ( HERE )

As you can see there were a number of cars with these rearends so finding one is not that hard and I've seen posts on the internet where guys with 2.28 gears are trying to find lower gears to improve performance so the 2.28 gear sets are out there,

Sum

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