Different gear ratio for our GTs

[jeff denton]

Thanks for the tip a few weeks ago about Specialty Gear, I forget who sent it but it seems to be promising. I e-mailed them and got a reply today, it reads: "Thanks for your inquiry. Yes, we can make you a 4.11 gear set, 1 set or a thousand! For competition we use 9310 material for strength and durability. I will need to see a sample set of gears, ratio makes no difference, before I can give you a final quote. Our production time is about 8 weeks. Glenn Sipe Gear and Machine Specialty Co. "
Cool, huh?
Question. Does anybody out there have an old un-needed gear set they can send to Glenn to check out? I will gladly pay you for your help and your name will be added to others in the "Thanks" area on the left rear quarter panel of Speedway GT.
Thank you!!

[Travis]

Sounds awful pricey. A few thousand maybe?

-Travis

[greensmurf20]

i have a rear axled i can get a ring and pinon out for you. its a earler style (70 and older) i dont know if this makes a difference or not. if you can use it i'll pull it.

[ConreroGT]

cool, I'm glad it worked out. I originaly found them in "Vintage Racecar" magazine.

I might put an order in with you then.

www.vintageracecar.com

~kyle

[jeff denton]

Thanks Kyle, it was you who turned me on to Specialty.
I hope the gears won't cost a few thousand, if so it would be years before I can order them...
Jared, don't pull apart a good rear end, I was hoping someone out there would have a set just laying around. I'm sure one will pop up, otherwise I can pull them out of my parts car, which is presently under a nice thick blanket of snow at zero degrees F.
Let's hope these gears will be reasonably priced, I would think that lots of GT owners will appreciate what some 4.11's would do the streetability of a hot Opel with a five speed!

[ConreroGT]

that was my next question...ARE the 4.11's good for the street?? Or are they a bit too high?

[greensmurf20]

i think the 4:11's would be too low for a 4 speed car, but with a getrag transmission, the 4:11's may be a good match with the overdrive. low gearing for acceleration and the overdrive to compensate.

[oldopelguy]

Do you just want to send them a regular Opel 4.125:1 rear to copy out of some stronger metal? I know I have at least one in my '67 with a 1.1, but I may actually have a NOS one too. I'll check tommorrow. I'm reasonably sure that it would be easier to make a stronger-metal copy than it would be to cut a new one from scratch, and you could run a 1.1 axle if you had to....

I love the 4.11 in my car, but the 1.1 really screams along at 65mph. It'd be rough to drive a 1.9 that hard.

I wonder if it would be possible to have the axle housing machined out to fit a GM 7.25 carrier? It wouldn't be easy or cheap, but it sounds like the gears won't be cheap either. Imagine the possibilities!

[namba209]

4.11:1 gears for street use? To give you an idea my 71 Dodge 1 ton van has a 4.10:1 "Trac-Loc" rear-end in it, up front is a maxed out 340 coupled to a 426 Hemi 727 auto tranny. At whatever speed you go divide by 2 and add 2 zeros, so basically at 70 MPH I'm turning 3500 RPM. Of course tire size also falls into this and I've got some tall tires they're 8.00 X 16.5, I don't know the circumference measurement, never really thought to take it. If I remember correctly the speedo only goes to 110 or 120, so the max RPM I can turn is 6000, well within the capability of the engine, but I'm not gonna say I've been there, done that. HTH.

[baz]

Originally Posted by greensmurf20 i think the 4:11's would be too low for a 4 speed car, but with a getrag transmission, the 4:11's may be a good match with the overdrive. low gearing for acceleration and the overdrive to compensate.

dont have the ratios for the getrag but using the gt's and a 5th i get these speeds on the number cruncher
those figs are 1ratio 2tyre width3tyre profile and rim diameter4diff ratio5revs after change 6 speedin kms 7 speed in mph

[GTJIM]

Originally Posted by oldopelguy Do you just want to send them a regular Opel 4.125:1 rear to copy out of some stronger metal? I know I have at least one in my '67 with a 1.1, but I may actually have a NOS one too. I'll check tommorrow. I'm reasonably sure that it would be easier to make a stronger-metal copy than it would be to cut a new one from scratch, and you could run a 1.1 axle if you had to.... I love the 4.11 in my car, but the 1.1 really screams along at 65mph. It'd be rough to drive a 1.9 that hard. I wonder if it would be possible to have the axle housing machined out to fit a GM 7.25 carrier? It wouldn't be easy or cheap, but it sounds like the gears won't be cheap either. Imagine the possibilities!

The 1.1 diff seems to be smaller thanthe 1.9 diff as parts are seperately listed for "93" and "94" serial number GTs. The Bearing shims for the 1.1 "93" are 46mm X 37.5mm and the ones for the 1.9 "94" are 50mm X 41mm. These are the shims for the diff case side bearings that hold the crownwheel & pinion carrier inside the housing. Thus the 4.11 gear set for the 1.1 motor GT would not fit into the 1.9 motor GT diff housing which has 3.44 gears.

The difference between the 4.11 gearing ( that won't fit) and the 3.9 gears from an Isuzu Impulse is so small that any difference could be compensated for by tyre diameter - particularly on the street. Racing may be a little different when trying to get the rev ranges through the gears "spot on"

THe GM 7.5" (and later 7.625" - which is very similar) conversion is starting to look more and more desirable due to the ease of availability for parts and a nice range of gear ratios. I will try to upload an article about the modification of a Rover Sailsbury diff for use in a Manta in the UK, which is a substantially similar idea.

[jeff denton]

I've run ideas thru the racetrack tech man who reminds me that the rules insist "stock production differential for car used. Any gear ratio allowed.
Carrier may be open, limited slip, aftermarket spool or welded spider gears."
I can't push my luck now. Swapping in the BMW 318 may still be my least illegal option, best option would be just getting the right gear ratio in the original rear end. He'll be curious how we got it fixed and will be inspecting it. Comparing it to what I don't know, GT's are kind of rare here!
4.11 gears seem to have always been the hot number for street rods, and
all my one ton work trucks use 4.10s. Like has been pointed out, tire size gets real important here!
Use BAZ's formula to figure it out, I've been to websites that give out formulas too. Somehow a year ago we did that and decided Speedway GT would be okay in second gear with stock 3.44 diff. Somehow it didn't work out that way, it was obvious in the very first test session we were not going fast enough at 7100 rpm... I wonder if tire slip is a factor I didn't consider?
Thanks, guys!

[oldopelguy]

Originally Posted by GTJIM THe GM 7.5" (and later 7.625" - which is very similar) conversion is starting to look more and more desirable due to the ease of availability for parts and a nice range of gear ratios. I will try to upload an article about the modification of a Rover Sailsbury diff for use in a Manta in the UK, which is a substantially similar idea.

What an interesting article, Jim, but not exactly what I had in mind. Would it be possible to machine out the inside of the Opel axle so that another carrier would fit inside? The casting is pretty thick, and a talented machinist might be able to work something.

I know once someone figured it out, we'd all be on the waiting list to get it done to ours too....

[GTJIM]

I do have a 7.65" Camaro Torsen LSD unit and a set of 3.42 gears here on the bench - never thought of fitting them into the GT case - now the "rats" are gnawing at my brain again ....... ghee - thanks!

[jeff denton]

Stephen's idea is what I've been pondering... Tomorrow I am going to bundle up and venture out into sub zero weather and get the diff out of Parts GT so I can disassemble it. This is territory I have not explored yet, I've never seen how the driveshaft/torque tube/pinion all work in an Opel. For all you know, some other kind of ring gear and pinion gear could be machined to fit the Opel case and couple to the stock driveshaft. The Salisbury option looks simple, but here again I don't clearly understand the whole torque tube situation. You are talking about a Chevrolet Camaro, the kind we have lots of even here in Montana, right? Did it ever get defined as per exactly what year of Camaro? I'm not up on Camaros since the sixties style, they had what we called a "ten bolt" diff.

[GTJIM]

Just arrived back from my workshop where I 'wrestled' with a couple of crownwheel & pinions.

This is the "good oil" : The late model Camaro/S10 diff carrier is 7.625" wide over the carrier bearings. The Opel diff is 6.5" - too big a difference for the larger Camaro carrier to fit into the Opel housing even with lots of machining and also the pinion cenre line is not the same either. The geometry between the centre of rotation of the crownwheel and that of the pinion is a critical dimension. Unfortunately quite different between the two types.
Also the axle splines are smaller in the Opel diff than in the Camaro/S10 diff.

Yes, the earlier Camaros did use a "10-bolt" diff but with 8.5" size - for the last 10 years or so the smaller 7.5" "10-bolt" has been used and recently the size has been increased to 7.625" but the same gears are used as in the 7.5" unit - they are not the same as the earlier 8.5" unit.

The interesting point is that the Opel diff has 10 bolts holding the rear cover on too so if the Camaro centre section was used it would look very much the same from behind (and that is where ALL your competitors will be; won't they?). However a steel torque tube would have to be made up to attach to the Camaro diff so it looked "standard" and a special set of axles made up too.

THe neat, street trick would be to use Isuzu Impulse axle tubes so that the spring cups and radius rod mountings would be there to fit the stock suspension - and the rear discs would be on the ends of them! Mind you the late Camaro (and S10 plus Firebird) 7.625" diffs often have discs and a Torsen limited slip unit stock from the assembly line ..... just got to keep those rats nibbbling at our brains

[jeff denton]

Thanks Jim, nice work. We'll rule out that possibility. About the center line of the pinion to the ring gear- are you saying the camaro pinion rides higher or lower on the ring gear than the Opel, or is it a side to side diference? I think you mean the former. Then the hypoids just aren't gonna mesh right...
We could scrutinize every make and model of differential in existence and find that little difference somewhere...
Using the entire center section is most feasible.
I'm still wondering just what that BMW 318 differential looks like. Supposedly the earlier ones are not IRS and bolt in to the GT. With strong 4.11 ratio...
Anybody got a BMW service manual that clearly shows the diff/axle/driveshaft both in the chassis and disassembled?
No auto dismantlers locally have any BMWs.

[RallyBob]

Originally Posted by jeff denton I'm still wondering just what that BMW 318 differential looks like. Supposedly the earlier ones are not IRS and bolt in to the GT. With strong 4.11 ratio...

Going way back to at least the '60's, from the BMW 1600, to the 2002, 320i, 318i, 325 etc.....they all have IRS rear diffs. That was one of their strong selling points.

From the point of view of stock appearances, I'd have to agree with going with something corporate GM. Probably an early S-10, since they're a bit narrower than the Camaro diffs. Swap the spring buckets and make a torque arm to replace the torque tube. Close enough....

[GTJIM]

Originally Posted by jeff denton Thanks Jim, nice work. We'll rule out that possibility. About the center line of the pinion to the ring gear- are you saying the camaro pinion rides higher or lower on the ring gear than the Opel, or is it a side to side diference? I think you mean the former. Then the hypoids just aren't gonna mesh right...

You are welcome Jeff, It was just wet and raining here - but quite warm. Couldn't bare to think of you under a car in the snow!

The Camaro crown wheel is bigger in diameter so the pinion ( while in a similar relationship to the crown wheel) is proportionally lower in the case so that it cannot line up with the Opel pinion bearing hole - the position of the crownwheel being fixed on the axle center line of course.

[baz]

jeff
a bit earlyer but the same system
theres also a thread about putting the bm axel in with pics and good info

[jeff denton]

Thanks guys, I got confused; I recall a post recently about the BMW rear being a bolt-in on the GT.
Am just now getting feeling back in my fingers enough to type. We have five inches of fresh snow, balmy 15 degrees out, and Parts GT is now minus a rear end. Now I see what a simple situation the torque tube/driveshaft/coupler/pinion is. After popping the cover and looking for the axle snap ring, I found there were none! Bonus, huh? The axles came out like a Ford rather than a Chevy. So, can these axle/bearing/retainers go into the earlier housing? And, just what are those shims for that I found inboard of the sealed bearing?
So, if the Chevette ring and pinion fit the Opel housing (they do, right?)
and I can locate one with 4.11 ratio, my problem is solved.
Or did I get confused again? Do I still need gears from Risse or custom built by Specialty to be legal and run the stock differential?
If the later bearings don't go in the early axle I'd have to cut the ends off all the tubes and do some grafting, which would be OK as I could weld them on a bit crooked/vertically offset and gain some rear wheel camber. (A super late model trick)
Sure is neat to have the advice of dedicated Opel experts. Thanks again!

[RallyBob]

So, can these axle/bearing/retainers go into the earlier housing?

Nope, the axle housings are different.

And, just what are those shims for that I found inboard of the sealed bearing?

They control the axle's lateral 'float' or endplay. If you leave them out completely, the axles will move side-to-side.

So, if the Chevette ring and pinion fit the Opel housing (they do, right?)
and I can locate one with 4.11 ratio, my problem is solved.

Nope, they are different. But, I know of numerous people who have grafted the complete Chevette rear axle into a circle-track Opel just for the gearing options (3.73 or 4.11). They are a bit smaller (weaker), but if you're welding the differential into a locker anyway, it will probably be fine unless you're pushing the HP levels a bit. The brakes are smaller too, but I imagine the Opel drums and backing plates could be grafted on rather easily.

If the later bearings don't go in the early axle I'd have to cut the ends off all the tubes and do some grafting, which would be OK as I could weld them on a bit crooked/vertically offset and gain some rear wheel camber. (A super late model trick)

I've tried the rear camber thing (about .75 degree negative) with our old circle track Ascona, and all it did is wear out the inner edges of the tires, and we didn't go any faster. And it wore wheel bearings quickly....

Haven't tried it in a GT chassis admittedly.

For some reason, we never even had to run a lot of stagger in an Opel, just 7/8" in front and 1 1/4" out back, and we had a welded diff. The Mustangs all ran 2.5" +. We found out a long time ago that a lot of the 'rules' or standard practices for tuning a roundy-round car didn't apply to our cars, but after a while we made up our own 'rules' which we stuck by.

Bob

[jeff denton]

Thanks, Bob. Interesting that the "textbook setup" didn't apply to an Opel!
My pit crew is gonna have to learn to speak German and wear chrome hats with a point on top. They still are shaking their heads about that front spring and the "Legere shackle" on the left side only.
We ran 2" rear stagger (like everybody else) on both test sessions and found tire heat to be even all across on the right side. Of course we were not quite up to speed and it was a chilly evening, so things may change...
Wish it was summer. Or we could run studded Hoosiers.

[baz]

Originally Posted by jeff denton Wish it was summer. Or we could run studded Hoosiers.

jeff have you been watching the ice racing?

[jeff denton]

Watch it? Hell, BAZ, this is Montana. I am IN it! My 412 cubic inch V8 powered S10 pickup is my daily driver this winter. Very squirrelly. Just an automatic upshift gets it sideways! Since half of english-speaking California moved here and bought new 4WD SUVs ya gotta forego common sense on the slippery streets to keep from getting lapped!