Hey all,

I'm strongly considering putting an a500 auto behind the slant in my swinger. The drivers and passenger floorpans were rotted and crappily repaired - the rest of the car is good, so I suspect rubber floor mats. So, I'll be cutting out sheet metal and putting new in; it'd be a good time to rework the trans tunnel & cross member for the larger OD trans.

Engine is a warmed up slant - 9:1, super six, mild port job. With the a500's .69 overdrive ratio, I'm thinking a 3.73 gear would be pretty good - give the slant some extra help in the lower gears, but keep the revs right near 2500 at 80mph (which is pretty regular on the freeways here in Atlanta). I'm thinking 4.11 would be just a little too much. Calculations done w/ a 26.7" tire.

Those of you with OD on your cars, what rear ratios are you running?

I ran 3.91s for quite a while behind my Slant/A500 and it was fine. 3.73s should be perfect. I have 3.23s in there now since I needed the 3.91s for my faster Dart, and the lower gears work fine too.

Lou

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Home of Slant6-powered fun machines since 1988

What is the start-off (1st gear) ratio on the A500?
Running the numbers in OD:
3.73 x .69 = 2.57
3.91 x .69 = 2.70
4.10 x .69 = 2.83
4.30 x .69 = 2.97

If first gear gets real low it becomes somewhat useless.
DD

I believe that the a500 got the 'wide ratio' gearing from the a999 trans - 2.74, 1.54, 1, then .69 OD. The a518 got the standard gearing from 904/727s - 2.45, 1.45, 1, .69.

I was figuring on the a500 both for the wide ratio (I figure it could only help a stockish /6) and the more efficient 904 base. 3.73 seems like a decent compromise on both the 1st and OD gear ratios. Given Lou's experience (and my own w/ a GearVendors on a 727), a 3.90 might not be bad, either.

I would take a look at your peak rpm and speed that you will be shifting at in each gear. If your engine peaks below 5000 rpm I would probably settle on something in the 3.4 to 3.5 range. With the lower first gear and good low to mid range torque it should like those gears. A lower rpm engine should be able to pull the overdrive on the highway without any lugging too. In any event I would stay at or below 3.73 unless you go to a taller tire.

I run a 4 barrel and 3.55's with the OD manual tranny and a set of 25.3" tires, and at 65mph I'm around 2300 rpm with a 3500 lb A-body, it gets OK mileage, but your auto tranny will have some slippage to deal with...if I had it to do over again with my 8.25" rear I'd go with either of the above ratios...(a 2.59 final OD ratio is kind of "granny" at 65...)

With your current tranny and a set of 2.76's in "Direct" is about 2250 rpms (plus or minus tranny slippage, and what carb/cam you are running...FYI a 3000 lb Valiant with a 1 barrel Holley and A-904 w/ 2.94's belts out 65 mph at about 2700 rpm due to restrictions in the exhaust and intake and some slippage in the stock old tranny).

The possibility exists with the "better" super six setup and some careful planning, you can get about 2000 rpm in OD with 3.55's, 2100 rpm with 3.73's, or about 2200 with 3.91's at about 60 mph (this will all vary by tire size, carb, cam, vehicle weight/load/etc...).

When you do make the change, if you have an 8.25" or a 9.25", I'd also plunk for a sure grip diff while in there (better than adding is much later down the road, and can help make nice uniform parallel stripes when conducting traction and rubber testing...)

Good luck on your decision, remember it's your car and you can always change it later,

-D.Idiot

I'll definately be using a sure grip in an 8 1/4 housing for this. They're just fun, and I've got one already.

How low of a first gear is 'too low'?

Also, since the (stock) /6 torque peak is at a diesel-like low rpm, I'm not too worried about lugging the engine. The motor has a pretty mild cam - the Comp 252H, so I expect it to remain pretty torquey.

The factory used a 10 to 1 start-off ratio for manual shift cars and 7 or 8 to 1 start-off ratio for automatics with some stall in the converter.

As a point of ref. think of a 3000 lb. A -Body with:
2.93 rear gears and 2.45 auto trans first gear ratio = 7.17 start-off multiplication.
The converter stall is rated at 1800 RPM and this combo is a little sluggish off the line.

3.23 with 2.45 low = 7.91 s.-off, this has a "crisp" off the line feel.

3.23 w/ 2.74 low = 8.85
3.55 w/ 2.45 low = 8.70
These combos get off the line quick and don't give much MPH before shifting to 2nd.

3.73 w/ 2.74 low = 10.22, this is getting pretty low for a light auto trans car with some converter.
Figure-out your current combo for a good point of ref.
May-be some others can comment on how they feel about the start-off gearing in their car.
DD

I have 3.23s in my car. With a comp 264 and 9.25 CR I find myself in 4th instaed of OD a lot because 60mph is too slow to be on the torque band in 5th. Ill prolly go to 3.55 or 3.73 but my 1st gear will be a stump puller.

The car actually suited the gears better when the engine was stock.

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Yeah....Im the one who destroyed this rare, vintage automobile.....

So it sounds like an 'ideal' combo would be a 518 gearset with a numerically higher OD (around .75 or so) and around a 3.40 gearset in the diff - keep the 1st gear ratio in the 8:1 range, and also keep from lugging the engine in overdrive. Alas, I don't have the tooling to try something like this.

I found a neat little tool on the 'net. I've populated it with the a518 and a500 ratios, an assumed shift point, gears and tire size. Top Speed Per Gear.

I think I'll lean towards a 3.55 rear end ratio with a a500, even though this puts the 1st gear pretty low. The whole project hinges on tracking down that /6-to-small block transmission adapter, though. I'm not cutting anything till I can track one of those down.