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Old 08-14-2011, 07:54 AM
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Budget rod: ax-15 with caddy 4.9

I've been looking into building something with junkyard parts - probably along the lines of an open wheel roadster with a self-built body.

I have a lot of experience with the Caddy 4.9 engine and it turns out, with the right bellhousing, it will bolt up to an AX-15 manual trans from a Dakota or Jeep. The trans is beefier than a T5 and should live fine behind the Caddy.

The engine has about 200HP @ 4000 rpm and 275 ftlbs of torque at 3000. I'm guessing the car will weigh about 2500-3000lbs. Redline's about 6000. The engine is very torquy at low RPMs - not like a huge engine, but it's not a high-rpm screamer.

The ratios on the AX-15 are:
1: 3.83
2: 2.33
3: 1.45
4: 1.00
5: 0.79

Probably run something like a 30-31" rear tire.

What would be the best rear end ratio to run with this combination as a fun driver/cruiser?

Is the gearing going to be useable or am I going to hate it? I'm willing to have something that's not perfect - this whole combo will cost me under $200, but if it's just awful to drive, then its not worth it. The first gear on this isn't as steep as some trucks, but it's still not really in car territory.

I haven't really played around with different manual trans ratios. Couldn't find any sort of estimator on the web that would crunch out these figures.

Any advice?

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Old 08-14-2011, 08:30 AM
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wheels up.

My roadster has 4.56 rear with about 2,4 first and will lift the front tires . with that low first you could probably use a 3.2 rear, plenty in first gear and still have a good cruze OD gear.
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Old 08-14-2011, 09:48 AM
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Thanks Timothale.

I was thinking a low numeric rear gear would help offset the low first.

Even though the cost of the parts is cheap, it's still a fair amount of work to build any rod. I'd hate to get it all together and find that either some of the gears are practically unuseable, or it's so darned boring to drive.

Seems like there ought to be a way to do some calculations or use some rules of thumb to give an idea of how a combo would perform before you put it together.


One thing I thought about was looking at the gear gaps.

For example, on the Ax-15, they work out to:

1-2: 1.5
2-3: .88
3-4: .45
4-5: .21

The T5 used on the TBI cars had the following ratios (those cars weighed in the 3100-3300lb range):
1: 2.75
2: 1.94
3: 1.34
4: 1.00
5: .74

so it's gaps were:

1-2: .81
2-3: .60
3-4: .34
4-5: .26

The 4.9 has a pretty broad torque curve, as I recall, being that it was designed to haul around some pretty heavy cars (around 3500/3600#).
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Old 08-14-2011, 10:11 AM
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To add a little more info...

the T5 that came behind the 2.8 V6 in an S-10 had the following ratios:

1: 4.03
2: 2.37
3: 1.49
4: 1.00
5: .73

But behind the 4.3 V6 was:

1: 3.50
2: 2.14
3: 1.39
4: 1.00
5: .73

The AX-15 falls between those two, but has better torque capacity.
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Old 08-15-2011, 09:25 AM
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The AX-15 is a good trans. The rule of thumb here at the shop is Axle Ratio x 1st Gear ratio should be in the 9-10 range depending on application. The heavier the vehicle or the desire for a deeper 1st, the closer to 10 we want to be. The problem with some 5 and 6 speeds is you have too much spread between the gears to be effective at times. Too low an axle ratio, and you can end up with a motor that is uncomfortable in OD, lugging, surging, and in worst case scenarios...doesn't make sufficient oil pressure in OD at high speeds and destroys itself (there was LOTS more wrong with this application...but it did happen).

Normally I would run a 3.42 or 3.55 and have a really fun 1st gear, and enough axle ratio to make up for the truck-trans RPM drop and still have enough Revs to make the OD useable. However, with your 31" tire I would suggest a 3.73
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Old 08-15-2011, 09:41 AM
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Thanks Autogear.

That's really helpful info. Much appreciated.
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Old 08-15-2011, 09:52 AM
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It may be good for you, depending on what you want to use this vehicle for; to find your donor engines cruise RPM (the original rpm at 60-65 with factory rims and tires in cruising gear); and try to dial in your axle ratio, with the OD and your tires...to put that little motor in its happy spot.
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Old 08-15-2011, 11:20 AM
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Good idea.

Going with the Eldorado TC in 92, the stock tires were 225/60r16, with 26.62" overall diameter.

The standard final drive ratio was 2.97:1 and the sport ratio was 3.33:1.

The 4t60e ratios were:

1: 2.92
2: 1.56
3: 1.00
4: 0.70

So, if I figured right, at 65 MPH the standard final drive gives a 1706 cruising rpm and the sport final drive gives a 1913 cruising rpm.

Running the same number through the Ax-15 with 31" tires gives me a 65mph engine rpm of 2110 with the 3.71, 1976 with the 3.55 and 1904 with the 3.42.

I'll have to check my figures when I have a little more time, but it looks like the 3.42 might be a good fit based on the high gear.

Multiplying that by the 3.83 first gives me 13 - would that be too far from 9-10?

3.08 and 3.23 were available in some S-10s (just picking a common rear axle). Not sure how common they are.

The 3.23 would put me at 1798 rpm at 65 and the 3.08 would put me at 1714. Both would be in the range of the two stock final drive options on the original car.

That would give me a first gear factor of 12.4 with the 3.23 and 11.8 with the 3.08.
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