Transmission Recommendation for a 318
Proud owner of a '68 convertible project car and a stock /6 that we are going to replace with a v8. Hoping people have some thoughts and insights to share regarding an automatic transmission with overdrive to pair with it (opinions welcomed too). Would like to spin the tires once-in-a-while, but trying to keep the highway cruising RPM's down.
For your stated useage, I think you have no choice but the A500/A518 series, with the .69 overdrive.
Here's why;
If you want to spin tires with a 318, you are kindof looking at a starter gear of 8.0 or greater, depending on your stall. Your stall rpm will depend on your quest for fuel-economy.
I know from experience that a lo-compression 318, a 2800TC and 3.23s will spin 245 tires. And if I can get the rpm up, right off the line, she will spin them a fair ways at WOT. But here's the thing;
3.23s and 2.45s will cruise at 65=2660@ zero-slip/ closer to 2750 on the tach. That will work together with the 2800TC, but at 2750, she will not pay back in fuel economy.
To get decent fuel economy, the rpm needs to get down closer to 2200 or less. But with a 904, that points to 2.67 gears, and your 318 will not even squawk those.
So with a 904 (2.45-1.45-1.00) you are gonna need to sacrifice one or the other.
Now the A999 has ratios of 2.74-1.54-1.00 and a Lock-up.
To get the 8.0 minimum starter gear you can use a rear of 2.94s which will get you 65=2420 in LU. That is a huge improvement over the previous 65=2750; being 13.6% lower, you can expect to get half of that back in fuel-economy so say 7%. That could be worth 1.4mpgs.
The A500 has those same ratios,namely 2.74-1.54-1.00, plus .69od and the LU.
But now you can run waay more starter gear than 8.0 and seriously smoke the tires and NOT have to go WOT to do it. And you can now set the cruise rpm to almost anything you want.
Lets work out one combo with 3.91s:
Your starter is 3.91 x 2.74=10.71, and you can now run 27/28" tall tires. With 27s your 65=2180 in LU. Over the A904/3.23s this is a reduction of 20.6% so an increase of 10.3% fuel economy could be expected; and that could be from 20, to 22 so 2.0mpgs. and because you have a LU TC, you can run whatever higher stall that you can get .
That starter gear of 10.71 is guaranteed to put a smile on your face, every time you take off. I had to run 4.30s with a 904, to get that, and with my 2800TC, that was serious fun.
There are only two downsides to running this A500.
1) yes you need to mod your tunnel. This is by now, no longer a pioneering endeavor because Cross-members are now available. Honestly it's not that big a deal anymore.
2) the gear splits on the A500 are quite a bit wider than on the A904. With 3.91s, this should not be a problem. The factory 5.2Ms had a cam with, IIRC an LSA of 111, and the injected ones had that marvelous long-ram intake, so between them, it covered the split. But when you carburate a 5.2M you lose that intake and along with it, the torque curve. So that is something to think about. Of course the 318 never had a long-ram,lol.
Now, I can tell you what I would do to make this a whole bunch less complicated. Just run a hi-torque-360 with any trans, and 2.94/2.76s ; problem solved.
