3.55 or 3.80???

gwb16 · 02-08-2009, 10:39 PM

Having a moment of indecision here, I really can't decide between the 3.55 gears or the 3.80 gears for my II. I just picked up an 8" rear end out of a 77 II, it has a 3.00 gear right now. But it's coming out....period. I'm also adding the Traction Lok or Detroit Locker....some sort of posi unit. Might be a Strange unit.....still undecided on that also. At any rate, car isn't a daily driver, won't be a daily driver. It's a play toy. May take her to the track occasionally to test the 1/4 mile waters, nothing serious, just for fun kind of stuff. Soooooooo, help me decide...which one?

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Norm Peterson · 02-09-2009, 12:51 PM

3.80's with a T5/TKO/AOD?

T-lok if street manners mean anything at all.

Norm

gwb16 · 02-09-2009, 03:11 PM

Quote:

Originally Posted by Norm Peterson

3.80's with a T5/TKO/AOD?

T-lok if street manners mean anything at all.

Norm

Sorry about that, it's a C-4, 2400 stall converter.
A friend of mine told me today to forget about the Detroit Locker. He said the first time it "CLUNKED" on me that I'd pull over and call a tow truck - out of fear that I broke it....LOL! He said T-Lok - 100%. He also said 3.80's for fun but build it with a new 3rd member and keep the 3.00's for cruising to car show's for fuel economy....if I was traveling to any long distance shows. Good idea, but I think once I put the new gears in, I'm not going to want to switch gears out with any frequency. I most likely won't be going to any shows more than hour or two away anyway.

Norm Peterson · 02-09-2009, 05:03 PM

Quote:

Originally Posted by gwb16

He also said 3.80's for fun but build it with a new 3rd member and keep the 3.00's for cruising to car show's for fuel economy....if I was traveling to any long distance shows. Good idea, but I think once I put the new gears in, I'm not going to want to switch gears out with any frequency.

That's why I was hinting at running any transmission with an overdrive. Then you choose a gear for the fun part and let the OD take care of the relaxed cruise and fuel economy issues.

I retrofitted a Tremec 3550 with 3.73's into my '79 Chevy Malibu a few years back, just so you know I'm not just trying to spend lots of somebody else's money with no direct experience to base the suggestion on.

Norm

nba1341 · 02-10-2009, 05:02 PM

i have 3.80s with a c4

gas mileage blows i stay going 65-70 on the freeway now but its really fun around town

totallywiredracing · 02-10-2009, 09:26 PM

i voted for the 3.50:1 gears based on the 1:1 drive ratio of the c-4 as well as the small (typically 26.0 inch) tire diameter. i was running a c-6 with 26 inch tires and was going through the traps at 8200 rpm with a 4.56 gear. now i have a 3.89 9 inch with a detroit locker (which i don't really like) with the same 26 inch tires and am going through the traps at 7400 rpm. the locker is a solid unit but as mentioned elsewhere in these replies, it is noisy and you better be holding on if you are cruising at a highway speed and let off the gas then punch it. they get a bit squirrely.

.boB · 02-10-2009, 10:51 PM

A lot depends on the engine you built. If it's a real torque monster, you can use a numerically low gear, like a 3.50. Then you get good take offs and reasonable cruising rpm's. But, if the engine is pretty close to stock, you'll need all the help you can get from the higher numbered gear.

As for the diff, I use a Detroit Gear (Eaton) TrueTrak. It's a worm gear LSD. You never have to worry about wearing out clutches. Torque split is about perfect. If you break an axle, it won't force you into a hard turn - which would be a disaster on the track. I've been beating the c rap out of mine for over three years without any complaints.

gwb16 · 02-11-2009, 06:21 AM

I wouldn't call it a torque monster, although it's fairly stout, my builder told me 330 ft/lbs @ 4000 rpm's and should be about 300+ hp, haven't been on the dyno with it, so I haven't been able to verify that yet. Maybe this spring I'll do that....
+1 on the Detroit Locker comment. My friend told me that if I went with the Detroit Locker to hold on for dear life if I had to lift on the gas and call back on it to merge onto interstate. Told me that things could get "real interesting - real quick"....especially considering my car only weighs 2600 lbs....well 2800 with my fat ass in the drivers seat....LOL!
My tire diameter is almost 24" now but near term plans are to move that up a bit also. Always modifying! I wish that I had just sucked it up and put the T5 in it now, I'll have a winter project in the future for that!
Thanks for the input, I appreciate the comments and suggestions!
I think I'm going to go with the 3.80's, I want that "WOW factor" when I launch it from a traffic light, nobody expects a Mustang II to set the road on fire!

.boB · 02-11-2009, 02:01 PM

That sounds like a good choice, especially since you'll be raising your tire diameter. SHould be a good match with that combo.

rms8 · 02-14-2009, 11:25 PM

I have a 9", but have had a 3.89, 4.30, and now running a 3.50.

The single most IMPORTANT thing you should do is NOT randomly ask what everyones opinion is regarding the gear ratio.

If this is something you are really going to do, do NOT leave it to chance. Having a 3rd member set up is not a cheap thing (normally runs $350-500). Do your home work.

You MUST know what sort of transmission you will be running.
You MUST know the approximate size tire you will be running.
You must know the redline of the motor.

Take these givens and plug them into any one of the dozens of online gear ratio calculators. Then you will know for sure the exact gear you should look for.

Opinions are fine, but don't leave these sorts of things to chance.

When my motor was NA, I had the 3.89. I then changed the cam and increased the CR. This necessitated a new gear to keep the car at the correct RPM going through the traps. This is when I went to a 4.30. Then I installed a Paxton Supercharger and had to once again revisit the calculator to ensure I was installing the right gear for my combo. This is when I went to the 3.50. I have a Detroit Locker too. Extremely stout, but can get a little wiggly at times.

If this is a street car with less than 400hp stick with the LSD. If you plan on visiting the track with anything more than 400hp, you're better off with the locker due to it's durability. I had a LSD for the longest time, but it just could hold up any longer once I installed the Paxton.