aaron7

iTrader: (7)

Hey guys, little overwhelmed with reading other threads and online articles so wanted to put my thoughts here and see what you think!

Wondering if the 3.42 gears I have in my Firebird are optimal for the short range daily driving I do. Car never sees highway and I'm looking for the best off-the-line acceleration I can get with this n/a V6. Don't care about gas mileage changes. I'd like a posi rear anyways so it's swap time!

Running an 87 T5 (1352-181): REV 3.76, 1st 4.03, 2nd 2.37, 3rd 1.49, 4th 1.00, 5th 0.76

245/50/16 tires; 25.6" diameter.

I know engine torque has a big play on gearing as well and everyone is discussing big power V8's.

Using this site: linky it came out to a 2.89 ... ?





That's as far as I've gotten. Thoughts?

big gear head

If you are never on the highway and want a V6 to feel strong then a 4.10 might work for you. Also look into the Eaton Posi, Yukon Dura Grip or Eaton Truetrac. Don't bother with the Auburn or Governor Lock. They are junk. You will also need a good installation kit. Tom's Differentials has the best kits, and US strange gears. He is also a Eaton distributor and has good prices on their products.

MoJoe

iTrader: (5)

I typically look at 1st gear x rear ratio. The "performance" gear offerings from the factory end up in the 10:1 range... Manual = 2.95 x 3.42 = 10.0, Auto = 3.08 x 3.23 = 9.94. The "standard" 2.77 rear gear factors out to 8.2 and 8.53 respectively... At least when applying to a V8's greater torque.

Your T5's 4.03 1st gear gives you a big boost already. It factors to x2.77 = 11.16, x3.42 = 13.78, x4.10 = 16.52.

Find a gear/tire/rpm/speed calculator and plug your redline (5000?) in with these numbers to see where you have to shift out of FIRST gear. Also plug in the RPM of your average shift (3000?). What is the MPH for both of those? With 4.10s and the 16.5:1 first ratio, do you really want to shift out of 1st gear at a ridiculous low speed?

I guess the point I am driving: Your V6 has the performance boost of a 4:1 first gear. The 3.23-3:43 might be snappy enough, and the 4.10 too much - a chore to drive from the other end of the argument.

sofakingdom

iTrader: (1)

Problem probably starts here:

245/50-16 tires, 25.6" tire diameter

Don't outsmart yourself. Go pull a measuring tape from the center of a rear wheel to the ground. I'm almost (but not quite, being a statistics professor) willing to bet that the "diameter" you're putting into your calculator is WRONG.

Diameter = 2 times what the tape measure reads.

Then, use something accurate for RPMs instead of the wavy-pointer trim package thing in the dash.

Put those NEW REAL REALITY ACTUAL TRUE numbers into a "calculator" and see what you get. Just like you don't have to worry about checking all the calculators on the shelf at the office supply place to make sure you pick up one that says that 2+2=4.

big gear head

Don't know what calculator you used, but it's WAY off. A 4.56 with your tires and transmission only puts you around 3100 at 65 MPH.
https://www.ringpinion.com/calculators/Calc_RPM.aspx

I hadn't noticed your 4.03 first gear. A 4.10 or 4.56 would make 1st gear almost useless. A 3.42 would be much better. Be careful with that 4.03 transmission. They are not very strong.

aaron7

iTrader: (7)

Thanks for the replies guys, got me headed in the right direction now.


I thought about the 4.10, but was worried about losing 1st gear as it has such a high ratio already! As for the posi unit, for now, I'll just be finding a good lower-mileage stocker in the ratio I decide on and refreshing it.

Ok, that is very helpful and confirms what I was worried about if I ran the 4.10 gears. Redline is 5500 and when getting on it (usual driving :P) I shift around 4500.

That said, I used those numbers and got very wrong numbers out of the calculator (like a 64.1 ratio...). Here's the link, maybe I'm just missing something here. http://hotrodworks.net/hotrodmath/quarter.html

And I guess this is why they stuck the V6 with the 3.42 rear from the factory! Just thought I could gain some acceleration at the same time as swapping in a posi rear. If I have to find a posi 3.42, so be it!

Ruler says 24" nearly on the dot. So yes, off 1.6" doesn't help with calculations.




Thanks for the link, good to have another tool to try. Seems I should stick with the 3.42 ratio I already have and just find a posi version to refurb & swap in!

As for the strength, if you're referring to the V6 transmission being weaker than the V8, that's been proven to be semi-false. The fault is the high 1st gear ratio paired with the torque of the V8; just too much stress. But the casing & gears are the same strength as the V8 variants.

big gear head

I've broke 5 of those transmissions with my 3.4 V6. The 4.03 first gear ratio makes the transmission weaker than other T5 transmissions. It's due to the size of the input gear, not the strength of the case or anything else. Any manual transmission gets weaker as the first gear ratio gets numerically higher. A 2.20 first gear is much stronger than a 4.03. Same thing goes for rear end gears. A 2.73 is stronger than a 4.56 due to the size of the pinion gear head.

Vanilla Ice

4.10s
4.33s
Something similar.

If you want low speed "max" performance and don't care about higher speed there's no reason not to go with 4.10s.

If you are concerned with the track then there's quite a bit more to take into account. Only concern is a short 1st gear. It's quite obnoxious shifting before you even get out of an intersection.

aaron7

iTrader: (7)

There's a guy with a 3:23 disc posi from an 87 Iroc that had 67k on it for a hundred bucks. Think I'll grab that and put some 4.10's in it.

With the 2.37 2nd, maybe I'll just start out in that! haha

jmd

iTrader: (4)

Those ratios would make me stab my eyeballs out with the 1-2 drop.

Changing from 3.42 to 4.10 would make it worse.

You definitely will start in 2nd gear.

Unfortunately, don't know of a parts mixture to build a hybrid GM WC T5 with 3.3x or 3.5x 1st gear. It's 3.7x first or 2.95 first; nothing in between.

sofakingdom

iTrader: (1)

4.10s is too much for a T-5...

Great for a T-56 though.

With a motor with good power, you'll be out of 1st gear in less than a second.

Rule of thumb is, you want your final drive ratio to be somewhere 10 ± 1 kind of range in 1st gear. There's nothing empirical about it; just kinda a rule of thumb. Of course tire size figures into it as well which further muddies the picture, but with the small tires these cars have, you want to keep from going too far off the high end. With a 2.95 1st, 3.73 rear gears put you at almost exactly 11 (2.95 x 3.73); perfect. 4.10s put you at over 12; a bit too short but possibly OK if you're willing to accept the amount of shifting you'll be doing while just driving around, and the constant high RPMs.

The ratios you gave sound like a 6-cyl transmission. You will HATE that thing. Besides just the extreme low 1st, the ratios are ALL real wide spread. Yuo'll have the feel of the 700 1st - 2nd shift in EVERY gear. I'd STRONGLY suggest putting the rear in as-is and just see how you like it. With that, and 4.10 in the rear, you'll end up with over a 16:1 final drive ratio; which renders 1st gear all but useless.

3.42 is a pretty good gear for most uses.