3.64 vs 3.91 diff comparison please....

[Guest]

Hey all. The diff on my soon to be home 73 took a nosedive and I 've found a 3.64 and 3.91 (open) to get it home with. The car is well sorted, 4-speed and will be used as a weekend autocrosser/fun driver. Having never driven at cruising speed yet in it, will the 3.91 be screaming? Gas milage is of no concern. The highest regular hwy speed will be 60-70 mph(100-110 km/h). Thanks, Chris.

[winstontj]

Hey all. The diff on my soon to be home 73 took a nosedive and I 've found a 3.64 and 3.91 (open) to get it home with. The car is well sorted, 4-speed and will be used as a weekend autocrosser/fun driver. Having never driven at cruising speed yet in it, will the 3.91 be screaming? Gas milage is of no concern. The highest regular hwy speed will be 60-70 mph(100-110 km/h). Thanks, Chris.

There is an RPM calculator on the FAQ's under the diff section. That will give you an idea of the differences between the diffs...

Not that much of a difference. Roughly a 6.9% difference.

For autox I'd go with the 3.9 but also for your purposes I'd use whatever is the most direct bolt in application without the need to purchase or modify anything (spacers or CV joints). Maybe keep both?

[Mike87]

Since you mentioned that you will be autocrossing, using the 3.91 is the best choice as winstontj said. Yes, with the 4 speed, you will be running a little faster at highway cruising speeds, but if your motor is up to it, then it will be fine.

You will notice the difference between the 3.64 and the 3.91, especially coming out of corners and starting from a light (or start line).

I chose the 3.64 primarily for mileage. I do a lot of highway driving and have a 3.64 LSD. It still gets out of corners quick, just not as quick as a 3.91 or 4.11.

[TobyB]

If you're not planning on long interstate cruises,

you can live with the 3.91 just fine- it's more

fun in town, it helps on MOST autocross tracks

(unless it means you have to shift to 3rd where a 3.64

could stay in 2nd)

Personally, I think you should look at it as a prelude to

your five-speed conversion!

t

[chrome68]

If your going for fun & autox, I'd say go with the 3.91. High rpm & a 4.11 are the biggest reasons a basicaly stock 1600 can still show up modern cars.

[JohnN]

club set? Brian Moon (in the SoCal area) kicks major ass in AutoX with a 3:45 differential. His group sets long and high speed courses and with the 3:45 he says he almost never has to go for third gear. Yes the car is a little slower off the line, but a 2002 is a momentum, not a point and squirt car.

It doesn't hurt that he can drive the wheels off of most any vehicle, but give it some thought before you swap.

Cheers!

John N

[jerry]

i'm curious.

would the recommendations differ any if the gearbox is a close ratio 5speed?

i have a C/R 5spd and was thinking a 3.64 might be the better match thinking it'd go faster in 5th gear.

i'm not an auto-crosser type per se (yet, maybe).

i have yet to install the C/R gearbox, btw so it's all academic at this point.

[winstontj]

i'm curious.

would the recommendations differ any if the gearbox is a close ratio 5speed?

i have a C/R 5spd and was thinking a 3.64 might be the better match thinking it'd go faster in 5th gear.

i'm not an auto-crosser type per se (yet, maybe).

i have yet to install the C/R gearbox, btw so it's all academic at this point.

In a CR-5 first and fourth (EDIT: FIRST AND FIFTH)are the same as in the stock 4-speed. The only difference is the middle gears. With a CR-5 gearbox the RPM's don't drop as much when you upshift and when you downshift they don't go up as much - that' it. I have a 3.45 and would LOVE a CR-5 because for me the gaps between gears (esp 2 & 3) are killer. I don't auto-x but if I did I'd look for something else.

In a CR box it's my opinion that you can get away with a lower rear end (3.45 or 3.64) because you can keep the engine wound up easier. With the OD-5 you need the higher ratio rear ends (3.91 & 4.11) so the diff will keep the revvs up rather than the OD-5 box.

I'd say that with the CR-5 keep the 3.64 and you'll be happy - if you have a stock 4-speed now the ONLY difference will be that you'll shift more with the CR-5 because you have an extra gear jammed in there....

Good luck!

[Guest Anonymous]

I think WinstonJ meant to say that the first and fifth gears in a c/r 5 speed are the same as the first and fourth in a 4 speed, therefore a 3.64 or 3.45 diff will work well with them.

[winstontj]

I think WinstonJ meant to say that the first and fifth

yeah sorry....

[Guest Anonymous]

no problem, Winston, I make worse mistakes almost daily lol

[Beaner7102]

will the 3.91 be screaming? The highest regular hwy speed will be 60-70 mph(100-110 km/h). Thanks, Chris.

At 60mph (100km) with a 3.64 the revs will be at 3300rpm, with a 3.91 they'll be 3500rpm approx - Not screeming in my book - 4500rpm now that is highway screaming. Beaner7102