Pick the gear

[Glensgages]

mr mctavish:

I'm just curious, but why make the change to 'only' 3.31s if you are also going to an OD transmission?

I'm not 'up' too-much on the aftermarket 5/6-speed transmissions offered, but I'm under the impression that the 1st gear is usually a bit deeper than the GM-transmissions, and that 5th/6th gear is well under 1:1, often .7/.6:1.

If the is true, 3.31s coupled to a .7:1 OD, would be 2.31 final-drive ratio, a full 17% taller than what you have now.

I'd think that 3.55s (2.48 final-drive), or even 3.73s (2.62 final-drive) would offer more 'spirited' performance, and yet deliver a 'taller' final-drive.

Would the lower 1st gear of the TKO, coupled with the 3.55/3.73 be 'too-much'?

Do you really NEED 17% more gear-ratio in every-day driving?

(Please bear with me: I'm just a bracket-racing, THM400-jamming fool...)

 

[mr mctavish]

Glensgages ------ I'm listening. [reading] Trying to learn. Perhaps your right.
I'm spooked about going too drastic, cause I like my top end. Downright scary.
Its a 383 with enough tourqe to make a higher rear work. but..... the 2.73 takes it out of the range. it starts to really pull a 3500. Im open to suggestions.
I learned alot just from reading this thread and your comments - thanks

 

[Glensgages]

I learned alot just from reading this thread...

You and me, both!!!

"Assuming" you ran 13.99, you gotta be close to 100 MPH;
with 2.72 gears, and a 27" tall tire (?), and factoring no TC-slippage (you run a stick), I'm 'guesstimating' that you are turning just 3360 RPM in 4th, or 4470 in 3rd gear.
Am I close?
:confused

Again, "assuming" that your 383" can pull to 5500 RPM in 4th gear, with your current 2.72:1 gears and 27" tall tires, I'm guessing your Vette would 'mathematically' nose-over at 163+ MPH.
:eek

(YOU know, and I know, that it probably WON'T hit 163 MPH, but just for this duscussion, we'll assume that it 'could'...)

With 3.31s and .7:1 OD, 5500 would 'mathematically-be' 192+ MPH;
3.55s w/ .7 OD would be 179+;
and 3.73s are 170+ (3.73 x .7 = 2.61, which is still 'longer-legged' than the present 2.72s)


Do you drive the pace-car at Talladega?:BOW

I guess my question is:

Fact #1:
You've got a car that runs 13's with 2.72 gears, and 'mathematically-could' run 160+ MPH at 'just' 5500 RPM 'as-is'...
Fact #2:
Adding with 3.31s and an aftermarket 5-speed, with OD being .7:1, may do 2 things:
(1) - with a 1st gear ratio of 3+:1 in the tranny, traction may be extremely-limited, and
(2) - the 2.32 (3.31 x .7 = 2.32) final-drive would offer even MORE top-end, or possibly (PROBABLY?) LESS chance to use 5th gear.....

Why-not consider swapping to 3.31s with the current transmission?
:confused

* By my calculations, 5500 in 4th would still be 134+ MPH (how much 'top-end' do you need?);
* the over-all 1st gear-ratio wouldn't be so deep that it'd over-power your tires (2.88 x 3.31 = 9.532) on the launch;
* the RPM-drop won't be as severe as the usually-wider gear-set of the aftermarket/OD transmissions (keeping the motor in it's power-band);
* one-less gear-shift to (possibly) miss, sending parts going in every direction at the strip;
* and you'd get to 'use-up' all the torque & power that 383" can make in the first 1320'...

Just a few idle ramblings.....

 

[mr mctavish]

 

[Glensgages]

mr mctavish:

Typical of a car needing gears, your car really runs once it gets going:
in-fact, you run the last 320' only .05-slower than my 12-second Z28.

Here is a comparison vs. my otherwise-stock '82 (with 3.73s) in Italics, and my Z28 in Bold:

0060': 2.118 / 1.851
0330': 6.157 / 5.319
0660': 9.562 / 8.216
MPH : 71.58 / 84.73
1000: 12.519 / 10.723
1320': 15.004 / 12.850
MPH : 90.89 / 104.76

I don't think you'll need a whole lot of gear to run mid-13s (or lower);
an old racing buddy had a '70 Camaro with a fairly-mild 9.5:1 406" SBC he'd drive 250 miles to the races in Reading, Pa., bolt-on stickie street-legal tires (26" x 8"), run 13-teens @ 98-100 (13.006 best) thru the full exhaust, swap tires again, and drive back, and he did this with 3.08 gears!!!

It's literally amazing what those long-arm SBC can do;
I shoulda known it when you said "13.99 with 2.72 gears..."

Having realized that you 'run-an-arm', I'd think that 3.31s will provide you enough 'grunt' to get it off the line quicker w/o hurting your top-end/mileage concerns.

If you go-by the time-honored theory of ".01 in the 60' equals .02 by the finish-line", and "assuming" that you can 60' on whatever tires you're running 'as-quick' (relative-term) as my CFI does on 'hard' 255/60R15 BFGs (2.118 vs. your 2.309), and with deeper gearing you should, you'd make-up almost .4-second in ET alone, based on your 60' times being quicker by .191-seconds.

Once you get that big ol' 383" rumbling, you might make-up another tenth-or-two:
13-seventies is almost a 'given'.

Your thoughts?

 

[mr mctavish]

It looks like I hijacked the thread- sorry. Im still paying for the eng. when its paid, I'll go for the rear. when its paid, the tko. until then I'm gathering information. I copied everything you posted for future reference when ready. Thanks for sharing info.
since were chating ,I stared another post about it breaking up at the moment.
its the 4g rpm one.

 

[81 Corvette]

Thanks for the info, and don't worry about hijacking the thread lol

I think I've decided to go with the 3.54. I ran the math and the 3.73 is just too much of a jump.

Engine RPM Rear Gear Gear Change vs. Stock % Change Tire RPM MPH
3,249 3.73 0.86 30.0% 871 70.0
3,084 3.54 0.67 23.3% 871 70.0
2,883 3.31 0.44 15.3% 871 70.0
2,500 2.87 871 70.0

Switching ot the 3.73 would be a 30% jump in rpm's at any given speed. I don't do a ton of highway driving, but when i do, i like to cruise at 75 and w/ a 3.54 I'll be pushing it, let alone the 3.73.

Thanks everyone for the input.

 

[Glensgages]

81 Corvette:
Great!!!

Keep us posted on your results!!!

 

[81 Corvette]

whooo hooo, goes in Thursday morning for the swap....hopefully back by saturday

 

[mr mctavish]

Waiting and watching