Gearing and drivetrain experts inside, please.

[ballistic69]

Here is a calculator I used to decide what rear gear ratio to use. You can enter tire size, gear ratios, max rpm and the calculator will give you mph at in all six gears.

My line of thought when I chose my gears (3.50) was if I want to run the 1/4 mile and use gears 1-4, use a calculator based on HP and weight of your car to estimate your trap speed. Play with gear selection until you have a trap speed in 4th gear about where you calculated your mph to be basted on your weight an HP.

You should be able to run in an "Open Road" race with 5th gear an reach drag limited top speed. The 6th gear is for good highway mileage.
(Doing 90mph and turning 1500rpm.)

http://www.geocities.com/z_design_st...ion_z28_6.html

Here is a link to more useful calculators, sign-up is FREE, 2 calculators require subscription to. Use the wind resistance calculator to determine Cd. It lists an '69 Camaro, so a '68 Cam should be close. Happy number crunching.
http://www.smokemup.com/auto_math/index.php

[Ralph LoGrasso]

Kirt,

Thanks for the reply. I found the exact calculator two days ago. It's pretty sweet! At this time, I'm looking at just running two seperate center sections, and swapping one for open road racing. Probably 4.10s on the Street/drag race, and then something like 3.08 for ORR. The racer that I talked to said he runs 3.08s, in his 4th gen, and he has run 197 before he was DQ'ed. So I'm thinking that the 3.08s will pull me to 185 hopefully, since I'm much worse off in the aero department. I'm still up in the air about what to do though.

How's your car coming along?!

[ballistic69]

Base on the "ideal" gearing:

Trans 1st gear X rear gear = 10
(Z= rear end ratio)
2.66 X Z = 10
Z= 3.7594 (This is the "ideal" best ratio for acceleration, remember tire diameter will affect this)

Your est HP & weight:
Vehicle Weight - 3200 (lbs)
Wheel HP - 600 (RWHP)
Trap Speed - 139.6 (mph @ end of 1/4 mi.)
E.T. - 10.17 (seconds @ end of 1/4 mi.)

For a 28" tall tire use 315/40/18.
I think you should use an 3.89 or 3.70 a 3.08 is way too tall.

My $0.02

[Ralph LoGrasso]

Kirt,

You're saying the 3.89 or 3.70 for 1/4 mile runs, or top speed runs? For top speed runs my tire is a 335/30/18 (26.1" tall), it is however 28" diameter for 1/4 mile duty.

[ballistic69]

For both. Also if you get a 345/35/18 tire, which I did since my slicks are 27.95", you won't have to worry about having to set your car up for 2 different tire hights since 345/35/18 are 27.5" tall.

[Ralph LoGrasso]

I wish I could fit those 345s! Unfortunately I don't have enough room. 335 is really the max width that you can go on a 67/68 without relocating the frame rails. I've mocked up my 335s, and they're pretty tight as it is, I don't think the 345s would fit. I agree, having two tires the same diameter or relatively close would make things much easier.

[RobM]

to me the need to swap out center sections kind of defeates the purpose of a 6 speed all together, because you could do the same with a 4 speed and have the same results! because basicly from what im understanding from this thread is the two sets of gears you would use would either make the first two gears usless or the top two gears usless. and that basicly gives you a 4 speed any way!

[Ralph LoGrasso]

Rob,

They won't really be useless. 1st and 2nd will be short with the 4.10s, but I'll still trap in 4th, and 5th/6th will be used for highway driving. A relatively high top speed will be attainable (155/160 or so).

With the 3.08s or 3.25s, all gears will be perfectly useful, and the car will reach top speed in 5th. The acceleration won't be as great as with the 4.10s, but it will still be plenty fast. 4th Gen LS1s have relatively tall rear gears from the factory, so the 3.08s will be pretty similar to a factory 4th gen setup.

[USAZR1]

Here are two examples that may or may not help you. On our 65 Corvette I can run about 180 with a 3:55 rear end, 600hp big block 2,800 pounds. On the 63 small block 430hp I can get up to 165 also with a 3:55 rear end before I have to break. We run 225/60/15 which comes out to something like 26 ½ This is at CMS just passed start & finish. Not brave enough to keep it pined going into the turn.
Both cars have stock trans in them

My 90 ZR-1 with 420rwhp LT5 would run 155mph in 4th gear(1:1 ratio)with 26" tall tires and 3.45 rear gear. Rpm at that speed was 7000.
To run 165 or 180mph with either of the above cars would require spinning their engines to around 8000rpm. I'm curious about your engine combos.

[USAZR1]

Rob,

They won't really be useless. 1st and 2nd will be short with the 4.10s, but I'll still trap in 4th, and 5th/6th will be used for highway driving. A relatively high top speed will be attainable (155/160 or so).

With the 3.08s or 3.25s, all gears will be perfectly useful, and the car will reach top speed in 5th. The acceleration won't be as great as with the 4.10s, but it will still be plenty fast. 4th Gen LS1s have relatively tall rear gears from the factory, so the 3.08s will be pretty similar to a factory 4th gen setup.

It's really hard to beat a T56/4.10 gear combo. Running a T56 with 3.08's or 3.25 cogs suck. I'm running a 3.25 cog in my Fab9 now and 6th gear is pretty tall,unless you like cruising at 80+mph.
4.10's or 4.30's will be going in the car,soon.

[Ralph LoGrasso]

It's really hard to beat a T56/4.10 gear combo. Running a T56 with 3.08's or 3.25 cogs suck. I'm running a 3.25 cog in my Fab9 now and 6th gear is pretty tall,unless you like cruising at 80+mph.
4.10's or 4.30's will be going in the car,soon.

Clint, I agree. I was doing some calculations to see what rpm I would be at with some of the taller gears, and it's pretty damn low. The 4.10s will be in the car 90% of the time. They will see the street and strip duty, the 3.08s or similar will only be for top speed events, so they won't really see all that much use.

[Blown353]

Don't forget driveline critical speed, something which I didn't see mentioned here. With low rear end gears this can become a major issue at high speeds. You don't want to jumprope a driveline!

Decide what gears you like, and then find out what kind of driveshaft RPM you will be spinning at the highest speed you plan on driving the car and pick a driveline that will give you some margin as far as critical speed goes-- the more margin, the better. Critical speed is a function of material, OD, wall thickness, length and a couple of other properties. Any driveshaft manufacturer should be able to provide you with this data, but here's a handy chart on Mark Williams' site: http://www.markwilliams.com/pdfcat/pg69.pdf

In a general sense, the following factors have the greatest effect on critical speed:
Outer diameter
Stiffness of material
Overall Length

Since the length is fixed per the application, your best options for increasing the critical speeds are to increase the driveline OD and also to change the material.

A carbon fiber driveshaft offers the best critical speed properties, but I wouldn't recommend one on a street car that racks up any miles. A piece of road debris that would likely bounce off a metal shaft can nick and grenade a carbon fiber piece. I had a friend lose a brand new CF driveshaft in his 4th gen Camaro to a very small piece of wood in the road. The nice thing about the CF driveshafts is that they shred and don't beat anything up when they fail... except your wallet.

Troy

[spanky the wondermuffin]

there is a discussion at openroadracing.com on this subject under the heading 'a little math help'

[kball]

I agree with zr1, in my case i'm running a 640hp 496 w/a tko600 close ratio. Unless you've got a really mild cam, the engine won't be happy in 6th gear below 100mph. Some consideration has to be given to what the minimum rpm the engine will like to cruise at. Thats the reason I chose the tko600 with the .82 od rather than .64 The engine in my chevelle won't like much less than 2700 rpm, so it's pretty impractical to have a .64od. That would put it at around 2100 rpm@65mph with 315/35/17 and 3.73's. It makes for more use of all the gears. Right now, with .82 and 3.73 gears, its 2550 @65mph. I'd love to drive 150 all the time, but that gets expensive quick. It still leaves me with a theoretical redline speed of 175 mph @ 7000 rpm and a first gear that's sort of useable with some throttle finesse.