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68ls1
10-07-2004, 01:00 AM
Ok, the million dollar question. My T56 has a .75 5th and .5 6th gear. I need some help figuring out what rear end ration to run. I have 3 choices, 3.73, 3.90 and 4.10:1.
My goals for this car are more based on open road, road course or auto x, although it will see the strip a little as well. I am leaning toward the 4.10:1 because of the really tall 6th I can still cruise at just under 1900 rpm at 70 mph but my concern there is that 1st gear will become all but unusable. Will traction be a complete nightmare in the first couple of gears. Will 6th gear even be of any use in the high speed runs or is it too tall for the amount of horsepower I have (+/- 400hp with a 125 shot N2O)
I would like to eventually race the car in some open road events and would like the car to be capable of 160-170 mph once my skills catch up.
Has anyone with this tranny used any of the above mentioned gears and if so what would you suggest.
I am running an LS1 that will have a cam and headers installed and a redline around 6200 rpm.

Appreciate the help. Noel.

Fuelie Fan
10-07-2004, 08:39 AM
Some calcs that may help guide your decision:

Assuming Cd = 0.40, frontal area = 2.33m^2, , air density = 1.293 kg/m^3, I get a terminal velocity is 79 m/s (about 177 mph) for 300kW

For 57%/43% weight distribution, 0.5m CG height, 1360kg mass, 2.74m wheelbase
I get a speed for power limited acceleration of 32 m/s (about 71 mph)

68ls1
10-07-2004, 10:03 AM
Holy S**t. Ok bro, you can't just drop bombs like that and walk away! hahaha. I got the top line, based on your assumptions I should be able to reach 177mph. Now, the second line...I'm not sure what a speed for power limited acceleration is all about. Can you explain it to me and tell me how this data will/should influence my rear gear decision. These assumptions don't take into account the rear gear ratio and how close it can come to putting me in my power band. Or does it. I'm lost, someone turn on a light.

Chill, Noel.

TurboLark
10-07-2004, 10:03 AM
I'd use the 4.10's. Even with 3.55's you will have traction problems with the power you have and low profile tires. I think the 4.10's will let you pull the high gears better.
Just an example from my experience :
I have a 77 Chevy 3/4 ton truck. Had a 4speed manual trans with 1-1 4th, and 4.10 gears. Pulled great on the freeway, although crap for milage. Swaped in a 5speed manual with .68 5th. Much better milage, but couldnt pull 5th on any hill or towing. Swaped in 4.56's, now its great for everything.
Goes to show, it doesnt take much to make a drastic difference.

harshman
10-07-2004, 10:13 AM
Some calcs that may help guide your decision:

Assuming Cd = 0.40, frontal area = 2.33m^2, , air density = 1.293 kg/m^3, I get a terminal velocity is 79 m/s (about 177 mph) for 300kW

For 57%/43% weight distribution, 0.5m CG height, 1360kg mass, 2.74m wheelbase
I get a speed for power limited acceleration of 32 m/s (about 71 mph)

Are we still talkin' gears here?

Iíve got a 396 BBC with a t56, 4:10 gearing and a 275/40/r17 tire. I cruise at 80 mph and 2k rpm. I have raced many cars on the highway and get it up to 155 mph in 5th. True the first gear is very short but with some drag radials (nitto 555 r to be exact) you will do fine. If you want to race at 160-170, make damn sure that your car is built for impact just in case.

All in all, Iím happy with mine. Low end torque is not my friend (I don't like my big block and will be switching to an ls1).

oh and by the way, terminal velocity is pointless when horse power is involved :drool: .

Ralph LoGrasso
10-07-2004, 12:57 PM
Noel,

Here is a thread on pretty much the same topic, with different variables.

http://66.70.234.198/forum/showthread.php?t=1335

You may find some info and opinions in there useful.

If you're wondering, I still haven't made a decision on what rear gears to run.

68ls1
10-07-2004, 07:21 PM
Thanks guys thats quality info. I think 4.10 will do the trick for me. When I get honest with myself, I realise my car may see 160+ once a year, if I'm able to get to any of these open raod races. These gears should be an absolute terror around town and still resonable everywhere else.
Thanks for the help, links and info.
PS, does anyone here have any experience with Yukon Gear?

Chill, Noel.

CAMAROBOY69
10-08-2004, 03:57 AM
I didnt see it asked yet but what size rear tire do you plan on running? That will determin a lot too. I have 4.11 gears and so far im happy with them. My tire size is about 26 dia.

68ls1
10-08-2004, 04:20 AM
315/30/18 Michelin. 25.5'' tall. Adam, how is your car to drive around town, do you find it revs far to quick in first or is it acceptable? Is it as much fun as I think it's going to be!

Thanks for the reply, Noel.

ballistic69
10-08-2004, 07:11 PM
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_studio/transmission_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
10-08-2004, 07:25 PM
Noel,

Kirt mentioned it already, your car with those gear (6th) ratios will almost definitely reach top speed in 5th gear.

I spoke with a person who open road races competitively in an LS1 4th gen Camaro yesterday. He told me 6th gear is useless because the drop is so big, it takes you out of your powerband. He's run his 4th gen at 197mph (he was DQ'ed at this speed) and I believe he runs either 3.25s, or 3.08s. Almost all 4th gens top out in 5th gear. So I would plan your top speed in 5th, that's what I'm doing.

68ls1
10-08-2004, 09:34 PM
Kirt, that is an excellent website and the rear gear calculator has been enlightening. Kind of reality checked myself, 4.10's are cool to say you have but in reality are too steep for what I want to accomplish. The fact the 6th is useless for top end pulls is another thing I was somewhat unaware of. I thought maybe...but no (more reality). 3.73's are perfect for both speed goals I have set, 160mph top end and low 12's/high11's in the quarter. The top end (6000rpm) of 5th gear will get me 158mph and on a 1/4 mile blast 4th would top out at 117 mph (6000rpm) . Anyway...thanks for straightening me out guys.
Chill, Noel.

ballistic69
10-09-2004, 09:39 AM
Remember if your using a Ford 9" you will go with a 3.70 not 3.73 (chevy). Also you stated your HP at +/- 400 w/ 125 spot of N2O. With the LS1 w/ a good head/cam pkg you can expct 425+ RWHP. Even a '68 Camaro with a Cd of 0.40 will hit 175-180 mph. Also these engine have no problem reving to 6500 and with new valve springs with a good head 7000 is no problem.

My point here is that you are under estimating your power plants ability, you will have a 180 mph car. You will need excellent roll cage and other safety equipment if you plan to go 180 though.

Be safe first, have fun second.

68ls1
10-09-2004, 10:13 AM
Kirt, thanks again for the input. For the record the rear is a Dana 44 IRS. I must say there is a first for everything, I can't remember the last time I UNDER estimated anything. Cam swap, valvetrain upgrade and headers will be the only engine mods for the first year, plus the nitrous. I'm hoping for 425 or so to the wheels. I may have to re-assess my roll bar/cage situation as well. And to get back to the original topic, I think the 3.73 gear will get me exactly what I want.
Thanks guys.

Noel.

Fuelie Fan
10-09-2004, 10:55 AM
Are we still talkin' gears here?
.

Perhaps I should expand.

Top speed is pretty easily to calculate.
Drag Force= 1/2*Cd*rho*A*V^2
rho=air density
A=frontal area
Cd=coefficient of drag
V=speed

Combine that with HP = F*V

And you get HP = 1/2*Cd*rho*A*V^3

Solve for V. It neglects effects tire losses as downforce changes, but it's a good start


If you want to gear for top speed (177 mph), you have to select gearing so that your engine is turning at peak hp RPM AT that speed. In other words, if your engine peaks at 6500 rpm, select a rear gear (matched with tires size) such that it is turning 6500 in 5th at 177 mph

The transition speed is the speed at which your car is no longer capable of, well, smokin 'em. It's kind of a PITA to derive, so unless you really want to see it I'm going to leave it at that. I also assumed (I forgot to mention) a coefficient of friction of 1.0, which might be a little optimistic. With a lower Cf,s that speed would increase. You use that speed if you are more concerned about acceleration and top speed. If you choose a gear that is too steep, such that you are already in third by that speed, then the previous two gears have been nothing but smoking tires.

It sounds like you've found a good compromise to meet your goals, best of luck with it. I like the approach of setting performance goals first (top speed and 1/4 times) and THEN building the car (engine, gearing, etc) to meet those goals. You'll be much more satisfied than the people that arbitrarily select a power goal and then "sees what she does". My very humble opinion is to leave power bragging to magazine cars. I'll climb down from my stump now

harshman
10-09-2004, 12:39 PM
Perhaps I should expand.

I don't think you need explanation. The formulas are very helpful - I however meant it as a "tongue n' cheek" comment as a gear head. "When you can't go any faster, you don't have enough horsepower". :)

Noel, I agree that the 3.73 will fit you best for better top end. The t56ís many gears will keep you smiling for an exceptionally long time. Just please make sure and build for safety.

68ls1
10-09-2004, 07:10 PM
Fueliefan, thanks for the explanation. I don't consider myself any threat to Einstein but you had me lost there! The reason I set all my performance goals at the outset is because I see so many people start projects without knowing really where they want to be and they end up either not finishing the project because it has changed direction so many times or spending WAY to much money.

Harshman - I had originally planned on an 8 point rollbar, now I'm not sure if that is really enough. I don't plan on these top end blast very often, I just want to be capable of them. I guess it only takes once, and without the proper cage I'd be fu####. Any cage in a kit recommendations. I don't have access to a tubing bender.

Chill, Noel.

awr68
10-10-2004, 10:22 AM
Noel, I have done some figuring and decided on a 3.89, I have a 315/35/17 tire, LT1 T56, and a Ford 9"...I beleive this is best for my setup, and a little assurance is that this is what Kyle used in the Twister Camaro.

Also, I just finished installing my roll bar kit from AME, as far as kits go I'm very pleased with how well it fit. Keep in mind that I only installed the 4-point and not the full cage though. The only draw back I see with the AME cage is that they don't use a hallow bar, but instead the side bars go clear from the main hoop forward and down to the floor, then have a bar between them near the top of the windshield. This (common) design works fine, it's just a little more limiting when it comes to getting the cage real tight to the interior (headliner). jp has this AME cage in II Much and was the one who brought this to my attention. Although this is exactly how Mark did the Mule's cage, so there's proof if designed properly it can work/fit great. All in all I love how AME's kit fit!

ballistic69
10-10-2004, 11:17 AM
Noel.
Here is an link http://www.openroadracing.com/ go to rules. You can base you safety needs on the speed class you run.

68ls1
10-10-2004, 06:31 PM
Thanks again guys. One question however...AME?? Art Morrison Enterprises? Am I right?

Kirt, cool website. Good info.

Noel.

awr68
10-10-2004, 08:21 PM
Yes, AME=Art Morrison

68ls1
10-11-2004, 01:32 AM
Thanks Anthony.