Trans and Diff ratio selection... Seeking advice...

Hi all!

My current project is a pro-street car... I'm having a tough time finding some practical info regarding trans ratios an diff ratios relative to car weight, tire size, etc...
Some basic info:
Finished vehicle weight will be approx. 3800 lbs.
Rear tires are MT 32" dia. radial street tires, (17.5" tread width).
9" Truetrack Differential.
Target engine hp to be approx. 800 to 850.
Target max engine rpm is 6500

Here is my dilemma...
I have a 4 speed Lenco for this project. I am adding a section to make this a 5 speed. I plan to go through it and change whatever I need, to get the ratios to be what I want/need for a streetable car. With respect to how the car will feel regarding take-off from a stop and low, (1st), gear drivability, I have little to no practical experience with what I'm building. In other words, I am not sure of what I will need to sort out both the trans and the diff ratios... I do want a lively car in 1st, 2nd, and maybe even 3rd gear. I don't want a "granny" 1st, nor do I want a 1st that requires a butt-load of clutch slip to get moving...

So here is where I started: (I'm working from memory and don't have my math sheet here in front of me so numbers are approx.)
I originally had a Tremec TKO 600 planned for this when the engine was going to be a small block... I'm currently building a blown 429, (Thanks Adney for that beautiful crank!) The Tremec 1st gear is 2.89:1 and 5th gear is .68:1. I figured that I wanted the engine to run approx. 2200 rpm at 70 mph in 5th gear. This gave me a 4.30:1 diff ratio. Working back to 1st gear, this gave me approx. 17 mph at 2000 rpm and about 50 mph at 6500 rpm. The first question is, are these reasonable numbers relative to a streetable car? Is 1st gear going to be too steep? Should I target something else?

So now with the Lenco, I have to start 5th gear at 1:1. Working that with 2200 rpm at 70 mph in 5th gear, I get a diff ratio of 3.10:1. And, if I follow the same basic ratio setup that the Tremec had, (working the numbers for the Lenco section percentages), I would have a 1st gear ratio of around 4.25:1...

Could anyone with some experience with sorting this tell me if I'm on track or out in the weeds? Maybe even tell me where to start if I'm working this backwards or if there are some rules-of-thumb to apply here... Maybe even a web page or two...

By the way, I wrote a nice excel spreadsheet for sorting all sorts of trans & diff ratios with relative mph and such. If anyone wants a copy, email me at upontwo@roadrunner.com and just ask for the "SpeedRacer" spreadsheet...

Thanks a bunch!
-Jay

I'm thinking something ini a 3.73-4.10 would be a good choice with a 32" tall tire.

in the middle A 3.91 will give you a 11.29:1 with first

Thanks for the reply dfree383, but I'm still a little unclear...
With respect to the 3.73-4.10 numbers, are you talking about 1st gear ratios or rear diff ratios?
If those are rear diff ratios, those won't work with the 1:1 final of the Lenco...
How did you arrive at the 11.29:1 number and what does that number represent? Is it a target number for a lively setup? If it's a target, what is the range, and what can I expect at each of the limits of the range?
Any info is appreciated!

Cheers,
-Jay

3.73 to 4.11 rear end ratio.

The 11.29 represents your first gear ratio of 2.89 x 3.91 and IMO thats decent with a big tire and not wanting to slip the clutch much, should be drivable.

??? Why wont those ratios work with a 1:1 ??? Your loosing me

With a 1:1 final trans ratio, a 3.91:1 Diff ratio, and a 32" tall tire, you get an engine speed of 2875 rpm at 70 mph... Cruisin' speed on the expressway... I'm looking for an engine rpm that's less harsh an easier on the fuel tank than that... 2000 to 2200 rpm works for me...

Cheers,
-Jay

you'll be needing an overdrive.the tall tire will need a fair low gear to have much low end "spunk".

I think you are trying to achieve a similar result to the overdrive 5 speed but with the lenco it has to be one to one in high. Makes sense to me.

So if the general consensus is that a nice driveable low gear with 4.10 in the back was a 2.89 or so that gives you a total low gear of 11.8. To achieve the same total gear with 3.1 gears in the back you need about 3.8 in low gear.

Doesnt matter where the gear is the rear axle rpm still works out the same. If you are going to err go to a lower number. With a big motor like you are talking about you will have plenty of torque to pull you away from the stop imho.

dkp

Let me give you a little something to think about........

you wish to drive 800 to 850 hp on the street @ 2200 rpm at high way cruising speed.....

at this hp rating the engine will still be in a just over loping and a rough RPM to stabilize, another words it will be jerky and really rough on parts......


just something to ponder...



Randy

Maybe I've misread your post, your going to use a 4.10 1st gear in the lenco and a 3.10 final drive???

Catching up with everyone...

Bosshoss - You are following me with my plan... Basically use the 1:1 final in the Lenco in the same manner as the overdrive in the Tremec... The Tremec is 32% overdriven, so shifting all of the Tremec's ratios by 32%, (relative to the Lenco) the Tremec's 2.89:1 1st gear would shift up to 4.25:1...
.68 / 1 = 1.47
1.47 x 2.89 = 4.25
So with the Lenco in 5th (1:1), a 32" tire, 2200 rpm, & 70 mph, I would need a 3.00:1 ring & pinion...
And with a target of 11.8:1 overall, 1st gear would need to be 3.93:1
With a target of 11.5:1 overall, 1st gear would need to be 3.83:1
With a target of 11.2:1 overall, 1st gear would need to be 3.73:1
At 4.25:1 1st gear and a 3.00:1 diff, the overall ratio would be 12.75:1... This sounds pretty steep...
I'm still not clear on where this target of 11.2-ish to 11.8-ish overall ratio comes from... Is this a rule-of-thumb?

Coug - I'm not sure I agree that the engine will not be at a good cruising rpm of 2000 to 2200... This will be a fuel injected, blown, 429... No radical cam profile, no huge stroke...