Hello All,

This is my first post at CamaroZ28.com. I chose this forum
because I recognize a few names from my regular Message
Boards that made a big impression on me.

There is a heated discussion at Fbody.com within the LT1
forum about gearing:

http://www.fbody.com/cgi-bin/lt1/read.cgi?msgid=64063

I am using the name Lukn4Trbl for the above thread. It's
a long thread, but it's worth the read.

With all my experience in racing and automotive tech., I'm
finding it difficult to get my point across.

The debate revolves around gearing a "STOCK" LT1 F-body
whether it be automatic, or manual transmission.

I'd like to see conclusive responses to this question from
users who:

a. Have experience setting up cars for 1/4 mile racing.
b. Who have late model LT1 F-body and have changed gears.
c. Who understand the prinicpals behind torque multipiers, shift points, and reading engine dyno charts, and gearing calculations.

Thanks in advance to anyone that can support or correct my
theory on LT1 gearing!

 

Simplest terms. You want the car to cross the finish line at or just before the red line in high gear 1:1, no OD.

You shift at the peak HP. If you're shifting at 5500 rpm with 26" tall tires and 3.42 gears and cross the finish line at 4500 rpm then you need more gear (3.73 etc). If you cross at 6000 then you need a taller tire or less gear.

There is no magic number as to what works best since no 2 cars are the same. It's not just LT1 gearing. Setting a car up for 1/4 mile racing is all the same. After knowing what the peak HP rpm is at you can calculate the tire size and gearing needed to cross the 1/4 mile in high gear. OD isn't recommended since the OD gears are weak and use more HP to turn. Transmission choice isn't really a factor. Different transmissions will just change the ET or MPH because of how they multiply torque differently. You're still looking for the same end result. A very high HP car doesn't need the torque mutiplication of low gears. That's why the 2 speed powerglide is used so much. There's so much brute torque off the line that low first gears are not needed.

I shift my car at 6700 rpm. If I bump the shift point to 6800, I can feel the car nose over as the hp goes over the curve. Using 4.56 gears and a 29.1" tall tire that grows to 30.26 (4% tire growth for a slick) I cross the finish line just as the shift light comes on.

 

Probably best to post in the appropriate forum... and this belongs in "Drag Racing". Although you may recognize some of the people who post here, most of us post on multiple forums... I know I post regularly on LT1 Tech, N2O Tech and the "Computer..." forum as well. Others will post on multiple forums also.

The answer above is the best one..... figure out what kind of trap speeds the HP will produce, and gear it to cross the finish line at the top of the rev band in the gear you want..... most people try to use a 1:1 final gear.... I'm trying the Gear Vendor O/D unit to make the TH400 more suitable to a heavy car.

Don't forget to consider things like convertor slip and tire growth. If you want, I have an Excel spreadsheet for the T56, and one for the TH400 that address tire size, gear ratio, slip (TH400) and growth. If you want, I could e-mail you a copy.

Moving to "Drag Racing"......

 

Something else to add...

Altitude will also play a part in gearing or tire selection. As I mentioned above, my combination is set up perfectly for the high altitude I race at. When I go to a track at a lower elevation, the engine produces more HP. I can still only shift at 6700 rpm but because of the extra HP, the car accellerates quicker. When I went to Yakima I noticed that I was at red line by the 1000 foot mark. To fix this problem I either need taller tires (no room) or less gear. 4.56 gears work well here but at lower elevations I need to switch to 4.33's (easy when you have a 9")

 

Thank you all. That's great information.

I think the original question was misread as I agree with
the gearing technique you all describe.

Please send me the Excel sheet for the TH400 and if you have
a decent formula to calculate the torque to the ground accounting
for flywheel, transmission ratio, differential ratio and tire size,
I'll take that as well!

Please and thanks!

BTW...You boys are good, been reading through some of your posts in Advance Tech.