Starting Line Ratio
Thread Starter
Teching In
Joined: Oct 2010
Posts: 5
Likes: 0
Starting Line Ratio Does anyone know how your launch ratio would change in relation to tire diameter. What would a correct ratio be for a street leagle 00 Trans Am WS6 (3570 w/driver, 345HP, T56 Trans, Ceramic Clutch, 26.5" M/T tire) used only for drag racing.
I am not entirely sure that I understand your question. Are you trying to figure out what rear end gears to run? Trying to calculate effective gear ratio vs. tire diameter?
Thread Starter
Teching In
Joined: Oct 2010
Posts: 5
Likes: 0
Launch Ratio It's my understanding that a good launch ratio is 10, that would be the rear gear times 1st gear (3.89 rg x 2.66 1st g = 10.3474), what I'm trying to find out is what size tire is the lunch ratio based on. If the ratio was based on a 26.5" tire I would have a lunch ratio of 10.3474, but if I were to run a 32" tire there would be a 1.21% difference between the two tires which would effectively change may rear gear (3.89/1.21) to 3.21 and my launch ratio to 8.55
That's my dilemma how to chose the correct rear gear and tire size to coordinate with 1st gear for a good launch.
That's my dilemma how to chose the correct rear gear and tire size to coordinate with 1st gear for a good launch.
12 Second Club
Joined: Feb 2006
Posts: 876
Likes: 0
From: Catlettsburg, Ky
I know the first 60' is supposed to be the most important, bit I've always made gear/tire selection based off of power and weight, then estimated what mph I SHOULD trap with an optimal setup. And select a gear/tire combo that puts me crossing the line 1-200rpm below where the motor falls on it's face.
On a 275/40/17, with a 6spd, swapping from a 3.23 to a 4.10 dropped my 60' .15 but dropped my 1/8 ET .5, and ultimately picked up 5mph.
That's probably the most extreme case tho, With the 3.23 I was crossing the 1/8th mike mark around 4500 and either bogging horribly, or spinning like mad on the launch
On a 275/40/17, with a 6spd, swapping from a 3.23 to a 4.10 dropped my 60' .15 but dropped my 1/8 ET .5, and ultimately picked up 5mph.
That's probably the most extreme case tho, With the 3.23 I was crossing the 1/8th mike mark around 4500 and either bogging horribly, or spinning like mad on the launch
Thread Starter
Teching In
Joined: Oct 2010
Posts: 5
Likes: 0
Thanks for the input, I don't feel my 10 bolt is long for this world so I'm trying to figure out whitch gear ratio will be best when the 9" is installed. Right now I'm running "Nitto" NT05R, P275/40R17LL, Drag Radials, 3:42 rear gear, I'm turning 13.21 @ 106.7, but my 60' time is 2.11. At that speed I should run 12.75, if I could get my 60' down to 1.65 or 1.70 I will be there. I'm in the process ot switching to M/T ET Street Bias-ply wrinkle wall 26x10.5x17 drag tires. The ceramic clutch does not slip pretty close to a transbreak, I have a 2 step on the car for launch if I go below 1800 it bogs anything above I blow off the tires. Hopefully the softer wall tire that I'm installing with rim screws will hold a higher RPM launch. I'm trying to deside between 3:75, 3:89, or 4:11.
Thread Starter
Teching In
Joined: Oct 2010
Posts: 5
Likes: 0
I think I may have figured it out if the starting line ratio was based on a 32" tall tire and I run a 26.5" tall tire the difference would be 32/26.5=1.21%, so if I run a rear gear of 3:00 x 1st 2.66 x 1.21% = 9.66 SLR
12 Second Club
Joined: Feb 2006
Posts: 876
Likes: 0
From: Catlettsburg, Ky
With a 26x10.50 tire on an n/a car that's not sub-3000lbs, I'd stick with a 4.11 gear, it'll put you right where you need to be (top of 3rd in the 1/8th, top of 4th in the 1/4)
Seems like stockish weight ls1's are linear in the rpm:mph/rwhp ratio. If you cam the car enough to run 120mph, it will usually build power high enough in the rpm range that you still have enough le-way to rev it out further thru the 1/4 mile.
If a stock car falls off at 5800-6000, it'll usually trap around 110-115 if it falls off around 63-6500, it's good for 115-118. 65-7k, somewhere around 120-125
My lt1 on the other hand, trapped 12mph faster than stock, but only built power for 500rpm later, and therefore, was trapping 114 at about 64-6500rpm but the motor was done for at 62-6300.
Seems like stockish weight ls1's are linear in the rpm:mph/rwhp ratio. If you cam the car enough to run 120mph, it will usually build power high enough in the rpm range that you still have enough le-way to rev it out further thru the 1/4 mile.
If a stock car falls off at 5800-6000, it'll usually trap around 110-115 if it falls off around 63-6500, it's good for 115-118. 65-7k, somewhere around 120-125
My lt1 on the other hand, trapped 12mph faster than stock, but only built power for 500rpm later, and therefore, was trapping 114 at about 64-6500rpm but the motor was done for at 62-6300.
