If you go to less or more rollout on the rear tires it changes the 'effective' rear gearing. The rear gear times your low gear is your "SLR". 3.14 × tire diameter=rollout.

Yes SLR changes with tire diameter effectively.

 

Right now I have it on a 28x10.5 slick
4.56 rear gears
2.46 1st gear

 

http://www.wallaceracing.com/calc-slr.php

 

Gear the car to cross the traps at what ever rpm and adjust the transmission gearing to get your SLR in a workable range.

 

Trap is more important then starting line when I pick the gear.
Unless you can spend the dough on a fancy trans, most of us run 1-1 high gear.
I did this test, with a 4.88 I had an awesome launch, but it was spinning too high at the end.
So I had to compromise so I didn't beat up the engine.


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OK so so what is the SLR for 2.98x4.56x31" tire? I wasn't quite getting the formula.

 

13.59 according to Wallace.

 

How do you know what your ratio should be? How do you know what's optimum for your combo?

 

Way too many factors to give an accurate answer. Is it a 3 speed automatic or a 5 speed manual? Big tire with great track prep or a smaller tire with so so prep? Radial or a slick?

How wide is the engines power band? Its one of those things when you need to balance everything. In a 3 speed automatic you can usually only choose 1st and 2nd gear ratio, often they are locked together (cant mix and match) so you need to balance starting line ratio with shift drop rpm. In my experience engines with larger power curves are less sensitive to the 1-2 drop then narrow power band pieces. We still want as much SLR as we can get so we can get the car moving, but if we dont pay attention to what happens when we pull 2nd we can knock the wind out of the engine 2 seconds into the run.

This is one of the reasons a proflite is such a popular transmission for high end/high rpm combos, there are so many ratio options available compared to other lightweight low drag 3 speeds

 

I don't know the exact numbers but obviously the lighter the car with lots of power is going to need less SLR than a heavy car with less power. At 2800 race weight and 950 hp I wouldn't think you would need a crazy SLR... with your weight hp rpm combo I'd gear it to cross the strip at peak rpm.

 

TH-400's accelerate I'n 1st gear and when they shift into 2nd gear, they have to completely stop a drum and then accelerate it again in the opposite direction.

This is the "lag" on the 1-2 shift. This affected by your 1st gear ratio. You can have too much or too little first gear and make the transmission more inefficient.

I ran everything from 1.97 - 2.68 and always ended up going back to one ratio.

I wish I had all of that money back......

 

Sheeeeet those are budget spindle mounts should got the santuffs when I first did the swap the strange ones were cheaper though now I have santuffs and he headers well I'm headed to elston here in a few weeks sooo...

 

Calvin is really a nice talented guy, does nice work and can explain some things about headers that will make you scratch your head and wonder???? I was impressed with the fact that he started on monday morn and by 2pm one side compete, said pick the car up tomorrow after 3. Wed morn headed home. Fast, precision work. .

 

I've been told by a couple old school guy's that ran many years N/A before all these fancy new ratios came out. Put in more gear then the car will take, make the car work better to take it, and just let the bitch spin out the back door where it ends up. Also have been told that the car should be geared to the 1,000' as well when racing in the 1/4?

 

I had a very sharp friend of mine say something similar; basically gear the trans and rear for the optimum starting line ratio and first half of the track, since that's where all the et is, and let the finsh line fall where it may.