When selecting the rear gear, where do you gear for in relation to horsepower peak, torque peak, and converter fallback? I'm taking some weight out of the car and will probably need to redo somethings.
Posts: 28 | Location: elsewhere | Registered: December 11, 2008
How are you figuring that shift point? My engines horsepower peak is at 6100. I wouldn't think that I would want to run too far past that at the finish line, but still want to have enough gear so it performs well on the starting line.
Posts: 28 | Location: elsewhere | Registered: December 11, 2008
I have never had dyno info for my stuff. But I think generally a shift point a few hun over peak hp and a fall back a few hun over peak torque gets you in the ballpark. Many start out thinking that the car should never see rpm over peak hp. what you really want is for most of the run to be spent between peak tq and peak hp. The shift point honestly is trial and error testing. Gearing a car so that it pulls the hardest at the finish line hamstrings the rest of the run. You gain little in et at the finish line. If it is a little out of breath there, that isn't a bad thing. My dragster with a glide is usually about 6800-7000 stall 7600 shift 8-8100 finish line That has always netted me the best et's....again, no dyno sheets on my junk.
Foxtrot Juliet Bravo
Posts: 6405 | Location: Illinois | Registered: July 08, 2004
Originally posted by can'thitthetree: How are you figuring that shift point? My engines horsepower peak is at 6100. I wouldn't think that I would want to run too far past that at the finish line, but still want to have enough gear so it performs well on the starting line.
Hypothetically speaking if you have a 3000 lb car with 580 hp it'll go 107 mph 1/8 mile. Hypothetically speaking if that 3000 lb car goes 107 mph with a 32" diameter tire, 4.56 gear it'll cross at 6100 rpm in the 1/8 mile if the converter slips 8.5%. The stall (drop back) is dependent on rpm at which peak torque occurs. Hypothetically speaking if peek torque is around 5200 rpm with a 3 speed trans you'd want the stall around 5000, shift 6200. With a 2 speed you'd want the stall around 5400, shift 6200.
1/4 mile hypothetically speaking all things equal ,a 4.10 gear crosses at 134 mph engine spinning 6100 rpms if the converter slips 5.5%
You want your shift light to come on before you cross the finish line.
So many things can factor in what's best. Something as simple as converter stall can change what gears should be in the diff.
I run 4.86 gears in my car and they work perfectly. I'd love to try some 5.00 gears but don't have a set to experiment with. I shift at 7400 and the rpms fall back to 6400 after the shift. I cross the finish line right around 7400. The camshaft/engine can easily go to 8000 but I don't spin it that high.
I tried some 4.57 gears once. Didn't change anything else. The car was a slug. The gearing may have worked better if I also tried a different converter. Put the 4.86 gears back in and the car picked right up again.
You can play "what if" all day long with software. You can go and put the car on a dyno to fine tune the setup but real world performance on a track will tell you what it's really doing.
Look at stock or superstock cars. They don't use a lot of high end exotic parts but they run faster than most street/bracket cars that have a lot of expensive parts on them. It's all about using the right combination of parts to get the best performance.
www.hardtail.com Stephen's Racing Page Best ET: 9.029 Best MPH: 150.45
Posts: 1352 | Location: Calgary | Registered: June 06, 2001
Originally posted by AlkyIROC: You want your shift light to come on before you cross the finish line.
So many things can factor in what's best. Something as simple as converter stall can change what gears should be in the diff.
I run 4.86 gears in my car and they work perfectly. I'd love to try some 5.00 gears but don't have a set to experiment with. I shift at 7400 and the rpms fall back to 6400 after the shift. I cross the finish line right around 7400. The camshaft/engine can easily go to 8000 but I don't spin it that high.
I tried some 4.57 gears once. Didn't change anything else. The car was a slug. The gearing may have worked better if I also tried a different converter. Put the 4.86 gears back in and the car picked right up again.
You can play "what if" all day long with software. You can go and put the car on a dyno to fine tune the setup but real world performance on a track will tell you what it's really doing.
Look at stock or superstock cars. They don't use a lot of high end exotic parts but they run faster than most street/bracket cars that have a lot of expensive parts on them. It's all about using the right combination of parts to get the best performance.
It's actually an equation. If you're crossing at 7400 rpm, 4.86 gear, with the converter slipping 7% with a 32" tire, it's going 135 mph. If it's slipping 9% it's going 133 mph. The only what if is the tire, MPH, rpm and gear. I have two 6200 stall converters, one spragless and one w/sprag. The spragless slips 7% and the sprag converter 13%. There's a quite a bit of difference in rpm at the stripe.This message has been edited. Last edited by: Mike Rietow,
