View Full Version : gear ratio
08-09-2006, 09:40 PM
gotta question about gear ratio's. i am running a 4 cylinder modified in the local circuit. the problem is, i got tons of straight away speed, but these guys are kiling me coming out of 2 and 4 turns. so to compensate, i wind up on the straight aways and dive into the corners, i mean i drive hard and deep into the corners. but at the expense of tire wear and loss of rpm's cause i am going in so hard.
i was running a 4.11 rear gear and then switched to a 4.83 to try for the corner shots. the new gear doesn't provide quite the acceleration i am looking for, but i also have some redline issues on the back straight away.
is there a program or something or a rule of thumb that i can go by? any help with this lagging 4 cylinder will be greatly appreciated.:sleeping:
08-09-2006, 10:13 PM
A general rule of thumb is: .1 (1/10) of a gear ratio is equal to 100 RPM, and .1 (1/10) second. Example, if you change from a 6.00 gear to a 6.20 gear you will gain about 200 RPM, and drop about .2 second off your lap times. These are pretty good generalizations, the only hard and fast rule is that there are no hard and fast rules.
Also, remember that your overall gear ratio isn't your differential ratio, it's your differential ratio times the transmission ratio, unless you're running in high gear (direct) in the tranny. Example, if your tranny ratio is 1.5 to one in 3rd gear, 1.5 times 4.11 = 6.165. 1.5 times 4.83 = 7.245. In this example you should gain 1000 - 1100 RPM. This all assumes that your engine is capable of turning the extra RPM. If you're turning 6800 RPM, and your engine will only turn 7000 RPM max, you aren't going to gain 1000 RPM.
You can also make a difference in your RPM by changine your tire sizes. Taller tires have the same effect as a little taller (smaller number) gear, smaller tires will have the effect as gearing lower (bigger number).
Hope this helps,
08-09-2006, 10:20 PM
ok, you say with your 4,11's you have tons of straight away speed. how much of that can you lose and be competetive on the straights?
i think you took too much of a jump going to the 4.83 gear. i would like to see you tune your engine for more torque, and maybe a 4.30 rear (maybe 4.56 if you dont have the redline problem and keep enough top end)
08-09-2006, 10:44 PM
what size tire?
4.56's sound ok
08-22-2006, 04:52 PM
sorry it took awhile. we are limited in tires to stock 225/ 60-15's. i went with the 4.56. my 4th gear is 1.10 and i'm just starting to to get a blink on the light. so i guess i am going to stick with it. thanks for the help. i am now trying to tune the car in with the set-ups. only problem is we can only practice on wed night for 3 hours. thats ok, only my first season. i am taking alot of notes every time i make a change to the car. but hey, i ain't in last place.
01-04-2007, 08:23 PM
Hard to answer without seeing you run but you talk about going really deep into the corners. Are the cars that ar beating you off the corners lifting sooner than you. You could be trying to go too deep into the corner. You could be getting beat out of the corner becuase the cars that enter the corners a little slower may be able to pick up the throttle sonner than you. on of of your practice nights find a car thats beating off the corner and play follow the leader for a few laps.
If you're going deeper than everbody else you're going to need a better handling and or brakes than everone else or you'll get passed coming out of the corner.As I said at the begining this is just something to take a look at it my not apply to you at all.
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