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Differential Question?
- Thread starter Dadandson
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GB1
Yoda
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The lower the number the less the engine needs to turn to spin the rear wheels at the same speed.
So a 4.22 is faster off the line, but on the highway it runs at higher RPM's.
The 3.9's is what is mostly used for street cars that will be driven on the highway. A 3.7 is probably better but they get pricey. A 3.9 over a 4.2 will reduce highway rpm's by about 300. This is preferred to keep the temps down.
For autocrossing, a 4.5 or higher is preferred to keep the rpm's in the power band at a lower speed.
For racing it greatly depends upon the track, the length of the straight, whats before it, how many twisties, etc.
But basically for a street car a 3.9 is the preferred rear ratio.
So a 4.22 is faster off the line, but on the highway it runs at higher RPM's.
The 3.9's is what is mostly used for street cars that will be driven on the highway. A 3.7 is probably better but they get pricey. A 3.9 over a 4.2 will reduce highway rpm's by about 300. This is preferred to keep the temps down.
For autocrossing, a 4.5 or higher is preferred to keep the rpm's in the power band at a lower speed.
For racing it greatly depends upon the track, the length of the straight, whats before it, how many twisties, etc.
But basically for a street car a 3.9 is the preferred rear ratio.
Roger
Luke Skywalker
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They signify the gear ratio. In other words, 4:1 means the input turns 4 times for every single turn of the output. A 3.75:1 (which I guess is what you meant) means the input turns 3.75 times for every turn of the output.
So a 4:1 is lower geared than a 3.75:1 - more RPM for the same MPH if you want to look at it that way. The 4.1:1 will give you a lower top speed than the 3.75:1 at maximum RPM, but will deliver more torque to the wheels.
Please also note that, though often used (or mis-used!) it's quite wrong to speak of the ratio of a differential. The differential is the component that allows one driven wheel to rotate at a different speed than the other, and it doesn't have a ratio per se. What you're referring to is the final drive ratio; pinion to ring gear is the term over here I think. I'd call it crown wheel and pinion ratio, being from across the pond.
Ocaasionally you'll see the ratios quoted as, e.g., 8:42. The means 8 teeth on the pinion and 42 on the crown wheel, for an overall 5.25:1 ratio, which will be familiar to most Austin Seven owners, incidentally.
So a 4:1 is lower geared than a 3.75:1 - more RPM for the same MPH if you want to look at it that way. The 4.1:1 will give you a lower top speed than the 3.75:1 at maximum RPM, but will deliver more torque to the wheels.
Please also note that, though often used (or mis-used!) it's quite wrong to speak of the ratio of a differential. The differential is the component that allows one driven wheel to rotate at a different speed than the other, and it doesn't have a ratio per se. What you're referring to is the final drive ratio; pinion to ring gear is the term over here I think. I'd call it crown wheel and pinion ratio, being from across the pond.
Ocaasionally you'll see the ratios quoted as, e.g., 8:42. The means 8 teeth on the pinion and 42 on the crown wheel, for an overall 5.25:1 ratio, which will be familiar to most Austin Seven owners, incidentally.
