Crazy to think about a R&P change?, the search for the ultimate gear ratios Options

[PlaysWithCars]

This is really more of an academic exercise, but hey, what else am I going to do this morning?

Looking at options for the elusive 'ultimate' gearbox for my car. The car is used primarily for autocrossing, but I want to be able to take it out for cruises on nice days so don't want some crazy low gears on the top end. I'm running a 2.4E motor is I have more torque than the cars originally came with.

First question: are the low gear ratios in a 914 an advantage or disadvantage in an autocross? Lower ratios contribute to quicker acceleration, but they also mandate a continuous rowing of the gears between 2nd and 3rd. I'm thinking that staying in one gear is ultimately an advantage. If you accept that being able to stay in one gear is an advantage versus shifting, then there are two choices: taller R&P to get longer legs into 2nd gear, or try and get a short enough 3rd that you can use it. If you can get a low enough 3rd though, it becomes a big compromise to street driveability.

This line of thinking is what's led me to look at alternate R&P ratios. The standard R&P for a 901 is 7:31 but there was an optional 7:27 available. With the optional R&P ratio it becomes conceivable to stay in 2nd on an autocross course and leaves nice 3rd/4th/5th options to get good gear splits and cruise ~3,000rpm at 70mph.

So, what do you guys think? Anyone using the taller R&P ratios in cars with higher torque motor swaps?

[Dave_Darling]

First question: are the low gear ratios in a 914 an advantage or disadvantage in an autocross?

Yes, it is.

Seriously, there is no one answer. I was able to do very well at a lot of autox events with short tires giving me short gearing, but there were a few events where I was between 2nd and 3rd all the time, where other 914s with taller tires just motored along in 2nd.

The alternate R&P is super spendy, if you can find one. Not to mention having to re-do all of the measurements for engagement depth and backlash and everything, which is kind of a pain.

Changing the R&P changes the overall gearing--something you can accomplish by changing overall tire diameter. Either just changing sidewall height, or changing wheel diameter. Guess which one is about a million times easier to do? (IMG:style_emoticons/default/wink.gif) Depending on which way you go, you can wind up with a lot bigger selection of tires, or a much smaller selection, so that does require consideration.

--DD

[PlaysWithCars]

Yeah, I went down the road of tire sizes initially. But, to get a tire to approximate the 7:27 gear ratio it would need to be 26" in diameter. Nothing in Hoosier's line up will get close to that until 275 width which isn't going to fit.

Good point about the effort and cost to install a different R&P. But it kind of surprises me that no one has done it. Especially given how common it has become to put big motors into the 914 platform. These motors are certainly generating enough torque that performance is being left on the table by using the common 7:31 R&P. Even a four cylinder car w/ short performance tires doesn't really need first gear. It seems that instead of putting taller 5th gears into the boxes to get a reasonable cruise RPM at the cost of sub-optimal gear splits, that it would be more common to change R&P.

[brant]

It’s been done many times
Still not cheaper or easy
In the old pca club racing production classes this was done because it was technically available as original equipment And gear changes were not legal

Unless you buy or have a friend with the expensive tool to set up the differential
Much cheaper to change gears.

[Dave_Darling]

Good point about the effort and cost to install a different R&P. But it kind of surprises me that no one has done it. Especially given how common it has become to put big motors into the 914 platform.

Usually those motors make enough torque that the 901-based box has trouble reliably transmitting it. So the change is to swap the whole gearbox, not just the R&P.

Either that, or just eliminating 1st gear and treating it as a four-speed box, while trying to be gentle with it.

--DD