PIR and Diff ratio question

[Earendil]

Simple question, what do people consider the better gear ratio for PIR, 3.73 or 4.10? I have the option of either, and am curious what those that run the circuit well think is the better gearing. What do HPDE guys generally run? What do the Pro3 guys run?

I've been on PIR once with a 3.73, but I wasn't going that fast, and I wasn't watching my speedo/tach that close in corners. I only know what the corners felt like as far as gearing, but I also wasn't pushing the car all the way either, so...

Thoughts?

[Tucker]

It partially depends on what you've done to your car. My answer below is assuming you have an 'i' motor.

PRO3 cars tend to use diffs closer to 4:10 and higher. At the same time the PRO3 track record at PIR was held for a long time by a car running a 3.73 and not using 5th.

Another way to look at it is that if you're a good driver and can get clean laps, the 3.73 might be a better diff. If you aren't absolutely excellent at keeping momentum and plan to do more W2W racing, then the 4.10 is preferable.

Also, I'd be happy to take your 3.73 off your hands if you decide on the 4.10. :)

[Earendil]

It partially depends on what you've done to your car. My answer below is assuming you have an 'i' motor.

That is the proper assumption. I almost listed what little things had been done to my car, but figured it wouldn't matter. That 50% increase in hp would though.

PRO3 cars tend to use diffs closer to 4:10 and higher. At the same time the PRO3 track record at PIR was held for a long time by a car running a 3.73 and not using 5th.

Interesting fact! That also makes the decision a lot closer than I was hoping it'd be :-)

Another way to look at it is that if you're a good driver and can get clean laps, the 3.73 might be a better diff. If you aren't absolutely excellent at keeping momentum and plan to do more W2W racing, then the 4.10 is preferable.

I keep forgetting that the way to archive a fast lap on an open track is not always the same way to archive a fast lap when W2W racing. Obviously being able to quick sprint to get in front of a slower guy will result in faster laps in W2W, even if you sacrifice an overall better ratio.

I was curious because with the option for either, I might as well learn PIR with the ratio that I might be using in the future if I shoot for Pro3 a couple years down the road. But perhaps if the closer and more realistic goal is fun HPDE days, the 3.73 will be better.

Also, I'd be happy to take your 3.73 off your hands if you decide on the 4.10. :)

I'll let you know :)
Right now the 3.73 is in the car, and the 4.10 has questionable internals. It might require a rebuild which is reason enough to stick with the 3.73 for now :(

Thanks for you awesome help and tips! I don't suppose you know the answer to that same question, but directed at

[Lance Racing]

The PRO3 guys are also choosing their rear diff ratios based on their tire size. 225-50-14 are running a 4.10 or 4.27. Guys on 225-50-15 are running 4.27 or 4.45. You'll pay a lot for a 4.27 gear set, and you'll pay even more if you can find a 4.45 for medium case.

So much of PIR is about being smooth through the turns, I'd concentrate on that. Until you are really worrying about lap times I wouldn't worry about the rear diff ratio. Just go with the 3.73 LSD. I ran my lightly prepped '87 325is street E30 for years at the track before getting into PRO3 and always ran the stock 3.73.

One thing you can do to measure your launches out of turns at PIR is to pick a point down the main straight ( I use start finish line) and watch what your RPMs are as you pass that point. You'll know you got a good launch if your RPMs are slightly higher, and know your launch out of Turn 12 was not a good if your RPMs were lower.

I'll have 4 or 5 points around the track that I measure my RPMs and compare to my personal baseline. It's a good way to judge your performance if you are not running a data acquisition system.