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I've been racing a Spec Miata w/Torsen 4.10 in ITA and it just isn't cutting it on the short technical tracks in my region (MidAmerica Motorplex (MAM) and Motorsports Park Hastings (MPH)).
Any advice as far as R&Ps go. I know a 4.8 has been proven to be potent at MAM. Heard others say 4.3 is better all round. Same with 4.77. Help!
Thanks,
Don't forget the 4.44 :)
Yep that's true, 4.44 is another.
I guess another way of asking the question is, how do you gauge which ratio to use? I'm looking for more acceleration out of the corner exits so.......
Here's my guess:
Short/technical tracks = low ratio (4.87)
Semi-fast tracks = higher (4.77, 4.44)
Fast tracks = even higher (4.30, 4.10)
Sound right or is there more to it?
I'm no expert, but won't let that stop me. I think there are two goals:
Pick the shortest available ratio that satisfies both goals.
- Reach your highest desired RPM in your top gear at the fastest point on the track.
- All shift points are at desirable points on the track. You don't want to hit your redline 50 yards before a braking zone, or be lugging pulling out of a corner.
What you want to look at is what is your top speed at any given track. Now looking at all 5 ratios, look at the percentage change between gears. If using 3 gears at that track, what are your 3 closest spaced gears. Probably 3rd to 4rth and 4rth to 5th. Now back into that the rear end ratio that gets you the correct top speed. My gearbox has a ,87 5th and my top speed on a short track is around 122mph with a 23" tall tire. Top rpm is 8200. So (.595)(82)(23/2)/.87(x) = 122 and solve. Turns out the 5.12 ratio is best. Just do the math.
Chris
Hey thanks for the replies! There are some good gear ratio calculators on the web. Top speed really isn't my issue, it's acceleration out of the corner exits. With the 4.10 there are several corners at both tracks where if in 2nd I'm at the top of the power band and if in 3rd I'm below it. Just looking for that sweet spot. Guess I have some experimenting to do.
The advice Chris is giving you isn't really about top speed, it's about getting the highest ratio possible so that you get that great acceleration out of the corners. You need to make sure that you won't over-rev the engine at the track that has the highest top speed so you need to make sure you don't go too far on your ratio. But you should go as far as you can short of that.Quote:
Originally Posted by red986s
Once you get the ratio set you can also make small adjustments by using a taller or shorter tire. When I run at California Speedway I need to run a 225/50 tire, all the other tracks I use a 225/45.
I've run a 4.1, 4.3 and a 4.77 in my ITA Miata. I ended on the 4.77 because it allows for a good top speed...and I can win races with it, where I couldn't win with the 4.1 or 4.3. I chose the 4.77 over the 4.87 because Mazdaspeed was back ordered on the 4.87 and I could buy the 4.77 from a local guy for $100 less.
I do run out of gear heading into some corners, but my lap times are lower and I'm closer to the front of the field, so I'm happy.
Thanks all for the advice! Really appreciate it.
I acquired a 95 SM that I'm in process of converting to ITA specs ... The former owner did not know if it had a 4.1 or 4.3 ... Most of his SM upgrades were done in the 06-07 range.
I'm guessing it's got a 4.1
How big of a job ( hours / tools / special info ) is the change to another R&P in the miata ?
Here's a link that describes the process: http://www.miata.net/garage/diff_rr.html
thanks for the link .... I may want to tackle that with some experience looking over my shoulderQuote:
Originally Posted by red986s
I put a new diff in our LeMons Miata yesterday- just swapping the entire diff- but it took probably 2 hours since I'd never worked on one before. Now that I've changed the diff and know the tricks, I can probably do it in 30-45min.
as for cracking it open and changing the r&P, I haven't gone there and don't have the tools. I'm taking the diff for my 240 to a friend's shop who has the tools and experience. easier to pay someone else $100 to set it up than it is to ruin $1000 of helical diff and new R&P