1st Gen Rear Drive Gear Ratio Question

[ishod]

My 1st Gen currently has a 4.88 ratio rear gear. I have a spare 3.90 ring and pinion. One of the tracks where I race is St. Louis where we use about 1/2 of the Nascar oval. A very long all-out pull. I'm wondering if it would make any sense to use the 3.90's at this track in an effort to get more speed on that portion of the track??? There are only two really tight turns on the infield and I know I would lose some there. The left hand turn onto the oval is the tightest and the slowest part of the circuit. Anyone have any suggestions???

Thanks,
Scott Goble
IT7 #03

[lateapex911]

I was having trouble with some sensors in November at the ARRCs when I ran Road Atlanta....but, with 4.88 gears, and 225/50-13 tires, I was still accelerating, (barely) at the end of the straight, but under my normal shift point of 8000 in 5th gear, as far as I could tell. Speed were in the 120 range.

That's just a comparision point, but I think that, in the most simple terms, you want to be at redline at the end of your longest straight. Now, shift points and lots of other variables can enter the equation, but thats a starting point.

Without knowing more info, my off the top of my head reaction is that no, 3.90s aren't the best choice.

That said, lets see what the smarter engineer types have to say!

[Black91n/aRX-7]

Maybe a 4.44 would be a better compromise. I know most times the power drops off a fair bit towards redline, so you may be able to get faster by keeping it closer to it's HP peak. It also leaves some room for improvement if you get a better launch onto the straight you're not running into redline.

[Hotshoe]

For what it's worth,

..... I run a 5:12 with the P225/50ZR-13 Hoosier, everywhere. The car is not capable of running out of breath because it cannot overcome the resistance. Charlotte is our fastest track in the SEDIV and I do not reach my Target RPM of 8250 by the time I reach the start finish line.

..... So , you might want to try running the P225/45ZR-13 Hoosier. The ratio turns out about the same with the 4:88.

..... Rule of Thumb, is like Jake mentioned. You want to have your rear gear peak out just before you reach the fastest point on the track. The lower the better 5:12 Vs 4:88 Vs 4:66 Vs 4:44. Reason is: A lower gear will give you better performance exiting the corners.

..... We have two tracks here in the SEDIV (CMP & Homestead) that the ideal set up is the 5:12 with the P225/45ZR-13 Hoosier just because there are no long straights.

..... So my answer to your question is. More Gear (4:88 w/ P225/45ZR-13) not higher ...... since we have No Torque

..... Here is a Circle Track point of view that I have always worked towards: http://www.circletrack.com/techarticles/ct...end_gear_ratio/ (http://www.circletrack.com/techarticles/ctrp_0606_best_rearend_gear_ratio/)

..... Rickey Thompson
... 2000,01,02,04,05 SEDIV SARRC IT7 Champion

[dickita15]

I run the 5:12 with the 225/45/13s which is fine for my two home tracks. I was worried when I went to the Glen but as Ricky said the acceleration is so slow at the top I never ran out of revs.
Now if I raced there more than once a year I might try a little narrower tire to help top speed.

[tlyttle43]

There is absolutely no place where a 3.90 is better than a 4.88 or 5.12. Unless you are running over your redline in top gear, shorter is always better, and rx7s don't have enough power to max out except maybe with a 5.12. With very short tires. At Daytona.

The main advantage of the very short diffs is that they allow you to stay in 4th and 5th gears which are close together, with only an occasional drop into 3rd gear. Since the gap between 3rd and 4th is so large, (1.42:1, I think) avoiding 3-4 shifts is a great thing for keeping the engine revving in its happy range. With a 3.90, you'll be using 3rd and even 2nd, which has an even bigger gap (over 1.5:1).

Tom Lyttle
IT7 #43

[Tak]

Out here in the SF region almost no one is running 5th gear--two of our 3 tracks have what would be a tough 5-4 downshift into a high speed sweeper. Are there any tricks or tips people are willing to share that makes the 5-4 shift less awkward?

Thanks,

Tak
# 29 ITA
SFR SCCA

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[lateapex911]

Hey Tak-

No tricks that I can share. I don't have any, LOL.. I get into 5th at most tracks I run, but I think at all of them, I downshift to 3rd. My only 5 -4 shift MIGHT be at the Glen, but I've never run there with the current rear gear (4.88) when I haven't been in so much traffic that I could actually try it.

I have done the 5-4-3 shift as well as the 5-3 shift and obviously, the 5-3 shift is less messy. I don't remember any difficulties going 5-4 though, but I also wasn't in the situation you are in, so I guess I'm no help, LOL.

With all the hills at Sears Point, I'd think you'd want all the gear you could have, but then I've never turned a lap there in a 7, ;) Is that one of the tracks with a 5-4 shift? Is it at the last turn in the esses? (is that 10?).

[joeracerx95]

I ran Sears at the last event in my newly-converted-to-ITA RX7. I ran a 4.88 rear end. Using 225/45/13 tires I needed to use 5th in the section you mentioned. I didn't need to drop to 4th because you don't slow down very much for that final right after the esses. Running at Willow Springs turn 2 can be run in 4th but as Tak related, I found the 5-4 shift so inconsistent that I just stayed in 5th most of the time.

-chuck-

[Hahn63]

There is absolutely no place where a 3.90 is better than a 4.88 or 5.12. Unless you are running over your redline in top gear, shorter is always better, and rx7s don't have enough power to max out except maybe with a 5.12. With very short tires. At Daytona.

The main advantage of the very short diffs is that they allow you to stay in 4th and 5th gears which are close together, with only an occasional drop into 3rd gear. Since the gap between 3rd and 4th is so large, (1.42:1, I think) avoiding 3-4 shifts is a great thing for keeping the engine revving in its happy range. With a 3.90, you'll be using 3rd and even 2nd, which has an even bigger gap (over 1.5:1).

Tom Lyttle
IT7 #43
[/b]


Not so quick on throwing out the 3.9. At Nelson Ledges we have found we can run just as quick with a 3.9 as with our 4.88. We also tend to get a bit better fuel economy as well which is great for the enduro's. If you are smooth you can make a 3.9 work for you , if your not it makes a good tool to teach you to be smooth and you will be faster once you put back in the other ratio's.


Roland

[Tak]

Thunderhill Turn 1 is the corner I would most fear with a 5-4 down shift. Willow Springs T2 as Chuck mentioned, Button Willow's Talledaga depending on the configuration. Possibly Laguna Seca T5, but I'm not sure on that one.
Chuck- I'm surprised you like 5th in T10 at sears. I've tried that and didn't really like it. For the race, didn't you have the 205/60 hoosiers?

Tak
#29 ITA

[joeracerx95]

Actually I ran both sizes that weekend. Bob Bradfield lent me a set of the taller 205s to try. They did allow me to hold a lower gear in several places including T10. My previous post was just mentioning that I didn't have a 5-4 downshift at that turn. If there was I think it wouldn't have worked well.

For other reasons I've swapped my transmission out and I'll be heading to Buttonwillow on the 19th. I'll report back on how it behaves there (although it is just the short West Loop configuration).