[cmartin]

I have a 915 w/ limited slip and a 7.3:1 R&P in my SC w/ a 3.6. At redline in 5th I top out at an indicated 120mph. I have thought about this for a while now and want to get some advice from the more experience and better informed.

I know 120mph is plenty fast, but I know I can go a lot faster in this car. I would expect ~140+ on the back straight of VIR and roughly 130 at summit.

This setup seems like overkill for a 3.6. It also has me worried about longevity, both of the transmission and the engine. Driving home at ~4k rpm for 5 plus hours surely puts more wear on the engine than at 3k rpm, right?

So, should I swap back in a standard 8.3:1? Change the 5th gear ratio? Do a complete gear change (cha ching) to work with the current diff?

Did I miss something? I have read a little about R&P changes and so far it seems this is a job for a real pro. Anyone done the switch themselves? Also, I've thought about selling the entire unit and buying another. This is a little scary, as the box shifts fine, and buying a used box could be a money pit.

Thanks for any insight

[Jazzbass]

(Disclaimer - the following is based on what I've read, not done. If anything is contradicted or refuted by someone with practical experience, well, you should know who to believe )

My understanding of the 7:31 is that its great for getting a little oomph off the line, which is good for a peaky early engine like a 2.4 that need to rev high to reach their powerband, but no necessary for a 3.6 with a lot of low end torque. It sounds like the first step would be to go to a 8:31 - which will give you a little more top end and be stronger to boot. I'd drive on that and see how it performs on the tracks you drive (Summit and VIR mostly, I'd imagine). My understanding is the 5th in a 915 is somewhat easy(er) to change, as it hangs outside the main transmission housing, so I'd look at that solution next. Marc seems to be our residen 915 expert, maybe he can comment on the difficuty of R&Ring this stuff.

All that said, I think we all know what the proper solution is: install coilovers, cut out the torsion bar tube, weld in some supports and get yourself a nice G50 6sp from a totaled 993

[cmartin]

Wow, 6sp would be ideal. That sound evil.

However, before I went that route I would buy a 996. Another ride in a pretty stock 996 with track tires had me rethinking my situation again. Imagine working A/C, 6 speed, awesome brakes and so on, in a stock car. Oh well, back to the question at hand.

[Jazzbass]

996? Now how are you going to get that "oil dripping on heat exchangers" smell in one of those?

[Charlie Stylianos]

Chris,

I think Noah recently swapped out his diff for a 7:31. I'm sure you can find the thread if you search on PP. If I remember correctly he had pretty detailed pics of the correct mesh pattern after he set lash. This is probably the easiest/cheapest way to get the gearing you want/need. The OEM diffs are easy to come by and save sought after 7:31 for another project.

Just MHO, but after reading your remarks I'm thinking the 7:31 is definitely not well suited (overkill) for the 3.6. Maybe the car was set up for tracks with more twisties and shorter straights, but our tracks like VIR, WG or even SP, you'll be missing some top end speed for sure. With the torque of the 3.6 and your light chassis, you'll still be way ahead of the game with an OEM diff. Engine and tranny = $$$$.

[jpnovak]

Chris, I think there are others around that have done a 3.6 conversion and had tranny issues. YOu should do a search on Pelican or Rennlist for some discussions.

one of the problems with the 7.3:1 is that it is not as strong as the 8.3:1. I am not sure it will hold up for repeated abuse of a 3.6 on the track. It will have blinding acceleration at low speeds and less top end as you have already found out.

I would suggest just getting a new transmission with a new RP size. It will be cheaper and easier than setting up your current box with a tall 5th. It will also last longer. You may want to also consider a cooler setup on the box to help reduce the stress loads. You are really headed down a slippery slope.

Is your tranny electric or mechanical speedo? I have a 915 box in my ST. I think the lower gears would work fine with the 3.0. It does not have the low end torque of your 3.6 and feels a bit over geared sometimes. Too bad there is no LSD or we could swap.

[Jase007]

Chris:

Others may differ (especially mathematicians and engineers that see my math ) and this is only one solution but, after years of doing this I think it is a good one. Especially if you add up the costs for the R&R of the R&P +, + + and you are left with only one box. Think about buying a re-mfg box with the 8:31.

What you have:

7:31 = 31/7 = 4.43
8:31 = 31/8 = 3.88

3.88 *1.14 = 4.4232 (close to 4.43) = ~14% difference.

6,200 (max rpm?) *14% = 868 rpm (roughly b/c I don't think this correlates directly)

Sooo.... if the gear stack is the same in the re-mfg. 915 box then you should be able to gain@ least something close to the ~ 868. I don't think it works out exactly like this but you get the idea. In our track boxes (not street 915s) we are geared for 800-1,100 rpm drops per gear (3rd, 4th 5th). This works for us b/c the motor is built to run there. You need to look at rpm drops between the gears you use the most (3rd, 4th, 5th ?), fat spot on your torque curve, and tire circumference.

This might create another problem b/c you already know final drive ratio isn't the only component to top end speed and doesn't correlate directly. You have less effective torque in each gear as each one is made taller by the 8:31 R&P stack. So, you may have less power in 5th to push against the air @ 120, 130, 140 than you did with the 7:31 R&P. Will the 3.6L make up for this...? I don't know. It probably has enough torque to overcome the aero resistance and the taller R&P.

How do you figure it out? I don't know. Some mathematician here can figure it out or you can get the gear charts from Bruce Anderson's book (?) and calculate hp req'd, available torque in rpm operating range, frontal area presented (drag coefficient) rolling resistance of tires, etc...

E.g when we swap from 10.5" and 14" F and R wheels to 10" and 12" wheels and switch the wide PCA CR bodywork (big air dam / splitter and 3.8 RSR double tail) back to the '73/'74 look RSR narrower bodywork (traditional '74 RSR nose and IROC tail) on our IROC we pick up a few hundred rpm and top end speed. However, in the process, we loose cornering speed as evidenced by doing segment timing. A good friend of ours runs a similar 3.4L RSR with narrow bodywork (no side pods, double tail) R-compounds (not slicks), '73 RSR nose. At the club race this past weekend at Sebring we were qualifying / running 2.19 and he was qualifying 2.20 ... on a 17 turn loooong track . During the sprint race we were 2:20.2 and he was 2:20.7. The two cars are very similar (even the drivers / style). I know there are a lot of variables in this comparison but I ran against him last year (same cars) and he was quicker on the straights, we were quicker in the infield / corners. Cat and mouse, speed v. handling. Wide-er body tires v. narrower.

Your other solution is swapping rear tires for different total circumference. Known as the "poor-man's R&P swap." This is what I did the last few years on the tub. Note: changes ride height as well and s-bars have to be adjusted - maybe).

E.g. I run 195/60/15 (1932mm Circ.) on a 6" rim on my tub. I get to about 7,200 rpm F an B straight of VIR. At SP I get to 7,000 rpm on F straight. Last CR @ SP I swapped to 205/50/15 (1841mm Circ) on 6s and picked up 1.5 - 2 .0 seconds

1932 - 1841 = 91mm
91 / 1932 = 4.7% (smaller)

7,000 * 1.047 = 7,329 (~7,400 rpm)

So, I was running up to 7,400 rpm in most gears b/c I shortened the tire circumference (too much). I ran out of rpms on the F straight. Tradeoff: wider rubber was better everywhere, higher rpm limited the advantages of wider rubber. I could have gone faster if I could have found a tire (not wheel b/c of rules) size in the middle of the two. This only works in this direction if you aren't running out of RPM after the swap.

At daytona I had the opposit problem. I would hit 7,400 rpm with the 195 / 60s and watch eveyone go by, or most everyone. I could count 11 or 12 "mississippis" sitting there at 7,400 on the banking. Absolute motor killer.

Your numbers (based on RA-1s - assuming not shaved):

225/50/16 = 1983.6mm Circ.
255/50/16 = 2077.85mm Circ.

2077.85 - 1983.6 = 94.25
94.25 / 1983.6 = ~4.75% (larger)

4.75% of 6,200 = 295 rpm

Gets you ~300rpm (assuming 3.6L has the torque to get you there AND you can fit & get 255/50/16 rubber).

So ...

8:31 R&P will get you ~14%
255/50/16s will get you ~5%


Play around with it and have fun.

Jase

[racer]

Well.. how fast do you want to go? I'm sure some others out there would love for you to only hit 120..

[Noah]

I did the 7:31 CWP swap and wrote a long, detailed tech piece about it for Pelican that has yet to be posted.

The 7:31 is overkill for a 3.6 -- that engine produces so much torque that an 8:31 box would work fine. In fact, the 7:31 is almost overkill for my car, which is about 240 HP and 2,250 pounds.

The 7:31 is not "weaker" than the 8:31, it just has a force vector that is skewed more toward the diff cover, whereas the 8:31's force vector runs more along the axis of the pinion shaft. You should have an updated (post-'79) reinforced diff cover and a one-piece bearing retainer in your tranny.

Hmmm...rather than re-hashing all of this, I should just post the tech piece. Hey Charlie, if I send you the tech piece and the associated photos, will you post them?

[Charlie Stylianos]

Hey Charlie, if I send you the tech piece and the associated photos, will you post them?

You got it.

charliestylianos at yahoo.com