Swapping to 7.31 R&P

[javadog]

I meant the gearbox case, not the motor case. I assume aluminum, but...

JR

[Trackrash]

Originally Posted by 65rsr

I'd like to badger Chris and Matt for fun, maybe another day on a different post.

No real science here, just my personal experience - I've been using a 7:31 with a high rpm / high hp (400hp / 300+ ft. lbs. 3.4l) in a 915 in the same gearbox for 13 years, I make no attempt to baby it, 3 to 4 and 4 to 5 clutchless up shifts. The case has all of the Wevo Race Case bearing mods, shot peened, lightened gears, cooler, spray bar, standard RSR flywheel, pressure plate, clutch, hydraulic VW clutch release, and it has a 935 spool instead of a LSD. I've never had any gearbox / ring or pinion failures, oil temp never an issue. Mobile 1 gear oil.

Good info, thanks. A couple of questions. What differential side plate are you using? Are you running stock sychros? Any mods to the ring and pinion gears?

[65rsr]

Whoops, yeah that's the engine case.

Aluminum transsmission case. Almost every part has been balanced, blusprinted, lightend, shot-peened, polished, whatever it took to make it durable. Stock R&P, backlash set properly. Steel syncros on some gears, brass on others, I'm not sure which or why. Not sure on exact gearing, (I have a chart at home on my computer) all gears are usable, high 1st that is removable, close ratios, I don't change them too often, nmainly because I have a short track box and a long track box. Wevo main case machine work, Wevo final drive work, Wevo bearing retainer. Over-sized CV's. Diff plate is either a 935 unit or an early Wevo. Cheap little electric pump through a cooler, RSR spray bar. I think Wevo sells basically the same set up. I only turn it on during long races if I see tranny temps rising. Solid mount to chassis. Other than that, it's bone stock.

Welcome to Windrush Evolutions - WEVO - Porsche Products

[Matt Monson]

Never seen a brass synchro for a 915 gearbox...

[65rsr]

These guys know the score:

Brian Copans

www.bmcg-gearbox.com

Paul Guard

Guard Transmission |

Williams Racing Development

Racing Transmissions

[65rsr]

Originally Posted by Matt Monson

Never seen a brass synchro for a 915 gearbox...

Ha. Neither have I, don't think I want to either. That's the street G50's. VW vs. Borg-Warner.

[Matt Monson]

65rsr,
Paul Guard is Gears and I am the owner of Guard Transmission now that he has retired. Just an FYI. I thought was obvious from the earlier discussion.

[65rsr]

Matt,

Awesome, I had no idea, been out of the loop, been playing with formula car gearboxes the last few year. I actually need to chat with you one of these days about a LSD for one of the transaxles, would like it to feel / act like the spool, but without the consequences of a broken axle. Multi-tasking from a phone without reading today, missed that post.

[Matt Monson]

At your convenience. A couple years back I did a small run of 935 LSDs with 80/80 and 90/90 ramps in them to replace the factory spools. They come complete with 935 four bolt stub axles. There's some pictures around here somewhere. I will dig up a link when I'm not on a phone.

Regards

Matt

[65rsr]

Thanks Matt. The spool gets me on the throttle and out out of the corner like nothing else, but..well, you know.

[Matt Monson]

Here's the thread on the 935 LSD.
A neat new product I thought I would share

Ps. Has a little ring and pinion content as well.

[Matt Monson]

I also want to go back to the strength question. Attached is a picture of a factory 8:31 (on the right) and our 8:35 (on the left). There's two things to consider here.


The first is the fact that running it against 35 teeth allows us to make the individual teeth on the pinion larger and therefore stronger.

The second, and often overlooked thing when people engage in these discusssions is the load spread across multiple teeth at one time. That's why going up from 31 teeth to 35, which are smaller teeth, doesn't hurt the strength. When we increase from 7 to 8 teeth while keeping the same basic end number (the ring count divided into the pinion count) we share the load more.

However it goes both ways. More is not always better. Our engineers (I am not one of them and would never pretend to be) calculates these things for us. As an example, the factory 996 GT3 Cup 3rd gear runs a 25:39 toothcount. That's a 1.56 ratio.

What we manufacture as a replacement gear for that position is a 20:31. That gear is stronger than the factory gear because the root width of each gear is wider than the stock ratio without compromising the shared load. But it is the opposite of why the 8:35 is stronger than the 7:31. It's all about finding the happy medium, where the combination of tooth size AND shared load on multiple teeth creates the strongest option.

I will be honest and tell you that with respect to the 8:35 option we never did the math. That's what Bill Verburg is talking about with load limit calculations back on page 1. On the gearsets, we always do that. Clearly a 2:4 tooth count 2nd gear of 2.00 isn't going to be as strong as a 3:6 tooth count 2.00. That intuitively make sense without doing the math. On the ring and pinion, this was an instance where we just trusted that the factory Porsche Motorsports people had done their homework and ran the numbers. Plus, it's homologated, which frequently influences our manufacturering decisions.

A 9:40 ring and pinion would also be really close to a 7:31 or 8:35 (4.44 versus 4.42 or 4.375) but if it doesn't meet FIA vintage homologation rules it's not something I can sell, at least in that market. Not only do parts have to be strong and durable, but they also have to be marketable.

Regards,

Matt