Gear Boxs

[bensam]

Hi Y'all

I'm currently rebuilding/restoring (replica) 1974 RSR. Last year rebuilt a 2.7 to full race specs but didn't touch the gear box.

This weekend guess the new engine was too much for it and now no gears at all.

Was going to rebuild anyway!

Question what choices do I have at this point? Intended use is PCA Club and Vintage Racing.

People talk about "trick" gear boxs - what are they? etc etc

Thanks all

[jluetjen]

What do the respective rules say? Do they allow lightened flywheels? Alternate CWP ratios? Alternate gear ratios? Dog ring conversions? External cooling? Alternate gear boxes?

Since you intend to race it, you need to start with the rules and then come up with a strategy to maximise your car within the rules. Either that or risk having someone protest your car and either going home early or spending a lot of money putting your gearbox back together after a protest and teardown.

[jluetjen]

Bensam;
Thanks for the PM. Assuming that the rules say something to the affect of...

the only requirement is that the gears fit within an OEM porsche transmission case.

My thought would be to start out with your engine charactoristics and tracks. What will be your engine's peak torque and peak HP speeds? What are your expected speeds on the tracks that you'll be driving? Specifically the maximum speed and the slowest corner speed(s). Extra credit if you know your expected speeds through the other corners.

If you assume that you'll be using 2nd gear (or in some cases first gear if you are really going all out) for the slowest corner, you want to make sure that you have a close set of gears that will cover both of those speeds without forcing you to shift in mid-corner. Close ratios like this will help most motors, especially those which are "peaky" like full-race motors. There are a number of spreadsheets referenced on this BBS that will help with the math. I'd do a search on "gears" and "spreadsheet" to find a copy. Installing close ratios may include getting an alternate mainshaft depending the gearbox that you are using.

Then you can look at a gear box cooler with potentially a spray bar system. You may also want to look at a limited slip differential if you think that it will help out some specific cornering issues like chronic corner exit oversteer and/or corner entry oversteer. Note that an LSD will significantly change your cars handling and you will most likely need a completely different set-up after you install it. The heat that it generates may also push you closer to needing a cooler.

Finally, if you really feel the need to burn money, you can consider converting to a dog-ring system in the gearbox for the ultimate in fast shifting. I don't think that Porsche ever used the system (although most non-Porsche race cars do) since they were interested in developing their synchro technology.

Finally, if cost is no object, there are a number of companies in the world now starting to make sequential conversions for different types of gear boxes. Sign up to be the lead customer for a sequential conversion for a 911 gearbox. You'll be going where most of us haven't and be the first guy in your neighborhood with such a configuration -- Just bring a big checkbook!

[bensam]

Hi

Wow - thanks. Exactly the sort of info I was looking for. Will look for the spreadsheets etc you suggest.

Thanks



Bensam
74 RSR