915 gearing question - another one!

[larrym]

i have a rebuilt 915/61 on the shelf - bought it ca 2010 out of a Santa Clara,CA bankruptcy for a planned 914-6/916 conversion - yup it's been on the shelf since that long ago

915 rebuild was done by Kahler's according to the guy i got it from, for a wide-SC track car that also never got finished (the 2008 recession killed the dream)

3yrs later when i finally got around to the 3.2 swap decided to stick with the 901 instead (yup, long term project)

(got my 3.2 from the same sale along with a flywheel & clutch package & e-speedo for the 915 - the guy had collected all the right parts, just ran out of job & money)

i'm never gonna do the 916 conversion, so am putting the Bott 916 kit up FS, but then i got to wondering about selling the 915 & FW package

first problem is i never got the Kahler's invoice showing the work done & the orig'l owner has long disappeared to somewhere, montana

problem 2 - this weekend, i got to wondering if the 915 maybe had different gears put in it for the track purposes? (would that have been a common mod back then?)


is the some relatively easy way & formula to do an external assessment of the gear ratios

p.s. - i also b'ot that SC roller project body - no market for it back then & storage was an issue - so i let it go 'way CHEAP

[dannobee]

We just went over this in the other thread. Two old racers quibbling about which was is easier, both methods work. One way requires a digital level, one requires guesstimating an angle. I’ll tell you my way. Put a clutch disc on the input shaft and block one axle stub. Carefully note your two starting points (Input shaft and axle stub). Count the number of turns of the clutch disc for exactly one turn of the axle stub. Do the math backward to determine each gear ratio. The assumption is that there are only a few ring and pinions available. Usually either 7:31 or 8:31. You’ll need to multiply your result by 2 because of the differential gears (assuming it’s not an LSD)

[larrym]

Originally Posted by dannobee

We just went over this in the other thread. Two old racers quibbling about which was is easier, both methods work. One way requires a digital level, one requires guesstimating an angle. I’ll tell you my way. Put a clutch disc on the input shaft and block one axle stub. Carefully note your two starting points (Input shaft and axle stub). Count the number of turns of the clutch disc for exactly one turn of the axle stub. Do the math backward to determine each gear ratio. The assumption is that there are only a few ring and pinions available. Usually either 7:31 or 8:31. You’ll need to multiply your result by 2 because of the differential gears (assuming it’s not an LSD)

thanks - my /61 is an 8:31 non-LSD (no LSD suggests it was not built to "winning" objectives)

[Walt Fricke]

915/61 ought to have an 8/31. It is stronger than the 7/31, so that's a plus for a race transmission. Plus it saves a lot of time and effort to leave the R&P the same.

However, using the stock 1977 gears, but switching to a 7/31 R&P, gives you a very nice fairly short gear box. Guys who have done this with street cars complain about the extra RPMs/noise for long highway drives, but the torque multiplication is very nice on the race track.

If you go to the other post you'll find a chart with all the math, and the various gears Porsche used. Others were available aftermarket.

Estimating angles doesn't seem like the way to go - just print a degree wheel you download from some website, paste it to a piece of cardboard, and attach that if you don't have a handy digital level. Rig up pointers. Should be accurate enough. Doing 4th (most of those are 1:1) and 5th should tell you what R&P is in there, and a shorter 5th is perhaps the most common track gear change. Or start with 1st - most stock 1sts are the same ratio. Though once you are set up you can check them all.

Old Racer.

[dannobee]

I thought about this over lunch today and came up with something most of us already have at hand, so it should work and be easy peasy.
Put the trans on its side, block it with wood or something else convenient so that the downward facing stub can't move and the trans itself is stable. Then put it in whatever gear you want to check. Then put a clutch disc on it and rig up a pointer with an old coat hanger and a nut and bolt through the engine mounting hole. Then open your phone's compass app and put it on the stub that's facing up. Turn the clutch disc until the compass reads N, 0 degrees. Now mark the clutch disc with a sharpie or a piece of tape to line up with the pointer. Now rotate the clutch disc exactly one turn, then read the compass heading. If it's a stock -61, the readings should be 1st, 58.4 deg, 2nd, 101.4 deg, 3rd, 147.4 deg, 4th, 187.4 deg, and 5th 226 deg.

Old Racer Too.