whats the difference between the 731 & 831 tranny??

artisticent · 03-09-2007, 09:06 PM

I have a 915 from a 1973 that is a 731 ration. I also have a 915 tranny from a 76. What is the difference and what are the advantages and disadvantages of each.
Thanks
Nathan

pieterk · 09:47 PM

I'm far from the experts on this board, but I'll give it a quick crack; please, Will, or Grady, or Dave or whomever please correct me if I've made a mistake.

The 7:31 ring and pinion has one less tooth on the pinion gear. Some refer to it as 'the poor man's short box,' meaning, it has the effect of shortening the gearing across all five gears of an otherwise stock geared box. It's not a true short(er) box, but on lower horsepower cars with somewhat smaller diameter tires, it has the effect of slightly increasing torque.

The 8:31 transmission has one more tooth on the pinion. With an otherwise identically geared box, it will slightly lengthen the gearing across all five gears relative to the 7:31. It's generally preferred on higher powered cars and/or cars with larger diameter tires (say on cars running 17" wheels).

The 7:31's advantage is somewhat enhanced acceleration lower on the power curve, especially with lower-powered cars. It doesn't actually equal a short gear box though as the spacing between gears is equal to the spacing on the 8:31 transmission. To genuinely shorten a gear box--irrespective of the ring and pinion--the gear sets must be swapped for closer ratios.

The 7:31's disadvantage is that it's weaker and more prone to failure under high loads, and that it has an overall lower top speed all other things being equal.

The 8:31's advantage is higher strength, and higher top speeds. With a bigger engine, and genuinely short gears, it is probably superior to the 7:31 in every respect, although a 7:31 with short gears is pretty fun.

My now dead 7:31 had only a shortened fifth gear. Mated to a 227 hp engine, it was a lot of fun.

In a few days, I'll be able to report on what it's like to mate the same car and engine to a genuinely shortened 3rd, 4th, and 5th gear, with an 8:31 ring and pinion, in a blueprinted box with 930 bearings and synchros, all Wevo plates and bearing mounts included, and transmission cooling with custom spray bar, front fender mounted cooler with pump and filter included, as well as a Kennedy lightweight aluminum racing pressure plate and lightened RSR flywheel (the PP is 9 pounds lighter than stock--from 15 to 6 lbs--and the flywheel loses just under two pounds).

artisticent · 03-09-2007, 10:09 PM

Thankyou for that. I ask because I have 2 cars I am building. A 76 with a 2.7 motor that is a C2 turbo look cab and a 73 911 built as an RSR. The RSR will eventually get a 3.6 or something of power. It has 315-50-16 on the rear so It needs to PULL. The 76 cab will have 18's maybe 255 or 275 but my 915 831 has 1st gear out so it needs repair. I am trying to match up the right tranny or figure out if I should not use the 731 fir either car and sell to buy a 831. It sounded like fun to use the 731 for the RSR but it sounds like a strong motor would kill it.

Thanks
Nathan

pieterk · 03-09-2007, 10:30 PM

What are your intended uses for each car?

If the cab is going to serve primarily as a street/daily driver, I think the 7:31 will be more than fine with a 2.7, and may in fact be preferable, since you'll be doing lots more accelerating from dead stops. It'll feel a little more lively in real-world driving conditions. Unless you're going to try to launch the car hard, the 7:31 should be quite fine durability wise (assuming it's in good condition to start with).

If the RSR is going to be more of a track toy, and you have an 8:31 already, it makes sense to use it in that car, especially if you're looking at moving into a 3.6 at some point. Generally speaking, as long as we're talking about 915s, 8:31s seem to be the choice for the higher torque motors like the 3.6s.

That said, there are some members here who run 7:31 boxes coupled with fairly high torque engines, and they seem to get by fine too, with proper care.

artisticent · 03-10-2007, 09:22 PM

The 73 rsr will have a powerful motor in it of some kind. I will be launching from stops and using it as a daily driver. The cab I am building will just be a pretty car with a 2.7 normal driving.
Thanks
Nathan

MBEngineering · 03-12-2007, 10:11 AM

HI Nathan if you must have a low ratio (7;31) and a high HP engine , then a 8;35 would be better and lot stronger as a 7;31 will not last on a high HP engine for standing starts from the lights.

regards mike

artisticent · 03-12-2007, 11:37 AM

Thanks mike,
Ill throw it in the car with the 2,7 then. Im from hastings originally and live in S. California. Im actuall as we speak having 6 pre 76 mini's imported over here. What a world
Nathan

JMatta · 03-12-2007, 02:06 PM

I concur with what pieterk brought to light. My '73 2.4 has "airport" (short 3rd, 4th and 5th) gears with a 7:31. The car is very responsive and quick, but runs out of gas at 135 mph @ 7000 rpm. This is great for every track I drive, but Road America. Three long straights at redline...ouch! But for "normal" tracks, I rarely drop below 3rd because the pull out of corners is so good.

artisticent · 03-12-2007, 02:28 PM

So I am to my understanding, I shouldnt really use this in either car. I will be racing the 73 rsr but also using it at top speeds and road use. The other car is only road use, so maybe I sell it and buy a 915 instead.
Thanks
Nathan

Bill Verburg · Bill Verburg's Garage

w/ the same tires top is about the same between a 7:31 and an 8:31

the biggest difference is in the gear split %, I prefer the 8:31 version

MBEngineering · 03-13-2007, 02:04 AM

HI Nathan I have 2 mini's in the garage at the moment and a rake of spares, one is a mini city and the other is a late cooper (original ) what are you getting shipped over ?? and good luck with the 911 project's. if you get stuck for bits on ither the mini's or the 911's give me a shout .

regards mike