Anyone running different ring and pinion
 

Just wondering if anyone is running the lower 7.3 to 1 R&P in their SC? I am interested in how it effects general all around driving and the 5 gear RPMs at highway speed.

Thanks

Drew Reed
79 SC

Drew,

Let me know what your rear tire diameter or size is and I can send you a plot (graph) of what you rpm vs. speed chart would be for all five gears, assuming 'regular' gearing of 3.18, 1.83, 1.26, 1.0, and 0.82 ...

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Warren Hall
1973 911S Targa

I have heard and seen stuff about this modification here are there...

what exactly does this do?
I am going to rebuild my 915 soon, maybe something I can use...

Replacing the ring and pinion gears with a set having a lower numerical ratio will multiply torque in all five of the drive gears. It will also increase the engine speed at any given rpm. This gives greater excelleration at the expense of fuel mileage, engine longevity, and durability due to the increased stresses.

Woops, I mean engine (RPM) speed will increase at a given vehicular (MPH) speed.

[This message has been edited by RarlyL8 (edited 12-19-2000).]

Thanks Warren

I'm running 245/45/16s
on the rear

Thanks

Drew Reed
79SC

Same effect as running smaller diameter tires.

[This message has been edited by wckrause (edited 12-19-2000).]

Does this mean that if I go from 15" to 16" wheels I will lose excelleration?

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Rob Fix
'78 SC Targa

Depends on what size tires you use.

185/70R15 are about the same diameter as 205/55R16's. Somebody had posted a webpage with a tire size calculator on it in a previous thread.

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Bill Krause
'79 911SC Euro

i'm running a 7/31 r&p currently in my 86 (same 915 'box)...great acceleration, but tachs at 3600 at 70 mph. gas mileage suffers about 3-5 pts on the highway, too. phenomenal mod for short track work, though.

I have the 7:31 (the original transaxle) mated to a 3.6. It's awesome.

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Jack Olsen
1973 911 T (3.6) sunroof coupe

Oh, I bet, Jack. I'm either gonna have great dreams, or nightmares, thinking about driving a 3.6 with a 7:31 tranny.

In a car like that, seats without headrests would be useless.

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'83 SC

so what is involved in changing from a reg 915 to this 7.3:1

When I rebuild my transmission what will have to go, and what new parts will I need...are all the mods in the tranny or are there other ones...

While on the subjects any clutch press plate, fly wheel upgrade sugesstions?

I am a daily driver, maybe some track time kinda car.

thanks

MJ

P.A.P. is selling the things on Ebay, it looks like. I don't know what their real price would be, but they're starting the bidding at $200.

http://cgi.ebay.com/ebaymotors/aw-cgi/eBayISAPI.dll?ViewItem&item=529564522&r=0&t=0&show Tutorial=0&ed=978129385&indexURL=0&rd=1

I always thought the best way to add it to a 915 would be to buy a rebuilt 73 unit, and use an in-need-of-a-rebuild 8:31 as a core.

[This message has been edited by JackOlsen (edited 12-20-2000).]

Jack / Drew.

This is exactly what I am going to do. I purchased a rebuilt 915 with standard r&p. After I realized the cost of shipping my core back, the resulting money I would get wasn't worth it.

So, after reading the posts from Warren and Superman, I decided to shelf my 'in-need-of-repair' 915 and rebuild it to the 7.3:1 r&p. Should be a good learning experience, and then I can take out the standard and keep it as a spare.

Gotta love spare 911 parts in my garage.

Ted


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'80 911-SC Targa

You guys are going to love this one; my 912E has, I believe a 7.3:1 R/P. My gearbox is called a 923 box, like the factory designation 923.XXX.... for the 912E-specific parts. I'm not sure if it's exactlythe same as the earlier 911 R/P, but, as to the merits of deeper R/P's, here's the funny part, my 912E with all of about 90HP has been characterized by my mechanic (who has an interesting background, including working at the factory, and tells great stories like doing 360s in the rain on the autobahn in 928 prototype #2 with a crazy factory test pilot!) as being faster to about 20MPH than a 911 turbo. So, how's that for a reason to go to the deeper R/P? I told you you'd laugh.

After 20mph, well... forget it.

happy holidays all...

Dave

My fairly stock 1983 SC 3.0 and 915 with 8:31 R/P has in might thought about as low of first gear as I would want, if it was any lower IMHO it would be wasted (if I wanted a compound gear I would buy a truck), and second gear is about right, I don't want fifth gear any shorter either. The motor by the end of January will be finished and with all the torque it will have I don,t need lower basement gears.

The 7:31 R/P are not as tough as the 8:31, thats why the factory went to 9:?? for the newer trannys, give the pinion gear more bulk. The 8:31 is something like 22% tougher thru the narrow part of the gear tooth.

It might be cool to have
some lower gears but the 7:31 is not the answer to everyone's problems. You need to spend some time and look at a gear chart and then at you wallet.

I'm pulling down my 915 for a rebuild and ZF limited slip, If I change any gears it will be third and fourth but not the R/P!

Randy Jones
1971 911

I might agree with Randy about the 1st gear.

Plus P.A.P. wants $1200 for the R&P on their site. Maybe better to just rebuild and change 2nd-4th gears if that is possible.

Drew Reed
79SC

I don't know how you guys drive your cars, but my SC runs just shy of 3200RPM @ 70MPH. That feels just about right to me. My tires are 24.5" tall and I presume the R&P is stock. First gear is already wickedly short. I have checked the speedo and it is accurate, I can only presume that the tach is good as well. Fifth gear runs strong to redline. One possibility for the Carrera owners would be to swap in an SC tranny. The Carrera trans has higher ratio gearing that the factory deemed appropriate for the extra power output.

RarlyL8, my (7:31) car is going 70 at about 3250 rpms (although my speedometer claims about 76), so we're not talking about such a huge difference if your speedo is accurate.

For my set-up (7:31 r&p, 245/45ZR16 tires in back), 3500 rpms = 18 mph in 1st, 32 mph in 2nd, 45 mph in 3rd, 60 in 4th, and 76 mph in 5th.

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Jack Olsen
1973 911 T (3.6) sunroof coupe

I guess I just don't understand all of this complaining about first gear at all!!! My idea of the perfect first gear is one that you can just let out the clutch at any rpm from idle upwards, have the tires bite and you take off without any fanfare ... then, depending on whether you are at a slalom or at the onramp of a freeway with 18-wheelers bearing down at 80 mph, you nail the throttle ... and all of the engine power is transferred into forward motion with most haste and no tire smoke or spinning of tires! Dealing with an upshift that occurs at 34 mph instead of 39 mph does not really seem like an issue worth complaining about, and, if you think changing out 4th and 5th gear sets is cheaper than a ring & pinion ... THINK AGAIN, as 915 gearsets are $500-$600 each! There is always the option of buying a used '72-'74 915 transaxle, which I did -- for under $1000, complete with 5 gearsets, and misc. parts!!! You MIGHT even get one that isn't trashed or needs new dogteeth!

In short, there are many options out there for the 915 ... even if you haven't thought of or considered them! I guess most of you would have gone berserk if I suggested the 7-to-37 (5.286:1) ring & pinion ... JEEZ, you would have to deal with upshifts at 29, 50, 72, and 90 mph ... as well as incredible pulling power up to a top-end of 110 mph!!! I guess that would be too much fun to even consider!

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Warren Hall
1973 911S Targa