911 RSR 3.6 conversion 915 gear ratios?

[CJMRSR]

Rebuilding a 72 911 intoan RSR replica, my 915 for the build has been cracked open and it requires a full rebuild.

1. Should I build back to original spec gearing or make changes to improve performance with the 993 3.6 non varioram I'm using which will be circa 300hp

2. Should I fit an LSD? Mainly a street driven car, might do a couple of track days a year.

Would appreciate hearing feedback for 3.6 conversion into early chassis owners with similar cars..

Thanks in advance..

[dwa917]

my tuppence... having just gone through that with my RSR style build - with a high butterfly MFI 3.0 - ~300HP engine...

ideally you want the engine torque/HP curves to optimise each gear ratio... the car is aerodynamically limited to about 150/160 MPH, so having a top gear that goes much above that is superfluous from a performance / circuit perspective... but, if the majority of your driving will be street, you might want a low RPM cruising gear for 5th...

a stock 3.6 non varioram has about the same torque that I ended up with on my 3.0, so potentially SCRS gearing would be good... but recommend you get other opinions with folks that have more exerpeince with the 3.6 and 915... LSD is personal and great for circuit...

Bill V might have some experience / expertise to share here. He was instrumental in helping me think through gearing...

Here is link to section where i look at gears...

https://forums.pelicanparts.com/porsche-911-technical-forum/1118487-my-life-porsche-my-latest-last-build-2-8-rsr-style-9111121235-a-17.html#post11754145

[shoooo32]

To me, short ratios make sense on race cars and engines built to rev to the moon. For your build (300hp 3.6 with lots of torque), I'd stick with the stock ratios. As for LSD, that's a worthy upgrade for any hotrod 911.

[nickelplated5s]

Maybe PeteKZ will chime in. His 3.2 has a taller 5th but I don't know what ratio. Makes it nice at freeway speeds. My 901 box does not and I wish I had done that when it was open.

[3rd_gear_Ted]

Tall 1st or short 2nd?
Pick your choice.
Sweet spot your 3rd & 4th.
Short 5th.
My 3.6 drives like its in sport mode.
My fave is 3rd gear of course

[CJMRSR]

Originally Posted by dwa917

my tuppence... having just gone through that with my RSR style build - with a high butterfly MFI 3.0 - ~300HP engine...

ideally you want the engine torque/HP curves to optimise each gear ratio... the car is aerodynamically limited to about 150/160 MPH, so having a top gear that goes much above that is superfluous from a performance / circuit perspective... but, if the majority of your driving will be street, you might want a low RPM cruising gear for 5th...

a stock 3.6 non varioram has about the same torque that I ended up with on my 3.0, so potentially SCRS gearing would be good... but recommend you get other opinions with folks that have more exerpeince with the 3.6 and 915... LSD is personal and great for circuit...

Bill V might have some experience / expertise to share here. He was instrumental in helping me think through gearing...

Here is link to section where i look at gears...

https://forums.pelicanparts.com/porsche-911-technical-forum/1118487-my-life-porsche-my-latest-last-build-2-8-rsr-style-9111121235-a-17.html#post11754145

I have seen your build thread, its excellent and the end result looks incredible. I was interested to see your exhaust setup I nearly went with the Eisenmann for mine but plumped for the Mittelmotor muffler. Ive got EB motorsport stuff on mine including the rear 15x11s they really do some great stuff.
I'll look at the SCRS gearing and go through it with my transmission guy, thanks for the link.
Im hoping Bill V will chime in, he's given me some great advice already on the suspension setup.

[CJMRSR]

Originally Posted by shoooo32

To me, short ratios make sense on race cars and engines built to rev to the moon. For your build (300hp 3.6 with lots of torque), I'd stick with the stock ratios. As for LSD, that's a worthy upgrade for any hotrod 911.

That was my initial thoughts. I just wanted to make sure I am not missing a trick, was curious as to whether there are some ratios out there that can improve performance and general street usage. Driver engagement and usability is paramount to me over shaving a few milli seconds off on 0-60s etc.

[CJMRSR]

Originally Posted by nickelplated5s

Maybe PeteKZ will chime in. His 3.2 has a taller 5th but I don't know what ratio. Makes it nice at freeway speeds. My 901 box does not and I wish I had done that when it was open.

Thats kinda the thing I'm trying to understand. I will be doing a lot of touring in her, so a taller 5th seems like a good idea for cruising intercontinental. I have a couple of weeks to get the gearing sorted then its getting closed up..

[Bill Verburg]

Originally Posted by CJMRSR

Rebuilding a 72 911 intoan RSR replica, my 915 for the build has been cracked open and it requires a full rebuild.

1. Should I build back to original spec gearing or make changes to improve performance with the 993 3.6 non varioram I'm using which will be circa 300hp

2. Should I fit an LSD? Mainly a street driven car, might do a couple of track days a year.

Would appreciate hearing feedback for 3.6 conversion into early chassis owners with similar cars..

Thanks in advance..

Depends on usage an driver preference but I found the 915/44 too short for street use, though it was a blast on a track w/ just a bas 964 3.6 w/ 245/45 x16 tires and a 915/02 from '72 is shorter yet. You don't mention tires they will make a significant difference in final gearing

The thing about 3.6 and bigger is the fat torque curves that they have even more so w/ vram. short gearing is for smaller revvy engines fat torque curves are good w/ taller gearing.

It's fuuny but the gearing on my 2 cars is eerily similar

the 993 has a 3.8RS engine running through g50/30 which was used in 993Cups and RSRs, my '76 has a 3.6nvr w/ cams and headers running through a915/67, both are limited to 6800 rpm

torque curves comparison


thrust comparison


both could use a little taller top gear for street use but not much taller

the tall 1& 2 in the /30 is great for track use while the shorter 1 in the '76 is great for stop lights

I'd definitely want an 8:31 cwp for strength w/ lsd GT asymmetric if possible ZF otherwise

the mg 915/02 case in ok but there were improvements as the years went by w/ the '76 915/44 being the best mg case, The std steel girdle is a big plus for the mg cases, if you went w/ a latter al. case best to have a Wevo girdle installed

[gled49]

You didn’t say which year gearbox you are using. 72 box should get input shaft seal mod. I always update like the factory did, with reinforced mainshaft, 1st & 2nd shift guide and slider, 3rd & 4th shift guide and slider, all associated synchro upgrades, clamping plate, side cover and LSD. Gear ratios at your choosing. Later model gearboxes include reinforced parts.

[spuggy]

My $0.02:

Ideally, raise both 1st and 2nd - (993RS used the same 1st ratio as turbo).

Raise 1st because it's not helpful being too short to be useful. Raise 2nd also as there's little/no advantage in it being very close to 1st.

The most useful place for you to pitch 2nd will depend on the characteristics of your motor, car weight and/or your usage. For a relatively light N/A car with a similar motor, 993RS is probably a good aiming point.

Make 2nd/3rd/4th relatively close by all means - as this is where you'll likely spend most of your time when not in traffic or on the freeway.

Should you do significant freeway miles, you'll love a tall 5th for a relaxed cruise.

I've never regretted fitting a motorsport 35/65 LSD - on both transmissions I've used. Worth every cent.

[shoooo32]

I run a 29/22 5th gear - 80mph is about 3200 RPM. Perfect for eating up freeway miles.

29/22 was the stock 5th gear ratio in '72-73 so there's plenty of them around.

29/21 was stock 5th in 1974, but was too big of a drop from 4th for my heavy car. I gave it to Chris Balich and his lightweight coupe has no issue, so your results may vary.

[Bill Verburg]

Here's wat you have in a '72, actual speeds in gear depends on rear tire spec


I found this too short w/ 245/45 x16 and stock 964, I actually had a 915/44 but that has an electric speedo, the /40 is the last mech. speedo and has the same gearing as the /44


this is what I settled w/ for a hot-rodded 3.6 993 and 3.8RS, 275/40 x17 tires were a little tall for the 3.8 and more so for 3.6, now using 255/40 x17 as a good fit

SC/RS gearing but doesn't show the true nature of the car is it has a much higher red line



just for comparison here are the 993 boxes
993RS, Cup and RSR optional trans


and 993tt the g50-51-51 is not the same as the other 993 g50s it uses a larger case to accommodate the larger cwp

[rswannabe]

Originally Posted by Bill Verburg

the mg 915/02 case in ok but there were improvements as the years went by w/ the '76 915/44 being the best mg case, The std steel girdle is a big plus for the mg cases, if you went w/ a latter al. case best to have a Wevo girdle installed

Why do you feel the 915/44 is the best mag case transmission? There were additional improvement introduced up through the last mag cased 915/61’s, so I thought those would be more desirable.

[Matt Monson]

Get the 7:31 ring and pinion out of there. It’s weaker and too short. You want an 8:31 from a later gearbox. All the rest is personal preference.

[Geneman]

What Matt said !!!

[PeteKz]

CJMRSR: It sounds like you will use it primarily as a street car. With a 3.6 engine, you have plenty of torque in a light car to pull higher gearing for a street car.

I have a high 5th gear in my 1973.5 car (not sure, but it's probably a 29:21). It has a built 3.2 engine and 915/40 (no LSD). I turn about 3000 RPM at 80MPH. Since I drive a lot on the highway, I really like having long legs. Lowering the highway RPM even a couple hundred makes the car feel much more relaxed. When I drive on a track, I don't get above 4th gear anyway, so a high 5th doesn't matter there.

[CJMRSR]

Originally Posted by gled49

You didn’t say which year gearbox you are using. 72 box should get input shaft seal mod. I always update like the factory did, with reinforced mainshaft, 1st & 2nd shift guide and slider, 3rd & 4th shift guide and slider, all associated synchro upgrades, clamping plate, side cover and LSD. Gear ratios at your choosing. Later model gearboxes include reinforced parts.

It's a 915/61 which is a later 78-79 box.

[CJMRSR]

Originally Posted by Bill Verburg

Depends on usage an driver preference but I found the 915/44 too short for street use, though it was a blast on a track w/ just a bas 964 3.6 w/ 245/45 x16 tires and a 915/02 from '72 is shorter yet. You don't mention tires they will make a significant difference in final gearing

The thing about 3.6 and bigger is the fat torque curves that they have even more so w/ vram. short gearing is for smaller revvy engines fat torque curves are good w/ taller gearing.

It's fuuny but the gearing on my 2 cars is eerily similar

the 993 has a 3.8RS engine running through g50/30 which was used in 993Cups and RSRs, my '76 has a 3.6nvr w/ cams and headers running through a915/67, both are limited to 6800 rpm

torque curves comparison


thrust comparison


both could use a little taller top gear for street use but not much taller

the tall 1& 2 in the /30 is great for track use while the shorter 1 in the '76 is great for stop lights

I'd definitely want an 8:31 cwp for strength w/ lsd GT asymmetric if possible ZF otherwise

the mg 915/02 case in ok but there were improvements as the years went by w/ the '76 915/44 being the best mg case, The std steel girdle is a big plus for the mg cases, if you went w/ a latter al. case best to have a Wevo girdle installed

Thanks for all the info Bill. It's a 915/61 box not a 72 MY box.. so it's a 78-79 reading on the forums..

Tyre wise it's Front - 18/60 R15 Rear - 26/61 R15 Michelin TB15s..

Sounds like a taller 5th is the way to go for cruising. What is your first gear spec on your 74?

[Bill Verburg]

Originally Posted by rswannabe

Why do you feel the 915/44 is the best mag case transmission? There were additional improvement introduced up through the last mag cased 915/61’s, so I thought those would be more desirable.

They went to aluminum/silicium for the '77 915/60 & 915/61 used for '78

no steel girdle w/ al. cases which is extremely desirable for a 3.6 and more so for a 3.8, there are aftermarket steel girdles available notably from Wevo

there were some worthwhile internal improvements for the /6x on but all that can be changed w/ a regear

there were some early '77s that had mg cases, I'm unsure of the material or even the name as the factory only lists /60 and /61 for '77 , '78 & '79 model 911s