915 gear ratios for 3.6 track use

Rusty911 · 07-09-2022, 01:08 PM

There quite a few threads on 915 gear ratios for track use in general but many are suggested for smaller displacement high revving engines like a 3 liter and some recommend ratios like the SC RS.

What I haven’t found are recommendations for a 3.6/3.8 track engine with a wider torque curve. I have read that some racers use a very tall first gear and then use closer gears of 2 thru 5.

If the engine powerband is from 4k to 7k shifting at 7.5k with a top speed of 160mph for top speed on straights what might be some good gear ratios combos for a 915 if using taller first gear?

Rusty911 · 01:13 PM

This would be for track use car.

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by Rusty911

There quite a few threads on 915 gear ratios for track use in general but many are suggested for smaller displacement high revving engines like a 3 liter and some recommend ratios like the SC RS.

What I haven’t found are recommendations for a 3.6/3.8 track engine with a wider torque curve. I have read that some racers use a very tall first gear and then use closer gears of 2 thru 5.

If the engine powerband is from 4k to 7k shifting at 7.5k with a top speed of 160mph for top speed on straights what might be some good gear ratios combos for a 915 if using taller first gear?

This is what a 4.0 w/ a g50/30 looks like, this as close to perfect as it gets

w/o 6 the top speed is around 162 w/ 285/30 x18 tire and 7250 red line, ~156@7500
cwp 3.44
1 3.154
2 2.00
3 1.52
4 1.212
5 1.027

Bill Verburg · Bill Verburg's Garage

Can't go wrong w/ SC/RS either

icarp · icarp's Garage

To hit 160 MPH you will need monster power , A Knight Race engine
Ian

3literpwr · 07-09-2022, 05:36 PM

I have SCRS gearing in my rally car with a hot 3.0. My redline is 7200, which gets me to about 140mph if I can recall. Great for a rally car, but maybe not for a track car on bigger straights.

Rusty911 · 09:09 PM

Thanks very much Bill for posting those ratios for my intended RPMs. Icarp, yes expecting about 400hp more or less.

3literpwr, what size rims:tires are you using?

I thought some racers used a much taller first gear that could go 60 mph or higher, almost like using the SCRS second gear as a first and then other gears would be close thereafter. I think theory is to make first a more usable gear on the track. Anyone familiar with a set up like that?

Evan Fullerton · 07-10-2022, 02:54 AM

400hp and 915 isn’t a recipe for longevity.

That said, an active 1st gear is something people do. Makes the car pretty awful for getting through the pits or onto a trailer though.

winders · 07-10-2022, 05:57 AM

I have almost two race seasons with a mag case 915 hooked up to a 431 HP/341 ft-lbs TQ William Knight 3.6L race engine spinning up to 8000 RPM regularly. It has an active 1st gear using a Guard Transmission mainshaft. It also has a Guard Transmission 930 LSD and gears 1 through 5 are Guard Transmission gears as well. I had Bill Rader re-gear it and go through it using his side cover and single piece bearing retaining plate.

I am not particularly nice to the 915. On warm up laps I am in 1st gear accelerating hard at least a couple of dozen times. Race starts are usually in 1st gear and are not nice to the box. I am amazed at what a well prepared 915 can handle. I just sent 915 to Bill Rader to refresh it as the 2nd gear synchro was having issues and it was time to look the 915 over.

I don’t have any issues driving the car around the pits or into the trailer with a 1st gear that does ~70 MPH at 8000 RPM. It helps to have ITBs and a bunch of torque.

Rusty911 · 08:50 AM

Thanks Scott for that info. That’s the other option I wanted to explore which might be more ideal. It sounds like the Guard main shaft and gears helped strengthen your gearbox as well as the other mods?

Your engine will be stronger than mine but maybe I could do something similar in the gearing. Maybe have a 60 mph first gear and then have custom gearing all the way through. Possibly my stock 5th might work out iif gears 2 thru 5 move up.

Let me ask you a general question as to what RPM drop do you see when you’re accelerating on a straight part and what top speed are you able to reach on the various tracks you race in?

I’m impressed with what you did to your engine to get that much output from a 3.6 but equally interested to learn about your gearbox since that aspect is interesting to me.

Sounds like there to approaches SCRS type or talk first and close 2 thru 5.

Thanks! Good stuff to learn about.

Bill Verburg · Bill Verburg's Garage

w/ a strong 3.8 to 4.0 you want a reinforced side plate and steel girdle for the shafts, Mg case has the girdle stock, aluminum needs a Wevo steel girdle to be installed., late 951/6x and /7x have the strongest stock side plates Wevo side is better yet

a Porsche Motorsports g50/30 uses the tallest factory 1st gear 10.862 overall good for 49mph @7500 w a 24" rear tire , 49mph@7500, I have this in my car and like it compared to the street cars 13.150 o/a ratio but it's more of a pit out gear, never gone lower than 2 on track, Monticello full course has 3 2nd corners

915 o/a 1st is 12.330 o/a w/ a 24" rear tire 43mph@7500

Gt used to offer 915 mineshafts from stock 3.182 all the way to 1.933 remember SC2nd is 1.833 so do a few starts in 2nd to see how you like it

w/ a torquey engine you don't want the gears to be as close or low as w/ a less torquey motor, lower gears means more shifting both up and down w/o a sequential this is slow

Rusty911 · 07-10-2022, 09:09 AM

Thanks Bill. That’s the info I was looking for. So, assuming I get a tall first, what would be good gear ratios for a torquey big motor like 3.8 with 76.4 crank? Would stock gear ratios from a 8.31 ratio work? I have a stock 75 915/40. I was already planning the side plate, bearing retainer, guard LSD and will send to Bill Raider most likely or Cal Motorsports.

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by Rusty911

Thanks Bill. That’s the info I was looking for. So, assuming I get a tall first, what would be good gear ratios for a torquey big motor like 3.8 with 76.4 crank? Would stock gear ratios from a 8.31 ratio work? I have a stock 75 915/40. I was already planning the side plate, bearing retainer, guard LSD and will send to Bill Raider most likely or Cal Motorsports.

the 0 his mg and already has the steel girdle for the shifts, good gearing too, the same as the /44 my C3 came w/. For street use I found it to be on the short side w/ a stockish 993 3.6, for track
@7500 w/ 24" rear tires
43 75 110 138 168 mph

I'd drive it that way and see how it works on the tracks you go to

perfect at WGI can be awkward in places at LRP

l

Rusty911 · 07-10-2022, 11:23 AM

Thanks for running those numbers and sharing your experiences using those ratios for track. Sounds doable and more cost effective.

winders · 07-10-2022, 12:28 PM

Quote:

Originally Posted by Rusty911

Thanks Scott for that info. That’s the other option I wanted to explore which might be more ideal. It sounds like the Guard main shaft and gears helped strengthen your gearbox as well as the other mods?

Your engine will be stronger than mine but maybe I could do something similar in the gearing. Maybe have a 60 mph first gear and then have custom gearing all the way through. Possibly my stock 5th might work out iif gears 2 thru 5 move up.

Let me ask you a general question as to what RPM drop do you see when you’re accelerating on a straight part and what top speed are you able to reach on the various tracks you race in?

I’m impressed with what you did to your engine to get that much output from a 3.6 but equally interested to learn about your gearbox since that aspect is interesting to me.

Sounds like there to approaches SCRS type or talk first and close 2 thru 5.

Thanks! Good stuff to learn about.

There is a lot of theory being thrown around here but let me tell you what I have found through experience racing 3.6L engines with a 915 gearbox in a race car:

If you can afford it, you want an active 1st gear. I have used 1st gear on all of the California race tracks I have raced on except Buttonwillow. I probably should use 1st there in one corner.

Why do you want an active 1st gear? So you can avoid having to shift into 5th gear. If you can stay in the H pattern you are much better off as shifting into 5th with a 915 takes too long.

I have never had to use 5th gear on California tracks. My 4th gear is good for 149.6 MPH.

With my 915, my biggest RPM drop is between 1st and 2nd at 2500 RPM. the drops get progressively smaller with 4th to 5th being a little less than 1000 RPM.

I am not going to disclose the ratios I am running now, but, I got there by examining qualifying and race videos to see what was happening on the tracks I race on. I used to race with a group called the PRC in their GTL class. The class had a fairly restricted 3.6L formula and I ran gearing recommend by the car builder. That gearing was based on an engine that made 320 HP and revved to 7000 RPM. With the new engine, I ended up changing 4 of the 5 forward gears.

Bill Verburg · Bill Verburg's Garage

here's a survey of some race 915s for use w/ big911motors

winders · 07-10-2022, 03:13 PM

That’s my old box…which was terrible with my current engine.

Mahler9th · 07-10-2022, 03:13 PM

I have a bunch of experience with a car similar to Winders' with both a 3.8 and 3.6 at Norcal tracks.

I have owned mine since 2001, and have gone through several gearing changes.

I have all of the gearing charts for my different gear sets and many from friends cars including Winders' car.

An active first can get you lap time, but it isn't a lot, depending on what you call a lot.

I have never run an active first-- could not justify the expense.

That is the key decision in my opinion and experience.

The parts from Guard Transmission-- the active first, and all the cogs and the excellent LSD are the cat's pajamas. So whatever you decide, GT's products are an awesome choice in my experience.

Winders' car and mine run the same size wheels and tires and were built by the same builder.

We run 16 inch wheels.

As far as things to do to make the transmission last-- we have had many, many cars racing with mag case 915's with high outputs.

Winders' is one of the highest if not the highest.

In my case, I'd guess my highest output was about 350-370 bhp with my old 3.8 and a huge single TB.

Basic things like alu side cover, thoughtful shifting and thoughtful and diligent maintenance have ensured that most of the folks I know have been able to run high output big motors and mag case 915s in NorCal sprint racing without too many crazy problems.

I do my own tranny work, except for ring and pinion set up-- for that I use Reno Rennsport.

One of the owners of that biz built my car and Winders' car and many others in our area.

They are a great resource for many of us here in Norcal.

Happy to share more off line if I can be of any help.

Rusty911 · 07-10-2022, 03:23 PM

Thanks again to all of you! This discussion has helped a lot.

Mahler9th · 07-10-2022, 03:29 PM

I should add that having to use fifth gear at Thunderhill has been no big issue for me.

BUT... at Sears Point it is has been a pain in the ass, all else equal. Even with that I could not justify buying the parts for active first and replacing all or most of the the rest of the cogs.

Nowadays, however, the used parts you may have with a mag case 915 could maybe bring more money to offset costs than when I was running a lot in anger. But of course the new cogs are way more expensive.

One thing I'd start planning for with any 915 with 8:31: those crown wheel and pinion sets are harder and harder to find used in "race application" condition.

Oh and of course for emphasis: "thoughtful" shifting. Especially for track use cases that ARE NOT wheel-to-wheel racing.