I realize it is probably torque that kills a transmission, but I don't know the torque output of my engine.

I am building a 901 transmission for my IMSA 914 and wonder what upgrades it will need. I have some possible upgrades I could do now.

First, I have a set of 930 stub axles, CVs and the corresponding sway-away axles for a 914. They are stronger and the CVs are available for $80 from pelican. Downside is I have to destroy the existing rear bearings in my axles to use them, which were rebuilt by PMB.

If I have 200 HP to the wheels are the 930 CVs a necessary or worthwhile upgrade?

Second, should I upgrade to the billet intermediate plate at this HP level?

If I do, will I have to reset the pinion depth? That is something I can't do myself and would need to find a shop here in Florida that can do it. I know this is overkill for a typical 914, but with 12 inch rear tires and a 2.5L six, is this when the upgrade is worth while?

I am not adverse to starting with the 914 axles and upgrading down the road. Pulling the engine and transmission is not that time consuming. I would hate to break the smaller axles or CVs the first time at the track though.

I have a stock gearset and know there are probably better gear choices for Sebring, Daytona, etc. No idea where to start with those.

I will be running a transmission cooler to start and may move to gear set spray bar over time. I have read that the mesh point of the gears is where to aim the cool oil sprays, not sure this is correct.

I appreciate any advice people have.

John

I would ask Glenn/GMS for suggestions

My Experiences:
I ran a 914 transmission in a dedicated track car for 23 years. Over time I upgraded the gear box to billet intermediate plate, steel synchros, modified gear set (AFNSY), external oil cooler, pump, and spray ports.

Heat is the death of transmissions and I was working it hard with a 3.4 liter twin plug at nearly 290 rwhp.

In 23 years I had two on-track failures and they were before the cooler and spray ports. One failure was ring and pinion, and one was 3rd gear. I did refresh the box about every 3 or four seasons depending how many events I did. A big year was 10 events.

I used 1st gear sparingly - only to get moving and loading the trailer. Most tracks involved using only gears 3,4,5.

My CV joints and axles were 100 mm CV's sourced from a 944. Direct bolt-in if you also source the trans output flange and hub flange from early 911/912. Never had a CV failure.

I used a thermo switch epoxy to the side of the case which energized a relay to turn on the cooler pump at 180 degrees F.

Sorry about the filthy condition of the parts in the photos - best I could find in my archives.



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A 914 race builder out here…. Recommends 205hp as the division between the 914 / 916 trans

I would blow my trans every other year without cooling
I added the intermediate plate and cooling after 4 rebuilds
Now 4 seasons on that box
And I’m Under 200 horse

Some good advice in here. Seems like you would be good with some upgrades to a 901. I have a little less track experience than others, but I have run over 200hp in my stock 901 box for over a dozen 20 minute sessions and have yet to have a problem.

For gear ratio selection, You want to be at redline in top gear at the braking point at the end of the longest straight. Obviously this applies to only one track, so unless you are going to have multiple gearboxes or are going to change gearsets for each track , you may have to compromise. I had two boxes a long and a short gear box (AFNSY and AFMSX) but always kept the long box installed and kept the short box in the trailer as a spare. I could hit redine in 5th on the front straight at Miller Motorsports (now UMC) which was 145-150 but was not at redline on most other tracks.

You have to know the tire diameter that you will be using, your redline, and the track with the longest straight to plan your gearset. I have an interactive excell file that lets you change the tire diameter and or gear ratios but I cannot attach an excell file so I've saved it as a JPG.

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Thanks for the great replies everyone!

Seems like the 901 should last a while with the right upgrades.

Love the gear ratio chart @slivel!

I plan to run the same cooling pump and radiator that were in the car when I got it.

Adding a spray to the diff seems like an easy upgrade and maybe a couple more, like 3rd, 4th, 5th gears.

Do most people use -10 AN lines?

If I use the billet intermediate plate, do I need to reset the pinion depth? That is a big question. If not, then I think it is a no brainer to get it.

Maybe the 108mm CVs from the 930 are overkill? Maybe the 100mm CVs would be enough?

John

I think that I used -8 lines on my car.

Regarding cv's, I put this together years ago:

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Thanks Steve, this is really a big help!

I have not picked a wheel and tire combination yet, so I will hold off on picking new gears. I imagine some of these gear sets are rare and expensive.

John

I'm at 215rwhp and run about 10 events per year on the 901's. Yes, driver error adds to transmission becoming a "consumable" like tires. I'm using two trannys now, one is AFMSX for short tracks like Roebling and Summit Point and the other is AFNSZA for VIR, Daytona, Sebring, Watkins Glen. I'm on 24 inch diameter rear tires. There's a really neat worksheet here:
http://www.914world.com/specs/regearing_901.php
--it's an excel spreadsheet that you can experiment with to choose tire diameter & gearing.
I pull the magnetic drain plug after each event & usually find a halo of "hair". ON occasion there will be a chip or sliver from driver clumsiness. I've found that one box or the other needs medical attention maybe once a year. In my opinion, get two trans's--keep one stock gearing and set the other with AFNS for 1-4th and find a ZA for 5th or just use a flipped N for 5th as a Z to start with. As you gain experience you'll likely try different combo's.
Not a "real" expert but I think if you replace intermediate plate then you'll need to set R/P depth again...

My racecar has ~260 hp and has run a 901 for years. It's generally held up well. After 80 hours, I did lose a couple gears, so time for a rebuild. Stock 914 CVs were replaced a few track hours ago - they held up pretty well.

Stock 914 cvs and axles are holding up well on the track with 260hp???

In my experience, the stock CV's will survive racing conditions and high HP, but you must use quality grease containing molybdenum and service (repack) regularly. Also and probably most important, avoid using the curbs and running over the gators at tracks. Off track excursions are deadly to the CV joint. The loading and unloading of the CV will destroy it.

Since keeping a 901 in one piece is an old and ongoing challenge for many of us, can I add my modifications for your review and suggestions for what I've missed? I've had my vintage race 2L single plug, narrow body 914 for almost 25yrs. For the first 5yrs I ignored advice to install an alum intermediate plate, as I never had an issue. Then the gearbox seized solid, mid corner, mid race, destroying the R&P and more. I discovered the mainshaft starts dancing in the loosening main bearing, and it all blew up.

Since then I've added the following [from front to back]:
[1] Renn shifter
[2] solid firewall bushing
[3] full Tangerine shift linkage kit
[4] Patrick clutch pulley
[5] CMS intermediate plate, side cover, and mainshaft bearing retainer
[6] and new this year, trans cooler with fan [i race mainly in Canada, on faster tracks, so super hot temps are less of a problem].

More on the cooler:
You can't see in the pic, but it has an enclosed fan on the backside. I know mounting it in the trunk without ducting isn't ideal for air circulation, but I wanted it where it would be protected from damage, and as far forward as possible. And because it's a biggie [it was given to me], mounting options were limited. It only squirts the R&P, through the side cover. Activated with a manual dash switch. Driving improvements: so far, it has eliminated the baulky shifting typical late in a race. Hopefully longevity will follow.

Anything I missed? Any changes?
Thanks!
Gavin
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... oops let's try for a more useful pic size

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Yep. About 80 hrs of use over the years before I lost two gears. And I think that may have happened due to a linkage misadjustment that caused less than full engagement of the (tall) first gear.

I have a friend who raced a 914/6 for years with 300+ and a 914 trans without even a cooler.

It's possible if you don't abuse the trans.

I think that high rpm also has an impact on transmission life in addition to hp and torque


I never had trans problems with my old race -4

But the 2.0/6 has a much higher red line

I do not think the trans bearings love 7 or 8k
More rpm creates more heat

Horsepower can be managed with cooling. Torque can exceed the physical strength of parts, cooling can't help with that.

Interesting point about RPM. I've assumed that also can be managed with cooling, don't have any data to support that though.

If you need to build a new race gear box, I would highly recommend REM finishing and WPC treatment of the internal gears and shafts. REM will knock about 15 deg off the temps due to friction and that will help keep the temps easier to manage.

As much as some people try to fight it, Swepco is also probably the best oil for these boxes. Redline is not a good answer, seems like the internals don't hold up as well with it compared to Swepco.

Man impressed if that is the case. I blew my cv apart this summer on my 2.4s -6 and now working on upgrading over the winter

In answer to the billet plate question, yes and yes. The intermediate plate takes a lot I of abuse as the thrust is directed at it. The billet upgrade is very worthwhile for that as well as having a superior (and new) int main bearing retainer. It is also wise to drill out (ideally on a milling machine) the case and use through bolts and Jet nuts as the threads in the case are also stressed.
As for gear lash set up; yes. The int plate swap is a complete tear down so all the tolerances will need to be checked and likely reset. Get used to doing this regularly. Nothing beats up gears of any kind more than excessive lash.
Traveling at the moment but will post pics of my rebuild when I get home.

My experience has been similar to above. My box had been built by John W at Ottos many years ago and was behind a relatively low torque 2.2 at first, then the significantly stouter 2.7. It has a 904 mainshaft, oiling mods, billet int plate, guard TBD and short gears.

By the last couple track events I did it was really shot including the R&P, several gears, synchros and various other bits. All freshened-up now including new clutch & flywheel, intermediate plate, and rebuilt diff as well but I still have no cooler on it. I actually have one in a box taken off a retired POC 914-6 racecar, but haven't pulled the trigger on the install and am not sure how much fab is entailed. It's definitely on my list as I don't want to have to rebuild that extensively again anytime soon (ever) and this car sees significant heat loads - high RPM in desert conditions.

In terms of the intermediate plate, my old one (Weltmeister) apparently had no bushings/bearings and was showing some galling, and also may have had some heat deformities, possibly was the culprit of the thing jamming in 4th prior to being replaced. The new one is bushed. Anyone have experience or thoughts on that?

I have it on rather good authority that the Intermediate Plate made for and sold by California Motorsports is of good quality. https://californiamotorsports.net/collectio...ermediate-plate

I hope so - that's the one I just got!