Question about verifying custom gears in a 915 trans

PcarPhil · 09-24-2009, 04:54 AM

Hey guys,

I've got a late 915 transmission sitting in my workshop that supposedly has short 3rd, 4th, and 5th gears installed, along with a OEM LSD. I'd like to verify that these gears are installed. In addition there is question of if this trans has a 7:31 or 8:31 R&P.

The PO said the trans topped out around 138mph near redline with his stock '95 3.6 engine and 255/40/17 rear tires.

I don't have any experience taking apart a 915. My experience is limited to changing the gear oil and adjusting the shift linkage.

Is there a fairly easy way for me to find out what gears are inside this trans? I don't mind taking it apart for inspection as long as it's fairly easy to put back together.

The trans is sitting on the floor of my workshop so I've got full access to it. I just don't know where to start!

Any suggestions??!!

Thanks for the help,

Matt Monson · 09-24-2009, 05:37 AM

Scott,
Finding out the ring and pinion ratio is fairly easy. Just pull the side cover.

5th gear is equally easy with just removal of the tail cone required. 3rd and 4th aren't so easy. You've got to pull apart 5th and reverse and take off the intermediate housing to get in there.

Based on the information you provided, there's a chance he's got a shorter 5th in there. Something like a 28:24 SO 5th is possible. Though we're really just guessing unless we know his rpms at 138mph. It's one thing if he's running it all the way up to redline to hit that number or if he's hitting that speed at 6000rpm and not going all the way to top. If he's doing that speed just shy of 6000 rpm that's likely a 28:23 SN which is a stock gear. This all assumes an 8:31 ring and pinion, which I think is likely.

175K911 · 09-24-2009, 06:29 AM

I have a 28:24 5th in my car with the 8:31 r&p and I'm pulling 140 at 6200. So if the seller is saying it tops at 138 at redline (what redline?, though with a modded box I have to assume a modded engine that will pull at least 6800 rpm) then I have to suspect either a 27:24 or the 7:31 r&p.

Before you can validate the gears, you'd need to confirm the r&p. Once you've done that, I'm sure you can then calculate the gearsets by turning the input shaft and measuring the number of turns at the axle flange. But you need to first know the r&p so that factor can be built into the equation.

PcarPhil · 10-05-2009, 06:47 AM

I found some time to check the input and output shaft ratios. Basically I marked both the input and output shafts, then while turning the output shaft one full turn I counted the number of turns of the input shaft.

Here's what I came up with. All of the ratios are # of turns of input shaft / # of turns of output shaft.

1. 14.2 / 1

2. 8.2 / 1

3. 5.6 / 1

4. 4.5 / 1

5. 3.6 / 1

Reverse 14.8 / 1 (this ratio may be slightly off - it was the first one I did)

Can any of you help me figure out what the R&P and individual gear ratios might be based on the info above? Also do 3rd, 4th, and 5th look different vs. a stock US-spec Carrera 915 box?

Thanks for the help!

175K911 · 07:23 AM

Scott- not sure you want to hear what I think but...

Based on my gear ratio notes, here's what I got:

If you assume a 7:31 R&P, your gear ratios would be like this:
1- 3.21
2- 1.85
3- 1.26
4- 1.02
5- 0.81

A stock SC trans has these gears:
1- 3.18
2- 1.78
3- 1.26
4- 1.00
5- 0.82

So other than 2nd gear, it looks like you might have a stock SC box with a 7:31 ring and pinion. And the delta in the 2nd gear could just be a bit of counting error on your end.

But....
If it's an 8:31, then every gear in the trans would have to be custom including the uber expensive 1st which is part of the mainshaft of the trans. Highly unlikely.

With the trans as I suspect, your road speed at 6000 should be about 125, or at 6800 it would be about 140.

PcarPhil · 10-05-2009, 07:35 AM

Ed,

Your information seems logical and honestly I'm not disappointed. There was vagueness in the original description with it possibly having a 7:31RP and short 3rd, 4th, and 5th gears. That description didn't make sense to me. I could see one or the other (the 7:31RP OR the short 3rd, 4th, and 5th gears) but not both.

Here's what smdubovsky over on Dorki had to say:

Quote:

Heres a list of ratios:
901&915Ratios

Assuming 1st is stock (which are all AZ 11:35 gears), then
8:31 R&P (3.375) * 11:35 AZ (3.182) = 10.74 rotations
7:31 R&P (4.429) * 11:35 AZ (3.182) = 14.09 rotations

So you've very likely got a 7:31 R&P. Go though the other ratios to determine the rest.

2nd: 8.2/4.429 = 1.851 (HX? = stock 2nd)
3rd: 5.6/4.429 = 1.264 (NT = stock 3rd)
4th: 4.5/4.429 = 1.02 (PQ or QQ?)
5th: 3.6/4.429 = 0.813 (SN??)

You can see that some of you math comes out close between two gears. Need to make an more exact measurement to know exactly which. Remember to only turn the box in one direction while measuring to eliminate backlash effects.

The trans has a Carrera case so perhaps it's a Carrera 915, stock gears, with a 7:31 R&P. It also has a LSD. This setup makes sense to me.

I may pull the output flange and side cover to verify the 7:31. I may also pull the nose cone and verify 5th gear since the trans has a slight leak between the nose cone and mid section. While I'm in there I'll replace the nose cone gasket.

BTW...thanks for your donation!

175K911 · 10-05-2009, 07:40 AM

Quote:

Originally Posted by Kaefer

Ed,

Your information seems logical and honestly I'm not disappointed. There was vagueness in the original description with it possibly having a 7:31RP and short 3rd, 4th, and 5th gears. That description didn't make sense to me. I could see one or the other (the 7:31RP OR the short 3rd, 4th, and 5th gears) but not both.

Here's what smdubovsky over on Dorki had to say:

The trans has a Carrera case so perhaps it's a Carrera 915, stock gears, with a 7:31 R&P. It also has a LSD. This setup makes sense to me.

I may pull the output flange and side cover to verify the 7:31. I may also pull the nose cone and verify 5th gear since the trans has a slight leak between the nose cone and mid section. While I'm in there I'll replace the nose cone gasket.

BTW...thanks for your donation!

Verifying the R&P is the best way to go about it. That's the critical element here. As far as a Carrera box, the stock 5th would be a 38:30 (0.79) so perhaps the PO took the stock Carrera box, swapped R&P and put an SC 5th in. That would line up pretty well with the rotation counts you have.

And happy to have made the donation. Very good cause!

PcarPhil · 10-05-2009, 07:45 AM

I'm kind of multi-tasking at the moment (I know...bad!) so my brain isn't wrapped around gear ratios...

Is a SC 5th a shorter gear vs. a Carrera 5th?

If SC 3-5 gears are slightly shorter vs. Carrera gears then perhaps this is where the description came from (Carrera case, 7:31 R&P, LSD, short 3rd, 4th, 5th gears).

Grady Clay · 10-05-2009, 09:48 AM

It is SO EASY to partially disassemble and reassemble a 915, why not take a look inside. You can then count the teeth. This also lets you visually inspect the condition of the R&P teeth, other gears and importantly the syncros and engagement dogs.

Using a new gasket/seal set from our host is worthwhile. The only other part I would use is the crush-nut for the pinion. This allows you access to the gears in the ‘tunnel’ housing.

There are many threads on all the details. No special tools are required.

Think of the pita resulting from discovering a problem after you have the transmission installed and test driving the car.

Additionally this gives you the opportunity to evaluate the existing gearing and possibly change ratios.

With the additional cost of 12 ZF LSD only ring-gear bolts and standard lock-tabs (from the gasket set), you can inspect, service and rearrange the clutches in the limited slip differential.

Yes, it is a ‘slippery slope’ but my feeling is it better to do these when you have the transmission on the bench than driving in the car – wishing you had.

Best,
Grady

PcarPhil · 10-06-2009, 08:18 AM

I pulled the side cover today and sure enough it has a 7:31 R&P. Turns out it also has a Quaife TBD instead of an OEM LSD.

Interesting... So as of now this is what we've come up with so far:

'84-'89 Carrera case
7:31 R&P
electronic speedo
Quaife TBD

The PPO mentioned winning an auto-x title with this trans. He wasn't sure of the specifics but he mentioned it being built by Auto Associates in CT.

I think I'm going to pull the nose cone and see what I have for 5th gear.

silver911rdb · silver911rdb's Garage

I also would be interested to see what was in that gearbox. I was the previous owner (actually 2 owners ago) who gave the mph speed info for the gearbox. I wasn't trying to be vague when I sold it, it's just that I purchased it from a guy who purchased it from a guy who rolled his car at Lime Rock. So I actually knew very little about the gearbox except that it had a 7:31 w/ limited slip and shorter gears. When I put it in my car...what a difference from my sponge-o-matic. But seriously, I did like the gearing for Lime Rock & Watkins Glen. It was excellent for auto x too. I did mention to Kaefer that I hit 138 mph with it at Watkins Glen. I didn't actually hit redline, but was close. I figure I was in the 6000-6100 rpm range. I had to brake to make the chicane at the end of the back straight,so I never actually hit the rev limiter. In the future I may build an early car and I would like to build another gearbox like that one. Any info would be great to have... just in case.

Thanks