Late 915 with early mech speedo ?? is it possible

Alan.UK · 06-24-2004, 02:11 AM

Hi all,

Is it possible to use a late 915 gear box with an early mech driven speedo, does anyone make a product that can convert between the two?

Thanks

Alan.UK

Alan.UK · 06-24-2004, 06:13 AM

Any ideas ?

Grady Clay · 06-24-2004, 06:26 AM

Alan,

It probably will be easier and less expensive to find a used electronic speedometer and wiring harness.

Guys, correct me if I’m wrong. You will need a nose casting with all the speedo drive parts and the gear on the end of the pinion shaft. What I don’t recall, does the later pinion have the extension for the gear? If not, that would involve swapping the ring & pinion.

If you have your old distressed 915, you should be able to swap parts. How easy (or not) depends on the pinion issue.

Best,
Grady

Alan.UK · 06-24-2004, 07:05 AM

Thanks Grady,

Can anyone send me any pics of the later speedo so I can compare how it looks against the mech one, and also will it look odd against all the other older style dials ?

Thanks

Alan

wevoid · wevoid's Garage

The pinion shaft does require the extension for the speedo drive gear, these were available in both 7:31 and 8:31.
The 8:31 version is pretty rare, only being used over one maybe two model years, '75 ish.

Other than that, as Grady explained, the full compliment of speedo parts from your existing front cover would be required, plus the drive gear for the end of the pinion shaft.

The 7:31 from an early transmission can be installed in the later case too, this would allow for a cable speedo.

Regards

Hayden

Grady Clay · 06-24-2004, 07:39 AM

Alan,

What year, model, etc. is your car?
What has been done previously?
What are you now undertaking?

You should be able to find a matching speedometer.
If not, you can get a new face installed.

Here are some illustrations to discuss.
"

"
(C) 1972 Porsche KG

If you buy used parts, be careful of corrosion on the threads
where the speedo cable attaches to the casting.
This is particularly true of the mag casting.
"

"
(C) 1972 Porsche AG

This is the drive gear on the pinion (my crude arrow.)
What I don't know is if there is this extension on the pinion for
this gear on the later electronic speedometer transmissions.

Best,
Grady

Alan.UK · 06-24-2004, 07:48 AM

Grady,

I am fitting a short stroke 3.2 into a 1971 T. The motor is having 46 pmos, S grind cams, HPX, and 98mm Max M.

I have a real nice early 915 box but I now have also access to a rebuilt later one with 8:31, which I feel may be better for me. I have S dials (7200rpm) going in and wanted to retain the original speedo, but if it looks the same then may be the later 915 is the best option.

Thanks

andyjboy · 06-24-2004, 08:48 AM

Alan - the simplest solution will be to get North Hollywood Speedo to change the face of an electronic speedo to match your existing gauges. You can take the opportunity to upgrade it to 180 mph at the same time:

Contact: Hartmut Behrens (info@nhspeedometer.com)

It will be cheaper if you find a non-working electronic speedo in the US & get it sent straight to him. Cost will be $150-$200.

Andy

Grady Clay · 06-24-2004, 09:08 AM

Alan,

Cool. When you open the deck lid it can still look close to original, yet really perform. That is the beauty of big engines in early 911s. Have you measured the weight difference? Curious.

I agree with Andy. One of the nice things about the 911 VDO gauges is that you can have any face you want. You can have it as original ‘71S or make one that reads to 350 km/hr. I have used Hollywood for almost 30 years.

I have even had some mis-calibrated with a 9000 RPM dial to indicate 8200 when the RPM was 7000. I had a “loaner” speedo that indicated 70 MPH when the speed was 85 MPH. We would swap into someone’s 911 and take in for inspection. The owner would take the report (hopefully not to court to get out of a speeding ticket) with him.

Please keep your early 915, you may want it someday for its light weight. Yes, an 8:31 is a more reliable ratio. I would prefer that R&P ratio and just change the individual gears to shorten the ratios. Porsche made a 6:29 for a 901 but the 6-tooth pinion was marginal (unsuitable?) even with a 2.0.

Your next task will be to wire the electronic instrument. Do you have access to a junk yard (err.. Porsche dismantler) where you can get the main wiring harness, magnetic speed sensor and other attendant pieces? You can make yourself a small sub-harness that looks original. It can have the correct wire color, connectors, and you can use black heat-shrink tubing to make it look proper. It can go up the tunnel, tied to the main harness, and not disturb any of your original wiring.

It is all in the details.

BTW, I can just see you blistering along some Dorset roads above the channel. Do you Brits have something like Mapquest?

Best,
Grady

KobaltBlau · 06-24-2004, 10:59 AM

Quote:

Originally posted by Alan.UK

I am fitting a short stroke 3.2 into a 1971 T. The motor is having 46 pmos, S grind cams, HPX, and 98mm Max M.

Have you verified valve clearance yet with early S cams and 98mm 3.2 SS Max Moritz? I was wondering...

Alan.UK · 06-25-2004, 08:10 AM

That’s for the info Andy, that is a great idea, so I am looking for a broken electronic Speedo in case anyone has one.

Thanks for the extra info Grady, and I am now going to fit the later 915 gearbox and wire the speedo in myself. I am really looking forward to getting this car done as the roads around here are fantastic, your more than welcome to join me if you wanted to pop across the pond. If you go to mapquest.co.uk and search for the town of "Poole" you will see exactly where I am, so come pop down, Andy to offer is also open to yourself. Whist your here maybe you can help with the RHD 70 S resto (grin)

Thanks again

Alan

PS. With regards to valve clearance, I asked that question to Andial and they said they had done it before, but gave no more info than that. When I get round to fitting it all together I will let you know.