911 Cam ID help needed

[Paul_D]

I bought a basket case 1968 euro S engine recently, and I'm trying to ID the cams. The numbers I get don't seem to match any of the cam data I find in my reference sources.
I'm measuring directly off the cam lobes using a dial indicator and v-blocks. I realize this gives only part of the picture, but I was hoping to confirm at least that the lift values were right for S cams. The table below shows the raw readings from the lobes, and in the next columns the same values multiplied by rocker arm lever ratio of 1.45 to represent the lift at the valve stem end, and in the end columns .050" is subtracted to arrive at the approximate values I would see by reading off the valve caps starting from .050" lift. The methodology and thus absolute values may not be exactly correct, but the unusually large spread between the intake and exhaust lifts are what stands out. I don't see any standard 911 cams with that much of a differential.
Any ideas what these are?


Thanks.
-Paul

[BURN-BROS]

.455 AND .399 is the lift for S cams. Looks like that is what you have

[2.70Racer]

Paul,
The rocker arm ratio varies depending upon whether the rocker is on the base circle or at the tip of the cam lobe. 1.45 is an average.
The ratio with the rocker at the tip of a cam lobe is very close to 1.5. I do not have the exact figure. Using 1.5 will get you very close.
The rocker ratio with the rocker on the base circle of a stock cam is very close to 1.4.
Published lifts are very close to the 1.5 ratio.
Secondly European cam durations use 1.0mm (.03937" rounded off to .040"), rather than .050" used for American cams.
This of course results in different duration numbers for the same cam, depending upon whether you are looking at an American manufacturer specifications or European manufacturer specifications.
It all just adds to the confusion.
Looking at your numbers, the cam lifts are very close to "S" numbers.
If you could verify the durations as being close to 268 degrees intake and 235 exhaust, I think you'll find these are an "S" grind.

[Paul_D]

Thanks for all the info Aaron, Doug.

The camshaft data table published in Wayne Dempsey's book on 911 engines gives values of "37.2" and "36.3" for Intake and Exhaust lift, respectively. Those can't me millimeters. I assumed the table was printed with the decimal point shifted 2 places to the right - that "37.2 actually meant .372
But that is not the case ? What units is he using?

[ajwans]

Do the cams have an 'S' stamped on the end of them?

andy

[2.70Racer]

Paul,
Yes, 37.2mm and 36.3mm cannot be lift dimensions.
For us US dimension folks that translates to 1.46" and 1.43" respectively.
I believe those numbers are from a caliper measurement around a cam lobe and the base circle.
For me these numbers really don't mean much unless you know the base circle dimension.
Take the above measurement then measure the diameter of the base circle. Subtract the base circle number from the number across the lobe tip. Now you have a useable number.
Along with the rocker arm ratio you can now come up with a figure comparable to those published by the manufacturer.
Fun huh?

[Paul_D]

Andy,

Just checked and sure enough there's a little "s" stamped on the end of each cam. So, that clinches it I'd say.

Thanks everybody for the help.

-Paul
'71 911T - perhaps to get S spec (or something close) motor soon

[Wayne 962]

These measurements are from the Porsche factory spec books and they are doubly confusing. I should have explained this in the notes at the bottom of page 207. The lift numbers include the "Base Circle Radius" according to Porsche, but in fact, it's not that either, it's the "Base Circle Diameter". This is 29.4mm on the 'S' cams.

There is a lot more good info on this particular thread here: 911RS Base Circle and Lift

-Wayne

[Walt Fricke]

Wayne - how did you come to supplement an 11 year old post?

However, it raises a pertinent question for me:

What are the factory max cam lobe lift specs, measured at the lobe. We all have copies of Tom Butler's chart of valve lifts, duration at 0.039" lift, and lobe separation - and assume Tom got all those from Porsche publications?

By measurement, I see SC/3.2 cam base circles being 1.265" (would be 32.131mm).

But I can't find the Porsche publications (or am overlooking what is there) which specify valve lift (SC/3.2 and 964 are of the most interest to me, but for guys verifying what swap meet cam they are looking at, others would be interesting to know) and base circle. Where should I be looking?

I've gotten lobe lift figures for SC/3.2 stock cams by measuring - both with a micrometer, and with the cam on V blocks. I'm using inches. I get variations between measurements of the same lobe 0 to 0.0005", which has to be technique (usually 0, but I've moved the indicator stand between measurements). And from lobe to lobe of one or two thousandths.

I need exhaust lobe dimensions because the Fritzelater (in car cam doctor, basically) measures exhaust lobe lift because that lobe can be accessed from the intake side (think the relative ease of pulling an intake valve cover at the track vs. the exhaust). Intakes get measured at the valve (retainer). Thanks to Keith Fritze's electronic acumen we have this system refined to where you can turn the engine by the starter for 2 + revolutions, and record lifts at each half degree, along with max lift and intake duration at 0.039", all showing up on a spread sheet.

When I get more proficient with graphing, we should be able to overlay the lobe profiles from known good cams onto the results from the cam being tested, to spot clever regrinds which have more valve open area with the same max lifts and durations.

I am working on the question of the tolerances of Porsche's cams. Measurement of cars at the track generally shows lifts less than spec. Often more than the 0.004" one might expect from lash (assuming Porsche's valve lift specs are with zero lash - I don't know that to be true).

Anyway, knowing what Porsche thought it had is of considerable interest.