Porsche Cam duration specs
 

Porsche lists, in the little white spec book for '78-81, the cam duration specs in a column whose heading is "Timing with 0.1 mm valve clearance."

Aftermarket cams are often speced at some specified lift, like maybe 0.050"? Maybe this is because below that/those lifts about nothing flows past the valve? Of course, they have to close fully to avoid burning seats or valves, and all sorts of other issues.

But my specific question is that when measuring valve duration to see if it is at stock duration specs - how many degrees is a valve open - do I use normal lash and nothing more, and Porsche's published figures?

I don't particularly care about where relative to TDC and BDC these events happen (other than perhaps trying to figure out if lobe separation is what it should be), since duration doesn't physically depend on that.

Walt - I'm wondering if the .1 mm and the .050" are either referring to different things, or if possibly the .1 mm is a misprint.

Cam duration specs are usually listed at either 1 mm or .050" lift. I've understood this to be because the flow at those low lifts is relatively inconsequential, and variations (in degrees) at those low lifts would be more likely due to differences in the ramps on the cam lobes to more gently load the rockers (or not). So to avoid misrepresenting the performance potential of a cam, they only count the duration from .050" through full lift and back to .050" again (or 1 mm, if that is used instead of .050").

Not surprisingly, American camgrinders seem to use .050" while Porsche and some others commonly use 1 mm. I think Porsche even used "full seat" timing specs in some cases (measuring duration from fully closed, through the cycle, and back to fully closed again). But never .1 mm as far as I know. Perhaps not coincidentally, valve lash is .1 mm, but you wouldn't normally specify that as part of cam timing? And I think I recall reading somewhere that there is a misprint in the spec book related to cam timing. Maybe they meant 1 mm and not .1 mm?

Cam timing is measured at 0.50" or 1mm because full seat is harder to measure with any precision.

Walt, you may need to measure several sets of cams to see how much degree variability there is at 0.050" (I.e. how repeatable is 0.050").



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The .1mm specification in Walts question is the valve clearance adjustment between the elephant foot adjuster and the top of the valve stem (.004").

Well, of course 0.1mm is the valve lash. However, that is also the heading for the column where all the duration BTDC, ABDC etcs are in the table headed Camshafts, timing on page 49. The only way to read that is that it governs, if you will, the specs below in that column. The 76-77 LWSB has it listed this way, and at 0.1 mm, also.

For instance, in the same Porsche little white spec book where this is found the next column to the left is headed "Intake valve stroke in over-lapping TDC with 0.1 mm valve clearance" That column gives data like 0.9-1.1, the stuff we all know and love when timing our cams.

I know I found at least one misprint at one time in this book, but it was for something which was easier to identify as a misprint. And misprints have a habit of carrying on in later editions. But still, ?

When I get confident in my technique, I can just measure. But part of working on technique is to know what the answer should be, and work to see what you are doing wrong because you aren't seeing what you should. I've got a stock cam as my specimen piece. My first attempt at measuring duration wasn't even close.

The Volume 1 Factory Workshop Manual gives the various BTDC etc cam figures at 1mm at supplement page SE2 for the 2.0S. Same thing for the 2.2s on page SE 36. When I look in Volume III of the workbook, which covers the 2.4s through the 3.0 SCs, I find 1mm where these specs are given. And I can line up the degrees from the LWSB with those for the '78SC, and the '81 SC in both Volume III and the little book. So I guess I'll have to assume one mm lift (which I guess I can simulate by setting the lash at 1mm to make reading things, especially valve closure, easier).

Ah, the '72-3 LWSB says 1mm, so the error crept in with the next edition of that otherwise handy booklet.

Yes there are errors generally all specifications are taken at 1.00mm where are US companies are listing at .050 and the SAE uses.006.

regards

The 0.050" spec is almost universally used in the US so that comparing cams is easier as cam lobe ramps can vary quite a bit. The duration at lash is usually called "advertised duration," and like said above, the SAE spec is 0.006". Porsche chooses 0.1mm (0.004") Gentle cam ramps are easier on valvetrain parts but often don't lend themselves to comparing one cam to another, hence the 0.050" readings.

If you're trying to compare cams as a motorsports official, first you'd need to define what a "stock" cam's specs are. In the old days, NHRA Stock eliminator cams had the same lift and duration as factory cams, but their duration at 0.050", 0.100", 0.200", and 0.300" lift were far greater than any factory cam (but still met the published factory specs of advertised duration and lift). Make no mistake, these Stock Eliminator cams were all out race cams because of the aggressiveness of the cam lobes. And were very hard on valvetrain parts. To show the complexity and difficulty in measuring duration, NHRA has since thrown out the whole measuring duration thing and allows cams of any duration, but limits the lift to factory specs.

Here is some data that I collected from John Dougherty a few years ago
993 Super Sport(mechanical)
lift .490I/.456E
Advertised duration 260/248
duration at .050" 240/230

Webcam 20/21
lift .485I/.452E
Advertised duration 258/246
duration at .050" 238/226

Just based on these 2 examples it looks like ~20° difference for .050" vs advertized ie seat duration

The ends of the cam lift curves have slopes that slowly approach 0, that means not much difference between .1mm and .050" even though .05" is ~10x larger than .1mm

Yes I agree. I looked at my 72-73 spec book to satisfy my curiosity. It mentions the .1mm spec in the leftmost column where lift is specified. The 1 mm spec is in the rightmost column as the clearance used for measuring the cam timing. Both make sense to me. Pic from my spec book below.


http://forums.pelicanparts.com/uploa...1585402271.jpg

When was your spec book published? Mine is 1st English Edition, Current as of May 31 1973.
Your third row with TDC overlap clearance (2.70-3.10), the valve timing is 48/8 for the exhaust. Mine has 38/8. It also has the models TV (T-USA), E, S, Carrera 2.7 listed in columns, not as rows.
Your 48 degree timing appears to be an outlier as the others are all near 40 degrees.

Is there a particular cam you are looking at and need info for ?

If you're referring to my post just above yours, my book also says 1st English edition, current as of May 31, 1973. The rows in the pic I posted are for (top to bottom) 911 T, T-CIS, E, S, and Carrera 2.7. Weird that my version is different from yours ...

Therein lies one of many of the issues that arise for a motorsports official. Imagine if you're ready to disqualify a competitor for a discrepancy in cam timing, then he produces a FACTORY publication showing his cam within specs.

http://forums.pelicanparts.com/uploa...1586369456.jpg
I have a little time this morning here is a profiled early 911s cam that shows the duration plus a few other specs at various lifts, my program will not directly allow me to transfer to this internet address so I copied it out of a scan.
I hope it works and you find it interesting.

regards