The problem when listening to Neil Harvey......is that YOU are not Neil Harvey. Yes he is 100% right. Yes there could be errors. But Porsche provides a spec which gives you decent timing in the end, decent enough to have a motor that runs for 100K miles or more on that timing spec without issue.

Throw in a dial gauge to determine #1 TDC instead of relying on the case parting line and you're even better.

Neil's experiences are bleeding edge race cars where they were trying to squeeze out every bit.

The average shade tree mechanic at home probably ought to just follow the manual.....

Nothing wrong with learning the finer points of cam timing instead of just blindly following the manual and hitting the timing spec. It's not that hard to employ a degree wheel and get your timing set up a different way. Plus the degree wheel process gives you assurance that your pulley marks are where they should be (or are not where they should be)

Let me tell you, Its a pain in the ass being Neil Harvey!!!

Point taken, but engines do run better when all cylinders have the same valve timing. Yes, Porsche gives a spec, but that works only if the cams are ground correctly and the rocker arms are refaced correctly. The way I suggested allows you to check.

Also, I do read these forums and often I read about unfortunate circumstances where a DIY made a mistake and the engine fails after rebuilding, or similar. The forum readers all give their sympathy and give the owner a thumbs up it will all be Ok.

Then you read about a shop having the same misfortune and the wrath of this forums lets loose on the shop. Double standard, maybe whichever position you are standing I guess. To eliminate this from happening we have chosen to build at a level where we make sure we cover all areas.

But we are human too and we do make mistakes. Its a matter of catching them before the engine leaves here. My past work history taught me one way only and one right only. Curse, maybe but it is humbling when the opportunities I had most will never have.

Totally agree. However, I am still looking for the correct spec for the lobe centerline to time my cams.

As a side note, Porsche used two or three different timing specs for their SC/ Carrera cam, right?

I have heard that the factory used different cam timing specs for the SC era engines. I think it was just the spec for setting of the cam timing that they changed, but I’m not sure of that. I think I read it in Bruce Anderson’s book.

As for the centerline spec for your cams, if I’m not mistaken you can calculate this if you have the intake and exhaust opening and closing event specs, which I’m sure is published for factory and aftermarket cams. Or find someone who has a cam doctor machine and have the cams measured.

Scott

I don't have the 911 SC spec book handy (because I have yet to cut one apart and scan it) so I only have the '84-'87 Carrera spec book that I scanned.

http://forums.pelicanparts.com/uploa...1550686720.jpg

I do have the '72 to '81 Workshop Manual handy and it has all the cam information for the engines. Here's the SC pages.

http://forums.pelicanparts.com/uploa...1550687063.jpg
http://forums.pelicanparts.com/uploa...1550687070.jpg

You can see the cam part numbers are different between SC & Carrera. But it's been said for a lot of years Carrera and SC cams are the same profile and the only physical difference is how the ends of the cams are fastened. That's sort of true since the specs of both cams are VERY close- note how the opening and closing degree descriptions are nearly the same for each.

But the point of my sharing this is to confirm the TDC overlap lift specs are indeed different for SC vs. Carrera according to the factory documentation. But they're quite close to being the same exact profile. Dougherty just lists one grind for his SC/Carrera cam

Dougherty Racing Cams Porsche 911, 930 and 964 camshaft profiles

But getting back to the factory info, I guess it shouldn't be surprising that the TDC overlap lift specs are different since we're talking about different engines with different displacement, different piston dome volumes and different compression ratios?

Lastly, Web Cam has an interesting page with a lot of info to digest (or ignore) for whoever's curious

Web Cam Racing Cams Inc. / Degreeing

From what KTL found, it looks like the durations of the SC and Carrera cams are the same at 234 deg intake and 226 exhaust. For some reason the SC specs say those measurements are taken at 1mm lift (one common value for listing cam duratioms) and the Carrera says .1mm. Who knows, it could be a misprint.

The opening and closing events are different due to the SC specs having the cams advanced by 3 degrees (hence the different TDC specs). Could be the same physical cam profile though.

Lobe center will be full cam lift/valve lift.

As for accuracy, that will depend upon your equipment and how clever you are at measuring true TDC and max lift.

A or symmetrical cams will both have the max lift at the designed lobe center. Cams with lobes that are more rounded, will appear to have a moving lobe center, but this is where you need to take two readings before and after full lift and average them out.

Alignment pins etc, will always have some slack in them. This is why it is important to always turn the engine in its normal rotation when taking measurements, so all parts are on their loaded side. Often the play in the alignment pins is enough to find the desired lobe center number. I cannot remember, but you could see 1° crank in the slop. I may be wrong here on the amount, but I do know its quite common to get the desired number just by taking the slop out of the pins.

How accurate do you need to be? We try to obtain a max of 0.25° ether side of the desired number. A lot comes down to the accuracy of the degree wheel too. Most of the mass produced wheels are Ok, but the larger diameter and the more expensive wheels are more accurate.

Many will think its not necessary to go to this extent. "The engine runs just fine". If that floats your boat, then that's Ok by me. We all have to choose our own level of expertise.

How well does the Stomski electronic degree wheel work is it as good
as a mechanical wheel ?

Any electronic rotary encoder will be vastly more accurate than a degree wheel.

This one sticks to the pulley with 4 neodymium magnets purchased from McMaster. You can turn it to adjust, but it takes a few pounds to pull it off. Stable.

I did a little analysis and I'm convinced you can stick an Iphone to the degree wheel, not even centered on the wheel and the Iphone will read the rotation accurately. I put a couple magnets in the Iphone plastic case.

http://forums.pelicanparts.com/uploa...1550757600.JPG

http://forums.pelicanparts.com/uploa...1550757922.JPG