911E cam timing, early or late????

[jim72911t]

Hi, I'm going to set the cam timing on my motor tonight, and have a question on which cam timing spec to use. The motor is an MFI '72T with 2.2E pistons and E cams. I am not sure if I have early or late cams, so I don't know whether to use the early ('69-'71) spec of 3.0-3.3 (3.15) or the late ('72-'73) spec of 2.70-3.10 (2.9).

I've done a bunch of research on the matter, and have come up with a couple of more questions, but no definite answers. I've had at least one person tell me that the early and late cams were the same but just timed differently to compensate for the differences in compression. I've also heard that the cams are indeed different, yet the part #'s for the early and late cams is the same, leading to more confusion.

One thought I had was to set the cams at right around 3.05, which would put me at the low end for the early spec and the high end for the late spec. This is a compromise, however. I'm also contemplating degreeing my cams to see what their profile is, but I don't have anything with which to compare my results.

Anyone have a solution?

Thanks,
Jim

[Wayne 962]

Same part number = same cams. Splitting the difference should work okay.

-Wayne

[Tim Walsh]

Wayne,
I'm not sure how they ended up with the same part numbers but according to BA's book the cams themselves are actually quite different. If I'm looking at this right the 2.4E cams are much closer to T cams and the early E cams are much closer to solex cams.

[jluetjen]

The specs that I've seen say that the early E cams are the same as the solex cams, except that they have a drive for the MFI. The late E cams have the same lift and duration, but the lobe angle is wider so that there is less overlap. This would account for the difference is timing specs (which is actually a lift measurement).

I didn't bother answering since Warren answered when you posted this on the main 911 Tech BBS, but it sounds like your engine isn't in the car yet. It really isn't such a big deal to map your cam now to get a pretty good idea what you have. You can do it with the cams assembled in the heads, but it is easier if you've replaced the stock springs with some light weight springs from the hardware store. Then you basically start at TDC, and measure the intake and exhaust lift on one of the cylinders every 5 or 10 degrees. I wouldn't install the rockers on the other 2 cylinders on that bank so that you don't have a fully loaded camshaft. You might need to take a couple of extra measurements at maximum lift to make sure that you catch the maximum.

That will tell you exactly what you have and then you will know for sure and won't need to depend on the opinions of the rest of us who to be honest have never even seen your engine or its cams.