1968 2.2 engine rebuild

oliverstrother · 12-17-2023, 10:27 AM

Tried posting this earlier, but now can't find it

I'm trying to get some advice on an engine rebuild, I've rebuilt it, but now gone to tune it at the rolling road an not got very far.

suspect is the cam timing, I think whats happened is I've timed it to a T spec, when the cam is an S spec.

A) does anyone know the cam profile for an S spec, anyone got any specifications of duration and lift?

B) if I time it to an S spec, does that mean I have to fit pistons with more relief than normal? There are Mahle ones fitted with a small amount (not very much). Obviously I don't want to do this, because I means taking the heads off

Thanks everyone,
Ollie.

john walker's workshop · 12-17-2023, 02:49 PM

Can't run S cams with T pistons. They need deeper valve reliefs than a T. Timing them to T specs obvoiusly won't work well as you found out. Probably easier to go with a lesser cam than change P&Cs.

oliverstrother · 12-18-2023, 04:09 AM

Thanks John, useful knowledge. Much appreciated.

I think the root cause is the left hand cam had a small amount of damage on the thread, and I changed it for a duplicate second hand part. (same part number: 901 105 133 OR) however it had been reground to an S without my knowledge.

Been trying to make the decision to either:
A) change the right cam for an S, and upgrade the pistons. OR
B) dress up the threads and put the old cam back in.

Given that I don't have to split the head to change the left hand cam, I'm going to go with option B.

I'm certainly not in search of 'S' performance at this time. Maybe in the future S pistons, rods, cams and re jetting will be fun.

Cheers!
Ollie.

oliverstrother · 12-19-2023, 12:57 PM

I've dug out the old cam, its part number 901.105.133 OR.

Where can I find a definitive specification for what to time it to? I have T style pistons in the engine, and as I've never timed a cam before, is there preferred method?

Googling a lot of things, I struggle to find out what specific details I should time this cam to.

Cheers!
Ollie.

john walker's workshop · 12-19-2023, 01:25 PM

T cam. 2.3 to 2.7mm, 2.5 suggested. Lots of cam timing threads if you search. You need a dial gauge and holder.

al lkosmal · 12-19-2023, 07:06 PM

pelican has a tech article for reference....
https://www.pelicanparts.com/techarticles/101_Projects_Porsche_911/15-Cam_Timing/15-Cam_Timing.htm

regards,
al

stownsen914 · 12-20-2023, 04:26 AM

If everything else about your engine is T spec (smaller valves, ports, and manifolds), you may see limited results upgrading cams and/or pistons to S spec.

How bad is the thread damage on your cam? It may be possible to dress it as you suggest. If it's not fixable, your replacement cam that's S spec can be ground to T spec. Which is fairly easy since a T spec lobe is smaller than the S lobes.

oliverstrother · 12-21-2023, 11:45 AM

Nice article thanks.

The thread damage is pretty bad, but... well.. it was working before!

yeah, not looking for any power, just looking to get the old girl on the road again.

I'm going to work on it tomo morning, and check with the mechanic who took the measurements. And convince myself the cam that is in there is a T or an S. If its the wrong one, its coming out. If its a T (as far as I can tell). Then I've got other problems.

Main symptom is its popping and missing at high RPM. (no load). Think the next two things I'll dig into are the carbs and the ignition.

Fit a 123 distributor and new coil, and be sure nothing else funky is going on.
Take the carbs apart (Again) and be sure nothing is blocked or not working.

Cheers!