1974 911S Cam Timing Issues

[Renner Larson]

I am doing an engine rebuild on a 1974 911 with a 2.7L case. I had to replace the pistons and got 90mm CIS pistons. These pistons were a different than the ones used before so I had to get new cams too due to the smaller piston-valve clearances created by the CIS pistons.

I got 1974 911S camshafts (911.105.143, 911.105.144), designed for CIS pistons. I put them in and timed them correctly, then started checking valve-piston clearances and the left camshaft (911.105.143) had more than 2mm of valve-piston clearance though the entire cycle. However, the right camshaft (911.105.144) has exhaust valve-piston piston contact near overlap at TDC.

I am positive it is timed correctly. I checked that the intake valve opens and closes at the correct angles as specified by Wayne Dempsey's Rebuild Book. However, the exhaust valve opens at around 5 degrees BBDC instead of 24 BBDC and closes 45 degrees ATDC instead of 2 BTDC.
I am incredibly confused.
What could be going on here?

[stownsen914]

Have you taken all the slack out of the cam chains? Using C clamps or spacers on the idlers. Slack in the chains can affect cam timing by a few degrees or more.

[356RS]

Sounds like you may have the 4-5-6 side with the pistons installed upside down. Dome up on all pistons right?

[gsxrken]

Originally Posted by 356RS

Sounds like you may have the 4-5-6 side with the pistons installed upside down. Dome up on all pistons right?

I thought of that but it doesn't explain his observation: "However, the exhaust valve opens at around 5 degrees BBDC instead of 24 BBDC and closes 45 degrees ATDC instead of 2 BTDC."

Either his cams are not as advertised or it's user-error here... over the internet its hard to say which. (I know that's not helpful.)

I'm also curious about the compression ratio you'll have. Are the CIS pistons pop-ups or flattops? Valve reliefs? Is this a common-enough combination that you know what to expect, or did you have to CC the heads and pistons and piston height etc?

[Renner Larson]

I am sure the piston orientation is correct.
The compression ratio is 8.5.
The CIS pistons I'm using are pop-ups with no valve reliefs. I didn't know flat top CIS pistons existed. The compression ratio was given by the manufacture (Mahle). I'm assuming this number is based on the stock rods, crank, heads, etc. used for 1974 models which I am using.

That being said, the pistons are the same on both sides of the engine but one side has piston-valve interference, and the other does not.

[Renner Larson]

Does the 1974 911S have special CIS pistons that have valve reliefs?
When I search 1974 911S pistons on the internet, I can't see a difference between regular 911 pistons and 911S ones.

[racing97]

so I had to get new cams too due to the smaller piston-valve clearances created by the CIS pistons.

it could be possible an error was made in the camshaft

regards

[stownsen914]

How repeatable are your cam timing numbers? If you turn over the engine through a revolution or two, do you get the same cam timing numbers again?

[dannobee]

Where did you get the cams? Is there a possibility that the right one has been reground to a different spec? Are there any numbers stamped into the back of the cam?

[Renner Larson]

I got them from DC Automotive, both used. The cam I am having issues with has WEB WP4 14 104 102 stamped in the back of it.

[boyt911sc]

Originally Posted by Renner Larson

I got them from DC Automotive, both used. The cam I am having issues with has WEB WP4 14 104 102 stamped in the back of it.

Renner,

If I were in your shoes, I would contact WebCam or any refutable cam grinder shop or Tom Butler (well known member) to help you verified the grind specification. Maybe the left cam is good and the right side cam is some what out of spec. (thinking loud). Good luck.

Tony

[356RS]

Looks like Web Cam # 104/102 which is close to a Solex grind. Intake valve lift is 11.43mm and cam timing is 4.2 - 4.6

[dannobee]

Originally Posted by Renner Larson

I got them from DC Automotive, both used. The cam I am having issues with has WEB WP4 14 104 102 stamped in the back of it.

There ya go. The right side cam has been reground. Contact who you got it from and see if they'll exchange it for the correct cam.

Otherwise contact web cams or dougherty and get a pair of the cams you want.

[Renner Larson]

That makes a lot more sense. Thank you everyone for the help!

[Renner Larson]

I ended up doing my own cam profiling for the two camshafts to try to make sense of things. Here are the results. Easy to see the right cam definitely has a different grind.

The specs call out that:
IVO @ 6 ATDC
IVC @ 50 ABDC = 230
EVO @ 24 BBDC = -204
EVC @ 2 BTDC = -2

[356RS]

Very nice cam profile chart. The overlay really displays the difference in the two cam specs.