Valve clearance, cam timing 3.0 with DC15 cams

[jjeffries]

I need some help, please.

82 3.0 re-assembling from cases up. Using Wayne's book as primary guide. 1st 911 engine for me. Engine is on a stand.

Issue: I am at the stage where. having timed the cams, I am checking for valve to piston clearance. I have rockers in for #1 and #4, intake and exhaust. As I check each valve's clearance, as per Wayne, I can only get to 1.0mm for both intakes; meaning, I can screw the intake adjuster in one complete turn, using the extended valve to look for how much clearance exists. Each does the same thing: anything more than 1.0 (tightening the valve adjuster screws), they lock-up at 360 degrees through their respective strokes. Wayne says you need at least 1.5mm clearance for intakes and 2.0mm for the exhausts. My exhaust valves clear fine, all the way up to 2.5mm.

Other info: the cases have not had any machine work done. Heads were done by Craig Garrett and have had 0.22mm removed. I only used one 0.25 copper base gasket under the cylinders during installation. The deck height looked good, although I did a regular measurement and NOT the solder-squish method. The cams were just done by John Dougherty and are his DC15 grind....they are the only departure from stock. Let me know if it would be helpful for me to provide the specs (lift/duration). JD's instructions are to time the cams at 1.8mm; mine came out more like 1.9mm. Is that an issue?

I'm being careful, and any advice or direction would be appreciated. If you need more/better info from me to help, please ask.

Many thanks, John, Tolland CT

[jjeffries]

Maybe I'm asking the question the wrong way.....

Wayne says there should 1.5mm clearance between each intake valve and its respective piston, measured from zero lash then probing forward by tightening the valve adjuster screw, up to this 1.5mm number.

I only have 1.0mm using Wayne's measuring technique.
- is that a problem? Do others agree with Wayne's 1.5mm number?
- can someone explain whether my cam timing may be affecting/exacerbating this?

I understand that I've chosen a slightly more aggressive cam but do y'all think that such a cam with an otherwise stock motor (albeit with heads cut 0.22mm) would need any special modifications, such as some reliefs cut in the pistons, as Wayne (may) suggest?

As mentioned already, the exhausts are fine, and then some.

Cheers, John

[cgarr]

What overlap are the cams set at? If they are the least bit off or if the cams can be changed without changing top end etc will make a big difference too:

[jjeffries]

Having received useful feedback from both Craig and John Dougherty, I will revisit my cam timing. The spec for the DC15 is 1.8, and I went on the fat side of that (a bit like Spinal Tap's "11" philosophy...). Will likely give me the sensible amount of clearance. John.

[Eagledriver]

As you noted, adjusting the cam timing slightly will give you more clearance. Acceptable numbers from Porsche are hard to find, but there is one reference in the original shop manual that specifies .8mm exhaust valve to piston clearance. I'm comfortable with anything over 1mm, especially on the intake valve.

-Andy

[jjeffries]

Problem solved. Per Messrs Dougherty and Garrett, I started fresh tonight and adjusted the cam timing to 1.6mm and that yielded the clearance valve to piston that Wayne's book suggests. Thanks gents, incl. Eagledriver, for helping me understand the geometry involved here. John.

[safe]

I think I have just 1mm on my intakes as well, its been running fine for years up to 6800 with stock stuff (except cams).

[cgarr]

Did John say what 1.6 will help with? If they are similar to mine it might give you a bit more top end?

[KTL]

Also important is making sure your pulley is indexed right (check for true TDC #1 at Z1 notch in the pulley) and that you're looking at the pulley consistently. Sounds like i'm splitting hairs but not really. As you can see, a slight adjustment in your overlap lift measurement makes all the difference. Everything starts from your pulley marks so you want to be sure you're right on with those. I like to use a razor blade edge held at the case parting line to "extend" the parting line out to your pulley.

I had tight P-V clearances when I installed a DC-20 cam on a bone stock '87 3.2 USA engine. The overlap spec for that cam is 2.2-2.4mm and I had to back it all the way down to 1.85mm to get 1mm of valve clearance. However I didn't check my TDC to ensure it was true.

If you don't like backing off your overlap spec to 1.6mm, you can have the pistons cut for slight valve reliefs. It's that "swirl" on the CIS piston top that's causing you to have tight clearance. So you need to machine away a little bit of that high area on the piston

https://www.youtube.com/watch?v=c4W2p_8aXxI

Or just overrev your engine after you're done building it and the intake valves will put little smiley grins on your pistons right where you need them!

[camgrinder]

Originally Posted by cgarr

Did John say what 1.6 will help with? If they are similar to mine it might give you a bit more top end?

You are slightly retarding the cam at 1.6mm move the power band up. Doubt the butt dyno would notice the change.