2.7 Rockers and valve adjustment 964

[Rexs-911T]

Just a novice here and my first Porsche 911-2.7 rebuild. Have a few issues that are just not clear in the rebuild book, at least I cant find it (novice)

- I have the following:
- CIS pistons
- 964 Dougherty cams (new)
- Heads shaved 0.035"

Issue #1- Do I need to set the "Intake Valve Overlap" before I can install and set the Exhaust rockers. (I put one exhaust in with one intake, set the 0.004 clearance on the intake adjusting screw at ZI TDC and ran through the 720 rotation. Then set the Exhaust to the 0.004 clearance with the valve adjusting screw and the exhaust valves hits the piston top about 3/4 the way through the 720 degree rotation.

Issue #2: I read through a few threads (since I don't have a copy of the specs for the overlap setting for these cams but fine conflicting information. The Dougherty Web site states 1.26mm, on some other threads I see is specified as between 2.2mm and 2.4mm). Which is it?

Issue #3: even though the end of the cams are stamped 964 and a dot on top the Intake lift is only 0.459" and the 964 spec states it should be 0.470. Is this a problem?

Issue #4: when I do the clearance test on the Intake and run it through the critical p to v point I get 0.047" clearance or about 1.19mm. Is that a problem.

I'm afraid to go any further until I can resolve thesee hurdles. Attached a pic of the 964 stamp ON he cam end and dot on top, not sure if it is clear but that what is there.

Thanks All: Rex

[Walt Fricke]

Well, you may be faced with adding back in the 0.035" you had shaved off of your heads! EBS sells 1mm cylinder base shims (for 3.0 motors, anyway, so I am assuming they have them for the 2.7 spigot also).

For a normal rebuild, as you have read, the drill is to install #1 and #4 intakes only, set their lash, and set the timing (overlap) on both cams. Then go on to install all the other rockers. But good for you to check clearance, since this is not a normal rebuild but a cam upgrade along with a shot at boosting the CR. Which means you have to worry about piston to head clearance (have to measure with clay or solder and installing/removing the head) as well as valve to piston clearance. You haven't mentioned if you did that.

If you reset the cams to a larger overlap number, you will reduce the intake valve clearance (which is close to minimal, seems to me, as it is), although the exhaust clearance will open up some. However, I'd doubt it will open up enough to give you the clearance you need.

First thing to do is retime the cams, since you don't have to take anything much apart to do that, and see what that does with your valve clearances.

In the race motor I just built I carefully measured the piston to head clearance, and ended up installing thicker base gaskets to get it where it needed to be. Forgot to check valve clearances until after it was all buttoned up. But good I did, as I had to take out the pistons and have intake pockets recut. I wouldn't have noticed actual interference dry cranking things because there was none, but with the engine running and hot it would have been pretty likely that I'd bend all the intakes on the dyno.

Count yourself lucky that your cams were ground on the older style blanks (had to be, of course, to fit your cam carrier bearing size), because it is so much easier to set and reset the timing with these than with the later 19mm bolt style.

[Rexs-911T]

Walt, thanks for all the good info, this makes sensed to me.

I will be checking the exhaust after setting the overlap this afternoon and will re post after. Want to resolve all this before I go too far.

To answer your question on the clearance, yes I did a solder test with pistion #1 before I did anything else and had 0.065" clearance on the solder for P&V #1. Satisfied with that I then went on to install all the heads and cam towers to start the next phase of clearance testing. That's when I ran across the 0.047" at TDC (After rotating the crank 360 D) would have assumed it would have been the same as the first test at 0.065" at TDC. But actually the 0.,047 again was after rotating the crank 360 degrees from the first TDC and just as the intake valve started to move and before the pistion retracted any. The first TDC test at 0 degrees it had 0.055" clearance but again about at 8 degrees before TDC at 360 Degrees rotation is when It got tighter to the 0.047".

Thanks again

[Rexs-911T]

Well, confirmed a few things and one correction,

Going through the first 360D rotation after TDC and (cam dot starting up) ended with the first intake valve movement off of "0" at about 6-8 degrees BTDC (Cam dot now at the bottom).

Confirmed that once I am at TDC and Cam Dot at bottom only have 1.14mm (0.045") V to P intake valve clearance (Was 1.19mm or 0.055" when TDC and Cam dot at top). So if the intake overlap for the 964 is asking for 1.26 (0.049") Then that will never happen without some relief cut into the CIS piston on both the Intake and Exhaust valve side. Tried to test the exhaust clearance but at 240 degrees the exhaust valve hits the top of the piston so, again, without valve reliefe cut into the pistion this set-up will never work.

Conclusion, "convinced the valve relief is required on both the intake and exhaust in the CIS pistions when using the 964 grind on a 2.7L motor!"

[Walt Fricke]

Rex

1) The 2.7 and 3.0 CIS motors aren't all that different in some ways, including cam and piston crown shapes. I'd do a Pelican search to see what guys who have put the 964 grind into their 3.0s report. Did they have to cut the pistons?

2) Before I went to cutting anything, I'd raise your cylinders back up at least to stock (reverse that 0.035 cut) and see what that does for your clearances. You can't just add/subtract deck height to valve clearance to calculate changes, because of the angles involved (at least I found the correlation was not linear - maybe I was wrong). But you can mock things up - keep a piston only in #1. Leave all of #2 out, and just put a cylinder in #3 to hold up that end of the cam carrier. So mock them up with thicker shims (you can just stack what you have - you can use the ones from the right bank too, as it is all going to have to come apart anyway. See what that does to valve clearances. And, since you don't have to do more than hold things fairly tight, you can check the P-V clearances with clay, so you will get the dynamic clearance, not just a static TDC clearance.

3) What is your piston to head clearance? That is tricky, as too little is bad, but so is too much. Anderson recommends 0.035 as a minimum. Max isn't a whole lot more.

Dempsey's valve clearances are 0.059 intake and 0.079 exhaust (inches = 1.5 and 2mm). But Anderson says you can run 0.060 on both. Some race motors run with less intake clearance. It is the exhaust which is way off. You can improve that some by retarding the cam more, though I have no idea just how that would affect performance. And what you gain with exhaust clearance you lose in intake clearance. However, you can juggle the deck height and valve timing to see where you can come closest. This should help you decide how much you may need to shave the pistons, if you need to do that at all.

4) Anderson cautions against cutting pistons to where the crown thickness goes below 0.200".

With luck maybe someone who has modified the 2.7 CIS will join the discussion. I built a carbureted 2.7 race motor with some funky aftermarket pistons and big cams, and basically had to set my own overlap, because there was a very small window between too little clearance on one valve, or too little on the other.

5) You might call/e-mail your cam grinder. He has a pretty good understanding of these factors.

[Rexs-911T]

Walt,

I will try moving the cylinder shims around to get some useful numbers there. Good idea considering the angle of the valves and such effecting the calculated clearance needed.

I did a V to P clearance check and found them to be between 0.062" to 0.065", I had read that min is 0.050" somewhere.

I had read and saw several people who had this set-up and find they had to cut the piston domes for the valve clearance

Here is a picture from a post that DohertyCM put out there on a 2.7 with CIS cams showing the relief he had to put into his pistons:




He reported great results in the end, only difference he was using the CIS fuel setup and I plan to use the Webers so we will see.

[Walt Fricke]

Tom Butler posted a cam spec chart on a nearby post today. It shows that the 2.7 had 0.405" lift, while the 964 has 0.470. Exhausts are 0.350 vs 0.430. So +0.065 and 0.040 more lift on the bigger cam. No wonder the clearances need attending to!

[Rexs-911T]

Thanks Walt, And then when you factor in the 0.035" shave on the heads there is even more of an interference issue. I ran through a bunch of cam settings yesterday to optimize the clearances on the #1 side, Getting ready now to do the same on the #4 to be surt they are in the correct position.

I managed to at get to a point where the piston will turn 720 degrees without hitting the piston by moving the pin a hole or 2 on the #1. Once I get through that I will pull it apart and start with adding the shims as you suggested as a reference point for machining if that's the way I go. Was mauling over the thought of just getting different pistions depending on how much the machine work is gonna cost me for cutting valve relief in the CIS's. I could probably buy some RS pistions and sell the CIS and come out even if I factor in the cost of machining probably. These are very good CIS pistions thats why I want to hold on to them for now.

Rex

[safe]

Rex, did you buy the cams from Dougherty? In that case he should have told you that they don't fit in a 2.7 without cutting the pistons, he told me that once.
SC cams are the only choice and the benifitis are marginal.

[Rexs-911T]

Safe,

I didn't buy these from John directly, got them from a second hand source. They are in fact John's grind though. Seller didn't mention the P to V clearance to me. He also sold me the pistion and Cylinders. Not a problem, just need to found someone who can cut these pistions valve reliefs for me. So far I have called several machine shops that others have recommended here on Pelican, finally got a good referal from Tim T for a machine shop in NJ who can do a good job for me. Gonna give them a shot.
(Thanks TimT)

Rex

[Walt Fricke]

Rex - when I needed existing pockets deepened, I went to a local (to me) guy who mostly does Detroit iron. But pockets are pockets, and he had no trouble at all.

Mind you, he didn't have to know the valve angles because he could just indicate off of what is there. However, you can hunt up these angles to give him, and bring a head with valves but no springs to him and he can easily enough determine where the valve would hit the piston to start each cut. Whatever pocket side clearance he is used to using for V8s is probably good enough.

If you can't find someone locally who machines 911 stuff.