2.7 with 964 Cams question

[Rexs-911T]

Thought I would pose the questioin hee on Technical Forum, no response on Engine Rebuilding

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 these 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

[jstobo]

According to Bruce Anderson's book the 964 cams had a lift 0f .464" and a duration of 240deg. The cam timing overlap for the 964 is 1.26mm. The MFI 72 911T had the 2.2-2.4mm overlap but the 73.5 CIS 911T had a 0.9-1.1mm overlap. Both engines were 2.4L. The stock 2.7L 74-77 had a spec of .4-.54mm overlap. The 78-83 3.0L SC specs were .9-1.1mm. What all the 74+ cars had in common was CIS. So, are you keeping the CIS? If so you probably cannot run much overlap as the above numbers show. That is why many people switch to Carbs when hot rodding a 2.7. Hope this helps.

[Rexs-911T]

Thanks JS, Yes, I will be keeping the CIS domed pistions going for the 9.8 to 1 CR. BUT!!!

I finally found (after 3 days of searching the forum) some of this info. Appears these pistions will have Valve to Piston issues, I was searching "2.7 with 964 cams" in the search field and got NOTHING!!. Amazing when I changed the search slightly (after the 3 days) and added an "L" after the 2.7 to "2.7L with 964 cams" you would have thought I stirred up a bees nest.

All kind of info (some very conflicting) out there when searching with this search in the field. Appears that I will need to cut some relief in the pistions to get the clearance I need for this setup.

I still have the 35mm intake bores in the heads though, wondering if I can open these up for better performance?

As far as the timing goes, with 0.047" (1.19mm) intake valve clearance at the very start with crank shaft at TDC, well when I advance it to the specified 1.26mm I will crash with the top of the intake pistion right from the start. At least this is what I think, will try it this afternoon to confirm. and offer feedback here.

Still have the question of porting the 35mm intake to something larger as I am planning on using 46mm Webers on this build.

Also since you brought up the question on keeping CIS or going to carbs (I'm going with carbs as mentioned above) How/why do Carbs change the advance on this setup. Seems I will still have clearance issues I think either way without the relief in the piston tops.

Any comments??

Thanks

[Targalid]

With CIS, fuel is constantly injected in front of the intake valves. With more valve overlap, the fuel is prematurely ignited in the intake manifold.

[Elombard]

Hmm going with carbs did you consider some cams a pistons for that application? maybe the 964 cams are good, I always here of the 'mod s' cams which I think are similar to what the RS came with.

Also the over lap cam timing deal is an issue for the CIS injection, it causes reversion in the intake which moves the flap in the CIS system and causes weird running problems (so I have read). With carbs its not an issue.

I wonder if it would be just as cheap to go with custom pistons from EBS or similar as it would to have yours notched? Can you notch pistons in your garage? Also, I think it mentions that the shape of the CIS pistons is not ideal for the carbs? Cant remember for sure.

[Bobboloo]

Originally Posted by Rexs-911T

Also since you brought up the question on keeping CIS or going to carbs (I'm going with carbs as mentioned above) How/why do Carbs change the advance on this setup. Seems I will still have clearance issues I think either way without the relief in the piston tops.

Any comments??

Thanks

The carbs don't change the advance but they do allow you to run more aggressive cams than you could if you had stayed with the CIS. The cam profile determines it's advance. You should set your cam timing to the cams spec 1.26mm.

Your problem is a mismatch of components. You have pistons designed for a 2.7 CIS cam and a cam designed for a 3.6 motronic piston.

So your choices are:

1. change pistons
2. change cams
3. clearance the pistons

Since your going with carbs you could go with an even more aggressive cam like a mod S provided you also changed to RS pistons but they may not be in your budget.

As for the ports I'd leave them alone for a street motor maybe 1mm larger at most.

[Rexs-911T]

Thanks all. Great comments. As mentioned I am going with the 46mm Webers since that's all I have in hand at the moment. NO CIS injection on this build. This setup was recommended to me and also I see that it is also one of Waynes recommendations on page 113 of his rebuild book.

I don't have the capability to do the piston clearance in my garage and will have to send them, out for that. NEED to find a reputable Porsche capable maching shop here in central Virginia if any one knows of one.

Pic on my CIS pistons below before any reliefe cuts, as they are today. If you look closely at the picture looks like the previous PO had some issue with valve to piston clearance as well by looking at the valve marks in the lower part of the piston (NOT ME... Yet, this the way I received them)


Thanks

Rex

[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) not After TDC as I mentioned earlier. (paying more attention now).

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!"

[Zeke]

I'm surprised John D. couldn't/ didn't tell you of the need.

[Elombard]

Waynes book recommends a 2.7 with CIS pistons and 964 cams? That does not sound right but I have not researched it that closely.

[Rexs-911T]

Elombard, You are correct, Waynes recommendation is JE Pistons on a 2.7L with 964 Cams, The fuel recommended with that set up is CIS in that listing. My mistake. Just saw the CIS and didnt pay attention to which column it was under. My mistake

Thanks, want to keep it correct.

[Rexs-911T]

Zeke,

Unfortunately I didn't buy the cams directly from John D. I got them second hand so can't blame him. Just a detail that was lost in the hand me down process I guess on the piston relief..

[Elombard]

What are you going to do?

You might do some reasearch on the CIS pistons (I think it is in Waynes Book?) They are a pretty serious compromise if I remember correctly. With all the trouble and expense you are going to it might be worth getting pistons and cams matched to the carbs. It will be a great motor if you do it right.

Since you are green at this, have you addressed the head stud pulling issues in the Mag case? Just want to make sure.

[Walt Fricke]

Rex has sinned - he also posted this on the 911 engine rebuilding forum, where perhaps is properly belongs.

My main issue is the 0.035 he (or someone) milled off the heads.

[Rexs-911T]

Walt,

I had it out there in the engine rebuild section for a few days and didn't get any response (look at the engine rebuild responses and the technical responses), once I posted it on Technical started getting stuff pretty quick, LOL...

Elombard,
I have case savers in this case from the guy I bought this from along with oil by-pass and piston squiters. There were several post from a search I did "2.7L with 964 cams" and found several people who had this setup with the CIS cams and had good results and good low end torque. From what I ready the Carbs should be the icing on the cake (I hope this is all true).

I am primarly going to run this on the street but wanted the torque and power this setup seems to offer. If nothing else I will hopefully post good results in the end and may be offer some insight to others who try this. Not saying I can offer any expert advise just some of the good and bad experiences I had doing this build.

[Zeke]

Seems to me that if the heads have been shaved a whole new bag is opened.

[Rexs-911T]

Zeke,

Probably so, gonna try to add back a 0.035" shim before I do anything else to see what happens there.

[Flat6pac]

The thought of 46mm Webers feeding a 2.7 sounds like overkill in the carb department.
Bruce

[Rexs-911T]

Flat6pac,

These 46mm Webers are coming off the current motor in the car now which is a 2.4L. And probably are too big for the 2.7 (not real sure what they are doing on the 2.4 really). Once I get the 2.4 out of the car I will see what cams, pistons etc are in it I'll know better.

The 2.4 runs good, except that it is setup for the power band to come on and be stable around 4,000 RPM (not the most fun on the street). Haven't taken it out yet. May have to tone down the 46mm as you mention though for the 2.7 as it is looking. The 2.4 is leaking oil pretty bad, that's why the 2.7 is going in so I can pull the 2.4 and work on it and still drive the car in between swaps.

[Bobboloo]

Originally Posted by Rexs-911T

Zeke,

Probably so, gonna try to add back a 0.035" shim before I do anything else to see what happens there.

What is your deck height? Deck height should be between .8 to 1.25mm.
Shoot for 1mm if you can.

Raising the deck for clearance is not the way to solve your issue unless your deck height is to small.