Valve pockets in CIS pistons?

[hasaramat]

I have a set of lower milage pistons from a 1980 911 SC but I want to upgrade the motor with carbs and cams. According to Bruce Andersons books the compression ratio is pretty good on these pistons,(9.3 to 1). I saw somewhere in here a post of someone who cut pockets in the CIS pistons and put solex cams in a 3.0 but I cannot find the post. Is this possible? Maybe I should be asking if this is smart? Any input?

[TT Oversteer]

I ran this setup on a 78Sc 3.0 with 46mm PMO's and 66 Solex cams. It ran great! I can't remember the cam timing I used as it was back in the '80's and I don't have any build notes from the project. The engine ran cool and never had any detonation problems running pump gas. Ted Robinson, who worked at Garretson at the time, cut the valve pockets for me. I'm no expert but Ted knows what he's doing and he recommended cutting the valve reliefs in the stock CIS pistons.

[Henry Schmidt]

Originally Posted by TT Oversteer

I ran this setup on a 78Sc 3.0 with 46mm PMO's and 66 Solex cams. It ran great! I can't remember the cam timing I used as it was back in the '80's and I don't have any build notes from the project. The engine ran cool and never had any detonation problems running pump gas. Ted Robinson, who worked at Garretson at the time, cut the valve pockets for me. I'm no expert but Ted knows what he's doing and he recommended cutting the valve reliefs in the stock CIS pistons.

We've done this process in the past with reasonable success.
You do loose a great deal of compression.
I would recommend just running a set of high performance CIS cams and you should be happy.
WebCam calls theirs a 2021
Dougherty calls his a DC 21 or 22 ?

[TT Oversteer]

I believe the benefit of the Solex cams over the CIS cams was the greater overlap between intake and exhaust. Carbs allow you to run this overlap while CIS or Motronic doesn't like it. Some run the stock SC cams with carbs and say it works fine. Carbs give you more options with cams though and the right cam can really wake up your engine. At the time that I built my 3.0 my budget didn't allow a new set of pistons and cylinders with higher compression and valve pockets. The machining of the CIS pistons allowed me to use more aggressive cams. It's true you will lose compression but I didn't cc the combustion chambers to find out how much. It wasn't a large amount of material removed. It would certainly be an interesting comparison to dyno two different engines: one with stock compression and CIS cams and one with hotter cams and slightly lower compression. On the plus side my engine didn't detonate on regular gas and always ran very cool. It also had great throttle response and plenty of power!

[Henry Schmidt]

Although both cams offer a compromise, I would tend to believe that the 20/21 will offer better performance through the low end with little or no loss at higher RPM. Keep in mind that 40 Webers (on a 3.0) and most heater boxes are not conducive to high RPM performance.
The high lift is the key.
Porsche trended towards low lift, high duration cams in a attempt to control valve float and valve train friction (heat which equals parasitic loss).

As you can see, the significant difference between the two cams is lift (greater in the 20/21) and lobe center.
The increased valve lift will greatly increase volumetric efficiency at all RPM ranges. (cylinder filling)
The wider lobe center on the 20/21 (113 vs 98) will reduce intake pulses and therefore created better cylinder filling at low end.

lift...........duration...........lobe center
20/21
485...........238...................113
452...........226
Solex
450...........250....................98
408...........236

Then you have to deal with the reduced compression related to valve pockets. If the valve pockets are only 3cc each ( a very small cut) the calculated compression would be just over 8.5:1. And you have to pay to do that.

For me it's a NO BRAINER but IMV.