What is the problem with CIS and aggessive cams?

[cstreit]

Remember also that there are different meanings for "Hot Cams".

THe reversion is the big issue... This can be cause by 1) overlap where intake valve is open at the same time as the exhaust/stroke and 2) duration of intake, same problem. It may also mean 3) higher lift, same duration, which doesn't necessarily cause a problem untill you start coil binding the springs or smashing valves into pistons... A particular problem with CIS pistons.

"Hot Cam" is the inital bad generalization.

[911-32]

So, three related questions:

1. What is the hottest cam you can run in a 3.2 Motronic (std engine, intake etc)?

2. What is the hottest cam you can run in a 3.2 with aftermarket management (say DTA etc)?

3. What is the hottest cam you can run in a 3.2 with aftermarket engine management and JE pistons with big valve cuts?

The common knowledge for 1) would suggest 964 or 20/21 type cam, but could you run a 3.8SS cam? Question 2 removes the flapper air flow metering from the equation and question 3 removes the valve to piston clearance issue.

If, for example, you thought of upgrading in stages and decided that ITBs were part of stage 2, what cam would you use in stage 1. That is what I am trying to answer.

Thanks,
Richard

[camgrinder]

3.0 with cis would be the 964 grind.

3.2 motronic gets the super cup grind

3.2 with factory pistons and aftermarket management my DC24 grind (Valve to piston clearance will be close and need piston notching)

3.2 with JE pistons and motec? anything goes

Remember hotter cams need more compression

[911-32]

Excellent - many thanks for the info.

[CARRERIC]

If I may ad my 0.2 cents. I have been down this road. Last year I had my 1975 911 Carrera (2.7L CIS) completely restored. When the engine was "completely" rebuilt. We used RS pistons, SC cam grind, Heads were fly cut yeilding 9.5 compression.

First I will tell you the SC cams made a HUGE difference in performance. However I did run into a few problems in the beginning. The car was leanning out causing bucking at low rpms and a stumble on the highway. I believe this was mainly do to a change in vacum that the new cams and RS pistons made. The RS pistons are forged not cast giving very tight clearences and a very strong piston. This is why porsche used forged pistons in there race cars. There is some information about this in Whaynes rebuild book.
The problem took six months and two engine drops by my shop $$$$$$ to figure out. On a lead from John Walker I disconected the vacum line from the WUR and plugged it, and the problem was solved. Disconnecting the vacum on the WUR did not effect any cold start. As I understand the vacum was for some sort of emissions purpose. The car runs fantastic! I will say the RS pistons may not burn as clean as CIS but is definetly a trade off. The SC cam profile on the 2.7L is a must. Best upgrade for the 2.7L engine. CO needs to be set a bit rich.

[BURN-BROS]

Originally posted by CARRERIC
The RS pistons are forged not cast giving very tight clearences and a very strong piston.

Forged pistons are more dense, requiring more clearance than a cast piston. They are stronger but need proper warm up to grow to its running tolerance.

I am glad you are happy with that engine. I bet it changed the character of the car.

[safe]

Originally posted by CARRERIC
We used RS pistons, SC cam grind, Heads were fly cut yeilding 9.5 compression.

The RS pistons have a 8.5 C/R, right? How much did you cut of the heads and how did you solve the timing chains?

Also, what kind of HP did you get?

[CARRERIC]

Safe,

I don't know exactly how much they were cut. Heads were cut more than once. Special gears were made for the timing chains by a company in California.

[SorenDK]

Know this thread is ancient but maybe some are still interested... I sure am :-)... Any new findings on this topic? I am building a 2,7 liter with JE 10,1:1 CR, DC40 "mod S" cam ... And want to try out a 3,0 liter CIS... :-)
I will let you how it goes... BTW plan B is EFI...
/S