Change of direction & piston choice

[coomo]

Engine is a 3.0SC big port Euro Model.
It will be 3.2SS with 964 Cams/SS heat exchangers.
Originally we were going to use uprated fuel metering head, and therefore keep the CIS.
things have changed and now engine will be on carbs,or maybe even EFI.
We purchased a set of Mahle PP98 -015 Pistons from EBS.
The flyer that circulated offered these for "911 3.0/3.2 CIS 10.0 CR"
Are these still going to be optimal now that induction system is NOT CIS?
We realise that 10.0 would usually dictate a twin plug engine.However we are in UK
and our fuel octane is higher.Any help, gratefully recieved.

[Focker]

You'll more than likely find that the 964 cam isn't going to work for you. I have the PS98-014 that are set up for carbs, i had to switch cams as the dynamic compression was going to be too high.

[coomo]

Originally Posted by Focker

You'll more than likely find that the 964 cam isn't going to work for you. I have the PS98-014 that are set up for carbs, i had to switch cams as the dynamic compression was going to be too high.

Hi Did you actually run the engine with the 964 cam or change before completing build?

[Focker]

Originally Posted by coomo

Hi Did you actually run the engine with the 964 cam or change before completing build?

change before based on advice from the engine architect I work with. I'm twin plug big port as well. Still in process of assembly.

Those much more knowledgeable than me can chime in, but as I understand it, the higher compression pistons raise static compression, but with less overlap and duration you could be reducing performance while also inviting preignition as the low overlap raises dynamic compression.

This is why just adding a big cam to a stock motor doesn't do much (lower dynamic compression) and why you don't see high compression motors with stock cams. One needs the other to be effective.

[coomo]

Originally Posted by Focker

change before based on advice from the engine architect I work with. I'm twin plug big port as well. Still in process of assembly.

Those much more knowledgeable than me can chime in, but as I understand it, the higher compression pistons raise static compression, but with less overlap and duration you could be reducing performance while also inviting preignition as the low overlap raises dynamic compression.

This is why just adding a big cam to a stock motor doesn't do much (lower dynamic compression) and why you don't see high compression motors with stock cams. One needs the other to be effective.

Thanks.I need input from some of the knowledgeable guys here.

[trond]

following this discussion with interest after having tried 10.5CR with stock cams (and twin spark). Liked the torque with these cams for the street but had temperature issues so took it apart again

[coomo]

Originally Posted by trond

following this discussion with interest after having tried 10.5CR with stock cams (and twin spark). Liked the torque with these cams for the street but had temperature issues so took it apart again

When I read you had temperature issues,I presumed you were in USA.Not Norway! Something wild going on.

[Focker]

Temp issues caused by the cams. I would speak with either William Knight or John Dougherty to assist with cam selection.

William helped me. I initially had the 964 cam for my build and quickly changed directions when I got the Mahle piston set

[coomo]

Originally Posted by Focker

Temp issues caused by the cams. I would speak with either William Knight or John Dougherty to assist with cam selection.

William helped me. I initially had the 964 cam for my build and quickly changed directions when I got the Mahle piston set

Mailed John few minutes ago.Still not sure if my pistons are compatible with a more agressive cam though?
Will drop EBS a mail

[icarp]

The 964 cam is an emissions cam, not an upgrade for performance . It adds only .015" lift on the intake. CIS pistons are the choke point for cams . The pistons that you chose should allow for an actual performance cam . William Knight is the man to speak with .
Ian

[trond]

it was indeed William knight who said temperature issuses would be expected with stock cams. What cams are suitable is unresolved. But then I have until next winter. But yes I had temperature issues. Even in Norway but I really suffered last summer in Italy and on Autobahn. Definitely not a good combination

[coomo]

Originally Posted by icarp

The 964 cam is an emissions cam, not an upgrade for performance . It adds only .015" lift on the intake. CIS pistons are the choke point for cams . The pistons that you chose should allow for an actual performance cam . William Knight is the man to speak with .
Ian

Thanks Ian.Don at EBS Thinks my pistons will work fine with a more aggressive cam.I will email William and see if he can supply cams.

[lindy 911]

I built a 3.0 SC with 10.5 : 1 JE pistons using GE 80 cams, twin plugs and 46 mm Webers. My first build and had some oil leaks. Live a learn. End result is that the motor rocked. No heat issues and very streetable.