CIS port sizes??
 

Hi,
Can someone explain what port sizes people refer to on the CIS engines? Are they talking about cylinder head port size or perhaps the air sensor plate?

What years/models are most desirable?

Thanks

The intake runners from the air box to the intakes are larger on SC 78-79 US models and on ROW SC Models this is to feed the larger intake ports. I believe the exhaust ports are the same on SC's.

The changes to smaller intakes were to meet fuel and emission requirements.

The larger ports are more desirable on power performance modified engines.

The smaller ports are better for fuel and emission performance.

So..Larger ports are better.

'73.5 CIS "T": (I) port = 30mm; (E) port = 33mm (I) valve = 46mm; (E) valve = 40mm
'74 911: (I) port = 32mm; (E) port = 32mm Valves 46mm/40mm
'74-77 911S: (I) port = 35mm; (E) port = 35mm Valves 46mm/40mm
'78/79 SC: (I) port = 39mm; (E) port = 35mm Valves 49mm/41.5mm
'80-83 SC: (I) port = 34mm; (E) port = 35mm Valves 49mm/41.5mm

Porting is a bit of a science, you can never know how a certain configuration will work. Aspects such as top end performance (somewhat negated by CIS), and low end torque must all be considered.

Great info guys!
Thanks!
Derek

Does anyone know if one of the 80-83 ports can be modified to the same value as the earleir year SC's?

Are large port SC heads available without air injection ports on the exhausts ?

I think the ROW heads didn't have air injection, however they may simply have been plugged from the factory.

Either way it is a simple option to plug them.

Thanks snbusch67