Cam timing on a 3.0 ltr SC

[burgundyben]

According to Wayne's book, the inlet valve lift with crank at Z1 for my 930/03 engine should be 1.4 to 1.7mm.

My pistons are from a 204 hp engine, a 930/10, the book says that for these engines the inlet lift at Z1 should be 0.9 to 1.1.

What should I go for? Both engines have the same cams, I have concluded that I should go for then 930/10 figure of 0.9 to 1.1 on the basis that as far as I know the main change between the 180 and 204 hp models is the pistons.

Comments?

[otto in norway]

I thought all SC cams should have 1 mm overlap lift... (+-0,1mm)
I know mine was supposed to have that. ('79)
My 964 cam, on the other hand should have 1,26 mm (+-0,1mm)

Maybe some changes was made for the '80 model?

[2.70Racer]

Ben,

Wayne's book is correct. The '80 SC's were timed 1.4 to 1.7. 7 degrees BTDC
Yes, SC cams and Carrera cams are indentical.
Porsche flopped around for several years timing these cams.
'78 & '79's were timed at .9 to 1.1. 1 degree BTDC
'80's as above, then back to .9 to 1.1 for '81 & '82.
Carreras were timed in between at 1.1 to 1.4. 4 degrees BTDC
All of this is from the factory manual.

[Aurel]

Same cams, but different port sizes on the 78-79 and 80-83 SCs. Could that explain the difference? The later SCs had small ports, which would make power at higher revs. To compensate for that, did they time the cams more advanced (advanced timing=power at low rpms, retarded timing=power at high rpms). This is just a wild guess that may need confirmation by the experts.

[Flieger]

Small ports move the power/torque band lower in the rpm range. Smaller diameter tubes speed gas flow but at high revs, gas flow approaches sonic limit sooner, thus the dropoff in efficiency and torque, therefore power.

[Aurel]

Originally Posted by Flieger

Small ports move the power/torque band lower in the rpm range. Smaller diameter tubes speed gas flow but at high revs, gas flow approaches sonic limit sooner, thus the dropoff in efficiency and torque, therefore power.

Ooops, thank you for the correction. Then, the cams would have been timed where the ports made the power, i.e low rpms. That would make sense too .

[Eagledriver]

Go by your port size not your pistons. If you have 34mm intake ports then use the advanced setting (1.4-1.7mm). If you have 39mm intake ports use the retarded setting (.9-1.2mm).


-Andy

[burgundyben]

I went for 1mm.

I have got no 1 inlet open 1mm with the crank at Z1, then one revolution of the crank has no 4 inlet open 1mm.

Seems to turn over ok.

[kenikh]

It is generally accepted that cam timing (within the accepted range) for the SC has no effect on performance.

[rsscotty]

We had a 3.0 spec. race engine on the engine dyno with the cams dialed in at 1.5 mm.
After re-setting the cam timing to .9mm we gained 7hp up on top. With it at 1.5 peak hp was at 5900 rpm. With cam timing at .9 peak hp is at 6050. This timing change for more top end hp resulted in a similar 6-7hp loss from 3500 to4800 rpm.
So that is what the Superflow says, and it does tend to be very repeatable and sensitive to changes.