What's the impact of a mistimed cam?

[axl911]

When you're doing cam timing on a rebuild, what's the impact of a mistimed cam? Meaning instead of setting the intake to 1.26mm (for a 964) BTDC, you set it at 1.15mm? Or 1.35mm? How would this affect the power or torque produced by the engine.

Assume everything stock.

--anthony

[304065]

If the setting is too high you have advanced the cam, which shifts the powerband lower in the rev range. If it is too low, you have retarded the cam, which moves the powerband higher.

[304065]

ADVANCED cam timing ?

[Flat6pac]

If you look at how the SC/Carrera cams were set up, and they are the same, you will see there is a long variant of settings as for the particular uses, USA, Euro, compression, etc.
Bruce

[lvporschepilot]

with a stock engine/compression, you do not want to retard the cam timing too much as this does a couple things:

1. the intake valve closes later reducing effective stroke (dynamic compresion ratio) causing the low end of the engine to suffer

2. if static compression is not adequate, the upper end will lose power resulting in power loss through the entire power band

8:1 - 8.5:1 dynamic compression is area to shoot for when timing cams according to static compression, cam duration, and more specifically intake valve closing angle.

Adverse to this is advancing which closes the intake valve more quickly causing the effective stroke to be longer creating more dynamic compression. More dynamic compression makes peak power at a lower rpm and because of the advance in dyn ratio, one cannot run as much total ignition timing.

It really is a balancing act. Make peak power at X rpm, but X rpm suffers. The idea when performing an upgrade is to make an upgrade that takes what is already there, and benefits throughout what you already have. Splitting hairs with retarding intake timing a couple degrees to gain 7bhp way up top is not really worth it.

[axl911]

Thanks all!!!

Next question would be what is the range of movement and its affect on power/torque. Would .1mm make a difference? And how much? Guesstimate is okay.

[Dr J]

The critical thing is to have adequate piston to valve clearance. I had advanced mine, but the test showed that I didn't have adequate clearance, so down it went until it was all clear.