Overlap, how much do you need?
On The Tree
Joined: May 2004
Posts: 104
Likes: 0
From: Denver CO.
Originally Posted by caddycarlo
to me overlap is a bad thing ..... with fuel injection if the vac in the motor drops the map will see it and the computer will think there is a load and try to up the fuel then the o2 sees the fuld and ajusts back down and you go back to square one and start over this can be a hard thing to program around becouse when you do firt put a load on the motor the vac will jump up so then the motor will lean out ...... so to me part of why the factory put a wide lobe center low overlap cam is to help the fuelinjecton work with little problem though most hot rod types will live with more problems them just joe driver
TECH Resident
Joined: Dec 2004
Posts: 845
Likes: 0
From: K-W, Ontario
I am of the understanding that turbo cams dont run any overlap
Since the plenum is pressurized, there is a good chance the chamber will be
above atmospheric pressure during the overlap period.
That means, IVC tuning and scavenging do not need to be 'excessive' in duration
to achieve good results.
I'm not aware of any boost cams that do not have any overlap. I would think
there is a need for even a few degrees to purge the chamber after each exhaust
stroke.
I'm referring to seat to seat timing when discussing overlap. I would also guess
that overlap duration is proportional to boost level in turbo/supercharged motor.
In other words, the gas speed through the valves at 15 PSI would need much
less time to scavenge the chamber than at 5 PSI (resulting in less overlap).
The only turbo cams that are likely to have zero to negative overlap are stock turbo cams like the on an audi. But it is a bandaid trying to compensate for the undersized turbines that they use and the resultant hi exhaust backpressure. This can lead to a situation where the pressure on the exhaust side is higher than the pressure in the intake manifold, if there was any overlap in an engine like this you would just be diluting your intake charge and heating up your intake valve.
