Josh B

I've got my car running - still sorting out an exhaust leak and exhaust mounting issue.

I've noticed that on idle (800 rpm) that where my car used to pull 14 or more inches of mercury I am just getting between 7 and 10. I pressure checked my intake system to 15 psi and squashed all the leaks. I have new intake gaskets on

I am wondering if the larger valves and modded cam could cause lower vaccum? Air to fuel ratio is good (14.7) as I am driving but at idle it is pretty lean (17 ish). Leaner than it was before. I have a piggeyback so I can richen that up a tad.

Anyone have any ideas? Intake system is pretty simple - no venturi or cycling valve. Fewer vacuum lines - no charcoal cannister etc.

V2Rocket

the valves are so big youve already combusted half the atmosphere.


thanks.

fast951

If you changed the cam to a performance cam, it could be the cause of the change in vacuum.

When running less vacuum at idle, the fuel pressure is higher at the rail. Seeing a change from 14.7 to 17.x:1AFR does not sound right. Have you checked your O2 sensor, engine temp sensor?

Josh B

I should've said that at idle before the modifications I was at 16ish AFR. O2 sensor and engine temp sensor are newer. I can tell the engine temp sensor is working too as the car runs richer when it is cold.

That is interesting about the cam - I guess it is extening the lift and so the engine isn't having to suck as much to get the air it needs? It is a webcam #274.

MM951

FWIW, John's car (porsssh951) had a cam and a done up head IRRC and his vacuum ( ~10") was a bit lower than my stock cam/head (18" ish) and his car ran like a top. Just my observation...

Chris White

Actually the lower vacuum in a performance cam is caused by greater cam lobe overlap. There is a period that both intake and exhaust valves are open to increase the scavenging effect (that only works at higher rpm). At idle this causes lower vacuum at idle. Not a problem....

2bridges

cam will absolutely alter vacuum.