C2 Base Engine Manifold Vacuum

[Dave K.]

I'm reading 21 in Hg on my 66 327/300 hp 4 speed engine. Engine was rebuilt about 30K miles ago. Vacuum is steady at 21 in.@ 600 rpm. I trust the vacuum gauge. Most of everything that I see posted on the base engine 4-speed says 17-19 in Hg at around 550-600 rpm.

I never thought too much vacuum was a problem?

Dave K.

[Duke W.]

Re: C2 Base Engine Manifold Vacuum

Higher idle speed yields higher manifold vacuum.

I don't know what the AMA specs recommended for your car, but for '63 the 327/250-300 is 500 RPM in neutral for manual transmissions and IIRC 450-475 for Powerglide in Drive. At 500 in neutral with a manual trans typical vacuum is 18-19", so drop your idle speed down and see what it reads.

The recommended idle speeds vary with year, and K-19 cars have a higher recommended idle speed than non-K-19.

My rule of thumb for idle speed is "the lowest commensurate with acceptable idle quality", and "acceptable idle quality" will vary among individuals.

I think the 250/300 HP idle speeds are reasonable, but I also think the 700 recommended for some SHP engines is unrealistically low.

Duke

[Dave K.]

Re: C2 Base Engine Manifold Vacuum

Duke,

It won't idle smoothly below 600 rpm. It surges about +/- 50 rpm below 600 rpm and vacuum surges.

[Duke W.]

Re: C2 Base Engine Manifold Vacuum

Yes, IIRC there was a previous discussion on the issue with no clear indication of a solution. If it won't idle below 600 just let it be.

Duke

[Dave K.]

Re: C2 Base Engine Manifold Vacuum

Duke,

That's my plan. However, this gives me little hot idle margin if I want to PV car. Specs call for 450-600 rpm for PV.