327/365 - How much vacuum at idle?

[Gary Wilkerson]

Recently I installed the correct 2818 carb and used the cold adjustment procedures obtained from the board for the 3030 cam (Thanks Duke).
It seems to idle ok and has that nice machine "sing" to it but I'm concerned about the vacuum level. At idle I see about 10 straight from the intake.
I seem to recall in some documentation that I can't find now saying I should have ~ 18.
The engine seems to run ok and I cannot find any vacuum leaks. Is 10 ok or should I look for some other condition?

Thanks

[John H.]

Re: 327/365 - How much vacuum at idle?

Gary -

I have a dead-stock original '69 Z/28, which is essentially the same engine you have, same "30-30" cam, just 25 cubes smaller, with a freshly rebuilt original Holley and blueprinted stock distributor, and the best I can do is 9-1/2" - 10" of manifold vacuum at 900-950 rpm also. The "30-30" cam is like that - 10" is about all you're going to get, regardless of what any Service Manual says.

[Duke W.]

Re: 327/365 - How much vacuum at idle?

Idle vacuum depends on idle speed, but because the 30-30 cam has a lot of effective overlap it doesn't generate much idle vacuum.

Depending on idle revs, 10" is about right. I would recommend you idle at no less than 900, and use an accurate test tach as the in car tach may not be that accurate.

That's why the second design '64 SHP/FI vacuum can is pulled to the stop delivering 16 degrees crankshaft advance at 8". That way you have a steady and high total idle advance of 16 plus initial, say 12, for a total of 28 degrees. It may actually be a bit more because the second design '64 SHP/FI distributors also had a very quick centrifugal that started at about 700.

The engine needs this aggressive idle advance and a fairly rich mixture because all the exhaust gas dilution slows flame propagation speed dramatically.

High and steady idle advance is the only way to get accepatable idle stability and quality, but 900 is probably the low limit.

Duke

[Gary Wilkerson]

Re: 327/365 - How much vacuum at idle?

Thanks John and Duke, I've got it idling well at 950, I checked the cetrifigal advance and it starts at about 800.
This combination seems to be working well. I'll drive it as is.

Thanks again!