390HP - Vacuum at Idle

[Douglas L.]

How much vacuum should a stock 427-390 hp motor pull at idle? I'm not getting any. Could this be right. The compression is fine and the motor starts and idles fine.

[Joe L.]

Re: 390HP - Vacuum at Idle

Doug-----

The only way you could get zero vacuum is if the engine is not running. There HAS TO BE vacuum when it's running. So, I expect that you're taking the vacuum off at the wrong place. Your best bet is to take it right off of the manifold at the fitting between the distributor and carburetor. If you have power brakes, there will be a hose connection here. Otherwise, it will be plugged. With power brakes, just remove the power brake hose and connect your vacuum gauge. Otherwise, remove the plug and install a fitting that you can connect the vacuum hose from the gauge to.

I would expect vacuum in the 16-19 inch range for your engine.

[Duke W.]

Re: 390HP - Vacuum at Idle

As far as I know your engine has full time vacuum advance, so teeing a vacuum gage into the vacuum advance signal line should provide full manifold vacuum.

Another source is to tee into the choke vacuum break hose.

Let us know what you read. Since idle vacuum is effected by idle speed, please list both observed vacuum and idle speed.

My guess would be about 14-16" at the OE recommended idle speed of 550 in Neutral for manual transmissons. If you prefer a higher idle speed, vacuum will be higher.

Duke

[Douglas L.]

Re: 390HP - Vacuum at Idle

Joe,

At this point, I am baffled. The engine idles and runs fine. Its a K19 engine with everything there. The brakes are not power. The carb is newly rebuilt. I put a vacuum guage inline with the hose from the carb to the distributor and get no reading. I disconnect the vacuum hose to the distributor and it has no effect on the motor running. I would expect at least a little stumble. There is no "hissing" sound that vacuum produces. So, the only thing I can surmise is I must have a plugged hose or it has something to do with the K19.

[Joe L.]

Re: 390HP - Vacuum at Idle

Doug-----

That's what I suspected; you're taking the vacuum reading at the wrong location. For a K-19-equipped car, the distributor advance vacuum is a PORTED vacuum source. Therefore, you won't see any vacuum at this connection at idle.

If you use the manifold vacuum port as I described earlier, you will get around any such problems and get a "true" reading. Let us know what you get.

[Douglas L.]

Re: 390HP - Vacuum at Idle

Joe,

Finally got the time to do the test right. Pulling 20 on the vacuum guage.

Thanks,

Doug Lee