Mopar Muscle Association UK

do I have to test the engine vacuum,or is there a basic size to start from? I have a standard 440,auto,at the moment 2.94 gears,but soon to be 3.9,I would have thought there would be a basic size,surely a stock 440 will have the same vac. reading as another stock 440,I wait to be told otherwise

The vacuum reading will depend on the state of tune and condition of the engine so you need to check it to be sure. If you read through Dave's carb tuning procedures in another post it will give you all the info you need. From memory most Holleys seem to come with a 6.5 power valve from the factory and it's highly likely it won't be the right one for your combo.

I see I guess I`ll have to get a vacuum gauge thanks for the info

i needed to change the powervalve in my carb this week. it is the same set-up as yours. i needed to put a 10.5 inch valve in. i had 18 inches at idle

The correct size power valve is more important for us guys with long duration camshafts than it is for you guys with stock or near stock engines.

The important thing is that manifold vacuum does not drop below the power valve reading at idle or at high speed.

If the valve opening pressure is too low it will never come open at all. however you can live with that. It is not as bad as the valve opening when you don't want it too.

Does changing from 2-plane to 1-plane inlet alter the vacuum? Could explain my slight bogging.

Yes! Typically you need a bigger pump shot when changing to single plane.

thanks for all the info guys

I have just checked the vacuum, i have 17 in at idle,snap throttle open it drops to about 5 in ,close it and it goes up to 22- 24 & slowly creeps back to a steady 17,do I have to run the car? or should I just hold the revs at about 2500?and take a reading?is this a decent amount of vacuum to have?

forgot to say,the readings are taken from the port in the base of carb (just above inlet manifold),and I checked the distributor port,at idle there is no vacuum.

Anything over 7 inches will work fine. That's what wor lass says anyway.

You want a valve rated about 2 inches above the lowest reading you get with normal driving.

If you have a slight hesitation when launching hard use a bigger squirter on the pump shot.

thanks for the info dave

Carrying on from this old post. Gone from a dual plane to a single plane on me 'bitty 318. I have got 9.5-10" Hg at idle taken from the port just above the manifold. I have a 3.5" power valve in there from the dual plane. As I understand it I will need a 7.5"-8" power valve. Is this correct? Would this explain why we had to go way large on the shooter to avoid an off the line bog when I first fitted the single plane.

Could be the case Kev.