1970 440/6 Engine Vacuum at Idle

[MoparHonda]

What vacuum should a 1970 440 six pack pull at 800 rpm idle? The engine is stone stock, except for an electronic performance distributor. The distributor has about 10 degrees advance at idle.

If my vacuum is below that amount, what steps can I take to increase the vacuum?

 

[Hemi Rebel]

Should be in the neighborhood of 17 inhg. Depending on the cam profile, it could be lower, but only by 1 or 2 inches really.

 

[MoparHonda]

HMMMM. Mine is in the 8 to 9 range at 800 RPM idle, what can I do to increase the vacuum? I have a stock cam.

It does increase to the 17 - 18 range, but only when I rev the engine.

 

[Hemi Rebel]

Idle is a bit low. Try 900 rpm. Where is the timing set? Get as much initial advance as you can get. Then set idle at 900. See how it idles then

 

[Meep-Meep]

8-9 in Hg with a stock cam!? Does it run well? Try a different gauge for starters. Agreed should be above 15 if all is well and stock.

 

[MoparHonda]

Thank you, Meep-Meep and Rebel.

Engine runs rough, new performance type distributor, new plugs.

The advance is now at 10-12 degrees, with the vacuum advance disconnected. I'll bump the advance up and set the idle at 900 RPM, see what happens. (Gauge should be OK, shows good vacuum on other cars.)

 

[69 GTX]

We had poor vacuum 8-9 till we set the timing from 8 to 16 then it went to 11-13. This is with a bigger cam.

 

[Hemi Rebel]

Also, make sure the linkage to the outboards is adjusted correctly. The throttle blades need to be closed all the way. If they are opened even a tiny little bit it will kill your idle vacuum

 

[MoparHonda]

Thank you all, I advanced timing to 15 degrees, set the idle at 900 RPM, checked the outboard linkages, vacuum is now in the 9 - 10 range.

I'll do some more tweaking this evening, and double check to ensure those outboards are closing all the way.

 

[GTX MATT]

Something is wrong, I'd start looking for a vacuum leak. If its really a stock or stock replacement cam you should have 17 inches or so. Are you sure the harmonic balancer is correct?

 

[MoparHonda]

HMMMM. How would I know which harmonic balancer is the correct one? Do you have a photo of the correct one?

 

[Mebsuta]

My dum 383 RR with factory pattern cam has 16 inches at 750 rpm.

440+6 should have the same valve events, and you have more displacement, so I think you should have a little more manifold vacuum if it is actually the factory pattern camshaft.

 

[440+6]

Mine makes 17 to 18 at 800 rpm, mr six pack cam, 9.5 compression, six bbl carbs.