15 In Vacuum on Intake??

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70DusterBob

70DusterBob

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Have a 75 360 CID, mild cam:

Lift Intake: 465
Lift Exhaust: 440

Duration Intake: 224
Duration Exhaust: 214

Performer Intake, Holley 750 Mech, Headers, 9.5:1 Compression

I changed out the intake a while back, but just now checked the vacuum and it is at 15.1-.2

I used an adapter, and a 3/8ths to 1/8th inch vacuum port fitting to go into the factory drilled and tapped port on the passenger side in the rear of the intake. I'm getting identical reading at the vac port under the carb in front.

Is this okay? Seems kinda low. Just timed it. It runs good, smooth and doesn't ping, just seems like a low vac to me.

Also, the air/fuel needles are only like 1 turn out from all the way in... That doesn't seem right either. Does it seem like I have a vacuum leak? I just changed out the rubberized cork grommet type deals that seal the air/fuel needles. I will try to re-torque the intake today... A lot of the bolts like 4 of the water pump bolts, the alt pivot bolt, the thermostat, and some of the hose clamps have backed off a bit, not like loose, but not fully tight. I re-tightened them all yesterday.

Thanks...
 
Any cam different from factory spec will directly affect engine vacuum. I'd be happy with 15" !
 
I have a BB and a big cam I get 5. Have to run a vac pump for the brake. !
15 is nice.
 
What those guys said, 15 is fine.

If it drives well and you're happy with it then don't worry!
 
Are your durations 224/214 @ .050" lift? Probably so. If that is the case, 15" is decent for that cam. That assumes that your idle vacuum is at around 750-800 RPM; is that the case? And if you had a vacuum leak, you would be opening up the idle mixture screws MORE, not less. So it sounds all OK but let us know your full duration spec and idle RPM's.

We got 14-15" idle vacuum at 850 RPM on a 340 with a cam that has almost identical duration specs at .050" lift, and get 14.5-15 inches of vacuum. We had to fix up the PCV to the right type and adjust on the secondary idle stop screw and tighten up the secondary linkage to get there.
 
The lift specs are

Lift Intake: 465
Lift Exhaust: 440

I suppose that is close to a 0.50, I'm not real certain on that. But from what everyone said 15.1 to 15.2 is good vacuum. I'm happy with it. It runs great, better than in a long time. I had taken it to a shop to get the oil pan gasket changed and they screwed everything up that they touched. I guess they had to take the dist out to lift the engine to get the pan off. It was Wayyyyyyyyyyyyyy retarded. I will have to put the light on it now to tell you what the advance is, but it is running great. I was surprised the air/fuel screws were so close to bottom but if that's okay, I don't mind.

The idle is bouncing between 850 and 900, somewhere in there, it isn't a super smooth idle, but again I guess that's the cam.

Thanks You Guys
 
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