Low Vacuum from crate 360

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Tjhoward84

Tjhoward84

1969 Barracuda Convertible
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Mar 19, 2014
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Bothell
I have a 1998 Mopar Preformance crate 360 installed in my 1969 Barracuda convertible. The engine is an LA block with Magnum heads. It has an Edlebrock Air Gap dual plane manifold and a Comp Cam (20-614-9) with the following measurements: 264/274, .480/.480, .210/.220 @ .050 lift, 112 deg lobe separation. The tappets are roller hydraulic type. The carb is a Holley 750.

My problem is there doesn't seem to be enough vacuum to run the power brake booster. The shop where the car is says the engine is only producing 11.5 inches of vacuum. They're going to check the timing at both Idle and at 2,500 rpm and also tune the carb to see if we can get closer to 16 inches of vacuum.

My question is: could I have a vacuum leak and if so, where would I check first?

Thanks for the help.
 
More info - the 11.5 reading was at idle (500 rpm) with the trans (Torqueflite 727) in gear. The timing is set at 12 deg at idle (no vacuum) and 34 deg at 2,500 rpm (no vacuum).
 
Hook a vacuum gauge to it and tune for the highest reading while idling in gear. I got mine to around 14, and the booster does work, just not as good as it did when stock.

I'm just converting to manual and eliminate the whole problem.
 
Comp Cams says that motor/intake/cam combo should produce 16-18 inches of vacuum when timed correctly. I would like to do hydro boost and be done with it but the cost is not in the budget. I have a Summit Racing vacuum pump but it never worked correctly.
 
Tjhoward84 said:
Comp Cams says that motor/intake/cam combo should produce 16-18 inches of vacuum when timed correctly.
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Of course they do. My bet is 14ish.

Granted, your initial could stand to come up some, but then you will need to adjust the mechanical to keep in the desired range.
 
More initial timing would be my first adjustment. Don't worry about total until you sort out the idle timing. The limit for idle timing will be when at operating temps, the engine won't start/drags starter or kicks back on the starter.

That set up should easily idle at 700-750RPM with minimal rpm drop when placed in gear.

It may have a bad vacuum leak somewhere as well. I'd isolate the intake tract from everything that is pulling vacuum and see if the idle issues clear up. If they do, reattach things one at a time until you find the offending system. Power brake boosters are a usual suspect in the leak area.
 
I'm really surprised looking at your cam specs that your having trouble obtaining enough vacuum. @210/220 duration @ .050 and .480 lift It seems pretty tame. I run a Hughes HEH 2328AL which is 223/228 @.050 and .504/.515 lift in my 360 LA with power brakes and have no issues.
 
SGBARRACUDA said:
I'm really surprised looking at your cam specs that your having trouble obtaining enough vacuum. @210/220 duration @ .050 and .480 lift It seems pretty tame. I run a Hughes HEH 2328AL which is 223/228 @.050 and .504/.515 lift in my 360 LA with power brakes and have no issues.
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I agree which is why I think there is some type of vacuum leak. We are going back to set the idle timing and idle rpm higher (700-750).
 
My big block idles at 5.5" of vacuum and the booster works fine. It's a 1970 car with the large diameter thinner one, and it was recently rebuilt.
 
even if you find a vacuum leak you need more initial timing! I would start at 20* or more.
 
Was the cam degreed or were the dots on the timing set just lined up?
 
SGBARRACUDA said:
That could be a problem. Straight up doesn't guarantee a thing. could be 120 or 100 CL. You definitely should check every cam when you install it.
Click to expand...

Exactly. Everything you are trying tuning-wise is just chasing your tail until you know where the cam is installed.
 
Got the car back and it runs great and the brakes work great. The low vacuum ended up being a leak in the base plate and linkage of the carb. We went ahead and changed the carb to a 670 Holley with primary and secondary metering blocks. The original carb was a 750 Holley that was too big for the crate 360 and it only had primary metering and no secondary metering. We also changed out the power brake booster from a single 7" to a dual 8" diaphragm. The vacuum is up to 16" and the brakes work great.
 
Glad you had an easy fix, always nice to find simple problem.

FYI, if you are looking for vacuum leak, spray some carb cleaner around intake/head union, carb base, vac lines at carb etc. If your car picks up/drops rpm, you have found your leak. The rpm change will vary depending on how your car is jetted. If you ever have to test booster, pinch line with needle nose pliers and see if RPM changes. Typically, you will get a slight rpm drop pinching line but not a big change.

Want another tip? Does your engine misfire under load but cant find bad plug wire? Put water in a spray bottle and mist wires while engine is running....bad wire will make itself very obvious.
 
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