Not as Much Vacuum as Expected

At what rpm , how far open are the primaries ? Lots of variables to be considered ..

I'm running 12.5" and it all works perfectly

2old2fast said:

At what rpm , how far open are the primaries ? Lots of variables to be considered ..

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750-800 rpm, transition slot showing square or about .020. Timing at 14 initial at idle. A few degrees either way makes little difference to the vacuum level. 4 corner idle screws tuned to best vacuum. 14" vac is the best I can get at any carb or timing setting. I know that is not too low. My 383 runs 7-8" vac, but it has a way bigger cam in it. Just curious.

I can say this. My cam has close to the same adv duration & 15+° more at .050 than yours . At 900 rpm mine has 13" of vacuum . I'll take a WAG that its related to valve timing , early opening. Late closing ??
Having the ring seal you have would make me very happy !!

I agree with 2old, Cam timing, factory cams are ground to run at machined timing specs, controlled only by mechanical setting off a pin at TDC. Aftermarket cams need to be dialed in to the spec for that cam, or you can throw the pin in and run with it. Because of higher lift and different LSA, a few degrees of timing shift in relation to crank position can make several inches of vacuum difference.
If you believe the intake is leaking in the valley, pull out your PCV, and spray flammable brake clean in the vent hole on each valve cover, while idling. A pick up off rpm would indicate a leak.
The 4 corner idle also bleeds more are than a standard 4bbL.

2old2fast said:

I can say this. My cam has close to the same adv duration & 15+° more at .050 than yours . At 900 rpm mine has 13" of vacuum . I'll take a WAG that its related to valve timing , early opening. Late closing ??
Having the ring seal you have would make me very happy !!

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Timing Events: intake open -1, close 36, exhaust open 48, close -4.
The zero overlap is why I thought there might be more vacuum.
On the ring seal. Yeah, I was very happy. This might be the tightest one I have ever put together. when I checked #1 and saw 1% leak, I had to stop and check all the equipment then check the leak twice more. Had 3 cylinders at 2%. the rest at 1%. I'd say the 1.5 thou piston to wall clearance has something to do with that. The machinist used some kind of special diamond hone. The finish looked real nice. I'd also say that ProMaxx did a good job with the valve seats and the valves were actually round. Any way, I'm real happy with this new motor. Runs just like I wanted. Just curious about the vacuum.

MGK said:

I agree with 2old, Cam timing, factory cams are ground to run at machined timing specs, controlled only by mechanical setting off a pin at TDC. Aftermarket cams need to be dialed in to the spec for that cam, or you can throw the pin in and run with it. Because of higher lift and different LSA, a few degrees of timing shift in relation to crank position can make several inches of vacuum difference.
If you believe the intake is leaking in the valley, pull out your PCV, and spray flammable brake clean in the vent hole on each valve cover, while idling. A pick up off rpm would indicate a leak.
The 4 corner idle also bleeds more are than a standard 4bbL.

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I degreed the cam in at 107.5 to the recommended 108 intake centerline. So within a half degree. Close enough.
I was trying to think of a way to get brake kleen to the valley to check the bottom of the intake gasket. Thanks for that.

You " could" run a piece of that plastic tubing like comes with gauges through the breather hole down into the valley for a temporary propane or other flammable gas leak check ??
You could call straub and ask about the vacuum ??

For the duration this sounds about where it should be. Even if you’re using no overlap, the vacuum is still sensitive to when the intake valve is closed in crank degrees. I this case the cam is starting to get into that late to close zone that will show a a small but steady vacuum drop.

Bogie

BogiesAnnex1 said:

For the duration this sounds about where it should be. Even if you’re using no overlap, the vacuum is still sensitive to when the intake valve is closed in crank degrees. I this case the cam is starting to get into that late to close zone that will show a a small but steady vacuum drop.

Bogie

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Thanks Bogie. That makes me feel better about the vacuum level.

jaw22w said:

I have about 500 miles on my new rebuilt 355 SBC. It has 10:1 CR. Compression test shows 175 PSI in all cylinders and 1-2% leakdown on all cylinders. The cam is a HR Straub. .050 duration is 215/224, advertised is 284/296, Lift is .470/.490. Centerlines 108/116, LSA is 112, Zero overlap. I am getting a steady 14" vacuum at idle. I kind of expected more vacuum at idle. I don't have power vacuum assist brakes, so it really doesn't hurt a thing. I just expected it to be higher. Just wondering if I may have leak. I can detect no external leaks, so if anything is leaking it would have to be at the bottom of the intake gaskets.
What vacuum would you guys expect from this setup?

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14 inches of vacuum at an idle isn’t bad at all. Run the engine and don’t worry about it.