vacuum readings
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vacuum readings
Shouldn't your vacuum reading be highest at idle? My Holley 390 on my 300ci has a reading of 15" vacuum at idle, but I see it go up to 24"when at a high cruise rpm just before the secondaries open.
Laughing Gas
Joined: Jul 1999
Posts: 1,048
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From: Lacey, WA
vacuum readings
Hi,
Generally speaking, You will achive the highest manifold Vac reading when you have high RPM with a closed throttle. Such as when downing shifting. The engine is spinning fairly fast and the throttle plates are closed preventing and air/fuel from entering.
What you have described does not sound to me like a manifold vacuum reading as vacuum should fall as the throotles are opened to WOT and then pick back up as RPM increases. I sounds to me like you are measuring "Ported" vacuum off of the carb.
Where is the gauge hooked up?
On a side note, my idle and steady cruise vacuum readings are almost the same.
Hope this helps,
Generally speaking, You will achive the highest manifold Vac reading when you have high RPM with a closed throttle. Such as when downing shifting. The engine is spinning fairly fast and the throttle plates are closed preventing and air/fuel from entering.
What you have described does not sound to me like a manifold vacuum reading as vacuum should fall as the throotles are opened to WOT and then pick back up as RPM increases. I sounds to me like you are measuring "Ported" vacuum off of the carb.
Where is the gauge hooked up?
On a side note, my idle and steady cruise vacuum readings are almost the same.
Hope this helps,
Hotshot
Joined: Oct 2002
Posts: 18,488
Likes: 22
From: The Big, Oregon
vacuum readings
If you have about a 270* duration cam in the engine, that sounds about right. At idle, the engine isn't spinning over fast enough to create much vacuum, due to the slow piston speed of these engines. At higher rpm, higher pistons speeds mean a stronger vacuum, and it is able to pull in more air. You may also want to check the timing, and your carb settings, to get the idle vacuum as high as possible.
Moderator
Joined: Dec 2001
Posts: 22,436
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From: Louisville,Ky.
vacuum readings
I agree, sounds like the vacuum is being checked at the ported vac. port on the side of the front metering block. Try checking it under the carb or straight off of the manifold. If it is on the ported vac port, then your throttle plates are open way too far at and idle. There should be no vacuum on that port until you start opening the throttle. Jimmy
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vacuum readings
I'm running the Comp Cams 268H cam with timing straight up, I also had the bowls in the head ported and polished with a 3 angle valve job. I have the vacuum hooked up to the lower vacuum port not the upper ported vacuum. When I get the higher vacuum reading the throttle blades are not yet fully open it's just at a high steady speed right before you would change gears like when climbing a gradual hill. On a steady cruise speed I have 15" of vacuum.
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