Engine vacuum
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Team Owner
Joined: Mar 2003
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From: Greenville, Indiana
Engine vacuum
I'm interested in knowing the manifold vacuum of both an L-79 and L-84 engine while they are being cranked on the starter motor. Assume a good tight engine and hot battery, throttle closed and off fast idle.
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Originally Posted by MikeM
I'm interested in knowing the manifold vacuum of both an L-79 and L-84 engine while they are being cranked on the starter motor. Assume a good tight engine and hot battery, throttle closed and off fast idle.
Heh Mike ..That's a good one... Don't think I ever heard of a Manifold Vacuume spec at cranking speed...????? But if ther is one..I bet some here will have the answer.
Good one..
Thread Starter
Team Owner
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From: Greenville, Indiana
It is somewhat of an odball request.
Originally Posted by Viet Nam Vett
Heh Mike ..That's a good one... Don't think I ever heard of a Manifold Vacuume spec at cranking speed...????? But if ther is one..I bet some here will have the answer.
Good one..
Good one..
I didn't think I would find the answer in the archives. I didn't even look. I hope somebody will take a couple of minutes and check their engine for me. Mine pulls only 1"of vacuum. I think it should be greater.
Safety Car
Joined: Feb 1999
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From: Fountain Hills AZ
I don't think such a comparison would be useful as there are too many variables. Ring seal and cranking speed would be the main ones. An engine that's cooler or hotter, or has sat longer, has different rings or different amount of wear may generate more or less vacuum. Cranking speed would probably make the biggest difference. Different starters and different batteries will have different cranking speeds. Why not just check it at idle?
TXSTICK. 9" sound low for the L79 cam, even with 1.6 rockers. I'd expect more like 13".
TXSTICK. 9" sound low for the L79 cam, even with 1.6 rockers. I'd expect more like 13".
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From: Greenville, Indiana
Originally Posted by Vetterodder
I don't think such a comparison would be useful as there are too many variables. Ring seal and cranking speed would be the main ones. An engine that's cooler or hotter, or has sat longer, has different rings or different amount of wear may generate more or less vacuum. Cranking speed would probably make the biggest difference. Different starters and different batteries will have different cranking speeds. Why not just check it at idle?
TXSTICK. 9" sound low for the L79 cam, even with 1.6 rockers. I'd expect more like 13".
TXSTICK. 9" sound low for the L79 cam, even with 1.6 rockers. I'd expect more like 13".
Idle vacuum would do me no good. I'm working on STARTING the car, not keeping it RUNNING.
A comparison would be VERY USEFUL for me. I'm working on a hard start problem on a Rochestor injector. You may or may not know that the cranking signal valve, which controls fuel flow during cranking, works off manifold vacuum. All the variables you mention are present on stock engines with the Rochestor unit installed on it.
I would much appreciate it if I could get a few snapshots of cranking vacuum from owners with the L-79 cam or the 30-30. Or, both.
Melting Slicks
Joined: Sep 2004
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From: NE Illinois IL
Mike,
I don't really know exactly what the issue you are having is but I've have a bit of experience with the Rochester FI unit, and others here have as well.
You don't need much vacuum to open the CSV and 1" should be enough. You can test the CSV by removing it and blowing through it. You should be able to do this without problem. Next try gently sucking through it. It should close (actually open). In doing so, the spill valve closes and all the fuel goes directly to the injector nozzles.
Hope this helps.
I don't really know exactly what the issue you are having is but I've have a bit of experience with the Rochester FI unit, and others here have as well.
You don't need much vacuum to open the CSV and 1" should be enough. You can test the CSV by removing it and blowing through it. You should be able to do this without problem. Next try gently sucking through it. It should close (actually open). In doing so, the spill valve closes and all the fuel goes directly to the injector nozzles.
Hope this helps.
Thread Starter
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From: Greenville, Indiana
Originally Posted by Joel Falk
Mike,
I don't really know exactly what the issue you are having is but I've have a bit of experience with the Rochester FI unit, and others here have as well.
You don't need much vacuum to open the CSV and 1" should be enough. You can test the CSV by removing it and blowing through it. You should be able to do this without problem. Next try gently sucking through it. It should close (actually open). In doing so, the spill valve closes and all the fuel goes directly to the injector nozzles.
Hope this helps.
I don't really know exactly what the issue you are having is but I've have a bit of experience with the Rochester FI unit, and others here have as well.
You don't need much vacuum to open the CSV and 1" should be enough. You can test the CSV by removing it and blowing through it. You should be able to do this without problem. Next try gently sucking through it. It should close (actually open). In doing so, the spill valve closes and all the fuel goes directly to the injector nozzles.
Hope this helps.
Joel:
I have a hard starting problem after sitting idle for a week or so.
I really didn't want to debate or discuss anything except the cranking vacuum. I just wanted to know what is normal with those two camshafts. All it takes is someone who has one of these "stock" engines, a vacuum guage and a few minutes to run the check. I would appreciate hearing from any of you.
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