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Quick question-vacuum

 
Old 03-19-2019, 04:08 PM
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htown81vette
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Default Quick question-vacuum

1982 Corvette, crossfire injection. 350 (290 hp crate motor), nothing special, 9:1 compression, mild cam. Everything else stock.

I hooked a vacuum gage up to it and got 12" of vacuum. Is this about what to expect?
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Old 03-19-2019, 04:23 PM
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MelWff
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What was the idle speed?
What do you have the initial timing set for?
I assume the computer is no longer controlling anything?
The 12" sounds low to me.
Are you sure the gauge is accurate?

Last edited by MelWff; 03-19-2019 at 04:26 PM.
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Old 03-19-2019, 04:35 PM
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7T1vette
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No. An idling stock '82 C3 engine should show 15-20" Hg vacuum. With yours being that low, either there are leaks in your vacuum system(s) or the timing is set improperly. If the engine has a more radical cam installed, 12" Hg might be all you can get.

To determine what vacuum level your engine is CAPABLE of producing, you need to disconnect and block-off ALL vacuum lines coming from the carb and intake manifold (so there can be NO LEAKS, other than the carb and intake system) and then start the car with vacuum gauge plumbed-in [to one of the carb or intake lines]. Once engine is warmed up and idling, adjust the carb idle screws for maximum vacuum; then adjust timing for maximum vacuum. Repeat those two steps until you get no further increase in vacuum reading.

THAT will be the highest vacuum your engine can produce [without mechanical repair]. Whether you want to leave it there...or not..is up to you. But at least you know whether you can generate enough vacuum for your accessories. Now plug the vacuum system line back on -- one at a time -- and notice if you still maintain that level of vacuum or get a significant drop in vacuum reading. If it drops, you need to find where in that "just added" system the leakage is occurring and fix it. Continue until all systems are attached and you have very little vacuum loss (1-2" Hg MAXIMUM).

Last edited by 7T1vette; 03-19-2019 at 04:36 PM.
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Old 03-19-2019, 05:29 PM
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wilcar
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You say it has a mild cam in it but the 290 hp crate motor has L79/L82 type cam in it which is not mild compared to the stock crossfire 202-206 @ 050 cam. If your crossfire is tuned perfectly I am not sure what kind of vacuum it would pull but I would guess at least 15. My carbed 84 c4 with 9.0 c r, large valve 72 cc heads and a 228-228 108 lsa isky solid cam pulls 15 inches of vacuum at 900 rpm. Even if you have no vac leaks with the crossfire set up your ecm may need some tuning with your 290 camshaft.

Last edited by wilcar; 03-19-2019 at 05:31 PM.
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Old 03-19-2019, 07:46 PM
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CanadaGrant
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Originally Posted by 7T1vette View Post
No. An idling stock '82 C3 engine should show 15-20" Hg vacuum. With yours being that low, either there are leaks in your vacuum system(s) or the timing is set improperly. If the engine has a more radical cam installed, 12" Hg might be all you can get.

To determine what vacuum level your engine is CAPABLE of producing, you need to disconnect and block-off ALL vacuum lines coming from the carb and intake manifold (so there can be NO LEAKS, other than the carb and intake system) and then start the car with vacuum gauge plumbed-in [to one of the carb or intake lines]. Once engine is warmed up and idling, adjust the carb idle screws for maximum vacuum; then adjust timing for maximum vacuum. Repeat those two steps until you get no further increase in vacuum reading.

THAT will be the highest vacuum your engine can produce [without mechanical repair]. Whether you want to leave it there...or not..is up to you. But at least you know whether you can generate enough vacuum for your accessories. Now plug the vacuum system line back on -- one at a time -- and notice if you still maintain that level of vacuum or get a significant drop in vacuum reading. If it drops, you need to find where in that "just added" system the leakage is occurring and fix it. Continue until all systems are attached and you have very little vacuum loss (1-2" Hg MAXIMUM).
The above is a very good diagnostic method of figuring out what's REALLY going on, what problems you have and what to correct. On a C3 you use vacuum for all kinds of things from power brakes, operating heater controls to raising and lowering your headlight doors and leaks are really common. Get a baseline first with everything disconnected and plugged, tune on that then check for e reduction in vacuum with each system you connect. Including the power brake line.
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Old 03-19-2019, 08:36 PM
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htown81vette
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Good, thanks
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Old 03-19-2019, 08:40 PM
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htown81vette
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Originally Posted by wilcar View Post
You say it has a mild cam in it but the 290 hp crate motor has L79/L82 type cam in it which is not mild compared to the stock crossfire 202-206 @ 050 cam.
I suppose mild is relative. For me, that is a mild cam. My LS in my 'bird has a 250+ cam. Now THAT is a big cam...
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