) anything between 10 and 15 and you're good to go.
How much vacuum is enough?
Thread Starter
Melting Slicks
Joined: Dec 2009
Posts: 3,220
Likes: 651
From: Thunder Bay
How much vacuum is enough?
The new 427 idles with about 12" of vacuum. The headlights work ok - maybe a little slower than before, but I won't be able to try the brakes for a few months until the snow melts. Is 12" of vacuum going to be enough?
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
I run about 11" with my 496-- haven't got it all dialed in yet, but I don't expect much more. Headlights are really slow- but the vacuum system there is still 69 vintage stuff that's never had anything done to it. The brakes seem fine so far, but I don't have many miles on it yet.
Safety Car
Joined: Oct 2007
Posts: 3,729
Likes: 267
From: Austin Texas
Corvette of the Year Winner 2017
2016 C3 of Year Finalist
in my mind, anything below 10" vacuum is not acceptable,
because of slow headlights, due to lower vacuum.
which also means it will also impact power brakes.
and on corvettes A/c and heater controls, and wipers. my recommendation 10" minimum.
prefer 12 min. on corvettes, this is exactly why I do not install large cam in vehicles,
with a lot of vacuum components, like the corvette.
because of slow headlights, due to lower vacuum.
which also means it will also impact power brakes.
and on corvettes A/c and heater controls, and wipers. my recommendation 10" minimum.
prefer 12 min. on corvettes, this is exactly why I do not install large cam in vehicles,
with a lot of vacuum components, like the corvette.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
12" Hg should be enough. But, you want to make sure that all vacuum leakages are found and repaired so that you have as much vacuum as your engine will produce. You might also experiment with timing settings. When set for 'performance', your vacuum level may be marginal; but, if you adjust timing a bit, that vacuum level may increase. If so, you could have two markings on your distributor flange: one for performance and one for normal operation (where you have higher vacuum).
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,170
From: Hermosa
On a nice hot day and in gear I only have about 10.5 to 11.0 inches of vacuum and everything still works fine.
Your vacuum can for timing advance may need to be changed or modified to work at that vacuum.
Your vacuum can for timing advance may need to be changed or modified to work at that vacuum.
Thread Starter
Melting Slicks
Joined: Dec 2009
Posts: 3,220
Likes: 651
From: Thunder Bay
Melting Slicks
Joined: Mar 2008
Posts: 2,666
Likes: 289
From: Spring Park MN
You guys are missing that as you cruise, even with a big cam and low idle vacuum, the vacuum goes higher. My engine idled at 9 inches in neutral and 7 inches in gear, but when I was cruising anywhere from 2000 rpm to 3000 rpm, the vacuum would go to over 20 inches. If you don't have any leaks and have good check valves on the vacuum tank and brake booster, that vacuum will be there when you need the brakes or need to open the headlights.
My brake worked great and my headlights snapped up. Main thing, no leaks and good check valves!
Mike
My brake worked great and my headlights snapped up. Main thing, no leaks and good check valves!
Mike
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