Intake Vacumn ?
Thread Starter
Burning Brakes
Joined: Oct 2001
Posts: 860
Likes: 40
From: Woodforest
Intake Vacumn ?
I have an 82 (Crossfire) and my intake vacumn at the manifold, with every vacumn line taken off and plugged is only 10 to 12 inches at idle (750 rpm). Is this typical or is it low? I have read that a stroker generally will give a lower vacumn and I was told this car was a 383 when I purchased it. Don't know that for sure, haven't taken any of the engine down yet.
I have some headlight problems that seem to be a result of low vacumn rather than parts issues. Thanks for any replies.
I have some headlight problems that seem to be a result of low vacumn rather than parts issues. Thanks for any replies.
Safety Car
Joined: Apr 2001
Posts: 4,795
Likes: 1
From: Regina Saskatchewan, Canada.
Re: Intake Vacumn ? (Woodstoc)
Wether it's a stroker or not doesn't mean it will have low vacuum readings. That's usually determined by the type of camshaft that the particular engine has. Low vacuum readings can make carb tuning difficult. Check your valves, maybe they have been set too tight. That would cause some vacuum loss. Normally, I think guys go anywhere from 1/4 turn to 1 1/2 turns. Of course, this is assuming you have a hydraulic cam.
[Modified by Stingy74, 4:01 PM 12/10/2001]
[Modified by Stingy74, 4:01 PM 12/10/2001]
Advanced
Joined: Jan 2000
Posts: 61
Likes: 0
From: Colorado Springs, CO
Re: Intake Vacumn ? (Stingy74)
I have 13 in. off my intake at 800 rpm in park. My headlights open ok. Make sure your headlight vacuum is coming off the manifold and not the carb. Also, be sure the intended vacuum line I.D. is not reduced by a fitting or pinched.
Jeff
Jeff
Drifting
Joined: Nov 2001
Posts: 1,469
Likes: 6
From: 68 427 4.11s Roadster
Re: Intake Vacumn ? (jeffs)
Ignition timing can affect the vacuum. Retarded I think wil do it.
You could check your compression or better do a leak down test.
You can check the stroke by pulling the plugs and checking the depth of the piston then rotate the engine and measure that. Do the math.
They messed with the ignition on those later cars so this may be normal for you.
You could check your compression or better do a leak down test.
You can check the stroke by pulling the plugs and checking the depth of the piston then rotate the engine and measure that. Do the math.
They messed with the ignition on those later cars so this may be normal for you.
Melting Slicks
Joined: Sep 2000
Posts: 2,404
Likes: 418
From: Chester Virginia
Re: Intake Vacumn ? (Woodstoc)
When I got my Vette; L-82, the intake vacuum was 9-1/2"Hg and idling was very rough. I check everything and I found a leak under the intake manifold. Replaced the intake gaskets and the vacuum went to 13-1/2" Hg. To test for a vacuum leak under the intake manifold I checked the vacuum in the crackcase by using the vacuum gage on the dipstick tube.
Roger
Roger
Thread Starter
Burning Brakes
Joined: Oct 2001
Posts: 860
Likes: 40
From: Woodforest
Re: Intake Vacumn ? (Woodstoc)
Thanks for the replies. I am off the manifold for the headlight circuit and get the same reading at the storage canister as at the manifold.
That is an interesting comment Techno about measuring to determine stroke. Care to elaborate? I am thinking you are talking about measuring from BDC to where the plug would be? And if so, what distance would normally be stock?
Sorry, stupid comment, didn't read your post properly, however, do you know the stock measurement?
[Modified by Woodstoc, 4:32 AM 12/11/2001]
That is an interesting comment Techno about measuring to determine stroke. Care to elaborate? I am thinking you are talking about measuring from BDC to where the plug would be? And if so, what distance would normally be stock?
Sorry, stupid comment, didn't read your post properly, however, do you know the stock measurement?
[Modified by Woodstoc, 4:32 AM 12/11/2001]
Drifting
Joined: Nov 2001
Posts: 1,469
Likes: 6
From: 68 427 4.11s Roadster
Re: Intake Vacumn ? (Woodstoc)
Right. From TDC to BDC would be the stroke.
3.25"=327
3.48"=350
3.75"=400 But that has a 4.126" bore compared to the 4.001" for the others.
You shouldn't need to be real accurate, just within a few tenths since they are pretty far apart.
.2" = ~3/16"
.48=7.6/16"
Put something soft like a dowel,straw,pen in the hole. Pick a reference and mark it. rotate the engine and mark it again. The distance between marks is the stroke. This will tell you if it has been stroked but might be a little crude for the exact amount.
A cheat would be just to stick it in the cylinder that is already at BDC from the one that you checked at TDC. There should be one.
3.25"=327
3.48"=350
3.75"=400 But that has a 4.126" bore compared to the 4.001" for the others.
You shouldn't need to be real accurate, just within a few tenths since they are pretty far apart.
.2" = ~3/16"
.48=7.6/16"
Put something soft like a dowel,straw,pen in the hole. Pick a reference and mark it. rotate the engine and mark it again. The distance between marks is the stroke. This will tell you if it has been stroked but might be a little crude for the exact amount.
A cheat would be just to stick it in the cylinder that is already at BDC from the one that you checked at TDC. There should be one.
Thread Starter
Burning Brakes
Joined: Oct 2001
Posts: 860
Likes: 40
From: Woodforest
Re: Intake Vacumn ? (Techno)
Excellent Techno. I don't know why I didn't think of that (possibly because I am seeing all the $$$ signs on everything else I have been fixing on this beast). Thanks.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Drifting
Joined: Feb 2000
Posts: 1,607
Likes: 0
From: Clarksville,Tn,USA TN
Re: Intake Vacumn ? (Woodstoc)
I am running about 17 Hg at 750-800 rpms in my 388ci stroke motor. At first my readings were about 12 and then I found out I had the vacuum gage plugged into the wrong port on my Holley.
Neil in Tenn
Neil in Tenn