Vacuum on 383 stroker
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As Kevin suggests, a lot of items can affect the amount of vacuum the engine creates.
Every toolbox should have a vacuum gauge - they are cheap. I've bought mine for $2 and best tuning tool.
The longer the valves stay open, the less vacuum created. A stock engine generally indicates 18"-21" at idle, increase RPM, might get up to 25".
More duration - longer valves stay open, less vacuum and also dependent upon LSA (Lobe Separation Angle) between lobe centerlines of intake & exhaust lobes on the same cylinder - wider 116 degrees and higher, 18"-21", 108 degrees and smaller, maybe 3"-5" at idle, but then longer duration and shorter LSA means cam desired for higher RPM, not idle.
Every toolbox should have a vacuum gauge - they are cheap. I've bought mine for $2 and best tuning tool.
The longer the valves stay open, the less vacuum created. A stock engine generally indicates 18"-21" at idle, increase RPM, might get up to 25".
More duration - longer valves stay open, less vacuum and also dependent upon LSA (Lobe Separation Angle) between lobe centerlines of intake & exhaust lobes on the same cylinder - wider 116 degrees and higher, 18"-21", 108 degrees and smaller, maybe 3"-5" at idle, but then longer duration and shorter LSA means cam desired for higher RPM, not idle.
I have no cam specs for the Camshaft, such as a tag, or sheet. Other than a engine build sheet (3) pages that says it is a special grind which may mean anything. I believe my vacuum advance is disconnected which leads me to wonder, Does it have a high lift enough cam to have low vacuum.
Flat tire says he has 11.5 with a canister. Meaning if his car didn’t have the canister the vacuum I assume would be really low? Low vacuum affects the power brakes and vacuum advance to the distributor correct? Will low vacuum affect the power brakes a great deal? What are the symptoms with low vacuum to the power brakes? Would the brake pedal feel be spongy or hard? If I added a vacuum canister where would I get my feed vacuum from. Manifold? Front or rear of manifold or from the carburetor and where would it feed from off the carb.?
I am assuming Flat tire added a vacuum canister to help with the low vacuum. Is this thenormal thing to do to help with the brakes?
Why do people block off or disconnect the advance at the distributor? Is the timing advanced a lot at idle to compensate for the advance not working? Wouldn’t this make a car a little hard to start but yet be ok for driving. Please educate me on this.
Flat tire says he has 11.5 with a canister. Meaning if his car didn’t have the canister the vacuum I assume would be really low? Low vacuum affects the power brakes and vacuum advance to the distributor correct? Will low vacuum affect the power brakes a great deal? What are the symptoms with low vacuum to the power brakes? Would the brake pedal feel be spongy or hard? If I added a vacuum canister where would I get my feed vacuum from. Manifold? Front or rear of manifold or from the carburetor and where would it feed from off the carb.?
I am assuming Flat tire added a vacuum canister to help with the low vacuum. Is this thenormal thing to do to help with the brakes?
Why do people block off or disconnect the advance at the distributor? Is the timing advanced a lot at idle to compensate for the advance not working? Wouldn’t this make a car a little hard to start but yet be ok for driving. Please educate me on this.
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Many with aggressive cams have a Vacuum canister so there is ample vacuum for the power brake booster to work properly. Low vacuum otherwise would render the brakes much harder to push as there is no "vacuum assist"
You want to tap into intake manifold vacuum either from a port behind carb on intake (one that feeds brake booster and often auto trans vac module) or the rear port of the carb itself.
If your brakes are working good you should have decent vacuum. A quick check with a gage would tell you what it is. Vacuum gauges are very handy in diagnosing several engine conditions. There is usually a sheet with gauge showing several examples of its use and meanings of what the needle on the gauge is showing.
Your vacuum advance to distributor is likely blocked off because you are set for "total", aka centrifugal, timing vs timing at idle then the timing advances when rpm increase with vacuum hooked up. I have mine set for total timing of 35 degrees at 2700 rpm. It is about 14-16 at idle
distributors using vacuum advance are often set up anywhere between 6-16 degrees of timing at idle (generally on the low side). The distributor vacuum advance generally nets a additional 14-16 degrees so if your idle timing was say 8 degrees and vacuum advance would only add another 14-16 then your "total" timing would be 22-24....you are leaving HP on the table as most SBC 4 barrel motors are happy with about 35 degrees of timing
To set total timing you block off vacuum to distributor (some use a BB in the vac line so it "looks" stock with a vac line attached on a stock distributor). You then (with car completely warmed up) hold rpm 2500-3000 RPM and with a timing light set advance to 35. The timing will drop to about 14-16 at idle but will get all the way to 35 by 2500-3000 rpm
Timing light, vac gauge, tach are all needed to "tune" your car and knowing how to use them is mandatory. As you set timing you often need to adjust idle and idle mixture initially on a new carb/motor as one affects the other. Many timing lights come with a rpm function and also a way to set it at a particular advance so with that advance being say 35 your timing mark on the damper and your timing pointer on block would be at zero as the timing light is pre-set to whatever # you want. That function is helpful for those who do not have timing mark tape (shows full spread of degrees) on their damper or aftermarket ones with the timing marks etched in. Generally -10 to +50
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