C3 Engine Vacuum
Thread Starter
Intermediate
Joined: Sep 2023
Posts: 41
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From: Gainesville GA USA
C3 Engine Vacuum
Hello Everyone,
General question here, what is everyone's (1) vacuum from the carb base (going to the lights, wipers, etc.) and from the intake manifold (going to the brake booster)? Just curious, because mine (organically) seems to be low. Sorry, I should mention my engine is a "crate" 383 engine with some upgraded cam (don't know the specs) and HEI ignition.
General question here, what is everyone's (1) vacuum from the carb base (going to the lights, wipers, etc.) and from the intake manifold (going to the brake booster)? Just curious, because mine (organically) seems to be low. Sorry, I should mention my engine is a "crate" 383 engine with some upgraded cam (don't know the specs) and HEI ignition.
Last edited by LTC_Ret; Apr 29, 2026 at 10:29 AM.
1967 Pedal Car Champion
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From: US-PNW
You know this is going to be different depending on the cam and valve timing of the engine, yes? Some will be as high as ~20 in/hg (stock cam) while a radical cam could be sub-10 in/hg. If your engine lopes at idle it's got overlapped valve timing and thus lowered vacuum available.
My basically stock LS-5 is in the 18-20 in/hg at both ports. (I don't believe there should be any significant difference between the two vacuum sources.)
My basically stock LS-5 is in the 18-20 in/hg at both ports. (I don't believe there should be any significant difference between the two vacuum sources.)
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Different cams, compression etc will vary idle vacuum.
My engine is at about 14.5 inches.
My engine is not stock.
My engine is at about 14.5 inches.
My engine is not stock.
Race Director
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From: Virginia
Can you post some photos of all sides of your engine/carb with the air cleaner assembly removed?
Getting the timing just right should raise your idle vacuum. Email @lars for his timing and Q-Jet papers, and make sure you are using manifold vacuum for your vacuum advance. The process may also help you find vacuum leaks.
Post 7 in this thread has his email, and some useful timing tips.
https://www.corvetteforum.com/forums...ing-light.html
Getting the timing just right should raise your idle vacuum. Email @lars for his timing and Q-Jet papers, and make sure you are using manifold vacuum for your vacuum advance. The process may also help you find vacuum leaks.
Post 7 in this thread has his email, and some useful timing tips.
https://www.corvetteforum.com/forums...ing-light.html
Thread Starter
Intermediate
Joined: Sep 2023
Posts: 41
Likes: 6
From: Gainesville GA USA
Yes, I realize cam and valve timing will effect vacuum. I wanted to get a since of an overall pattern, My engine is a "crate" 383. There is a upgraded cam in mime but I don't know the specs. I had the valves set per the 72 Service Manual. Still, my vacuum is low and it is impacting how my headlights work. The previous owner installed an electric vacuum pump but it only supplied the brake booster.
I've divided the electric vacuum pump's vacuum and sent it to both systems.
I've divided the electric vacuum pump's vacuum and sent it to both systems.
Last edited by LTC_Ret; Apr 29, 2026 at 10:28 AM.
Thread Starter
Intermediate
Joined: Sep 2023
Posts: 41
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From: Gainesville GA USA
Perfect, I emailed the address given for their technical papers on timing and Q-jets. I also read most of the thread on timing. Hey, I've never heard of using manifold vacuum for distributor advance, where does that come from?
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1967 Pedal Car Champion
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From: US-PNW
Yes, I realize cam and valve timing will effect vacuum. I wanted to get a since of an overall pattern, My engine is a "crate" 383. There is a upgraded cam in mime but I don't know the specs. I had the valves set per the 72 Service Manual. Still, my vacuum is low and it is impacting how my headlights work. The previous owner installed an electric vacuum pump but it only supplied the brake booster.
I've divided the electric vacuum pump's vacuum and sent it to both systems.
I've divided the electric vacuum pump's vacuum and sent it to both systems.
Valve timing is only adjustable with camshaft swaps (can also be how the timing chain crank sprocket is oriented). For this discussion, valve timing is determined by how each of the lobes are ground and the relationship of the lobe peak with its partner. What is your idle vacuum when the gauge is connected to the manifold T-fitting or the carb's brake booster port?
Manifold vacuum is a more responsive, always "on" source. The manifold T-fitting is constant manifold vacuum. The very base of the carb usually has manifold vacuum ports while the upper ports are "timed" sources that only activate with rises in rpm. To find a manifold vac port on the carb, while at hot idle, remove each cap/line and feel the port end with your finger. If it sucks on your finger, it's manifold vac, if no suck, it's "timed" vac.
Thread Starter
Intermediate
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From: Gainesville GA USA
Understand, Valve timing, Valver lash and ignition timing are different. My mechanic friend set the valve lash (per the service manual). I know ignition timing (which we've checked) is connecting a timing light the #1 spark plug wire, illuminating the crankshaft pully and reading the attached bracket and adkjusting as necessary with turning the distributor. If setting the valve lash is different than setting the valve timing - I'm unclear.
The ignition timing specs (I've found) for the 383 is at high RPMs (2500-3500) with initial timing 18 BTDC (vacuum disconnected). Total mechnical timing 36-28 (same RPM, disconnected).
The ignition timing specs (I've found) for the 383 is at high RPMs (2500-3500) with initial timing 18 BTDC (vacuum disconnected). Total mechnical timing 36-28 (same RPM, disconnected).
1967 Pedal Car Champion
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From: US-PNW
Understand, Valve timing, Valver lash and ignition timing are different. My mechanic friend set the valve lash (per the service manual). I know ignition timing (which we've checked) is connecting a timing light the #1 spark plug wire, illuminating the crankshaft pully and reading the attached bracket and adkjusting as necessary with turning the distributor. If setting the valve lash is different than setting the valve timing - I'm unclear.
The ignition timing specs (I've found) for the 383 is at high RPMs (2500-3500) with initial timing 18 BTDC (vacuum disconnected). Total mechnical timing 36-28 (same RPM, disconnected).
The ignition timing specs (I've found) for the 383 is at high RPMs (2500-3500) with initial timing 18 BTDC (vacuum disconnected). Total mechnical timing 36-28 (same RPM, disconnected).
Have you measured your hot idle manifold vacuum?
Valve timing has to do with how the camshaft is designed: how tall the lobes are (lift), how gradual or steep the sides are to the peak of the lobe (ramp), the top curve of the lobe peak, and how the lobe center lines are oriented. It's also tied into the crankshaft rotation and piston cycles by way of the timing chain. If you have low manifold vacuum, it may very well be the camshaft causing it — how the openings and closings and rates of change (lobe overlap/separation angle) are designed. Large angles allow the characteristics of each lobe & valve to behave more independently rather than be influenced by its partner. @leigh1322 understands this stuff way better than I do and can explain it better too!
Thread Starter
Intermediate
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From: Gainesville GA USA
Thanks, yes, I meant "36-38". Hot idle manifold vacuum is 10-12. I undersand that an aftermarket cam can impact the vacuum. I've checked carefully and I have no vacuum leaks. Unfortnuately, I know of no way of determining the installed cam specs wihthout removing and and using a micrometer. I have emailed @leigh1322 and received his "away message". Hopefully, he can provide more inmformation when he gets back. At this point, its a bit achedemic since I've already split the electric auxiliary vacuum between the two systems. When I go back to the garage next week, I'll measure the improved vacuum for both the brake system and HL/Wipers system and post it here. The previous low vacuum was causing my headlights to raise at random times.
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If you just want to solve the problem, electric actuators exist for your headlights, and wiper tray. These are the ones to get. Made in USA!
https://www.ebay.com/str/richard454
Many folks convert to manual brakes, too, but perhaps your current system can keep up with that.
https://www.ebay.com/str/richard454
Many folks convert to manual brakes, too, but perhaps your current system can keep up with that.
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If your headlights rise inter acceleration. It's not your vacuum level that's the problem at all. It's a bad check valve. 10 inches of vacuum is enough for your headlights to work properly.
Melting Slicks
Joined: Dec 2009
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From: Thunder Bay
427 small block. I get about 12" of vacuum at idle. Headlights work fine. Your problem isn't low vacuum.
Old Pro Solo Guy
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From: Marlton NJ
Mild smooth idle cams give 18" at idle.
Mild loping performance cams like a 350-350 HP L46 give about 15" at idle with 224* duration. Lt1 and L71 solid lifter cams are over 230*, and say 12* vac. I ran an even bigger cam with 255* and had 10.5" vac. And rough idle. But I still had zero problems with power brakes because the vac quickly climbs over 15" as soon as you pass 1300rpm. Your problems are elsewhere not the cam.
Mild loping performance cams like a 350-350 HP L46 give about 15" at idle with 224* duration. Lt1 and L71 solid lifter cams are over 230*, and say 12* vac. I ran an even bigger cam with 255* and had 10.5" vac. And rough idle. But I still had zero problems with power brakes because the vac quickly climbs over 15" as soon as you pass 1300rpm. Your problems are elsewhere not the cam.