Engine Timing With Vacuum Gauge - Setting Timing For Good

Tynan918

Royal Smart Person
Aug 2, 2021
1,120
63
Okay I know I played a lot with timing and I've been told a million times to set it until it runs right and leave it alone.. however I have just realized that every time I have done this procedure I have never used a vacuum gauge attached to a full manifold vacuum source on the carburetor to set the timing...

So I'm going to try it today..

I understand how to use the vacuum gauge to set base / initial timing, but how would that work when I'm setting total timing by putting the RPMs at 3000 ?

Doesn't vacuum drop when I flick the throttle? If so what vacuum level should I be looking at while I'm setting total timing ?

And for clarity and assurance, the vacuum advance canister should be disconnected until both base timing and total timing is set, correct ?

Right now I have that set up, I have the vacuum canister disconnected and plugged off, I have my vacuum gauge connected at the full manifold port on the carburetor where the vacuum canister was plugged in, and I have my adjustable timing light connected...

IMG_20220929_121328615.jpg


For starters, I'm going to try to document here in this thread what my vacuum readings are at initial timing from 6° BTDC to 20° BTDC and tell you whether it gets better or worse as I go up the initial degrees BTDC.
 

78Delta88

Master Mechanic
Supporting Member
May 23, 2022
406
43
SW Arizona
I'll try to answer some basic questions.

When setting base timing unhook vacuum can, temporarily plug off the vac signal.

Street car/daily driver moderate CR use ported/timed vacuum advance signal.

Race use manifold vac or none.

When throttle is closed and everything set right and no vac leaks in system the manifold vac signal will be at its highest value.

At idle there should be no ported vac signal

On a race unit vac to manifold brings in max timing that the vacuum actuator can do at the distributor. There is no mechanical advance as the RPM value is still low. At launch you go from Idle to WOT and you lose vac signal to the distributor. Mechanical advance comes on full and vac advance is no longer in play.

On a daily driver manifold vac is high at idle, no vac signal at the ported vacuum. Distributor attached to ported vacuum. Most vacuum Cans start moving at or about 6 inches and full on at or about 12 inches of vacuum.

As you accelerate normally in normal traffic, ported signal comes on and starts applying advance needed to accelerate and not lug the engine. As you give more gas, throttle plates continues to open and vac signal will decrease, but RPMs are up and mechanical advance starts coming in.

On a Q Jet (as an example) as you really stand on the gas and secondaries open, there no longer is a vac signal. Manifold vacuum at this point is at or about 1.5 inches of mercury vacuum, sometimes as little as .75 inches. RPM is high and all your timing is mechanical advance.

Where ever your max total timing is set, it is set at that RPM. That is the max and it will not go any higher. The only thing you can do at that point is WHEN you get total timing.

Heavier weights and lighter springs makes total timing come on sooner. While lighter weights and heavier springs will make timing come on later.

Your static CR will set your base timing. High compression needs lower base timing 0 to 6 BTDC, while lower Static CR will have higher base timing 12 to 14 BTDC.

Your cam lift and duration, valve overlap, dynamic compression and heat control at the head will affect detonation (pinging); when and how much, for any given octane rating. Higher octane fuel will have better properties to resist detonation, (knock/pinging). While poorer quality and lower octane rated fuels will knock, ping and create engine damage.
 
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Tynan918

Royal Smart Person
Thread starter
Aug 2, 2021
1,120
63
I'll try to answer some basic questions.

When setting base timing unhook vacuum can, temporarily plug off the vac signal.

Street car/daily driver moderate CR use ported/timed vacuum advance signal.

Race use manifold vac or none.

When throttle is closed and everything set right and no vac leaks in system the manifold vac signal will be at its highest value.

At idle there should be no ported vac signal

On a race unit vac to manifold brings in max timing that the vacuum actuator can do at the distributor. There is no mechanical advance as the RPM value is still low. At launch you go from Idle to WOT and you lose vac signal to the distributor. Mechanical advance comes on full and vac advance is no longer in play.

On a daily driver manifold vac is high at idle, no vac signal at the ported vacuum. Distributor attached to ported vacuum. Most vacuum Cans start moving at or about 6 inches and full on at or about 12 inches of vacuum.

As you accelerate normally in normal traffic, ported signal comes on and starts applying advance needed to accelerate and not lug the engine. As you give more gas, throttle plates continues to open and vac signal will decrease, but RPMs are up and mechanical advance starts coming in.

On a Q Jet (as an example) as you really stand on the gas and secondaries open, there no longer is a vac signal. Manifold vacuum at this point is at or about 1.5 inches of mercury vacuum, sometimes as little as .75 inches. RPM is high and all your timing is mechanical advance.

Where ever your max total timing is set, it is set at that RPM. That is the max and it will not go any higher. The only thing you can do at that point is WHEN you get total timing.

Heavier weights and lighter springs makes total timing come on sooner. While lighter weights and heavier springs will make timing come on later.

Your static CR will set your base timing. High compression needs lower base timing 0 to 6 BTDC, while lower Static CR will have higher base timing 12 to 14 BTDC.

Your cam lift and duration, valve overlap, dynamic compression and heat control at the head will affect detonation (pinging); when and how much, for any given octane rating. Higher octane fuel will have better properties to resist detonation, (knock/pinging). While poorer quality and lower octane rated fuels will knock, ping and create engine damage.
Great information.
 

Tynan918

Royal Smart Person
Thread starter
Aug 2, 2021
1,120
63
When setting base timing unhook vacuum can, temporarily plug off the vac signal.
Done.
Street car/daily driver moderate CR use ported/timed vacuum advance signal.

Race use manifold vac or none.
My emissions have been gutted out and I have no computer. I'm told because of this that the vacuum advance must be ran off of full manifold, and not ported...true or false?
When throttle is closed and everything set right and no vac leaks in system the manifold vac signal will be at its highest value.
So I had a confusing issue with reading the vacuum levels. I have two vacuum gauges, I have one as a tool that I can hook up anywhere in the engine bay and I have one that I bought on eBay to use for the inside of the car. Since I have installed the one in the car, I have been going off its reading and not using the one I bought as a tool, and it's been giving me a reading of 13 to 14 Hg... However, yesterday when I pulled out the tool and hooked it up to the full manifold vacuum source on my carburetor base plate and the engine at operating temperature, I get a steady vacuum reading of 21 Hg... Clearly the interior gauge is off by 4 or 5 Hg levels, the hose that came with it was a transparent hose (like a fish tank hose and I'm not sure if it pinches or not, but the damn thing reads wrong for sure.
IMG_20220929_140034899_HDR.jpg



On a race unit vac to manifold brings in max timing that the vacuum actuator can do at the distributor. There is no mechanical advance as the RPM value is still low. At launch you go from Idle to WOT and you lose vac signal to the distributor. Mechanical advance comes on full and vac advance is no longer in play.

On a daily driver manifold vac is high at idle, no vac signal at the ported vacuum. Distributor attached to ported vacuum. Most vacuum Cans start moving at or about 6 inches and full on at or about 12 inches of vacuum.

As you accelerate normally in normal traffic, ported signal comes on and starts applying advance needed to accelerate and not lug the engine. As you give more gas, throttle plates continues to open and vac signal will decrease, but RPMs are up and mechanical advance starts coming in.
That goes back to my original question about my emissions and computer being ripped out.. should I run full manifold or should I run ported manifold ? Would ported still save me on gas without the emissions and computer being part of my system ?


Where ever your max total timing is set, it is set at that RPM. That is the max and it will not go any higher. The only thing you can do at that point is WHEN you get total timing.
Total timing should be set at 3000 to 3500... Why these numbers and which one is good for daily driving and racing ? I don't intend on doing any racing but I don't like to turn down a challenge lol... what's good for daily driving but also for a little racing when I need to ?


Heavier weights and lighter springs makes total timing come on sooner. While lighter weights and heavier springs will make timing come on later.
I bought a spring and weight recurve kit for my distributor.. but I did what I was told, I was told to only switch out the springs with lighter springs and to leave the stock weights in.. I still have the kit if you think I should put in the heavier weights with those lighter springs.


Your static CR will set your base timing. High compression needs lower base timing 0 to 6 BTDC, while lower Static CR will have higher base timing 12 to 14 BTDC.
I love this answer here because I've never been told this before and it clears up a lot of things, but this is where I'm at a standstill because I don't know what my compression ratio is which is why I was asking about a manometer and another thread so that way hi I can get my compression ratio without disassembling and/or removing the engine. 👇🏾
Screenshot_20220929-104557-318.png
Screenshot_20220929-104627-130.png
Screenshot_20220929-104647-421.png



Your cam lift and duration, valve overlap, dynamic compression and heat control at the head will affect detonation (pinging); when and how much, for any given octane rating. Higher octane fuel will have better properties to resist detonation, (knock/pinging). While poorer quality and lower octane rated fuels will knock, ping and create engine damage.
I'm not for sure about cam lift and duration, valve overlap, or dynamic compression... I do know that the engine doesn't overheat because I don't have drivability issues or engine smoking. I flushed out my entire cooling system not too long ago and I replaced the water with coolant. I made my own mixture by buying pure green antifreeze and mixing it half and half with distilled water.

I have played with 91 gas before, but it had 10% ethanol in it and I think that killed the original carburetor that was on the motor when I bought it. The engine did run a lot better and didn't ping as much or nearly at all on the 91 then the 87 that has no ethanol.. currently trying to find a gas station with 91 to 93 octane no ethanol fuel that I can use regularly.

There is a couple of Kum & goes in my area that have 93 gas but I'm not for sure if it's got ethanol in it or not. I guess I could call those gas stations and ask them specifically. If do contain ethanol, there's a race track about 40 miles from me that has a gas station with up to 110 octane fuel with no ethanol. But damn that's 40 miles away.
 
Last edited:

78Delta88

Master Mechanic
Supporting Member
May 23, 2022
406
43
SW Arizona
I'll have to flesh out more later but.

On 87 fuel back down initial to 10 to 12
Ported vacuum
Your listening to some that are sending you the wrong way. I've been doing this 40+ years and usually get to fix what the experts screw up
Full on advance by 2500 not 3500
 
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78Delta88

Master Mechanic
Supporting Member
May 23, 2022
406
43
SW Arizona
Looking at your vac gauge its 21 and solid, that is very good for street car, but what it does show is you don't have a large cam. Your LSA is around 114 based on what you show. Mother Thumper is LSA about 106/104.

I think if you open up the engine you will find something close to 212 duration very mild lift. This would be something closer to SUM-1102.

150 psi is going to be closer to 8.5:1. Once you get up around 10:1 your in the 175 - 190 psi ball park.

Basically you have a "top heavy" car. You have everything for 2600 - 6000 street smoker, but that mild cam you have is not going to let you open it up and this is where you main problem is.

For what you got up top plus 3.73 rear gears, you need .488 lift to 286 duration or more if you want in higher rpm range. Possible go to .520 lift and 303 duration but not with the dual plane you have you need Victor junior or other single plane, plus change to 3000 stall TQ.

Basically you have what you need For start of really nice street car but wimpy cam.
 
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Tynan918

Royal Smart Person
Thread starter
Aug 2, 2021
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Last edited:

Tynan918

Royal Smart Person
Thread starter
Aug 2, 2021
1,120
63
Looking at your vac gauge its 21 and solid, that is very good for street car, but what it does show is you don't have a large cam. Your LSA is around 114 based on what you show.
Well at least that's out the way and now we know what cam is in there 😎

150 psi is going to be closer to 8.5:1. Once you get up around 10:1 your in the 175 - 190 psi ball park.
Last time I did a compression test with all spark plugs removed (almost an exact year ago) the ratings were 175-185... Is that up around 10:1 ?


Screenshot_20220929-075802-098.png


Basically you have a "top heavy" car. You have everything for 2600 - 6000 street smoker, but that mild cam you have is not going to let you open it up and this is where you main problem is.
Ok, so new camshaft... should I also buy lifters, rods, and rocker arms to match the new camshaft ?

I don't have the money for it all this month around, but I will next month, around $400...
Possible go to .520 lift and 303 duration but not with the dual plane you have you need Victor junior or other single plane,
Funny, it had a Victor Jr. single plane intake when I bought it, I was told it was too big for my stock setup so I removed it and sold it...I been sold that almost a year ago.


For what you got up top plus 3.73 rear gears, you need .488 lift to 286 duration or more if you want in higher rpm range.
I don't have 3.73 rear gears, tires spin in the opposite directions when they're off the ground and turned by hand.

What's the highest I can go with the stall and dual plane intake I currently have ?
 

Rt Jam

Master Mechanic
Mar 30, 2020
340
63
Ontario Canada
There is not right on wrong answer on port vs manifold. Just understand the difference. During idle and light throttle cruise the manifold vacuum will be high, ported 0.
Once the throttle is cracked slightly the ported one will raise up and both disappear during deep throttle.

There are a dozen pros and cons but one advantage that makes me run ported. Mileage, off idle torque, idle quality is better at high timing, than low timing. This can ONLY be achieved with manifold vacuum advance.

On a stock car. You can set the initial to factory specs and everything should fall into place but if it's modified or has a different distributor. Follow below.

Ignore all past advice that you set it where it runs best. A properly tuned distributor needs at least 3 things set.

1.Initial, this is typically the timing at idle with no vacuum advance connected. 5 to 15 deg.
2. Mechanical advance. This is where the engine makes the most power. You will need a dyno or trap speed to set this. Most older engines are in the upper 30's. Modern lower 30's. Say 35 deg.
3. Vacuum advance. This adds timing to your initial timing. Since you do not want to idle and light throttle cruise at initial. The engine during lean conditions needs way more timing. During cruise, this can be way over 40.
 
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Tynan918

Royal Smart Person
Thread starter
Aug 2, 2021
1,120
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I have base timing at 12°, so I guess I'm going to put 22 at 2500, making total to make it 34 total timing.

Right now I'm adjusting the idle mixture screws using a vacuum gauge. When I'm done with that I'm going to do total timing, reconnect the distributor to ported and take it down the street and back and see how it does.

Should I go ahead and throw in the heavier weights in the distributor since I already put in the lighter springs or should I just use the weights that came with the aftermarket HEI distributor ?
 
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