Vacuum advance.
Thread Starter
Team Owner
Joined: Nov 2004
Posts: 20,715
Likes: 25
From: Sorrento Italy
Vacuum advance.
Actually I know the 'vacuum advance' but I've never really understood what it actually is and how it is involved in an engine advance.
Any of you guys can clarify it to me?
Any of you guys can clarify it to me?
Melting Slicks
Joined: Apr 2021
Posts: 2,717
Likes: 2,163
From: Minnesota
It’s a vacuum diaphragm that can further advance your engine’s ignition timing when there is little to no load on the engine such as idle or cruising, increasing fuel economy, reducing engine bay temperatures, and improving throttle response.
If you’re looking for the best way to set up your vacuum advance and ignition timing I would email Lars at v8fastcars@msn.com and ask for his timing papers.
If you’re looking for the best way to set up your vacuum advance and ignition timing I would email Lars at v8fastcars@msn.com and ask for his timing papers.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,218
Likes: 9,356
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
to add, when the factory used it to detune the engien and make it pass emmisions it was run off of ported vacuum and set really low around 6-8*. For performance you want to run it off a manifold vacuum port and it is usually going to end up around 14-16 degrees. But get Lars paper for all the adjustments that will get you there.
1967 Pedal Car Champion
Joined: Mar 2020
Posts: 9,137
Likes: 4,248
From: US-PNW
When the vacuum advance canister is activated, it pulls it's arm inward, which pulls on the plate the points are mounted on. This counterclockwise rotation of the points plate moves the points to open earlier in the engine's cycle than how they are set without being rotated. In addition to the info above, I believe the engine has an easier job of spinning and starting the less advance it has.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Just e-mail me a request for my Vacuum Advance tech paper - it covers the vacuum advance system operation in detail and provides all info for setting it up correctly.
Lars
V8FastCars@msn.com
Lars
V8FastCars@msn.com
Instructor
Joined: Dec 2022
Posts: 157
Likes: 97
From: Virginia
“How They Work”
“Ported vacuum: With that thought in mind consider if the distributor hooked to Ported Vac, as air speed increases the ported vac activates and starts to pull more and more timing in the motor as velocity increases. So if you set you total timing to 34* at 3000RPM that's when the ported starts to do it's job and advances the the timing to the total stroke of the vacuum can arm, usually 12-18* (without Limiters) so now have your total of 34* PLUS the stroke of the Canister arm of say 16* net result=50* of total timing under hard acceleration and your motor WILL "Detonate". If you had a OBD1 computer it would pull that timing back to the 34* as all the sensors feed information to it.... and no detonation.
Now to Constant: At idle/part throttle cruise you have high vacuum, the carb is nearly closed causing a restriction which creates the high vacuum level. Under this light load condition and lean AF ratios the motor needs more timing to burn the fuel (Lean Mixtures take Longer to Burn than rich mixtures) so you need more timing at idle and cruise to burn the fuel correctly and completely. When you stomp the throttle you have NO manifold Vacuum so you have NO vacuum timing and at NO time under high load will it ever advance more than the mechanical "All In" numbers. Stop pointing your finger at the carb for rich idle and top end lean conditions.. It's In Your Distributor Tuning! If your Buddy tells you to hook your Hot Rod distributor to Ported Vacuum, find a new friend because that guys advice is going to blow your crankshaft through the oil pan. Same goes for the guy who say's to disconnect it, they obviously have no idea of how it works or what it does or why we use it.
This not hear-say or an opinion its engineering, based on physics formulas, calculated by people who are a lot smarter than me and I know that so I do what they tell me. They come up with the numbers, we set them up accordingly.
If the Motor Makes 10" of vacuum or less at Idle with 30* of timing in it then it needs a properly calibrated Vacuum Can, we have Mopar and GM vacuum cans that will read 5" of vacuum.”
(From FBO ignitions)
“Ported vacuum: With that thought in mind consider if the distributor hooked to Ported Vac, as air speed increases the ported vac activates and starts to pull more and more timing in the motor as velocity increases. So if you set you total timing to 34* at 3000RPM that's when the ported starts to do it's job and advances the the timing to the total stroke of the vacuum can arm, usually 12-18* (without Limiters) so now have your total of 34* PLUS the stroke of the Canister arm of say 16* net result=50* of total timing under hard acceleration and your motor WILL "Detonate". If you had a OBD1 computer it would pull that timing back to the 34* as all the sensors feed information to it.... and no detonation.
Now to Constant: At idle/part throttle cruise you have high vacuum, the carb is nearly closed causing a restriction which creates the high vacuum level. Under this light load condition and lean AF ratios the motor needs more timing to burn the fuel (Lean Mixtures take Longer to Burn than rich mixtures) so you need more timing at idle and cruise to burn the fuel correctly and completely. When you stomp the throttle you have NO manifold Vacuum so you have NO vacuum timing and at NO time under high load will it ever advance more than the mechanical "All In" numbers. Stop pointing your finger at the carb for rich idle and top end lean conditions.. It's In Your Distributor Tuning! If your Buddy tells you to hook your Hot Rod distributor to Ported Vacuum, find a new friend because that guys advice is going to blow your crankshaft through the oil pan. Same goes for the guy who say's to disconnect it, they obviously have no idea of how it works or what it does or why we use it.
This not hear-say or an opinion its engineering, based on physics formulas, calculated by people who are a lot smarter than me and I know that so I do what they tell me. They come up with the numbers, we set them up accordingly.
If the Motor Makes 10" of vacuum or less at Idle with 30* of timing in it then it needs a properly calibrated Vacuum Can, we have Mopar and GM vacuum cans that will read 5" of vacuum.”
(From FBO ignitions)
Tech Contributor
Joined: Jun 2004
Posts: 20,910
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
“How They Work”
“Ported vacuum: With that thought in mind consider if the distributor hooked to Ported Vac, as air speed increases the ported vac activates and starts to pull more and more timing in the motor as velocity increases. So if you set you total timing to 34* at 3000RPM that's when the ported starts to do it's job and advances the the timing to the total stroke of the vacuum can arm, usually 12-18* (without Limiters) so now have your total of 34* PLUS the stroke of the Canister arm of say 16* net result=50* of total timing under hard acceleration and your motor WILL "Detonate". If you had a OBD1 computer it would pull that timing back to the 34* as all the sensors feed information to it.... and no detonation.
Now to Constant: At idle/part throttle cruise you have high vacuum, the carb is nearly closed causing a restriction which creates the high vacuum level. Under this light load condition and lean AF ratios the motor needs more timing to burn the fuel (Lean Mixtures take Longer to Burn than rich mixtures) so you need more timing at idle and cruise to burn the fuel correctly and completely. When you stomp the throttle you have NO manifold Vacuum so you have NO vacuum timing and at NO time under high load will it ever advance more than the mechanical "All In" numbers. Stop pointing your finger at the carb for rich idle and top end lean conditions.. It's In Your Distributor Tuning! If your Buddy tells you to hook your Hot Rod distributor to Ported Vacuum, find a new friend because that guys advice is going to blow your crankshaft through the oil pan. Same goes for the guy who say's to disconnect it, they obviously have no idea of how it works or what it does or why we use it.
This not hear-say or an opinion its engineering, based on physics formulas, calculated by people who are a lot smarter than me and I know that so I do what they tell me. They come up with the numbers, we set them up accordingly.
If the Motor Makes 10" of vacuum or less at Idle with 30* of timing in it then it needs a properly calibrated Vacuum Can, we have Mopar and GM vacuum cans that will read 5" of vacuum.”
(From FBO ignitions)
“Ported vacuum: With that thought in mind consider if the distributor hooked to Ported Vac, as air speed increases the ported vac activates and starts to pull more and more timing in the motor as velocity increases. So if you set you total timing to 34* at 3000RPM that's when the ported starts to do it's job and advances the the timing to the total stroke of the vacuum can arm, usually 12-18* (without Limiters) so now have your total of 34* PLUS the stroke of the Canister arm of say 16* net result=50* of total timing under hard acceleration and your motor WILL "Detonate". If you had a OBD1 computer it would pull that timing back to the 34* as all the sensors feed information to it.... and no detonation.
Now to Constant: At idle/part throttle cruise you have high vacuum, the carb is nearly closed causing a restriction which creates the high vacuum level. Under this light load condition and lean AF ratios the motor needs more timing to burn the fuel (Lean Mixtures take Longer to Burn than rich mixtures) so you need more timing at idle and cruise to burn the fuel correctly and completely. When you stomp the throttle you have NO manifold Vacuum so you have NO vacuum timing and at NO time under high load will it ever advance more than the mechanical "All In" numbers. Stop pointing your finger at the carb for rich idle and top end lean conditions.. It's In Your Distributor Tuning! If your Buddy tells you to hook your Hot Rod distributor to Ported Vacuum, find a new friend because that guys advice is going to blow your crankshaft through the oil pan. Same goes for the guy who say's to disconnect it, they obviously have no idea of how it works or what it does or why we use it.
This not hear-say or an opinion its engineering, based on physics formulas, calculated by people who are a lot smarter than me and I know that so I do what they tell me. They come up with the numbers, we set them up accordingly.
If the Motor Makes 10" of vacuum or less at Idle with 30* of timing in it then it needs a properly calibrated Vacuum Can, we have Mopar and GM vacuum cans that will read 5" of vacuum.”
(From FBO ignitions)
Respectfully, FBO is FOS. The descriptions above are not at all accurate regarding how the distributor vacuum systems operate.
Race Director
Joined: Nov 2006
Posts: 13,251
Likes: 7,840
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
I liked the bit about , find a new friend as your crankshaft is about to go through your oil pan!
Made me laugh.
Made me laugh.
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
Burning Brakes
Joined: Dec 2005
Posts: 1,153
Likes: 341
About Vacuum Advance
What kind of distributor do you have now? Factory Stock? Aftermarket? HEI?
Adjusting ignition timing "on the fly" can give you more power, easier starting, better gas mileage, a host of benefits.
Usually this is done with a distributor that has built in vacuum advance. One vacuum line from the manifold to the distributor.
I went from a vintage mechanical-advance only Mallory distributor (350 crate motor) to a $100 generic HEI with vacuum advance. Got a noticeably smoother idle and
a minimum of 3 MPG's improvement. Night and day!
If I ever have time to follow Lars' prescription for timing adjustment I could probably get even more power / MPG's.
Adjusting ignition timing "on the fly" can give you more power, easier starting, better gas mileage, a host of benefits.
Usually this is done with a distributor that has built in vacuum advance. One vacuum line from the manifold to the distributor.
I went from a vintage mechanical-advance only Mallory distributor (350 crate motor) to a $100 generic HEI with vacuum advance. Got a noticeably smoother idle and
a minimum of 3 MPG's improvement. Night and day!
If I ever have time to follow Lars' prescription for timing adjustment I could probably get even more power / MPG's.
Team Owner
Joined: Jan 2006
Posts: 39,390
Likes: 171
From: Into the Mystic And yet, despite the look on my face, you're still talking TN
St. Jude Donor '09 thru '25
What kind of distributor do you have now? Factory Stock? Aftermarket? HEI?
Adjusting ignition timing "on the fly" can give you more power, easier starting, better gas mileage, a host of benefits.
Usually this is done with a distributor that has built in vacuum advance. One vacuum line from the manifold to the distributor.
I went from a vintage mechanical-advance only Mallory distributor (350 crate motor) to a $100 generic HEI with vacuum advance. Got a noticeably smoother idle and
a minimum of 3 MPG's improvement. Night and day!
If I ever have time to follow Lars' prescription for timing adjustment I could probably get even more power / MPG's.
Adjusting ignition timing "on the fly" can give you more power, easier starting, better gas mileage, a host of benefits.
Usually this is done with a distributor that has built in vacuum advance. One vacuum line from the manifold to the distributor.
I went from a vintage mechanical-advance only Mallory distributor (350 crate motor) to a $100 generic HEI with vacuum advance. Got a noticeably smoother idle and
a minimum of 3 MPG's improvement. Night and day!
If I ever have time to follow Lars' prescription for timing adjustment I could probably get even more power / MPG's.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
“How They Work”
“Ported vacuum: With that thought in mind consider if the distributor hooked to Ported Vac, as air speed increases the ported vac activates and starts to pull more and more timing in the motor as velocity increases. So if you set you total timing to 34* at 3000RPM that's when the ported starts to do it's job and advances the the timing to the total stroke of the vacuum can arm, usually 12-18* (without Limiters) so now have your total of 34* PLUS the stroke of the Canister arm of say 16* net result=50* of total timing under hard acceleration and your motor WILL "Detonate". If you had a OBD1 computer it would pull that timing back to the 34* as all the sensors feed information to it.... and no detonation.
Now to Constant: At idle/part throttle cruise you have high vacuum, the carb is nearly closed causing a restriction which creates the high vacuum level. Under this light load condition and lean AF ratios the motor needs more timing to burn the fuel (Lean Mixtures take Longer to Burn than rich mixtures) so you need more timing at idle and cruise to burn the fuel correctly and completely. When you stomp the throttle you have NO manifold Vacuum so you have NO vacuum timing and at NO time under high load will it ever advance more than the mechanical "All In" numbers. Stop pointing your finger at the carb for rich idle and top end lean conditions.. It's In Your Distributor Tuning! If your Buddy tells you to hook your Hot Rod distributor to Ported Vacuum, find a new friend because that guys advice is going to blow your crankshaft through the oil pan. Same goes for the guy who say's to disconnect it, they obviously have no idea of how it works or what it does or why we use it.
This not hear-say or an opinion its engineering, based on physics formulas, calculated by people who are a lot smarter than me and I know that so I do what they tell me. They come up with the numbers, we set them up accordingly.
If the Motor Makes 10" of vacuum or less at Idle with 30* of timing in it then it needs a properly calibrated Vacuum Can, we have Mopar and GM vacuum cans that will read 5" of vacuum.”
(From FBO ignitions)
“Ported vacuum: With that thought in mind consider if the distributor hooked to Ported Vac, as air speed increases the ported vac activates and starts to pull more and more timing in the motor as velocity increases. So if you set you total timing to 34* at 3000RPM that's when the ported starts to do it's job and advances the the timing to the total stroke of the vacuum can arm, usually 12-18* (without Limiters) so now have your total of 34* PLUS the stroke of the Canister arm of say 16* net result=50* of total timing under hard acceleration and your motor WILL "Detonate". If you had a OBD1 computer it would pull that timing back to the 34* as all the sensors feed information to it.... and no detonation.
Now to Constant: At idle/part throttle cruise you have high vacuum, the carb is nearly closed causing a restriction which creates the high vacuum level. Under this light load condition and lean AF ratios the motor needs more timing to burn the fuel (Lean Mixtures take Longer to Burn than rich mixtures) so you need more timing at idle and cruise to burn the fuel correctly and completely. When you stomp the throttle you have NO manifold Vacuum so you have NO vacuum timing and at NO time under high load will it ever advance more than the mechanical "All In" numbers. Stop pointing your finger at the carb for rich idle and top end lean conditions.. It's In Your Distributor Tuning! If your Buddy tells you to hook your Hot Rod distributor to Ported Vacuum, find a new friend because that guys advice is going to blow your crankshaft through the oil pan. Same goes for the guy who say's to disconnect it, they obviously have no idea of how it works or what it does or why we use it.
This not hear-say or an opinion its engineering, based on physics formulas, calculated by people who are a lot smarter than me and I know that so I do what they tell me. They come up with the numbers, we set them up accordingly.
If the Motor Makes 10" of vacuum or less at Idle with 30* of timing in it then it needs a properly calibrated Vacuum Can, we have Mopar and GM vacuum cans that will read 5" of vacuum.”
(From FBO ignitions)
Instructor
Joined: Dec 2022
Posts: 157
Likes: 97
From: Virginia
Just keeping things simple for the new mechanics here.
A lot of hobbyists come to the forum asking the same questions regarding timing in one form or another. The answer is definitely not coming across. Here’s an elementary explanation of what they want to know without nuance.
Corvette owners ultimately want to just drive their cars, not get lectured.
Last edited by Chagjr; Oct 26, 2024 at 07:28 PM.
Race Director
Joined: Nov 2006
Posts: 13,251
Likes: 7,840
From: Sunshine Coast, Queensland, Australia
2025 C3 of the Year Finalist - Modified
2024 C3 of the Year Finalist- Modified
2022 C3 of the Year Finalist - Modified
Cruise-In VIII Veteran
Of course they don't want to be lectured. But the OP did ask a question. And perhaps a basic answer is in order. But how do you really do that?
Lars has taken the time to write up a paper that describes how the system is meant to work, why it works, how it works and how best to tune it to your engine. And is willing to send that paper to pretty much anyone who asks. But to sit and retype it all everytime someone asks. Well, that's a big ask.
So, the short answer.
Basically, engine ignition timing is a process that is much like shooting ducks.
In theory, we want our spark to occur just as our fuel mixture is at it's peak compression. Right on Top Dead Center or TDC as it were.
BUT.
The fuel doesn't really burn instantaneously. It takes time for the burn or explosion to actually happen. How much time?? AHHH, thus the mystery.
So, we actually time the spark so the burn is perfectly complete precisely as the piston hits TDC. So, the faster the engine is spinning, the more we need to lead that timing out.
Back to shooting ducks.
Consider your shooting at a duck high in the sky. You don't aim at the duck. You'll miss him as your shot will fly behind him. It takes time for your shot to reach the duck just as it takes time for the explosion to happen in your cylinder. So, you aim in front of the duck to "Time" the arrival of the shot to meet the duck.
The higher the duck is in the sky, the more you aim in front. The faster the duck is flying. The more you aim in front. And then, just to keep you on your toes. There is windage!
OK, so, we have centrifugal advance to advance the timing as the engine spins faster. So the burn is completed right on TDC.
So why vacuum advance? During periods of very light load, light throttle operation,, like cruising down the highway in top gear. Or idling the engine will accept much more timing advance without causing spark knock or pre ignition. This is the windage!
The engine is pulling high vacuum during these periods. Thus, adding timing using a vacuum method makes perfect sense. Adding timing while idling, (very low engine load) or cruising down the highway, (ever notice your throttle is barely moved on the highway) anyway, adding timing at these very low engine load times will help the engine run cooler, and more efficiently. And as soon as you step on the throttle, the butterfly's in your carb open up, the vacuum drops right off, vacuum advance let's go and your back strictly on the mechanical advance.
And that is the short answer.
Lars has taken the time to write up a paper that describes how the system is meant to work, why it works, how it works and how best to tune it to your engine. And is willing to send that paper to pretty much anyone who asks. But to sit and retype it all everytime someone asks. Well, that's a big ask.
So, the short answer.
Basically, engine ignition timing is a process that is much like shooting ducks.
In theory, we want our spark to occur just as our fuel mixture is at it's peak compression. Right on Top Dead Center or TDC as it were.
BUT.
The fuel doesn't really burn instantaneously. It takes time for the burn or explosion to actually happen. How much time?? AHHH, thus the mystery.
So, we actually time the spark so the burn is perfectly complete precisely as the piston hits TDC. So, the faster the engine is spinning, the more we need to lead that timing out.
Back to shooting ducks.
Consider your shooting at a duck high in the sky. You don't aim at the duck. You'll miss him as your shot will fly behind him. It takes time for your shot to reach the duck just as it takes time for the explosion to happen in your cylinder. So, you aim in front of the duck to "Time" the arrival of the shot to meet the duck.
The higher the duck is in the sky, the more you aim in front. The faster the duck is flying. The more you aim in front. And then, just to keep you on your toes. There is windage!
OK, so, we have centrifugal advance to advance the timing as the engine spins faster. So the burn is completed right on TDC.
So why vacuum advance? During periods of very light load, light throttle operation,, like cruising down the highway in top gear. Or idling the engine will accept much more timing advance without causing spark knock or pre ignition. This is the windage!
The engine is pulling high vacuum during these periods. Thus, adding timing using a vacuum method makes perfect sense. Adding timing while idling, (very low engine load) or cruising down the highway, (ever notice your throttle is barely moved on the highway) anyway, adding timing at these very low engine load times will help the engine run cooler, and more efficiently. And as soon as you step on the throttle, the butterfly's in your carb open up, the vacuum drops right off, vacuum advance let's go and your back strictly on the mechanical advance.
And that is the short answer.
Thread Starter
Team Owner
Joined: Nov 2004
Posts: 20,715
Likes: 25
From: Sorrento Italy
Just e-mail me a request for my Vacuum Advance tech paper - it covers the vacuum advance system operation in detail and provides all info for setting it up correctly.
Lars
V8FastCars@msn.com
Lars
V8FastCars@msn.com
Instructor
Joined: Dec 2022
Posts: 157
Likes: 97
From: Virginia
Of course they don't want to be lectured. But the OP did ask a question. And perhaps a basic answer is in order. But how do you really do that?
Lars has taken the time to write up a paper that describes how the system is meant to work, why it works, how it works and how best to tune it to your engine. And is willing to send that paper to pretty much anyone who asks. But to sit and retype it all everytime someone asks. Well, that's a big ask.
(C’mon really, how can anyone take you guys serious)
So, the short answer.
Basically, engine ignition timing is a process that is much like shooting ducks.
In theory, we want our spark to occur just as our fuel mixture is at it's peak compression. Right on Top Dead Center or TDC as it were.
BUT.
The fuel doesn't really burn instantaneously. It takes time for the burn or explosion to actually happen. How much time?? AHHH, thus the mystery.
So, we actually time the spark so the burn is perfectly complete precisely as the piston hits TDC. So, the faster the engine is spinning, the more we need to lead that timing out.
Back to shooting ducks.
Consider your shooting at a duck high in the sky. You don't aim at the duck. You'll miss him as your shot will fly behind him. It takes time for your shot to reach the duck just as it takes time for the explosion to happen in your cylinder. So, you aim in front of the duck to "Time" the arrival of the shot to meet the duck.
The higher the duck is in the sky, the more you aim in front. The faster the duck is flying. The more you aim in front. And then, just to keep you on your toes. There is windage!
OK, so, we have centrifugal advance to advance the timing as the engine spins faster. So the burn is completed right on TDC.
So why vacuum advance? During periods of very light load, light throttle operation,, like cruising down the highway in top gear. Or idling the engine will accept much more timing advance without causing spark knock or pre ignition. This is the windage!
The engine is pulling high vacuum during these periods. Thus, adding timing using a vacuum method makes perfect sense. Adding timing while idling, (very low engine load) or cruising down the highway, (ever notice your throttle is barely moved on the highway) anyway, adding timing at these very low engine load times will help the engine run cooler, and more efficiently. And as soon as you step on the throttle, the butterfly's in your carb open up, the vacuum drops right off, vacuum advance let's go and your back strictly on the mechanical advance.
(Another Lecture)
And that is the short answer.
Lars has taken the time to write up a paper that describes how the system is meant to work, why it works, how it works and how best to tune it to your engine. And is willing to send that paper to pretty much anyone who asks. But to sit and retype it all everytime someone asks. Well, that's a big ask.
(C’mon really, how can anyone take you guys serious)
So, the short answer.
Basically, engine ignition timing is a process that is much like shooting ducks.
In theory, we want our spark to occur just as our fuel mixture is at it's peak compression. Right on Top Dead Center or TDC as it were.
BUT.
The fuel doesn't really burn instantaneously. It takes time for the burn or explosion to actually happen. How much time?? AHHH, thus the mystery.
So, we actually time the spark so the burn is perfectly complete precisely as the piston hits TDC. So, the faster the engine is spinning, the more we need to lead that timing out.
Back to shooting ducks.
Consider your shooting at a duck high in the sky. You don't aim at the duck. You'll miss him as your shot will fly behind him. It takes time for your shot to reach the duck just as it takes time for the explosion to happen in your cylinder. So, you aim in front of the duck to "Time" the arrival of the shot to meet the duck.
The higher the duck is in the sky, the more you aim in front. The faster the duck is flying. The more you aim in front. And then, just to keep you on your toes. There is windage!
OK, so, we have centrifugal advance to advance the timing as the engine spins faster. So the burn is completed right on TDC.
So why vacuum advance? During periods of very light load, light throttle operation,, like cruising down the highway in top gear. Or idling the engine will accept much more timing advance without causing spark knock or pre ignition. This is the windage!
The engine is pulling high vacuum during these periods. Thus, adding timing using a vacuum method makes perfect sense. Adding timing while idling, (very low engine load) or cruising down the highway, (ever notice your throttle is barely moved on the highway) anyway, adding timing at these very low engine load times will help the engine run cooler, and more efficiently. And as soon as you step on the throttle, the butterfly's in your carb open up, the vacuum drops right off, vacuum advance let's go and your back strictly on the mechanical advance.
(Another Lecture)
And that is the short answer.
I get the one stop shop Lars provides for this forum, Great!
Model A Fords had a Timing advance/retard lever right at the steering wheel. This stuff is not rocket science.
The engines that are in the Corvettes are Chevrolet engines installed in passenger cars and trucks.
The information regarding ignition timing is available on any muscle car, hot rod enthusiasts website. And it is the same for all V8 engines, Chrysler, Ford, GM.
Heres a great link from Hot Rod regarding ignition timing.
https://www.motortrend.com/how-to/ig...-hp-heres-get/
Tech Contributor
Joined: Jun 2004
Posts: 20,910
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Just keeping things simple for the new mechanics here.
A lot of hobbyists come to the forum asking the same questions regarding timing in one form or another. The answer is definitely not coming across. Here’s an elementary explanation of what they want to know without nuance.
Corvette owners ultimately want to just drive their cars, not get lectured.
A lot of hobbyists come to the forum asking the same questions regarding timing in one form or another. The answer is definitely not coming across. Here’s an elementary explanation of what they want to know without nuance.
Corvette owners ultimately want to just drive their cars, not get lectured.