Im an open book. I love learning new information. So feel free to explain your responce
Have a timing ??
- Thread starter bertscova
- Start date
- Status
Some general info and terms :
'Base timing' - also known as initial degrees of advance at idle; 'mechanical advance' - also referred to as centrifugal advance is the amount of advance supplied via the weight & spring system in the distributor; 'all in rpm' is the rpm at which the mechanical advance is maxed out; 'all in timing' is the max timing advance once the all in rpm is achieved; 'vacuum advance' is the amount of degrees of advance supplied by the vacuum canister.
The all in timing will always be the sum of the base + mechanical advance. The base timing must be set at an rpm low enough to ensure that there is no mechanical advance be supplied. Most performance builds/motors will run best with the mechanical advance being supplied by 2800-3200 rpm's. All steps of curving the the distributor, except the last step, are done with the vacuum canister line unhooked from the distributor and plugged.
1st - get the motor to operating temperature and set the base timing and the appropriate rpm. Pick somewhere in the 750-850. I would expect your cam would be pushing the higher end and maybe upto 900. Adjust the carb idle settings as needed to achieve the highest vacuum (via gauge). 12-16 is a good starting point. Whatever you choose is fine, just record it.
2nd - determine the all in rpm. This is done with a timing light hooked up, you'll need either a dial back timing light, timing tape for the balancer or a balancer that is scribed with timing marks. Slowly rev the motor until the timing stops advancing. Adjust this to 2800-3200 as previously mentioned via springs on the weights. The lighter the spring, then the quicker it will advance. Once adjusted double check via revving to 4000-4500 rpm's to ensure that it doesn't advance any further past your all in rpm.
3rd - set the all in timing (this is the fun part). Start at 32 degrees. This is done by turning the distributor to achieve 32 degrees when the engine speed is at or above your all in rpm. Obviously, this might, and most likely will, change your base setting - this isn't a problem. Now go drive the car through some WOT pulls from below 1500 rpms upto 4000ish. If performance is good, no pinging, then reset the all in timing up 1-2 degrees and perform another WOT pull. Continue this until you feel a slight loss of power or can hear pinging (detonation). Once that is detected, then knock the all in timing back 2 degrees and this will be your all in timing.
4th - subtract the base timing from the all in timing and this is your amount of mechanical advance needed. Now you need to adjust the distributor to allow only the amount of mechanical advance needed. Re-adjust your base timing to your previously determined number and recheck your all in number to make sure you made the correct adjustments.
5th - go for a drive and make a couple more WOT pulls to make sure all is operating well. Hopefully your base timing is low enough to allow for smooth starts when hot, implying no kick back of the starter (ask what this is if you don't know.) Now hookup the vacuum advance and drive it in rpm ranges from 1500-4000 and ensure that there is no detonation heard during light throttle cruising and when decelerating. If any is heard, then you need to limit your vacuum advance until it is no longer heard.
6th - determine whether to use ported or full manifold vacuum advance. I have always used full manifold inorder to maintain a smoother idle. If you do use full manifold vacuum, then you'll need to adjust the idle speed again with the goal to maintain the highest possible vacuum reading at your idle speed.
A couple of things about vacuum advance. Using full manifold advance allows the engine start easier as a general rule. Usually a healthy cam'd motor will like more initial timing - as much 18-22 degrees, but this makes it hard starting which is tough on the starter and the block where the starter mounts. A solution if running a 'locked' distributor or ported vacuum is to install an interrupt switch for the distributor power so that you can crank the motor with no voltage to the distributor (no spark) and then turn the switch on while cranking. I didn't mention increasing the vacuum advance during the driving test, but you can adjust it higher. More vacuum advance, without any detonation, will improve mileage substantially when cruising. Another advantage to using full manifold vacuum is the motor will generally idle at lower coolant temp.
All of this can be done quicker on a chassis dyno and when doing this you don't have to listen for detonation as you will see a power loss prior to hearing detonation during the all in adjustments. Also yo can see the HP increases as you get it dialed in. Every degree of all in timing up to the highest point is worth 10-15 HP.
===
You mentioned you have an MSD distributor. These make this process easy because they are designed to be adjusted. But keep in mind that all this can be done with a stock HEI, it just takes a bit longer and requires a bit more craftiness - lol. My guess for your motor will be 34 degrees all in at 2800 with 16 degrees base and 9-11 vacuum advance. Let us know what you end up with and good luck - Jim
'Base timing' - also known as initial degrees of advance at idle; 'mechanical advance' - also referred to as centrifugal advance is the amount of advance supplied via the weight & spring system in the distributor; 'all in rpm' is the rpm at which the mechanical advance is maxed out; 'all in timing' is the max timing advance once the all in rpm is achieved; 'vacuum advance' is the amount of degrees of advance supplied by the vacuum canister.
The all in timing will always be the sum of the base + mechanical advance. The base timing must be set at an rpm low enough to ensure that there is no mechanical advance be supplied. Most performance builds/motors will run best with the mechanical advance being supplied by 2800-3200 rpm's. All steps of curving the the distributor, except the last step, are done with the vacuum canister line unhooked from the distributor and plugged.
1st - get the motor to operating temperature and set the base timing and the appropriate rpm. Pick somewhere in the 750-850. I would expect your cam would be pushing the higher end and maybe upto 900. Adjust the carb idle settings as needed to achieve the highest vacuum (via gauge). 12-16 is a good starting point. Whatever you choose is fine, just record it.
2nd - determine the all in rpm. This is done with a timing light hooked up, you'll need either a dial back timing light, timing tape for the balancer or a balancer that is scribed with timing marks. Slowly rev the motor until the timing stops advancing. Adjust this to 2800-3200 as previously mentioned via springs on the weights. The lighter the spring, then the quicker it will advance. Once adjusted double check via revving to 4000-4500 rpm's to ensure that it doesn't advance any further past your all in rpm.
3rd - set the all in timing (this is the fun part). Start at 32 degrees. This is done by turning the distributor to achieve 32 degrees when the engine speed is at or above your all in rpm. Obviously, this might, and most likely will, change your base setting - this isn't a problem. Now go drive the car through some WOT pulls from below 1500 rpms upto 4000ish. If performance is good, no pinging, then reset the all in timing up 1-2 degrees and perform another WOT pull. Continue this until you feel a slight loss of power or can hear pinging (detonation). Once that is detected, then knock the all in timing back 2 degrees and this will be your all in timing.
4th - subtract the base timing from the all in timing and this is your amount of mechanical advance needed. Now you need to adjust the distributor to allow only the amount of mechanical advance needed. Re-adjust your base timing to your previously determined number and recheck your all in number to make sure you made the correct adjustments.
5th - go for a drive and make a couple more WOT pulls to make sure all is operating well. Hopefully your base timing is low enough to allow for smooth starts when hot, implying no kick back of the starter (ask what this is if you don't know.) Now hookup the vacuum advance and drive it in rpm ranges from 1500-4000 and ensure that there is no detonation heard during light throttle cruising and when decelerating. If any is heard, then you need to limit your vacuum advance until it is no longer heard.
6th - determine whether to use ported or full manifold vacuum advance. I have always used full manifold inorder to maintain a smoother idle. If you do use full manifold vacuum, then you'll need to adjust the idle speed again with the goal to maintain the highest possible vacuum reading at your idle speed.
A couple of things about vacuum advance. Using full manifold advance allows the engine start easier as a general rule. Usually a healthy cam'd motor will like more initial timing - as much 18-22 degrees, but this makes it hard starting which is tough on the starter and the block where the starter mounts. A solution if running a 'locked' distributor or ported vacuum is to install an interrupt switch for the distributor power so that you can crank the motor with no voltage to the distributor (no spark) and then turn the switch on while cranking. I didn't mention increasing the vacuum advance during the driving test, but you can adjust it higher. More vacuum advance, without any detonation, will improve mileage substantially when cruising. Another advantage to using full manifold vacuum is the motor will generally idle at lower coolant temp.
All of this can be done quicker on a chassis dyno and when doing this you don't have to listen for detonation as you will see a power loss prior to hearing detonation during the all in adjustments. Also yo can see the HP increases as you get it dialed in. Every degree of all in timing up to the highest point is worth 10-15 HP.
===
You mentioned you have an MSD distributor. These make this process easy because they are designed to be adjusted. But keep in mind that all this can be done with a stock HEI, it just takes a bit longer and requires a bit more craftiness - lol. My guess for your motor will be 34 degrees all in at 2800 with 16 degrees base and 9-11 vacuum advance. Let us know what you end up with and good luck - Jim
Here is a good read
TIMING AND VACUUM ADVANCE 101
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.
If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
TIMING AND VACUUM ADVANCE 101
The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.
The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.
At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).
When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.
The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.
Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.
If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.
What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.
Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
That's a little bit of a bummer if you intend to do much street driving. Not the end of the world, but the vacuum advance is helpful.
Ok the distributor i currently have is the msd pro billet 85551 w/o vacuum advance. So would you suggest the msd pro billet 8361. Basically the same distributor with vacuum advance i have the 6al digital box. Or should i go with a standard hei msd distributor
Last edited:
I'm not an MSD box fan for street drivers because when they crap out you will be sitting along side the road waiting for a wrecker/rollback. But if your street driving is going to be minimal, then they are fine (always a short tow when they crap out), and they do give a bump in HP. But for a street driver I prefer to be able to buy parts as needed.
Without knowing your intended use I can't make a recommendation. And FWIW, alot of people think I'm silly about my dislike for the 6A boxes, but what do they know - lol.My goal is to drive it home - always.
Also, lilbowtie's article that he attached offers a great description of what's cooking with timing, fuel burn, etc.
Without knowing your intended use I can't make a recommendation. And FWIW, alot of people think I'm silly about my dislike for the 6A boxes, but what do they know - lol.My goal is to drive it home - always.
Also, lilbowtie's article that he attached offers a great description of what's cooking with timing, fuel burn, etc.
Well im an over the road truck driver and this car is my toy to play with a week at a time when im home. So not really a daily. Maybe 75 to 150 miles every other month. Mostly to the track and back. I built the motor, trans and rear end for the 1/4 mile.. ive always used a street fire msd hei with good reliability. But read that the trigger box style ignition is much more accurate and you can get more hp out of them
You're definitely not the first one that I've seen with that opinion of MSD boxes. Although - FWIW: I ran an MSD Digital-6 Plus for around two years, on the street, without a single problem. I liked the built-in rev limiter feature, since traction was a major issue with that car.
Then use the non vacuum advance distributor with the MSD box - it makes sense for your application IMO.
- Status
Similar threads
