440roadrunner
Well-Known Member
You can start one HELL of an argument with that statement.
Drilling throttle plates?
- Thread starter 71_Duster
- Start date
You can start one HELL of an argument with that statement.
RustyRatRod
Well-Known Member
Probably so, but those arguing against manifold vacuum would still be just as WRONG.
- Local time
- 10:51 AM
- Joined
- Nov 21, 2008
- Messages
- 20,230
- Reaction score
- 50,837
- Location
- In GTXtacy Illinois
oh boy...lol
RustyRatRod
Well-Known Member
I always liked this article. Some will say "it's a chebbie thing", but the vacuum advance doesn't give a damn what kinda engine it's on. This guy was dead nuts on the money. I love it when so called "forum gurus" come out and argue with it, but I've tested it in the real world, and he's right 100%.
Written by a former GM engineer.
As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.
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.
Written by a former GM engineer.
As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.
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.
Garys1969RR
Well-Known Member
You need someone that has experience tuning those carbs, or just keep trying different adjustments till you figure it out. I still think a tighter wind up on the secondary air door spring will help alot. And its quite easy to adjust. Just remove the air filter, and its right in front of you. Seems like there is a lock screw, and then the larger slotted screw is what adjusts the spring wind up. Try backing it off all the way until the air valve just opens up. Then wind up the spring 3 1/2 turns, and tighten the lock screw. That should be a good starting point.
71_Duster
Well-Known Member
Heres the current state of things.
Timing as rechecked and set with a friends digital advance light is 18 initial.
I had a look see at the springs on the rods while idleing to see if they where staying down or danceing. They hold nice and steady.
I didn't read the post on manifold vs vacuum until now so will swap that over tomorrow.
Pulled the dist cap off and it was all oil spattered inside, someone decided to over oil the little felt, cleaned that up dried it out squeezed the felt so it was moist not soaked and fired it up.
Idle held much steadier and smoother. Idle mixture screws still pretty useless some effect but not a lot just about shutting them off got the most noticeable rpm gains.
Air door had also been monkey'd with so I set it back to factory first which states 2 1/2 turns.
Go for a cruise getting snappier off the line but still not happy about taking WOT off the line or at cruise. Feel like you hit a brick wall you slow so bad and popping can be heard to me sounds like through the intake. Possibly loading up on fuel?
Going to pick up a tuning kit tomorrow, engine shop I deal with still thinks it's too rich. I went from the as bought set up of 2 stages rich back to base stock, it cleaned the sooty tail pipes up a bit but not completely and the car stinks and waters the eyes at idle a bit.
So a jet and rod change 2 stakes leaner then stock and I'll see what that gets me and then I'll tighten up the air door a 1/4 at a time?
Timing as rechecked and set with a friends digital advance light is 18 initial.
I had a look see at the springs on the rods while idleing to see if they where staying down or danceing. They hold nice and steady.
I didn't read the post on manifold vs vacuum until now so will swap that over tomorrow.
Pulled the dist cap off and it was all oil spattered inside, someone decided to over oil the little felt, cleaned that up dried it out squeezed the felt so it was moist not soaked and fired it up.
Idle held much steadier and smoother. Idle mixture screws still pretty useless some effect but not a lot just about shutting them off got the most noticeable rpm gains.
Air door had also been monkey'd with so I set it back to factory first which states 2 1/2 turns.
Go for a cruise getting snappier off the line but still not happy about taking WOT off the line or at cruise. Feel like you hit a brick wall you slow so bad and popping can be heard to me sounds like through the intake. Possibly loading up on fuel?
Going to pick up a tuning kit tomorrow, engine shop I deal with still thinks it's too rich. I went from the as bought set up of 2 stages rich back to base stock, it cleaned the sooty tail pipes up a bit but not completely and the car stinks and waters the eyes at idle a bit.
So a jet and rod change 2 stakes leaner then stock and I'll see what that gets me and then I'll tighten up the air door a 1/4 at a time?
Meep-Meep
дворянин
One thing to understand about carbs is they like air velocity, which is the only thing that's responsible for fuel movement. It's the low pressure at the venturi that allows the atmosphere pressure in the fuel bowl to push the fuel out. Since air is much easier to move than fuel, it's counter intuitive that a big carb can actually go lean, but it does. I'm also of the opinion that if one has to make huge modifications to a carb just so it will run you have the wrong carb or something else is wrong. High overlap cams will mess with low speed drivability too. Put on a different and/or smaller carb or consider changing the cam.
Adding holes in the throttle blades will allow you to back of the blade to uncover less of the transfer slot. The hole should be added opposite the idle discharge passage.
Adding holes in the throttle blades will allow you to back of the blade to uncover less of the transfer slot. The hole should be added opposite the idle discharge passage.
Garys1969RR
Well-Known Member
Hi Duster, well keep trying, eventually you will get it sorted out. If nothing else, try a carb change. You said you have a Holley 3310 there, that one may work better, maybe worth a try. Thats a vacumn actuated secondary, which really helps to eliminate bog. And I have a 700 dbl pumper on my 451 which will bog when cold but its fine when a little warmed up. But even then it will still bog if you floor it from a stop. And if your interested in a carb trade, I have a 750 CFM Eddy AFB, and that one never bogs, even off the line. I would really like to try a 3310 Holley 750 vac secondary On my 69 RR , 451. And on that I use intake manifold vacumn for the vac advance, seems to work better for me. I'm taking mine to the track on Wed the 22nd to see how she will E T in the 1/4 mile. May have to swap out the dbl pumper for the Eddy 750 if I cant get it to leave the line with out bogging. And when it bogs that is a momentary lean condition while the fuel is trying to catch up with the air. Ok good luck
71_Duster
Well-Known Member
Update:
Swapped the vac advance back to manifold, now jetted 2% leaner then box stock which is position 22 on the chart.
I have yet to do a plug check but at stock position they where black as hell. Car starts and idles better hot and cold and the idle mixture screws are starting to get more responsive.
Still stumbles and pops badly when 3/4- full throttle happens quickly. I gave the air door a half turn tighter a little better it tries to pull through it now but still popping and stumbleing.
Tried putting the accelerator pump back in the middle position, thats a no go a rev with the aircleaner off got me a nice backfire through the carb, figure the intake must be loading up badly with fuel as the timing has no issues anyother time. Back to the bottom hole and no problem.
Testing springs on the rods as per Diamondback engines tech papers, rods where fluttering a bit when in gear so went from pink to orange. Flutter is gone so I tried to check with the in gear throttle snaps but after the throttle is snapped it stalls.
Went to go for a drive and after a nice crashign banging sound one of the caliper bolts on my SSBC front brakes decided to sheer itself off, glad I was close to home. Got the old bolt out and the center of it looks like cheap porous junk.
So no more test and tune untill I get some quality grade 8 hardware.
Swapped the vac advance back to manifold, now jetted 2% leaner then box stock which is position 22 on the chart.
I have yet to do a plug check but at stock position they where black as hell. Car starts and idles better hot and cold and the idle mixture screws are starting to get more responsive.
Still stumbles and pops badly when 3/4- full throttle happens quickly. I gave the air door a half turn tighter a little better it tries to pull through it now but still popping and stumbleing.
Tried putting the accelerator pump back in the middle position, thats a no go a rev with the aircleaner off got me a nice backfire through the carb, figure the intake must be loading up badly with fuel as the timing has no issues anyother time. Back to the bottom hole and no problem.
Testing springs on the rods as per Diamondback engines tech papers, rods where fluttering a bit when in gear so went from pink to orange. Flutter is gone so I tried to check with the in gear throttle snaps but after the throttle is snapped it stalls.
Went to go for a drive and after a nice crashign banging sound one of the caliper bolts on my SSBC front brakes decided to sheer itself off, glad I was close to home. Got the old bolt out and the center of it looks like cheap porous junk.
So no more test and tune untill I get some quality grade 8 hardware.
Similar threads
