Ignition Advance

[wacker]

Bumping up a an old thread here.

Whats the status on your engines/timing nowdays? Im pretty sure that the ideas has changed during the build of the engines.
Alleycats ide of blocking the centrifugal seems to be a good idé, has anyone played around with the springs and weights?
Im planning to rebuild my distributor this winter and try to figure out the best setup. Will also do some work on the head in the same time.

What else have you done that might change the timing (mill etc).

[alleycat]

This is still a perplexing issue. It is the gas we have to use. It's different in the summer from the winter, north to south and around the world. The combustion chamber is really the determining factor. Big open, as cast, chambers are the worst, small machined, the best. And gas will take more compression than ya might think, in fact, what I've found is, this stuff must be pushed right to the edge of detonation/pre-ignition to work best, if not , the performance really falls off fast. So, what I do is polish the piston top, valve faces, chamber to the highest polish I can get, like chrome, if possible. Run the hottest ignition, like a MSD 6A or a hei of some sort. I've given up on the stock distributor. I got off ebay a Spalding flamethrower distributor, never used, gotta be the only 1 in the world. I'll probably change out the points for a pertronix module and we'll see how that works, sure looks cool! I use every trick I can to jack up the compression, mill the deck in order to get the piston about .008 out of the deck, .012 shim head gasket. Mill the head, perch the valves as high on the seat as possiable. But still...the str8 has just about the worst chamber ever, so a lot of initial advance is unavoidable. I don't have a degreed flywheel, takes a gap bed lathe to make one, I don't have one. I think my str8's like about 20-22 initial, I don't know what the mechanical advance is going to work out to, a vac-advance is a must as under cruse/light load leanish air/fuel and 8 to 1 comp, so the vac has to pull in the last of the timing but drop it out fast if stomp it. Total may work out to be about 38-40, I would sure like less, but the engine does'ent care what I like! alleycat

[bruno]

I have a 1950 263, basically stock except for a log intake with 3 Stromberg 97s. Exhaust will be 8 straight pipes. The engine has not run yet with this setup. When you say lock out the centrifugal does that mean removing the springs and using electrical cables to hold it open? Then run the vacuum advance through the top of the log manifold and start advancing the dist. I am not real sharp when it comes to timing . Will this be ok on a light weight street driven fenderless pickup running a Saginaw 4spd? Thanks.

[telekenfun]

Bumping up a an old thread here.

Whats the status on your engines/timing nowdays? Im pretty sure that the ideas has changed during the build of the engines.
Alleycats ide of blocking the centrifugal seems to be a good idé, has anyone played around with the springs and weights?
Im planning to rebuild my distributor this winter and try to figure out the best setup. Will also do some work on the head in the same time.

What else have you done that might change the timing (mill etc).

Well, Wacker, to get an idea of my engine look up my entries in "Anything new------" around page 50 I think. After I built up a new 263 for our 49 sedanette, it was a dog at first. It was certainly better than the tired 248 but way below its potential. The first thing that was obvious was it ran too rich, it stank like unburned gas. That was partly because the timing ran so late in the combustion cycle. Advancing helped the power and stink a bit but it sounded what I call burbling (rich). I was up to about 36 degrees according to advance timing light. The next thing I did was installed a sideband Bosch O2 meter and recurved the enrichment circuits of the carbs to get it richer @ low to zero vacuum conditions (wide open throttle) and downsized the main jets and emulsion tubes to lean out the high & intermediate vacuum conditions for idle & cruise settings of the throttle. This eliminated the stink and the exhaust note began to take on some verve & crackle. The air fuel ratios varied from the low 15s:1 @ idle and deceleration to the low 12s:1 @ WOT, close to ideal.<br>However, it wasn't very tractable. I sometimes needed to feather the throttle to get past flat spots at certain RPM, advancing the timing would benefit low range but pinging and preignition would occur at more open throttle positions under acceleration. Retarding would eliminate the preignition and also the power.<br>A little understanding of fuel stochiometrics and flame front speeds and how they vary in rich and lean mixtures will give us insight into what timing curve requirements are necessary with different engine operating conditions.The flame front burns slowly in lean mixtures and faster in richer mixtures. The more closed the throttle the lower the fuel charge entering the cylinders (low dynamic compression and lean mixture requiring extra timing advance). The more open the throttle the higher the fuel charge enters the cylinders (higher dynamic compression, requiring less timing advance). Adding complication to the timing equation of a modern fuel efficient and powerful engine is the fact that the flame front always travels at the same speed in a given stochiometric charge. However, the amount of time for a combustion to occur gets shorter with increasing RPM. This requires increasing timing advance if it is to occur at the optimum moment in the engine cycle for best effect.<br>These are the conditions that must be met by the distributor.<br>I was able to borrow for a few weeks an old King distributor machine. With the use of the machine I was able to see the effect on advance that the vacuum advance mechanism had. I could see the effect the centrifical advance had. The vacuum only allow 9-10 degrees and started @2 ins Hg&nbsp;vacuum, but maxed out @about 10 insHg. The implication of this is; unless the carbs are @ near WOT the vacuum advance is alway in play. The solution I used was to remove material that limited the number of degrees the vacuum canister could move the vacuum plate. This got me nearly 20 degrees and changes in advance between 10 ins Hg & 20 ins Hg which is what my engine idles at. This strategy allowed me to reduce base timing and still have enough advance @ idle and lean cruise. I still had preignition symptoms as the centrifical advance in the stock distributor started advancing @ around 800 RPM even with the stiffest spring available in an MSD distributor kit. Also the stock distributor only allows 10 degrees of centrifical advance. My solution was to first eliminate the bob weight stop to allow for about 23 degrees advance. Next, I needed to delay to higher rpm the onset of centrifical advance to eliminate midband preignition. Old Chevy hot rodders would grind the bob weights to lighten and there by reduce centrifical force exerted on the advance cam. I thought this may lead to a more unpredictable action versus the stock weights. I instead opted to modify the advance cam profile so the bob weights moved out but didn't start effecting advance until around 2200. The ability to make changes and see their effect on the timing curve is essential and only possible through the use of a distributor machine. Otherwise you must rely on blind intuition. The target curve I was shooting for was to set optimum base timing in normal operating RPM range of 2200 with the vacuum @ about 8-10 ins Hg. With a static advance of 10 degrees and add 23 with the centrifical all in the total advance becomes 33degrees which is fairly close to most modern motors.<br>The results have been quite rewarding, with very good power and economy. There is no stumbling or hesitation anymore and the gassy smell is gone. I don't think it is quite there yet though as I feel it could pull a little harder with some more advance in the 3500-4000 range. I think that can be gotten by adding to the flanks of the bob weight cams.<br>So that's some of the stuff you need to go through when modifying a vintage motor.<br>Good Luck with all your endeavors, Best Regards, Telekenfun

[wacker]

Thanks Telefunken for some good advices.


Right now my distributor is stock and set up with a temporary high initial(dont remember what it is). This gives a lot better response at low revs but got pre-ignitions at higher revs as expected.
I have bought a couple of distributors on ebay that I will send it to someone in the US with a distributor machine. Someone with a distributor machine in Sweden seems to be impossible to find.
Now im just looking into what guidelines to give to the distributor shop.

[Jenz38]

....telekenfun thats very Interesting ,especially your expierience with the Dist Maschine .I have also a Problem in the midrange ( Cruisingspeed), after read this I can imagine the reason isn't ONLY a Carb problem . I think in the future I have to pay more attention to my Distributor ...

[39CENT]

I have been very interested about the straight 8 ignition for quite awhile. I think we are getting closer to the problem but still needing to continue investigating. I read something that was kinda interesting if I can find it I will post it. I'm not an expert in ignitions but am learning. I have been trying to find out about using manifold or ported vacuum advance, and how that works. Many say use manifold and many say ported, however most Buicks used manifold vac. About all the cars went to ported advance when the smog rules arrived. Now Jyrki and others say manf vac doesnt work, but our straight 8,s are using ported so who am I to say which is right or wrong. I do think its got a lot to do with todays fuel and that we need to investigate all we can about that. Most agree that the intake, and combustion chambers are not very good, but as the recent Bonneville runs lately over 160 and 200 mph using better ports Injection and supercharging they have to make over 300 to 400hp or more. So i think we should try things like filling the combustion chambers like they did back the old days, porting,fuel injection tuning and some more ignion tricks etc. One thing tho Bonneville shows that the old 8,s can 'take a licking and keep on ticking'.

[telekenfun]

I have been very interested about the straight 8 ignition for quite awhile. I think we are getting closer to the problem but still needing to continue investigating. I read something that was kinda interesting if I can find it I will post it. I'm not an expert in ignitions but am learning. I have been trying to find out about using manifold or ported vacuum advance, and how that works. Many say use manifold and many say ported, however most Buicks used manifold vac. About all the cars went to ported advance when the smog rules arrived. Now Jyrki and others say manf vac doesnt work, but our straight 8,s are using ported so who am I to say which is right or wrong. I do think its got a lot to do with todays fuel and that we need to investigate all we can about that. Most agree that the intake, and combustion chambers are not very good, but as the recent Bonneville runs lately over 160 and 200 mph using better ports Injection and supercharging they have to make over 300 to 400hp or more. So i think we should try things like filling the combustion chambers like they did back the old days, porting,fuel injection tuning and some more ignion tricks etc. One thing tho Bonneville shows that the old 8,s can 'take a licking and keep on ticking'.

39CENTS,I'm just a neophyte about these timing issues, but by reading and applying the principles of combustion and what causes combustion to vary has helped me develop strategies to approach optimum timing in my modernized old motors.
Regarding ported versus manifold vacuum, I recently deduced that the reason manufactures used ported vacuum in the old days was in part because of poor starting systems. Reflecting to the hand crank motors those vehicles had spark advance /retard controls right at the steering wheel. To start those motors the operator needed to retard the timing and in order to get enough to move the vehicle the timing was then advanced. Going forward in time to the electric start era, vacuum advance systems were developed to automatically advance the timing once the engine was running. To generate that vacuum a port was created in the carburetor just above the throttle plate so at closed throttle there was no vacuum sent to the distributor advance mechanism until the throttle was cracked open. I believe this was to benefit easy starting with the sorry 6 volt starters and still apply enough advance to power the vehicle to move. Today with 12 volt systems and modern starters engines start much better while being advanced. However if we continue to use ported vacuum our engines can have too much advance as the throttle is opened for acceleration from a stop. This results is pinging and preignition caused by higher combustion pressures of greater air/fuel charge that doesn't need the advance firing of the lean idle charge. The solution is to switch to manifold vacuum so the timing advance diminishes as the throttle is opened. This allows the use of a lean mixture for economy at idle and cruise with enough advance timing to fire its slow moving flame front and still lose the advance when an open throttle's greater air/fuel charge and resultant fast flame front to occur without preignition.

[Bob]

I don't believe vacuum advance has anything to do with starting. Electric starters were around a long time before vacuum advance was added. I think you will find that vacuum advance has only to do with improving fuel economy, and nothing to do with performance as once you put your petal to the metal you have no vacuum advance. Before the engine is running, there is no vacuum advance, no matter where you plum it in. Once above an idle both manifold and ported vacuum advances work the same. I suspect the reason for running manifold vacuum may be an idling engine is under low load and can use up a lot more timing without problems for the purpose of emissions and fuel economy.

[telekenfun]

I don't believe vacuum advance has anything to do with starting. Electric starters were around a long time before vacuum advance was added. I think you will find that vacuum advance has only to do with improving fuel economy, and nothing to do with performance as once you put your petal to the metal you have no vacuum advance. Before the engine is running, there is no vacuum advance, no matter where you plum it in. Once above an idle both manifold and ported vacuum advances work the same. I suspect the reason for running manifold vacuum may be an idling engine is under low load and can use up a lot more timing without problems for the purpose of emissions and fuel economy.

Bob I think we are in agreement at least conceptually. However in the dynamic realm of our imaginary engine(air pump) there are no absolutes. if the engines is in motion and everything mechanical is working properly there will be a vacuum created, be the throttle completely closed or fully open. Zero vacuum in a running engine could be approached in a well designed full race type with 100% volumetric efficiency. In such a perfect engine there would be no difference in manifold pressure (vacuum) and atmospheric pressure. Under this condition a carburetor can't function as it requires a pressure differential to work, thus a restriction (venturi) is placed in the carburetor to create a pressure difference. Therefore it must be understood that if the engine is turning over even if it isn't running it is creating vacuum, how else could it draw fuel to start. A stock Buick S-8 distributor specifies the onset of advance occurs @ 2Lbs Hg while generating 10Lbs Hg while cranking.
Regarding manifold and ported vacuum, there is a difference. Manifold vacuum is somewhat linear in its behavior as the strict difference between atmosphere and manifold pressure, while ported vacuum is generated by accelerated air passing by an elongated vertical orfice located just at and to slightly above the closed throttle plate. The vacuum thus created has a more logarithmic nature as it is velocity driven. The result is that for any given part throttle position the ported vacuum signal will always be the greater. Because of this ported vacuum will influence the advance mechanism into a higher RPM and throttle position.
Also in the real world there are no absolutes, nothing is instantianeous, everything has a reaction time. Vacuum doesn't go from zero to twenty instantly nor does RPM change from zero to 5000 instantly. Likewise timing mechanisms and carburetor enrichment circuits have their own response times. Our challenge is to design or modify these mechanisms to optimize the performance of our engines. Understanding the characteristics of each of the elements and their interrelationships is essential toward meeting that goal.
Good Luck with all your endeavors and best regards, KB.