Engine and Transmission Tech

Max and I have beaten this about to death, yet a question remains. Provided the same base timing is used in either case, and provided the engine tolerates whatever advance it has at idle without misbehavior, then wouldn't it be safe to say that the engine gets exactly the same amount of timing advance under every operating condition, EXCEPT during idle?

The particular case of concern applies to Edith's V6 TR-7 (Buick) with HEI. As OEM, this engine used thermo-switched ported vacuum. In fact, it used two vacuum lines from the carb to the thermo switch on the manifold. The stated intention of the engineers who designed the system was to improve warm-up, thereby lowering idle emissions. Base timing is 15 and vacuum advance is 20, so at idle the engine sees 15 degrees, and the extra 20 only kicks in at cruise after warm-up. Mechanical is of course unchanged and can be expected to be somewhere around 15-20, I have yet to measure it.

But the point is, switching to manifold vacuum should only affect the idle, correct? The mechanical will be the same, and cruise will be the same. With base time unchanged starting should be no different. So if the engine tolerates more advance at idle all should be good. It seems to tolerate it very well indeed. Idle speed goes up dramatically. The Idle speed screw is backed out to near the end of it's travel. I have yet to check the adjustment of the mixture screws but there is some suspicion that this carb may have the infamous reverse mixture circuits in which case they can be expected to be largely ineffective but we'll see. The test drive showed no hint of ping or bad behavior.

My personal conclusion is that this is the way to configure the advance, but I'd like differing opinions to weigh in, I may be missing something.

Jim

I have always run ported on everything I've owned and havent really had a problem, I tend to find the lightest dashpot so it comes in quicker and have run the adjustable cans as well. I guess I am set in my ways to try it any other way.

You'll get conflicting ideas on this one - personally, I've always run full vacuum......AFAIK, ported vacuum was introduced for idle emission considerations some years ago.
Googled this good write-up from the net
[www.corvetteactioncenter.com]

I concur with Graham. My 215 has always used manifold vacuum.

I'm all for ported vacuum. For several reasons.
The first is idle stability.
You should be running a can that is somewhere near your max idle vacuum. This allows the timing to back off under engine load so that you can run the most advance possible under cruise for maximum fuel mileage and driveability.
Lets say that you have 17in. vacuum @idle and you have a 16in. can. You set everything up nicely to idle @ 800 rpm. And it runs well.
You then drop the car into gear and the manifold referenced can vacuum drops to 13in and takes 4' of timing with it. The idle drops and pulls a few more ins. and timing out of the engine and it stalls.
Or the engine misses and the vacuum drops for a second, the timing falls off and the vacuum drops some more and once again it stalls or goes into a surge cycle that drives you crazy.
Ported vacuum leaves the idle timing rock solid and gives a much more stable idle.
The second is "tip in performance".
Very often an off idle stumble is caused by a timing deficiency. But is thought to be a lean condition that is "adjusted out" by more accelerator pump, power valve adjustments or bigger jets.
This "adjustment" just uses more fuel needlessly. And really doesn't make the engine run better.
What is really happening is on tip in the vacuum once again drops off and the manifold referenced can drops timing causing a stumble.
Ported vacuum shows the can vacuum on tip in which pulls up the timing accordingly and makes the engine much more responsive.
Lastly, manifold referenced vacuum @ idle causes you to use far more timing at idle than is required. This raises the HC and Nox levels by a huge factor. Needlessly polluting the atmosphere. It also slows engine warm up times which pollute even more and shorten engine life. Optimum timing @ cruise is unobtainable, wasting fuel.
The only reason that manifold referenced vacuum ever existed was to allow big inch v8s to crank over on the weak starters that we had back in the day. Remember your 396 bucking the starter when you tried to start it hot? Back off the initial and let the vacuum cover it up and it started fine. Now we have much more powerfull starters and the old crutches are no longer required.
More modern engines did use some manifold vacuum but only in conjunction with a heavy dose of egr. (something to consider as a well setup egr will let you use regular fuel on your high compression engine).
Lots of guys run manifold vacuum but I think that they are leaving a lot on the table.
Cheers
Fred

Fred, that's the most well-stated argument for ported vacuum advance I've ever read.

I agree with Carl, quite impressive. Clearly Fred, you are a thinker. It makes a lot of sense, and I always thought there was more to it than what I was hearing. But I'm not drinking the kool-aid just yet, I still have questions. I doubt ported vacuum was used for timing control before straight manifold vacuum was tried, somewhere along about when they got away from timing controls on the steering column. So I'm pretty sure the first use wasn't for big engines, though that may have given it a boost. And I think the overall effectiveness of either system has a whole lot more to do with how much advance is used and when, than anything else. For instance, I'm not sure the claim that you wouldn't be able to use full advance at cruise is accurate. It doesn't appear to obviously be so and I can't think of why it would be. Undoubtedly there are some cases where that would be true but I don't see why it would be in all cases. The stack up of advance seems to be more the issue, and the quantities chosen for each category.

Different engines act differently of course. The argument for a steady idle is a good one, as is the one for environmental responsibility and I wouldn't want to take anything away from that. But at the same time I have a little trouble with the whole concept of burning more fuel in order to reduce emissions, especially if the car is not fitted with cats. Seems to me that the overall level of pollutants would necessarily have to go up any time a larger throttle opening is needed to maintain the same idle speed. But just as clearly, we're dealing with a range here rather than one or two discreet points, so for instance the optimal advance for idle is anything but a given, and covers a range where the desired characteristics blend and blur, shifting from an inefficient but stable idle speed at one end to an efficient but unstable one at the other for example. And the manufacturers have further confused matters by using all manner of static advance figures. The Buick smog motor for instance is to be set at 15 degrees which is quite a lot for something out of the mid 80's yet it is perfectly happy to idle smoothly and steadily along with no ill manners whatsoever under a combined 35 degrees when connected to straight manifold vacuum. Yet even with the improved efficiency, the (very) obviously less labored running and the reduced transfer of heat to the exhaust system and to the water jackets and radiator, we have nitrous oxides to be concerned about (although I can hardly see how unburned hydrocarbons and hydrocarbon byproducts could be any higher with less fuel being burned).

It's a very complex problem for those of us of the more-or-less shade tree variety to be sorting it out on our own. I'd say that if there is an ideal solution available now, it lies in the electronic control of the ignition system, where we have a good bit more flexibility in setting things up. There, for instance, we can set idle advance to come up to an optimum level as soon as the engine fires, while still keeping it low during cranking and initial light off. Then we can advance it as much as we want and put it wherever we want it based on load and speed, using manifold pressure or not, as we see fit. Too bad we don't all have this option.

So in some cases one approach is going to be best and in some the other, or it appears so to me. In my particular example one aspect is marginal cooling which shifts the decision towards manifold vacuum with it's markedly lower heat output at idle. All else being equal. an engine that does not overheat is much better than one that does.

Jim

Hey Jim,

Just a couple of comments in my defense.
Ported vacuum control has been used for a very long time.
A couple that come to mind right away are some of the flat head v8 and the early VWs. They used ported advance exclusively, no mechanical advance at all.
A number of earlier cars used manifold vacuum in order to facilitate hand cranking.
I agree absolutely, that the whole object is to provide the optimum timing for all operating situations.
How you get there doesn't really matter to the engine. It needs what it needs and that's it.
Electronic controls get us there much more easily and allow for more precise control of the advance curve.
But the good old vacuum can and mechanical advance are still able to be made to work very well.
What I meant about the timing @ cruise was that the timing that the engine will tolerate at idle and the timing at cruise are very different.
Enough vacuum advance to provide optimum running at cruise is going to be far too much at idle. Especially on a low compression small cammed engine.
I would never advocate running a richer mixture than is necessary. Most hobbyist engines run way too rich to cover up other deficiencies.
Power and fuel mileage are lost and pollutants go way up.
As far as that goes an engine with less timing and a larger throttle opening is actually cleaner than one with more timing and less throttle.
Efficiency goes up rapidly as the throttle is opened.
Something that is easily demonstrated on an exhaust gas analyser. CO and HC levels drop due to the increased airflow and the Nox drops due to the reduced combustion temp.
That brings us to over heating. More timing puts more heat into the combustion chamber. Less timing puts the heat further out the exhaust.
Thats why OEMs use retarded timing on cold starts. The heat in the exhaust system lights off the cats much sooner.
Your engine design will determine how heat reactive it is to timing.
Long exhaust ports such as the old flat head V8s are very sensitive. Retarded timing would put heat into the exhaust and it would overwhelm the cooling system.
Short ported engines for the most part are the opposite.
In the end Jim, as you said every situation is unique. And expectations differ.
I drink my flavor kool-aid and you drink yours, they're both good.
Cheers
Fred

Thanks Fred, plenty fair enough. We seem in pretty general agreement other than that bit about timing vs throttle opening. Perhaps it is more a matter of semantics, but to my mind a larger throttle opening always means more fuel, and more fuel at the same speed always means less efficiency. I just can't seem to get my head around the idea of it being opposite to that. Now I'll grant you, some pollutants may go up. I could see that happening. But overall it still seems that more fuel equals more by products no matter what else you do, especially since we rarely put cats on these cars.

It just occurred to me... if timing retard is used to throttle the engine back (which seems to be the case here) then the entire system is changed. Intake vacuum would necessarily drop, requiring a larger throttle opening to deliver the same volume of mixture, however the vacuum signal at the carb would be weaker, tending towards less effective atomization one would think. However, the actual volume of fuel being burnt might be increased only enough to account for the waste heat in the exhaust, which, aiding warm-up as you mentioned could ultimately give an overall efficiency increase, depending on the driving pattern. So maybe I see what you are getting at. An imperfect band-aid but it does make some sense. Obviously EFI and timing control offers a much better option, but I'd be willing to bet that a few of the old Model T drivers used retard to throttle down their engines.

Fondest regards,
Jim

I have tried both ported and manifold vacuum, now I am not using vacuum at all because of distributor clearance issues (vacuum advance distributors are generally taller). I did run a Crane electronic distributor with vacuum advance for a while but the electronic programs did not seem to work well; the vacuum advance did not drop out fast enough on acceleration so I ended up not using the advance program at all. Earlier I had a Mallory with a vacuum advance can that worked very well, but the plug wires rubbed on the inside of the hood.

I found that the idle was less stable on manifold vacuum, but the engine ran much cooler at idle. This is because of less fuel used as demonstrated by being able to turn down the idle speed screw on manifold vacuum. It made a significant difference when I was running a stock MG radiator with my V8 but after switching to an aluminum radiator it was no longer a problem. Some of the early emissions controlled cars with ported vacuum had a thermal switch that allowed manifold vacuum if the engine started to overheat. So my advice is to use ported vacuum unless there is an overheating problem. Even then, it might be possible to get one of the early thermal switches.

It was known since the 1920's that retarding the timing at idle made for cleaner emissions, and emissions control regulations seemed to be the driving factor in adopting ported vacuum on most cars. That is not to say it is a bad thing, just like EGR actually seems to be beneficial to performance engines.

Quote:

...now I am not using vacuum at all

According to David Vizard....."When I am modifying an engine, I go to great lengths to retain a functional vacuum advance system. The return in highway fuel efficiency numbers alone is worth the effort"
Vacuum advance..."is all too often deleted from modified high-performance street and race engines. Making such a move does nothing to help total power output, but it does serve to increase part throttle fuel consumption and reduce driveability."
Vizard does not specify whether that vacuum advance is manifold or ported.

Pontiac used full manifold vacuum in the 60's. My 65 Bonneville 389 and 67 Firebird 400 both used it stock. The Firebird only had a 8 degree vacuum can and used 12 degrees initial. I used manifold vacuum on other cars (Ford) and had erratic fluctuations and high idle. I would reccomend the lower vacuum can(Pontiac) or adjustable for Jim's Buick V6.

One other thing that '60s cars had, was a vacuum dashpot or throttle jack. This nifty little device pulled in under vacuum and was spring loaded with enough force to push the throttle open when the vacuum went away, such as in the event of a misfire or maybe turning on the A/C or putting the car in gear. Often extremely effective in maintaining a stable idle, they apparently fell into disfavor with the advent of smog devices and tuning. I could probably use one on the TR-7, but it already has a solenoid type throttle jack for the A/C. It would be most helpful during warm up as the rich mixture control is abysmal, but once warm it isn't needed.

Jim, the shorter vacuum can is a good suggestion, but it brings in the scenario Fred was describing. Although 15 initial plus 8 gives 23 degrees at idle which the engine will like rather well, should idle smooth with reduced emissions, run reasonably cool and be pretty responsive and stable, at cruise that pulls 12 degrees of advance out, meaning poor highway economy. The only practical way to satisfy both ends might be to crank in more static advance and run ported, but most motors don't like a lot of cranking advance so getting up to around 23-24 degrees is pretty iffy at best even though that is the point where most motors seem to run the smoothest and most evenly at idle.

Now I've read that engineering book on the otto cycle where the author built all of those test engines and ran countless test cycles under all sorts of conditions. I can't claim to have memorized it all, but what I do recall is that there is always a range involved when discussing emissions, and always speed and power output considerations. It is convenient to say things like NOX levels go up dramatically with timing advance, but it is essentially a meaningless statement unless you include the other parameters such as total volume of exhaust gasses, total amount of fuel consumed, engine rpm, and total wattage output during the test. Even then, unless you precisely define the degrees of advance it isn't of much use, and even still is not necessarily portable to any other engine. And even after you do all that you are still left with a range where the most you can say is that as timing advance increases the proportion of NOX to other combustion byproducts increases. But does this mean that the proportion of NOX to power output increases as well? Possibly, possibly not. That's one of those things I can't remember. And it's worth considering that wattage or average power output is probably a better standard than horsepower at these levels.

So again, although I'm not by any means adverse to cleaning up the exhaust, there is a lot more to the picture. Because the NOX output is a range, it's more a matter of deciding what part of that range is an acceptable operating envelope, and because we are not running complete OEM systems, the OEM range does not apply. Plus, now we have to consider the NOX in proportion to total byproduct emissions, and not just at idle, but in relation to total and complete engine byproducts from every part of the operating envelope. Obviously if the car rarely idles for instance, NOX at idle is much less of a concern than with an engine that rarely does anything but idle. And then there are a host of other but still important considerations. One little side benefit to ported vacuum for instance that makes an absolutely huge difference in cold climates is fast warm up. But in warm climates it is a detriment unless you turn it off as soon as the engine is warm. Somebody mentioned a thermo switch that goes to manifold vacuum under overheat conditions, but for maximum benefit this really should switch over after initial warm up IF there were a way to limit vacuum advance at idle but not at cruise.

And right about here is where the OEMs punted, leaving us with no easy answer unless you are willing to go to electronic timing control. Personally, I'll take a timing map any day but with this older technology sometimes it's just not possible, so you compromise and choose your poison. In my case it looks like that will just have to be idle NOX. I apologize for that, but the pollution issue is outweighed by the overheating issue, and although a car that overheats can't be driven, one that pollutes can. This may not be as insensitive as it seems however. The car does not get driven daily, is not obviously worn out, does not smoke, does not make your eyes water or your nose burn to be around it when it is running, and only goes through about half as much fuel as my daily driver to cover the same distance. So again, we're on that slippery sliding scale where the costs and benefits are usually so oddly balanced. If in the final analysis that balance benefits me slightly more than it benefits society as a whole, I don't think society can be seen to complain. After all, who was it that provided the labor and wherewithall to improve the operating condition of the engine from the infinitely worse state of tune at which it was running when I got it? Society? I think not. And even if it had, it would have come from people like me in the first place, reduced a thousandfold by the burden of government. That clearly unsustainable picture is what got into the mess we are in today, a much clearer and more obvious danger than air pollution ever was.

Jim

Jim B,This GM HEI link recommends 12 degrees for vacuum can and 16 if you have EGR.

[www.73-87.com]

Good tip Jim. The reasoning for the reduced vacuum advance was because the author installed weaker springs for faster mechanical advance. This creates more total advance at freeway cruise speeds and makes the total too high. (Bearing in mind that total advance at higher rpm may not be excessive) I have left the springs alone so I'm having no trouble in that area (so far) but it is a good tip to keep in mind if I do.

Jim