TR6 TR6 vacuum retard, counter intuitive to me

[TR6oldtimer]

I was reading through an article on TIMING AND VACUUM ADVANCE. The author pointed out that the centrifugal advance was a function of engine RPM and advances the firing point as RPM increases to compensate for the faster piston speed in relation to the "steady" burn time of the fuel/air mixture, thereby keeping the maximum combustion pressure in the sweet spot. This makes sense to me.

He went on to describe that a lean mixture, which occurs with high intake manifold pressure, burns slower then a rich mixture. So, to keep the maximum combustion pressure in the sweet spot, you need to advance the spark when the mixture is lean, which is the purpose of a vacuum advance. Again, this makes sense to me.

So, with higher intake vacuum, you have a leaner mixture and need more advance. When you open the throttle, vacuum drops, the mixture becomes richer, so the spark is retarded. That is the vacuum advance decreases with a lower manifold pressure. Again, this makes sense to me.

This is what I find to be counter intuitive. My '73 has only a vacuum retard that is always in play unless a high engine coolant situation is encountered, and then it is disabled. So, when you open the inlet valves, it creates a lower manifold pressure, which means a richer mixture, a faster fuel burn time, implying the timing needs to be retarded. The '73 setup ADVANCES the timing (lower vacuum, less retard).

Here is the spec on the vacuum retard:

No retard before 2 1/2 inHg. Fully retarded, 7+/- 1 deg. at 10 inHg.

It seems to me, this engine setup is deliberately retarding the timing to lower NOX at the expense of peak performance.

Is my assumption correct, or what am I missing?

The article:

https://www.corvette-restoration.com/resources/technical_papers/Timing101.pdf

 

[hondo402000]

Ray I belive you would be correct, it reduces admissions at expense of power, you could send your distributor out to jeff at Advance distributors, he can set it up for mechanical advance only and add a thumb wheel on the dist so you can tweek the advance and then set your timing to, I dont have my book but 12-14 degree BTDC and plug all those vacuum lines and it should run really well, Unless I am missing something but mine is a 1971 model year TR6

JMO

Hondo

 

[SkinnedKnuckles]

I think you have it right. It was one of those "quick fixes" to reduce emissions at idle. Lowering compression was another.

 

[TR6oldtimer]

All that you guys have stated is true. But here is the question that came to mind while reading the article. You are driving along a 60 MPH on a flat road and come to a hill. To maintain speed, you give the engine more throttle to maintain speed. The centrifugal advance stays the same because the RPM's have not changed, but with more throttle, the fuel air mixture is richer, and the manifold vacuum is lower. According to the article, an engine with a vacuum advance would sense the decrease in vacuum, suggesting a richer mixture, and retard the spark.

Now I do not know what the manifold vacuum is when doing 60 mph, but I doubt it is below 2.5 inches of mercury. So the retard unit is still in play. This means that spark will be advanced as you give the engine more throttle, just the opposite of what should happen.

Now for those of you who have only a centrifugal advance, opening the throttle going up the hill will not change your timing even though a richer mixture would mean the faster burning of the fuel should have a slight decrease in timing.

Maybe the answer is that with an engine design that realistically is maxed at 150hp, it does not really matter...

What it does suggest is that you need to tune your car to the type of driving you do, and to do that, it is Dyno time.

If I had the time and money, this would be fun see if an engine with a vacuum advance would make a significant change in performance.

Oh how I long for the days when I owned a TR4, where this was not an issue.

 

[TR3driver]

Ray, you've gotten the wrong information about the vacuum retard. It should be connected to a special carb port, that only supplies vacuum when the throttle plates are closed. Thus the retard should only be active at idle and when decelerating sharply. (TR250 and early TR6 had a mechanically operated valve that did the same thing.)

Also, the primary reason for vacuum advance is not due to the change in mixture, but the change in charge density when the cylinder fires. With the throttle only slightly open (cruise conditions), the cylinder pressure at the moment of ignition is much lower. The lower pressure results in a slower flame front, meaning the spark needs to happen earlier to put peak pressure at the right point in the cycle (where it produces the most power).

Vacuum advance mostly affects fuel economy, although it does supply a little extra part-throttle performance. The reason it was deleted on later TR6 is because it also increases NOx emissions (because it results in higher peak cylinder temperatures under cruise conditions, and it is the high temperature that causes nitrogen to oxidize).

That's a simplification, of course, but I think captures the essence.

 

[TR6oldtimer]

Randall, you are correct on where the vacuum retard senses manifold vacuum. It is indeed just behind the butterfly valve, which as it opens passes over the port, thus sensing only the slightly lower pressure due to the air filters. So it has no effect on timing at speed.

As to what the vacuum advance does, we are talking about the same effect, that is the effect of the speed of the flame front under different conditions and that the vacuum advance adjusts timing for optimum output. My references to lean and rich were from the author of the article, who dumbed it down so even I could understand what was happening.

The point of eliminating the advance to reduce NOx emissions is something we are both in agreement on.

What is still unanswered is if there is a noticeable difference in performance with or without a vacuum advance. Poor fuel economy implies an inefficiency in the performance of the engine. Many on this and other forums put a lot of money into their lumps to get more power, or efficiencies if you will. So I was just wondering if anyone built an engine with a properly curved distributor with a vacuum advance and compared it to one without.

I have a TR250 dizzy that needs to be refurbished, who knows, maybe at some point I'll give it a go.

 

[poolboy]

There's a pretty good explanation of the Vacuum Advance on Lucas distributors about halfway down this page.
https://mgaguru.com/mgtech/books/lts_otcc/Session3_CoilIgnition.pdf

 

[TR3driver]

What is still unanswered is if there is a noticeable difference in performance with or without a vacuum advance. Poor fuel economy implies an inefficiency in the performance of the engine. Many on this and other forums put a lot of money into their lumps to get more power, or efficiencies if you will. So I was just wondering if anyone built an engine with a properly curved distributor with a vacuum advance and compared it to one without.

Sorry, I thought I covered that. Vacuum advance has no effect on full throttle power, as it is not active then. It's primary benefit is under cruise conditions, although it does seem (to me) to make the engine more responsive at part throttle. IOW it does produce more power for the same throttle & rpm, but only when the throttle is not open all the way.

The centrifugal advance has to cover the engine's characteristics at full throttle, hence the optimum centrifugal curve shouldn't change without vacuum advance. And I have tested with vs without, by the simple expedient of having the vacuum module fail. It nets about 10% better fuel mileage on the highway (ie only 2-3 mpg difference) and a slightly more 'eager' engine at mid-throttle. With the failed module, the engine felt sluggish, kind of like it needed a tune-up, even after just getting one.

A friend of mine also experimented with this, on a TR4 with DCOEs. Being unwilling to drill his expensive carbs for a proper advance port, he used an electrical valve & a microswitch to gate manifold vacuum to the advance module. (Somewhat similar to the valve used for retard on TR250 & early 6, but his was arranged to supply vacuum only when the throttle was opened.) He also reported about 10% improvement.

 

[SkinnedKnuckles]

"The centrifugal advance stays the same because the RPM's have not changed, but with more throttle, the fuel air mixture is richer, and the manifold vacuum is lower."

The vacuum retard really shouldn't be in play there. The enrichment is only temporary to supply more power, but as the air valve opens the "goal" F/A mix comes back.

Mr. Good used to change the vacuum retard to advance. I did that years ago and really like it.

 

[SkinnedKnuckles]

Maybe you're thinking about it too hard. Fuel has changed alot in the past 35 years - lead was used to keep flame propagation rates low which reduced knock. Now there are oxygenates (10% ethanol) in there. I don't know how all the change effects flame speed in the cylinder. Don't know how the computers in modern cars even optimize things besides O2 in the exhaust....

 

[TR6oldtimer]

Hmmm, not really thinking about it too hard, just wondering.

PB, that was a good link. I especially liked this statement, "The general effect of the vacuum unit (advance) is to increase the 'liveliness' of the engine..."

Here is what I learned.

The vacuum retard on the 6's is self eliminating as the engine speed increases due to the location of the port just behind the carb valve. So it makes no difference when the car is at speed.

The vacuum advance makes no difference when the throttle is wide open, which we all knew.

The centrifugal advance, in conjunction with the effect of a vacuum advance is curved to provide the proper engine timing at any given engine rpm.

Now this is were the efficiency comes in, which Randall has pointed out as well as the article and PB's link, and that is at driving speed, like on a highway where changes in throttle are frequent. So back to my example,

"You are driving along a 60 MPH on a flat road and come to a hill. To maintain speed, you give the engine more throttle. The centrifugal advance stays the same because the RPM's have not changed, but with more throttle, the manifold vacuum lowers. According to the article, an engine with a vacuum advance would sense the decrease in vacuum, and retard the spark."

So, there is an advantage to a vacuum advance. But, as I learned (again from PB's link) is that just dropping a TR250 dizzy with a vacuum advance will not do the trick, unless I also have a carb with the properly located port. That means I would also need the 250 carbs (which I have), and that sets up a whole different set of things to deal with...

Thanks guys, this was fun.

 

[TR3driver]

But, as I learned (again from PB's link) is that just dropping a TR250 dizzy with a vacuum advance will not do the trick, unless I also have a carb with the properly located port.

Or, as I mentioned, some creativity with plumbing and/or electricity. (DCOEs also do not have the advance port.)

Personally, I intend to drill & tap the advance port on one of my 73 ZS carbs, eventually. If I bollux it up too badly, I can just plug the hole with JB Weld and tackle the other carb. And I've got spares if that doesn't work either :laugh:

I've got some photos taken by a fellow Stag owner of where the port goes, if anyone is interested. Of course I've also got the 71 carbs on my other Stag, which have the port.

 

[Brosky]

I totally agree with Randall's assessment of the vacuum operation. But as the devil's advocate, I have been running mine with full centrifugal advance for two years now and I'm very happy with the performance.

 

[TR6oldtimer]

I totally agree with Randall's assessment of the vacuum operation. But as the devil's advocate, <span style="font-weight: bold">I have been running mine with full centrifugal advance for two years now and <span style="text-decoration: underline">I'm very happy with the performance.</span> </span>

That is what the virgin guy said about his wife. :laugh:

In reality, what you haven't tried, you will never miss.

Sorry, I just could not help myself...

At some point, I will take a look at those 250 carbs, and perhaps drill the advance port in the later ones. What the heck, if I can get a more responsive throttle and increased mileage, it is a very inexpensive improvement.

 

[angelfj1]

Ray: You might consider a vacuum gauge, either as a temporary measure for experimentation or as a permanent installation.

 

[angelfj1]

Ray: I don't want to hijack this thread BUT

I've been operating the TR250 with the stock set up since 1985. This is the distributor with two vacuum capsules, one for advance and one for retard. The biggest problem I have experienced is the poor quality of the US made vacuum valve. They are plastic and don't last long because they get cooked by all that heat. They tend to become just another source for vacuum leaks over time.

Does anyone know of a higher quality substitute?

Thanks

 

[TR3driver]

Does anyone know of a higher quality substitute?

Frank, why not just punt the vacuum retard? As we've already discussed, it wastes fuel and increases overheating and CO2 emissions. You could leave everything in place for appearances sake and just plug the line from the carbs. Readjust the idle and you're good to go.

Remember, "If it's not leaking, it's empty" applies to that valve too!

 

[TR6oldtimer]

Does anyone know of a higher quality substitute?

Frank, why not just punt the vacuum retard? As we've already discussed, it wastes fuel and increases overheating and CO2 emissions. You could leave everything in place for appearances sake and just plug the line from the carbs. Readjust the idle and you're good to go.

Remember, "If it's not leaking, it's empty" applies to that valve too!

Yep, if I were in your situation, I would do that as well...

 

[hondo402000]

if I am not mistaken on SU and Stromburg carbs just because you open the butterflys all the way it takes an increase in vacuum to lift the dash pots and raise the needles so actually the car goes lean till the needles catch up to the open throttle plates and pardon the miss spelled words and I i know it emissions not admission


thats why they are called CD or constant depression

Hondo

 

[TR3driver]

I'm not quite sure of your point, Hondo. With a properly setup SU or ZS carb, the mixture will actually go somewhat rich immediately after opening the throttle, because the damper resists the upward motion of the piston. This increases the velocity through the venturi, and sucks more fuel out of the jet. This is roughly analogous to the function of the accelerator pump on conventional carbs.

Then the needle taper also varies slightly, to give a slightly richer mixture all the time at higher flow rates. Very roughly analogous to the power valve on conventional carbs.

This is all because best fuel economy is with the mixture slightly lean (from 'perfect'), while best power is with the mixture slightly rich (again from 'perfect' aka stoichiometric).