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Sat Sep 05, 2015 2:10 pm

Can a retard of 10 degrees in the timing cause your car to run hot?

Sat Sep 05, 2015 2:44 pm

Retarding can cause it to run hotter. It is better than running too advanced though.

Running too retarded causes the pressure to build up late in the combustion cycle, which causes less heat to turn into power. This results in poor efficiency and the consumption of more fuel. The more fuel you burn the hotter the engine.

Also running too retarded can cause the flame to still be burning by the time the exhaust valve opens. This can burn the exhaust valve and causes hotter exhaust gases.

Most VWs running a mechanical advanced distributor cruse retarded. That's because mechanical advance is advanced for full throttle operation. The vacuum advance or load advance usually would advance the timing another 10° or more at cruise giving it a much more efficient timing.

PS. That was the whole idea behind DVDA distributors. The VW engine trends to idle too cold, so these distributors made the engine idle warmer by retarding the timing at idle.

Sat Sep 05, 2015 3:50 pm

From another forum, not my words:
hemi (Automotive)25 Aug 15 22:35
It depends a lot on the engine design. Peak combustion temperature will be lower, but this period is very short. Heat rejection to the entire power cylinder including head will be greater due to less extraction of mechanical energy from a given amount of heat release. Exhaust gas temperature will be higher also and assuming exhaust ports in the cylinder head, this will drive more heat into the head. I would say it would take an exceptional engine design to run cooler cylinder head temperature with retarded spark timing, everything else being equal. Yes, exhaust valves will "take a beating", at any rate.
Now if there is detonation occuring with the reference spark timing, retarding timing away from detonation could reduce head temperature, due to the effect of detonation on heat transfer to the chamber surface.
I suppose you could retard the timing to the point of making the cylinder more-or-less a gas generator, with the majority of the heat energy going into the exhaust, but this would be an unrealistic operating point, except in certain racing applications where such a mode may be useful for goosing the turbo very briefly, prior to acceleration. Still, with so much heat in the exhaust port, it's not clear that cylinder head temperature would go down; and as mentioned, this would only make sense as a very transient condition, where there would be little time for bulk temperature change in the head anyway.

Sat Sep 05, 2015 8:45 pm

Isn't the #3 terminal on a stock vw distributor spaced farther to make that cylinder run cooler?

Sat Sep 05, 2015 9:16 pm

theKbStockpiler wrote: Isn't the #3 terminal on a stock vw distributor spaced farther to make that cylinder run cooler?

No, that was to prevent pinging or detonation.

On the single port engines the #3 cylinder had everything against it. First, it was fed in a way that it and the #1 cylinder tended to get more fuel and air than the #2 and #4 cylinders did. Second, it and the #4 cylinder were on the side that had the oil cooler in the shroud. And third, it and the #2 cylinder were positioned farther out from the middle of the cooling shroud getting less air than the #1 and #4 cylinders did.

Because of these reasons it tended to run hotter. A hotter cylinder would tend to ping easier than the others. On modern cars pinging is controlled by a knock sensor that retards the particular cylinder a couple or few degrees. VW just set this one back a few degrees.

Now there is a difference between a few degrees and 10 degrees on an aftermarket distributor that is already likely to be running several degrees more retarded than it should.

Sat Sep 05, 2015 9:53 pm

A lot of this depends on the air fuel ratio. Lean mixtures burn fast. Lean burn idle systems need the timing to be retarded 5deg or so at idle on Type 1 engines with 34 PICT 3 carburetors, or other lean burn idle systems. Some of the T4 engines actually were retarded back to as much as 10deg ATDC for proper running. Obviously the distributor is matched closely to the fuel system, so use the correct distributor for the job. You don't want such a fast burning charge banging away too early on ya. :wink:

Sun Sep 06, 2015 1:43 am

Retarded timing really heats up the exhaust valves!
It certainly will make SOME parts a lot hotter.

Sun Sep 06, 2015 5:18 am

Vw did not want cylinder #3 to dictate the octane of the gas required :wink:

If the combustion process is incomplete, there is still fuel, fuel/mixture in the cylinder acting as an insulator. I think the relationship is actually that lean mixtures burn longer because of the space between the molecules, the longer the distance , the longer the time. :? If the exhaust valve is open it cant transfer heat to the head and the combustion cycle is still occurring.

Sun Sep 06, 2015 5:20 am

Vanapplebomb wrote: A lot of this depends on the air fuel ratio. Lean mixtures burn fast. Lean burn idle systems need the timing to be retarded 5deg or so at idle on Type 1 engines with 34 PICT 3 carburetors, or other lean burn idle systems. Some of the T4 engines actually were retarded back to as much as 10deg ATDC for proper running. Obviously the distributor is matched closely to the fuel system, so use the correct distributor for the job. You don't want such a fast burning charge banging away too early on ya. :wink:

Air fuel ratios do change the timing needs of a car but I don't think that has anything to do with retarding it so much at idle. You could advance it to just about anywhere and probably never get it to ping at idle. In my own experiments the coolest running idle timing is much more advanced than even SVDA timing. I've even contemplated adding a thermovalve that would change the vacuum signal from port to manifold vacuum in order get full advance at idle to help cool an overheated engine. Well, at least I had thought about doing that until I read about thermo shock caused by rapid cooling in an air cooled engine.

Sun Sep 06, 2015 5:32 am

theKbStockpiler wrote: Vw did not want cylinder #3 to dictate the octane of the gas required :wink:

If the combustion process is incomplete, there is still fuel, fuel/mixture in the cylinder acting as an insulator. I think the relationship is actually that lean mixtures burn longer because of the space between the molecules, the longer the distance , the longer the time. :? If the exhaust valve is open it cant transfer heat to the head and the combustion cycle is still occurring.

Like I said. The #3 cylinder is only rewarded a few degrees. Yes, that will help it run cooler than if it were pinging all over the place. It's better to run a little retarded than a little advanced. But if you were to retard all for cylinders a few degrees then the engine would consume more fuel. More fuel= more heat. Also remember that most of the heating of the head usually takes place in the exhaust ports. The combustion chamber is getting cooled with every intake charge. The more you retard the less heat is transferred into workable energy. So therefore you get hotter exhaust gases sitting in the exhaust ports.

Like I said,a few degrees isn't going to make that much of a difference. But 10 degrees likely will run hotter.

Sun Sep 06, 2015 5:55 am

One's i run into with dropped valve always was #3. After the 40hp eng mine were updated to dh cooling. I may use a 40 later but it will have an external oil cooler to help protect 3/4 cyls.As usual no warranty offered at this time.

Sun Sep 06, 2015 8:32 am

Juanito84 wrote: Vanapplebomb wrote: A lot of this depends on the air fuel ratio. Lean mixtures burn fast. Lean burn idle systems need the timing to be retarded 5deg or so at idle on Type 1 engines with 34 PICT 3 carburetors, or other lean burn idle systems. Some of the T4 engines actually were retarded back to as much as 10deg ATDC for proper running. Obviously the distributor is matched closely to the fuel system, so use the correct distributor for the job. You don't want such a fast burning charge banging away too early on ya. :wink:

Air fuel ratios do change the timing needs of a car but I don't think that has anything to do with retarding it so much at idle. You could advance it to just about anywhere and probably never get it to ping at idle. In my own experiments the coolest running idle timing is much more advanced than even SVDA timing. I've even contemplated adding a thermovalve that would change the vacuum signal from port to manifold vacuum in order get full advance at idle to help cool an overheated engine. Well, at least I had thought about doing that until I read about thermo shock caused by rapid cooling in an air cooled engine.

Be aware that retarding the timing and retarding the bulk of the combustions flame front are actually two different things.

Because the lean idle mixture burns faster, retarding the timing keeps the bulk of the flame front at approximately the same point in the cycle. This can be observed with special high flicker rate combustion chamber cameras. Most automotive engine labs use these cameras to observe the combustion process in order to develope engine management systems. Pretty cool stuff. I have some college buddies that did/do this at Ford and Toyota.

Mon Sep 07, 2015 7:41 am

Vanapplebomb wrote: Be aware that retarding the timing and retarding the bulk of the combustions flame front are actually two different things.

Because the lean idle mixture burns faster, retarding the timing keeps the bulk of the flame front at approximately the same point in the cycle. This can be observed with special high flicker rate combustion chamber cameras. Most automotive engine labs use these cameras to observe the combustion process in order to develope engine management systems. Pretty cool stuff. I have some college buddies that did/do this at Ford and Toyota.

This is exactly what has to do with the OP's question. The goal when the engine is producing power is to get the flame to end a the same point every time, usually about 10° to 15° ATDC. Sooner than that and you are now liable to ping, knock, and simply create too much back pressure. This can cause the heads and piston to heat up from within the combustion chamber, but exhaust temps will actually go down. This can cause holes to melt right through the piston.

But on the other hand if the flame is too late it never builds up maximum pressure, in stead the flame chases the piston down the cylinder and can even still be burning by the time it passes through exhaust valve. Exhaust temperatures will be higher and these temps will enter the head through the exhaust port.

When the timing is right on you get the most power out of your mix. Not only does that translate into more power, but also better fuel economy.

Of course you have to realize that the ignition timing event needs to be constantly changing in order to get the flame in the sweet spot. If you increase the engine speed the timing will need to advance in order to get the flame to end at the same place. Likewise, if you increase the engine load the ignition timing will need to retard to get the flame to end at the same time.

You can do a simple experiment at idle to prove this. Idle is always retarded and therefore inefficient on these old cars. If you advance the distributor the idle speed increases, even though your throttle hasn't changed. When you get to the point of highest idle speed you just found the sweet spot for that particular RPM and LOAD. If you continue to advance the distributor idle speed will then decrease, showing you past the point of best timing. If you were to set your initial timing there (you'd need programmable timing, or connect an SVDA advance to your intake manifold) and then change the idle speed back by adjusting the carburetor your idle temps would be much lower than SVDA timing. And DVDA timing will produce even warmer idle temps.