To disable cooling flaps or not, The endless debate Options

[Superhawk996]

ditch the stat.

Cabman - love ya to death but please stop propagating the ditch the thermostat advice.

The flaps and thermostat are there for a reason.

The majority of engine wear occurs during the warmup phase of operation this is true of all engines and is backed by decades of SAE research and documentation.

Disabling flaps and delaying warmup results in accelerated wear.

Even worse, air cooled engines depend on expansion of the heads to increase clamp load between the cylinder and the head as the engine heats up. Delaying warmup results in more likelihood of blowby between the head and cylinder and between the cylinder and the case.

And even worse yet on a carbureted car where the delayed warmup reduces fuel atomization, and increases cylinder wash down - really accelerating wear and diluting oil.

I’ve read all the anecdotal stuff of “my engine is fine”. You may think so but the reality is you are only looking at a tiny snapshot in time of the engine life and wear. As a hobbyist, you likely won’t ever have the opportunity to compare engine life of engine A that ran properly vs engine B that has the flaps disabled.

And I’ll concede that because a hobby car only gets driven infrequently - that partially negates the wear argument(s).

But let’s not keep propagating the shade tree mindset that the thermostat & flaps aren’t necessary for proper operation and longevity of T4 engines.

Once he’s in there deep enough to identify the cause, it should be fixed properly not jury rigged.

[brant]

Full load redline is the most important for mixture

Something around 12.5 or close to that is rich enough to increase cooling on the heads and ideal for max power

Leaner usually raises head temps.


Also idle doesn’t matter
Cruise barely
AFR becomes important at high rpm
Race cars are jetted for red line

[JamesM]

Ok, you said I was still on topic so... here goes, you asked for it (IMG:style_emoticons/default/biggrin.gif)

Leaner usually raises head temps.

This is what you generally read on the internet but the reality is a little less one dimensional and to be a little more scientific you need to give a frame of reference.

Specifically leaner than what?

Head temperature response to AFR is not a straight line, it is a bell curve. Peak head temperatures typically occur near stoichiometric mixtures, with temperatures falling as the mixture becomes either richer or leaner. So say you are running at 12.5:1 at a given load, it is true that running leaner say 13.5 or 14.5 will produce hotter temps BUT that is only true to a point. If your starting point of reference is ~14.7:1 and you run leaner, things get cooler.

The primary source of heat in a combustion engine is the chemical energy released during combustion. Maximum combustion temperatures generally occur near stoichiometric mixtures (around 14.7 AFR). To keep heads cooler you either need to increase cooling OR put less heat into them to start with. Running richer than 14.7 you are still roughly burning the same amount of fuel, now just with a little left over (fuel rich) Because nearly the same amount of fuel is still being burned, the total chemical energy released changes very little. The excess fuel acts as a heat sink through vaporization and by absorbing energy that would otherwise increase combustion and component temperatures.

The other way to get less heat in the system, is to generate less heat to begin with. 14.7(give or take) is stoichiometric, the maximum amount of fuel that can be completely combusted. Burning a leaner mixture than that generally results in less total heat being released into the engine. From a cooling standpoint its more efficient to not generate the heat in the first place than it is to put the heat in and then cool it off.

Now I know at this point I have raised a LOT of questions, but hang on because there is a lot more to explain here that may answer them...

Most car guys on the internet only care about power and when they are talking about rich vs lean its always in the unmentioned context of full load/wide open throttle. In this context, they are correct, leaning out your mixture would most likely be very undesirable for multiple reasons. Heat, potential detonation, but possibly the most important in the context of full throttle, POWER.

Going richer than stoichiometric you will notice power output vs AFR does not change much largely in part due to the fact that you are not extracting any more energy from the gasoline the more you add. On the lean side of 14.7 however power drops off FAST, so if you lean burn under high load, assuming your engine doesn't detonate, you are significantly down on power under the operating conditions (full throttle) where you are looking for maximum power.

But under conditions where you do not need the power (Cruise, decel, light acceleration you can get away with not burning the extra fuel and it is in fact preferable because you are 1. saving fuel and 2. generating less heat.

BUT in order to do it there are other things that have to be considered, specifically flame speed. Say for example your engine is tuned for max power and at a specific RPM, say 3500 that happens to be from 12.8:1 AFR @27 degrees advance. This happens to be the AFR where you are typically going to have the fastest flame front. Assume a centrifugal advance distributor so your timing at 3500 doesn't change (27 degrees) you will lose power going leaner OR richer from here but initially its not due to combusting less gasoline because anything richer than 14.7 you are already utilizing essentially all of the available oxygen for combustion. Instead the power output changes because the change in flame speed means the peak cylinder pressure is no longer occurring at the optimal time. Then once you go leaner than stoichiometric factors contributing to power loss are twofold 1. you are now starting to burn less fuel than the available oxygen allows for, so less energy into the system and 2. The flame front continues to get progressively slower meaning the fuel you are burning is ALSO creating sub optimal cylinder pressure. You will generally find that with a stock distributor/timing curve you can be pretty much anywhere between 14-11:1 afr without noticing much, but things become undriveable pretty quickly as you go leaner than 14.7. You will most likely be bucking and on the edge of even having enough power to go down the road, popping out the exhaust as the flame front is slow enough that fuel is still in the process of combusting by the time the exhaust valve opens.

So the actual drivability problem of trying to run leaner than 14.7 under light load conditions (which in a street driven 914 is like 90%+ of your driving) isn't that there is not enough energy in the amount of fuel, its that the energy you are producing is all flying out your tailpipe because you are unable to optimize when peak cylinder pressure is occurring. This can be solved however by igniting the fuel sooner and is a fairly easy problem to solve with a programable three dimensional timing map.

The short take away here, to generate the necessary power needed at light loads you can either burn rich, throwing more fuel than then you have available air to burn and removing some excess heat with the extra fuel OR add advance to account for a slower flame front and burn less fuel than the available air


Congratulations if you made it this far!

Its a multidimensional problem and takes a little while to wrap your head around.

Ive been playing with EFI on 914s for 20+ years now and its only been in the last couple where I have really started to nail down some of this stuff. You will find older posts where I say you cant run lean, it was because I didn't know better.