I've been wondering today:

After doing a hard run and you let off and DFCO kicks in, would that cause a rapid heating-cooling cycle that could cause piston metal fatigue (and therefore eventual cracking) over time?

This morning for instance, it was below freezing outside. The pistons are exposed to several hundred degrees in temp even on a normal driving basis and then when I let off and DFCO kicks on, it's got below freezing air suddenly freely flowing through there for a while if you stay decelerating. Then when you add throttle or DFCO kicks off you've got several hundred degrees in there again.

When you do a hard run and then let off, the temperature difference becomes even greater as well (esp being a turbocharged car) as is the time period that you're in DFCO probably. Hypereutectic pistons have a higher silicon content, to make them harder... but that also makes them more brittle too.

As it is, if I am getting off the highway and coming to a light for instance my coolant temp can come down quite a bit through the cooling effect alone.

Just wondering what you all thought about that. I wonder if many large and rapid heat cycles like that could cause metal fatigue, warping, uneven wear, and/or cracking. That would also explain why we suddenly start hearing about a lot of cracking piston cases so suddenly as soon as it starts getting really cold out, even when on GMS1.

 

What the hell is dfco

 

Guessing it stands for something like Decelerating Fuel Cut Off.....or something to that effect. I definitely notice it with mine being stock.

 

I don't think you would ever see metal failure due to the quick temperature change.

God I would hope our engines are much stronger than that.

 

Cool air going into the cylinder is not going to be enough to cause rapid cooling of the cylinder walls or pistons.

 

Think about how many RPMS the engine is running.

You are taking in the air and compressing it in fractions of a second. It would heat up in the bat of an eye.

 

Correct

That also means fresh cold air comes in and cools the piston/cylinder through convection that many more times too. Either way, it's a lot colder in there than when a combustion event takes place though, and the air also quickly takes a lot of heat out with it (as seen on the coolant temp). I'm just wondering if going from a high-pressure/temp turbo-compressed, compressed again, and then ignited temps, to sub-freezing temps in a little over a second can cause an issue. I don't have an answer, that's why I'm throwing it out there for people to think over.

If this is a possible issue, I could see it being maybe fixed by delaying DFCO slighty through tuning. That way it lets down the temps slower through some lower temps through light combustion before the real cold air starts coming in and keeps coming in.

 

Cars have been using DFCO for years, and I've never heard of an engine blowing because of it. I'm not sure exactly what makes you think it's all of a sudden "cold" because you let off the gas while coasting.

 

I understand. Most of those cars aren't pushing boost, much less the amounts we are though (meaning much higher cylinder/exhaust temps), and on hypereutectic pistons for us to have much information to fall back on though.

When you let off the gas getting off the highway for instance, the engine cuts fuel, combustion stops, and the engine basically windmills the incoming air through the engine until either you add gas again, or the DFCO turns off again due to it's set RPM (based on what gear you're in).

The only cars similar in power to a GMS1+ TC that I know of (Turbo-4 of similar power) would be an STI or EVO, and I've found cracked piston and ring issues with them as well.

We can't throw out yet that it could just be people running too much boost or on gnarly tunes, but I guess I just find it interesting that this is just now surfacing all at once in the winter time and to LNFs of any tune.

 

wouldnt pistons in all cars do that then?

 

No. I'm basing the idea off of the fact that during WOT we're making much higher cylinder/exhaust temps than N/A cars (due to the fact we're forced induction and such a high boost on top of that) and they also don't have the more brittle hypereutectic pistons in them either most likely.

 

Air is not a very efficient cooling method. The cool air will be heated before the cylinder wall is ever cooled. You would have to spray cold water into the combustion chamber to cool it fast enough to actually cause a thermal stress crack.

(Water is much, much more efficient at cooling than air is)

 

I can think of a couple 20 year old cars with factory turbo's and dfco and never heard of piston failure due to it.

 

I will say one benefit tauted from GM on the LNF is "very aggressive DFCO" which I feel is too aggressive.....Never had a car that would let you know it was in DFCO

I try to never let the car go into DFCO...not because I feel its bad (lower emissions is always a good thing, and mpg). It just doesnt make sense on a manual car imo. If I'm slowing down I just press in the clutch which does not allow the DFCO to come on, one of the requirements is the car has to be in gear. First time I saw it was when I saw my engine had an out of this world lean condition. I just press the clutch now if I'm cruising in traffic...a few low speed crawls in gear w/ a buck here and there reminds me to just press the clutch in.

 

Turbo T/A & Buick Grand National?

 

so to avoid dfco, you hold the clutch??? you know that's not good, right? what exactly about it doesn't make sense? The engine runs crazy "lean" because the injectors are not firing... doesn't mean it's not safe though.

If your car bucks while driving, you're doing something wrong. Simply letting off the gas slower will fix the "problem".

 

Never said I hold the clutch, nor did I say it was unsafe, nor state I do not understand what DFCO....I stated on a manual I simply do not see the benefit as much as I see it on automatics

GM went too aggressive on it, its as simple as that. You should never "feel" it nor should you have to vary letting off the gas slowly to not feel the engine buck (even though that has no effect). The only indication should be instant mpg readings shooting to 99+. I've felt it in other LNF (hhr ss, and my friends rl)

 

How much boost do they each push from the factory?

Alright, well it seems that the concensus is that it's not even plausible, so I'll let it rest then. It was one of those things I thought of when I was brainstorming.

I think he meant that he pushes in the clutch and lets the engine idle as he decelerates, not continuously slips the clutch while he decels.

 

really, not trying to be a dick here.... but those statements totally contradict each other. manual cars will give a buck if you quickly let off the gas. it's not a gm thing, it's a manual thing. is this your first manual?

 

I tuned my LSJ DFCO very aggressive, and I loved getting an extra 2 - 3 MPG because of it.

 

I was thinking more of the dsm/silvia/mr2, but the gnx would do just fine.

Most of them were in the 10-15psi range, but with a huge aftermarket/modding community none of the pistons were failing due to dfco.

 

Correct, air has a very low specific heat capacity, which means it does not take much heat transfer (energy transfer) to change the temperature of a given amont (mass) of air. Air is heated and cooled very easily and very quickly. Water on the other hand has a very high specific heat capacity which is why they use water so often for cooling applications. It takes a ton more heat transfer (energy tranfer) to change the temperature of a certain amount (mass) of water vs the same amount (mass) of air. Think about how quickly you freeze to death in 40* water vs 40* air. Air is a very poor conductor of heat. It is the air trapped between the strands of fiberglass insulation that gives it its insulating properties.