The Piston Killer

[Dirmaid]

I've been wondering for a while now, what actually destroys a piston in a FI system? Is it cylinder pressures, heat or what? I know running lean will eat up an engine, i'm talking about a properly tuned engine.

I guess what set off this thought was reading posts about max boost before problems occur. I'd think a mix of pressure and heat are the culprits, thus why high compression engines take to boost like potassium takes to water. This is only speculation though and i would love for either an explanation or someone elses theories. I'm talking in general terms, nothing specific to any particular engine if anyone is wondering

 

[Hahnsb2]

Heat and pressure, mainly. From running lean, too much timing, too much compression/boost, ect. A big killer is an improper (small) ring gap, the top ring gets hot under boost, the ends meet, the ring locks to the cylinder and rips the ring land right off the piston.

 

[RangerSVT]

Actual strength of the piston has an effect too. That's why boosted engines use forged internals instead of cast...

SVT

 

[Insanejughead]

I bought a turbo motor that was almost junk because it had been run hard and cracked part of the head (or head gasket, I can't remember now) but it let coolant get into cylinder 1 and just that started to melt the outer edge of that piston.

There were bits of aluminum scored into the cylinder wall; it sucked.




However, the main consensus like everyone else is saying (and more closely to what you're asking about) is that compression equals heat and some people don't monitor their motor's performance as they up the ante on things that affect boost.

So, I will add ignorance to the list here.

 

[Dirmaid]

So if heat is a major issue in blown motors, could water/methanol injection in tandem with an intercooler raise how much boost you could pump in since it absorbs heat? And wouldn't lowering heat levels decrease cylinder pressures too?

I'm not sure how to solve that ignorance issue though. As a wise redneck once said, you can't fix stupid

 

[85_Ranger4x4]

I bought a turbo motor that was almost junk because it had been run hard and cracked part of the head (or head gasket, I can't remember now) but it let coolant get into cylinder 1 and just that started to melt the outer edge of that piston.

There were bits of aluminum scored into the cylinder wall; it sucked.

Burning antifreeze eats pistons boosted or not.

IIRC my old Laser (4G63T) had cast aluminum pistons with steel supports cast into them, it ran 11lbs of boost from the factory (and people greatly increase that) They also have piston cooling jets too though.

 

[srteach]

All the other previous replies are correct. Without detracting from them, here is a fairly technical explanation:

Heat is molecular movement energy, and will always transfer from hot to cool environments. That being said:

Compression slows molecular movement (squeezing heat out of the gasses being compressed) and forces it into the surrounding environment (cylinder walls, air, or liquids), which transfers heat to the surrounding environment. Anything that absorbs heat and assists its' transfer faster (water / methanol or intercooler) will help with longevity of mechanical systems. The construction of the mechanical parts (how they are made / what they are made of) determines how much heat load they can absorb before failure occurs. The more dense they are, the more heat load the parts can handle (cast vs. forged).
YES, an intercooler and methanol injection will help compression, and will cool the cylinder a very small amount. This just means the ignition points (spark plugs) are forced to handle more power for the same speed of ignition of fuel. Lowering the temperature of the cylinder will lower the pressure only marginally, due to the small volume of space in the cylinder

 

[Insanejughead]

Burning antifreeze eats pistons boosted or not.

IIRC my old Laser (4G63T) had cast aluminum pistons with steel supports cast into them, it ran 11lbs of boost from the factory (and people greatly increase that) They also have piston cooling jets too though.

Yep, you're right. I just think that the coolant coupled with higher compression ratios caused my motor to wear down quicker.


I'd like to try cast pistons with steel inside of them...

 

[85_Ranger4x4]

Yep, you're right. I just think that the coolant coupled with higher compression ratios caused my motor to wear down quicker.


I'd like to try cast pistons with steel inside of them...

It would be offset somewhat by leaking compression into the cooling system too though... not sure how that would work out. I know even on 6.5:1 tractors it is pretty hard on aluminum pistons though.

Those little 2.0's are really a pretty neat little engine once you get to looking at them, if I could I would LOVE to buy another car and have more fun with it this time around (and to be honest I do wonder why I kept the Ranger instead of it)

 

[stmitch]

So if heat is a major issue in blown motors, could water/methanol injection in tandem with an intercooler raise how much boost you could pump in since it absorbs heat? And wouldn't lowering heat levels decrease cylinder pressures too?

Exactly. The primary killer for boosted motors is detonation, or unexpected combustion in the combustion chamber. Detonation is caused by a lack of fuel, to much boost, or even carbon buildup and roughness in the combustion chamber.
You can fight detonation by
1. running less boost (not likely)
2. cooling the intake air (intercooler or water/meth)
3. running a cooler burning fuel (e85, alcohol, etc)
4. removing any roughness in the combustion chamber where carbon deposits can develop (avoids hot spots)
5. having the engine properly tuned by a professional (altering the amount of fuel or timing to avoid a lean condition)

 

[AlaskanRider]

I was surprised to learn that US spec subaru sti pistons are cast. My friend is pushin 470 whp on his motor, 2.5 liters. Hes blown it up a couple times but both times it was connecting rods that let loose. Once it threw the rod through the block and another time it bent a rod. Massive detonation both occasions. Pistons were OK both times. Most recently when he bent a rod he re-used the block and pistons. He is running water/meth injection.

The motor that melted its pistons on me was an old IH 345. We advanced its timing for more power, and apparently it was too much for it and it holed a piston. Never had any real obvious detonation. It was a really mellow engine, probably south of 200 HP on any day.

Ive heard detonation disrupts a protective gas layer that insulates your engine from the heat of combustion, which is what caused

 

[Dirmaid]

Wow thats a lot of boosted power to ask of cast pistons. Especially with an engine so small. What all did he do to it to get it to handle that kind of power?

 

[AlaskanRider]

Wow thats a lot of boosted power to ask of cast pistons. Especially with an engine so small. What all did he do to it to get it to handle that kind of power?

It was a completely stock motor, with some ARP head studs. He has some opensource software for tuning it, and a wideband 02. It is his DD. They are just really tough little engines. He did have water/meth injection on it.

The Jap spec pistons are forged.

He thought it was some bad gas that did it in, just rompin on it one day and experienced massive detonation, and bent a rod. He said 470 whp was pretty mild in terms of subaru tuning performance.

Its fun to watch it go, scary fast. Like a spaceship engaging warp drive, it just gets small and far away all the sudden when i try to race him.

I guess im just trying to say that I think that consistent lean conditions/advanced timing is what melts pistons, not just too much boost.

 

[85_Ranger4x4]

I guess im just trying to say that I think that consistent lean conditions/advanced timing is what melts pistons, not just too much boost.

You could call a lean condition too much boost though...

I used to jokeingly call the turbo in my car the hyperdrive actually, that is pretty much what it felt like when she started boosting.

It is crazy the power they get out of those little turboed 4cyl engines. In a lot of ways they do almost outdo a V8 (the hp your friend is running is just on the otherside of the 302 comfort zone for splitting a factory block in half)

 

[Hahnsb2]

Most people would be surprised what you can get away with on cast/hyper pistons with a good tune. There's more than a few stock bottom end Magnum V8s in the 5-700hp range, and LS motors with even more.