Z300, z250

[boscoe99]

Interesting that these modern ecu computers cannot compensate for the load and autocorrect for the drag that "blows" these motors. Some sort of vacuum sensor?

A number of Yamaha's incorporate a knock sensor that detects the onset of detonation. The ECU can either retard the timing a bit, add more fuel, come back on the throttle to reduce the power, or a bit of each, to try and prevent detonation from damaging the motor.

As I said, some Yamaha's prior to the HPDI have them. The OX66 for example. Later four strokes (but not all) have them. The HPDI never got one. And it seemed to need it the most.

[rodbolt17]

and I do not believe it was detonation.
I believe it is erosion due to the fuel still burning when the ex port uncovers.
I have seen a lot of that damage some worse and some just starting.
that is why I do not think it is detonation.
remember this motor does NOT rely on primary compression to move fuel.

[boscoe99]

and I do not believe it was detonation.
I believe it is erosion due to the fuel still burning when the ex port uncovers.
I have seen a lot of that damage some worse and some just starting.
that is why I do not think it is detonation.
remember this motor does NOT rely on primary compression to move fuel.

Why is it seen so much more so in the HPDI? It is being given less fuel to burn (say compared to an OX66) so one might think there is less fuel still being burned when the exhaust port is uncovered by the piston, while the OX66 in theory should have more fuel still being burned.

Now less fuel might indicate a hot burning exhaust gas temperature. Is it the hotter exhaust gas that is doing the erosion?

What in your opinion is causing the piston to fail, with either chunks off the sides or holes in the middle, when it fails like that? Or metal to become molten and making a mess of things?

[99yam40]

I myself always though running lean and too hot are what most holes in pistons came from.

I had heard running to hot of a plug could do that, but do not understand how a wrong heat range plug would do that unless it got hot enough to fire off the fuel before the spark called for it to

now with the edge of piston going away I am not real sure, but believe the exhaust was too hot if that is where the piston is going away.
still Burning fuel heading out the exhaust is the only thing that makes any sense or like you say maybe lean burn is hotter temp

[boscoe99]

I myself always though running lean and too hot are what most holes in pistons came from.

I had heard running to hot of a plug could do that, but do not understand how a wrong heat range plug would do that unless it got hot enough to fire off the fuel before the spark called for it to

now with the edge of piston going away I am not real sure, but believe the exhaust was too hot if that is where the piston is going away.
still Burning fuel heading out the exhaust is the only thing that makes any sense or like you say maybe lean burn is hotter temp

Depends upon the degree of "leanliness". As the air/fuel ratio is leaned from what is normal the combustion gas temperatures will rise. But at some point, they will begin to fall. Think of an injector that is stuck closed. The air/fuel mix is completely lean. But the EGT will be low since no fire is taking place. No harm will befall that cylinder.

Many many airplane motors are flown way on the lean side of normal combustion temperatures with no ill effects. To get more range. But, only at reduced power levels. If the motor is operating at high power then a lean mixture can raise the temperatures to the point where bad stuff starts to happen. Detonation. It is detonation that is doing the damage. It is leanliness that might be resulting in the detonation.

https://www.ngksparkplugs.com/about-...-is-detonation