PWC Engine Failure diagnostics - related to "The story of tuning a turbocharged jetski"

SUB-TOPIC - relates to The story of tuning a turbocharged jetski

By the way - offtopic question to anyone who has built engines for PWC's. Since you have the best possible cooling in the real world, how does that affect the engine specs, especially piston to wall clearance when building a motor?

Recently I've seen a failure of a Hydrospace S4 (800cc 2 cylinder, 85mm piston) engine where the piston siezed on the intake side primarily and destroyed itself. The crown of the piston was intact, neither the piston nor the head showed zero signs of knock, and the logs showed the lambda and temperatures well within margins of safe (0,83 - 0,84 lambda, IAT around 30 degrees and coolant temp never going anywhere above 45 deg C) The piston to wall clearance on this specific engine was 0,09mm using wiseco forged pistons. (I have no way to confirm the clearance was really this, since I never saw the engine being built) but that's the data I was give regarding the build, the logs I checked over myself. The owner of the jet and the guy who built the motor both accounted it to "water getting in to the cylinders and creating inappropriate cooling of the engine block" but that sounds like a load of bs to me. Apparently there was some water inside the jet so it's actually possible that some water would have been spraying through the intake into the engine (saltwater) but I have my doubts that this could cause the sieze of the engine.

Notes:

- The "W" wiseco sign was the intake side of the cylinder.

- The head "spots" were apparently salt because they literally cleaned off with water - not detonation damage.

- The pistons showed ZERO signs of any knock event occuring

- The block that is smashed up is the intake side

I've had almost nothing to do with either building or tuning engines in PWC so it's quite possible some other users may offer better insight here. I'll give my 2c though. On the face of it the failure appears to be a piston seizure most likely relating to insufficient piston to bore clearance or perhaps the piston running too hot and expanding too much in the bore creating the same result.

I'm not familiar with the Hydrospace S4 however a quick google suggests they are turbocharged. If so there's a little more information that is probably relevant. First of all I seldom use Wiesco pistons and there are often slight differences in the alloy used for the piston forging from one manufacturer to another, however my gut feel is that 0.09mm is a little on the tight side without knowing specifically anything about your boost level and usage. For an 85 mm bore I'd normally be very slightly looser - Around 0.10 mm / 0.004". Of course Wiesco will have their recommendation and if you know no better then I'd use that above all else.

Next up is the lambda. Again I don't really have enough information to make a call here. My lambda target would depend on the ultimate boost level as well as the ski usage - For example is this likely to see extended periods of WOT use or is it for more recreational use? If the engine is running reasonably high boost and seeing extended WOT operation I'd be inclined to target a slightly richer lambda.

Lastly the ECT is concerning. The piston to bore clearance is measured cold (normally at something like 20 deg C) and the aim is to produce a cold clearance that will tighten to the desired operating clearance when both the piston and block are at operating temperature. This relies on both the piston and the engine block/bore expanding with heat. With the coolant temp only running at 45 deg C, this would limit the heat expansion of the block/bore and could result in a tighter than expected clearance. I have no idea if 45 deg C is a typical ECT for a PWC - If it is then this would need to be factored in to the clearances when the engine was built. Wiesco's technical help line should be able to offer some advice around this specific application though and perhaps a recommendation on tweaks to the piston to bore clearance.

I look forward to seeing what else is added to this thread.

The additional info about this jet would be:

It's running 1,2 bar of boost, while IAT staying between 30-40 degrees, depending on how rough the Waters are meaning - how fast the water changes in the intercooler. It lives most of it's life in the X > 7000 rpm section and more or less an "on off" driving experience so it's either off gas or WOT. In terms of lambda - I am considering running it lower for next year but on the other hand - the same jets in limited class run 0,87-0,90 lambda (no joke) and make it through the runs - however all of them have a thermostat as it's not allowed to remove in the limited class.

There's also an issue of having a limited quantity of gasoline. We have a 19 liter tank topped off completely and it needs to work for a warm up lap, start procedure - the possibility of false starts mean there can be 2 repeat starts before you are allowed to refuel before the third one - and after that 15 minutes + 1 lap of pretty much WOT racing. So running a richer lambda actually in the GP class can lead to running out of fuel. I might switch them to racegas just for the comfort of running a leaner mixture - and it's also the reason we don't use E85, because you simply don't have enough fuel for a race.

Regarding the ECT - the stock ski (however running 0,4 bar and producing just north of 100hp) has a thermostat of 75C however everyone takes out their thermostat for better cooling. When presented with the question why they do it, I recieved the same answer from everyone - "because everyone does it and it aids cooling". They're a "back in time" community and in the build we're doing for next year, there Will be a thermostat in it.

I've also sent a question to Wiseco in terms of PTW clearances and I Will post the answer here when I do get one, but as I have limited experience with Wisecos too I recommended CP pistons for our nextyear project as I am familiar with those much better.