Pistons, Clearances, Compression Ratios and Fuel

[apardal]

Hi again!

Sorry for the bunch of topics regarding similar subjects, but each one has new different questions...

I've just bought a set of wiseco pistons and I'll be receiving them next week, I hope. Ready to come is also some ARP bolts and studs for conrod, crank and flywheel. Going to use new OEM head bolts.

Crankshaft is going to be balanced along with the flywheel, pressure plate and crank pulley.

Block will be bored to fit the 81.5mm wiseco forged pistons.

OEM pistons broke after some 100k miles and this is how they looked like.





Engine setup was HKS 264/264 8.1mm lift cams, tsudo header and complete exhaust with 2.25", stock intake manifold with monsterflow air filter, ported head with polished exhaust, bronze valve guides, 3 angle valve job, 0.6mm decked head, 0.1mm decked block and 0.5mm TRD head gasket. Tuned ECU to raise rev limit at around ~8400rpm. I also have a piggy back (Apexi SAFC) ready to assemble if my wideband O2 sensor starts reading lean AFR's.

With this in mind, I googled a compression ratio calculator and found this one:

http://www.csgnetwork.com/compcalc.html

I measured my cylinder chamber volume and it is around 34cc's (not to precise, but helps to have an idea.

With 5.9 cc piston dome volume and 0.76mm deck clearance from Wiseco product sheet, I calculated 11.9:1 CR with a 1mm (minus 0.1mm for block skim) HKS metal head gasket that has 82mm of bore.

I really don't know if this will bring me piston to head clearance problems, might have to check on that when pistons arrive but hoping it will all be ok, I'm a little worried that 11.9 might be a little too much compression.

What do you guys think, and what fuel will cope with this new compression? In Portugal, we have available 95, 98 and 100 octane at almost all gas stations. Or would you just put a higher head gasket (1.6mm for example) and shave the head 0.1-0.3mm?

Best regards

[Lipinhu]

I would go for a 0.8 HG. Seems a bit too high CR.

[NoHachi]

I'd first try and find out what happened to the original pistons, do you have an idea?
This kind of damage to the ringland is pretty uncomon.

[apardal]

Lipinhu, you should read the post again. My calculations are for 1mm head gasket. Thinner head gasket would raise the compression even more. My question is between 1 and 1.6mm.

NoHachi - NoIdea!

Engine wasn't running lean. Was indeed running higher RPM. Might have ran with the ignition timing too advanced a long time ago that could have damaged the pistons. It wasn't water temperature. Oil temperature, I don't know.

Anyway... what I really want to know is what compression you guys think I should run? And with which gasoline?

Best regards

[NoHachi]

Its just that it doesn't seem like a detonation incurred failure. Closest thing I've seen to this was the time that I didn't allign the pistonrings correctly on my 50cc bike and ended up with similar damage. I'd do a extensive google search on piston ring failure before installing the new pistons.

Check this website for more info on compression ratio's:
http://www.guy-croft.com/viewtopic.php?t=164

Lots more info to find there, pretty cool little site.

[Rasteirinho]

Well, 11.9 is no problem if you have a good cooling and lubrication system.
Also good bolts and bearings should be fitted.

Because the temperature and pressure inside the combustion chamber will increase a lot compared to the original values.

Well, if i had the time i could calculate the new pressure and temperature... lol

However, also the ignition timing and fuel injection will have a big impact on the pressure and temperature values. So a good mapping is also requested.

[Ivan141]

Dont listen to NoHachi, this kind of damage to the ringlands is not uncommon on old 4age pistons.. I have 2 just like those here in a box from my first engine. Seems like an overheating failure, as the nr. 3 and 4 pistons got it a lot worse than the others. The ringlands are the weakest point so that's where they break first when things get tight.

[Denmark]

Yep,
That is pretty commen for all cars, that run too much advance,this is alsmost always what happens,

/Skassa