redmaro1

To make a long story short, after taking apart the engine, one piston is damaged and the rings are broken, but the cylinder wall is intact. Can I replace this piston by itself? And just leave the other ones how they are, Id just keep the stock cylinder size.

Also whats the max compression for a 4.6L with PI heads?



1998 4.6 GT

GreyStang

What happened to the piston? So there's absolutely NO damage to the cylinder walls? Not even scratches?

Chopper

Just replacing one piston and rings without at least honing the cylinder will lead to oil consumption problems in that cylinder.You need to at least hone the cylinder walls to get a good seal with the new rings.Honing a cylinder without tearing down the engine can allow abrasive from the hone to get into the engine with disasterous results.

The piston damage could have been caused by a slightly agressive tune with the nitrous.That cylinder could have been detonating,causing the ring grooves to tighten up and breaking the rings.

Max compression running 93 octane pump gas would be somewhere around 10.5-1 or so.This would be static compression only.Dynamic compression changes with engine speed,intake mods,cam timing and characteristics,and exhaust changes.

redmaro1

Well after putting the new mods on the engine listed on the bottom, the engine ran very good for a whole week. On the weekend after taking it to the tuner in the morning to get it tuned, we then went back in the afternoon, and he told us it had a tapping sound. So after a few days we took it apart and found a found the damaged piston. THere were also like reallly smal chips on the other pistons, and only one piston was damaged. The tappin sound seems to come by the valves hitting the piston slightly.

Mods intstalled:

Ported & Polished PI Head SWAP
Crower Stage 2 plus Cams

And all the mods in my sig were on the car already.


It was being tuned by an "authorized" Diablo Sport tuner.


So if I hone the cylinder and just replace it with another piston and new rings it should be fine?

GreyStang

Geez dude ALL the pistons have TINY chips in them?? Either the cams or valvetrain aren't compatible with your engine or someone revved the SNOT out of the engine causing the valves to float & hit the pistons?

"tiny" imperfections in the pistons could lead to hotspots and potentially detonation

redmaro1

Well the engine was working great for a whole week, with no problems at all. All the problems were there since the tuner gave us the car back. Probably over revved it or just tuned it wrong and moved timing too much or something.

Chopper

Yep,since the tuner had it when it blew I say it's his fault.
Getting them to fix it will be another story though.

By "chips" if you mean there were particles left in the engine when you took it apart the whole engine neds to be rebuilt to be reliable.
If there were just little marks on the piston tops and in the combustion chambers it could have been caused by debris working it's way around the engine.
I've had this happen myself.The broken piston crown came off and after smashing the front cylinder sparkplug gap closed,went through the intake and into the rear cylinder and finished it off too.This was in my Harley BTW.
Nothing made it to the bottom end however,and the crankcase wasn't rebuilt.
Since the cylinders don't come off a V8 you ain't going to be so lucky.I'd try to work something out with the tuner and get a discount on some engine work.It sounds like they got a little too aggressive with the tune and broke the engine.They might try to say that since you weren't there to supervise,they didn't know how far to take the tune,and was just trying to get the most out of your engine.
I don't agree with that.It sounds like they were "in school" with your engine and the lesson went sour.

Don't be surprised if there's a sign somewhere in the shop releasing them from damage claims when dyno tuning a car.You might have even signed your name underneath some fine print containing the same release language.