I pulled my stock 99 head off a couple days ago and wasn't all that excited about the way the pistons looked. My old motor looked brand new when it came apart but this one has a lot of carbon on it. Is this the result of a bad tune? Detonation? Any advise on what to change?

Stock 99 motor w/ 130k miles, gt2560 @ 12psi the last 5000 miles or so. I'm running lean at cruise, around 16:1 then it tapers down to 11.5:1 at full boost.

 

there looks to maybe be det on the edges and middle of the piston tops.

 

grimy...

 

How did your plugs look during the times of running this tune? Your spark would also be helpful for cruise and full boost. Do you run WI, intercooler etc?

I'm running 11.8:1 at full boost (only 7psi though a charger) but 15.5:1 at cruise. My plugs are a nice tan colour.

 

I'd take a coarse cloth and wipe the carbon off, that's the only way you'll really see the condition of the metal.

Mine went from-


EnginePiston3Flash by AnonymousNamelss, on Flickr

To this after a wipe down-


IMG_8350sm by AnonymousNamelss, on Flickr


I know I had a lot of carbon...from running rich and burning oil...and yes, the 01+ 10:1 comp. pistons are supposed to look like that. They don't have the nice machine marks like all the other 1.8/1.6 pistons. The only detonation are the very light pot marks on the right cylinder between the exhaust valves. The other piston is det. free.

 

I see detonation, maybe you should pull some spark or run a tick richer on cruise.

 

That was my biggest surprise, when I pulled the motor out of my old car it looked almost new. I was concerned to see so much carbon junk.

The plugs looked fine to me, but I'm no expert. I can post some pics later. Spark is 32-36 at cruise (depending on RPM) and tapers to 16 degrees at WOT. This is on 93 octane (which looks to convert to 98-99 on your side of the pond) so I don't think it's terribly aggressive. I've got an intercooler, no WI.

I'm putting in my '95 bottom end so hopefully the slight compression drop helps a bit. I think I am going to add a bit more gas at cruise though. Thanks for the input guys.

 

I don't see evidence of detonation but it does look like you are burning oil. Dopple is right about removing carbon to really see what the pistons look like. What does your IM look like inside? Is it oily? Are you drawing oil through your PCV? Do the intake valve stems have carbon build up on them? Are you running adequate crankcase ventilation?

Carbon build up on your pistons will cause hot spots that will promote detonation.

 

Double check TDC. IF you aren't really at 10 degrees of timing when you set your MS to 10 degrees; You could end up with big problems. It looked like det to me, but if not, that's a lot of carbon for only 5k.

 

I'm interested to find out for sure if it's det or not. I run 38-39* of timing during cruise and my boost is not as significant as yours at only 7psi but I also don't have a huge restriction in the exhaust path either