Forged pistons and cold mornings

[Pyrotechnic]

I have Mahle forged pistons. When it's cold out, I get some rough running and some oil smoke out the exhaust on start up for about 2 - 3 minutes. Not excessive smoke, but there is some. During this time, it's very unwilling to rev and basically undrivable. It feels rough, but it stays running just fine. After this time passes, you can hear and feel the motor start to smooth out and the idle speed starts to increase and the oil smoke stops. At this point it's ready to drive. No smoke at any other times other than a cold start on a cold morning. Motor runs excellent and makes great power. On a cold start on a hot day, I do not have this issue at all.

What it seems like is the rings don't seal completely till the piston heats up and swells. Anyone else ever experience this ? Just making sure this is normal behavior for forged pistons.

I also have an RPM Air Gap intake which I suspect doesn't help the cause, being that fuel will tend to puddle rather than atomize in the cold. One thing I did was put a stock air cleaner on with the hot air tube against a header tube and it does seem to reduce my warm up time a little. The plan is to eventually make an enclosure for my open element air cleaner that will give maximum flow but allow for the hot air tube on startup.

[hotrod 80]

My TRWs do the same thing .

[custom72c20]

Your getting piston slap. The piston is bouncing off the skirts. Its not uncommon for forged pistons to create piston slap when they are cold, that's why most vehicles that are daily drivers don't use forged pistons.

[Marv D]

The Mahle forged, coated pistons I used for the 383 I built for my 94 tow vehicle is the same way with a LOT of piston slap in the mornings. I wanted to blame it on the guy that designed / burned the chip missing the startup tune-up. I'm getting black/blue smoke in the cold mornings. But,, the piston slap says it's more an issue of piston/wall tolerance. I would also like to blame it on the machinist, but more and more people are talking about QC in the Mahle pistons being a little 'loose' lately. I don't know,,, not anything I can't live with, I'm good about building temp before I start demanding from my vehicles. But sure is frustrating ain't it.

[swb85]

Forged pistons expand more than non-forged, thus requiring more piston-cyl wall clearance. When it's cold, you've got excessive clearance and the skirts slap a little. Perfectly normal. Not sure about the whole smoking thing, think you have something else going on there.

[hotrod 80]

Im with Marv on the warm up before work thing . I usually do this when i warm the gear ( probably not a problem in AZ ) I also drive a powerstroke , so im used to warming up first . My wife and i go out to dinner and she's like " What are you waiting on? " . Oh , and the machinist should have finished honed the cylinders to each piston as they will all have variences even if to the 10.000th .

[Pyrotechnic]

Well this motor is a 383 with 6 inch rods, so these pistons don't have much of a skirt. They are a very short piston. I suppose this doesn't help the piston slap either ? It seems like there is a temperature threshold for it to happen which makes me think expansion/contraction of metal. Even if it sat overnight, if it's a warm and sunny day, it starts and runs like a stock motor.

The smoke isn't too bad. When I build that custom air cleaner, I'm planning on using dual hot air tubes, one on each side, to see if I can speed up my warm up time anymore.

[scatesracing]

Yeah it sounds like hell..... nature of the beast mine did it to... the funny thing is my 99 gmc has done it since it was new.... it now has 120,000 miles on it