Tech Question - Why does my Porsche smoke but not VW's

[daepp]

I have heard many times that a "puff" of smoke is normal with an early 911. And mine has recently been rebuilt. Some of the most professional wrencehs around here have said that a new rebulit should not smoke at all, while others say some smoke is normal due to the peculiarities of the 911 - i.e. dry sump tank position, flat six design, or simply, all air-cooled cars smoke.

So that got me to thinking - none of the beetles or buses I've owned never smoked. What's the (technical) difference.

Thanks,

David

[pwd72s]

Dry Sump lube system on the 911...also, forged 911 pistons don't fit as "tight" as cast pistons like you'd find in a VW engine. Forged pistons need to be looser to allow for expansion...this allows oil to seep down into the bottoms of the cylinder, which burns on start-up, which means smoke.

[Todd Simpson]

I'm going to guess it's because the valves in the VW engine are parallel to the cylinder.

The 911 engine, OTOH, has intake valves above the engine. Gravity can pull oil past the valve stem seals into the combustion chamber.

[daepp]

Very interesting - I hadn't thought of either of these. How much do your cars smoke? And do they run rich?

[stlrj]

VW's don't have oil tanks full of oil that drains back oil into the crankcase when the engine shuts down filling it to the level of the cylinders. When a VW shuts down, the crankcase oil level never reaches the cylinders.

Cheers,

Joe

[RSBob]

Must be the same with the 914 motors based on Vdubs design (duh) 'cause mine never smokes on a start-up even after prolonged sitting.