Burn Pattern

ChrisU · Post by **ChrisU** » Fri Jun 23, 2006 8:24 am

Doing a mid season check up on an engine the other day I was a little perplexed about the burn patter on the piston. It's a flat top piston, 15 degree Comp type head.... sbc.

There is a cresent moon shaped clean area on the intake side of the piston.... the burn pattern is only encompassing about 75 percent of the piston.

Is this fuel wash? Or Burn pattern? I'd like to address this... no pics had to put it back together on the spot.

Dan Barton · Post by **Dan Barton** » Fri Jun 23, 2006 3:58 pm

In my opinion, a clean spot is an indication of nothing there to burn, I think that the so called fuel wash theory that some have of the burn pattern is incorrect, I would be interested to see the fuel curve of this engine on the dyno and to know what the timing requirements are, this would help to understand how efficient this combination is and if it is somewhat out of whack, a high mass fuel flow and more timing that what would be considered normal for a given combination can help to point you in the direction of a problem with the design of the induction system.

automotive breath · Post by **automotive breath** » Fri Jun 23, 2006 4:43 pm

In theory the ideal mixture for engines is about 14.7 parts air and 1 part fuel, which should result in no oxygen and no unburned fuel going out the header. But we've long known that a richer mixture will produce more power. The reason is the charge contains mixture stratification or of varying air/fuel mixtures. The result is in lean areas in the chamber, some of the oxygen does not find fuel quickly enough to burn, and they remain unused or unburned. In other areas the air/fuel ratio could be too rich to burn causing wash. Another possibility is the mixture in the clean area does burn at a very lean air/fuel ratio, Darin Morgan suggest this in a recent article. This effects boundary layer, the mixture burns completely to the combustion chamber surface leaving no traces of combustion. In reality all of these conditions can exist in the same combustion chamber.

All this said we haven’t talked about piston to head clearance at RPM, squish velocity, wet flow, ignition timing and the conditions in the chamber when the flame front actually reaches the air/fuel mixture. Agreeing with Dan, more information is needed to determine if changes are needed.