angle top pistons

[dynajohn]

Mike I have a question concerning when to use angle top pistons versus flat tops. In large bore high displacement Harleys (4-1/8 & 4-1/4 inch bores) is there any advantage using angle top pistons with a larger matching combustion chamber over flat tops with a smaller chamber? The assumption is that both sets of heads are set up properly.

[Mike]

Typically with large bore motors we try to put in fairly large valves (2.00-2.500 dia). The large valves in the small oval chamber end up being shrouded on either side by the proximity of the chamber walls, reducing flow. When we pull back the walls to regain the flow we then loose the quench area and gain cc'c (lowering compression). By going to pistons with an angled, pop up, crown we can regain the compression, unshroud the valves and still retain a respectable quench area. A 15 degree angle is generally the best but does not give enough pop up for compression or provide enough quench area so the 20 or 30 degree angles are used even though it's something of a compromise (thinking flame paths). Our head porter (Baisley Hi-Performance) has a CNC lathe program to match his head chamber shapes to most piston configurations we use. Sometimes we buy unfinished pistons and machine for our chamber shape (we means Baisley) or have specials made, as in the case of the 110's. All of this requires specialty machine work, adds cost, time and must be done by someone who REALLY know's what they are doing.
Mike

[dynajohn]

Using the 103+ HD head with 2.080 intakes and 1.623 exhausts and retaining the stock valve sizes as a basis for porting work for a 4-1/8 or 4-1/4 bore motor with a 4-3/8 stroker crank (117 or 124 inches) would you choose to use angle top pistons instead of flat tops? Would the 4-1/4 bore motor be more likely to call for the angle top pistons than the 4-1/8 bore motor in this application? The application is for a dyna and the cam choice is to either use a Woods W-68G advanced 4 degrees with 1.7 Baisley rockers or a T-Man 725-G cams at 11.5/1 compression. The stock 103+ chambers which are 98cc would have to be milled and the chamber volume reduced with flat tops to get enough static comp. to get these cams to work. In you're opinion would these smaller chambers mask the valves to much with the flat tops in this application and would angle tops be a better choice?

[Mike]

I would go with the angle tops to achieve the compression, quench and unshrouding. Even better as bore size goes up.
Mike

[dynajohn]

thanks mike, sorry for the repeated questioning on this subject.

[Mike]

No problem. Mike