My point exactly Randy, we must be talking past each other. That's Chamber 101, and there are many ways to skin the cat, but in the end, there is no such thing as an "FE chamber". Heads alone vary in design, but more than that, quench distance, piston design, builds are all over the map, and it's not just an FE thing. I would also tell you that with an FE chamber, the common 38 degrees is likely due more to the fuel than anything else...reason being is "it doesn't get much
worse than that" in terms of needing that much lead at WOT, even in other engine designs.
When someone thinks chamber, they need to think in 3 dimensions, shape, as well as how those moving shapes interact, and not only with flame expansion but also with churn and airflow. Although that is not the discussion here.
One very important word police point, there is no explosion in an engine, its a controlled burn with heat expansion that pushes the piston down. I am usually not a definition guy but 40 years ago, when I was about 12 years old, I used to argue with my dad about that during his builds, and guess who was wrong...not him LOL There were many times that I have admitted that over the years...
Back to the real story...if anything in my post made you think that I advocate more advance over less for some reason,
I do not. I advocate for the
right advance,
across the whole RPM and load range, which makes a significant difference drivability and power. The "right" advance for a compression, chamber design, fuel, rpm, not to mention other inputs like cam design, even elevation, etc, will never fight the piston. That was my point exactly, the burn will not expand fully until the piston is at the top of it's stroke, not when the plug fires, (unless you have too much advance, which you shouldn't, or pre-ignition) so my position is that pre-TDC rate of chamber volume change is less a factor with a long rod than dwell and the immediate start of expansion.
However, even THAT position I think is minimal, I think there is MUCH more to be gained with a tight quench and proper upper and lower quench pad cooperation and 3D chamber shape than with the slight change in dwell and acceleration with a rod change. However this post isn't going to change two old dogs that have slightly different opinions on how to skin the same cat with chamber design
FYI - It's going to be very hard to directly compare the two engines coming up on the dyno, but one is your 24 cc D-cup with a crappy D2 iron chamber and .041 quench, the other is your 24 cc dish with a TFS chamber and .041 quench. It will be hard to draw a conclusion based on piston alone, because all of the different variables in the engine, compression and cam alone are worlds apart, despite the cylinder volume (less the head, being identical) but it still may give us a little info on two very different chamber designs. It would be neat if total timing on both was lower than 36-38, no doubt the TFS head wsill be, but we will see what the iron head will do as a comparison. The issue of course is we know the TFS chamber works better, so we really won't know how much the piston contributes...however, we will know that a standard FE likes 36-38, and if it comes in on the low side, the quench and D-cup will have helped. We will have that info for both on 15 Nov.
If you want to BS about that, I'd rather put it in one of the "guess the HP" posts, or start a new post and not hijack here
