Quote:
Originally Posted by cuong-nutz
One thing I am still wondering: could the sharp shroud edge inbetween the intake valves cause predetonation? I would think being that it's on the intake side it would be cooler than on the exhaust side.
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I had talked with a couple of old (both in age but most importantly in experience) engine builders that used to prepare small and medium size race engines for the beginning of the 4-stroke era and when I showed them the 250 head one of them laughed... the other got quite angry with my ignorance...
they both told me -more or less- the same thing :
"...You spent so much time, effort and money porting the intake while this thing still has the same awful chamber the eliminator had... if you really want flow through your chamber, less scavenge and negative work on the piston face you have to get rid of those edges and smoothen it out like a bowl...you will lose some compression but you can put it back either by milling it (the head) or by using the zzr250 pistons (which is the cheapest solution for getting higher compression pistons since they have a higher dome) or both if necessary...(of course they added that aftermarket pistons would be even better)"
but the whole point is that if the chamber does not have a good flow through for exhaust fumes to exit with a proper vacuum effect so that they help pump in the next intake mixture wave (and have that "wash off" the exhaust gas remnants during the overlap phase), then both flow AND chamber charge is poor so lots of power is not even produced while the potential is there...
needless to say, all that definetely can (and most probably will) cause predetonation among other unwanted stuff (swirl or tumble blown in pieces before even reaching the piston top, hot and cold spots, weird flame travel etc)some of which I don't remember now and will have to ask them (again) while at the same time will have to listen to them (again) cursing at me...
so this time, when i get my 300 head in hand, instead of growing the ports, I will just take off some sharp edges and grit and work on the inside-the chamber and round the valve seats area to make it more "fluent"...
P.S: I asked around about the notch Cuong-nutz was wondering about (so was I...) and the answer is quite as simple and evident as it looks after all... it is for relieving air pressure under the piston, to move more easily from under the one coming down to the other going up, helping a bit with revving and reducing power losses, just like in the bigger 4-cyl engines....
seems there are many tiny but significant improvements in the 300 engine after all !