^^+1
anyone actually run and engine with these yet?
4 so far, all work fine. Adds significant torque, and make less heat than the Wiseco pistons. Do get the HD studs from Big Jay (or the same ones from Z1 Enterprises) if you go this route, to reduce tendency toward oil leaks at the base gasket and head gasket in the future.
Hondaman do you want to wade in once with an opinion on these pistons once and for all in an F2-F3 application so maybe it can be put into the record and maybe to bed also ??
Well, maybe....I don't think I'm anyone's "final authority" when various opinions pop up.
The 836 pistons are what I call 90%-ers. By this, I mean they need a little TLC in the finishing department because of some flash and rough edges here and there, and sometimes some weight-balancing work (if you are as picky as I am). The oiling holes on them should all be hand-chamfered a bit, as they are drilled, but not fully finished like ART pistons. And, most of the rings must be hand-filed on these pistons, as they are about 0.5mm too big. I've got a post around here somewhere about all this, when Z1 Enterprises first introduced the 0.5mm and 1.00mm oversized ones from this same Thailand vendor: the ones from Z1 are better-finished, with fitted rings, and look like "completed" versions of the ones sold by CycleX as "Bobber or chopper" pistons (which resemble these 836 versions in roughness). I've had some discussion with Z1 as to whether they might venture into providing the 836 versions in a more finished style, but nothing is settled over it, at least not yet: they didn't sell many of the oversized ones this last year and have lots of CB750 inventory on hand as the result.
I've heard all sorts of chamber volume numbers here and there, which hasn't helped the discussion: the stock K0-K6 heads are 22.5cc or so, which yields a final static compression ratio of [9x((836/4)+22.5)/(836/4)]=9.97:1, all else being equal on a K0-K6 engine. This leaves lots of room to add a quench band (about 0.7cc extra) and trim back the swirl shrouds (perhaps another 0.3cc) and still have more CR than you started with. Typically, the measured compression on one of these 836 engines after break-in is around 140 PSI, while the Wiseco (10.25:1 advertised) versions make about 155 PSI.
Part of the published compression ratio of the "F" engines is not from static compression, but from delayed cam timing (5 degrees later than K0-K6). In the various versions of the F0/F1/K7/K8 heads I've been working lately, the chambers are a hair over 23cc, stock. Those engines' pistons domes are approximately 0.7cc in volume, which would resemble more a K0-K6 engine with a 0.008" milled head than anything else. The delayed cam adds a dynamic compression increase at engine speeds above where the incoming charge can force its way in against the exhaust pressures during overlap, which gives those later bikes that "rush" feeling at 6000+ RPM (that's about when the cam starts getting effective).
The F2/3 engines are the 'special case', where the larger chamber volume comes from the bigger valve(s) being squeezed in. Honda wasp-waisted the chamber a little to try to keep this volume down, but in some ways it wasn't enough for even the stock engine to work as they had probably hoped: it meant they had to remove most of the swirl-charge ridges to prevent an incoming air dam below 5000 RPM that would have made the engine fall more flat than it does (in the F0/F1) at those speeds. The K7/8 solved this by using smaller valves.
So, all of that said: I have an untouched F2 head sitting on the floor in my room, will measure the chamber volume this week to see how these flat-domed 836 pistons would work out in CR. This was planned anyway, for the upcoming book on CB750 Performance, which is to be the hotrod supplement to my first text. So far, I have only put Honda pistons in these F2/3 engines in the past, simply because they don't often show up in my stable for performance work. In the last 3 years, I've done more K7 engines than anything else, oddly enough.
My personal philosophy is to make HP with flow, not extra compression that isn't strictly needed (in other words, I like to make touring bikes). The extra CR always adds heat up high in the engine: while the 750 cools (overcools) exceptionally well in this regard, the heat is responsible for the shorter valve guide life in the later engines (along with their crummy guide materials). Compression does not return as much HP as flow: for a 10% CR increase you might get 3% more HP with hotter heads, but for a 10% flow increase you will get nearly 10% more HP, with cooler heads and cylinders. More displacement moves this heat downward, into the cylinders, so it is managed differently overall. These things all matter together, in the end.
