I see
You want to muck about with the stoke to gain a little more swept area in the hope of getting more power and then shove a FZS400 engine in a FZR250, right?
Partially correct, we're planning to basically overbore the FZR400 from 56mm to 62mm with either YZF600 or FZS600 cylinders & YZF600 cylinder head - various rod/piston combinations/choices to get compression height spot on - this may go into an FZR250 or an FZR400 - TBD
Most gains are had by increasing the bore and shortening the stroke as Yamaha did with the YZF600 R6, going from 62 x49.6 for the YZS600 (Tcat as we know it) to 67 x 42.5 for the R6. Or decreasing the squash band, sort of increasing compression but gives you better mixing. Where as, I think you want to decrease (destroking) the stroke, to increase the swept area (by using YZF conrods/pistons or a mix of FZS parts) but that could well also lower the compression and will change the squash area? Or have I not read you right?
Correct a straight bolt on would drop compression.
We're taking an FZR400 crank for the 40.5mm stroke, then a possible combination of cylinder block and conrod and pistons @62mm bore - fortunately there's quite a few pistons available in 62mm, because a straight swap of say YZF600 top end, with the same squish band with the reduced swept area would see compression fall from 12:1 -> 9.78:1 - not good!
Just on the numbers before any actual measurement of the combustion chamber volume there's a 2.3cc combustion chamber volume change (reduction) required to keep the same compression ratio
YZF600 @ normal bore/stoke ratio has 12.5cc combustion chamber volumeYZF600 @ 62mm bore 40.5mm stroke needs 10.18cc combustion chamber volume to maintain 12:1 compression ratio
Fortunately we know the basic FZR400 crank as deployed in other bikes is good for well over 120HP, crank mains and conrod big end diameters remain consistent for ~30 yearsI've read of people, taking the YZF600 stroke to 54.8 with first release FZR600 cranks and 64mm bore for something well over 700cc and over 120HP
Silliness?
You make no mention of swapping the block 600 for 400?
The YZF600 and FZS600 blocks are essentially the same for our purposes, the 62mm Nikasil bore, cylinder head will be YZF600, as will carbs airbox etc
The difference in cylinder block height is of interest, we know with some accuracy that the YZF600 cylinder block is 87mm high, apparently the FZS600 cylinder block is a few millimetres shorter - TBD - this difference may assist us with a rod/piston combination that doesn't need any sillyness further in.
Hogging 4.55m off of the YZF600 block will mean cam chain tensioner blades become a problem as well as cam chain length coming into play, not to mention re-timing cams, which will get done anyway further in, but let's keep the mods, and potential problems to a minimum to start with...
Other than point 1 of your list, it makes no sense, the gains will be a minimal for the work and you'll more than likely have to play about with the fueling to get it to run right and produce power across the rev range. The power increase in the Tcat (15hp ish) over the FZS is all mainly in the head and intake (the block is angled forward slightly and the intakes are 90* to the ground i.e. straight down and in) plus the T cat revs faster. Basically from the crankcase up the YZF is a completely different engine. The R6 is totally different in all areas.
Everything from the engine cases up will be YZF600 - with the cylinder block dependent upon deck height and TBD
BTW, R6 rods share a common bottom end bore diameter @33mm, rod width unknown at this point and fortunately some of our potential candidate pistons for recovering the compression ratio are 16mm diameter also
That's the beauty of Yamaha, basically the FZR600/FZR400 engine cases were so well designed they're been deployed with all manner of crank throws for ~30 years in various bikes, and there's also consistency among those various cranks, with the stroke variations
I'm seeing someone in the early to mid '80's with the foresight to plan a roadmap that has advanced for decades and build accordingly
The only limit in there is the bore spacing, consider that the original FZR600 was 59mm bore and the only thing preventing them going larger than 64mm is bore spacing of adjacent bores
Unless it's all about point 1 I'd be more inclined to shove an R6 into the 250 frame now your talking, quite a bit a fabrication but it will go
See now, this is what happens, a few guys get to chatting on a forum...
Not motorbike specific, but definitely related, part of what reinvigorated this was project binky on youtube - I am assuming you may have seen it, fair warning, if you haven't it's addictive
I won't post a link so that I can remain blame free