> Mikey Antonakakis Wrote:
> No, if they milled the topd of the cylinder it would only change your
> compression, not timing. Unless they cut down the base of the cylinder
> the timing is the same, and if they did cut the base, your timing would
> be less and your bike would be slower as a result.
They did mill the top of the cylinder so with a single base gasket it would increase compression, however since I'm running 5 base gaskets the compression stays the same, the ports are just raised so they're fully exposed at BDC
> Higher compression ratio is generally a solid benefit for a stock-ish
> setup, you might see about a 10-15% improvement in power across much of
> the rev range if the stock compression ratio was really low. But that's
> not going to make you much faster, that's worth maybe 3 or 4 mph on the
> top end, and slightly better acceleration.
I used a digital caliper to compare the stock FZ50 head to the milled FA50 head and it looks like they took off right around 2mm. Taking the stock compression of 6.5:1 and factoring in the 2mm reduction it increases compression to 8.8:1 (if I did my math and equations right: 6.5x=37.4+x where 37.4 is the stroke. Subtract x from both sides and you get 5.5x=37.4 then divide by 5.5 and x=6.8mm at TDC. Subtract 2mm that was milled off the head and you get 4.8mm. Then the equation for the new compression ratio would be x(4.8)=37.4+4.8 >> x(4.8)=42.2 divide by 4.8 and get x=8.8). I understand this is still not the 11.5:1 that my Aprilia has but there's not much of a way to get any more compression without a different head design (the piston crown protrudes above the top surface of the cylinder as it is right now)
> To make more power, you need to get more air into and out of the
> cylinder, and that only happens with improvements to ports, reeds,
> carbs, and pipes. If you add a base spacer, you can bump up the timing
> of all the ports, but that's not the ideal way to do it because then
> your port floors don't line up with the piston crown at BDC, and that
> hurts the flow (more important for transfers than exhaust, though).
I did add a base spacer in the form of 5 base gaskets because in stock form the port floors sit 2.6mm below the piston crown at BDC (per the Performance Tuning wiki). With the 5 base gaskets the ports are fully exposed at BDC
> One relatively easy way to get a good bump in power is to raise the roof
> of the exhaust port (and widen it as much as possible - 70% of bore
> diameter max). But this will give you a peakier power band because it
> increase your blowdown. This strengthens the effect of your pipe, but in
> a narrower band of revs - at resonance you make more power, but outside
> of it, you make less. If you have CVT then it's not a big deal, but with
> a one or two speed you can pretty easily end up with a power band that
> is far too narrow, that makes so little power outside of the peak range
> that it doesn't let you accelerate up to the range that the pipe hits,
> and even perfect clutch tuning can't fix it. I can draw a graph if you'd
> like, but as you accelerate, the power required increases exponentially
> (as a factor of speed cubed). Even if your peak power is enough to
> sustain whatever max speed you're geared for, if at any point below that
> you make less than the required power, you won't be able to get to that
> top speed.
This makes total sense, but I think for now I'm going to avoid raising the port roof of the exhaust port to keep a wider RPM range and not have such a peaky powerband. If I find I'm not making enough power to pull through the gearing and get up to speed then I might try raising the exhaust port roof at that time, after first doing as much clutch tuning as possible.
> So as I've said before, the cylinder you've got *might* be capable of
> supporting a pretty high top speed - 70 is out of the question, 60 might
> be theoretically possible if you cut additional transfer ports into the
> cylinder - but your power band would probably end up so narrow that you
> wouldn't be able to accelerate up to that speed, even with a two speed.
> This is why I'm curious to see what your bike is capable of the way it's
> set up right now, having that baseline is critical to figuring out how
> to go faster. I'm guess you'll be right around 40mph in a flat road/both
> directions/no wind/no drafting/GPS situation. If so, we could probably
> help get you to maybe 50 if you're lucky, but to go faster than that
> you'll really want to find a better cylinder.
I'm also curious to see where I'm at currently so as soon as the weather breaks I'll definitely get on with the test rides. Since the FZ50 with these mods can hit 40 with the taller final gearing I'm thinking I'll probably be more around 50 with the FA final gearing and properly tuned clutches, with 60 being the end goal