Anyone make high-compression pistons for a 300?

[Guest]

Just putting feelers out for now, but if I do the 250 cams on the 300 it'll fare a lot better with something better than the stock 8.9:1 compression. The 650 has the same bore, but it's a 2-valve. Google has been of no use in this search.

[chef1366]

What the piston size and wrist pin size?

[GregT]

If it's the same as a GS650 it's 62.0 bore.
Some options, research needed, piston catalogues to be consulted...

Honda - CBX550 is 59.2 so look for overbores. CBR600 1st version is 63.0...
Suxuki GSX600F - effectively the first version GSXR600 - is 62.6.
Later GSXR600's may be worth looking at too.

[Ric]

Sorry, been a whirlwind of activity lately.

Yes, the cylinder bore is 62.0mm, and the pin dimensions are 16 x 49.5mm.

[John Park]

On a cylinder this small a mm is a lot. By my calculations, the combustion chamber should be a bit more than 15cc and a 1mm skim is 3cc so .5mm would get you somewhere close to 10:1 like the 250s. That won't remotely upset the cam timing or drive.

There is a question of valve interference even if you don't change anything due to the greater cam duration. Subbing in the cams will determine that, and if the valves don't hit with the head gasket off you can take a 20 thou skim.

Something to consider, anyway.

[Ric]

John Park, I like the way you think!

The lift on the 250 cams is identical to the 300 cams - only difference is the extra 20° duration. So it's all in the camshaft timing as to whether the pistons contact the valves. I definitely wanna get that right. If possible, it sounds like a thinner head gasket would be enough to up the compression to where the hotter cams would be more effective.

[GregT]

On a cylinder this small a mm is a lot. By my calculations, the combustion chamber should be a bit more than 15cc and a 1mm skim is 3cc so .5mm would get you somewhere close to 10:1 like the 250s. That won't remotely upset the cam timing or drive.

There is a question of valve interference even if you don't change anything due to the greater cam duration. Subbing in the cams will determine that, and if the valves don't hit with the head gasket off you can take a 20 thou skim.

Something to consider, anyway.

A 20 thou skim will retard the cams around 3 degrees (as a rough estimate) It would be worth dialling them in to 105/105 lobe centers too.
Do a trial assembly - you're looking for around .035in valve to piston clearance when assembled at running clearances.

I had a customer GSXR1000 race engine in, on which a previous owner had taken 20 thou off the head...All the inlet valves had touched the pistons.
To recover it, it was full set of valves, recut the pistons to suit - and put the valve springs in at the correct height which hadn't been done either...
All because of an unthinking 20 thou off the head...

[John Park]

John Park, I like the way you think!

The lift on the 250 cams is identical to the 300 cams - only difference is the extra 20° duration. So it's all in the camshaft timing as to whether the pistons contact the valves. I definitely wanna get that right. If possible, it sounds like a thinner head gasket would be enough to up the compression to where the hotter cams would be more effective.

While GregT is absolutely right; you must do the piston valve clearance check, I'm not so concerned that 3 degrees is worth degreeing the cams for. For racing, sure, but retarding the cams slightly isn't going to do that much and the cam timing shifts a bit as the block expands anyway - the other direction. Piston clearance increases as well with heat, but don't count on that.

GSXRs are a far more radical thing than a GS300 and I'm expecting that there will be enough room; OTOH, those valve pockets in the pistons are there for a reason. Two valve hemis can have the valves tangle with each other, but 4 valves have the valves pointing down at the piston.

Strangely, the GSX head gaskets, 400 anyway, don't have any sealing rings at the outer stud holes. They don't seem to leak; perhaps because there is no rubber there to break down. The 400 ones measure .045 and .050 at the fire rings. If you have time to tinker you could fashion a gasket from .020 copper just like the old British twin gaskets were made. At the worst, you would get some leakage but get enough info as to whether you should skim the head.

Skimming the head is a one way deal and spare heads are probably scarce by now. You also should consider that 9:1 is a respectable street compression and the 400s [10:1] needed 92 octane to avoid ping. At least mine did. But they also probably tend to run hotter. A 3 degree retard of the intake cam will reduce effective compression a bit which may be more octane friendly.

I also wonder what it was they did to lower the compression on the 300 compared to the 250. Boring it larger will increase compression, so they probably reduced the piston dome. Whether that increases the valve to piston clearance is the question.

I wouldn't expect an increase in low end torque with 250 cams - maybe the opposite. But you already know that these motors don't get very productive under 6k anyway.

Bravo on the 6 speed; wonder why they bothered to leave it out.

[Ric]

On the 6-speed, I have no clue why. Unless they had the crazy notion that folks wouldn't take the 300 much over 45 mph? :P

I'm no professional engine builder, but piston/valve clearance is something I'm very aware of thanks to Car Craft and Hot Rod magazines. If I'm not mistaken, the 3° retard will put the power curve higher in the rpm range. I'd be quite okay with that, seeing how even liter engines aren't built with off-idle torque in mind.

Looking at stock photos, the 250 piston does have more of a dome than the 300 does. The 300 is almost a flat top - I like that actually, since it makes for fewer hot spots and doesn't shroud the flame front like the 250 piston does.

And while I wouldn't push it like they do, some of the folks over on the Vmax forums say they've ran 87 gas in bikes with something like 12:1 compression with no noticeable effects. Worst case, the oversquare design of the 300 has more of a "squeeze" compression stroke than a "slam", so it's already pretty tolerant of low octane gas. And if it does start pinging I have access to 100% 93 octane here, and $.50/gal difference in price doesn't really break the bank when you fill up on 3.5 gallons.