Originally Posted by Sportbike_Mike
You're close BR. It's also highly dependent upon how much boost there is, what octane fuel and most importantly the compression.
I read somewhere (at least for cages) that if your CR is more than like 8.8:1, you need to lower it or you need to keep it under 4 lbs of boost.
Not sure how well it would work on bikes, but one way to lower the CR in a cage is to either put dished pistons and/or a thicker head gasket. Not sure a thicker head gasket on one of these bikes would be feasible given how tight their tolerances are otherwise.
BR, the one that blew on the dyno, do you remember what octane fuel they were running?
Compression ratios don't work like that. What matters is the dynamic compression ratio which is determined by volumetric efficiency, how well you can fill the cylinder on the intake stroke. If you have a poor breathing engine you might be able to run a 15:1 static ratio and not experience detonation at all simply because the cylinder is never completely filled. A really good breather might detonate at 12:1 or less.
The compression ratio at which detonation occurs is different depending on a lot of factors including piston area, cylinder capacity, RPM, combustion chamber temperature, and above all how the engine is used. In a huge V8 car engine the compression is going to have to be a lot lower than a 600cc inline-four motorcycle engine.
Yes, you can use spacer gaskets but they're not the best way to do it.
I personally don't like reducing static compression ratio as you lose the off-boost engine performance, I would prefer to keep the stock engine performance and use less boost. Ride the turbo and non-turbo GPZ750R for an example, off-boost compression ratio of 7.8 and 10.5 - an inline-four 750 at 7.8:1 makes for a very sedate ride, even my old KZ750L has 9.5:1 :-)
But if you're chasing big hp then you need more boost and you must reduce static compression ratio to allow that.
Nope, but I'm sure both were using good fuel. I think the one that went bang on the dyno was increasing boost chasing bigger numbers, a very easy trap to fall into. Lots of turboed engines last just fine simply because the owners keep the boost below safe limits.