How can bikes be comfortable with high compression ratios.?

Question

Most bikes that I look up have close to 11:1 compression ratios, yet they can still use 87RON petrol. My car has 10:1 and the manufacturer recommends 91 at least, 96 at most.
Why can bikes have that high CR and still be comfortable with regular fuel, while car engines will knock?

Answer 1

This all has to do with valve timing. I remember cars in the early 70s with 12:1 compression ratios - but compression ratios are based on pure math, bot the actual amount of compression. It's basically the swept volume of the cylinder being compared to the area left inside the combustion chamber (plus a little to allow for the thickness of the head gasket.)

However, the engine doesn't actually start compressing from the bottom of the piston stroke. The intake valve doesn't close until after the piston has already started up the bore - the air's inertia means that the partial vacuum in the cylinder that sucks the air in has not completely gone away, so leaving the valve open a little longer lets the cylinder fill as completely as it can, thus giving the most power.

This is what gives motorcycle engines such amazing power for their size - but it's only efficient at high rpm (which explains why car engines aren't as powerful - they need low rpm torque as well as high rpm power.) At low rpm, the motorcycle engine will actually push some of its compression back out the intake valve. At high rpm, they are much more efficient - but it's still hard to get the air to move that fast, so they still don't actually fill completely - there always tend to be more inefficiencies at higher rpm.

Remember, octane reflects how much actual pressure the fuel can handle before exploding - which is only roughly hinted at by the compression ratio.

Answer 2

lower octane numbers burn faster and with the added force of high compression the compressed air fuel mix slaps the piston down causing spark knock. the higher the octane rating the slower the gas burns thus reducing spark knock.
I think that when the air fuel mix pushes down on the piston in a car it takes so much more force to start the piston moving because of the heavy weight of the car. but a motorcycle's lighter weight allows the piston to be pushed down easy without a big slap. so I'm guessing it's the weight of the vehicle as why.