Supraman786

Just like the topic states I am looking for more of a technical answer as to why bikes can get compression ratios of 12:1 and more where as car motors start detonating around there. I mean what gives bikes that ability to run 91 octane at those compression ratios? Any technical knowledge on the topic would be great. Its really been boggling my mind for quite some time now. Thanks in advance.

Slows2k

Most bike engines have much smaller bores than auto engines. The smaller chamber and bore size is more detonation resistant than larger ones.

xviper

FYI, there is a "Bike Talk" forum for this kind of topic.

RazorV3

exactly. plus the smaller size engines makes it easier for them to rev higher than automobile engines(mostly true).

mvf4s

smaller and lighter parts. compare a rc motor to a regular motor and it's even more evident.

VTEC_Junkie

i think you guys are misunderstanding. he is talking about an engine's compression ratio, not how high the engine can rev. i think what the original poster wants to know is how motorcycle engines can manage to use a higher engine compression ratio with 91 octane gas and not have problems with detonation, as opposed to car motors.

jsalicru

As stated by the others... smaller bore allows higher compression ratio as well as superior design to the combustion chamber itself. Motorcycle heads are typically designed to be very, very efficient.

Motorcycles use pretty interesting technology if you actually look deep into those motors.. pretty cool stuff.

Chui

Surface Area of Combustion Chamber: Bore Diameter

chrissa

It's actually a heat issue. Detonation occurs because the cylinder temperature gets too high and the gas spontaneously starts burning before the sparkplug even fires. The smaller motorcycle engines are much better at moving the heat out of the combustion chamber. Most of the bike engines are made completely out of aluminum which is better at transferring heat than cast iron is. They also have incredible cooling systems which are extremely efficient. Size really isn't the issue. There's no reason why you couldn't build a large high compression engine, as long as you could include a large enough and properly designed cooling system, and use the right materials for the engine.

Just taking a quick look online, I see the Honda 600RR has a compression ratio of 12.0:0, a 1000RR engine has a ratio of 11.9:1, the S2000 has a ratio of 11.1:1 (the 2.2L), the Ford Mustang 4.6 V8 (DOHC) has a ratio of 10.1:1, and BMW's 6.0L V12 has a ratio of 11.3:1. All have aluminum blocks and heads. The difference between the ratios is not as large as they used to be like back in the days of cast iron big blocks. The old rule of thumb that only small engines can have high compression is today becoming a myth. I think you're right that car engines still have a little catching up to do with the bikes, but not by a lot. Give it time; I bet we will see 12:1 compresstion ratios showing up in cars soon.

For a comparison, the Honda RC-51 is also a 1000cc engine (a v-twin), but it has the two most dinky radiators I have ever seen. It "only" has a ratio of 10.8:1, and let me tell you, it gets plenty more hot than any inline four I have ever thrown a leg over. Their full-on superbike ran a ratio of 12.7:1, had a massive front radiator four time the size of stock, and they used race gas in those bikes. Comments?

Chris.

E36Ap1

But bikes don't last as long as cars and damn they rev high.