Compression ratios - Comparing Specs

Port8080

Port8080

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I was hoping someone could explain a bit more about compression ratios and how it can help when looking for a bike.

For example, The 2009 Buell lightning has a compression ratio of 10:0:1 and the 2009 CBR 600rr has a compression ratio of 12:2:1.

Which is better?

What does it say about the engine?/ Performance?

Newbie <-------- lol.
 
The compression ratio is just the ratio of the volume between the piston face and top of the cylinder at the bottom of the stroke versus the same volume at the top of the stroke. A higher number generally implies the motor is in a higher state of tune and can theoretically extract a greater amount of the potential energy from the combustion process. You’ll typically see “sportier” motors with higher compression ratios.

Shouldn’t even factor into your buying decision at all. Are you looking at these 2 bikes as first bikes? I would recommend against either of them in that case. Spend $1-$2K on a used 750 cc or less non-sportbike, checked out by a trustworthy mechanic or knowledgable friend, ride it (and drop it) for a season or two, then decide what bike you want to get.
 
Very good question.

A higher compression ratio will enable the engine to develop more torque across the power band. But one has to consider the engine configuration; eg: V2 vs. I4 and air cooled vs. liquid cooled, before bumping the compression up. There needs to be enough capacity in the cooling systems to handle the higher compression, be it; liquid, air or oil. Typically an air cooled engine will have a lower compression ratio than a similar liquid cooled one. The reason being the liquid cooled motor will have fewer and smaller hot spots, if any at all. An example of this is a air cooled narrow V twin; Harley. To performance tune one of these the rear cylinder runs hotter due to less air flow so the tuner will add more fuel to the rear cylinder to help it run cooler.

Another factor of how high compression ratios can go is squish area. That is the area of the head of the motor that "collects" the air/fule mixture. The squish area is perhaps the most important aspect of a motor. A poor squish area will not concentrate the air/fuel mixture well and be a cause of early detonation. To counter this timing is retarded and the compression ratio is dropped. Most new Japanese and European bikes have very good squish areas and cooling so the CRs are pretty high.

If you want to read more pick up a copy of Four Stroke Performance Tuning, excellent read.

[ame=http://www.amazon.com/Four-Stroke-Performance-Tuning-Bell/dp/1859604358]Amazon.com: Four-Stroke Performance Tuning (9781859604359): A. Bell: Books[/ame]
 
If compression ratio is used exclusively as a means of determining if a bike is what you need or like, you'll be "shooting in the blind". As already mentioned, the higher compression bikes are usually the sport bikes and a bit higher strung than the standard or cruiser style bikes, although it seems like more and more of the standard style bikes are running compression ratios above 11:1 and I've seen quite a few v-twins running 12:1. You should be more concerned with the style of the bike and whether or not it fits you (physically) and your riding style, if you have developed one. Also as already stated, buy a used bike in decent running condition, ride it for a season or two, then get more serious about buying something you plan on keeping and riding for a while. The main reason there are so many good used bikes out in the market is because the average newbie sees someone else with a bike who thinks it's the cat's meow, buys one, then discovers it's not what he/she really wants and many times, decides the motorcycling thing isn't either. Be patient, maybe take a few rides or demos if you are licensed or have friends who would trust you taking a spin on their bike. You'll be glad you took your time and diddn't fritter yur money away.
 
I was just curious on compression ratios and they are not a deciding factor in me purchasing another bike.

To me, when I get off of my bike, all I want to do is get back on it. Last saturday I road for a total of 8 hours. I just couldn't resist getting back on the bike. No matter what bike I ride I have always had the urge to get back on it again and I hope that will always be the case. Whether it be a cruiser , sportbike or supersport.

Last saturday at around 3am I realized I wanted a bike that was a little more nimble and lighter. I want to upgrade or downgrade to a bike that is meant to redline ( push high rpms before shifts) and has a lot of torque.

After bumping into some other bikers at the local hang out spot, we started talking about bikes and one guy brought up how the CBR 600rr would be a possible option. After trying out the seat and things it seems more comfortable then the r6, gsx6r and the zx6r.

Any opinions?

would going from an 07 suzuki katana 750 to a 07 CBR 600rr be a downgrade in terms of speed?
 
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