Advantage to low compression engine?

Question

I just got a yamaha Road star today and i noticed the compression ratio is 8.38 to 1 and the other cruisers in the star family have 9.0 to 1 or higher compression, it also has lower fuel economy. What is the advantages/disadvantages to a lower compression engine on a motorcycle.

Answer 1

The engine is essentially an air compressor. The engine is gaining air, mixes it with fuel and then the mixture is compressed and ignited. The additional pressure created by burning fuel, increases a factor directly related to the magnitude of compression: increasing compression increases power output. But there is some limit to the possible compression. Too much compression is too rapid combustion of the fuel mixture, and it comes back to us like a detonation scenario with too hot spark plug.





The value of compression is determined by the number and thickness of shims between the head of the engine and the top liner. In fact, it is determined by many other factors, but only one easily changed is the rise of the head through the pads. More seals = less compression, less seals = more compression. Removing or replacing gaskets on a thin gasket will increase compression. Some engines have only one seal, in this case would be unwise to run it at all without pads. Moderation is the key. Move slowly and make small changes that minimize the risk of engine damage. First, make sure that the piston stops against the head of the engine, if you remove the pad (or pads).



NOTE: After adjusting the compression with a gasket may need to replace incandescent candles. Engines with higher compression prefer colder candles and engines with lower compression prefer a hot candle.

Answer 2

Low Compression Ratio

Answer 3

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Actually all European cars run on a higher compression. The reason is that we drive a fuel with a higher "knocking resistivity" (higher octane) so it is possible to compress more, hereby delivering more torque without having the dangerous uncontrolled explosions (instead of a controled fast combustion) which damage the motor in the long term. The octane 95 or 98 is really standard. Moreover we drive a lot of diesels which have an even much higher compression, meaning that by producing biodiesel instead of bioethanol we will be able to drive more "green miles". Don´t forget that ethanOL means it is an alcohol and therefore contains oxygen. In turn you have a lower air consumption but a higher fuel consumption (since it contains the oxygen which recombines). That is the reason why ethanol has a lower energy density. Flexfuel means that you adapt to a lot of different fuels... including the regular US gas which is not good regarding its low knocking resistivity. Several systems, especially electronic ones can nevertheless correct this like an earlier ignation. I personally believe E85 is not really the way to go and (bio)diesel is still and will remain superior.

Answer 4

You can run it on crap fuel. going to mexico anytime soon? Your bike will run just fine on what they sell there.



In the US, you can run it on the lowest octane (cheapest) gas at the pump. Keeps more money in your wallet.



That's really all it means. Fuel economy comes from your right wrist more than anything else. Most bikes can easily get 40mpg if ridden with economy in mind. Some quite a bit more, but that has more to do with weight, gearing, aerodynamics, speed, use of acceleration, etc than compression ratio.



Higher compression alone does not automatically equate to higher RPM, higher fuel economy, or anything else. It's simply a measure of the combustion chamber pressures. Compression ratio taken together with all of the other parameters of an engine, can mean better or worse fuel mileage, more or less power, more or less torque, more or less driveability, etc. Mostly what it means for you as an end user is what octane of fuel you need to use. Low compression ratio generally means low octane fuel can and should be used. High compression ratio (generally over 10:1 or 11:1 means you need to be using a higher octane fuel to prevent knocking and damage to the engine.



Engines with turbos or blowers have lower compression ratios for a variety of reasons. High RPM engines often have high compression ratios for a variety of reasons. Big VTwins with oversquare combustion chambers often have low compression ratios for a variety of reasons. Engine architecture is very complicated and any one change usually means a variety of other changes for that combination to work in that engine.



If you really want to learn about all this stuff, start reading about how engines are made. Follow their history. Follow the early builders and what they found worked and what didn't work and why. It's rather complicated, but fascinating.

Answer 5

Don't worry about it and ride is my answer.



I recently had to do a top end overhaul on my old BMW R100RT. In the process I dropped the compression ratio from 9 to 1 down to 8.2 to 1. The only reason I did that was because I already had the parts, so I saved a bunch of money. The drop in performance was barely noticeable, and my gas mileage didn't change a bit. The big advantage is that now I can run 87 octane, and my engine doesn't ping.

Answer 6

The above answers are all correct. In addition, cost and weight are a factor. All other things held constant, a higher compression motor puts more stress on the crank, cylnider walls, etc. The decision by the manufacturer is to add stronger parts at more weight and more cost, or accept a higher failure rate, or some combination of both. Most people who buy cruisers don't buy them for power and won't push them to the limit, which is why most of the cruiser market is low compression.



To specifically answer why your bike has lower compression than the rest of the family, the Road Star is their "entry level" large cruiser and entry level competes on price, so you're less likely to get performance parts. Companies also make an effort to have seperation between their entry level and their higher end models. So you could have some on purpose detuning to make the Roadliner, etc. more attractive to buyers...you're looking at 62hp v 89hp between the two bikes.

Answer 7

A lower compression ratio engine can run a low octane fuel and some times they also have a higher rev limit. But this is at the expense of horse power and torque.

Answer 8

a higher compression engine almost has to run higher octance, where with a low compression your not limited to a super high octane rating,