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I always assumed that the less "square" (or more "undersquare") a motorcycle engine was, the higher the compression ratios it can have. But a survey of several bikes -- from cruisers to supersports, seems to reveal that it's not necessarily the case!

So, if not bore and stroke, then what are the most significant factors determining an engine's compression ratio?

*Edit: supposedly the higher the compression ratio, the higher the octane your fuel should be. But with all our advanced EFI and ignition timing electronics, is this still an issue?

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I always assumed that the less "square" (or more "undersquare") a motorcycle engine was, the higher the compression ratios it can have.

In order to maintain the same in-cylinder displacement a smaller bore will require a larger stroke, so the distance between top-dead-center (TDC) and bottom-dead-center (BDC) will be larger.

However, this does not necessarily equate to a higher compression ratio (CR):

CR = ( V_L + V_H ) / V_L

where V_L = cylinder volume @ TDC, V_H = bore * stroke

So cylinder volume at TDC also influences the compression ratio; it isn't just the stroke (which impacts V_H).


supposedly the higher the compression ratio, the higher the octane your fuel should be. But with all our advanced EFI and ignition timing electronics, is this still an issue?

EFI or not, fuel is fuel; the physics of auto-ignition doesn't change. In the case of gasoline/petrol engines, detonation remains a concern, which is why there is a limit to how much CR can be designed into a gasoline engine.

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  • Well then, I can see why some places sell, albeit separately, a higher octane but more expensive gasoline (e.g., $9.99/gallon) called "racing fuel" : ) Granted, I also don't own any vehicle with an engine that can fully take advantage of that either! – ManRow May 24 '17 at 4:25
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Put simply, an engine's compression ratio is the ratio between the volume of a cylinder with the piston at the down position (position 1 and 4 in the pic below) and the volume of the same cylinder with the piston in the up position (position 2 and 3 in the pic below). enter image description here

So, basically, the volume of the cylinder with the piston in the down position is the volume of fuel and air that the cylinder can ingest (on the intake stroke).

Then, this mixture gets squeezed to a much smaller volume (during the compression stroke) before ignition, and that volume is the cylinder volume with the piston in the up position.

The ratio of the 2 is how much the fuel-air mix is getting compressed. 9 to 10 fold is typical.

Now, regarding octane ratings, put simply, a fuel's octane rating is it's ability to resist what we call detonation, which is the fuel starting to burn all by itself just because it got squeezed too much and got too hot. With higher compression, the fuel-air mixture is getting squeezed more and it will get hotter (it's a property of gases).

All the electronic engine controls are doing is, in the case of detonation, which means you are running the wrong fuel, the engine timing will be retarded and the fuel-air mix will be altered in order to avoid detonation. This isn't magic though: the fuel-air mix will not be optimal for the engine, and neither will be the timing. The engine will produce less power if running the wrong fuel and the engine management does it's thing.

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  • +1, but to give figures, 9 to 10 fold is typical for engines running on 95 to 98 octane fuel. With 87 octane, you have to stick at 7 fold, racing fuel with 108 octane allows up to 12 fold. – Janka May 28 '17 at 13:19
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Caveat lector: I am neither an engineer nor engine builder. I just read a lot.

You have received correct descriptions of static CR. But you framed the question with concerns about preignition and how it is effected by under- or over- square configurations and fuel quality. Unfortunately no one is going to be able to answer the questions to your satisfaction.

Most people point to calculated static CR as an indicator of fuel requirements and an engine's propensity to ping, knock, rattle itself to death. That is a rule of thumb that used to be true for most applications. Engines are not the same now as they were back in the day. Much more is known about air flow and fuel dispersal and how to control it. There are programs available to give a graphic indication of the swirls and eddies and paths of an air charge and the propagation of the flame front under dynamic conditions.

My Fiesta ST makes 190 HP with 1.6L and a turbocharger. With a mild tune to the ECU and no other changes it will make over 200 HP. With only bolt-on modifications it will make over 240 HP. Determined tuners are achieving more than 300 HP with major changes. The EcoTech 1.6 has a static CR of 10.5:1.

Just a decade ago Top Fuel engines were making 1000 HP per cylinder. Today the mark to beat is 1388 HP per cylinder. That mind boggling HP level is achieved with a static CR of 6.5:1.

There is no way to say flatly that a specific static CR will cause x, y, or z. You need more information to have an idea of the personality of the engine. I have read about tuner built Honda engines that are streetable with a calculated static CR of 16:1.

Beside static CR there is also dynamic CR. While the static CR can be determined with measurements of the volumes at TDC and BDC, dynamic CR requires knowledge of several other measurements. Valve timing and piston speed are perhaps the most dominant determinants of dynamic CR. But there others; including temperature and barometric pressure.

Even a discussion of preignition in over- or under- square engines has to include more than bore and stroke. How long are the rods? What is the quench clearance? Domed pistons? Maximum piston speed? Combustion chamber shape? Position of the spark plug? Multivalve? Reversion and scavenging characteristics of the exhaust stream? Length of the intake runners?

To better understand preignition and the relationships of valve timing, ignition timing, A/F ratios, piston and dome shapes, VE, BSFC, pressure pulses, scavenging, and on and on, read everything you can find on the subject. I am especially fond of articles written by The Old One. Just Google theoldone. Energy Dynamics is the very first return. But I warn you, for a gearhead that site is a candy shop.

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There's actually another factor to consider in addition to the bore and stroke - the size of the space above the piston when the piston is at TDC is also a factor. (that dome shaped area you see within the cylinder head when the head is off). The smaller that space, the higher the compression ratio, even when bore and stroke is the same.

Think of it this way... Let's say you have a piston that travels from one extreme to the other, and that travel takes up 100cc. That's the displacement. But, the cylinder head still had some extra space above the piston when the piston is at TDC.. Let's say that space is another 10cc. So, the total volume of space when the piston is all the way down is 110cc. The piston travel will never consume ALL of the space (otherwise compression would be astronomically high).

In this example, the piston will compress all the air and gas that fits into a space of 110cc down to just 10cc... 11:1 ratio...

If the space in the cylinder head were larger, the compression ratio goes down... For example, if the space in the cylinder head was 20cc, and the piston travel still displaces 100cc, then: the piston would compress all the air and gas that fits into a space of 120cc down to just 20cc. 120:20 = 12:2 = 6:1 ratio.

One other interesting factor to consider is that even though the total volume in the example above is 120cc, the piston still only "displaces" 100cc... meaning that at the end of the exhaust stroke, after the piston has pushed out as much burned (and now inert) gas as it can, there is still 20cc of burned inert gas still remaining in the cylinder, and by the end of the intake stroke, the piston will have sucked in 100cc of clean air/fuel mixture... So, at the bottom of it's intake stroke, it now has a mixture of 100cc of fresh air and gas, and 20cc of old inert exhaust gas... (the point being: even though there's 120cc of space, it can only ever get 100cc of fresh air and gas, thus why we always refer to the displacement, not the total volume).

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  • It's called the "combustion chamber." – 3Dave May 30 '17 at 19:27
  • Meh. Combustion only happens in that area, because the piston as at TDC when the spark plug fires. The resulting gas expansion pushes the piston down, but the combustion happens in the cavity in the head. Thus, "combusion chamber." Of course, if you're running shorter rods / pistons for reduced compression, things change. – 3Dave May 30 '17 at 19:44
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I am often amused by the discussion of compression ratios. Why will a lower compression ratio engine push the compression gauge to a higher number than a high performance, high compression engine? In discussing compression ratios you have a Nominal and an Absolute. The nominal compression ratio is the one that is calculated as the ratio from the total volume of the cylinder at Bottom Center to the total volume of the cylinder at top center. The Absolute Compression ratio is calculated from the point of volume where all valves are closed (because the engine is only making compression when all valves and/or ports are closed) to the volume of the cylinder at Top Center. Many of the high performance engines have such radical valve overlap that they are really not making compression for as long as a mildly tuned engine. The problem of 'ping' is more dependent upon combustion chamber design than on compression ratio.

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