# Answers to questions like ## And how is the stock compression ratio 7.1 : 1 with a 0.9mm squish gap ?! Here is how I got the figures for squish gap volume and compression ratio for different squish gaps. These figures are for the 'high compression' Y2 head, which is by far the most common, and a stock bore. The lower-compression Y1 head has a squish band that is not the same thickness throughout, which makes it harder to measure up.

The picture on the left shows the piston at the top, or TDC (Top Dead Center), as they say in the trade. I have divided the compressed volume into three parts:

• Volume A, the red bit, is the volume of the combustion chamber proper. This is the volume that would remain if the squish gap was reduced to zero. This volume can easily be measured.
• Volume B, the blue bit, is the additional volume under the combustion chamber due to the squish gap. This volume is not actually in the squish gap and so it will still play a full part in the combustion. It can easily be calculated for a given squish gap because its diameter can be measured directly.
• Volume C, the orange bit, is the volume of mixture in the squish gap. This will not play a part in the combustion process. That volume can also be calculated easily because its dimensions are known.
• First, I'll give the volumes A, B and C for a squish gap of 0.9 mm, and for a squish gap of 1.9 mm and I'll plug them into the simple calculations to give the percentage of mixture in the squish gap, and the compression ratio. After that, I'll show how the volumes A, B & C are found. I'll also throw in a value of 88.4 cm3 for the swept volume and explain that later. This swept volume may surprise some, after all it's a 350cc twin engine so you might think that it should be 175cm3 per cylinder.

### Squish gap of 1.9mm

• Volume A = 11.5 cm3
• Volume B = 2.95 cm3
• Volume C = 3.17 cm3
• Swept volume = 88.4 cm3

The percentage of the total compressed volume taken by the squish gap is:

100 times Volume-of-squish-gap Divided By Total-compressed-volume

= 100 x C / ( A + B + C )
= 100 x 3.17 / ( 11.5 + 2.95 + 3.17 )
= 18.0 %

The compression ratio is:

( Swept-volume plus compressed-volume ) Divided by compressed-volume

= ( Swept-volume + A + B + C ) / ( A + B + C )
= (88.4 + 11.5 + 2.95 + 3.17 ) / ( 11.5 + 2.95 + 3.17 )
= 6.02 : 1

### Squish gap of 0.9mm

• Volume A = 11.5 cm3
• Volume B = 1.39 cm3
• Volume C = 1.51 cm3
• Swept volume = 88.4 cm3

The percentage of the total compressed volume taken by the squish gap is:

100 times Volume-of-squish-gap Divided By Total-compressed-volume

= 100 x C / ( A + B + C )
= 100 x 1.51 / ( 11.5 + 1.39 + 1.51 )
= 10.5 %

The compression ratio is:

( Swept-volume plus compressed-volume ) Divided by compressed-volume

= ( Swept-volume + A + B + C ) / ( A + B + C )
= (88.4 + 11.5 + 1.39 + 1.51 ) / ( 11.5 + 1.39 + 1.51 )
= 7.14 : 1

So, nice, simple calculations. And the stock ratio that I measured of 6.02:1 agrees well with Yamahas claimed compression ratio of 6:1. All we need to do now is to see how we got the volumes.

### Measuring volume A, the main combustion chamber This is the compressed volume that we would have if the squish gap were zero. This is easily found while the head is off the bike and you have a piston to hand. You'll also need a hypodermic syringe with a reasonably fine needle. The ones used to refill ink cartridges are ideal and have a safe, blunt needle. Oh, and a drop of water.

Put a spark plug in the head and prop the head upside-down so that you can fill it with water. Use the piston to squish excess water out of the head by putting the top of it snugly into the squish gap, just as if it where in the engine at TDC and the squish gap was zero. Blow away the excess water that squidges out to get rid of it. When you remove the piston, the water remaining in the combustion chamber will have the volume we are looking for. Volume A ! To measure the volume of the water, simply draw it into the syringe. You'll need a needle on the syringe to probe down to the recesses of the spark plug. I sucked 11.5 cm3 of water out of this Y2 head. Et voila ! Or QED as my old math's teacher would have it.

Note that you cannot measure this volume while the head is on the bike in the time-honored manner of filling up the combustion chamber with oil. All this will tell you is the total of volumes A, B and C. It will also be out by about 0.3 cm3 because the stock BR8ES spark plug adds about 0.3 cm3 to the volume.

### Finding volume B, the extra combustion volume This volume obviously depends on the squish gap. In fact it has the same volume as a cylinder with a height of the squish gap and with a cross-section area the same as the bottom of the combustion chamber. The picture opposite shows how to measure the diameter of the bottom of the combustion chamber. The only hazy bit is where exactly the combustion chamber ends and where the squish gap starts in the head. However, this is fairly clearly defined on the YPVS. Note that this volume is not actually a cylinder in reality: the bottom is bowed up due to the dome of the piston crown. However, the top is also bowed up in exactly the same way so two distortions cancel out and the volume is exactly the same a cylinder with the height of the squish gap.

So, I measured this diameter at 44.4 mm with a vernier gauge as in the photo. Lets work in cm because that give volumes directly in cm3, so the diameter is 4.44 cm and the radius is half this, 2.22 cm. Now, my old math's teacher would be proud of me for remembering that the area of a circle is Pi times Radius Squared, and the volume of a cylinder is this times its height. The height is the squish gap: 0.09 cm for a 9 mm gap and 0.19 cm for a 1.9 mm gap. So B is:

= Pi x r Squared x height
= 3.142 x 2.22 squared x squish-gap
= 15.5 x squish gap
= 1.39 cm3 with a 0.9 mm squish gap
= 2.95 cm3 with a 1.9 mm squish gap

### Finding volume C, the squish gap volume

This volume obviously depends on the squish gap, too. In fact it has the same volume as a cylinder the height of the squish gap and with the circular cross-section of the bore, minus the volume B we found above.

The standard bore is 64.0 mm, so the radius of the bore is 3.20 cm. Using the same math's as before, the volume C is:

( Pi times Bore-radius squared times squish-gap ) minus Volume B.

= ( 3.142 x 3.20 squared x squish-gap ) - B
= ( 32.2 x squish gap ) - B
= (32.2 x 0.09) - 1.39 = 1.51 cm3 with a 0.9 mm squish gap
= (32.2 x 0.19) - 2.95 = 3.17 cm3 with a 1.9 mm squish gap

### Hey, where did the 88.4 cm3 swept volume come from ?

There are ways of measuring the compression ratio in a two stroke, and there are other ways !

On a four-stroke, the swept volume is obviously the volume swept by the piston, ie the area of the bore times its stroke. If we used this value on the YPVS, we ought to get 175 cm3 per cylinder because it is a 350 cm3 engine, which by definition means that the displacement is 175 cm3 per cylinder.

Ah, but it's not fair to use this figure on a two-stroke to calculate compression. After all, a two-stroke has a great big hole in the bottom half of the bore called the exhaust port. And this hole obviously lets out a good lot of that mixture you're trying to compress, until the piston closes the port when it is about half way up the bore. Actually, as you probably know, we can use resonance effects from the exhaust to effectively partially bung this hole. So, to be comparable to a four-stroke compression ratio, the equivalent swept volume is some really complicated value from part-way down the exhaust port. And it will change with rpm. To give a simple, objective, compression ratio that takes into account the exhaust port, one way is simply to measure the swept volume above the top of the exhaust port. This is what the Japanese engineers, including Yamaha, do. That value won't be comparable to a four-stroke, it will obviously be lower. But so what ? It will allow us to objectively compare the state of tune of two different YPVS engines, for example. It will also allow us to state rules-of-thumb like "don't exceed a compression ratio of 7.5 : 1 on a YPVS with 95 octane petrol.".

So, the stock YPVS has an exhaust port that starts 2.75 cm down the bore, from when the piston is at TDC. The stock bore radius is 3.20 cm so the swept volume, above the exhaust port is:

Pi x 3.20 squared x 2.75 = 88.5 cm3

If you raise the exhaust port to start at, say 26 mm, instead of the stock 27.5 mm, you will lower the compression ratio as well as altering the power delivery. You might want to do this modification purely to lower compression if you have aggressively skimmed the head and got a squish band below 0.9mm, or if you have a large overbore, both of which will increase the compression ratio.