The International Cessna 170 Association

The higher piston reduces the total volume of mixture in the cylinder. The piston is raised the same distance (stroke), the bore is unchanged and so the same volume of displacement but into a smaller combustion chamber. Hence the compression ratio is increased. Displacement is unchanged, compression ratio is increased.

daedaluscan wrote:The higher piston reduces the total volume of mixture in the cylinder….

..which is displacement…correct? (Not trying to argue the point…just trying to wrap my small mind around it.)

Displacement is the volume moved by the piston. Either the volume sucked in through the intake valve (assuming perfect conditions of course), the volume displaced by the piston on the compression stroke, or the volume pushed out through the exhaust valve. Simply bore cross sectional area x stroke (x number of cylinders).

GAHorn wrote:

daedaluscan wrote:The higher piston reduces the total volume of mixture in the cylinder….

..which is displacement…correct? (Not trying to argue the point…just trying to wrap my small mind around it.)

This is the displacement PLUS the volume of the combustion chamber at TDC.

So how does this all equal 35 extra HP ?

lowNslow wrote:So how does this all equal 35 extra HP ?

It probably doesn't. If we are being generous a bump from 7.0:1 to 9.5:1 might yield 10% so 159HP.

Porting, polishing, and flow matching all of the cylinders is where the rest of the gains are probably supposed to come from.

Even a 170HP would be a welcome upgrade from 145HP (if that is really what my tired old engine is making).

daedaluscan wrote:Displacement is the volume moved by the piston. Either the volume sucked in through the intake valve (assuming perfect conditions of course), the volume displaced by the piston on the compression stroke, or the volume pushed out through the exhaust valve. Simply bore cross sectional area x stroke (x number of cylinders).

If the piston is taller…then it takes up more space in the cylinder…no matter where it sits. If a short piston is removed…and a tall piston is installed… then the available space above that piston is reduced. So..it seems to me that if less space exists in the cylinder because the newly installed piston is larger….then the capacity (displacement) of the air inside the cylinder is less. Yes, because the taller piston moves closer to the top of the cylinder…. It Compresses the air to a greater degree (raises compression ratio)…. But the amount of air being compressed is less.

THAT is what I always thought of as displacement. It’s the definition of displacement which is not making sense to me.

If an original piston sitting at the bottom of an original cylinder…times six…equals 300 cu. inches…. then how can a taller piston which takes up more space inside the cylinder…. also equal 300 cu. Inches.

George, you are thinking correct. A engine with a taller piston vs the same engine with a shorter piston, ie length from the center of the piston pin to the too of the piston, has a smaller displacement.

Bruce Fenstermacher wrote:George, you are thinking correct. A engine with a taller piston vs the same engine with a shorter piston, ie length from the center of the piston pin to the too of the piston, has a smaller displacement.

That is contrary to my understanding of displacement!

The overall displacement for a typical reciprocating piston engine is calculated by multiplying together three values; the distance travelled by the piston (the stroke length), the circular area of the cylinder, and the number of cylinders in the whole engine.

You can change combustion chamber volume (which changes compression ratio), by changing cylinder head volume or piston height, without changing displacement.

Bruce Fenstermacher wrote:George, you are thinking correct. A engine with a taller piston vs the same engine with a shorter piston, ie length from the center of the piston pin to the too of the piston, has a smaller displacement.

This is incorrect. Both you and GAHorn are confusing cylinder volume with displacement volume. The sum of the displacement volume and the clearance volume (combustion chamber volume) is the cylinder volume. You are both correct that increasing the height of the piston decreases the cylinder volume. However, the term displacement refers to how much volume is DISPLACED by the piston over its stroke length.

From Wikipedia: Engine displacement is the measure of the cylinder volume swept by all of the pistons of a piston engine, excluding the combustion chambers."

https://en.wikipedia.org/wiki/Engine_displacement

Well…we may be confused on terminology… but the fact is (and what I was attempting to point out)….. a taller piston will begin to compress a smaller volume of fuel/air than would a shorter piston…. all other aspects being equal. If a taller piston is installed in-place of a shorter piston….then a smaller amount of fuel/air mixture will be used in the combustion process. The increase in compression may allow a higher-energy fuel (higher octane) to be utilized and it may produce more HP…. but there is also a reduction of thermal capacity due to the reduction of fuel/air volume being used in the process.

GAHorn wrote:Well…we may be confused on terminology… but the fact is (and what I was attempting to point out)….. a taller piston will begin to compress a smaller volume of fuel/air than would a shorter piston…. all other aspects being equal. If a taller piston is installed in-place of a shorter piston….then a smaller amount of fuel/air mixture will be used in the combustion process. The increase in compression may allow a higher-energy fuel (higher octane) to be utilized and it may produce more HP…. but there is also a reduction of thermal capacity due to the reduction of fuel/air volume being used in the process.

The same amount of fuel/air mixture will be pulled into the cylinder because the piston area is the same and it travels the same distance. If you think of it as a pump, it is the area times the stroke. It is pulling that same amount of fuel/air mixture into a smaller overall cavity.

GAHorn wrote:... If a taller piston is installed in-place of a shorter piston….then a smaller amount of fuel/air mixture will be used in the combustion process. ...

Closer but still not correct. The number of molecules of air/fuel is determined by the sucking volume of the piston, not by the volume into which it's sucked. No more air/fuel can enter the cylinder than is pulled by the piston. You can change the clearance volume all you like but it won't change the number of air/fuel molecules. To change the number of combustion molecules you must change either bore or stroke.

Displacement is a zero-velocity constant-pressure simplification that describes how much air/fuel is consumed with each power event. It's measured in hotrod shops by putting the piston at bottom center, then filling the cylinder with liquid, then bringing the piston up to top center and measuring the amount of liquid expelled.

GAHorn wrote:Well…we may be confused on terminology… but the fact is (and what I was attempting to point out)….. a taller piston will begin to compress a smaller volume of fuel/air than would a shorter piston…. all other aspects being equal. If a taller piston is installed in-place of a shorter piston….then a smaller amount of fuel/air mixture will be used in the combustion process. The increase in compression may allow a higher-energy fuel (higher octane) to be utilized and it may produce more HP…. but there is also a reduction of thermal capacity due to the reduction of fuel/air volume being used in the process.

NO! NO! NO! Displacement is not the volume of the cylinder at any one piston position; it is the DIFFERENCE between the cylinder volume at the top of the stroke and the cylinder volume at the bottom of the stroke, and is the volume of fuel/air mixture pulled into the cylinder. Displacement of a cylinder is determined ONLY by the bore and stroke; it’s independent of the height of the piston. The formula for a single cylinder’s displacement is (stroke)*(pi)*(bore^2)/4. Put the numbers for an O-300 engine in this formula and you will come out with slightly over 50 cubic inches. Each cylinder of our O-300 pulls in 50 cubic inches of fuel/air mixture on each intake stroke, period. Nowhere does height of the piston enter into it.

What increasing the height of the piston does do is compress that 50 cubic inches into a smaller volume (higher compression ratio). Using math that is too complex to go into here, it can be shown that increasing compression ratio increases efficiency, thus allowing more energy to be extracted from the same amount of fuel.

Also, higher octane fuel IS NOT higher energy than lower octane fuel. Higher octane fuel simply allows a higher compression ratio without causing pre-ignition or detonation, and the resulting engine damage. A given engine produces no more power with higher octane fuel than it does with the octane for which it is rated.

GAHorn wrote:

daedaluscan wrote:Displacement is the volume moved by the piston. Either the volume sucked in through the intake valve (assuming perfect conditions of course), the volume displaced by the piston on the compression stroke, or the volume pushed out through the exhaust valve. Simply bore cross sectional area x stroke (x number of cylinders).

If the piston is taller…then it takes up more space in the cylinder…no matter where it sits. If a short piston is removed…and a tall piston is installed… then the available space above that piston is reduced. So..it seems to me that if less space exists in the cylinder because the newly installed piston is larger….then the capacity (displacement) of the air inside the cylinder is less. Yes, because the taller piston moves closer to the top of the cylinder…. It Compresses the air to a greater degree (raises compression ratio)…. But the amount of air being compressed is less.

THAT is what I always thought of as displacement. It’s the definition of displacement which is not making sense to me.

If an original piston sitting at the bottom of an original cylinder…times six…equals 300 cu. inches…. then how can a taller piston which takes up more space inside the cylinder…. also equal 300 cu. Inches.

You are confusing displacement - the volume "displaced" by the piston with the total volume of the cylinder. The volume of the cylinder is displacement plus combustion chamber volume (the volume above the piston at top dead centre). The taller piston will reduce combustion chamber volume but not change the displacement (bore cross section x stroke).

Think of the 0-300 as having six pistons which each displace (move) about 50cu in. on each stroke. It doesnt matter at all what the remaining volume in the cylinders is.

Taller pistons will increase compression ratio (The ratio of (displacement + combustion chamber volume) / (combustion chamber volume)) as the combustion chamber volume is smaller. Higher compression ratio gives more power, as well as more risk of detonation and more load on the bottom end.