Displacement increased by stroke?

How is displacement calculated? Bore X Stroke...

 

most engine - or stroker kits - are increasing displacement through a reground or new crank, for this specific purpose, since many engines are limited to a fairly minimal amount of bore diameter, unless re-sleeved, which is much more costly, so stroking an engine is much more cost effective

 

The swept volume of an engine determines the engine displacement. Rotate a crankshaft and the volume from Top Dead Center to Bottom Dead Center of the cylinder is the displacement. The compression is the ratio between the volume at BDC and what it is at TDC with the head on. Compression is determined by head cc, piston height from piston pin bore, piston dome volume, gasket thickness, and block deck hight.

 

The engine capacity is essentially its ability to pump air, the greater the cylinder volume the more air and fuel that can be inhaled and exhaled, generally resulting in more power.

Volume is the height multiplied by the cross sectional area. Engine capacity is this for each cylinder multiplied by the number of cylinders. The bore cannot be a measure of engine capacity because it does not take into account the number of cylinders or the volume of the cylinders.

Nothing misleading about the concept, may not be apparent to some who don't know is all.

 

Nothing untrue here! Think of a syringe that your doctor uses. Its displacment is a function of how big in diameter it is or the bore and how much he pulls the plunger back or stroke. Change either one and your changing its displacment. All just basic geometry.

 

If a stroke is increased the piston will sit lower at BDC and higher at TDC increasing the swept volume or displacement. I think your thinking that just because the piston will be higher at TDC that you've lost displacement.
Imagine increasing a stroke by a foot and the distance it would have the piston travel in the cylinder. That is a huge increase in volume.

 

i'm sure you do Gregg, but displacement is not a static measurement, it's a dynamic measurement, the volumetric displacement of each cylinder in motion per cycle

an engine with twice the stroke - given same bore - is going to draw in twice the air (not technically accurate, but let's say so for simplicity sake) per cycle, so that increase in volumetric processing is the gain ...the engine being an air pump

to be most accurate, would be to compute the actual volumetric displacement of each cylinder for a single cycle, added together for a total ....but since each engine is unique in airflow rates, due to cylinder heads, then using just the bare engine block volumetric displacement is the only way to make a consistent measure across the board of all engine types, compared to one another

 

To have a volume, or displacement, there are 3 factors that determine it. Length, width and height. The length and width are determined by the bore (actually the diameter of the bore accounts for both as it's a circle), and the height is determined by the stroke. If you increase the stroke, you increase the height and hence the volume.

The combustion chamber volume should not change at all if you stroke an engine properly (you either decrease the rod length, change the piston pin height or, if the stroke increase is really small, shave the top of the piston so that the top of the piston still ends up at the exact same spot), but you are cramming more volume into that same combustion chamber, hence the higher compression.

As to what it does to power by increasing the stroke, the only gain is slightly more compression and a lower rpm that the power is generated at. The power should be identical as you are not increasing the airflow potential through the heads. Increasing the bore, however, usually results in better breathing as the valves are a little less shrouded and hence can provide a little more power. If you have an engine that is rpm limited and not airflow limited (ie, valvetrain can't go any higher but it has more potential airflow), then increasing the displacement any way you can will help as it will pull more air at lower rpms. This is usually not the case with Ferrari engines in stock form, so if you want to gain power with a displacement increase you also have to to increase the airflow potential and often you can increase power by doing that alone and just revving a little higher instead of having to increase displacement.

 

You say it is still a 3.5L syringe because it is. You did not change anything. Modify either the dia or length it can be drawn back and now it has a new displacment.

 

No. If you mean a 350 chev is still a 350 even though it now is a 383 because it was stroked then I get where your coming from but it's still wrong. A motors displacment gives it its name rounded to the nicest sounding number +\- 2 cu in.

 

I'll be honest, not really. If you have a basic understanding of how a piston engine works, you should understand that moving the piston further increases the volume, exactly like the syringe example described.

 

You better never look at Pontiac engines then. The 400, 428 and 455 all have roughly the same bore, only difference is the stroke. 3.75" vs. 4" vs. 4.21"

That's why the engine is labeled by it's displacement and not simply it's bore.

 

Well said. It's like a glass of beer - perhaps both have the came circumference (bore), but one is taller than the other (stroke) - the taller one holds more beer, hence it has higher displacement.

 

He's just thinking of the volume of the combustion chamber at TDC, not the volume of the whole cylinder at BDC. It'll sink in ...............

 

Some good animations...

Stroker Motors Basics and Theory - FordMuscle

CW

 

But if you increase stroke you increase the cars torque. This is whats makes a car come off the line quicker and you use to pull tree stumps

FYI HP is calculated as the square root of Torque times RPM. So if you increase stroke as per your example you increase HP by difinition.

This is why Ruropean and American cars with the same HP feel so much different. My 458 has a rpm red line of 9500 revs where most American cars are 5500 give or take.

best

Lee

 

Incorrect, you won't gain any more power unless you are rpm limited and not airflow limited. A larger stroke doesn't overcome any kind of top end (head or intake flow) limitation. You just lower the rpm the power is made at (which by definition increases torque).

 

Gregg, we do realize where you're coming from - but you're coming from misunderstanding the definition of displacement.

Others have described it well, but to try to help clarify - engine displacement actually has nothing to do with combustion chamber volume. The only determination of engine displacement is the volume of air displaced by the piston as it moves through it's stroke.

To illustrate - consider an engine with a cylinder bore of 3", you then get a piston top surface area of diameter x 3.14159 (pi), 3x3.14=9.42 square inches. That's area, in square inches. To get volume, you need to move that area to displace air - if the stroke of that engine is 4", then the displacement is 4"x9.42 square inches =37.68 cubic inches. If you have 8 cylinders, the displacement for the engine becomes 8x37.68= 301.4 cubic inches.

Now increase that same engine's stroke to 5" - the cylinder bore is still 9.42 square inches, the cylinder displacement is 5" stroke x 9.42 square inches = 47.2 cubic inches instead of 37.68, and the 8 cylinder displacement would be 8x47.2 = 376.8 cubic inches.

So, changing the stroke of your V8 from 4" to 5" increased your displacement from 301 cubic inches to 377 cubic inches.

 

Yes. You are coming from Houston. And either you are jerking our chains or you are misunderstanding stroke.

If you put in longer connecting rods, you will be correct. The motion of the piston up and down in the bore will be the same, but higher up in the cylinder. By keeping the same rods and changing the crank to one with a larger diameter, you will be moving the piston higher in the bore at the top of the stroke and lower in the bore at the bottom of the stroke, thus increasing displacement. Wish I wasn't so lazy, a picture is worth a thousand words but I don't want to draw it out for you when you have Google and the links already provided.