Suggestion for Correction of a Course Lesson – My E-mails of 2/21/2026

To: Ben Silcock, Co-founder, HP Academy

Andre Simon, Co-Founder, HP Academy

Cc: Marcin Lipski, Customer Support, HP Academy

Dear Ben Silcock and Andre Simon:

On your Learn to Modify Your Car – Online Automotive Training Courses webpage it states that the courses are “Always Relevant,” and that “Your course is regularly updated” so that we can “Stay on the cutting edge of new technology and practices.” I would like to provide you with my feedback here so that you can do just that by correcting and updating one of your unclear and confusing videos as per my course notes copied here:

EFI Tuning Fundamentals

Fundamental Engine Principles—Engine Principles—Pressure Volume Cycle Video

Firstly, at 02:57 you said “For the sake of simplicity, we'll assume that despite these different compression ratios, the peak cylinder pressure is equal.” And then, at 03:04 you said “Let's assume our first engine has a compression ratio of 10:1 while the second engine has a compression ratio of 5:1.”

This assumption is an oxymoron because you cannot have the peak cylinder pressures being equal and at the very same time having two different compressions! Obviously, pressure and compression are the very same thing!

Secondly, at 03:13 you said that “What we find is that as the piston moves down the bore, the shape of the pressure graph looks very different between these two engines with the pressure dropping much faster in the engine with the higher compression ratio as the piston moves towards BDC.” And then at 03:27 “This is because as the piston moves down the bore, the cylinder volume increases faster compared to the lower compression engine, giving the combustion gases more room to expand.”

YES, the pressure drops much faster in the engine with the higher compression ratio; and NO, the cylinder volume cannot increase as the volumes of the cylinders, as the piston descends, increase identically in both examples. However, the mass of the combustion gases are different due to the difference in their initial compression despite your false assertion that the “peak cylinder pressure” of the two are equal!

So your “For the sake of simplicity” postulate is also false: it wasn't a valid simplification, but a confusing and vexing complication!

Therefore the higher pressure combustion gas of the higher compression ratio must expand and decrease in pressure more rapidly than the lower pressure combustion gas of the lower compression ratio as the volume of the cylinders increase with the decent of the pistons.

You really had me perplexed here Andre, with my having to assume that you, as the authority, must always be correct. And with my having to first try and figure out how all this could be as you've described you've caused me much time, trouble, and perturbation in resolving this confounding gobbledygook!

The reason that I went to the trouble of this in-depth analysis was because, as a multiple award-winning, published, and patent-holding Mechanical Design Engineer, I needed to understand this important dynamic of internal combustion engines for my own understanding, satisfaction, and to get this damned cognitive dissonance out of my head!

Please excuse my boldness here,

Steven D. Johann

P.S.: If I am mistaken in my analysis, please kindly show me where: I am totally new to this engine tuning business.

Pressure and compression are not the same thing.

Compression in this case is over a swept volume, that as a result creates a certain amount of pressure, but that pressure can be different depending on how much cylinder fill is happening, which is a result of other variables, like cam design, engine RPM, how efficient the intake and exhaust tract are, throttle position, etc. The compression of the cylinder fill is consistent, due to it being because of the swept volume in the cylinder is teh same regardless of cylinder fill.

The pressure drop as the piston moving down, being different between different compression ratios, has to do with the change in volume with the pistons at comparable locations in the cylinder bore. The lower compression engine will have more volume, due to the function of how a lower compression ratio is achieved, and therefore more volume for the same theoretical pressure to dissipate into.

It's always a tightrope trying to achieve a suitable level of detail on a particular concept and present it at a level where it can easily be digested and understood by our average member. I also need to weigh up the specific importance of a topic - For example, thousands of tuners around the world have been tuning engines for decades without ever seeing a pressure-volume graph, or hearing the term 'expansion ratio', nor given more than a passing thought to the effect of compression ratio beyond the obvious impact on knock sensitivity.

In this instance I've attempted to simplify a concept to a secondary school level. I'm comfortable trading absolute technical accuracy for a simplification in this instance that allows our average user to understand the concept, if not the absolute specifics and nuance. You'll note I have front footed this simplification between 2:30 and 3:00 within this video.

I havent watched the video being discussed so I may be missing some context, but there does seem to be some factors you are missing in some of your (Steven) statements.

"This assumption is an oxymoron because you cannot have the peak cylinder pressures being equal and at the very same time having two different compressions! Obviously, pressure and compression are the very same thing!"

Your term "different compressions" is potentially ambiguous, so to clarify - we are talking about "Compression ratio" - not compression. Compression ratio is just the ratio between the maximum combustion chamber volume (ie at BDC) and the minimum combustion chamber volume (ie at TDC). There are many factors besides compression ratio alone that determine the peak cylinder pressure, so it is certainly possible to have the same peak cylinder pressure with two different very compression ratios even in the same engine. Throttle opening and spark timing for example have a much larger effect on cylinder pressure than compression ratio.

"Secondly, at 03:13 you said that “What we find is that as the piston moves down the bore, the shape of the pressure graph looks very different between these two engines with the pressure dropping much faster in the engine with the higher compression ratio as the piston moves towards BDC.” And then at 03:27 “This is because as the piston moves down the bore, the cylinder volume increases faster compared to the lower compression engine, giving the combustion gases more room to expand.”

YES, the pressure drops much faster in the engine with the higher compression ratio; and NO, the cylinder volume cannot increase as the volumes of the cylinders, as the piston descends, increase identically in both examples."

Chris covered this quite well in his reply above, but I thought putting some example numbers together might make it clearer:

At TDC the combustion chamber volume of the 5:1CR engine will be roughly twice that of the engine with the 10:1CR. To add some example numbers, say we have a 500cc swept volume, to achieve a 5:1CR your chamber volume would be about 125cc, and for 10:1CR it would be about 55cc.

If we then consider the expansion that occurs when the piston starts to travel down the bore lets pick a point say when the piston has traveled down about 25% of its stroke (25% of 500cc = 125cc) - that means our chamber volume in the 5:1CR engine has expanded 100% by that point - ie from 125cc to 250cc. For the 10:1CR engine there would be 227% expansion by the same point (from 55cc at TDC to 180cc at 25% down). So this is what is meant by Andre's statement "the volume increases faster".

Six_Shooter, Chris Rook

I already knew what compression was Chris, however they are directly related as you've noted. Where Andre said at 02:57, "For the sake of simplicity, we'll assume that despite these different compression ratios, the peak cylinder pressure is equal,” pressure and compression were conflated as being the same thing at the same point of peak pressure.

This was confusing to me because you can't have identical peak pressures with the same engine when they have different compression ratios. However, two different compression ratios in identical engines can be made to produce identical peak pressures when not comparing apples to oranges. That is, by supplying a higher (above atmospheric) pressure intake gas to the lower compression ratio engine in order to create an equivalent peak pressure to the higher compression ratio engine.

Therefore, where you wrote that "pressure [as a result of compression] can be different depending on how much cylinder fill is happening" is irrelevant because the engines and conditions in this example were deemed to be identical.

And for the same reason, this is also irrelevant: "The compression of the cylinder fill is consistent, due to it being because of the swept volume in the cylinder is the same regardless of cylinder fill." Or more clearly stated: “The compression of the cylinder fill is consistent [produces the same compression ratio] ("due to it being" is confusing and redundant) because (omit “of,” bad grammar) the swept volume [of the same density of gas] in the cylinder is the same regardless of cylinder fill."

"The pressure drop as the piston moving down, being different between different compression ratios, has to do with the change in volume with the pistons at comparable locations in the cylinder bore. The lower compression engine will have more volume, due to the function of how a lower compression ratio is achieved, and therefore more volume for the same theoretical pressure to dissipate into."

1. The pistons are not "at comparable locations in the cylinder bore" because the higher compression engine—and we are talking about identical engines with the same cylinder head to crank journal lengths—has a longer stroke and therefore its piston is always in a different relative position, and at a different speed.

2. "The lower compression engine will have more volume, due to the function of how a lower compression ratio is achieved, and therefore more volume for the same theoretical pressure to dissipate into." You as well are comparing apples to oranges here Chris! The cylinder volume above the lower compression engine—which is at the same peak pressure (gas density) as in the higher compression engine—is both incorrect and irrelevant! It is irrelevant because the pressure drop is into the expanding volume of the piston's stroke in the bore, which does not include the volume above the piston. It is incorrect because the expanding volume of the piston's stroke in the bore for the lower compression engine is less than, not greater than that of the higher compression engine.

3. The lower compression engine's equivalent peak pressure gas will have less volume to dissipate into since its stroke is shorter than the higher compression engine's, hence it has less swept cylinder volume to flow into. To Quote Andre at 03:27: "This is because as the piston moves down the bore [in the engine with the higher compression ratio], the cylinder volume increases faster compared to the lower compression engine, giving the combustion gases more room to expand." This is disregarding the fact that the lower compression engine has a bit more mass, with this same pressure, above its piston to help propel it downward. I mention this because you included the volume above this piston in your disputing, however, this is also irrelevant.

Adam@Link_ECU, Adam Walmsley

Because you haven't watched the video, you are just shooting in the dark here Adam! To repeat, it's the "EFI Tuning Fundamentals – Fundamental Engine Principles – Engine Principles – Pressure Volume Cycle" video, and:

See the attached Pressure Volume Cycle Graph

About your writing below, no comment:

"Your term "different compressions" is potentially ambiguous, so to clarify - we are talking about "Compression ratio" - not compression. Compression ratio is just the ratio between the maximum combustion chamber volume (ie at BDC) and the minimum combustion chamber volume (ie at TDC). There are many factors besides compression ratio alone that determine the peak cylinder pressure, so it is certainly possible to have the same peak cylinder pressure with two different very compression ratios even in the same engine. Throttle opening and spark timing for example have a much larger effect on cylinder pressure than compression ratio."

When you wrote that these were my words below, they are not! These are the words of Andre Simon, the Tutor of the video:

"Secondly, at 03:13 you said that “What we find is that as the piston moves down the bore, the shape of the pressure graph looks very different between these two engines with the pressure dropping much faster in the engine with the higher compression ratio as the piston moves towards BDC.” And then at 03:27 “This is because as the piston moves down the bore, the cylinder volume increases faster compared to the lower compression engine, giving the combustion gases more room to expand.”

I did write: "YES, the pressure drops much faster in the engine with the higher compression ratio; and NO, the cylinder volume cannot increase as the volumes of the cylinders, as the piston descends, increase identically in both examples." I was thinking that for this simplified example the pistons, both being at peak pressure, were in the same position in the bore past TDC; then would descend to the same position at BDC. See my reply to Chris Rook for a literal example of what's happening here.

I disagree with you, in your confirmation of Chris’ analysis, in my dispute with his analysis as per the above: that "the expanding volume of the piston's stroke in the bore for the lower compression engine is less than, not greater than that of the higher compression engine.” Accordingly, I also dispute the logic of your example below.

"Chris covered this quite well in his reply above, but I thought putting some example numbers together might make it clearer:

"At TDC the combustion chamber volume of the 5:1CR engine will be roughly twice that of the engine with the 10:1CR. To add some example numbers, say we have a 500cc swept volume, to achieve a 5:1CR your chamber volume would be about 125cc, and for 10:1CR it would be about 55cc.

"If we then consider the expansion that occurs when the piston starts to travel down the bore lets pick a point say when the piston has traveled down about 25% of its stroke (25% of 500cc = 125cc) - that means our chamber volume in the 5:1CR engine has expanded 100% by that point - ie from 125cc to 250cc. For the 10:1CR engine there would be 227% expansion by the same point (from 55cc at TDC to 180cc at 25% down). So this is what is meant by Andre's statement "the volume increases faster."

See the attached Pressure Volume Diagram

I certainly hope this resolves this issue—we've been beating it to death here—and it's cost me a great deal of time to deconstruct these many false assumptions all the while trying to help Andre to understand the need to clarify his "simplified," but instead complicating of an unfamiliar subject.

A simple clarification is all that is needed to his “For the sake of simplicity, we'll assume that despite these different compression ratios, the peak cylinder pressure is equal,” by adding "The peak cylinder pressures are equalized here by supplying a higher (above atmospheric) pressure intake gas to the lower compression ratio engine." This eliminates all confusion so that even the average user can understand the concept and not have to be "trading absolute technical accuracy for a simplification" as Andre put it. It's my belief that even a novice user, like myself, deserves to know the truth, to know how things actually work! And, to save guys like you and me from such a terrible heap of confusion!

"A simple clarification is all that is needed to his “For the sake of simplicity, we'll assume that despite these different compression ratios, the peak cylinder pressure is equal,” by adding "The peak cylinder pressures are equalized here by supplying a higher (above atmospheric) pressure intake gas to the lower compression ratio engine."

It seems like you are still missing some fundamental? Why do you think it would require higher than atmospheric pressure on the LC engine to achieve the same peak cyl pressure? You can simply vary peak cylinder pressure of both engines just by changing the throttle opening, there is no requirement for "supercharging" to make Andre's statement true.

Your "Pressure-Volume_Diagram-V4" shows a change of stroke and states "ignore cylinder head volume"? That would be a change of swept volume, not specifically a change of compression ratio. We have only been only discussing the effects of different compression ratio's here so why are you now changing the swept volume as well? Typically to change the compression ratio you only change the volume of the combustion chamber - ie cylinder head chamber volume or piston dome/bowl volume (or sometimes head gasket thickness for small adjustments).

Although changing bore or stroke without changing combustion chamber volume would certainly result in a different compression ratio as well, that is usually more considered a consequence rather than the intent, and that is going way off the context being discussed in the pressure-volume cycle video example which was only comparing the effects of compression ratio.

Adam@Link_ECU, Adam Walmsley

Again, no comment. I consider this case resolved.

I also considered this thread resolved after my previous post and I'm not sure there's much more to add to this. As per my previous post, in this instance I've attempted to simplify a concept to a secondary school level. I'm comfortable trading absolute technical accuracy for a simplification in this instance that allows our average user to understand the concept if not the absolute specifics and nuance. You'll note I have front footed this simplification between 2:30 and 3:00 within this video.

I don't see a justification to make any changes to the course module on this basis.

Andre.Simon, Andre SimonTutor

Thank you again Andre, for responding to this chronic issue.

This was, in point of fact, a suggestion to supplement your “For the sake of simplicity, we'll assume that despite these different compression ratios, the peak cylinder pressure is equal” with the addition of "The peak cylinder pressures are equalized here by supplying a higher (above atmospheric) pressure intake gas to the lower compression ratio engine," to supply what I thought was some needed clarification to explain how this could, in actuality, be so.

However I see that you disagree, so I won't bother you, or anyone else further. And I won’t be responding to any more disputes of this, or any other posts that I should make, if in fact I do. 'Nuff said—over and out—I have better uses to make of my valuable time! So as a result, no further suggestions to improve, or to clarify HP Academy's lessons—for their students—will be forthcoming on this forum, or anywhere else.