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".