Regular or Premium Gas: Is Engine Still High Compression After Supercharger Disconnect

Whom are you asking?

JoeMario

If you search the net for dynamic compression ratio, you’ll see various discussions - some including pressure, while most do not.

Trying to add pressure to the calculation for dynamic compression ratio (DCR) begins to make it meaningless. Once you add inlet pressure to the equation, you must also add piston speed, piston top shape, valve timing and overlap, valve size, scavaging ability, temperature, and more. If you’re saying that pressure should be included in DCR, then you must also include it and the rest of the variables for both DCR and SCR.

All those variables are very relevant to what happens in the combustion chamber. They are just not relevant in a ratio which compares one volume to another volume.

Well, for a while I thot I understood, then we got different answers. I understand the principal of compression in a cylinder but am not sure that a S-charger would change that. I can see where the S-charger might add to the pressure in the cylinder.

OK JoeMario, does supercharging an engine make it need a higher fuel grade or not?

What you call “dynamic compression ratio” was called “corrected compression ratio” back in the heyday of two stroke motorcycle engines, where the compression ratio was calculated from port closing instead of bottom dead center by the Japanese manufacturers but the European makers quoted compression ratios calculated from bottom dead center. That meant comparing Eropean to Japanese two stroke compression ratios was comparing apples to oranges. Funny thing is nobody quoted “corrected displacement”.

It’s all just semantics. The fact is, with a supercharger, the air is already pre-compressed before the piston even begins to compress the air.

Maybe we should use the Brayton Cycle term of “pressure ratio”, the ratio of absolute pressure of the air before it enters the engine and after the engine has finished compressing it. Note, a 15 to 1 pressure ratio is not the same as a 15 to 1 compression ratio because both the compression and the heating resulting from compression adds to the pressure.

OK JoeMario, does supercharging an engine make it need a higher
fuel grade or not?

Yes. We all agree that supercharging will increase the need for a higher octane fuel.

I think we can all agree that the whether it’s SCR or DCR, the ratio of the numerator and denominator never changes.

Adding a supercharger influences the initial pressure at the beginning of the power stroke. So do many other variables (like atmospheric pressure, exhaust restriction, temperature). I suspect we all agree on that too.

As BLE noted, we’re differing on semantics. It’s not worth having a heated discussion about.

The supercharger improves the volumetric efficiency of the engine and for many years I have been lumping that into dynamic compression ratio. My bad. Even more correctly, it alters the mass efficiency. Once again, I found this discussion enlightening for me and thanks to those who contributed.

The power is like a normal car without the supercharger. They removed the belt drive. I didn’t have any symptoms. I brought the car to the dealer for other work and that’s when they told me the supercharger was starting to act up and it should be connected. I haven’t gotten a quote for a new one.