Compression Ratio Vs Cylinder Fill

[lucky]

Hello everyone,
Most have known/ heard that compression ratio, albeit static or dynamic, can and does have a significant bearing on specific output for our given engine combinations. Extreme examples of this are engines with restricted induction systems, or on the other hand, engines without unrestricted induction systems.
Can an increased compression ratio make up for a limited cylinder fill in some cases?/ most cases?/ up to when?

[Rowdy Yates]

Compression is always a plus. When dealing with a induction restriction you can add RPM to help but.... Why not just do the correct thing in picking a good induction with higher compression . If it's class limited or something along those lines you just have to work with the rules. Are you dealing with class rules?

[Roundybout]

In a restricted engine increased compression ratio will definitely help recover the power lost compared to unrestricted in most cases. As much compression as the driving conditions and fuel allow is always good in my book, with a small safety margin. How small your mileage may vary

[David Redszus]

A drop in inlet pressure due to a restriction can be offset by use of higher dynamic compression.

But the question lingers; how much increase in compression is needed per psi of inlet pressure drop?

While the increased compression will be present across the entire rpm range, the inlet pressure drop will be a function of local rpm.

[LSP]

Can an increased compression ratio make up for a limited cylinder fill in some cases?

If ultimate acceleration and track performance is the end goal, then yes.

So can opening the intake valve earlier, and closing it later than what is considered the outer limit or norm.

[mag2555]

You want to make use of the depression ( many times greater then atmospheric!) a well tuned Exh system can place across the open valves during overlap to give a early start to the Intake cycle, and the smaller the chamber the further into the Intake tract the Exh depression can reach so to speak!

[Rick360]

You want to make use of the depression ( many times greater then atmospheric!) a well tuned Exh system can place across the open valves during overlap to give a early start to the Intake cycle, and the smaller the chamber the further into the Intake tract the Exh depression can reach so to speak!

The highlighted part is wrong, a depression can only be as much as atmospheric pressure.

Compression certainly HELPS make up for a lack of cylinder fill. How much it helps depends on how bad the cylinder fill is and how much compression you can get and use. CR is there at low rpm too when cylinder fill may still be good. Make sure to use adequate fuel for the load, CR and cylinder fill you will have.

Rick

[mag2555]

Not by along shot!

[Rick360]

Not by along shot!

Yes ...
0 psia or zero absolute pressure is the lack of all pressure. Nothing can be less pressure than that. Atmospheric pressure is the pressure of our atmosphere in absolute pressure units (ie how much above 0 psia) Depression is how much below atmosphere a pressure is.

How can a depression be more than atmospheric pressure?

maybe you can explain it to me ...

Rick

[Frankshaft]

Not by along shot!

Yes ...
0 psia or zero absolute pressure is the lack of all pressure. Nothing can be less pressure than that. Atmospheric pressure is the pressure of our atmosphere in absolute pressure units (ie how much above 0 psia) Depression is how much below atmosphere a pressure is.

How can a depression be more than atmospheric pressure?

maybe you can explain it to me ...

Rick

With boost.

[RevTheory]

Not by along shot!

Yes ...
0 psia or zero absolute pressure is the lack of all pressure. Nothing can be less pressure than that. Atmospheric pressure is the pressure of our atmosphere in absolute pressure units (ie how much above 0 psia) Depression is how much below atmosphere a pressure is.

How can a depression be more than atmospheric pressure?

maybe you can explain it to me ...

Rick

Unless I read him wrong, I think he was answering the original question.

[Stan Weiss]

What is the cylinder pressure relative to atmospheric pressure @ IVC @ wot @ peak HP RPM for an N/A engine?

Stan

[modok]

Probably, very near atmospheric

[randy331]

So,.. can the depression be "many times greater than atmospheric" with a well tuned ex. or not ?

Some of us would still like to know.

Mag didn't come back on here and explain it to us all so I (and others) just wanted to know.

Randy

[Stan Weiss]

Probably, very near atmospheric

I am sure someone will correct me if I got the calculations wrong.
Bore = 4.155"
Stroke = 3.5"
Rod = 6"
IVC = 85 ABDC
VE = 115%

cylinder pressure around 61.5" hg @ IVC

Stan