Compression Ratio Vs Cylinder Fill

General engine tech -- Drag Racing to Circle Track

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lucky
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Compression Ratio Vs Cylinder Fill

Post by lucky » Mon Oct 01, 2018 10:09 pm

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?

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Re: Compression Ratio Vs Cylinder Fill

Post by Rowdy Yates » Tue Oct 02, 2018 12:09 am

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?

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Re: Compression Ratio Vs Cylinder Fill

Post by Roundybout » Tue Oct 02, 2018 12:26 am

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 :wink:

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Re: Compression Ratio Vs Cylinder Fill

Post by David Redszus » Tue Oct 02, 2018 12:45 am

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.

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Re: Compression Ratio Vs Cylinder Fill

Post by LSP » Tue Oct 02, 2018 5:51 am

lucky wrote:
Mon Oct 01, 2018 10:09 pm
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.
Wish I had time to be a Guru

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Re: Compression Ratio Vs Cylinder Fill

Post by mag2555 » Tue Oct 02, 2018 6:12 am

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!

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Re: Compression Ratio Vs Cylinder Fill

Post by Rick360 » Tue Oct 02, 2018 9:54 am

mag2555 wrote:
Tue Oct 02, 2018 6:12 am
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

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Re: Compression Ratio Vs Cylinder Fill

Post by mag2555 » Tue Oct 02, 2018 11:07 am

Not by along shot!

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Re: Compression Ratio Vs Cylinder Fill

Post by Rick360 » Tue Oct 02, 2018 11:48 am

mag2555 wrote:
Tue Oct 02, 2018 11:07 am
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 ... :-k ](*,)

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Re: Compression Ratio Vs Cylinder Fill

Post by Frankshaft » Tue Oct 02, 2018 12:17 pm

Rick360 wrote:
Tue Oct 02, 2018 11:48 am
mag2555 wrote:
Tue Oct 02, 2018 11:07 am
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 ... :-k ](*,)

Rick
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Re: Compression Ratio Vs Cylinder Fill

Post by RevTheory » Tue Oct 02, 2018 12:39 pm

Rick360 wrote:
Tue Oct 02, 2018 11:48 am
mag2555 wrote:
Tue Oct 02, 2018 11:07 am
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 ... :-k ](*,)

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

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Re: Compression Ratio Vs Cylinder Fill

Post by Stan Weiss » Tue Oct 02, 2018 1:33 pm

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

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Re: Compression Ratio Vs Cylinder Fill

Post by modok » Wed Oct 03, 2018 7:55 pm

Probably, very near atmospheric
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Re: Compression Ratio Vs Cylinder Fill

Post by randy331 » Wed Oct 03, 2018 11:28 pm

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

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Re: Compression Ratio Vs Cylinder Fill

Post by Stan Weiss » Wed Oct 03, 2018 11:47 pm

modok wrote:
Wed Oct 03, 2018 7:55 pm
Probably, very near atmospheric
I am sure someone will correct me if I got the calculations wrong. :lol:
Bore = 4.155"
Stroke = 3.5"
Rod = 6"
IVC = 85 ABDC
VE = 115%

cylinder pressure around 61.5" hg @ IVC

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Offering Performance Software Since 1987
Do you use engine simulation software that uses cylinder head flow files?
We have a package of more than 3000 DFW or FLW Files

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