Compression ratio vs Octane
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Road Captain
Joined: May 2014
Posts: 555
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From: France
Compression ratio vs Octane
Hi guys,
I need some help from engine gurus to understand some stuffs
I understand the difference betwen Static Compression and Corrected Compression (Intake valve closing), but :
- Does the motor ever see the Static Compression, or it only see the Corrected CR ? I know CCR is used for Low RPMs (2000) but what happen on High Rpms ?
- Wich compression ratio determin if the engine can be run on pump gas (91aki right ?) ? Is it the CCR (around 9.2 max) or the Static (around 10.5), or both (CCR at low RPM and Static at high RPM)?
Thanks !
I need some help from engine gurus to understand some stuffs
I understand the difference betwen Static Compression and Corrected Compression (Intake valve closing), but :
- Does the motor ever see the Static Compression, or it only see the Corrected CR ? I know CCR is used for Low RPMs (2000) but what happen on High Rpms ?
- Wich compression ratio determin if the engine can be run on pump gas (91aki right ?) ? Is it the CCR (around 9.2 max) or the Static (around 10.5), or both (CCR at low RPM and Static at high RPM)?
Thanks !
Last edited by Frédéric CM; Dec 16, 2015 at 01:20 PM.
HDF Community Team
Joined: Jun 2006
Posts: 12,763
Likes: 2,597
From: san antonio
Hi guys,
I need some help from engine gurus to understand some stuffs
I understand the difference betwen Static Compression and Corrected Compression (Intake valve closing), but :
- Does the motor ever see the Static Compression, or it only see the Corrected CR ? I know CCR is used for Low RPMs (2000) but what happen on High Rpms ?
- Wich compression ratio determin if the engine can be run on pump gas (91aki right ?) ? Is it the CCR (around 9.2 max) or the Static (around 10.5), or both (CCR at low RPM and Static at high RPM)?
Thanks !
I need some help from engine gurus to understand some stuffs
I understand the difference betwen Static Compression and Corrected Compression (Intake valve closing), but :
- Does the motor ever see the Static Compression, or it only see the Corrected CR ? I know CCR is used for Low RPMs (2000) but what happen on High Rpms ?
- Wich compression ratio determin if the engine can be run on pump gas (91aki right ?) ? Is it the CCR (around 9.2 max) or the Static (around 10.5), or both (CCR at low RPM and Static at high RPM)?
Thanks !
EDIT:As an example, a motor with 9.5 corrected CR and CCP at 175psi at 5000 feet would still have corrected CR of 9.5 at sea level but CCP would jump up to 200psi. So while that motor might get by on 89 octane at 5000 feet, now way does it run on 89 octane at sea level. Comprende??
Last edited by djl; Dec 16, 2015 at 04:02 PM.
Ultimate HDF Member
Joined: Jul 2014
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From: Windwardside, Saba N.A.
Static compression is just a number; the motor never sees static. Cold Cranking Pressure (CCP) is what you should be looking at with regard to what octane the motor will tolerate. There are other factors, like ambient temperature, timing, etc. that also have an impact. Generally speaking CCP less than 185psi will tolerate lower octane. CCP higher than 190 will generally require higher octane fuel. Having said that, you should run the lowest octane that the motor will tolerate without detonation.
EDIT:As an example, a motor with 9.5 corrected CR and CCP at 175psi at 5000 feet would still have corrected CR of 9.5 at sea level but CCP would jump up to 200psi. So while that motor might get by on 89 octane at 5000 feet, now way does it run on 89 octane at sea level. Comprende??
EDIT:As an example, a motor with 9.5 corrected CR and CCP at 175psi at 5000 feet would still have corrected CR of 9.5 at sea level but CCP would jump up to 200psi. So while that motor might get by on 89 octane at 5000 feet, now way does it run on 89 octane at sea level. Comprende??
Thread Starter
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Road Captain
Joined: May 2014
Posts: 555
Likes: 39
From: France
Okay, thanks for the explanations guys !!
So if i understand it right, you can have (for example) a static of 14:1, but as long as the CCP is at 190 or less (at sea level) it can run with 91 without pinging?
But what happen when the intake reversion stop (when the cam "kick on") ? Engine is no longer seeing the CCP but something higher ?
So if i understand it right, you can have (for example) a static of 14:1, but as long as the CCP is at 190 or less (at sea level) it can run with 91 without pinging?
But what happen when the intake reversion stop (when the cam "kick on") ? Engine is no longer seeing the CCP but something higher ?
Last edited by Frédéric CM; Dec 17, 2015 at 11:50 AM.
HDF Community Team
Joined: Jun 2006
Posts: 12,763
Likes: 2,597
From: san antonio
Okay, thanks for the explanations guys !!
So if i understand it right, you can have (for example) a static of 14:1, but as long as the CCP is at 190 or less (at sea level) it can run with 91 without pinging?
But what happen when the intake reversion stop (when the cam "kick on") ? Engine is no longer seeing the CCP but something higher ?
So if i understand it right, you can have (for example) a static of 14:1, but as long as the CCP is at 190 or less (at sea level) it can run with 91 without pinging?
But what happen when the intake reversion stop (when the cam "kick on") ? Engine is no longer seeing the CCP but something higher ?
Very unlikely you would ever see 14:1 static and CCP of 190psi but the answer to the question is yes.I don't even understand the second question; do you? I don't see any situation where the motor will ever see more than CCP.
Road Captain
Joined: Mar 2006
Posts: 741
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From: West Central WI
So much more to it than just a ccp reading, good heads with proper squish will take more compression and require less ignition timing to make the same power. Proper cooling will have the most effect in reducing detonation. All cam timing events will have influence on dynamic(motor running) compression as will ignition timing.
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Road Captain
Joined: May 2014
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From: France
I'm sorry, my English may not be as good as yours 
I was meaning, when the cam is on it's "full torque" spot (VE of 100 or more), will it see a higher number ?
I was meaning, when the cam is on it's "full torque" spot (VE of 100 or more), will it see a higher number ?
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Tourer
Joined: Jul 2007
Posts: 497
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From: New Egypt, NJ
I think I understand what you're asking. The answer is no, the valve timing events do not change regardless of engine rpm (in HD engines - there are some engines with variable valve timing, but no need to get into that and confuse the issue). Since valve timing events do not change, the compression ratio the engine sees does not change.
In some cases, well designed and built engines can actually exceed 100% VE, but now I'm getting in over my head... This has to do with cylinder fill resulting from intake velocity, exhaust scavenging, etc. But like I said I'm getting in over my head now.
In some cases, well designed and built engines can actually exceed 100% VE, but now I'm getting in over my head... This has to do with cylinder fill resulting from intake velocity, exhaust scavenging, etc. But like I said I'm getting in over my head now.
Last edited by northeastconfederate; Dec 18, 2015 at 06:52 PM.
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Road Captain
Joined: May 2014
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From: France
I was thinking about what i read in this paragraph :
So far, we’ve discussed corrected compression ratios at low RPM’s. What happens as the RPM’s go up? The corrected compression ratio continues to change throughout the RPM range. In a well-designed motor, as the RPM’s increase, the corrected compression ratio also continues to rise. At some point, it becomes equal to and (in a well-balanced motor) will surpass the static ratio by a certain percentage. This is called “overstuffing.†Engine builders have been trying to overstuff cylinders since the combustion engine was first designed by using things like cam timing, turbo charging, blowers, nitrous, and ram air to push more air/fuel into the cylinder to produce higher horsepower in all RPM ranges.
From the book - How to Build a Harley-Davidson Torque Monster by Bill Rook
So far, we’ve discussed corrected compression ratios at low RPM’s. What happens as the RPM’s go up? The corrected compression ratio continues to change throughout the RPM range. In a well-designed motor, as the RPM’s increase, the corrected compression ratio also continues to rise. At some point, it becomes equal to and (in a well-balanced motor) will surpass the static ratio by a certain percentage. This is called “overstuffing.†Engine builders have been trying to overstuff cylinders since the combustion engine was first designed by using things like cam timing, turbo charging, blowers, nitrous, and ram air to push more air/fuel into the cylinder to produce higher horsepower in all RPM ranges.
From the book - How to Build a Harley-Davidson Torque Monster by Bill Rook