11.7:1 to high compression?
6th Gear Member
Joined: Dec 2006
Posts: 8,007
From: Citrus County, FL
my 302 was 11+:1 compression and i have cast iron heads. 36 degrees total timing and i had to blend 93 and 100+ to keep it from pinging when i got on it.
they should have other piston choices available, if not y ou need to be looking at a different stroker kit
they should have other piston choices available, if not y ou need to be looking at a different stroker kit
5th Gear Member
Joined: Apr 2007
Posts: 2,100
From: Texas Hill Country
Static compression and dynamic compression are two different things. You can calculate static compression ratio, but your engine responds and sees dynamic compression. You can have 11:1 static compression, but your cam have a profile that makes the engine's dynamic compression be much lower. It really depends. In general, low to mid 10s will require 93 octane in the summer. If you go into the 11s, your engine makes good compression, and your cam is normal, you will probably need race gas. I believe many EFI cars run 9:1 or so. I have a 10.2:1 383W stroker with a 280* cam. I have to run 93 octane to keep the denonation down. I might could run a bunch less timing and get by with 89, but this is a hotrod not a DD.
5th Gear Member
Joined: Apr 2007
Posts: 2,100
From: Texas Hill Country
This is copied from Wikipedia:
The calculated compression ratio, as given above, presumes that the cylinder is sealed at the bottom of the stroke (bottom dead centre - BDC), and that the volume compressed is the actual volume.
However: intake valve closure (sealing the cylinder) always takes place after BDC, which causes some of the intake charge to be compressed backwards out of the cylinder by the rising piston at very low speeds; only the percentage of the stroke after intake valve closure is compressed. This "corrected" compression ratio is commonly called the "dynamic compression ratio".
This ratio is higher with more conservative (i.e., earlier, soon after BDC) intake cam timing, and lower with more radical (i.e., later, long after BDC) intake cam timing, but always lower than the static or "nominal" compression ratio.
The actual position of the piston can be determined by trigonometry, using the stroke length and the connecting rod length (measured between centers). The absolute cylinder pressure is the result of an exponent of the dynamic compression ratio. This exponent is a polytropic value for the ratio of variable heats for air and similar gases at the temperatures present. This compensates for the temperature rise caused by compression, as well as heat lost to the cylinder. Under ideal (adiabatic) conditions, the exponent would be 1.4, but a lower value, generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison. For example, if the static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be (7.5)^1.3 × atmospheric pressure, or 13.7 bar. (× 14.7 psi at sea level = 201.8 psi. The pressure shown on a gauge would be the absolute pressure less atmospheric pressure, or 187.1 psi.)
The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength. An engine with high static compression ratio and late intake valve closure will have a DCR similar to an engine with lower compression but earlier intake valve closure.
The calculated compression ratio, as given above, presumes that the cylinder is sealed at the bottom of the stroke (bottom dead centre - BDC), and that the volume compressed is the actual volume.
However: intake valve closure (sealing the cylinder) always takes place after BDC, which causes some of the intake charge to be compressed backwards out of the cylinder by the rising piston at very low speeds; only the percentage of the stroke after intake valve closure is compressed. This "corrected" compression ratio is commonly called the "dynamic compression ratio".
This ratio is higher with more conservative (i.e., earlier, soon after BDC) intake cam timing, and lower with more radical (i.e., later, long after BDC) intake cam timing, but always lower than the static or "nominal" compression ratio.
The actual position of the piston can be determined by trigonometry, using the stroke length and the connecting rod length (measured between centers). The absolute cylinder pressure is the result of an exponent of the dynamic compression ratio. This exponent is a polytropic value for the ratio of variable heats for air and similar gases at the temperatures present. This compensates for the temperature rise caused by compression, as well as heat lost to the cylinder. Under ideal (adiabatic) conditions, the exponent would be 1.4, but a lower value, generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison. For example, if the static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be (7.5)^1.3 × atmospheric pressure, or 13.7 bar. (× 14.7 psi at sea level = 201.8 psi. The pressure shown on a gauge would be the absolute pressure less atmospheric pressure, or 187.1 psi.)
The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength. An engine with high static compression ratio and late intake valve closure will have a DCR similar to an engine with lower compression but earlier intake valve closure.
6th Gear Member
Joined: Aug 2007
Posts: 5,896
From: Elk Grove, CA
This is why I can run 89 octane in my 9.3:1 331 with 14* initial advance
My cam has a fair amount of valve overlap which keeps dynamic compression down at lower rpm's, where high dynamic compression would usually start to cause detonation issues.
My cam has a fair amount of valve overlap which keeps dynamic compression down at lower rpm's, where high dynamic compression would usually start to cause detonation issues.
5th Gear Member
Joined: Apr 2007
Posts: 2,100
From: Texas Hill Country
Dynamic compression depends on cam event timing. When the intake opens and closes and when the exhaust opens and closes with relation to the piston position. In a way the duration and overlap affects it, but those are statistics durived from the cam events. In general, a larger duration cam is going to give less compression at idle but more compression at high rpm (moves the power band up) because at lower rpm it is pushing air out of the cylinder but at high rpm it is allowing enough time for more air charge to enter the cylinder. Small duration cams do not push as much air out of the cylinder at idle, but at high rpms they do not allow for as much time for the air charge to enter the cylinder. I hope that makes sense.
My main point is compression ratio is a good thing to build against and should be considered, but it is not the end all be all. A 9:1 compression motor may have the same cylinder pressure as a 10:1 motor depending on what cams are in both and what rpm range you want to run them in.
Now I am going to throw something else in the ring. Unless you are experiencing pre-ignition (air gas charge exploding before spark), you can control your engine within reason at most compression ratios with spark timing, octane, and cam timing. Detonation is the charge exploding instead of burning after the spark is initiated. This is usually cause by heat or high pressure, or both. This is pinging and can be minimized through reducing timing and/or increasing octane. Pre-iginition is very bad and cannot be fixed this way. You can run 11:1 CR and minimize pinging, but you may have to run race gas (high octane) and/or run less than preferred timing. More timing means more power, but you cannot run more timing than your CR will allow for.
In Conclusion, you have to consider compression ratio, timing, octane gas, and cam timing when thinking about how you want your engine to run. Most people make decisions like I want to run pump gas, which then limit how much timing, what cam, and what compression ratio they can use without hurting the motor.
My main point is compression ratio is a good thing to build against and should be considered, but it is not the end all be all. A 9:1 compression motor may have the same cylinder pressure as a 10:1 motor depending on what cams are in both and what rpm range you want to run them in.
Now I am going to throw something else in the ring. Unless you are experiencing pre-ignition (air gas charge exploding before spark), you can control your engine within reason at most compression ratios with spark timing, octane, and cam timing. Detonation is the charge exploding instead of burning after the spark is initiated. This is usually cause by heat or high pressure, or both. This is pinging and can be minimized through reducing timing and/or increasing octane. Pre-iginition is very bad and cannot be fixed this way. You can run 11:1 CR and minimize pinging, but you may have to run race gas (high octane) and/or run less than preferred timing. More timing means more power, but you cannot run more timing than your CR will allow for.
In Conclusion, you have to consider compression ratio, timing, octane gas, and cam timing when thinking about how you want your engine to run. Most people make decisions like I want to run pump gas, which then limit how much timing, what cam, and what compression ratio they can use without hurting the motor.
Last edited by urban_cowboy; Oct 2, 2008 at 04:24 PM.
5th Gear Member
Joined: Mar 2005
Posts: 3,566
From: Marquette Mi
Variables that affect pre-ignition include, but is not limited to:
Static compression ratio.
Bore size.
Chamber shape.
Piston design.
Cylinder head material.
Octane of the fuel.
Oil in the combustion chamber.
Altitude.
Humidity.
Ambient air temperature.
Coolant temperature.
Fuel temperature.
Air fuel ratio.
Cam timing.
Ignition timing.
Load on the engine:
a. vehicle weight
b. gear ratio
c. low vacuum part throttle/transitional conditions
Carbon buildup in combustion chamber.
Also what an engine will tolerate at idle, or just cruising with low throttle angle on a cool night is not what it will tolerate under a high load situation such as a WOT run on a dragstrip, or passing a slower vehicle on a two lane road in 90F heat.
So if you have abundant overlap that bleeds cylinder pressure at low rpm, it will not have the same effect at a higher rpm. thus you will still need octane commensurate with what the static compression ratio requires.
Not all detonation is audible. Inaudible detonation kills engines just as easily and quickly as audible detonation. If its audible that means its quite severe and you know to back off, if you dont hear it its still doing damage. You can flatten the upper rod bearings quite quickly, or in more severe cases break a ring land on the piston, that will allow more oil in the chamber that would require higher octane fuel.
Lastly, unless you find out what the actual cc of the chamber, valve reliefs, and combustion chamber is, you could think you have 11:1 but in reality its 9:1 or lower. Just having a flat top piston does not an 11:1 engine make. The farther into a head the valves are sunk after a valve job also increases chamber volume. So whatever you hear about this that or whoever is running 13:1 with 87 octane, only in very rare instances does high compression work with low octane gasoline.
Yes its a complicated thing, no there are no hard and fast rules, other than if you dont know what you are doing, chances are you will break something sooner than later. General guidelines were not just pulled out of someones posterior, there is a reason why people shy away from more than 10:1 with iron heads and pump gas. Yes it can be done, but chances are a novice will not tune it well enough to make it work right.
Static compression ratio.
Bore size.
Chamber shape.
Piston design.
Cylinder head material.
Octane of the fuel.
Oil in the combustion chamber.
Altitude.
Humidity.
Ambient air temperature.
Coolant temperature.
Fuel temperature.
Air fuel ratio.
Cam timing.
Ignition timing.
Load on the engine:
a. vehicle weight
b. gear ratio
c. low vacuum part throttle/transitional conditions
Carbon buildup in combustion chamber.
Also what an engine will tolerate at idle, or just cruising with low throttle angle on a cool night is not what it will tolerate under a high load situation such as a WOT run on a dragstrip, or passing a slower vehicle on a two lane road in 90F heat.
So if you have abundant overlap that bleeds cylinder pressure at low rpm, it will not have the same effect at a higher rpm. thus you will still need octane commensurate with what the static compression ratio requires.
Not all detonation is audible. Inaudible detonation kills engines just as easily and quickly as audible detonation. If its audible that means its quite severe and you know to back off, if you dont hear it its still doing damage. You can flatten the upper rod bearings quite quickly, or in more severe cases break a ring land on the piston, that will allow more oil in the chamber that would require higher octane fuel.
Lastly, unless you find out what the actual cc of the chamber, valve reliefs, and combustion chamber is, you could think you have 11:1 but in reality its 9:1 or lower. Just having a flat top piston does not an 11:1 engine make. The farther into a head the valves are sunk after a valve job also increases chamber volume. So whatever you hear about this that or whoever is running 13:1 with 87 octane, only in very rare instances does high compression work with low octane gasoline.
Yes its a complicated thing, no there are no hard and fast rules, other than if you dont know what you are doing, chances are you will break something sooner than later.
General guidelines were not just pulled out of someones posterior, there is a reason why people shy away from more than 10:1 with iron heads and pump gas. Yes it can be done, but chances are a novice will not tune it well enough to make it work right.