Compression ratio in relation to octane requirements?
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From: Silver Spring ,Md.
Compression ratio in relation to octane requirements?
I'm wondering how auto manufacturers are now able to get away with 12.5 and 13-1 compression ratios on production cars with pump gas? My belief was that anything over 11.5-1 required higher octane than what could be bought at the pump.
If it boils down to the designs of the cams being used in these motors, what are the advantages of going that high with the compression if you're going to lose the compression through valve timing anyway?
How much compression can we get away with while still being able to get away with 93 octane fuel?
Does it boil down to DCR? Please explain. Thanks.
If it boils down to the designs of the cams being used in these motors, what are the advantages of going that high with the compression if you're going to lose the compression through valve timing anyway?
How much compression can we get away with while still being able to get away with 93 octane fuel?
Does it boil down to DCR? Please explain. Thanks.
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Compare an 11:1 SBC from the late 60s to the 11:1 LS7 mentioned by rskrause. Both have the same compression ratio but have different fuel requirements.
Many design differences exist; IMO none have a larger impact on detonation than the combustion space itself (shape, material of construction and mixture movement), second would come fuel/spark management.
Last edited by automotivebreath; Dec 28, 2006 at 12:39 PM.
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yea new heads have smooth chambers and are alluminum, and have better valves designed to transfer heat, and better cooling systems, and anti knock sensors that retard timing........ so less hotspots, and better heat transfer and better tech = less detonation
however you can only compress gas so far before it will just detonate, usually somewhere in the 8.5-1 DCR and 180 psi range??? dont quote me on that but it does boil down to DCR mainly because it doesnt matter if you compress it back through the intake... that gas wont detonate, higher compression and more cam duration = more power at high rpm, think of it like a hydraulic, the lower the piston the higher the suction. but if the cylinder doesnt have to completely fill, it will be more efficient when it's running faster, because even though the piston is headed upwards, there still isnt a full cylinder of air.... am i making sense? anyway yea, more duration requires more compression
however you can only compress gas so far before it will just detonate, usually somewhere in the 8.5-1 DCR and 180 psi range??? dont quote me on that but it does boil down to DCR mainly because it doesnt matter if you compress it back through the intake... that gas wont detonate, higher compression and more cam duration = more power at high rpm, think of it like a hydraulic, the lower the piston the higher the suction. but if the cylinder doesnt have to completely fill, it will be more efficient when it's running faster, because even though the piston is headed upwards, there still isnt a full cylinder of air.... am i making sense? anyway yea, more duration requires more compression
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Yes, you can only compresses the mixture so far before it detonates. That limit has been and continues to be raised with improved design. GM got into the hunt for higher compression and increased efficiently sometime in the 90's. Before then the SBC was using 50's technology.
This comparison displays some of the improvements that GM has made in the last 15 years. It is also worth noting the iron head uses a dome piston to generate compression, the newer design uses a flat top.
This comparison displays some of the improvements that GM has made in the last 15 years. It is also worth noting the iron head uses a dome piston to generate compression, the newer design uses a flat top.
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Thanks for the replies. One thing I've noticed is the manufacturers who have gone with the higher compression ratios generally have done it with smaller cube motors (excluding the new Z06). Is there a relation there or am I heading in the wrong direction?
If higher compression means more power and better efficiency, why are more manufacturers not doing it?
If higher compression means more power and better efficiency, why are more manufacturers not doing it?
Last edited by big dave; Dec 29, 2006 at 06:27 AM.
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I am by no means an expert, but I have been around this site for the past 6 years. I have read about many guys with 11,12.1 compression. Not many 12.1 but many 11.5-1 guys running around on the street with just 93 in the tank. I also have been reading what Rich and other knowledgeable guys post. I trust what they say, so I guess my question is how do they do it? Is it the size of cam? a forged bottom end that just handles it? How can some do it and others can't?
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We're talking about two different things here. One is the gradual rise in CR of OEM applications. CR are now approaching the levels of the sixties when you could buy 110 octane at the pump! The 11:1 LS7 is the highest for an OHV V-8 for some time. Some of the technical features which allow these kinds of CR have been mentioned. We will see them go even higher when direct injection comes into widespread use.
When it comes to modified engiones, that's something else again. As you mentioned, larger cams are used and this allows higher compression. Also, OEM levels of reliability are not required, nor do emissions standards need to be met. Consequently, modified motors use considerably higher CR than OEM's.
Rich
When it comes to modified engiones, that's something else again. As you mentioned, larger cams are used and this allows higher compression. Also, OEM levels of reliability are not required, nor do emissions standards need to be met. Consequently, modified motors use considerably higher CR than OEM's.
Rich
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Some of the needs for high compression on pump gas are evident in the LT1 head.
Compact combustion chamber
Aluminum construction for fast heat transfer
Reverse flow cooling.
High levels of mixture movement for fast burn
Ideal spark plug location
Compact combustion chamber
Aluminum construction for fast heat transfer
Reverse flow cooling.
High levels of mixture movement for fast burn
Ideal spark plug location
Last edited by automotivebreath; Dec 30, 2006 at 04:01 AM.
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another thing is the smoothness of the combustion chamber, smoother flow and less EDIT: air friction reduces pinging as well.
the size and shape of the chamber and piston will affect the ability of the engine to run and not ping, but he main thing is too much pressure will destabilize the gas and it will detonate, the cam timing allows for the pressure to bleed back out
the size and shape of the chamber and piston will affect the ability of the engine to run and not ping, but he main thing is too much pressure will destabilize the gas and it will detonate, the cam timing allows for the pressure to bleed back out
Last edited by 84firebird; Dec 30, 2006 at 05:49 PM. Reason: better word
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Here's a quote from another forum, this member has a very clear understanding of combustion, detonation and the importance of turbulence during the combustion process.
84firebird is correct by saying we want a smooth combustion space with little obstruction, but mixture movement it is also very important.
Spark ignition (SI) engines all work on the principle of a turbulent flame front (TFF) consuming the air-fuel charge. A normal combustion event would be considered one where the spark ignites the air fuel mixture (a complex process in itself) and the flame propagates throughout the air-fuel mixture with this turbulent flame front. Skip the next paragraph if you know how stuff burns
I think most people have a pretty good idea of how a TFF works, but let’s go through some of the details. First, think of a chamber (say a cube or a disc for simplicity) full of nice calm, still air-fuel mixture. A spark in the middle ignites, and begins to consume the mixture. Ideally, the flame front (in this case a laminar flame front, since the mixture is still) would form a spherical shell, as it progresses. Now, this flame front is propelled by a couple of forces. First, the mixture in the wake of the flame front is obviously heated by the combustion. This heat translates to an increase in pressure. This higher pressure burned gas compresses the mixture ahead of the flame front. Since the volume of the burned gases expands it helps accelerate the flame front. Think of blowing up a balloon. The compression of the end gas also raises its temperature. Flame speeds are higher in higher temperature mixtures.
For a turbulent flame front, consider instead of a calm chamber, a chamber full of turbulent eddies, of all size scales. As the flame front approaches one of these swirling eddies, the flame edge is 'torn' and spun around by the eddy, into fresh mixture. This helps to shred up the flame front, and helps to progress the burn of the mixture. In short, this is really why SI engines work at all, lol.
Now, this burning action is really a race. As you compress the combustible end gas, you get ever closer to the auto-ignition temperature. Auto-ignition is a process by where a series of branching chemical reactions (which mainly all have a very strong dependence on temperature) result in the combustion of a mixture, with no flame front. These reactions begin to oxide the mixture simply due to the thermal energy available from the high temperature. Now don't get me wrong, this is a VERY complex series of chemical reactions, but some of the basics help give a good understanding.
The auto-ignition is very dependent on the time history of the mixture. If you hold the mixture at a low temperature, it may not auto-ignite for a long time. Raise the temperature, and it ignites sooner. So if the TFF takes a long time consuming the mixture, the compression effects of the TFF are present for a longer time and hence the temp. of the end gas is higher for longer.
If the end gas does auto-ignite before the TFF reaches it, or before the relatively cool cylinder wall quenches the flame, the end gas auto-ignites, or detonates. Now, if one corner of this chamber is the last to receive the flame front, and indeed does detonate, what is the result? The auto-igniting mixture essentially 'explodes' and sends a pressure wave across the chamber at the local speed of sound in the cylinder. This pressure wave is what is heard as 'knock' Oddly enough your ear picks up short pulses of a tone, as a 'knock', and not a ringing sound.
84firebird is correct by saying we want a smooth combustion space with little obstruction, but mixture movement it is also very important.
Spark ignition (SI) engines all work on the principle of a turbulent flame front (TFF) consuming the air-fuel charge. A normal combustion event would be considered one where the spark ignites the air fuel mixture (a complex process in itself) and the flame propagates throughout the air-fuel mixture with this turbulent flame front. Skip the next paragraph if you know how stuff burns
I think most people have a pretty good idea of how a TFF works, but let’s go through some of the details. First, think of a chamber (say a cube or a disc for simplicity) full of nice calm, still air-fuel mixture. A spark in the middle ignites, and begins to consume the mixture. Ideally, the flame front (in this case a laminar flame front, since the mixture is still) would form a spherical shell, as it progresses. Now, this flame front is propelled by a couple of forces. First, the mixture in the wake of the flame front is obviously heated by the combustion. This heat translates to an increase in pressure. This higher pressure burned gas compresses the mixture ahead of the flame front. Since the volume of the burned gases expands it helps accelerate the flame front. Think of blowing up a balloon. The compression of the end gas also raises its temperature. Flame speeds are higher in higher temperature mixtures.
For a turbulent flame front, consider instead of a calm chamber, a chamber full of turbulent eddies, of all size scales. As the flame front approaches one of these swirling eddies, the flame edge is 'torn' and spun around by the eddy, into fresh mixture. This helps to shred up the flame front, and helps to progress the burn of the mixture. In short, this is really why SI engines work at all, lol.
Now, this burning action is really a race. As you compress the combustible end gas, you get ever closer to the auto-ignition temperature. Auto-ignition is a process by where a series of branching chemical reactions (which mainly all have a very strong dependence on temperature) result in the combustion of a mixture, with no flame front. These reactions begin to oxide the mixture simply due to the thermal energy available from the high temperature. Now don't get me wrong, this is a VERY complex series of chemical reactions, but some of the basics help give a good understanding.
The auto-ignition is very dependent on the time history of the mixture. If you hold the mixture at a low temperature, it may not auto-ignite for a long time. Raise the temperature, and it ignites sooner. So if the TFF takes a long time consuming the mixture, the compression effects of the TFF are present for a longer time and hence the temp. of the end gas is higher for longer.
If the end gas does auto-ignite before the TFF reaches it, or before the relatively cool cylinder wall quenches the flame, the end gas auto-ignites, or detonates. Now, if one corner of this chamber is the last to receive the flame front, and indeed does detonate, what is the result? The auto-igniting mixture essentially 'explodes' and sends a pressure wave across the chamber at the local speed of sound in the cylinder. This pressure wave is what is heard as 'knock' Oddly enough your ear picks up short pulses of a tone, as a 'knock', and not a ringing sound.
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... So if the TFF takes a long time consuming the mixture, the compression effects of the TFF are present for a longer time and hence the temp. of the end gas is higher for longer....
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yea, i didnt mean turbulence like in a valve bowl or an airplane, more like an 'obstruction' (gonna try and use some terminology ) on the chamber surface, the way i understand it is the air friction there can get the molecules to 'auto-ignite' and once one does the chain reaction is....bang
but yea turbulence like you're thinking and probably most the modern world, is good to keep the air fuel mixture mixed, and any inconsistency will probably slow the 'flame front' down as well... i guess that's probably the reasoning for why incomplete mixture is bad
sry for the misunderstanding
) on the chamber surface, the way i understand it is the air friction there can get the molecules to 'auto-ignite' and once one does the chain reaction is....bangbut yea turbulence like you're thinking and probably most the modern world, is good to keep the air fuel mixture mixed, and any inconsistency will probably slow the 'flame front' down as well... i guess that's probably the reasoning for why incomplete mixture is bad
sry for the misunderstanding