Compression Ratio VS. Fuel Octane
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You would need a very sophisticated model to calculate cylinder pressures over the course of the piston stroke. However, average (mean) effective cylinder pressure can be estimated by MEP = (HP X 792,000) / (displacement X rpm) or MEP = (torque X 150.8) / displacement
I haven't read this thread in detail, but from skimming, it seems there are some misconceptions. High cylinder pressures are good as long as they occur at the right time in terms of the piston position and aren't so high as to exceed the mechanical strength of the components. When high cylinder pressures occur before or immediately after TDC power from the combustion process is lost. You want peak pressure at ~15-20 degrees after TDC to gain the maximum power from a given combustion cycle. Too early and you are "fighting" the piston as it continues to move toward TDC or is simply "dwelling" near TDC as occurs at the top of the piston sroke. Too late and you lose maximal mechanical advantage as the piston accelerates down the bore and there is not enough time for combustion to complete as ths piston approachs or passes BDC. Since the combustion process takes a certain amount of time, regardless of engine rpm, ignition timing must be advanced as rpm rises to keep the combustion events occuring at the optimal time in terms of piston position.
The problem with high compression and/or low octane is that if the CR is sufficiently high the air:fuel mixture will ignite (actually pre-ignite) too early based on heating from the compression process itself. Preignition is thus a type of detonation that cannot be prevented by retarding ignition timing. Preignition is probably not best thought of as being "caused" by high compression, the most proximate cause is usually hot spots in the combustion chamber or piston crown. But a high CR will contribute to it by "preheating" the air:fuel mixture.
Since compression heats the air:fuel mixture, it contributes to detonation even if preignition does not occur. This type of detonation happens after spark ignition. The flame front initiated by the spark is suppposed to move smoothly across the combustion chamber. If the unburned air:fuel mixture reaches a temperature and/or pressure at which it is no longer stable, it will ignite in an second, abnormal location causing a new flame front to collide with the one that originated at the spark plug. Maximum cylinder pressure occurs early and very high pressures and temparatures may be generated in a local area with resulting damage or heating of parts. A vicious cycle may result as parts heat up, causing more detonation, which causes more heat. And, of course, overheated parts become weak and may melt (especially piston crowns). This type of detonation may respond to retarded ignition timing.
Both types of abnormal combustion are more likely with a high CR and helped by high octane. A high CR makes the air:fuel mixture hotter and thus increases the chances of both types of abnormal combustion. Since "high octane" fuel is by definition harder to ignite (requires a higher temperature) it will help prevent both types of abnormal combustion. It will therefore produce more power if and only if it is needed to prevent detonation (which robs power) or if it allows optimal ignition timing to be used. In terms of CR, higher CR is also good in that it promotes efficient combustion up the point where it causes detonation or requires excessively retarded ignition to prevent it. Running an overly rich fuel mixture is another trick for preventing detonation. The excess fuel has a cooling effect on the air:fuel mixture.
The are models that allow maximum compression for a given octane to be calculated. But unless you have much more data about your motor and much more sophisticated software than most of us have available, I am not sure how much use they are. Find one at http://www.dunegoon.org/compression.html The problem with a simple calculator like this is that there are many more variables including: the shape of the combustion chamber, amount of swirl and tumble in the incoming air:fuel mixture, thermal characteristics of the materials used, efficiency of the cooling system, etc., etc. Therefore experience is probably the best guide for a given engine type. I'll bet that even the major OEMs determine CR by actual experimentation and not by modeling.
Rich Krause
I haven't read this thread in detail, but from skimming, it seems there are some misconceptions. High cylinder pressures are good as long as they occur at the right time in terms of the piston position and aren't so high as to exceed the mechanical strength of the components. When high cylinder pressures occur before or immediately after TDC power from the combustion process is lost. You want peak pressure at ~15-20 degrees after TDC to gain the maximum power from a given combustion cycle. Too early and you are "fighting" the piston as it continues to move toward TDC or is simply "dwelling" near TDC as occurs at the top of the piston sroke. Too late and you lose maximal mechanical advantage as the piston accelerates down the bore and there is not enough time for combustion to complete as ths piston approachs or passes BDC. Since the combustion process takes a certain amount of time, regardless of engine rpm, ignition timing must be advanced as rpm rises to keep the combustion events occuring at the optimal time in terms of piston position.
The problem with high compression and/or low octane is that if the CR is sufficiently high the air:fuel mixture will ignite (actually pre-ignite) too early based on heating from the compression process itself. Preignition is thus a type of detonation that cannot be prevented by retarding ignition timing. Preignition is probably not best thought of as being "caused" by high compression, the most proximate cause is usually hot spots in the combustion chamber or piston crown. But a high CR will contribute to it by "preheating" the air:fuel mixture.
Since compression heats the air:fuel mixture, it contributes to detonation even if preignition does not occur. This type of detonation happens after spark ignition. The flame front initiated by the spark is suppposed to move smoothly across the combustion chamber. If the unburned air:fuel mixture reaches a temperature and/or pressure at which it is no longer stable, it will ignite in an second, abnormal location causing a new flame front to collide with the one that originated at the spark plug. Maximum cylinder pressure occurs early and very high pressures and temparatures may be generated in a local area with resulting damage or heating of parts. A vicious cycle may result as parts heat up, causing more detonation, which causes more heat. And, of course, overheated parts become weak and may melt (especially piston crowns). This type of detonation may respond to retarded ignition timing.
Both types of abnormal combustion are more likely with a high CR and helped by high octane. A high CR makes the air:fuel mixture hotter and thus increases the chances of both types of abnormal combustion. Since "high octane" fuel is by definition harder to ignite (requires a higher temperature) it will help prevent both types of abnormal combustion. It will therefore produce more power if and only if it is needed to prevent detonation (which robs power) or if it allows optimal ignition timing to be used. In terms of CR, higher CR is also good in that it promotes efficient combustion up the point where it causes detonation or requires excessively retarded ignition to prevent it. Running an overly rich fuel mixture is another trick for preventing detonation. The excess fuel has a cooling effect on the air:fuel mixture.
The are models that allow maximum compression for a given octane to be calculated. But unless you have much more data about your motor and much more sophisticated software than most of us have available, I am not sure how much use they are. Find one at http://www.dunegoon.org/compression.html The problem with a simple calculator like this is that there are many more variables including: the shape of the combustion chamber, amount of swirl and tumble in the incoming air:fuel mixture, thermal characteristics of the materials used, efficiency of the cooling system, etc., etc. Therefore experience is probably the best guide for a given engine type. I'll bet that even the major OEMs determine CR by actual experimentation and not by modeling.
Rich Krause
Last edited by rskrause; Sep 24, 2002 at 09:32 PM.
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Originally posted by kmook
Wow thanks for the good info Rich
Wow thanks for the good info Rich
Rich Krause
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From: Jacksonville, FL
Re: Compression Ratio VS. Fuel Octane
HeavyChevySS
Hi I I think you posted this also in the L-town board ,,,
I thought I would add to the subject
I found running 11 to 1 is pushing it on pump gas for the street. You are going to have keep the timing on the conservative side.
What most of you are not considering is that you might be ok running 94 but we all know that what is advertises is not is always what you get at the pump. Every once and a while I run into a bad tank of gas and for my blower combo it can be deadly (I already pop a head gasket due to this) .
10 to 1 should be a safe margin to help against detonation especially during those hot summer months. Even the LT1 and LS1 have problems keeping the detonation under control with 10 to 1 compression ratios during the hot summer months.
If you are stuck with this head and piston combo I would consider a couple of things:
1) A thicker gasket to bring down the compression
2) Some sort of timing control device from MSD . A least you can set your initial timing for max power when the condition is right and have to option to pull timing out with a turn of the ****.
3) Selecting a cam that would help bleed off cylinder pressure would also allow you to run higher compression (only one draw back with a cam like trhis that you would kill low-end power which is not to friendly on the street. If you like to drive it everyday)
4) Also if you choose to keep your cam set up , you and can try to play with your cam timing (2 – 4 degrees retard) this will help bleed off some pressure but, this will also move your power band up and kill low end torque and power (not sure of your gear and torque converter set up)
Remember you need to consider the combination of mechanical and static compression (this is based on the cam choice) which will determine whether are not your motor will be friendly with pump gas with.
I have seen guys with 9 to 1 compression running into part throttle detonation problems. It turned out to be a poor cam choice for their combo. (Incorrect centerline (cam to much advanced or retarded) or to much or to little over lap etc)
Hope this makes sense
Jim
Hi I I think you posted this also in the L-town board ,,,
I thought I would add to the subject
I found running 11 to 1 is pushing it on pump gas for the street. You are going to have keep the timing on the conservative side.
What most of you are not considering is that you might be ok running 94 but we all know that what is advertises is not is always what you get at the pump. Every once and a while I run into a bad tank of gas and for my blower combo it can be deadly (I already pop a head gasket due to this) .
10 to 1 should be a safe margin to help against detonation especially during those hot summer months. Even the LT1 and LS1 have problems keeping the detonation under control with 10 to 1 compression ratios during the hot summer months.
If you are stuck with this head and piston combo I would consider a couple of things:
1) A thicker gasket to bring down the compression
2) Some sort of timing control device from MSD . A least you can set your initial timing for max power when the condition is right and have to option to pull timing out with a turn of the ****.
3) Selecting a cam that would help bleed off cylinder pressure would also allow you to run higher compression (only one draw back with a cam like trhis that you would kill low-end power which is not to friendly on the street. If you like to drive it everyday)
4) Also if you choose to keep your cam set up , you and can try to play with your cam timing (2 – 4 degrees retard) this will help bleed off some pressure but, this will also move your power band up and kill low end torque and power (not sure of your gear and torque converter set up)
Remember you need to consider the combination of mechanical and static compression (this is based on the cam choice) which will determine whether are not your motor will be friendly with pump gas with.
I have seen guys with 9 to 1 compression running into part throttle detonation problems. It turned out to be a poor cam choice for their combo. (Incorrect centerline (cam to much advanced or retarded) or to much or to little over lap etc)
Hope this makes sense
Jim
Last edited by Maldo; Sep 27, 2002 at 10:46 AM.
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Posts: 691
From: Jacksonville, FL
[QUOTE]Originally posted by HeavyChevySS
[B]Mikael--I don't see how that's possible...12.5 to 1 on pump gas. Maybe with severe timing retard. And was it on an LT-1 (reverse cooling)?
Striker- if you really want to use that low of an octane fuel, my suggestion is to run your gas tank as close to dry as possible and then put in 5-6 gallons and then drive around with the radio off and the windows down. LISTEN for engine knock! If you hear it then you got your answer.
Hi, when you can here the motor pinging ,,,,, it's wayyyyy to late.....
Knock occurs way before you can hear it. Just because you can not hear it does not mean tha it's is not happening....
A close examination of the plugs will usually give you a good indication of what's going on in the combustion chamber.
I think anyone who is runs any kind of high performance motor should frequently check the plugs for signs of detention or improper mixture.
[B]Mikael--I don't see how that's possible...12.5 to 1 on pump gas. Maybe with severe timing retard. And was it on an LT-1 (reverse cooling)?
Striker- if you really want to use that low of an octane fuel, my suggestion is to run your gas tank as close to dry as possible and then put in 5-6 gallons and then drive around with the radio off and the windows down. LISTEN for engine knock! If you hear it then you got your answer.
Hi, when you can here the motor pinging ,,,,, it's wayyyyy to late.....
Knock occurs way before you can hear it. Just because you can not hear it does not mean tha it's is not happening....
A close examination of the plugs will usually give you a good indication of what's going on in the combustion chamber.
I think anyone who is runs any kind of high performance motor should frequently check the plugs for signs of detention or improper mixture.
Last edited by Maldo; Mar 15, 2005 at 08:48 PM.
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Posts: 132
From: Lewiston, ID
Originally posted by HeavyChevySS
The cam is a Comp Cam, solid, 248 258 @ 050, lift is somewhere in the high 500's (I don't know exact numbers)
The cam is a Comp Cam, solid, 248 258 @ 050, lift is somewhere in the high 500's (I don't know exact numbers)
Did you ever run a compression test on the motor before? That'll tell you alot more. over 200psi and you're definately asking for it with pump gas
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From: Newark, DE
First off...thanks again for all the replies on this! I really do appreciate it!!
Maldo....YES I did post this on the Ltown board cause I wanted max response on this. And I decided that I am going to switch the heads from 66cc chambers to 74cc chambers. The reason being is...the engine (before this blown head gasket!) even with conservative timing, ran very strong! So I think I won't loose that much by going to a larger comb. chamber size and lowering the compression to about 10.2 to 1. The engine is still going to run very strong because my combo (cam, heads w/ porting, intake, carb) is pretty well matched. After the new heads go on, I think we'll be able to set the timing up around 15-16 degrees initial but that is just a guess. We will just have to play with it to get it perfect.
Another note. After I pulled the other head of the block, the gasket was not damaged at all, it was perfect.
So that leads me to believe the problem was caused by the loosening of the head bolts on that one side. When we put the new heads on, we will make sure this doesn't happen again !!!! ( I didn't put this engine together the first time either. 
)
Blue eyes, I know the cam isn't the biggest but compared to an "average" 300 - 350 hp street engine cam; yes it is. Compared to a 9 second drag car; no it's not. It's puney (sp?). Thanks.
I'll post an update on my situation after I get the new heads installed and the engine tuned up!
It's going to be pretty hard to tell if I lost anything by lowering my compression because I also yanked the old crappy pegleg 2.73 10-bolt rear and installed a REFURBISHED 12 BOLT AXLE W/ POSI & 3:73'S !!!! OH MY...I can't wait to actually launch this car at 3 grand!
Late.... Heavy
Maldo....YES I did post this on the Ltown board cause I wanted max response on this. And I decided that I am going to switch the heads from 66cc chambers to 74cc chambers. The reason being is...the engine (before this blown head gasket!) even with conservative timing, ran very strong! So I think I won't loose that much by going to a larger comb. chamber size and lowering the compression to about 10.2 to 1. The engine is still going to run very strong because my combo (cam, heads w/ porting, intake, carb) is pretty well matched. After the new heads go on, I think we'll be able to set the timing up around 15-16 degrees initial but that is just a guess. We will just have to play with it to get it perfect.
Another note. After I pulled the other head of the block, the gasket was not damaged at all, it was perfect.
So that leads me to believe the problem was caused by the loosening of the head bolts on that one side. When we put the new heads on, we will make sure this doesn't happen again !!!! ( I didn't put this engine together the first time either. )Blue eyes, I know the cam isn't the biggest but compared to an "average" 300 - 350 hp street engine cam; yes it is. Compared to a 9 second drag car; no it's not. It's puney (sp?). Thanks.
I'll post an update on my situation after I get the new heads installed and the engine tuned up!
It's going to be pretty hard to tell if I lost anything by lowering my compression because I also yanked the old crappy pegleg 2.73 10-bolt rear and installed a REFURBISHED 12 BOLT AXLE W/ POSI & 3:73'S !!!! OH MY...I can't wait to actually launch this car at 3 grand!
Late.... Heavy
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Posts: 132
From: Lewiston, ID
Originally posted by HeavyChevySS
Blue eyes, I know the cam isn't the biggest but compared to an "average" 300 - 350 hp street engine cam; yes it is. Compared to a 9 second drag car; no it's not. It's puney (sp?). Thanks.
Blue eyes, I know the cam isn't the biggest but compared to an "average" 300 - 350 hp street engine cam; yes it is. Compared to a 9 second drag car; no it's not. It's puney (sp?). Thanks.
Let's take the vintage LT1 cam - 242/254@.05, 116LSA, 6* advanced
working the math, the intake valve closes 51*ABDC@.05"
(that's theoretical @zero lash, of course, so it would really close earlier). It did have iron heads, but it was with 100 octane gas.
Your cam: 248/258@.05. Since it's comp, I'm guessing 110LSA, 4* advanced?
Intake valve closes 50*ABDC@.05". (again, minus lash). Now, you *do* have aluminum heads, but also *alot less* octane.
Big cam - definately (I'll bet it sounds wicked!) but 11:1 on pump gas big? Definately borderline... (well, you had proof of that to begin with
). You've got to weigh the thermodynamic advantages of aluminum against 8 or so fewer octane points... A wider LSA will prolonge the intake closing, reducing cranking pressure, and tendancy to detonate. It'll also increase idle vacuum some, but make the engine sound tamer.
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Of course, what really counts isn't static compression ratio but the so-called dynamic compression ratio. Static CR a simple mathematical comparison between the volume above the piston at BDC and TDC. Dynamic compression ratio is more difficult to measure. It depends on how efficiently the cylinders are filled when the engine is running. A major determinate of dynamic CR is cam timing (especially the intake valve closing point). On a FI motor, boost pressure will have a huge impact and even on an NA setup the tuning of the induction system have an impact. Dynamic CR will vary with rpm.
So, the cam cannot be ignored as it can have a major impact on dynamic CR. The effect is not so simple as "big cam = low dynamic CR" and "small cam = high dynamic CR". Consider identical setups except for the cam. The engine with the shorter camshaft duration and earlier intake closing (less overlap) will typically have higher dynamic compression at low rpm because the intake valve closes earlier on the compression stroke. If the duration is increased (and the intake closes later with more overlap) there will be less dynamic compression at low rpm. However, at high rpm, the long-duration cam may fill the cylinder better and the dynamic compression ratio may be higher. If you have ever wondered why installing a large cam in a stock shortblock may lead to crappy low and mid-range performance, the answer is in the effect of the cam on dynamic CR. A big cam needs a higher CR.
As far as how to get the desired CR (once you have figured out what it is!) be careful of thicker head gaskets as a way of lowering CR. While it's better than allowing detonation, this will decrease turbulence in the combustion chamber. Turbulence in the chamber helps prevent detonation by creating a finely atomized, homogeneous fuel/air mixture that burns quickly. A thicker head gasket will decrease the "squish" between the head and piston that is essential to producing turbulence. A better atomized induction charge will also make more power, as the fuel is burned more efficiently.
Unfortunately, while these theoretical considerations add to our understanding, they can't be used in a practical way by us amatuers. We just don't have the capability to model our combos with sufficient power to be useful. And we also don't have the means to try a bunch of different things to see what works. That's why experience is so valuable. If someone reliable has a combo that works, there is every reason to build up a similar setup and not try and reinvent the wheel. That's why a forum like this board is so valuable.
Rich Krause
So, the cam cannot be ignored as it can have a major impact on dynamic CR. The effect is not so simple as "big cam = low dynamic CR" and "small cam = high dynamic CR". Consider identical setups except for the cam. The engine with the shorter camshaft duration and earlier intake closing (less overlap) will typically have higher dynamic compression at low rpm because the intake valve closes earlier on the compression stroke. If the duration is increased (and the intake closes later with more overlap) there will be less dynamic compression at low rpm. However, at high rpm, the long-duration cam may fill the cylinder better and the dynamic compression ratio may be higher. If you have ever wondered why installing a large cam in a stock shortblock may lead to crappy low and mid-range performance, the answer is in the effect of the cam on dynamic CR. A big cam needs a higher CR.
As far as how to get the desired CR (once you have figured out what it is!) be careful of thicker head gaskets as a way of lowering CR. While it's better than allowing detonation, this will decrease turbulence in the combustion chamber. Turbulence in the chamber helps prevent detonation by creating a finely atomized, homogeneous fuel/air mixture that burns quickly. A thicker head gasket will decrease the "squish" between the head and piston that is essential to producing turbulence. A better atomized induction charge will also make more power, as the fuel is burned more efficiently.
Unfortunately, while these theoretical considerations add to our understanding, they can't be used in a practical way by us amatuers. We just don't have the capability to model our combos with sufficient power to be useful. And we also don't have the means to try a bunch of different things to see what works. That's why experience is so valuable. If someone reliable has a combo that works, there is every reason to build up a similar setup and not try and reinvent the wheel. That's why a forum like this board is so valuable.
Rich Krause
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Re: Compression Ratio VS. Fuel Octane
Can I ask you what your quench clearance is?
I've been trying to make it a point to ask that question whenever I hear someone is having detonation issues; and I'm hoping to see if the pattern I expect to see appears.
I've been trying to make it a point to ask that question whenever I hear someone is having detonation issues; and I'm hoping to see if the pattern I expect to see appears.
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Re: Compression Ratio VS. Fuel Octane
I'm glad to see people are using the "search" feature, even if it means dredging up 2.5-year old threads.......
Hopefully, HeavyChevySS has solved his problem by now.
Hopefully, HeavyChevySS has solved his problem by now.
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Re: Compression Ratio VS. Fuel Octane
Originally Posted by LameRandomName
Can I ask you what your quench clearance is?
I've been trying to make it a point to ask that question whenever I hear someone is having detonation issues; and I'm hoping to see if the pattern I expect to see appears.
I've been trying to make it a point to ask that question whenever I hear someone is having detonation issues; and I'm hoping to see if the pattern I expect to see appears.
I am having a detonation problem with 10.8 Static compression and the hot cam with water temps around 180. My quench is at .054" I am planning on switching out to a .026" gasket to lower this number to .041". By doing this I will also be raising CR to 11.2, So I will keep my fingers crossed and hope it works.
Also would a colder plug be worth investegating? And how do you go about figuring out what plug runs the best?
