Effective Compression
06-23-2004, 09:52 AM
#1
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Join Date: Nov 2002
Location: Central KY
Posts: 292
Since high compression and boost come up here frequently here is more information that may be helpful. I got this from a piston only page but the math is the same.
Anyone who has run higher compression rotors with boost please post here. Let us know: PSI as well as CFM, turbo or S/C, fuel octane used, intake charge temps, exhaust gas temps, intercooled or aftercooled, or if your system is water injected.
Experience only please, no hypotheticals or BS. I want this to be a resource for board members. Maybe we can learn from other's mistakes instead of making costly ones ourselves.
Cheers
Compression -vs- Boost
Anyone who has run higher compression rotors with boost please post here. Let us know: PSI as well as CFM, turbo or S/C, fuel octane used, intake charge temps, exhaust gas temps, intercooled or aftercooled, or if your system is water injected.
Experience only please, no hypotheticals or BS. I want this to be a resource for board members. Maybe we can learn from other's mistakes instead of making costly ones ourselves.
Cheers
Compression -vs- Boost
Compression -vs- Boost
--------------------------------------------------------------------------------
Almost as fast as a supercharger can be bolted on, the question of how much boost can be run is instantly a concern. When building up a motor to be supercharged, you've also got the question of just how much compression to run. Both of these questions relate to essentially the same set of equations. Assuming that all of the other requirements of the motor are satisfied, the compression -vs- boost aspect is not all that difficult.
The only way to make more power is to increase cylinder pressure and burn more fuel. The main purpose of the supercharger is to supply the motor with a more dense air charge, which allows for the ability to burn the additional fuel. By adding a supercharger, additional air should no longer be a problem. Ensuring that there will be enough additional fuel to maintain the proper air to fuel ratio will be the key to using the maximum effective compression.
All motors have a static compression ratio. This is the amount that the air inside the cylinder is compressed. It is a ratio of the cylinder volume at BDC to the volume at TDC. When a supercharger is added, additional air is forced into the cylinder effectively raising the compression ratio. The result of this is called effective compression. The formula for finding the effective compression is very easy:
((boost psi / 14.7) + 1) x motor compression = effective compression.
The effective compression allows a supercharged motor to be compared to a normally aspirated motor. For the most part, a supercharged motor with the same effective compression as a (similar) normally aspirated motor with the same static compression should have about the same overall power.
This may bring up the question that if the overall power should be about the same, why go with a supercharger? The main advantage of the supercharger is that it allows for a moderate compression level during normal driving while allowing for very high compression levels when needed. Obviously a high compression motor of about 14:1 makes a lot of power, but it would never survive daily driving. A lower compression motor is great for daily driving, but greatly reduces the potential for power. The supercharger allows for higher compression levels than could be used without a supercharger, while still offering the benifits of a standard compression motor. Many street supercharged systems will go beyond 18:1 effective compression under boost. Under race conditions, many supercharged race motors will go well beyond 22:1 effective compression. Both of these levels are far beyond what could be done reliably or cost effectively without a supercharger.
This brings us back to the question of just how much boost or compression can be run. Obviously there can't be a simple number that could be used for every application. This is why it's so critical to chose the proper components. It's not necessary to build a low compression motor to use a supercharger, but the correct parts are still necessary. The biggest factors will be in things like head bolts (or preferably studs), gaskets, and the strength of the other engine components. It goes without saying that the incredible power that a supercharger can add, can easily start breaking things. It is very important that as the boost levels rise, the need for a stronger crank, rods, pistons, etc... becomes very critical. Many people forget this as the motor itself is relatively mild, while the supercharger pushes it well beyond the practical limits it was intended for.
Now, back to the compression issue. Anyone who has looked into supercharging has heard that you need a low (static) compression motor. This may have been true once upon a time, when roots type (positive displacement) superchargers ruled the land, but it's not so necessary now. The problem with a low compression motor is that it relies heavily on the supercharger for its power. An 8:1 motor is definitely not going to be a power house. Sure, you can throw 18 lbs of boost on it and get some real power, but why? A higher compression motor of 9.5:1 will have much more power without the blower. Then, with less boost you could easily have the same overall power - only it would be much more usable. Both of the motors (8:1 with 18 lbs boost and 9.5:1 with 12 lbs boost) will have almost the same effective compression and about the same overall power. The big difference will be where you see the power, and how much of a demand will be placed on the supercharger. Obviously, the 9.5:1 motor is going to have far greater torque and low end power as the boost is only starting to come in. It is also going to be much easier to find a blower to survive only 12 lbs of boost -vs- one that would have to put out 18 lbs. It is now very easy to see why a higher compression motor with lower boost is becoming so popular.
Please understand that when I say higher compression and lower boost, there are limits to each. Going over about 10:1 will make the amount of boost that is usable drop quickly to the point that the supercharger is somewhat wasted. In my opinion, anything less than 8 lbs of boost is a waste of a supercharger. Going over 10:1 will also make daily driving with pump gas much more difficult. In this same way, compression levels much under 9:1 will require substantial boost levels to make massive power gains. This would require boost levels that are very demanding of a supercharger. This is truly unnecessary. This isn't to say that the lower compression / higher boost set-up doesn't have a slightly higher potential for power, because it does. A lower compression motor has the ability to contain more volume. This can be an advantage, but is such a minor one that it's not necessarily worth the effort - unless it's for an all out race motor. Even then there are limits for the same reasons as the street / strip motor.
Once again, the compression -vs- boost issue. For a car that will see the streets (actually for most applications), the best thing to do is start with a motor compression that is high enough to make the horsepower you want for normal driving. Don't rely on your supercharger to make all your horsepower. With a good motor compression, add as much boost as is safe for your particular application. Decide on a final effective compression, and work your way back through the formula to find your maximum boost level: ((effective compression / motor compression) - 1) x 14.7 = boost. With the proper fuel system and related engine components, an effective compression of 16:1 to 18:1 should be more than workable. For heavily modified cars, effective compressions over 20:1 should be very carefully considered. Remember, even Indy cars only run about 18 Lbs of boost and reasonable static compression levels. Technology has come a long way and modern day supercharging should take full advantage of this.
While these opinions are not exactly the most popular, they are based on facts and real world performance. While there will always be those who continue with tradition and stick with what was done in the past, it is those who reach for something more that are winning races. Often times, some of the best advice can be found from those who have done what you want to do. All too often it is those who know the least that offer the most advice. After having been involved in supercharging for many years, I have heard it all. Most of it was worthless. It was often the least mentioned things and trail and error that have been the most rewarding. Hopefully this information will help to explain some of the often misunderstood aspects of supercharging.
--------------------------------------------------------------------------------
CopyrightŠ 2001, MotorSports Digest
msd@stic.net
--------------------------------------------------------------------------------
Almost as fast as a supercharger can be bolted on, the question of how much boost can be run is instantly a concern. When building up a motor to be supercharged, you've also got the question of just how much compression to run. Both of these questions relate to essentially the same set of equations. Assuming that all of the other requirements of the motor are satisfied, the compression -vs- boost aspect is not all that difficult.
The only way to make more power is to increase cylinder pressure and burn more fuel. The main purpose of the supercharger is to supply the motor with a more dense air charge, which allows for the ability to burn the additional fuel. By adding a supercharger, additional air should no longer be a problem. Ensuring that there will be enough additional fuel to maintain the proper air to fuel ratio will be the key to using the maximum effective compression.
All motors have a static compression ratio. This is the amount that the air inside the cylinder is compressed. It is a ratio of the cylinder volume at BDC to the volume at TDC. When a supercharger is added, additional air is forced into the cylinder effectively raising the compression ratio. The result of this is called effective compression. The formula for finding the effective compression is very easy:
((boost psi / 14.7) + 1) x motor compression = effective compression.
The effective compression allows a supercharged motor to be compared to a normally aspirated motor. For the most part, a supercharged motor with the same effective compression as a (similar) normally aspirated motor with the same static compression should have about the same overall power.
This may bring up the question that if the overall power should be about the same, why go with a supercharger? The main advantage of the supercharger is that it allows for a moderate compression level during normal driving while allowing for very high compression levels when needed. Obviously a high compression motor of about 14:1 makes a lot of power, but it would never survive daily driving. A lower compression motor is great for daily driving, but greatly reduces the potential for power. The supercharger allows for higher compression levels than could be used without a supercharger, while still offering the benifits of a standard compression motor. Many street supercharged systems will go beyond 18:1 effective compression under boost. Under race conditions, many supercharged race motors will go well beyond 22:1 effective compression. Both of these levels are far beyond what could be done reliably or cost effectively without a supercharger.
This brings us back to the question of just how much boost or compression can be run. Obviously there can't be a simple number that could be used for every application. This is why it's so critical to chose the proper components. It's not necessary to build a low compression motor to use a supercharger, but the correct parts are still necessary. The biggest factors will be in things like head bolts (or preferably studs), gaskets, and the strength of the other engine components. It goes without saying that the incredible power that a supercharger can add, can easily start breaking things. It is very important that as the boost levels rise, the need for a stronger crank, rods, pistons, etc... becomes very critical. Many people forget this as the motor itself is relatively mild, while the supercharger pushes it well beyond the practical limits it was intended for.
Now, back to the compression issue. Anyone who has looked into supercharging has heard that you need a low (static) compression motor. This may have been true once upon a time, when roots type (positive displacement) superchargers ruled the land, but it's not so necessary now. The problem with a low compression motor is that it relies heavily on the supercharger for its power. An 8:1 motor is definitely not going to be a power house. Sure, you can throw 18 lbs of boost on it and get some real power, but why? A higher compression motor of 9.5:1 will have much more power without the blower. Then, with less boost you could easily have the same overall power - only it would be much more usable. Both of the motors (8:1 with 18 lbs boost and 9.5:1 with 12 lbs boost) will have almost the same effective compression and about the same overall power. The big difference will be where you see the power, and how much of a demand will be placed on the supercharger. Obviously, the 9.5:1 motor is going to have far greater torque and low end power as the boost is only starting to come in. It is also going to be much easier to find a blower to survive only 12 lbs of boost -vs- one that would have to put out 18 lbs. It is now very easy to see why a higher compression motor with lower boost is becoming so popular.
Please understand that when I say higher compression and lower boost, there are limits to each. Going over about 10:1 will make the amount of boost that is usable drop quickly to the point that the supercharger is somewhat wasted. In my opinion, anything less than 8 lbs of boost is a waste of a supercharger. Going over 10:1 will also make daily driving with pump gas much more difficult. In this same way, compression levels much under 9:1 will require substantial boost levels to make massive power gains. This would require boost levels that are very demanding of a supercharger. This is truly unnecessary. This isn't to say that the lower compression / higher boost set-up doesn't have a slightly higher potential for power, because it does. A lower compression motor has the ability to contain more volume. This can be an advantage, but is such a minor one that it's not necessarily worth the effort - unless it's for an all out race motor. Even then there are limits for the same reasons as the street / strip motor.
Once again, the compression -vs- boost issue. For a car that will see the streets (actually for most applications), the best thing to do is start with a motor compression that is high enough to make the horsepower you want for normal driving. Don't rely on your supercharger to make all your horsepower. With a good motor compression, add as much boost as is safe for your particular application. Decide on a final effective compression, and work your way back through the formula to find your maximum boost level: ((effective compression / motor compression) - 1) x 14.7 = boost. With the proper fuel system and related engine components, an effective compression of 16:1 to 18:1 should be more than workable. For heavily modified cars, effective compressions over 20:1 should be very carefully considered. Remember, even Indy cars only run about 18 Lbs of boost and reasonable static compression levels. Technology has come a long way and modern day supercharging should take full advantage of this.
While these opinions are not exactly the most popular, they are based on facts and real world performance. While there will always be those who continue with tradition and stick with what was done in the past, it is those who reach for something more that are winning races. Often times, some of the best advice can be found from those who have done what you want to do. All too often it is those who know the least that offer the most advice. After having been involved in supercharging for many years, I have heard it all. Most of it was worthless. It was often the least mentioned things and trail and error that have been the most rewarding. Hopefully this information will help to explain some of the often misunderstood aspects of supercharging.
--------------------------------------------------------------------------------
CopyrightŠ 2001, MotorSports Digest
msd@stic.net
06-24-2004, 05:13 PM
#2
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Join Date: Nov 2003
Posts: 480
Here's some info excerpted from a book on turbocharging:
Twe engines:
1 - normally aspirated 10.5:1 compression
2 - 7.0:1 compression at 10 psi boost
Compression pressure on engine #1 is 340 psia
Compression pressure on engine #2 is 320 psia
engine #2 consumes over 50% more air and produces over 50% more power
than engine #1 - despite having less pressure at peak compression.
This would lead one to believe that boost and compression really don't
interchange per se.
Twe engines:
1 - normally aspirated 10.5:1 compression
2 - 7.0:1 compression at 10 psi boost
Compression pressure on engine #1 is 340 psia
Compression pressure on engine #2 is 320 psia
engine #2 consumes over 50% more air and produces over 50% more power
than engine #1 - despite having less pressure at peak compression.
This would lead one to believe that boost and compression really don't
interchange per se.
06-24-2004, 06:44 PM
#3
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Join Date: Oct 2003
Location: Boston Ma.
Posts: 1,420
Are you building a supercharged rotary? If not, very little of that article applies to turbocharging. The best example;
The ability to contain more volume in a SC application may have marginal gains. But in a turbocharged engine the benefits of the higher volume are more considerable.
But you may choose to dismiss this post because i have no experience with *shudders* with a turbocharged N/A motor.
"This is truly unnecessary. This isn't to say that the lower compression / higher boost set-up doesn't have a slightly higher potential for power, because it does. A lower compression motor has the ability to contain more volume. This can be an advantage, but is such a minor one that it's not necessarily worth the effort "
But you may choose to dismiss this post because i have no experience with *shudders* with a turbocharged N/A motor.
06-25-2004, 06:58 AM
#4
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Originally Posted by RONIN FC' date='Jun 24 2004, 06:44 PM
Are you building a supercharged rotary? If not, very little of that article applies to turbocharging. The best example;
The ability to contain more volume in a SC application may have marginal gains. But in a turbocharged engine the benefits of the higher volume are more considerable.
The ability to contain more volume in a SC application may have marginal gains. But in a turbocharged engine the benefits of the higher volume are more considerable.
15.79:1 for a 10 PSI S/C on 9.4:1 rotors.
15.80:1 for a 12 PSI turbo on 8.7:1 rotors.
By no means am I plugging S/C over turbo, I just wanted this to be a resource for rotors heads. Whether they wanted to think out of the box or use tried and true techniques.
06-25-2004, 10:36 PM
#5
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Posts: 524
Originally Posted by twstdmtl' date='Jun 25 2004, 03:58 AM
Don't miss my point here. This is an "across the board" formula for determining compression ratios under boost.
What you want is "dynamic compression"... and that is a WHOLE different ball of wax, because it is dependent on compressor efficiency, atmospheric conditions, port shape/size, engine RPM, sealing efficiency, not to mention the intake systems and the exhaust systems and how they all interrelate!
You can't say "12psi with 8.5 = xpsi with 9.4" because there are WAAAAYYY too many variables. Just look at the people making more power at the same RPM using a single with less boost, compared to stock single or stock twins! Or more power with the same turbo and same boost but different porting.
THERE IS NO EASY WAY TO SLICE IT. I am sure you could work up some formula but it will have to be insanely complex because you'll have to take into account ALL variables... and I doubt that even the R&D departments of the big OEMs even have ever bothered to work them up!
06-27-2004, 11:30 AM
#6
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Location: Boston Ma.
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Originally Posted by twstdmtl' date='Jun 25 2004, 06:58 AM
for example, the kid who put 10 PSI of boost from an S/C on 9.4:1 N/A rotors is not that much different from 12PSI on 8.7:1 Turbo rotors.
15.79:1 for a 10 PSI S/C on 9.4:1 rotors.
15.80:1 for a 12 PSI turbo on 8.7:1 rotors.
15.79:1 for a 10 PSI S/C on 9.4:1 rotors.
15.80:1 for a 12 PSI turbo on 8.7:1 rotors.
In my *oppinion*, (and this is gathered from people who have done it in this board and others) that 10 psi on 9.7s is at, or beyond its limit. Where 12 psi on 8.7s or 9.0s are not even close to its limits.
06-27-2004, 12:07 PM
#7
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Originally Posted by RONIN FC' date='Jun 27 2004, 11:30 AM
Not to echo heretic, but your thowing even more variables in there by comparing a S/C to turbo, then putting different rotors and psi for each.
In my *oppinion*, (and this is gathered from people who have done it in this board and others) that 10 psi on 9.7s is at, or beyond its limit. Where 12 psi on 8.7s or 9.0s are not even close to its limits.
In my *oppinion*, (and this is gathered from people who have done it in this board and others) that 10 psi on 9.7s is at, or beyond its limit. Where 12 psi on 8.7s or 9.0s are not even close to its limits.
Have you ever experienced (or known some one who has experienced) 2 bar or (14.7 PSI * 2) = 29.4PSI or more on 8.X CR rotors?!? There are people on this very forum that have! That to me seems a little "at or, beyond it's limits." as you put it... Or nitrous oxide systems that exceed 250 HP shots. The effective compression ratio in a n20 rotary is just unimaginable. Yet people are doing it here.
You all are right, there are a lot of other variables in the mix, too many to list. My goal here was just to present a real world proven mathematical formula to explain the difference and similarities between low compression/high boost and high compression/low boost scenarios. Regardless of "other variables" the largest single factor in determining power is compression ratio... We boost (S/C or turbo) or spray (n20) to get a better compression ratio to get more air/fuel into the combustion process hopefully without going POP
Anytime some one says SUPERCHARGER on this forum they are just asking for trouble. You don't have to like it, just try to respect that someone else wants to do or does something different. I have personally invited two local gentlemen to post their S/C rotaries and experience here. Both of them politefully declined after seeing how the community is pro-turbo to the point of being anti-supercharger. I have SEEN 9.X CR rotaries running 10-12 PSI from positive displacement roots style superchargers. They may fall a little short of a Tii on the dyno but will eat a Tii in the quarter. It's just different strokes for different folks.
I started the thread (potential flame session
Cheers
no hate, no foul, i only have love for my rotary brothers
06-27-2004, 12:34 PM
#8
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Join Date: Nov 2002
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Originally Posted by heretic' date='Jun 25 2004, 10:36 PM
Compression *ratio* is fixed and is the mathematical difference between maximum chamber size and minimum chamber size.
What you want is "dynamic compression"... and that is a WHOLE different ball of wax, because it is dependent on compressor efficiency, atmospheric conditions, port shape/size, engine RPM, sealing efficiency, not to mention the intake systems and the exhaust systems and how they all interrelate!
You can't say "12psi with 8.5 = xpsi with 9.4" because there are WAAAAYYY too many variables. Just look at the people making more power at the same RPM using a single with less boost, compared to stock single or stock twins! Or more power with the same turbo and same boost but different porting.
THERE IS NO EASY WAY TO SLICE IT. I am sure you could work up some formula but it will have to be insanely complex because you'll have to take into account ALL variables... and I doubt that even the R&D departments of the big OEMs even have ever bothered to work them up!
What you want is "dynamic compression"... and that is a WHOLE different ball of wax, because it is dependent on compressor efficiency, atmospheric conditions, port shape/size, engine RPM, sealing efficiency, not to mention the intake systems and the exhaust systems and how they all interrelate!
You can't say "12psi with 8.5 = xpsi with 9.4" because there are WAAAAYYY too many variables. Just look at the people making more power at the same RPM using a single with less boost, compared to stock single or stock twins! Or more power with the same turbo and same boost but different porting.
THERE IS NO EASY WAY TO SLICE IT. I am sure you could work up some formula but it will have to be insanely complex because you'll have to take into account ALL variables... and I doubt that even the R&D departments of the big OEMs even have ever bothered to work them up!
I am not trying to over simply anything but... But do you eat an apple or do you eat a vegetatively created conglomeration of elements and molecules more specifically H2O, glucose, fructose, sucrose, and primarily carbon based constituents? Not to be silly but I am just proving a point, that anyone can over complicate anything.
06-28-2004, 08:47 PM
#9
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Join Date: Jun 2004
Posts: 524
An engine can never run and have a static compression ratio. Again, it's just the difference between the chamber's maximum size and minimum size. For a 9.4:1 13B, the numbers are 731.86cc and 77.86cc. The chamber is 731.86cc at its largest, 77.86cc at its smallest, and if you notice, 731.86 minus 77.86 is 654, which is the displacement of one chamber. Pure math.
Dynamic compression (it's not really a "ratio" anymore, since you're not comparing max size to min size) is dependent on waaaay too many factors, as outlined above.
And then there's real-world application: How detonation-prone is it/how much power can it make? This I think is what the point of your query. And it's even more complex - you have to figure in the different tuning required, different air temperatures (which affect air density as well as detonation resistance) which is related to compressor efficiency and intercooler efficiency.
Dynamic compression (it's not really a "ratio" anymore, since you're not comparing max size to min size) is dependent on waaaay too many factors, as outlined above.
And then there's real-world application: How detonation-prone is it/how much power can it make? This I think is what the point of your query. And it's even more complex - you have to figure in the different tuning required, different air temperatures (which affect air density as well as detonation resistance) which is related to compressor efficiency and intercooler efficiency.
06-29-2004, 06:46 AM
#10
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Join Date: Nov 2002
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Posts: 292
An engine can never run and have a static compression ratio.
Anyone who has run higher compression rotors with boost please post here. Let us know: PSI as well as CFM, turbo or S/C, fuel octane used, intake charge temps, exhaust gas temps, intercooled or aftercooled, or if your system is water injected.
What compression ratio and how many pounds of boost did you say your running?