Originally posted by David Coleman, cleaned up by myself for readability purposes.
The K series motor has a forged crank, oil squirters on the pistons, short stroke, great rod ratio, huge quench area, miniscule valve angle, well cooled aluminum heads, low mean piston speed, low combustion chamber surface area, extra oil galleys on block, and many other features that lend itself directly to boosting. I know guys who have run 14psi+ on daily driven KLDE engines. The first roadblock on a boosted KLDE or KLZE is fuel, or lack thereof with teh stock injectors. That is going to hold you around 6-9psi, depending on setup as far as compressor and IC size. 6-9psi on 10:1 CR is still streetable on pump gas if you can control your ignition advance and injector pulsewidths. You'll make more power on a high compression motor at 8psi than a lower compression motor at 9psi, along with having better MPG and a quicker spool.
FAQ Rewrite: Compression and Boost
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ScooterBovine
- Supporting Member
- Posts: 502
- Joined: May 9th, 2004, 2:01 am
- Location: Blacklick, Ohio
FAQ Rewrite: Compression and Boost
No more MX-3.
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ScooterBovine
- Supporting Member
- Posts: 502
- Joined: May 9th, 2004, 2:01 am
- Location: Blacklick, Ohio
This was written by a Honda guy, but the theories cross over.
There has been a long-standing myth with turbocharged Honda cars. For as long as I can remember, people have been quite concerned about having too high of a compression ratio, on their turbocharged Honda. The myth is: the lower the compression...the better. Well, this is not entirely true.
Why is a low compression motor good to turbocharge?
In essence, a low compression motor is good to turbocharge, because it is just easier to do so...and not worry about it. But is that what we really want to do as enthusiasts? Do we want to just slap a turbocharger on a car, and call it a day? Absolutely not. With turbocharging, comes the responsibility of tuning and care. If you are just going to turbocharge your car, and call it a day, then frankly, you don't deserve the luxery of boost!
As well, might I add...that simply because it is easy to slap a turbocharger on a low compression motor and not worry about it, does not mean that it is right. I can guarantee you, that if you turbocharge a low compression motor, and fail to tune it correctly, you will end up on the side of the road, with blue smoke coming from your exhaust pipe.
Generally, a lower compression motor affords you more margin of error, when tuning. A slightly imperfect a:f ratio probably won't lead to the demise of your motor...unless you drive like a total jerk.
Why should I consider a higher compression motor, while making my decision of what motor will suit my needs?
Different from a low compression motor, a higher compression motor will not give you a large margin of error, when tuning. As stated before, if you expect to slap on a turbocharger, and call it a day, well then stay away from the higher compression motors all together. A higher compression motor demands slightly more TLC than a lower compression motor. But oh, the rewards are plenty.
Bottom line...a higher compression motor, pound for pound, will make more power, than a low compression motor. This works along the same lines as naturally aspirated Honda motors. You never see an NA enthusiasts spouting off about low compression do you? No, one of the keys to NA performance is high compression. So, why should it be any differnt for turbocharged applications? Of course, the routes to high compression are different (NA uses lightweight rods and pistons, that a turbocharged application would simply tear to shreds), but the end goal is the same.
Let's make an observation here. I am going to throw a scenerio for you...
Car A:
B18B (stock 9.2:1 CR)
Rev Hard Stage II
Thermal 3" exhaust
MSD 6AL
MSD Pro Cap
MSD Blaster 3 Coil
Holley 255lph in-tank fuel pump
AEM fuel rail
RC440cc injectors
A'pex S-AFC
Car B:
B18B (JE pistons, 10.5:1 CR)
Rev Hard Stage II
Thermal 3" exhaust
MSD 6AL
MSD Pro Cap
MSD Blaster 3 Coil
Holley 255lph in-tank fuel pump
AEM fuel rail
RC440cc injectors
A'pex S-AFC
Now for the sake of argument, let's say that both cars are well tuned, by the same tuner, both have an identical weight, both have the same slicks, both are boosting 1.0bar, and both are being driven by the same guy...at the same time (yeah, yeah, yeah...just roll with me here)!
Now, which one do you think is going to come across the finish line first? Mythological thinking says that the guy with the low compression is best suited to win...right? Wrong. The guy with the 10.5:1 compression ratio is going to smoke the guy with the low compression.
Another point I would like to bring up is the misuse of the term "high compression" when it comes to Honda motors, and turbocharging them. In all honesty...10.6:1, 11.0:1, etc...aren't even really that high. Most NA monsters utilize 12.5:1 or higher...and some even as high as 14.0:1, in cases of extreme race. If you do some snooping around, you will realize that most of the really fast Hondas, and I'm talking sub-10 second monsters, utilize high compression setups, to achieve enourmous horsepower goals. Most of these guys won't openly discuss it though, so you are likely to come across terms such as "undisclosed compression", or something along that line. As I stated before, the myth is that low compression is key...so these guys want to stay on top, and the way to do it is hide the fact that they are using high compression, turbocharged motors...to propel themselves into sub-10's.
Reliability, and compression...
This is a regular question that pops up, and is quite valid. Frankly, as soon as you modify a Honda at all...reliability becomes an issue. A lot of us can testify that the proverbial "can of worms" sprung open as soon as we made our first modification. Many times, Honda enthusiasts will spout off the reliability factor, in Import vs. Domestic wars, when in actuality, they fail to realize that we are in just as much danger as those old pushrod V8's are, as soon as we modify our Honda engines.
Now, what is the solution? I have been saying it for a long time, as have several others such as Tuan, neouser, texan, and st00pid...TUNING IS KEY! That's the bottom line.
A well tuned higher compression motor will last just as long as any low compression motor. Tuning is not a factor that can afford to be sacraficed. You will not survive with an untuned higher compression motor, just as you will not survive with an untuned low compression motor.
So, with all of that said, it basically boils down to your own personal choice. Do not fear the B16A, the B18C1, or the H22A...etc. If you have the monitary requirement to turbocharge a higher compression VTEC motor, I would say, do not hesitate to do so.
I hope that you have a slightly better understanding of how compression and turbochargers co-exist. If you have any questions, feel free to make a new thread, and inquire. Thanks for reading!
There has been a long-standing myth with turbocharged Honda cars. For as long as I can remember, people have been quite concerned about having too high of a compression ratio, on their turbocharged Honda. The myth is: the lower the compression...the better. Well, this is not entirely true.
Why is a low compression motor good to turbocharge?
In essence, a low compression motor is good to turbocharge, because it is just easier to do so...and not worry about it. But is that what we really want to do as enthusiasts? Do we want to just slap a turbocharger on a car, and call it a day? Absolutely not. With turbocharging, comes the responsibility of tuning and care. If you are just going to turbocharge your car, and call it a day, then frankly, you don't deserve the luxery of boost!
As well, might I add...that simply because it is easy to slap a turbocharger on a low compression motor and not worry about it, does not mean that it is right. I can guarantee you, that if you turbocharge a low compression motor, and fail to tune it correctly, you will end up on the side of the road, with blue smoke coming from your exhaust pipe.
Generally, a lower compression motor affords you more margin of error, when tuning. A slightly imperfect a:f ratio probably won't lead to the demise of your motor...unless you drive like a total jerk.
Why should I consider a higher compression motor, while making my decision of what motor will suit my needs?
Different from a low compression motor, a higher compression motor will not give you a large margin of error, when tuning. As stated before, if you expect to slap on a turbocharger, and call it a day, well then stay away from the higher compression motors all together. A higher compression motor demands slightly more TLC than a lower compression motor. But oh, the rewards are plenty.
Bottom line...a higher compression motor, pound for pound, will make more power, than a low compression motor. This works along the same lines as naturally aspirated Honda motors. You never see an NA enthusiasts spouting off about low compression do you? No, one of the keys to NA performance is high compression. So, why should it be any differnt for turbocharged applications? Of course, the routes to high compression are different (NA uses lightweight rods and pistons, that a turbocharged application would simply tear to shreds), but the end goal is the same.
Let's make an observation here. I am going to throw a scenerio for you...
Car A:
B18B (stock 9.2:1 CR)
Rev Hard Stage II
Thermal 3" exhaust
MSD 6AL
MSD Pro Cap
MSD Blaster 3 Coil
Holley 255lph in-tank fuel pump
AEM fuel rail
RC440cc injectors
A'pex S-AFC
Car B:
B18B (JE pistons, 10.5:1 CR)
Rev Hard Stage II
Thermal 3" exhaust
MSD 6AL
MSD Pro Cap
MSD Blaster 3 Coil
Holley 255lph in-tank fuel pump
AEM fuel rail
RC440cc injectors
A'pex S-AFC
Now for the sake of argument, let's say that both cars are well tuned, by the same tuner, both have an identical weight, both have the same slicks, both are boosting 1.0bar, and both are being driven by the same guy...at the same time (yeah, yeah, yeah...just roll with me here)!
Now, which one do you think is going to come across the finish line first? Mythological thinking says that the guy with the low compression is best suited to win...right? Wrong. The guy with the 10.5:1 compression ratio is going to smoke the guy with the low compression.
Another point I would like to bring up is the misuse of the term "high compression" when it comes to Honda motors, and turbocharging them. In all honesty...10.6:1, 11.0:1, etc...aren't even really that high. Most NA monsters utilize 12.5:1 or higher...and some even as high as 14.0:1, in cases of extreme race. If you do some snooping around, you will realize that most of the really fast Hondas, and I'm talking sub-10 second monsters, utilize high compression setups, to achieve enourmous horsepower goals. Most of these guys won't openly discuss it though, so you are likely to come across terms such as "undisclosed compression", or something along that line. As I stated before, the myth is that low compression is key...so these guys want to stay on top, and the way to do it is hide the fact that they are using high compression, turbocharged motors...to propel themselves into sub-10's.
Reliability, and compression...
This is a regular question that pops up, and is quite valid. Frankly, as soon as you modify a Honda at all...reliability becomes an issue. A lot of us can testify that the proverbial "can of worms" sprung open as soon as we made our first modification. Many times, Honda enthusiasts will spout off the reliability factor, in Import vs. Domestic wars, when in actuality, they fail to realize that we are in just as much danger as those old pushrod V8's are, as soon as we modify our Honda engines.
Now, what is the solution? I have been saying it for a long time, as have several others such as Tuan, neouser, texan, and st00pid...TUNING IS KEY! That's the bottom line.
A well tuned higher compression motor will last just as long as any low compression motor. Tuning is not a factor that can afford to be sacraficed. You will not survive with an untuned higher compression motor, just as you will not survive with an untuned low compression motor.
So, with all of that said, it basically boils down to your own personal choice. Do not fear the B16A, the B18C1, or the H22A...etc. If you have the monitary requirement to turbocharge a higher compression VTEC motor, I would say, do not hesitate to do so.
I hope that you have a slightly better understanding of how compression and turbochargers co-exist. If you have any questions, feel free to make a new thread, and inquire. Thanks for reading!
No more MX-3.
-
ScooterBovine
- Supporting Member
- Posts: 502
- Joined: May 9th, 2004, 2:01 am
- Location: Blacklick, Ohio
One main concern in power production with forced induction is effective compression.
Effective compression is the sum of the motors static compression, plus the additional compression added by the forced induction tool.
A B18C1 (also B16A) motor will have a higher effective compression than a B18B motor will, on the same boost...therefore, pound for pound, it will make more power.
The next argument that people usually bring up is that a higher compression is bad for turbocharging. Well, if you understand the concept of effective compression, then you should understand that this statement is entirely incorrect. A higher compression engine makes more power in NA form. So, why do you turbo guys think that a lower compression turbo motor makes more power? Does that make any sense when you really think about it? A turbocharger is a power adder? So why deplete power that was there to begin with? The answer I usually get to that is "So I can run more boost!" Well, sorry to rain on your parade, but more boost does not always equal more power. Check out this mathematical example of effective compression:
A motor with a 10.0:1 static CR boosting 10psi
10psi/14.7psi = .68
.68 + 1 = 1.68
1.68 x 10 = 16.8 effective CR
A motor with an 8.5:1 static CR boosting 10psi
10psi/14.7psi = .68
.68 + 1 = 1.68
1.68 x 8.5 = 14.28 effective CR
Now tell me who is going to make more power? The higher CR motor, or the lower CR motor?
So, maybe add more boost to the lower CR motor, right? Wrong...
A motor with an 8.5:1 static CR boosting 13psi
13psi/14.7psi = .88
.88 + 1 = 1.88
1.88 x 8.5 = 15.98 effective CR
Now you see, even adding 3psi of boost, still does not equal the effective CR of the higher compression, lower boost motor.
Effective compression is not the only advantage of the B16A/B18C1 either. The B16A/B18C1 has a stronger, better flowing cylinder head. It can rev much higher, making it that much more effective, and it flows great to handle all of the extra volume. The block has oil squirters to help support the bottom end assembly at high RPM. It takes more than a valvetrain upgrade to make a B18B safe at 8k. The higher compression also aids in spooling the turbo faster too.
Both motors have similar tolerances though. Both motors pretty much top out at around 350-400hp on stock motors, very well tuned. The B18C1 will make it far more efficiently for you though. It takes less boost to do so, it has more safeguards...and the bottom line on any Honda motor is tuning. If it is well tuned, you will be set. That goes for both motors. YOU ARE A FOOL if you think for one second that just because your B18B has a lower compression, you can substitute that for proper tuning.
A lot of people like to lower their motors compression when they build their motor. I used to think it was a good idea before I understood about tuning, and the positive aspects of compression. In the mathematical representation below, I will show you how a low compression motor must boost more to equal the output of a higher compression, lower boost motor:
Motor: stock B16A2 boosting 7psi.
Static Compression Ratio: 10.4:1
((boost psi / 14.7) + 1) x motor compression = effective compression
Stock motor (10.4:1 CR) on 7psi:
7psi/14.7psi = .47
.47 + 1 = 1.47
1.47 x 10.4 = 15.288 effective CR
Built motor (9.0:1 CR) on 7psi:
7psi/14.7psi = .47
.47 + 1 = 1.47
1.47 x 9 = 13.23 effective CR
You will lose 2.058 points from your effective compression ratio, this translates to a significant power loss.
In order to gain back that power, you have to do this:
Built motor (9.0:1 CR) on 10.5psi:
10.5psi/14.7psi = .71
.71 + 1 = 1.71
1.71 x 9 = 15.39 effective CR
Add 3.5psi to what you were boosting before, and you should be able to make around the same power as before, granted you haven't done any other kinds of modifications port/polish, cams, etc...
As you can see, considering all things stay equal (bore/stroke/cylinder head/etc...), you must add 3.5psi to make the motors perform similarly. You just spent about $2,500 to build your bottom end, and make your car slow.
By now we all should understand the positive aspects of compression, and how when teamed with the faster spoolng turbo, more efficient output, better flowing B-series VTEC cylinder heads, better low end spool time, stock oil squirters, higher redline, etc...you should see that turbocharging B-series VTEC motors is clearly not dangerous, and highly adviseable. I love a good turbo B16A!
Effective compression is the sum of the motors static compression, plus the additional compression added by the forced induction tool.
A B18C1 (also B16A) motor will have a higher effective compression than a B18B motor will, on the same boost...therefore, pound for pound, it will make more power.
The next argument that people usually bring up is that a higher compression is bad for turbocharging. Well, if you understand the concept of effective compression, then you should understand that this statement is entirely incorrect. A higher compression engine makes more power in NA form. So, why do you turbo guys think that a lower compression turbo motor makes more power? Does that make any sense when you really think about it? A turbocharger is a power adder? So why deplete power that was there to begin with? The answer I usually get to that is "So I can run more boost!" Well, sorry to rain on your parade, but more boost does not always equal more power. Check out this mathematical example of effective compression:
A motor with a 10.0:1 static CR boosting 10psi
10psi/14.7psi = .68
.68 + 1 = 1.68
1.68 x 10 = 16.8 effective CR
A motor with an 8.5:1 static CR boosting 10psi
10psi/14.7psi = .68
.68 + 1 = 1.68
1.68 x 8.5 = 14.28 effective CR
Now tell me who is going to make more power? The higher CR motor, or the lower CR motor?
So, maybe add more boost to the lower CR motor, right? Wrong...
A motor with an 8.5:1 static CR boosting 13psi
13psi/14.7psi = .88
.88 + 1 = 1.88
1.88 x 8.5 = 15.98 effective CR
Now you see, even adding 3psi of boost, still does not equal the effective CR of the higher compression, lower boost motor.
Effective compression is not the only advantage of the B16A/B18C1 either. The B16A/B18C1 has a stronger, better flowing cylinder head. It can rev much higher, making it that much more effective, and it flows great to handle all of the extra volume. The block has oil squirters to help support the bottom end assembly at high RPM. It takes more than a valvetrain upgrade to make a B18B safe at 8k. The higher compression also aids in spooling the turbo faster too.
Both motors have similar tolerances though. Both motors pretty much top out at around 350-400hp on stock motors, very well tuned. The B18C1 will make it far more efficiently for you though. It takes less boost to do so, it has more safeguards...and the bottom line on any Honda motor is tuning. If it is well tuned, you will be set. That goes for both motors. YOU ARE A FOOL if you think for one second that just because your B18B has a lower compression, you can substitute that for proper tuning.
A lot of people like to lower their motors compression when they build their motor. I used to think it was a good idea before I understood about tuning, and the positive aspects of compression. In the mathematical representation below, I will show you how a low compression motor must boost more to equal the output of a higher compression, lower boost motor:
Motor: stock B16A2 boosting 7psi.
Static Compression Ratio: 10.4:1
((boost psi / 14.7) + 1) x motor compression = effective compression
Stock motor (10.4:1 CR) on 7psi:
7psi/14.7psi = .47
.47 + 1 = 1.47
1.47 x 10.4 = 15.288 effective CR
Built motor (9.0:1 CR) on 7psi:
7psi/14.7psi = .47
.47 + 1 = 1.47
1.47 x 9 = 13.23 effective CR
You will lose 2.058 points from your effective compression ratio, this translates to a significant power loss.
In order to gain back that power, you have to do this:
Built motor (9.0:1 CR) on 10.5psi:
10.5psi/14.7psi = .71
.71 + 1 = 1.71
1.71 x 9 = 15.39 effective CR
Add 3.5psi to what you were boosting before, and you should be able to make around the same power as before, granted you haven't done any other kinds of modifications port/polish, cams, etc...
As you can see, considering all things stay equal (bore/stroke/cylinder head/etc...), you must add 3.5psi to make the motors perform similarly. You just spent about $2,500 to build your bottom end, and make your car slow.
By now we all should understand the positive aspects of compression, and how when teamed with the faster spoolng turbo, more efficient output, better flowing B-series VTEC cylinder heads, better low end spool time, stock oil squirters, higher redline, etc...you should see that turbocharging B-series VTEC motors is clearly not dangerous, and highly adviseable. I love a good turbo B16A!
No more MX-3.
-
ScooterBovine
- Supporting Member
- Posts: 502
- Joined: May 9th, 2004, 2:01 am
- Location: Blacklick, Ohio
From a smart Mustang guy; remember turbochargers are exhaust driven centrifugal superchargers:
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.
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.
No more MX-3.
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ScooterBovine
- Supporting Member
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- Joined: May 9th, 2004, 2:01 am
- Location: Blacklick, Ohio