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  • Compression vs Boost FYI

    ok check this out if you got 2 engines with the following set-ups:

    9:2.1 CR + 12psi = 16.7 DynamicCR
    8:1.1 CR + 15.7psi = 16.7 DynamicCR


    are both engines producing the same POWER. even if one engine allmost 4psi more boost but lower compression.
    1993 built ZE/DE n2o 2380lbs probe GT = parted out 2004
    1996 klze mx-3

  • #2
    As I recall the lower compression/higher boost engine will produce more power.

    Comment


    • #3
      PGTracer is right. Lower comp + higher boost obviously nets more powa. Just don't ask how much...

      Mici
      Mazda MX-6 LS-T http://www.cardomain.com/ride/466742
      Audi TT-R 1.8TQ http://www.cardomain.com/ride/2486942
      Toyota MR2 Spyder http://blopster.galleria.fi/kuvat/MR2Only/
      Aprilia RSV1000RR Factory

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      • #4
        that's what i'm hoping
        1993 built ZE/DE n2o 2380lbs probe GT = parted out 2004
        1996 klze mx-3

        Comment


        • #5
          well if you have more hp to begin with cuz of higher compression would that net you almost or as much as the lower compression engine with more boost?
          Nathan
          1994 GT/KLZE/NOS (old car)
          1986 Corolla GTS(AE86) (new car)

          Comment


          • #6
            why does lower compression + high boost make more power? That's absurd. Go w/ the HIGHER Compression piston, if you aren't making a track-only car, which will never see RPMS below, say, 4K.

            Comment


            • #7
              There really is no why to answer that question. . . . because you cannot compare the two.

              I am no expert, but first understand the differences between high compression and low compression motors. They offer different characteristics. They will have different torque and power curves. . . bottomline. High compression is alway more advantageous, but fueling and ignition timing must be more precise, and with the high compression motor you have higher cylinder pressures, meaning you need to control that timing and fueling to avoid knocking and detonation.

              Add a turbo and it doesn't change much. High compression and turbo just means cylinder pressures will increase more. You will always be able to extract more power from a high compression turbo motor than a low compression turbo motor. The problem is in the tuning. low compression motor are more tolerate of improper tuning, while a high compression turbo motor must have precise ignition timing and fueling to run properly and reliably.

              Again I do not think you can compare the two situations you mentioned. Its like trying to compare apples and oranges. Just because you fix the dynamic CR's of two motors does not mean they will produce similiar outputs. They are still two different animals. If I had to take a stab at it, I would probably side with the low CR high boost case, as I feel the extra manifold pressure would probably be more beneficial than the CR, but without seeing the two situation on the dyno side by side we'll never know. And I assure you we will probably never see these two situation on a dyno side by side.

              Enjoy,

              Bryan Pendleton | LeMons Racing Blog | BPi Flow Stacks | Facebook Anyone?

              Comment


              • #8
                how much can you loose from droppinf 1point of compression. 10-15ftlbs of torque????

                but the extra 4psi should get you 25-35ftlbs, which would be 15-20ft/lbs that you wouldn't be able to see going the high compression route. the power curve would be 4k and above only. ??????

                i don't care if i don't care if there's no power under 4K driving around in the city. as long as i got crazy power at 4K and up.



                1993 built ZE/DE n2o 2380lbs probe GT = parted out 2004
                1996 klze mx-3

                Comment


                • #9
                  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.

                  [I got this from the archives....search is good]
                  David Coleman #1891 - Fastest of any Gen @ MMIV:
                  1993 Mazda MX-3 GSR Special Edition - PINK POWA! - KLZE powered!

                  The Idiot Post Patrol :
                  Fighting ignorance one post at a time [this public service brought to you by Nikki and David]

                  Comment


                  • #10
                    I plan on reducing my compression, about one point and was wondering, if since I was doing this then could I run more boost? I'm not talking like a ton more but could I run 12 safely all the time?

                    Comment


                    • #11
                      On Tuesday October 30, 2001 3:26 PM, INTENSE MOTORSPORTS wrote:
                      I plan on reducing my compression, about one point and was wondering, if since I was doing this then could I run more boost? I'm not talking like a ton more but could I run 12 safely all the time?
                      You could run 12psi on a STOCK motor...If you want to replace your pistons/rods, get the same CR, maybe even .3 highter, but forged...run 12psi on that :smile:

                      YUM! :smile: If you can get fuel, I would have very little trouble believing you'd see VERY close to 300whp....all other things being taken care of.

                      Comment


                      • #12
                        1) Compression (of any sort) creates heat. If you use a compressor to generate boost before feeding it to the engine, then you can intercool it. If you increase your compression ratio then the heat is generated in the cylinder directly and the only way to cool it is with water injection (or by cooling the intake air below ambient).

                        2) For a cylinder of fixed size you will injest more air if you are running a compressor. Air mass is a function of pressure and volume after all, and since the volume is fixed by the cylinder size the only way to get more air in is by increasing the pressure. Since power is primarily a function of the amount of gas you burn, and that is largely determined by the amount of air in the cylinder. I think that this means that directly comparing the so-called "dynamic CR" doesn't work... there is a more subtle relationship at work here. If I had to guess I'd say that the higher CR case above would develop more power.

                        3) Without a lot of engineering (and perhaps experimentation) you cannot say whether any given combination of boost and CR will work in a given engine. As a group we have a fair bit of collective knowledge about the stock CR (since it is relatively easy to change the boost level once you have a compressor), but very little knowledge about how changing the CR affects the situation (it is rarely done and other factors usually change at the same time). As a practical matter if you change the CR of your engine and then boost it, you will end up starting low and turning up the boost until you experience a little detonation... then you'll turn it back down a little and try to figure out ways to cool the intake charge or combustion chamber (unless you blew the engine again, of course). How high you can turn the boost always depends primarily on fueling, ignition timing, octane, intake charge temperature... regardless of the CR. The CR will limit how far you can push the boost based on how well you control the other factors... but if you read most of the messages in this forum, it will become obvious that there is a great deal of room to play with these other factors and this is where you'll make the most progress. Once you set your CR, changing it is pretty expensive.

                        Personally I wouldn't change the CR too much from stock unless you're going to change the engine management system as well. Even if you do put an aftermarket EMS in, I'd probably leave the CR alone. If it was going to change (most likely because I was going for stronger/tougher pistons and they couldn't match the stock CR exactly), I would stick as close to stock as possible without going higher.

                        Former PGT-turbo owner... now 2010 VW Golf TDI

                        Comment


                        • #13
                          On Tuesday October 30, 2001 10:24 AM, David Coleman wrote:
                          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.

                          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.
                          I am going to have to slap this down with a BIG NO. Just because two motors have the same effective CR does NOT mean they will have the same power. They cannot be compared on the basis of effective CR. If you truly investigate/research and study the DYNAMICS of a high compression motor and a low CR boost motor the are very very different. You will have different output and responses. They cannot be comprared.

                          You are going to yeild higher returns with manifold pressure than from increasing the CR. Under boost you are burning a larger air/fuel mixture which will create more usable enengy than simply compressing a smaller mixture more.

                          Also, you will quickly reach the limitations of pump gas by simply increasing static CR. You could push a boosted application to some absurd effective compression ratios.

                          High compression motors are more drivable, but you also have less room for ignition and fueling errors. Unless you are building a trailor queen I would stick with the stock CR, and boost that. With proper engine management you will be able to run some high boost pressures, and will not sacrifice drivability by dropping the CR.

                          Bryan Pendleton | LeMons Racing Blog | BPi Flow Stacks | Facebook Anyone?

                          Comment


                          • #14
                            On Tuesday October 30, 2001 3:05 PM, BryanPendleton wrote:

                            I am going to have to slap this down with a BIG NO. Just because two motors have the same effective CR does NOT mean they will have the same power. They cannot be compared on the basis of effective CR. If you truly investigate/research and study the DYNAMICS of a high compression motor and a low CR boost motor the are very very different. You will have different output and responses. They cannot be comprared.

                            You are going to yeild higher returns with manifold pressure than from increasing the CR. Under boost you are burning a larger air/fuel mixture which will create more usable enengy than simply compressing a smaller mixture more.

                            Also, you will quickly reach the limitations of pump gas by simply increasing static CR. You could push a boosted application to some absurd effective compression ratios.

                            High compression motors are more drivable, but you also have less room for ignition and fueling errors. Unless you are building a trailor queen I would stick with the stock CR, and boost that. With proper engine management you will be able to run some high boost pressures, and will not sacrifice drivability by dropping the CR.
                            Turning up the boost is all fine and dandy if you can get the fuel to go with it. I know FSD's are quite different as they have top feeds, but fuel is the #1 limiting factor on KLD's. So if fuel [assuming you don't have the ~3kUSD for a stand alone and new injectors and dyno tuning] is the limiting factor, then boost is also limited too. If boost is limited but you're still not experiencing detonation, why not increase your engine's effeciency and up the CR? Less lag, more power off boost, and better mpg are qualities that I know I would strive for in a street motor. I agree that for all out power, increasing the pressure ratio will make your mdot values increase greatly, much more so than increasing effeciency by increasing CR. But when you can get some new pistons for ~$600USD and some rods for the same [in stock size] you've got a strong motor that gets good gas milage and has good off boost power, which happens to have a turbo on it.

                            So, for a generic motor which upgrading fuel is easy, I agree with you. However, for the KL, where going above ~9psi [sans extra injectors] is really the limit of the stock fuel system, I'd have to say ~1500USD for new rods and pistons is a viable option if you're building a street car on a budget. If you're not a) on a budget b) building a street car or c) boosting a KL, then ignore all this.
                            David Coleman #1891 - Fastest of any Gen @ MMIV:
                            1993 Mazda MX-3 GSR Special Edition - PINK POWA! - KLZE powered!

                            The Idiot Post Patrol :
                            Fighting ignorance one post at a time [this public service brought to you by Nikki and David]

                            Comment


                            • #15
                              I thought this was more of a theoretical discussion than a KL-specific discussion. There was no specific motor listed in the original post.

                              Staying on the theoretical side: how could you create two engines that were completely identical *except* for their static CR? Does that mean they have the same bore & stroke, but different piston shape? To keep everything else the same, you need to change the combustion chamber volume. How do you do this, and still call the two engines "otherwise identical" ?

                              Assuming you have an answer to that question, then clearly the higher boost engine will produce more power at a fixed dynamic CR, because it ingests more air mass than the other.

                              In reality, as was stated before, it's an apples to oranges comparison because you can't create two engines that are identical *except* for static CR. Many other characteristics will change in order to bring about the change in CR...
                              -- Howard Chu Chief Architect, Symas Corp. Director, Highland Sun
                              17x8" BBS RC/Leda 24-way adj. coilovers/Hye-Dra-Cyl Big Brakes: Wilwood 4-piston calipers+12.2x1.25" rotors/RRE FSTB/Top-End Racing FW/MazdaSpeed-Quaife LSD/65mm TB/Autophysics CAI/Groundz/Bi-Xenon HID headlamps/Euro Clear-corners+amber LEDs/Blazer amber fogs/JVC KDMX3000+CHX1200/Pioneer 6x9s/BCT7 Scanner/etc...

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