boost question

[krisw]

alright so ive had this question for a while and havent been able to figure it out. how is boost pressure different from supercharger to supercharger? for example, if you have an eaton m45 and an m90 and theyre both pushing 7psi, will the m90 be flowing more air? the first thought would be that since the eaton m90 flows twice as much air per revolution that it would then be flowing twice as much air at 7psi. but that doesnt seem possible, how can you have the exact same amount of pressure with twice the airflow? so then that means that to achieve the 7psi, the m90 must be rotating half the speed of the m45? will the m90 make more power than the m45 at 7psi?

also, since the amount the supercharger flows increases as rpms increase, then how do you have a pulley that "sets" the amount of boost you run? as the engine revolutions increase shouldnt the amount of boost?

[SkylineUSA]

psi does not measure volume.

Good question, I will let the other guys awnser.

[cheap5.0]

I also had the same question awhile back and read a really good page explaining it. I can't explain it myself, but i can explain your pulley question. Even though a pulley will produce more boost than another, they are both limited by what the blow off valve allows them to produce. lets say you have a centrifugal blower making 6 psi. You then add a different pulley that is capable of 10 psi. The supercharger is now making 10 psi, but it is beeing "bled" out of the system via the blow off valve (BOV) back down to 6psi. If you want more boost you can adjust the BOV (some have a dial, others require stiffer springs).

[boosted331]

Boost pressure is irrelevant, but airflow is not. Positive displacement blowers are hard to explain, but yes the bigger blower will make more power at the same power, more power at less boost. A pulley for a positive displacement blower basically dictates the amount of airflow going into the motor, and the reistance put forth by the motor determines the boost level. That's why if you run the same S-trim pulley on a 302, or a 302 with AFR 185's and a victor 5.0 intake the one with heads will see a few pounds less boost, but make more power.

[krisw]

yes i get all these concepts. but the amount of air a supercharger flows and the pressure within the intake manifold are directly related to one another. with the engine setup remaining constant, a certain amount of air in the intake track will provide a certain amount of pressure pushing outward... aka Xairflow=Xpsi, at least thats how i understand it. so therefore no matter what the supercharger size, to attain a certain psi within the intake manifold you have to flow a certain amount of air. so therefore 13psi is 13psi so it doesnt matter what supercharger your using, youll always make the same amount of power because youll always be flowing the same amount of air. the only way i can see one supercharger making more power than anohter supercharger at the same boost level would be if one were more efficient, be it operating at a lower temperature or requiring less power to turn the rotors. as for blow off valves setting boost level, as i understand it thats a poor practice, maybe im wrong. seems like it would just be needless work being put thru the supercharger. lacking a description on this thread, can someone at least point me to a site that can explain this?

[boosted331]

No, you apparently havn't grasped it.

IE, fastest LT1 car in the world (best example I could think of ) has gone low 8's at over 170 MPH. Turbo? 98MM large frame, at 23 pounds of boost. Now, a 98MM turbo isn't grossly efficient at 23 pounds of boost, it's actually quite low on the map, but do you think even a big turbo like a T76 would make that kind of power at 23 psi, even though 23 psi into a motor like that is right at the peak of the T76's efficiency island? No, because it doesn't move the same amount of air at 23 psi as a bigger turbo/compressor does.

So when trying to understand turbos/blowers, throw that boost pressure shit all the way out the window because it doesn't mean shit for understanding why bigger blowers make more power. An M90 moves 90 cubic inches of air per revolution. An M45 moves 45 inches, simple. So, you need to spin the M45 roughly twice as fast to move the same amount of air into the motor. Spinning that blower twice as fast not only causes a power loss due to extra heat produced and drag on the crankshaft, but the blower is spinning faster, and has to work much harder to keep up with the motor. Faster moving blower = faster rotors, which means the air is compressed more for the given volume. It's hard to explain and I'm not the best at it. Talk to a SAE person, they'll lay it out easy. Either way you cut the cake, for equal boost pressures the larger blower is moving more air and making more power.

[krisw]

anyone have a link that can explain this to me?

[SkylineUSA]

Originally Posted by krisw

anyone have a link that can explain this to me?

Buy a book, Corky Bell's Maximum Boost.

boosted331 explained it pretty simple terms, if you want to get a more tech explaination, buy that book. It goes into great detail of what you want to know.

[krisw]

well i already own supercharged by corky bell and that doesnt really explain it. it doesnt make sense to me that you can be pushing more air but still be producing the same amount of pressure? i mean whats the point of even talking about it in terms of boost, why wouldnt people talk about it in terms of airflow?

[cuda_dude]

Bigger blower(m90) = less revolutions to = desired psi
Smaller blower(m45) = more revoultions/ more strain on the engine/ more heat generated to = same desired psi

PSI being regulated by the BOV

So the m90 takes less revolutions to = desired psi which means it is more efficent because it takes less power from the engine to generate the same power as the m45

[krisw]

so then 13 psi on an m90 will flow the same amount of air as 13 psi on an m45

[boosted331]

Originally Posted by krisw

so then 13 psi on an m90 will flow the same amount of air as 13 psi on an m45

No, because the motor poses more of a restriction to the M40 than the M45. 13 psi on an M90 is a lot more power than on an M45.

[duplox]

How about this... you have a 5 gallon air compressor tank, and a 30 gallon air compressor tank. Fill them both to 10psi. Which one has more air in it? Obviously the 30 gallon. Same pressure, different volume. More air means more power.
A certain engine(say, a 5.0L ford v8) can only take in so many cubic feet of air per revolution. Lets say for our imaginary engine that this is 500cfm. Now we put a supercharger on the car, running 1 bar of pressure(twice the atmospheric pressure - twice as much air molecules in a cubic foot). With the engine taking in 500cfm of air at 1bar pressure, the supercharger will be taking in 1,000cfm of air at 0 bar pressure. Volume and pressure.
Now lets take an imaginary 2.5L engine that can take in 250cfm. Slap a supercharger on that, running 1 bar as well. Double the pressure, double the airflow, the supercharger is taking in 500cfm. Same pressure, different volume. Different power output.
Ultimately, it all comes down to how much air your engine can take in, in a given ammount of time (cubic feet per minute, cfm). This is the ultimate goal. Pressure(psi) is just another measurement of an engine's system. It is difficult to measure CFM, so people use pressure(easy to measure) so people can get a general idea of how much air the engine is taking in. Combine pressure, engine displacement, and engine redline, and I'd have a good general idea of how much power the engine is making.

[krisw]

Originally Posted by duplox

How about this... you have a 5 gallon air compressor tank, and a 30 gallon air compressor tank. Fill them both to 10psi. Which one has more air in it? Obviously the 30 gallon. Same pressure, different volume. More air means more power.
A certain engine(say, a 5.0L ford v8) can only take in so many cubic feet of air per revolution. Lets say for our imaginary engine that this is 500cfm. Now we put a supercharger on the car, running 1 bar of pressure(twice the atmospheric pressure - twice as much air molecules in a cubic foot). With the engine taking in 500cfm of air at 1bar pressure, the supercharger will be taking in 1,000cfm of air at 0 bar pressure. Volume and pressure.
Now lets take an imaginary 2.5L engine that can take in 250cfm. Slap a supercharger on that, running 1 bar as well. Double the pressure, double the airflow, the supercharger is taking in 500cfm. Same pressure, different volume. Different power output.
Ultimately, it all comes down to how much air your engine can take in, in a given ammount of time (cubic feet per minute, cfm). This is the ultimate goal. Pressure(psi) is just another measurement of an engine's system. It is difficult to measure CFM, so people use pressure(easy to measure) so people can get a general idea of how much air the engine is taking in. Combine pressure, engine displacement, and engine redline, and I'd have a good general idea of how much power the engine is making.

your first analogy doesnt really make sense, i mean it does but in no way does it relate to what im talking about. im talking if you take the exact same engine, and put two different superchargers on it pushing the exact same psi which one is going to flow more air. now pretty much everyone on here says that obviously the larger supercharger will be flowing more air, which seems to make sense if you just take it at that. but psi is the pressure within the intake system, now if your putting more air into the intake system then would it not make sense that there would be more pressure (psi) as the air would be more tightly packed? can someone explain to me how a supercharger can flow more air into the intake system and not as a result produce more pressure within that intake system? can someone post a link or something and point me in the right direction?

[-Jayson-]

ive got a good analgy for this guy. ok lets say you have two ballons. One is a normal sized party Ballon the other is a tiny water ballon. Now you fill each balloon to eactly 4 PSI. Now the smaller balloon occupies much less space than the bigger balloon. So it order to make it reach 4 PSI it doesnt take nearly as much air. Where as the big balloon take sup alot of space, and reaching 4 PSI will take alot more air.

I honestly dont think you know what PSI is, or understand the physics of air volume/air pressure/ and air mass.