Close

Page 3 of 3 FirstFirst 123
Results 51 to 65 of 65
  1. #51
    Join Date
    Jan 2010
    Location
    SoCal
    Posts
    119,444
    Rep Points
    32,146.1
    Mentioned
    2108 Post(s)
    Rep Power
    322


    Yes Reputation No
    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
    i get that's the only differing factor, but i was always lead to believe that a supercharger of a certain size with a certain pulley etc. will always (well, in theory) produce the same amount of boost and airflow..
    Same amount of airflow, yes, same boost, no. You can change the boost with just an exhaust.

  2. #52
    Join Date
    Sep 2012
    Location
    Brisbane, Australia
    Posts
    3,310
    Rep Points
    1,439.4
    Mentioned
    54 Post(s)
    Rep Power
    15


    Yes Reputation No
    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    Same amount of airflow, yes, same boost, no. You can change the boost with just an exhaust.
    can you explain this at all? (or tell me a phrase to google on the subject, i'm cool with that)

    i thought it was just a compressor, that pumps out air into the intake manifold?

  3. #53
    Join Date
    Jan 2010
    Location
    SoCal
    Posts
    119,444
    Rep Points
    32,146.1
    Mentioned
    2108 Post(s)
    Rep Power
    322


    Yes Reputation No
    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
    can you explain this at all? (or tell me a phrase to google on the subject, i'm cool with that)

    i thought it was just a compressor, that pumps out air into the intake manifold?
    Yes but boost is the measure of backpressure not airflow.

  4. #54
    Join Date
    Sep 2012
    Location
    Brisbane, Australia
    Posts
    3,310
    Rep Points
    1,439.4
    Mentioned
    54 Post(s)
    Rep Power
    15


    Yes Reputation No
    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    Yes but boost is the measure of backpressure not airflow.
    ... if there was zero backpressure in a cylinder, but 1 bar of boost being put out by the blower, wouldn't it still be the same amount of 'boost' pressure as if it were positive or negative in the cylinder?

    i thought boost was the measure of pressure over atmospheric as measured at the compressor?.. anything over the 14.5psi is positive, so boost? :/

    i know that 14.5psi from one source of forced induction can be flowing more or less air than another one as even 1 bar to 1 bar, it could be 100 liters or 95 liters going through at a given time... but identical compressors in identical setups :/

  5. #55
    Join Date
    Nov 2010
    Posts
    937
    Rep Points
    562.7
    Mentioned
    52 Post(s)
    Rep Power
    6


    Yes Reputation No
    I to don't understand the change in boost with CR being the only difference in that article. I'm guessing the dyno environment changed, or it has to do with the different hardware altering flow somehow (separate from CR). DFBIU could explain the possibilities.

    Exhaust BP should not change with CR... still same air mass.


    If you were running 100% E85 and octane was not an issue then ign timing for MBT would be different with different CRs... ie. You would have to change timing. I believe roughly you need peak pressure at 15 to 20deg ATDC for MBT no matter the CR. Higher CR should reduce burn time.

    If octane is not an issue (which generally its not with e85) then CR will directly effect power with all other parameters being the same: MAF, boost, optimal timing for MBT, etc.

  6. #56
    Join Date
    Jan 2010
    Location
    SoCal
    Posts
    119,444
    Rep Points
    32,146.1
    Mentioned
    2108 Post(s)
    Rep Power
    322


    Yes Reputation No
    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
    ... if there was zero backpressure in a cylinder, but 1 bar of boost being put out by the blower, wouldn't it still be the same amount of 'boost' pressure as if it were positive or negative in the cylinder?
    You're thinking of boost as the airflow. If there is 1 bar of boost measure, then there has to be positive boost pressure wherever it is being measured to give that reading.

    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
    i thought boost was the measure of pressure over atmospheric as measured at the compressor?.. anything over the 14.5psi is positive, so boost? :/
    Yes it should really be indicating the amount you are increasing past atmospheric pressure.

  7. #57
    Join Date
    Jan 2010
    Location
    SoCal
    Posts
    119,444
    Rep Points
    32,146.1
    Mentioned
    2108 Post(s)
    Rep Power
    322


    Yes Reputation No
    Click here to enlarge Originally Posted by JoshBoody Click here to enlarge
    I to don't understand the change in boost with CR being the only difference in that article. I'm guessing the dyno environment changed, or it has to do with the different hardware altering flow somehow (separate from CR). DFBIU could explain the possibilities.
    @DBFIU does changing compression ratio affect boost pressure.

    Click here to enlarge Originally Posted by JoshBoody Click here to enlarge
    Exhaust BP should not change with CR... still same air mass.
    Why wouldn't the compression ratio change affect the exhaust?

  8. #58
    Join Date
    Dec 2011
    Location
    Roanoke VA
    Posts
    1,632
    Rep Points
    2,248.3
    Mentioned
    54 Post(s)
    Rep Power
    23


    Yes Reputation No
    Just me thinking out loud but reducing CR should reduce the amount/pressure of reversion occuring in the intake during valve overlap.
    Click here to enlarge
    MOTIV750, MOTIV P-1000 PI, MOTIV/FUEL-IT! low pressure fuel system, AEM EMS/COBB AP, Aquamist HFS-3, ETS FMIC, SPEC stage 3+ clutch/SS flywheel, BC Racing coilovers and VMR wheels wrapped in Hankook RS3s.

  9. #59
    Join Date
    Sep 2012
    Location
    Brisbane, Australia
    Posts
    3,310
    Rep Points
    1,439.4
    Mentioned
    54 Post(s)
    Rep Power
    15


    Yes Reputation No
    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    You're thinking of boost as the airflow. If there is 1 bar of boost measure, then there has to be positive boost pressure wherever it is being measured to give that reading.



    Yes it should really be indicating the amount you are increasing past atmospheric pressure.
    nah, i know that whether it's 1000 psi or 10 psi, if the compressor can only flow 'x' cfm, it's still only x cfm

  10. #60
    Join Date
    Jan 2010
    Location
    SoCal
    Posts
    119,444
    Rep Points
    32,146.1
    Mentioned
    2108 Post(s)
    Rep Power
    322


    Yes Reputation No
    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
    nah, i know that whether it's 1000 psi or 10 psi, if the compressor can only flow 'x' cfm, it's still only x cfm
    Ya so the cfm is really the airflow rating.

  11. #61
    Join Date
    Sep 2012
    Location
    Brisbane, Australia
    Posts
    3,310
    Rep Points
    1,439.4
    Mentioned
    54 Post(s)
    Rep Power
    15


    Yes Reputation No
    Click here to enlarge Originally Posted by rader1 Click here to enlarge
    Just me thinking out loud but reducing CR should reduce the amount/pressure of reversion occuring in the intake during valve overlap.
    In the same turn, wouldn't it also effect how effectively it pumps out exhaust gases... Almost a net 0 effect?

  12. #62
    Join Date
    Apr 2010
    Posts
    4,614
    Rep Points
    3,236.6
    Mentioned
    24 Post(s)
    Rep Power
    33


    Yes Reputation No
    I didnt read the whole thread but increasing your compression ratio will increase your in-cylinder pressure, and for the same amount of fuel and air, holding all other values constant (ignition advance included) it will produce more power. In theory your brake specific fuel consumption will get better with higher compression, more power with the same amount of air and fuel assuming all other values constant...

    Now the can of worms opens when you raise compression and run the same amount of boost but use a different fuel and still want the same amount of timing... Lots of factors here, cant predict $#@! because too many variables going on at once. Need to test the engine to really know.

    Simple answer though is manifold pressure does not change with compression ratio, static compression is a function of swept volume of the cylinder and the overall volume of the cylinder. Increasing compressrion ratio does increase power if you can take advantage of it with the right fuel and the computer is happy.

    Increases in compression can produce higher EGTs if all other variables are held constatn, because you are starting out with a higher in-cylinder temperature due to the higher in-cyl pressure (PV=nRT right??).

    Some people will offset this with higher octane fuels that burn slower and cooler, just so they can run more air mass through the engine. More airmass = more power, hope that makes sense.
    Some people live long, meaningful lives.

    Other people eat shit and die.

    I'm not racist, I hate everybody equally; especially fat people.


    Click here to enlarge

  13. #63
    Join Date
    Apr 2010
    Posts
    4,614
    Rep Points
    3,236.6
    Mentioned
    24 Post(s)
    Rep Power
    33


    Yes Reputation No
    Click here to enlarge Originally Posted by rader1 Click here to enlarge
    Just me thinking out loud but reducing CR should reduce the amount/pressure of reversion occuring in the intake during valve overlap.
    Intuition would be that you are right. As the piston is on the upstroke in the cylinder, a lower CR engine the piston would have effectively 'stopped short' compared to a higher CR engine, and because intake valves stay open long after the piston is TDC the fact that the cylinder has lower CR would reduce reversion. So a change in camshafts would be needed with drastic changes in CR, thats my opinion...
    Some people live long, meaningful lives.

    Other people eat shit and die.

    I'm not racist, I hate everybody equally; especially fat people.


    Click here to enlarge

  14. #64
    Join Date
    Aug 2010
    Location
    over there
    Posts
    288
    Rep Points
    481.1
    Mentioned
    14 Post(s)
    Rep Power
    5



    Yes Reputation No
    Click here to enlarge Originally Posted by DBFIU Click here to enlarge
    I didnt read the whole thread but increasing your compression ratio will increase your in-cylinder pressure, and for the same amount of fuel and air, holding all other values constant (ignition advance included) it will produce more power. In theory your brake specific fuel consumption will get better with higher compression, more power with the same amount of air and fuel assuming all other values constant...

    Now the can of worms opens when you raise compression and run the same amount of boost but use a different fuel and still want the same amount of timing... Lots of factors here, cant predict $#@! because too many variables going on at once. Need to test the engine to really know.

    Simple answer though is manifold pressure does not change with compression ratio, static compression is a function of swept volume of the cylinder and the overall volume of the cylinder. Increasing compressrion ratio does increase power if you can take advantage of it with the right fuel and the computer is happy.

    Increases in compression can produce higher EGTs if all other variables are held constatn, because you are starting out with a higher in-cylinder temperature due to the higher in-cyl pressure (PV=nRT right??).

    Some people will offset this with higher octane fuels that burn slower and cooler, just so they can run more air mass through the engine. More airmass = more power, hope that makes sense.
    so to reduce some variables here lets say i keep all the same and just raise compression results in me gaining power

    running e85 raising compression and keeping all else the same = more power as well

    so when would running raised compression at the same boost psi be a bad thing?
    Turbo lag is the on ramp to the highway which is power.

  15. #65
    Join Date
    Apr 2010
    Posts
    4,614
    Rep Points
    3,236.6
    Mentioned
    24 Post(s)
    Rep Power
    33


    Yes Reputation No
    Click here to enlarge Originally Posted by WDBi Click here to enlarge
    so to reduce some variables here lets say i keep all the same and just raise compression results in me gaining power

    running e85 raising compression and keeping all else the same = more power as well

    so when would running raised compression at the same boost psi be a bad thing?
    It would be a bad thing if your engine has to pull too much ignition advance and then you are not going to make as much power as you thought, or even less then you started with because the engine is trying to prevent detonation.

    E85 will help with that, but there is no way to know if it can prevent detonation alltogether with a high CR N54 and the same boost. At least, I dont know if it can and you are probably going to be the first person to do this.
    Some people live long, meaningful lives.

    Other people eat shit and die.

    I'm not racist, I hate everybody equally; especially fat people.


    Click here to enlarge

Page 3 of 3 FirstFirst 123

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may post replies
  • You may not post attachments
  • You may not edit your posts
  •