Increasing CR vs boost pressure - Page 2

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  1. #16
    Premium Member Array black2003cobra's Avatar
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    03yllwcobra - With a centri blower, I can understand why you wanted to bump up the CR. Since boost at low rpm is virtually non-existent, they tend to be doggy down low. I had one, so I understand. With a positive-displacement blower, that isn't so much of an issue. (The bypass valve is not very big, so it will come out of vacuum fairly easily.) Even at part throttle, my '03 is significantly more responsive than my '01 was, which had a CR of 9.85 IIRC. If you're detecting signs of detonation, there are a few things you can do. 1) reduce spark advance, 2) enrichen the air/fuel mixture, 3) reduce boost pressure, or 4) up the fuel octane. Options 1-3 will result in a loss of power, of course. And there are some that fancy alcohol injection w/ centri blowers.

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  3. #17
    Senior Member Array 03yllwcobra's Avatar
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    Quote Originally Posted by black2003cobra View Post
    03yllwcobra - With a centri blower, I can understand why you wanted to bump up the CR. Since boost at low rpm is virtually non-existent, they tend to be doggy down low. I had one, so I understand. With a positive-displacement blower, that isn't so much of an issue. (The bypass valve is not very big, so it will come out of vacuum fairly easily.) Even at part throttle, my '03 is significantly more responsive than my '01 was, which had a CR of 9.85 IIRC. If you're detecting signs of detonation, there are a few things you can do. 1) reduce spark advance, 2) enrichen the air/fuel mixture, 3) reduce boost pressure, or 4) up the fuel octane. Options 1-3 will result in a loss of power, of course. And there are some that fancy alcohol injection w/ centri blowers.

    The Highest timing im going to run is going to be 14* but the lowest boost psi i can run is 23 psi unless procharger makes a bigger cog pulley, i usually run 110 in the car all the time but i think that what caused my stock motor to blow up because the o2s werent switching fast enough to keep a/f

  4. #18
    Premium Member Array black2003cobra's Avatar
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    I'm not a tuner, but I would suspect for those conditions on 110, you would be OK. I assume you're running an intercooler.

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  6. #19

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    Great read


    cheers

  7. #20
    VMP Tuning Array Justin@VMP's Avatar
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    I worked on a TT mach1, dual 50 trims I believe. With the stock 10+:1 motor it made about 440rwhp at 11:1, 7psi, and 14*.

    With an 03 cobra shortblock it made 380RWHP with the same tune, even with 5-6* more timing cranked in, it still made only 420rwhp, though I could have run 2-4* more timing at such a low boost level, I did not feel like there were great gains from doing so, gains were starting to taper off.

    That goes to show how much power lies in compression.

    Now in his case he just turned up the boost to 12psi and made over 600rwhp, this goes to show how inefficient the compressors were at low boost.

  8. #21
    Premium Member Array black2003cobra's Avatar
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    Thanks for the data Justin.

    Going from 8.5:1 to 11:1 increases the thermal conversion efficiency by roughly 8.3%, (using gamma = 1.3). So assuming all else is equal, scaling gives ... 380*1.083 = 411 at the same rpm, which isn't too terrible considering it's just a basic, back-of-the-envelope estimate. (~6.6% error.) If I get ambitious, maybe I'll run it through the more detailed thermodynamic model I have.

    I'm not clear which motor made 600 going from 7 to 12 psi, but the turbo 03 & 04's seem to gain about 23 rwHP per psi, (=> Ref link). So a 5 psi increase would translate to 115 rwHP. So in either case, the engine definitely picked up more than what is typical!
    Last edited by black2003cobra; 07-23-2009 at 03:48 PM. Reason: Dyslexia!

  9. #22
    Premium Member Array black2003cobra's Avatar
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    fpv_gtp - Thank you.

  10. #23
    Premium Member Array black2003cobra's Avatar
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    Copied relevant post below from CAT600's thread:
    Link => Compression ratio and Boost

    Quote Originally Posted by 03_StangGuy View Post
    I agree with you, but the statement I was replaying to sounded like he was talking NA motors where a higher CR motor would do better, cuz it makes more pressure and more power. This is tru which is why I was saying if you know take a low CR motor and cram air in with a blower there is twice the air to make power with. When you get into it even more the lower air temps allow even more power.
    Yes, that's correct. With the same inlet pressure, power and torque go up with increasing CR due to the commensurate increase in thermal-conversion efficiency. (Power is proportional to mass-air flow rate times therm-conv efficiency.)

    There is a very nice write up on a test that MM&FF did at their site. (Link => MM&FF Ref ) On a 5.4L NA motor, they raised CR from 8.3 to 11.8, and observed power and torque to go from 478 hp and 395 lb-ft of torque, to 543 hp and 439 lb-ft of torque. Again, that's due to the increase in thermal-conversion efficiency. Assuming an effective ratio of cp/cv = 1.3 over the entire cycle, when one does the math, the thermal-conversion efficiency will be seen to go up by ~11.3%, from CR = 8.3 to 11.8. So theoretically, (first order), the baseline power of 478 scales to 478*1.113 = 532 hp, and baseline tq of 395 scales to 395*1.113 = 439.6 lb-ft, (where the error is a mere 2%, and 0.1%, respectively.)

    Summarizing, by going from a CR of 8.3 to 11.8, the measured and calc'd results are:
    Code:
    Parm	Meas	Calc	Error
    Power	543	532	2%
    Torq	439	439.6	0.1%
    So it's not magic or BS theory, and the experimental numbers agree. The article also rightly points out the dangers of increased risk of knock, and what this means as far as the octane requirements of the engine. Make no mistake about it...power/torque absolutely do go up with CR, (all else being equal). And so does cylinder pressure. So one needs to be cognizant of limits such as an octane (knock) limit, strength-of-parts limit, etc.

    And of course the equal-inlet-pressure case they tested isn't being discussed here. They didn't address the low CR/high boost vs high CR/low boost comparison at the same peak cyl pressure. As far as that case goes, SlowSVT is certainly on the right track...more air "wins" over the lower therm-conv efficiency of the low CR/hi boost case, at the same peak combustion pressure as the hi CR/low boost case...
    Last edited by black2003cobra; 10-07-2009 at 09:50 PM. Reason: Summary table

  11. #24

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    nice read

  12. #25

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    Excellent Info.. Thanks for posting.
    Makes perfect sense why i was seeing detonation after upping compression to 9:0 1

    Had to reduce boost to 13lbs and timing to 15 degrees up top to clear things up,(Had to go even lower from 1000 - 4000)..

  13. #26

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    I just had a flashback to my undergrad Mechanical Engineering courses. If only all professors were gearheads, I would've paid more attention!

    Always good to dig back into my old books though after reading a post like this and refresh my memory - now if I could only find a job where I could apply this type of knowledge!

  14. #27

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    great read, thanks for posting that.

  15. #28
    Senior Member Array Apicia's Avatar
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    I am curious what your opinion is on how much boost I can run using 9.3:1 compression on pump gas (93 octane)? I would guess 14 psi? There are variables such as heads, cams, timing, etc., but in general I need to know to dial in my next street tune. Want to keep her safe, but not strip all the gusto out of her. I keep reading more boost and less timing is better. My race tune has 22psi and 9.3:1 compression, but octane is being supplemented by Torco and 100% VP M1 methanol.

  16. #29

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    What is the limit - or the best power for a given setup? I am not yet familiar with the math required to do this myself. But by manipulating the following equation that you provided: Pb' = [(CR/CR')(1 + Pboost/Patm) -1]Patm = [(10/8.5)(1 + 15/14.7) -1]14.7 = 20.2 psi, I could build a table of CRs and Pbs. I don't know how to relate this back to hp as you did in the example. As you point out nt goes to zero at CR=1, so somewhere there is a best power for this setup...a graph of HP vs CR/Pboost would be enlightening. I don't care about "drivability"...just steady state hp at 1 engine speed and 1 manifold setting. FYI: we are at 6:1 and lots of boost with great success, but we don't know where to go from here or even how far we can go with this setup.

  17. #30
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    Quote Originally Posted by todd.rudberg View Post
    FYI: we are at 6:1 and lots of boost with great success, but we don't know where to go from here or even how far we can go with this setup.
    6:1 static? What is the setup?

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