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Thread: Is all boost created equal?

  1. #1

    Is all boost created equal?

    After being convinced that the turbo on my mini is too big, I have a question: is 10 psi from a T3/T4 .57 trim the same as 10 psi from a Garrett TII turbo? I mean, is the same amount of cfm getting into the chamber?
    I understand (now, anyways) that the .57 trim is not the best turbo for an 8-valve. But what I can't understand is why at 15 psi with a Mitsu turbo (stock) the mini pulled pretty well, but with this .57 trim at 15 psi it feels like an n/a. It seems to me like the cfm would have to be the same in that both compressors are feeding enough air to creat 15 psi of back pressure. Is this right?
    What am I missing? Keep in mind that I am not a turbo guru, which is why I am asking. I am reading "Maximum Boost" for the third time and get a little bit more each time I read, so don't flame me. I just need to know why! This turbo came off of my Miata, and it dyno's 263 rwhp at 19 psi. Granted, it took a while to spool, but when it did it pulled like a madman. I took it off of the Miata with about 5000 miles on it, and I put it on the van. But this van ain't pullin' at all (well, very slightly).
    How are the boost levels different? Thanks!

    -Les

  2. #2
    Garrett booster
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    Re: Is all boost created equal?

    10 psi is 10 psi regardless of turbo. Now, different turbos move air faster or slower and have different limitations of course, but a 10 psi reading in the system is pressurized air and nothing more.

  3. #3
    Garrett booster
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    Re: Is all boost created equal?

    Quote Originally Posted by neonsox View Post
    10 psi is 10 psi regardless of turbo. Now, different turbos move air faster or slower and have different limitations of course, but a 10 psi reading in the system is pressurized air and nothing more.
    yes the psi tells you how much pressure there is in the pipe, not the amount of air.

    I have a question: is 10 psi from a T3/T4 .57 trim the same as 10 psi from a Garrett TII turbo? I mean, is the same amount of cfm getting into the chamber?
    The short answer is no.
    The smaller turbo will hit the 'boost setpoint' at a lower rpm and provide (x) amount of air efficiently to a certain rpm, as the rpm rise so does the air demand. which pushes the turbo out of its 'zone'.
    your 57 trim wont spool till later but has the capability to feed alot more cfm at a cooler temperature. (my 57t4 /t3 is spooled<wheel is spinning fast enough to build pressure> by 3700 <i checked yesterday)
    when it finally catches up, the tq should hit like a freight train.

    Generally More air can get in the cylinders with the larger turbo because the air is cooler.
    This is why people can run 14 pounds on the mitsu and not be lean, but garrett guys lean can out at 11-12. The carrett can produce the same amount of air as the mitsu at a lower pressure and temperature


    Your running the 57 out of its cfm 'zone'
    A) the compressor wheel does'nt spinup to speed till late
    B) When it does, your not taking advantage of its potential to push air
    C) The mini is heavy, so the loss of low tq hurts


    If your intending to only run 10psi...
    a mitzu is great to 170whp
    a stock garret can fill the air demands to about 250whp
    a super 60 can hit 280-300hp

    for more you need to look into the hybirds / gt / Hx-hy cummins ect.

  4. #4

    Re: Is all boost created equal?

    So, then, shouldn't the .57 trim be putting more air into the chamber at 10 psi because the air is cooler, as the .57 trim wouldn't need to spin as fast as the Mitsu to get to 10 psi?

    -Les

  5. #5

    Re: Is all boost created equal?

    Or is it that the .57 trim is moving air so slowly that the air is not being crammed into the chamber fast enough?

    -Les

  6. #6
    Garrett booster
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    Re: Is all boost created equal?

    The problem is that the 57 cant push air in until 3500 4500 rpm

    where as the mistu has been humming along and feeding air since 1200 rpm

    (the 'hit' of pressure from the 57 should be overwhelming the tires at 14psi though which is why i think your centerline is off)

  7. #7
    Banned Turbo Mopar Contributor
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    Re: Is all boost created equal?

    Quote Originally Posted by Sloride View Post
    The problem is that the 57 cant push air in until 3500 4500 rpm

    where as the mistu has been humming along and feeding air since 1200 rpm

    (the 'hit' of pressure from the 57 should be overwhelming the tires at 14psi though which is why i think your centerline is off)


    i cant find the other thread...but is the van a 2.5??
    the 2.5 would help spool the bigger turbo faster, but once its spooled you dont have much room to work with as the 2.5 likes bottom end power.

    not only does it take the 57 trim turbo to spool longer, but you have to run MORE boost to get it into its "zone" as mentioned b4.

    this turbo is capable of pushing to to 400hp....at 10 psi your lucky to be cracking 200hp

    it sounds like youd be much better off suited with a more streetable turbo, such as the S60 or a 46trim E wheel on a stage 1 exhaust wheel with .63 housing

  8. #8

    Re: Is all boost created equal?

    You guys are right, and I am switching turbos, but I just needed it explained to me. I can stall the van with the e-brake on and let the boost come up to 15 psi and the van will drag itself, but not turn tires. WTF?
    Anyway, I am probably going to buy a new turbo or send this one to Chris at TU to get a compressor/compressor housing swap to a .50 or .46 trim.
    I got some advice from Simon about checking to see if my cam is off by using a timing light. If I don't have a cover, how would I do that?

    Thanks again for all of your help! I don't want it to sound like I am beating a dead horse, but I am just trying to understand this problem fully, that way I can build on it in future diagnostics. Thanks for taking the time, guys!

    -Les

  9. #9

    Re: Is all boost created equal?

    And yes, the van is a 2.5. I don't have the tach hooked up yet; I spent all day yesterday hooking up the fuel pressure guage.

    -Les

  10. #10
    Banned Turbo Mopar Contributor
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    Re: Is all boost created equal?

    ya unless your 2.5 is heavily modded, that turbo is going to hit when your already falling out of the torque range

    u want the cam "straight up" with straight up being in relevance to the motor, and not up into the sky the motor tilts backwards some.

    i think u want the cam position hole after the 3rd from the front (rad), rib on the valve cover...been 6 months since ive looked at a valve cover lol....

    if you have a timing belt cover, unplug the grommet, and u should see the timing hole on the cam right in the middle (but kind of up some i think)...i dont run them....


    this is the best pic i could find 4 u..sorry no timing belt....just follow centerline!

  11. #11

    Re: Is all boost created equal?

    IMO for most of us anywhere from a 40 trim to 50 trim is the way to go on a t3/t4 turbo

  12. #12

    Re: Is all boost created equal?

    Thanks, Juggy! I have done a compression test and the pressures are as follows (from 1-4, respectively): 155, 150, 152, 155.
    Now, if I move my cam won't the numbers decrease? I am running 4* of advance on my adjustable cam sprocket, and that is where it was set when I did my compression test.

    -Les

  13. #13

    Re: Is all boost created equal?

    Woops, wrong thread!

    -Les

  14. #14
    Banned Turbo Mopar Contributor
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    Re: Is all boost created equal?

    Quote Originally Posted by fastasleep View Post
    Thanks, Juggy! I have done a compression test and the pressures are as follows (from 1-4, respectively): 155, 150, 152, 155.
    Now, if I move my cam won't the numbers decrease? I am running 4* of advance on my adjustable cam sprocket, and that is where it was set when I did my compression test.

    -Les
    with compression like that your cam timing sounds fine
    looks like a healthy motor u got there

  15. #15
    turbo addict
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    Re: Is all boost created equal?

    I went with a 54 trim compressor on a .63 stage 2 housing for my daytona. Hopefully it will make good power. I also plan on running 22psi.

  16. #16
    Supporting Member Turbo Mopar Contributor mcglsr2's Avatar
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    Re: Is all boost created equal?

    i have some thoughts on your question. i have read that book (and severl others) quite a few times, and here is my understanding of things. bear in mind that i could be completely wrong WARNING, long post...

    Quote Originally Posted by fastasleep View Post
    After being convinced that the turbo on my mini is too big, I have a question: is 10 psi from a T3/T4 .57 trim the same as 10 psi from a Garrett TII turbo? I mean, is the same amount of cfm getting into the chamber?
    essentially yes. we are talking about flow vs. pressure ratio (10psi). thinking that different turbos produce different cfms at the same boost is a misconception IMO (unless i am way off on my thinking - if so please correct me!). to see that the flow is the same, just look at the two compressor maps. here's an exercise: using Frank's turbo calc, put in some data points for your car (values don't really matter). adjust the HP until the calc tells you you need 10psi. then select some different turbos, like the 40, 46, 57 and 60 (under the T04E Series). take a look at the first 1.68 (10psi) point in each map, then look down at the airflow. this is the amount of cfm (in lbs/min) that the turbo will flow. notice for each turbo, that point is at the same cfm. thus different turbos will (roughly) produce the same cfm at a given pressure ratio.

    Quote Originally Posted by fastasleep View Post
    I understand (now, anyways) that the .57 trim is not the best turbo for an 8-valve. But what I can't understand is why at 15 psi with a Mitsu turbo (stock) the mini pulled pretty well, but with this .57 trim at 15 psi it feels like an n/a. It seems to me like the cfm would have to be the same in that both compressors are feeding enough air to creat 15 psi of back pressure. Is this right?
    okay, so if 10psi = 10psi, why do some turbos pull better or produce more power? the reason is because of the adiabatic efficiency of the the turbo at a specific pressure ratio (PR) and the RPMs of the shaft. if you look back at those maps, you'll notice that the efficiency "islands" shift around the points. if a point on one turbo is lower in efficiency then the same point on another turbo, the first turbo will heat up the charge more then the second. and heat = less dense charge = less fuel that can be injected = less power. essentially, a smaller turbo operating in it's peak efficiency range will produce MORE power then a larger turbo operating way outside it's peak efficiency range at a given PR. this is because the larger turbo is heating the charge more as it compresses the air.

    also, you have to take the shaft speed into account. in order for a turbo to produce the desired PR (1.68, 10psi), the compressor and turbine must be spinning at a certain RPM. exhaust is what gets this shaft spinning. larger turbos [in general] don't need to spin as fast to produce the desired PR. so why doesn't my larger turbo produce boost faster then, since it can spin SLOWER and make the same PR!? the reason is because the weight of the compressor wheel is a lot heavier then that of a smaller wheel, which means it takes more exhaust energy to get it spinning. so even though the larger turbo can spin slower, it takes LONGER to get to that slower speed then it does for a smaller turbo. this why BB turbos are great - they can have the benefit of the larger compressor wheel (and slower shaft speed), but since the resistance to get the compressor wheel spinning is lower (thanks to the ball bearings), it can spool faster and create boost sooner.

    so, given the above, the reason you feel a difference is that the mitsu is small, so therefore it doesn't take long to produce boost. so, based off the seat-of-the-pants feel, it feels like it pulls hard. cause your butt tells you so. the larger turbo, though, takes a lot longer to produce the same boost (because of the compressor wheel weight). so boost onset is more gradual, and your butt-o-meter says i'm not accelerating as much. question: are your above statements based off seat-of-the-pants feel, or your took the car to a dyno or drag stip? also, you will get max boost on the mitsu SOONER in the rpm range then the larger turbo. this is where your engine/cam come into play. if you make most of your power/torque lower in the rpm range, the mitsu which gives you max boost sooner, will have a larger affect on overall performance (because it's boosting more power under the curve), whereas the larger turbo reaches max boost later on in the range, quite possibly where your engine is running out of breath - thus no power for the turbo to boost. this is why OEMs typically use smaller turbos on stock street-driven cars, because the cars spend most of their life at lower RPMs and it gives a more "peppy, i have crap loads of power" feel (and then falls on it's face at higher RPMs).

    in summary: 10psi=10psi at roughly the same flow, but that does NOT equal the same power (due to charge air temp, density ratio, etc). a larger turbo will require more exhaust engery to get going, and thusly will require more time (or more flow) to get going.

    all of this is why proper turbo-sizing is important.

    thoughts everyone? i hope this helps...assuming i'm not completely wrong. do you guys think i am way off here?
    Last edited by mcglsr2; 04-08-2009 at 03:30 PM.
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  17. #17

    Re: Is all boost created equal?

    Wow, good read and thanks for the input.

    -Les

  18. #18
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    Re: Is all boost created equal?

    This is an easier way to understand. Ok, lets say you have a 1/4" pipe, this the garrett, so with the tap fully open, its coming out fast and with some high pressure, now change to a 2" pipe, tap fully open, what happens, you have the same amount of water but its not coming out very fast and volume is lower. Now if you put a pump inline and try again, the 1/4" will be busting at the seams and you'll strain or damage the pump, but with the 2" pipe, its all good now, Make sense?
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  19. #19

    Re: Is all boost created equal?

    Yes, it does. Thanks!

    -Les

  20. #20
    Supporting Member II Turbo Mopar Contributor minigts's Avatar
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    Re: Is all boost created equal?

    Quote Originally Posted by turbovanman View Post
    This is an easier way to understand. Ok, lets say you have a 1/4" pipe, this the garrett, so with the tap fully open, its coming out fast and with some high pressure, now change to a 2" pipe, tap fully open, what happens, you have the same amount of water but its not coming out very fast and volume is lower. Now if you put a pump inline and try again, the 1/4" will be busting at the seams and you'll strain or damage the pump, but with the 2" pipe, its all good now, Make sense?
    Who's running 1/4" piping? That seems a little small....I'm confused now.


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