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high compression and boost

This is a discussion on high compression and boost within the Forced Induction forums, part of the LSx Technical Help Section category; I think for the street turbo sizing can be very important. You could run low c/r, pack in lots of ...

  1. #21
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    I think for the street turbo sizing can be very important. You could run low c/r, pack in lots of fuel and air and be pulling hard from 2000 rpm'swith small turbo's. It would be inefficient off boost though. I've got a 5.7 here waiting to go on the dyno. It's a custom build. I've no idea what it will make but it's on 7:1 c/r with a titchy pair of GT 28's. On paper and on flow charts they should flow enough. It's destined for a TR8 racer and with head changes I can increase c/r if I need to.

    If you don't use boost very often then stock c/r and low boost will be a happier experience but with less of a rush

  2. #22
    Awaiting Activation Liquifire's Avatar
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    I would also like to know this as I plan on building a motor with the max that I can get out of it on pump gas. I don't plan on going with an intercooler or with large boost(6-9 psi) on a D1SC, and I was wondering what the max CR I could use with a forged bottom end and have it reliable for the long term. Anyone have any idea?

  3. #23
    Ratdaddy07
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    I am no expert but I do know a little physics and it seems to me that the answer is not the same for everyone. You want balls to the wall power and don't care if you blow her then go with high comp. and much boost. You want a daily driver that is capable of making people on the street shit every now and then then low comp, and a nice little boost, from those two extremes decide how far you are willing to go. I'm thinking for my street car 8.5/1 maybe 9.5 and 6 to 7 pounds. Forged 383 bottom end.

  4. #24
    Ratdaddy07
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    Although I am still not 100% sure I wont just go with nitrous instead of turbo, so much cheaper just don't like the idea of limited amount before you have to refill the bottle. My luck I'll be on the way to fill the bottle when I finally get on side a new Z06 at the light!

  5. #25
    LS1 FTW! 85mcss's Avatar
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    keep the informative replys coming guys!

  6. #26
    Member z28misfit's Avatar
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    I would think that high compression and low boost theoretically will work,but everyone always changes their setup so I would go with the lower comp and high boost. Boost already adds compression and if you have a high comp motor you may start have some problems I would think.Ex-head gaskets may not take the high cylinder pressure. At least with low comp pistons you can compensate for this. I believe Weind has a catalog with compression readings in it with different boost settings. You'll be surprised what little boost it takes too bring the c/r ratio up, especially on a high compression engine!
    99 z28 cam,m6 ,S60 4.56gears,Fast 90/90 full boltons,more mods one day.
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  7. #27
    Junior Member jakesz28's Avatar
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    Well I still feel that a 10 to1 motor can handle a fair amout of boost. The important thing would be sizing the turbos and intercooler correctly. The faster the turbo has to spin to feed a motor the more it raises the air temp. If the air can be kept cooler it helps keep you away from detonation.

    Two things I find important when running a higher compression motor on boost are compressor and intercooler efficincy.

  8. #28
    Junior Member Dave Stubbs's Avatar
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    Just a thought, correct me if I'm wrong. The only way to lower compression is to change the size of the combustion chamber (eg dished pistons, Larger CC heads, Thicker head gaskets, etc..). This would change your diplacement (more CC). So if you went with low compression high boost I believe you would see perfomance gians from a larger combustion chamber (bigger displacement)

  9. #29
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    Those are deffinately ways to lower compression but don't add displacement.

  10. #30
    Car RamRod 98Camarod's Avatar
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    I think it would help to see the dyno graphs of a low comp-high boost combo and a high comp-low boost combo.

  11. #31
    LS1 FTW! 85mcss's Avatar
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  12. #32
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    a higher cr motor will always make more power at the same boost level as a lower cr engine.Given they both have identical setups.The lower compression motor will be able to handle more boost without detonating,but it will take the extra boost to make the same power as the hi cr motor does at a lower boost level.
    With the higher cr motor u have more power and better driveability off boost.
    A higher cr motor will also spool up a turbo faster.
    The only thing that really comes into to play is detonation, its easier to get a higher compression motor to detonate.well of course unless ur runing way lean or something in the low compression motor lol.
    The low cr motor is more forgiving to a bad tune
    my first boosted motor was an 8.2-1 v6 in my 3rdgen which i ran 18 psi
    my second motor was identical except for the fact the cr was 9.2-1, the second motor made the same power at 12 psi as the first did at 18 psi

  13. #33
    Senior Member predator's Avatar
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    Quote Originally Posted by Boosted89rs View Post
    a higher cr motor will always make more power at the same boost level as a lower cr engine.Given they both have identical setups.The lower compression motor will be able to handle more boost without detonating,but it will take the extra boost to make the same power as the hi cr motor does at a lower boost level.
    With the higher cr motor u have more power and better driveability off boost.
    A higher cr motor will also spool up a turbo faster.
    The only thing that really comes into to play is detonation, its easier to get a higher compression motor to detonate.well of course unless ur runing way lean or something in the low compression motor lol.
    The low cr motor is more forgiving to a bad tune
    my first boosted motor was an 8.2-1 v6 in my 3rdgen which i ran 18 psi
    my second motor was identical except for the fact the cr was 9.2-1, the second motor made the same power at 12 psi as the first did at 18 psi
    my question to you is which one had the better curve... sure both might have had say 350 hp peak, but which gave you more in the non peak range... im gonna guess that higher compression lower boost is gonna give you a flatter, more useable curve.

    Is it more logical to just max out compression then to even bother with boost for a street engine? or is High Comp with just a few psi the way to go?

    Surely there is a magical c/r and boost # that gives you the best under curve HP for a given setup.

    -me

  14. #34
    Member Boosted89rs's Avatar
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    hands down the higher cr motor.

    cr is just that the ratio of which the pistion compresses the air/fuel in the cyl,
    the more u compress it the more power it makes.on a supercharged/turbocharged motor ur increasing the ve of the motor,i.e more air/fuel to compress.which means more heat.lowering the cr helps get rid of some of this heat,avoiding detonation.now with a properly sized and efficiant intercooler,keeping the turbo within its max efficiancy range, u keep the charge cooler allowing for more compression.

    when deciding on a combo/cr to use u have to look at everything.head type aluminum vs iron,compressor efficiancy/intercooler. etc.
    u cant just have soone say oh u can run x amount of cr with so many pounds of boost.

    ill use my motor for an example
    3.1L 9.2-1 cr
    i run a 61mm turbo and it sits right in the 74% eff range air temps after my intercooler are 110 ish range my motor dosent detonate.
    now remove my 61mm and put on a smaller turbo that sits at say 65% eff
    well now my intake air temps prolly just jumped up 80+* motor will prolly detonate

    hopefully u see were im going with this

    boost preasure is meaningless actually most ppl ask how many pounds are u running. well psi is just a measurement of restriction really.

    take 2 5.7 motors identical shortblocks,now on one say a bone stock upper end l98 heads tpi intake
    on the second say a set of ported heads,and ported manifold base with aftermarket runners etc.

    identical turbo setups on each, well say a t61 turbo each set for 5 psi well they may both be at 5 psi but ones surley gonna make alot more power due to the fact it moves more air.

    hence why psi is a meaningless term

  15. #35
    Senior Member predator's Avatar
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    i dont totally follow you on this... i understand where you are comming from on the psi is basically a measurement of restriction, but that would only be true if they both running down an open ended tube...

    since the exhaust valves cause a blockage in both motors no matter what the restrictions are between the valve and the turbo it will build psi because there is nowhere for the air to go... not because the air is slowing down on it way out...

    -me

  16. #36
    Member Boosted89rs's Avatar
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    Quote Originally Posted by predator View Post
    i dont totally follow you on this... i understand where you are comming from on the psi is basically a measurement of restriction, but that would only be true if they both running down an open ended tube...

    since the exhaust valves cause a blockage in both motors no matter what the restrictions are between the valve and the turbo it will build psi because there is nowhere for the air to go... not because the air is slowing down on it way out...

    -me

    this is why i gave the example of the 2 5.7 l98 engines
    both are running the same psi but one makes alot more power even though the boost gauge reads the same
    the one motor just happens to move more air at the same psi.

    this is why refering to psi is meaningless.it all comes down to the amoutn of air moved, wether it be measured in cfm,ina cuper/turbocharged cars case its lbs per min

    one other thing id liek to point out is a motor with a smaller bore also has better flame front control, then a larger bore,this also helps with detonation and heat issues.allowing for more compression

  17. #37
    doesn'tplaywellwithothers superchargedblackgto's Avatar
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    here is my .02 worth, I have a stock ls1 motor with no forged parts yet, Magnacharger running 7psi making 447.4rwhp, Now this fall I will be stroking and forging motor dropping c/r 9.1 and putting a smaller pulley on it. I have been told with a proper tune a stock ls1 is maxed out 10-12psi, because of blowby and other problems. Hope this helps a little
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  18. #38
    Awaiting Activation Liquifire's Avatar
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    I would not run ten to twelve psi on a stock motor.....I would be nervous about dependability.

  19. #39
    doesn'tplaywellwithothers superchargedblackgto's Avatar
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    I have been told by 3 different tuners "experts in their fields" that with the proper tune it was "safe" I don't know but I would lower comp. and forge internals first....thats just me

  20. #40
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    We always build blower/turbo motors with NO more than 9:1 for many reasons...... If you did do a 10:1 plus motor with that kind of boost you would need to get some custom pistons made for that combo with a lowered ring pack. Most blower pistons that are a dished head have lower ring glands and a thicker face to help with all the heat produced by boost.
    And if you are going to do that high compression with lots of boost your NOT going to get away with pump gas unless it is stupidly de tuned. Camshaft choice is going to be VERY crucial also with that kind of cylinder pressure.
    Everything will need to work as a complete unit once you get cyclinder pressures that high. You'll be finding ANY weak link quickly.
    You'll also need a COMPLETE fuel system to support that kind of damand.
    Don't be afraid of the bottle!!! Be afraid of your tune!!!

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