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    lower CR piston..

    what r the benefits of lowering the CR, let's say the stock CR is 10.2 on 20psi
    if i lower the CR to 9.6 at the same boost 20psi?

    any gains?Click here to enlarge Click here to enlarge

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    It'll allow you to run a lot higher boost while lowering the potential for to detonation caused by lack/too low octane.
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    ^ Pretty much that.

    You can only run so much boost with a given fuel before either detonation is unavoidable, or you have to retard timing so much to avoid detonation that you are not making any more power.

    Or more specifically if you push timing too low actual peak cylinder pressure will be too late to push the piston down. Eventually you are just heating up your cylinder head, exhaust valves, exhaust manifold and turbo since combustion is still occurring as the charge goes out the exhaust valve. That's bad, mkay!

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    Click here to enlarge Originally Posted by benzy89 Click here to enlarge
    It'll allow you to run a lot higher boost while lowering the potential for to detonation caused by lack/too low octane.
    so give us the reason you wouldn't want to do this.




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    Click here to enlarge Originally Posted by Freon Click here to enlarge
    ^ Pretty much that.

    You can only run so much boost with a given fuel before either detonation is unavoidable, or you have to retard timing so much to avoid detonation that you are not making any more power.

    Or more specifically if you push timing too low actual peak cylinder pressure will be too late to push the piston down. Eventually you are just heating up your cylinder head, exhaust valves, exhaust manifold and turbo since combustion is still occurring as the charge goes out the exhaust valve. That's bad, mkay!
    so lower CR can run higher boost with lower octane?

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    Click here to enlarge Originally Posted by E90SoFlo Click here to enlarge
    so give us the reason you wouldn't want to do this.
    If you have the octane then you're only decreasing performance by lowering compression. I.e an e85 car or a race car that runs strictly on race gas. The higher the compression the better on these. When I build my next project car it will not be a DD. it will be an e85 car with at least 10.5:1 and a huge turbo Click here to enlarge
    2011 335is DCT, moving to Italy, looking for new car friends Click here to enlarge

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    Click here to enlarge Originally Posted by 135idct Click here to enlarge
    so lower CR can run higher boost with lower octane?
    exactly. Spool is usually negatively affected though by going big turbo, lower compression and pump gas.
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    Another downside -- not necessarily significant -- is softer response (ie., less torque) at low rpm.

    Neil

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    any expert here can tell us more about advantages and dis about the lower CR piston??

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    advantages: more boost + timing on any given fuel. safer tuning. better potential peak power with the right setup.

    disadvantages: when not octane limited (already lots of timing) less power.. also slightly less spool.. and less hp/lb of boost.
    boop

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    2 out of 2 members liked this post. Yes Reputation No
    You will lose gas mileage as well - Not that anyone cares about efficiency at this point, but it will cause a substantial drop.

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    I drove my S52 @ 8.5:1 for about 10k naturally aspirated (before turbo) It didn't loose a lot of gas mileage, but it sure as hell lost all its "snap". It dynoed 185 Lolz
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    Click here to enlarge Originally Posted by E90SoFlo Click here to enlarge
    so give us the reason you wouldn't want to do this.
    You're not pushing the performance of the motor (regardless of the combination of boost/ignition advance) beyond the octane's capability.

    EX: Car A only runs on 120 octane & has been aggressively tuned to run 20 psi & 12 degrees advance. Car B is the exact same setup, but runs on 93 octane; to run the same boost & ignition advance safely, the CR must be decreased.
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    Click here to enlarge Originally Posted by E90SoFlo Click here to enlarge
    so give us the reason you wouldn't want to do this.
    Direct injection's cooling effect allows a comparatively higher compression ratio than port injection before detonation on pump gas. Lowering the compression ratio will affect off boost power and spool. I wouldn't lower it.
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    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
    and less hp/lb of boost.
    This one. I hate this one.
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    Click here to enlarge Originally Posted by MDORPHN Click here to enlarge
    Another downside -- not necessarily significant -- is softer response (ie., less torque) at low rpm.

    Neil
    which is usually overcome by the massive advance in timing assuming you're octane limited.

    ED: unless you're running E85, you're pretty much 100% for sure octane limited on the N54. they run fairly high compression, and the turbo's aren't exactly run at efficient levels lol

    it still needs a little more proof, but from what i can tell, at least a thicker head gasket is what the N54 needs in terms of compression drop

    and if you're running a big turbo, even with E85, you'll probably want to drop the compression slightly as well.

    turns out E85 isn't yet common in my city, E70 is a little more common, but i can get a ton of local E10 100RON (~95-96AKI)... so for me, i'm still strongly thinking 9.5:1 is the way i want to go

    but that's just me haha.
    boop

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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    This one. I hate this one.
    it's all a balancing act

    you want low enough compression you can throw a lot of boost and a lot of timing for whatever fuel you want

    but you want high enough compression that you aren't throwing away power//spool in turn

    the N54 still needs experimentation before anyone goes 'no need to change from 10.2:1' or if anyone can say 'lower compression is a really good idea'

    just theories, and looking at other platforms/thinking sorta logically.

    if you want max power you can possibly get, and want to push, drop compression, and in future, maybe experiment with raising.

    if you have a specific reasonable power GOAL (500-600 say), keep stock or raise. 10.5:1 with a big upgraded turbo + good fuel = solid solid performer

    9.5:1 with a big turbo running bagillions of psi of boost = numbers.
    boop

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    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
    it's all a balancing act

    you want low enough compression you can throw a lot of boost and a lot of timing for whatever fuel you want

    but you want high enough compression that you aren't throwing away power//spool in turn

    the N54 still needs experimentation before anyone goes 'no need to change from 10.2:1' or if anyone can say 'lower compression is a really good idea'

    just theories, and looking at other platforms/thinking sorta logically.

    if you want max power you can possibly get, and want to push, drop compression, and in future, maybe experiment with raising.

    if you have a specific reasonable power GOAL (500-600 say), keep stock or raise. 10.5:1 with a big upgraded turbo + good fuel = solid solid performer

    9.5:1 with a big turbo running bagillions of psi of boost = numbers.
    In my opinion the compression is already low enough and dropping it further just increases lag and spool essentially amplifying negative characteristics of turbo motors. 10.2:1 is more than low enough to make big, big power. Fueling will be a restriction way before the compression ratio ever would be.
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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    In my opinion the compression is already low enough and dropping it further just increases lag and spool essentially amplifying negative characteristics of turbo motors. 10.2:1 is more than low enough to make big, big power. Fueling will be a restriction way before the compression ratio ever would be.
    10.2:1 is still FAIRLY high

    i mean, on the 725hp run, slightly (not even 9.5 necessarily) lower compression would have seen solid gains on the fuel used, simply because of the amount of additional timing with no other changes..

    fuelling is already a restriction, if you want to run E85 lol. hence why most people will (at least for the foreseeable future) be running pump, which will benefit from said lower compression.. say 10.0:1 with the option of the thicker head gasket for.. 9.6-9.7:1 and that's as much as i'd assume you'd need.. not too much downside with such a small drop, with the option to drop lower if you find benefit... again, without any solid proof or testing Click here to enlarge.

    E85 or E70, yeah don't bother, 93 -- it's not completely stupid.

    and running race gas day to day is simply unrealistic, unless you're incredibly wealthy.. heck, even for a race meet/day, it's simply too expensive for me (especially at australian prices), and i assume, many other people.
    boop

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    10.2 is honestly a very high compression.
    lots of "high compression" engines are 10.5:1.

    the only reason they can get away with it so easily is DI.

    i cut down from 10.5 to 8.5 in anticipation for turbo.
    i turbo'ed it, ran out of octane on 93, and ended up switching to E85.
    looking back, would I have dropped compression to run E85? No. Not really.
    does the car run bad because of it? Hell no Click here to enlarge


    and I HIGHLY recommend NOT using a thicker head gasket, but replacing pistons.
    lifting the head KILLS quench.
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    +1

    Neil
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    Eh, the latest round of NA DI motors are well above 12:1. The FR-S/BRZ is 12.5:1 for instance. Pretty insane for a car that is rated for 91 octane. It really does seem all the DI motors are capable of running significantly higher compression ratio.

    Compare to an STI/WRX/EVO that run 8.2-9.2 or so with standard multiport injection. There are more differences that the DI/MPI fueling system, but feel free to look around at a wide swath of factory equipment cars with DI and without and come to your own conclusion.

    If you really wanted to know if you have to lower your compression you might start with EGT measurements and dyno time to see if the extra boost you are adding is adding power. I.e. if you tune your XYZ turbo kit N54 to 18 psi, retune best timing without knock, make 450hp with EGT of 1750F. If you bump to 22psi, retune timing to avoid knock, but only make 465hp (only +15hp) and EGT is now 1900F you ran out of turbo airflow (which you can check against your compressor maps), or the compression may be too high, or you are missing something else in the tune, or maybe the turbine is too small... There's no silver bullet so those developing big turbo kits should be doing that sort of research. Part of that is doing the best maths you can up front, building the best setup you can, then possibly some trial and error.

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    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
    10.2:1 is still FAIRLY high

    i mean, on the 725hp run, slightly (not even 9.5 necessarily) lower compression would have seen solid gains on the fuel used, simply because of the amount of additional timing with no other changes..

    fuelling is already a restriction, if you want to run E85 lol. hence why most people will (at least for the foreseeable future) be running pump, which will benefit from said lower compression.. say 10.0:1 with the option of the thicker head gasket for.. 9.6-9.7:1 and that's as much as i'd assume you'd need.. not too much downside with such a small drop, with the option to drop lower if you find benefit... again, without any solid proof or testing Click here to enlarge.

    E85 or E70, yeah don't bother, 93 -- it's not completely stupid.

    and running race gas day to day is simply unrealistic, unless you're incredibly wealthy.. heck, even for a race meet/day, it's simply too expensive for me (especially at australian prices), and i assume, many other people.
    No it isn't. The compression ratio stuff is kind of an old school approach these days. Weistec boosts the stock compression ratio even with their built motors. From what I understand managing cylinder temps is more important. If your internals are strong and you tune well, you want higher compression as you will make more power per pound of boost plus spool faster and have greater volumetric efficiency.

    That's why the m3 is so strong with just a little boost. Plus, the less air your have to compress the less heat.

    It isn't a question of race gas. 10.2:1 with direct injection is more than low enough. As stated, there are other problem areas with the N54 to address well before this.
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    Well, if you want custom pistons for anything let me know. I can get JE or weisco pistons for anything.
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    What's the relation between lower CR and destroke?

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