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  1. #1
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    raising compression?

    ive been considering buying cp pistons and carrillo rods for my car as well as porting the head. I know really none of this is needed but i really would like to push the upper limits of the car and be sure nothing will break. I was looking at cp pistons and of course they let you get a custom compression and this brought up the idea.

    Would it be beneficial to raise compression to say 11:1 or maybe higher and still run stock turbos?

    i think e85 + 19psi on stock turbos + 11:1 compression + a head port would be pretty amazing
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    Why spend all that money and go through all those problems and then run the stock turbos? You'll make way more power and spend less money just throwing on a set of hybrid snails.

    However, back on topic:(assuming you aren't knocking/DME isn't pulling timing) the more compression = the more power.

    However, can e85 support 11:1@19psi(do you mean a flat 19psi across the board or the usual boost taper?) with an aggressive timing curve? No one knows. This is another area where the stock turbos are going to be an issue. All the back pressure/heat generated by the tiny turbos just makes knock more likely to occure(e85 helps here, but again, how much???)
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    Click here to enlarge Originally Posted by rader1 Click here to enlarge
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    Why spend all that money and go through all those problems and then run the stock turbos?
    This.

    Why go through the trouble to pull the motor and build it if you are going to stay with the stockers? By the time you even get the motor together Vargas will probably have their S3 out, just put that one. Building a motor on stock turbos seems like a waste of money.

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    If you are building it fo strickly E85, why not go with a higher CR? I'd say 12.5:1 minimum.
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    Click here to enlarge Originally Posted by WDBi Click here to enlarge
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    ive been considering buying cp pistons and carrillo rods for my car as well as porting the head. I know really none of this is needed but i really would like to push the upper limits of the car and be sure nothing will break. I was looking at cp pistons and of course they let you get a custom compression and this brought up the idea.
    Oxymoron.

    You are better off getting some sort of upgraded turbo(s) and making power with the built setup.

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    Click here to enlarge Originally Posted by lulz_m3 Click here to enlarge
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    If you are building it fo strickly E85, why not go with a higher CR? I'd say 12.5:1 minimum.
    because you'll still end up with worse top end power, and probably mid range power (can't have anywhere near as much boost), than a low or stock CR build

    basically, if you can push a turbo on a given compression ratio to max efficiency/max boost, without knock, raising the compression ratio simply advances the point where you have to pull timing to avoid detonaton.. where lowering CR lets you add more timing, while keeping the turbo at hte desired level

    raising the CR can be a good idea if your turbos are maxed out with no detonation and you have plenty more octane to spare, or if you really really want more off-boost power

    stock turbos already need meth it seem for stock CR, raising it wouldn't help anything.


    got this from another forum:

    "Forced induction raises your effective compression ratio. I don't know the PLAN numbers on the SR. But an cylinder can only hold so much pressure. You can increase cylinder pressure with a change in C:R, Cam Timing or adding forced induction.

    The engine is going to produce the same power at the same effective C:R. Whether it is a high C:R engine and low CFM turbo or a Low C:R engine, high CFM turbo.


    The advantage to running a high C:R is that it is always a high C:R. You don't have to wait on a turbo to spool up to raise the cylinder pressure. The exhaust velocity's will be faster and the turbo will spool faster.


    The benefit is also the problem. With a high C:R it is harder to tune cause your margins are much closer and the effective C:R raises faster with boost. It is a lot easier and safer to run low C:R, high boost."

    example in a perfect world, ignoring the rest of physics:

    say at 10:1 and 20 PSI you'l end up with 200psi effective assume that's 100% as high as you can go, if you raise to 11:1, you'll only be able to run 18.18 PSI, if you went 12.5:1, 16psi

    but at 10psi, 10:1 would have 100, 11:1 would have 110, and more power produced

    so you'll have more power available at boosts lower than 18.18/16PSI respectively, but your peak power will suffer.
    Last edited by Flinchy; 12-06-2012 at 07:05 PM.

  7. #7
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    That's all assumption, without knowing exactly what the limits of the N54 we don't know where that point is.

    However, the more air you can fit into the cylinder and the tighter you squeeze it the more power it will make.
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    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
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    but your peak power will suffer.
    Why will peak power suffer in this case? I don't know anything about this, but it seems like if your effective compression ratio was the same in all scenarios then you'd be able to get the same peak power, all else being equal.

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    Click here to enlarge Originally Posted by trufus Click here to enlarge
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    Why will peak power suffer in this case? I don't know anything about this, but it seems like if your effective compression ratio was the same in all scenarios then you'd be able to get the same peak power, all else being equal.
    well the N54 at 10.2:1 needs more octane, so you'd be instantly restricted to alcohol fuels (semingly not a problem for the OP though).. add to that the high back pressure of the stock turbos, and it's even hotter in there, and will be made even more so.. .then if you decide to go with bigger turbos (RB or something) you'll be limited in how much boost you can run in comparison, since you could turn it up much higher

    effective compression ratios being the same at 16 psi high comp as the low comp at 20psi.. the 20psi motor is pushing more air, soyou can burn more fuel, and generate more power.

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    Click here to enlarge Originally Posted by rader1 Click here to enlarge
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    That's all assumption, without knowing exactly what the limits of the N54 we don't know where that point is.

    However, the more air you can fit into the cylinder and the tighter you squeeze it the more power it will make.
    well, stock turbos, stock CR, they need meth with pump to raise the octane (or E85, if the fuelling system could handle it better?) already.. stock turbos and higher CR, you wouldn't have a choice in anything bar E85

    If there were more numbers available, i could put some of it together (more with help lol) :/

    at what pressure levels knock starts to get really bad, temperatures, all sorts of data (and testing) would be needed

  11. #11
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    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
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    well the N54 at 10.2:1 needs more octane, so you'd be instantly restricted to alcohol fuels (semingly not a problem for the OP though).. add to that the high back pressure of the stock turbos, and it's even hotter in there, and will be made even more so.. .then if you decide to go with bigger turbos (RB or something) you'll be limited in how much boost you can run in comparison, since you could turn it up much higher

    effective compression ratios being the same at 16 psi high comp as the low comp at 20psi.. the 20psi motor is pushing more air, soyou can burn more fuel, and generate more power.
    Thanks for explaining some of the practical limitations.

    What I'm still stuck on is that if octane, turbo limitations, etc. are taken out of the equation, you're left with a ratio that describes the amount of air moving through the cylinders. So how does a low CR, high boost application move more air than a high CR, low boost engine given an equal "effective CR"?

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    @Flinchy, with E85, we are already beyond max power the factory turbos can sustain at around an E50-E60 blend. Higher compression plus 100% E85 would yield even more power.
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    Click here to enlarge Originally Posted by trufus Click here to enlarge
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    Thanks for explaining some of the practical limitations.

    What I'm still stuck on is that if octane, turbo limitations, etc. are taken out of the equation, you're left with a ratio that describes the amount of air moving through the cylinders. So how does a low CR, high boost application move more air than a high CR, low boost engine given an equal "effective CR"?
    as i said above, just an example that your cylinder can only take 200psi

    10:1 factory, 20psi,
    11:1 increased, 18.18 psi

    18.18<20 ,there's less air crammed in per square inch.

    Click here to enlarge Originally Posted by lulz_m3 Click here to enlarge
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    Click here to enlarge Originally Posted by lulz_m3 Click here to enlarge
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    Flinchy , with E85, we are already beyond max power the factory turbos can sustain at around an E50-E60 blend. Higher compression plus 100% E85 would yield even more power.

    it's a bit more complicated though, how much absolute pressure can the stock rods handle? how much more heat, therefore less efficiency, will the stock turbo have put into it?

    more compression = more heat = hotter turbo = less efficiency in the compression

    however, if, yes, the factory turbo can take more compression, all well and good, but going the high comp build will cost as much (more) than RB/VargasII turbos, and you'll still end up with less power, and you'll 100% defintely be restricting them by then.

    so, yeah, if you could run more compression on stock turbos with more E85, go right ahead, but upgrading the turbos is cheaper, more effective, and will work BETTER on lower compression.

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    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
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    because you'll still end up with worse top end power, and probably mid range power (can't have anywhere near as much boost), than a low or stock CR build
    Based on what?

    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
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    basically, if you can push a turbo on a given compression ratio to max efficiency/max boost, without knock, raising the compression ratio simply advances the point where you have to pull timing to avoid detonaton.. where lowering CR lets you add more timing, while keeping the turbo at hte desired level
    From what tuners have told me keeping compression higher is becoming more and more common with modern forced induction builds as managing the cylinder pressure is key. And if you keep the boost the same between two setups one with higher versus lower compression, the higher compression car is simply going to be more fun to drive everywhere in the rev range.

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    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
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    as i said above, just an example that your cylinder can only take 200psi

    10:1 factory, 20psi,
    11:1 increased, 18.18 psi

    18.18<20 ,there's less air crammed in per square inch.
    I got it now. Compression ratio is a ratio of volume. I was thinking about it wrong.

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    CR just like everything else in a proper build needs to be figured into the big picture. There's a place and time for everything but unfortunately with several other limitations of the stock N54 setup CR is not something you should really be worried about. Regardless of CR, fixing the restrictions that will lead to inefficient boosting like undersized bottle necked turbos and low flowing heads that also lead to overheating intake and combustion chamber temps compared to efficiently sized turbos and a head/cam setup for flows properly. Once you have a properly flowing and boosting setup then you can start to look at what kind of power and power curve you want the more power you want given the lower displacement your going to have to deal with a smaller power range and a not so flat torque curve. But if you wanted a nice flat torque, medium power street setup you can go with a higher CR and medium boost level where the higher HP and less power band setup would typically have a lower CR higher boost setup. The fueling options help as you can get more peak or flatter power band. At this stage of the N54 I don't see any benefit of going higher CR's without bigger turbos and head work. Most shops I've seen still go lower CR but some shops like MSP and their 7.06 ET supra used to run 10+:1 CR but Paul had the money to try about every setup from 7.5:1 to 12:1 and blow motors just to see how they would run.

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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
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    Based on what?
    General guidelines on compression ratios and turbocharged cars... What i already kinda knew, and everything i could research

    some results from the STI crowd said that keeping boost the same (16psi say) and raising the CR from 8.55 to 9.1 (or something) lost them 30hp... and it's a theme for those motors, high compression hurts them for whatever reason.

    Click here to enlarge Originally Posted by Sticky Click here to enlarge
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    From what tuners have told me keeping compression higher is becoming more and more common with modern forced induction builds as managing the cylinder pressure is key. And if you keep the boost the same between two setups one with higher versus lower compression, the higher compression car is simply going to be more fun to drive everywhere in the rev range.
    Yes, this is true if you can keep boost/turbo/everything identical between setups, the higher comp will be plenty more fun, but the lower compression car with more boost... well it'll produce more power, drive differently, horses for courses almost... fact is, less compression can handle more boost, as long as you have the turbo setup to provide it...

    if you don't want GOBS more power, don't want new turbos (or can't fit bigger), but have plenty of octane left to tune with, and want more response and a little more power raise it then.. you'd have a much more pleasant off boost car.

    though if your goal is power, low compression, boost, and more timing... without going TOO low, so you do keep your mid>upper mid in tact


    i wish more people had messed wit the CR's in these motors, want more real world info Click here to enlarge

    @WDBi - you try 11:1, i'll try 9.5:1, we'll compare notes? Click here to enlarge

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    Click here to enlarge Originally Posted by trufus Click here to enlarge
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    I got it now. Compression ratio is a ratio of volume. I was thinking about it wrong.
    yeah, i'm having trouble putting it further into words, but, yeah. how much boost is the air content, compression is how much smaller that air becomes.

    thoug, it's not just the air content as you add more psi, just like adding static CR, you're adding more compression in the form of the additional air pressure, which just like static CR< is power adding in itself.


    @brusk - yeah, raising compression tends to be a lot more expensive than lowering.. that shop would have set a boost target, an ET etc. target and gone for as much compression as they can to keep up midrange.. that's DAMN expensive to do

    sigh, now i'm having second thoughts on exactly what i want for my engine in terms of compression ratio >_<.. go for easy power low CR, or possibly lose a bit of top end for more midrange and keep stock or around stock...
    Last edited by Flinchy; 12-07-2012 at 12:28 AM.

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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
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    Based on what?
    I'll take an educated guess. Running higher compression and high boost normally requires a reduction in timing at high RPM. As Flinchy alluded to earlier, this timing reduction can cost much more power than is gained by increasing compression. There is no free lunch here.

    A better, easier place to reap benefits on an N54 (according to Dzenno's testing) would be improving cylinder head flow.

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    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
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    yeah, i'm having trouble putting it further into words, but, yeah. how much boost is the air content, compression is how much smaller that air becomes.

    thoug, it's not just the air content as you add more psi, just like adding static CR, you're adding more compression in the form of the additional air pressure, which just like static CR< is power adding in itself.
    No it makes sense. [My phone is a sack. I've tried to post this three times. Sorry if it shows up more than once].

    The limiting factor here is the maximum pressure occurring at the lowest point in the engine stroke (an octane limitation so far on the N54, potentially an engine block limitation at high boost #s??). The intake air is at X PSI and is further compressed by the engine stroke (not an adiabatic process so I don't think it is safe to assume CR * PSI = "equivalent CR" at all intake PSI and CR).

    I think I'm on the same page now but your terminology threw me off a bit. The real world implications of running a higher CR are slightly more complicated than the "ideal" physics would imply.

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    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
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    some results from the STI crowd said that keeping boost the same (16psi say) and raising the CR from 8.55 to 9.1 (or something) lost them 30hp... and it's a theme for those motors, high compression hurts them for whatever reason.
    How did they raise compression, keep boost the same, and lose power?

    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
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    Yes, this is true if you can keep boost/turbo/everything identical between setups, the higher comp will be plenty more fun, but the lower
    compression car with more boost... well it'll produce more power, drive differently, horses for courses almost... fact is, less compression can handle more boost, as long as you have the turbo setup to provide it...
    The lower compression car should make less power per pound of boost. Less compression still should have the same max cylinder pressure limit. So ultimately the main issue seemingly is cylinder pressure.

    I'm going higher compression than I originally intended.

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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
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    How did they raise compression, keep boost the same, and lose power?



    The lower compression car should make less power per pound of boost. Less compression still should have the same max cylinder pressure limit. So ultimately the main issue seemingly is cylinder pressure.

    I'm going higher compression than I originally intended.
    Timing. Given a constant octane, and assuming you achieve max timing for the octane at X:1 CR, it is feasible to have a decrease in power at X+1:1 CR because you can't run the same timing at the increased maximum pressure occurring at the higher CR.

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    Click here to enlarge Originally Posted by trufus Click here to enlarge
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    Timing. Given a constant octane, and assuming you achieve max timing for the octane at X:1 CR, it is feasible to have a decrease in power at X+1:1 CR because you can't run the same timing at the increased maximum pressure occurring at the higher CR.
    Oh well of course, I assumed all things were equal other than the compression in the example.

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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
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    How did they raise compression, keep boost the same, and lose power?



    The lower compression car should make less power per pound of boost. Less compression still should have the same max cylinder pressure limit. So ultimately the main issue seemingly is cylinder pressure.

    I'm going higher compression than I originally intended.
    i'm not sure, there were reasons (something to do with the stroke and cams and timing, somewhat engine specific they said... ) i could find the thread again more than likely, if you'd like?.. possibly multiple threads?


    I'm pretty sure i'm going to lower the compression from 10.2 to something, just so i can be sure i'm able to throw ALL the boost at it

    the issues isn't JUST cylinder pressures though, just the most simple one to articulate... it's also heat and .. probably plenty of other variables i'm not educated enough to know about... the cooler you can get your cylinders, the more pressure you can add (heat) before the fuels knock point will occur, the 'max' could vary a fair bit depending on specific conditions and variables you're comparing

    i think we're starting to get into slightly other topics specifically to do with the tuning though?

    Click here to enlarge Originally Posted by trufus Click here to enlarge
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    Timing. Given a constant octane, and assuming you achieve max timing for the octane at X:1 CR, it is feasible to have a decrease in power at X+1:1 CR because you can't run the same timing at the increased maximum pressure occurring at the higher CR.
    ah well yeah that makes sense, so yeah, you can have more power off boost and down low, but makes sense to not have power higher up in cases like that?

    that reminds me, i'm going to go do some research on timing because that's the thing i know least about right now..

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    Click here to enlarge Originally Posted by trufus Click here to enlarge
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    No it makes sense. [My phone is a sack. I've tried to post this three times. Sorry if it shows up more than once].

    The limiting factor here is the maximum pressure occurring at the lowest point in the engine stroke (an octane limitation so far on the N54, potentially an engine block limitation at high boost #s??). The intake air is at X PSI and is further compressed by the engine stroke (not an adiabatic process so I don't think it is safe to assume CR * PSI = "equivalent CR" at all intake PSI and CR).

    I think I'm on the same page now but your terminology threw me off a bit. The real world implications of running a higher CR are slightly more complicated than the "ideal" physics would imply.
    Octane isn't the root of the problem though, as much as the fuel pump can't handle 100% E85/flex fuel (for those that want to run the stuff)?.. meth injection has excellent charge cooling properties, but the octane of the actual stuff burning is roughly the same as ethanol anyway.. so pure E85 would have a better octane rating.. until fuelling is sorted, octane KINDA is a limitation yeah..

    yes, perfect world physics are a lot different to real world physics, but they're good to understand things roughly.. as above, it's not JUST the compression ratio that influences how things perform, timing, temperatures, how you hold your tongue at 9:13pm on october 25th 2013... I'd never heard of adiabatic process before now, but no, heat definitely comes from all over the place in an engine lol

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