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  1. #1
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    Discussion purposes only... N54 rev limit - 7500 too high?

    Obviously there's no point in running an N54 to 7500 with stock turbos or even RB/VTT stock-frame turbos, but if you're running a single turbo or potentially stage 3 VTT's, would the engine have any trouble at 7500 rather than 7200?

    If the motor can maintain 400-425 ft-lbs to 7500rpm, that would be ~590rwhp at a very conservative torque figure.

    Even if the turbos aren't fully spooled until 4000rpm, 4000-7500 is a very broad power band, particularly if the response above 4000rpm is good.

    Seems like all things considered this is a much lower-stress way to make the horsepower rather than trying to run 25psi @ 3000rpm and going for 650 ft-lbs of torque.

    Considerations:

    This will be slower off the line due to the lack of that 25psi midrange. Click here to enlarge Which is fine with me, I don't drag race anyway.
    This will not need such extreme octane/fuel because it's never making over 425ft-lbs of torque (stock turbos make this much easily, even on pump gas. They just don't maintain it to redline.)

    Will the motor hold up fine at 7500rpm? Anybody tried it? Any data at all?

    Thanks

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    Cobb raises redline to 7200 safely, so maybe? The piston speeds aren't very bad there, depends on the valvetrain and headflow/cams i'd say. How does the vanos like being spun fast, anybody know?

    It seems the motors grenade over 8k, fwiw, although mechanical over revving is not exactly the same.

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    Ive Revved My Car To 8K For About 30Seconds Dont Ask why
    Turbo lag is the on ramp to the highway which is power.

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    3 out of 3 members liked this post. Reputation: Yes | No
    We've seen logs of CupertinoSteve's car as high as 8700rpm. Momentary and not intentional. A redline of 7500rpm isn't a problem as far as pistons/rods go.

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    Click here to enlarge Originally Posted by WDBi Click here to enlarge
    Ive Revved My Car To 8K For About 30Seconds Dont Ask why
    Actually I'm curious, specifically about the 30 seconds tim span Click here to enlarge
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    It Just Kinda Happened Car Didn'tShift Then I Let Off Everything Seemed fine
    Turbo lag is the on ramp to the highway which is power.

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    Click here to enlarge Originally Posted by shiv@vishnu Click here to enlarge
    We've seen logs of CupertinoSteve's car as high as 8700rpm. Momentary and not intentional. A redline of 7500rpm isn't a problem as far as pistons/rods go.
    So then airflow from heads and cams sound like the biggest hurdle after big turbo upgrades for higher rpm. Maybe oiling.

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    If you are still making power I don't see any reason why you would not want to if the fuel, internals, etc., can support it. I mean to really take advantage I would think someone needs to look into cams.

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    I think 7500 might be doable long term. The N54 seems to be built pretty strong. A little over 7k is the highest I've seen in logs, so can't speak from experience.

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    I was thinking of setting the rev limit at 7500 but then shifting in the low 7's (7100-7200) well short of 7500, just having that headroom there would be nice.

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    What's the max lift of the stock cam? You'd need to calculate that the intake diameter and lift is enough for not choking at that rpm. The bore is not too wide for being able to fit a lot bigger valves. Also the rod and stroke are quite long for making the power up high.

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    It'd just be up to the dyno sheets... on the single turbo cars at least, the torque is a pretty flat line straight across (once spooled) so a couple hundred more rpm would benefit it greatly.

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    Generally, the rev limit is defined by the capability of the valve train and the balancing of all heavy rotating elements such as crank shafts.

    If the engine spins too fast, the valve springs may not have enough time to pull back the valves. This is a situation known as "floating valve". The valve just ends up "floating" because the spring is not expanding fast enough.

    In the case of the crankshaft, it does not matter how strong it is built. It matters how well it is balanced and how good is the design of the vibration dampener (crank pulley). It may be the case where above certain engine speed, the crank develops second order vibrations which are very very bad and can lead to complete engine failure.

    All DI motors have one other complication: the actual time window during which the fuel injectors have a chance to inject the proper amount of fuel gets shorter and shorter as the engine rotates faster. There is theoretical threshold above which, the fuel injector pulse will take longer time than it is allotted by the sheer mechanics of the piston moving up, the valves opening and closing and the spark being delivered. Do not forget that you also need a small amount of time for the fuel to actually mix with the air and form homogeneous mixture.
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    i'm with other people, i can't see 7500 being a big issue as long as the motor doesn't live there for too long

    with headwork/lighter valves/stiffer springs etc., balanced crank/forged internals, then cams... i can't imagine any valve float or bad vibration up to 8k easily.. everything weighs so much less, i'd imagine there being barely any more, if not similar, rotating force with 1000rpm extra

    that's talking in the thousands and thousands for a 1000rpm increase AFTER a bigger turbo and all supporting mods have been fitted - at this stage realistically the LAST thing you would ever do to this engine value for money wise.

    but, if you got to that stage and had a flat torque curve that would hold... that's a straight up 14% increase in power... 600whp at 7000rpm (super super conservative at this level i'd say) ->684whp at 8000...

    Click here to enlarge Originally Posted by vasillalov Click here to enlarge
    All DI motors have one other complication: the actual time window during which the fuel injectors have a chance to inject the proper amount of fuel gets shorter and shorter as the engine rotates faster. There is theoretical threshold above which, the fuel injector pulse will take longer time than it is allotted by the sheer mechanics of the piston moving up, the valves opening and closing and the spark being delivered. Do not forget that you also need a small amount of time for the fuel to actually mix with the air and form homogeneous mixture.
    any idea what frequency the injectors can operate at? 7000rpm being 116/sec already... regular injectors don't have a worry revving pretty high for the most part, i think they shouldn't be a huge issue here?

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    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
    any idea what frequency the injectors can operate at? 7000rpm being 116/sec already... regular injectors don't have a worry revving pretty high for the most part, i think they shouldn't be a huge issue here?
    The max operating frequency of the injectors is defined by the properties of the piezoelectric elements inside them. This, unfortunately, is not a hard number, but more of a gray area because each piezoelectric element has slightly different physical properties due to impurities in their crystalline structure. This creates a somewhat interesting phenomenon in which some piezoelectric components may operate at relatively higher frequency than others. Some may operate at very high frequency, but unreliably. So, I would assume that some sort of a statistical formula is used to determine that 99.9999% of all piezoelectric components will operate RELIABLY and PREDICTABLY below a certain frequency and at what voltage/power levels. This is one of the parameters that you actually program into the ECU when injectors are replaced. What those numbers are, can be answered by either Delphi or BOSCH.

    All I do know, is that every single car maker who had a high-revving engine that switched to DI, had to reduce the engine speeds because of this injector pulse window limitation at high RPMS. I think Ferrari and Porsche are two car companies who have done this exercise already.

    I wish I had a more solid answer to give you, but I just don't. I kind of trust Cobb with their reasoning as to capping the rev limit to 7200 rpms.
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    Click here to enlarge Originally Posted by vasillalov Click here to enlarge
    All DI motors have one other complication: the actual time window during which the fuel injectors have a chance to inject the proper amount of fuel gets shorter and shorter as the engine rotates faster. There is theoretical threshold above which, the fuel injector pulse will take longer time than it is allotted by the sheer mechanics of the piston moving up, the valves opening and closing and the spark being delivered. Do not forget that you also need a small amount of time for the fuel to actually mix with the air and form homogeneous mixture.
    I was thinking this exact thing last night. But I haven't done the math and don't know the numbers off the top of my head well enough to comment. At 80% total duty cycle on port fuel, you get so much more time to inject vs 100% effective duty cycle on DI due to this window. I wonder if cams could make this worse.

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    Click here to enlarge Originally Posted by V8Bait Click here to enlarge
    I was thinking this exact thing last night. But I haven't done the math and don't know the numbers off the top of my head well enough to comment. At 80% total duty cycle on port fuel, you get so much more time to inject vs 100% effective duty cycle on DI due to this window. I wonder if cams could make this worse.

    Agreed. I am EXTREMELY curious to see what BMW will set as a rev limiter on the new M3. Traditionally the M3 engines have always been revving higher than their 3-series brethren.
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    Click here to enlarge Originally Posted by vasillalov Click here to enlarge
    The max operating frequency of the injectors is defined by the properties of the piezoelectric elements inside them. This, unfortunately, is not a hard number, but more of a gray area because each piezoelectric element has slightly different physical properties due to impurities in their crystalline structure. This creates a somewhat interesting phenomenon in which some piezoelectric components may operate at relatively higher frequency than others. Some may operate at very high frequency, but unreliably. So, I would assume that some sort of a statistical formula is used to determine that 99.9999% of all piezoelectric components will operate RELIABLY and PREDICTABLY below a certain frequency and at what voltage/power levels. This is one of the parameters that you actually program into the ECU when injectors are replaced. What those numbers are, can be answered by either Delphi or BOSCH.

    All I do know, is that every single car maker who had a high-revving engine that switched to DI, had to reduce the engine speeds because of this injector pulse window limitation at high RPMS. I think Ferrari and Porsche are two car companies who have done this exercise already.

    I wish I had a more solid answer to give you, but I just don't. I kind of trust Cobb with their reasoning as to capping the rev limit to 7200 rpms.
    that does make sense

    hmm well i guess it's something that will need to be found out - i'll do a bit of homework in an hour or so to see if i can dig up any relevant information - 7000RPM on DI seems like it's a pretty normal number for most manufacturers except audi with 8000~ on their V8's



    i believe cobb's logic to a 7200 RPM limiter is when you hit the limiter, timing is retarded.. in certain states of tune you can get CLOSE to 7000rpm peak acceleration in gear. so the 7200rpm is a safety for timing reasons rather than safety for engine longevity over raising the limiter.. when cobb changed it to 7200, it wasn't even a blip on the radar that the N54 could possibly make power over 7000rpm.

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    Click here to enlarge Originally Posted by V8Bait Click here to enlarge
    Cobb raises redline to 7200 safely
    This is an assumption.

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    Click here to enlarge Originally Posted by cstavaru Click here to enlarge
    This is an assumption.
    well, that's assuming that 7000rpm isn't 100% max load, which it isn't, because manufacturers aren't silly enough to do that, or you'd find tons of broken engines all the time. a 2.8% rpm increase shouldn't be a big deal stress wise.. especially as most people won't even shift at 7200, as discussed only a stock tune or big turbo build makes power for acceleration over 7000rpm.

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    Click here to enlarge Originally Posted by cstavaru Click here to enlarge
    This is an assumption.
    An assumption? More like a scientific theory, which happens to be backed up by evidence. From a pessimistic outlook turning the engine over is bad for it. Your comment isn't very helpful if you aren't going to elaborate on your statement.

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    I think you would have a serious problem maintaining 400+ ft-lbs at 7000+ RPM - I would see no reason without turbos though.

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    Click here to enlarge Originally Posted by Flinchy Click here to enlarge
    as discussed only a stock tune or big turbo build makes power for acceleration over 7000rpm.
    Or RBs or Stage 2s right?

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    I'm sure 7500 is fine, but I wouldn't want to be the one to find out. BMW non M motors have a history of being poorly balanced.

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    Click here to enlarge Originally Posted by inlineS54B32 Click here to enlarge
    I think you would have a serious problem maintaining 400+ ft-lbs at 7000+ RPM - I would see no reason without turbos though.
    Click here to enlarge

    The lower line is the single turbo on pump gas no meth... it maintained 400+ right up to 7000rpm. It looks like it was tapering downwards (or they were in the process of letting off the gas?) but thats the kind of thing I'm picturing making 7500rpm viable.

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