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    Question downpipe questions

    hi guys

    I was told by another vr4 owner that installing a downpipe on my car would cause my car to eventually over boost and blow my stock turbos prematurely.. Can some light be shed on that please?
    Last edited by cee; 14-04-2008 at 05:43 AM. Reason: notifications

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    Quote Originally Posted by cee
    hi guys

    I was told by another vr4 owner that installing a downpipe on my car would cause my car to eventually over boost and blow my stock turbos prematurely.. Can some light be shed on that please?
    I have seen no evidence to support that theory.
    However the fact of the matter is that if you place something under more stress, it will break sooner.

    In the end it is a toss up between the chance of the engine and or turbos breaking and the fun the extra power and response you gain. Personally, down pipes all the way!

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    Haven't heard of it causing turbos to blow up within 5 minutes put it that way. I've been pushing my turbos hard with 227.9kw ATW for some time now, I have a full downpipe + decat + rest of the way exhaust system and haven't managed to kill them!

    As Kenneth said though, ultimately any gain in power is going to put more stress on components.

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    yeah so basically what kenneth said, if you place more stress on something, its going to wear (and break?) at a faster rate.

    as i understand it the larger free'er-flowing downpipes coupled with a bigger exhaust allow the exhaust gases to exit the engine more quickly, this in turn will spool the turbos faster (not really a problem in itself) the problem is keeping the rising boost in check.

    if you were to graph your boost before and after the modification you would most likely see a much steeper curve peaking above the value its settling on, the whole system is essentially a feedback control system, when throttle is open, exhaust gases flow and allow the turbos to spool allowing even more gases to flow, when the desired boost level is reached the waste-gate is opened to allow some of the exhaust gases to bi-pass the turbines and halt the increase.

    the response times of the boost solidnoid and wastegate(i think these are the components that essentially 'control' the boost) are designed for the 'stock' spool times and will not change unless you upgrade them or other components (like fit an Apexi AVCR), as such they may be unable to act fast enough to the rapid increase in boost, its likely your boost level will overshoot(boost spikes) the desired value briefly (a problem in pretty much any feedback based control system). even without any kind of boost controller asking for a higher than stock value, the downpipes and exhaust upgrade will still contribute to some boost spiking.

    seeing as our stock boost level is relatively low the spike is likely within the safe limits and isn't necessarily bad, infact it can be a desired effect (though you would usually try to achieve it only in a particular gear or something with your electronic boost controller), its when your going way above what you consider safe that you should worry.

    so if your planning on pushing your desired boost value right to the edge of safe(stock) margins (.9-1.2bar??) then its likely you will over-shoot the upper limit of what is considered safe and be putting the component in danger of immediate failure.

    apparently you lose a little low-end torque when you lower backpressure (i.e upgrade exhaust) not sure how that works exactly, maybe because temperatures are lower? but im sure its more than offset by the increase in turbo response :-)

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    bit off topic but since there was a gap in my knowledge i did some digging arround why you lose low end torque..
    made sense to me!?

    The whole idea with the faster moving exhaust (vs. slower moving but larger cross-sectional area) is that the momentum of the gases will create a vacuum behind it as it's leaving between exhaust pulses. This literally "sucks" out the gas from the next cylinder when it's exhaust port opens. That's why a well designed header will give you more power, the exhaust pulses reach the collector just as the next cylinder opens, effectively sucking the exhaust gases out of the other cylinder. This reduces the work the piston has to do on the exhaust stroke, and you get more power.

    so is quite important to NA cars...

    so for turbos its been noticed when you replaced you stock exhaust with a new high flow 3", you lost a bit of power down low, but this was greatly out weighed by the gain once the rev rose above 3000rpm???

    This is due to the extra scavaging from the exhaust, helping to pull the gasses out of the cylender, however once the turbo starts moving, the spent gasses have enough charge to force itself out of the cylender faster than any negative pressure in the exhaust could try to pull it out, thus it is better to have a low speed/high volume exhaust than use gas speed to create the volume (smaller piping), as the negitive pressure will actually slow the high(er) pressure gases trying to get out.

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    In a VR-4 you might lose a little bit of power below 2000rpm, but once you get above that point and the turbos are working, you want as little back pressure as possible

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    rosco_nz the reason why you loss low down torque with the downpipe is just because of the fact you have made less back pressure, so the turbos take a little longer to spool. but one spooling they are able to breath better hence the better performance in the top end

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    Quote Originally Posted by Subaru ETA
    rosco_nz the reason why you loss low down torque with the downpipe is just because of the fact you have made less back pressure, so the turbos take a little longer to spool. but one spooling they are able to breath better hence the better performance in the top end
    made less back pressure? that just doesn't make sense(to me anyway) as to why it makes turbos spool slower... how does a turbo spool? exhaust gases passing the turbine... a slower velocity stream of exhaust gases is more the reason for slower spool.. not backpressure. backpressure is pressure which resists the motion of the piston, as the pressure of exhaust gas which does not find free outlet

    "backpressure is good" is a myth, the whole point of equal length headers is to reduce back-pressure by making them equal length so the flow of the gases (or pulses from each cyclinder) dont interupt each other, and indeed can actually help each other through the scavenge effect.

    When contemplating a modified exhaust system there are those who want the biggest diameter pipe that can be had. Their idea must be that fatter pipes are more effective at venting than narrower pipes. This sounds reasonable but it is not quite correct. Sure wider pipes have greater volume and higher flow capacity, but that is just half of the story. Capacity is one consideration but gas velocity is the other factor.

    An experienced exhaust designer knows that the best exhaust is one that balances flow capacity with velocity. A given volume/time of gasses will travel faster through a 2" pipe than the same volume of gas passing through a 3" pipe. So when taken to its extremes we can see that a too narrow pipe will create backpressure (restrictions to positive flow) problems and a too wide pipe will cause a very slow flow with no backpressure. (yet the backpressure itself isn't a good thing its the fact that the gas is flowing fast)

    The optimum is where the fastest velocity is achieved with the least constriction possible.

    It is easy to see how this misunderstanding arises. Lets’ say that Max puts a 3-inch system on his normally aspirated car. He soon realises that he has lost power right through the power band. The connection is made in his throbbing brain…. put on 3" pipe = loss of backpressure = loss of power. Max erroneously concludes that you need backpressure to retain performance. He has ignored the need for exhaust gas velocity to get that scavenge effect.

    Forced induction (turbo or supercharged) perform better with the high volume pipes (2 1/2" to 3"), while on boost, but off boost they suffer the same effect of reduced escape velocity and hence turbo spool times..Or so the theory goes, as I understand it (two disclaimers there).

    on a side note it kinda leads into anti-lag systems, which is another cool topic on its on.. http://www.rallycars.com/Cars/bangbang.html

    p.s. most of the ideas and stuff are copy paste (change to explain how i understand it) from below (being a good student and giving my references as to not plagerise...)
    http://warnertechnology.com/Cars/backpressure.shtml
    http://www.miata.net/garage/KnowYourCar/S4_Back.html
    http://www.uucmotorwerks.com/html_pr...torquemyth.htm
    Last edited by Rosco_NZ; 14-04-2008 at 10:16 AM.

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    only one question, who is Max?

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    Turbos work from GAS FLOW ACROSS THE EXHAUST TURBINE.

    That means that the hot gasses from the engine hit the turbine, move past it to the low pressure area on the other side. This movement through the turbine blades cause the turbine blades to spin.

    Therefore, the higher the pressure DIFFERENCE, the faster and more EFFICIENT the turbine will spin.

    A big exhaust means less pressure on the open side of the turbine, therefore the high pressure gas moves faster across the turbine blades causing them to start spinning sooner and faster.

    This is why with down pipes and decat you can get nearly 10psi of boost at 2500RPM. With standard downpipes and cat you get that at 3000RPM instead.

    It may be possible to loose a little bottom end torque with a big exhaust, but that is far out weighed by the increase in response from the turbo. (because the ones on the 6A13 are so small, any loss is just not an issue.)

    Bigger is better with turbo. Yes, it is possible that if your turbo is huge then a huge exhaust may mean that you loose some torque while not on boost, but again the extra response and flow means that you will eventually make up that and more. Unless your turbo is far to big for the application.

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    Quote Originally Posted by Kenneth
    Turbos work from GAS FLOW ACROSS THE EXHAUST TURBINE.

    That means that the hot gasses from the engine hit the turbine, move past it to the low pressure area on the other side. This movement through the turbine blades cause the turbine blades to spin.

    Therefore, the higher the pressure DIFFERENCE, the faster and more EFFICIENT the turbine will spin.

    A big exhaust means less pressure on the open side of the turbine, therefore the high pressure gas moves faster across the turbine blades causing them to start spinning sooner and faster.

    This is why with down pipes and decat you can get nearly 10psi of boost at 2500RPM. With standard downpipes and cat you get that at 3000RPM instead.

    It may be possible to loose a little bottom end torque with a big exhaust, but that is far out weighed by the increase in response from the turbo. (because the ones on the 6A13 are so small, any loss is just not an issue.)

    Bigger is better with turbo. Yes, it is possible that if your turbo is huge then a huge exhaust may mean that you loose some torque while not on boost, but again the extra response and flow means that you will eventually make up that and more. Unless your turbo is far to big for the application.
    yep id agree with that

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    Any theories as to how having two separate exhaust would behave? Assuming say 2.5" from turbos all the way back?
    I'm not arguing...I'm just offering an endless series of contrary points of view.

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    Quote Originally Posted by cee
    hi guys

    I was told by another vr4 owner that installing a downpipe on my car would cause my car to eventually over boost and blow my stock turbos prematurely.. Can some light be shed on that please?

    Cee; Has the above answered your question for you?

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    Quote Originally Posted by djb160
    Any theories as to how having two separate exhaust would behave? Assuming say 2.5" from turbos all the way back?
    It would work well, but you would have to use a standalone ECU or use a H or X pipe to balance the pressure.

    If you think of each bank as being a separate unit, one bank may have a little less pressure (say the turbo runs a bit better) in the exhaust which means it runs a bit more efficiently. If both banks ran straight out, the ECU wouldn't know that one bank was running better than the other, as such you could run lean on one bank and blow it up.

    So, to have 2 separate pipes you need 2 separate O2 sensors and the ECU to be able to trim each bank of injectors separately so that you get an even AFR across both banks.

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    I figured there would be something I didn't think of. I just thought it would be a good way of reducing restriction, coz even custom downpipes aren't exactly great, big improvement over stock but still pretty crap.

    Also having two would allow for greater control over noise wouldn't it?

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    the above has answered my question so far..

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    Dale, you're right about restriction, it would give you the ability to run slightly more restrictive mufflers and such to make the system quieter, but still letting you move a lot of air through the system.

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    Kenneth, whilst I agree the ECU should be changed, the pressure shouldn't be an issue: the actuator spring pressure should automatically compensate for any differences in flow between the turbos: if the pressure in both actuators is the same, but the pressure behind the flapper of one is greater, it will open slightly more correcting the flow.
    It does compromise the boost stability slightly, but on such small turbos I'd be surprised if it was significant.
    This is exactly the mechanism 300ZXs run on.
    As you quite correctly state, though, the O2 sensor is where things come unstuck: You CAN simply monitor one bank, which should remain fairly accurate as it's not relating to a table, it's more "self correcting". So if one half is running stoich, the other half should be too (injectors behaving the same etc) so you COULD do it if you wanted. That said, I'd feel nervous only having the one O2 sensor, and at the very least would want to fit a wideband in the other pipe.

    In all honesty, though, I think it would be a huge amount of effort on the VR-4, and simply isn't worth it.
    Interestingly, most of the aftermarket exhaust kits for the 300ZX (which runs a pipe-per-turbo from the factory) introduce a mix pipe towards the middle of the car. Even more confoundingly, it runs an O2 sensor in each downpipe, but cat-temp sensor only on one side...and not at all on UK cars.

    The off-boost torque thing is something of a myth: the headers have far more impact on this than the exhaust, which is completely disrupted by the restriction of the turbo anyway.
    I've even had personal experience of this having driven a turbo'd car home, taken the factory exhaust off and then having to drive it up to a friends house, you notice that the drive is almost identical.

    The point where the exhaust starts to make a difference is when the downpipe (and headers) are both good and hot, which keeps the gas flow faster.
    Allowing the gas to cool as it leaves the turbo slows the gas, which stalls the turbo, slowing spool and potentially reducing target boost (though I've never seen the latter). Lagging (or ceramic coating) your downpipe can make a massive difference!

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    Quote Originally Posted by djb160
    I figured there would be something I didn't think of. I just thought it would be a good way of reducing restriction, coz even custom downpipes aren't exactly great, big improvement over stock but still pretty crap.
    You can't have seen my down pipes then?

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    Quote Originally Posted by Turbo_Steve
    Kenneth, whilst I agree the ECU should be changed, the pressure shouldn't be an issue: the actuator spring pressure should automatically compensate for any differences in flow between the turbos: if the pressure in both actuators is the same, but the pressure behind the flapper of one is greater, it will open slightly more correcting the flow.
    It does compromise the boost stability slightly, but on such small turbos I'd be surprised if it was significant.
    This is exactly the mechanism 300ZXs run on.
    ...
    Interestingly, most of the aftermarket exhaust kits for the 300ZX (which runs a pipe-per-turbo from the factory) introduce a mix pipe towards the middle of the car. Even more confoundingly, it runs an O2 sensor in each downpipe, but cat-temp sensor only on one side...and not at all on UK cars.
    I have to respectfully disagree with you on this one. As you point out, the 300ZX uses a "mix pipe" which will even up the exhaust pressure. This is the same as an H or X pipe really and it joins up both exhausts to prevent one running with significantly more freedom than the other.
    You also say that the 300ZX has 2 O2 sensors, which means it does have the capability of detecting abnormal closed loop operation.

    Quote Originally Posted by Turbo_Steve
    The point where the exhaust starts to make a difference is when the downpipe (and headers) are both good and hot, which keeps the gas flow faster.
    Allowing the gas to cool as it leaves the turbo slows the gas, which stalls the turbo, slowing spool and potentially reducing target boost (though I've never seen the latter). Lagging (or ceramic coating) your downpipe can make a massive difference!
    I don't fully agree, but I think I would be arguing technicalities which was probably not your point... I also know I get annoyed when people argue with me and it ends up being we are saying the same thing in a slightly different way

    I will say though...

    Personally I would consider spending money on a mild steel performance exhaust a waste of money. Pay the extra and get Stainless Steel which has much better resistance to heat absorption. This will work to prevent the transfer of gas energy into the pipe work. which is also what ceramic coating is doing.

    I don't know much about ceramic coating... What is it like on stainless steel?

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