[s-cars] RE:Wastegate spring tightening

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Minhea:
I'm confused when I read your two posts vs the RS2 turbo map itself.  The RS2 is quite capable of 25psi (I prefer Pressure Ratios myself = 2.7PR), and in fact isn't a heat pump at that output, it's actually quite efficient (like 73%) at that pressure.  The turbo MAP also indicates that at 2.7PR turbo speed is a relatively safe (albeit close to max) 140,000rpm, not double that. Are you sure you aren't referencing a k24 turbo here?

Pipe diameter could be argued as a contributing factor, but the design of the RS2 may offset that somewhat with a revised (less restrictive) wheel design.  Ultimately pipe diameter is a mathmatical reality, but a large factor is the spinning turbine you design into it.

All that said, the RS2 is a relatively small turbine (for high HP AND high torque), so heat soak will eventually cause either CAT boost reduction, or knock boost reduction.  I suspect most that see a reduction in boost based on CAT, since knock interventon, IMO, tends to be a bit more dramatic a change.

Exhaust manifold pressures are usually a function of the design of the Nozzles and throat ratios, not so much the backpressure of the turbocharger hot side.  I'd certainly propose that if one is using the RS2 exhaust manifold, the RS2 turbocharger unit probably isn't the restriction...  Yet.

However, drop an RS2 turbocharger onto a audisport manifold or a tubular manifold, you could easily argue the point.

HTH and my .02

Scott Justusson







In a message dated 10/15/2004 8:50:48 AM Eastern Daylight Time, Mihnea Cotet <mihnea.cotet at easynet.be> writes:

>Pizzo, thanks for the input. My point is that these limits (boost and 
>timing limits) depend on the backpressure generated by the exhaust. If the 
>EM/turbo hot side/DP/exhaust are too small and acting as bottlenecks in 
>terms of flow, thus if they produce high backpressure, it's gonna be good 
>for getting high boost at low RPMs, but it's also gonna be a great way of 
>getting pinging at the top end, which will automatically limit the power. 
>All this is aside from all cold side efficiency considerations, if the 
>turbo runs at 250k RPM and 0% efficiency with 28psi at the redline (the RS2 
>isn't far from that with that much boost but it's just an example), then 
>it's totally pointless to even consider running that much boost, nevermind 
>your turbo won't even last for a couple of hours.
>
>The key word is back pressure. The more back pressure, the more efforts the 
>engine has to make to get the gasses out (pump losses for the MEs out 
>there), which means lower knock threshold, higher EGTs, higher heat 
>transfer from the hot side to the cold side, yadda yadda yadda, i.e. less 
>overall power and more risks involved in ruining the engine.
>
>
>HTH,
>
>Mihnea
>
>At 08:31 15/10/2004 -0400, Joe Pizzimenti wrote:
>> From the other side of the world...
>>
>>I've found in tooning my own car, even though the Evo has a lower
>>compression ratio (8.8:1 vs. 9.3:1), on pump gas the limit seems to be
>>22psi before timing starts to get pulled back due to detonation.  Ask
>>around and most other tuners will tell you the same thing.  Unless the
>>timing advance is really, well, retarded, running high boost on pump
>>gas is a great way to remove your cylinder head without lifting a
>>wrench.
>>
>>Personally, I think there's more to be gained with moderate boost
>>levels and good timing advance rather than the other way around.
>>
>>FWIW, YMMV, etc.
>>
>>Joe, I'm not a real tooner, but I play one on the Internet, Pizzo
>>
>>
>>
>>
>>On Fri, 15 Oct 2004 14:16:31 +0200, Mihnea Cotet
>><mihnea.cotet at easynet.be> wrote:
>> > Guys,
>> >
>> > Just a little info here: 28 psi all the way to 6k RPM and 25 psi up to the
>> > redline are both "suicidal" boost levels for an RS2 turbo.
>> >
>> > Apart from being bad on the turbo itself (overspinning at high RPMs), it's
>> > also totally unefficient and turns the turbo into a large heat pump.
>> > Efficiency must be close to 40% at these sort of boost levels and such high
>> > RPMs. Which means that your cars will be making lots of power at relatively
>> > low RPMs (4k to 4.5k) and then the power will simply tail off, because of
>> > the increase in heat on the intake side, and also because of the stupidly
>> > high backpressure levels in the exhaust, which in turn makes the engines
>> > ping a lot easier, so the ECUs will dial the timing advance back to keep
>> > the engines from knocking.
>> >
>> > Together with my german tuner colleagues, we have a phrase to sum all this
>> > up: manchmal ist weniger mehr, or in english: sometimes less is more.
>> >
>> > Just a thought...
>> >
>> > HTH,
>> >
>> > Mihnea
>> >
>> >
>> >
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>
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