[s-cars] Re: RS2 TURBOCHARGER CALCULATIONS - LONG - definitive answers

[Mihnea Cotet]

Scott, I'm not giving up on this, because I do think there's something 
wrong with your maths.

Here's what I think:

At 01:14 18/10/2004 -0400, QSHIPQ at aol.com wrote:
>A call to give away the store?
>Minhea, comments inserted
>SJ
>
>In a message dated 10/15/2004 5:39:38 PM Central Daylight Time, 
>mihnea.cotet at easynet.be writes:
>MC>>Scott, in my first post, I was talking about 28 psi (3.0 PR) at 6000 RPM
> >>and 25 psi (2.7PR) at the redline (7000 RPM). Plot the engine flow values
> >>on the compressor map assuming 85% VE for 6k RPM and 80% VE for 7k RPM (I
> >>quite frankly don't think a standard 20vt has more efficiency than that,
> >>even the RS2) and see the results for yourself.
>
>Ok, I'm game.  I do share your VE opinions Minhea
>For 6000rpm
>135.8ci * 6000rpm * .5 * .85VE/1728
>200cfm with no turbo (PR = 1.0)
>Assuming a Density Ratio of 1 (DR = 1) and no restrictions elsewhere, you 
>multiply
>200cfm * 2bar Pressure (29psi or 3.0PR)
>400cfm

WRONG, see below.

>
>Now, over to the RS2 MAP (2672 cold side)
>.20 m^3 (* 35.315 * 60) = 423 cfm
>.22 m^3 = 466cfm
>.25 m^3 = 529cfm (the absolule peak of the RS2 at 70% Turbo Efficiency)
>
>Following the PR line to 3.0 I see we are right at the edge of 72% TE
>with an output of ~440cfm and a turbo speed of ~140krpm (150krpm 
>max).  Looks ok to me, I doubt one is going to see this as time on this 
>boost increases (read: heat soak), but the turbo and the engine seem well 
>matched so far.  In fact, I'd even say the wastegate is still opening very 
>slightly...  and you are about to shift anyhow :).
>
>For 7000rpm
>135.8 * 7000 * .80/1728
>220cfm * 1.7bar Pressure (25psi or 2.7PR) = 374cfm (DR  = 1)
>On the RS2 turbo map that's 73% TE and ~440cfm and a more leisurely 
>130krpm turbo speed.  I'd say there is good reason why a bunch of folks 
>run the 26psi software with the RS2 goodies.
>

Nope, nope and nope.

In my excel spreadsheet for the 2226cm^3 engine, I have the following values:

rpm             cm^3/min        cm^3/s          unboosted m^3/s (including 
VE)  VE       PR     boosted airflow m^3/s

6000            6678000         111300                  0.0946 
     85%       3.0    0.0946*3.0=0.283815 (!!!!)
6500            7234500         120575                  0.0988 
     82%       2.9    0.0988*2.9=0.286787
7000            7791000 
129850                  0.1039                  80%       2.7 
0.1039*2.7=0.280584

Even assuming there's no pressure drop between the compressor outlet and 
the intake manifold (and there is unfortunately), running these boost 
pressures is right outside the map. I have done these calculations many 
times, for 2.7Ts, 1.8Ts and of course 20vt's, and this time I have done 
them over by hand to make sure I wasn't wrong somewhere. If you do see 
something that's not consistent in my maths, please let me know so I know 
I'm wrong...

**************I think however that I got it why you are so sure about your 
maths and why you think I am so wrong, it is simply because you are 
multiplying unboosted airflow by boost pressure, not pressure ratio. The 
formula is "Cfm boosted= Cfm unboosted x pressure ratio"*******************

I'm just expecting you to understand where your mistake was. I have no 
problem admitting my own mistakes, when I do make them, but as a good 
friend of mine says, "I niver meak mistreaks"....

Anyway, IMHO and IME, 2.0 Bar manifold pressure at 6000 RPM is not boost 
you want to run with an RS2 turbo!!!!! It's way outside the map at 0.2838 
m^3/s, which means it's very bad for the turbo and the compressor won't be 
flowing much air at those engine flow rates anyway....

By the way, when you say there's a bunch of folks running the 26psi 
software with the RS2 goodies, yes there are sir, but they are only running 
26psi peak boost, none of those that I have seen run that much boost at 
7000 RPMs. Those chips (Thomas Schmalz's 551B RS2 boost chip and Heinz 
Lehmann's 551AA AAN timing chip) don't run as much boost as you claim, and 
in the second variation of that chip (Lehmann's both chips basically), they 
only run 11psi at the redline, as measured on several otherwise good cars 
that had been tested for boost leaks etc etc etc....

Oh, an example to illustrate that the boost levels you're suggesting as 
"safe" aren't, a guy I know of in France was running some "expert's" chips 
in his RS2. I went for a ride with him and took a look at the boost gauge. 
Boost pressure was: 2.5 bar relative peak boost (36.75psig) anywhere 
between 3500 and 4200 RPM, 2.0 Bar at 5000, 1.7 at 6000 and 1.5 at 7000 
RPM. The car was a real screamer and I can only guesstimate the power 
levels to somewhere between 390 and 410 crank HP. After using those chips 
for 30,000 kilometers, his turbo has started to blow blue smoke on startup 
and to whine like it's on its last legs. It has been replaced last week, 
the shaft had excessive play in it and the hot side was also slightly 
cracked, which means it's not going to be rebuilt.


> >> If I remember my
> >>calculations correctly, both of them will either be outside the compressor
> >>map or at very critical points in that map. And again, I'm talking about
> >>the 2672 cold side, not the 2470 cold side used on the standard 20vt K24
> >>turbocharger.
>
>See above

Hmmm, see above again :-))))



>SJ >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.
>
>MC>>The RS2 turbine wheel is a very good one, but it's not ideal IMHO. 
>Perfect
> >>for anything below 380 real crank HP, but no good for anything above
> >>that... This is based off of first hand experience, not hearsay or other
> >>people's experience BTW.
>
>My first hand experience shows that it can achieve 300rwhp with the stock 
>IC and RS2 bits.  My thinking is that the turbo isn't the limiting factor 
>above that.

My thinking/experience is that the turbo IS a limiting factor above that. 
Otherwise, why would MTM, Sportec, Lehmann and such be selling big turbo 
kits with K27/29 combinations and so on??? Just because it's cool to rip 
people off and make money off their backs? I'm not saying it's not the case 
to some extent for some of the aforementioned companies in some cases 
(RS4), but it's not the point here, an RS2 turbo can do about 370-375 crank 
HP but not much more, period, BTDT, etc etc etc....


>SJ>All that said, the RS2 is a relatively small turbine (for high HP AND high
> >torque), <snip> and the
> >>ECU is trying to do its best to retard the timing, boost is never 
> dropped a
> >>single psi compared to what it is when no pinging occurs. I am 100% 
> sure of
> >>these statements as I've been able to verify them by myself intentionally,
> >>on my own engine.
>
>Interesting Minhea, I'll quote from Audis own Documentation on the 20vt 
>motronic: Knock Sensors I & II, G61 and G66...  "The ignition timing of an 
>individual cylinder can be retarded if knock is detected.  If knock 
>continues, a richer fuel mixture will be used and the charge pressure is 
>reduced.  Knock control is now adaptive which means that the system will 
>adapt to fuel quality and driving conditions."
>
>Not sure if this calls into question your BTDT, or if boost modifications 
>might adversely affect this feature,  but one can easily argue the way the 
>ECU was designed, a knock intevention would cause a reduction in 
>boost.  Why?  Because VE drops dramatically with knock.  Less VE, less 
>exhaust energy, less exhaust energy, less boost.

Hmmm, if you put it that way (VE thing) it's ok for me, but I am absolutely 
positive the ECU doesn't use knock correction on short-term boost control. 
I could post the real formulas/block diagrams for boost control to you so 
you can see for yourself, but if this is to happen, it's gonna be off-list.


>SJ>Exhaust manifold pressures are usually a function of the design of the
> >Nozzles and <snip> above what an RS2 turbo can deliver with no
> >>pinging and any action from the ECU to retard the timing, again, even if
> >>only on one cylinder.
>
>The world of tradeoffs, but I might argue my point better with some actual 
>PD measures Minhea.  Let's compare the exducer ("pipe diameters") of 3 
>turbos, the k24, the k26 #6 and the k26RS2.  The k26 RS2 PD sits between 
>the k24 and the k26#6.  Using the PD vs HP output, here is a *massive* 
>differential in flow and HP between the k24 and the RS2 with a very slight 
>increase in PD.  For the *same* slight increase in PD going from the k26 
>RS2  to the #6 26, there is only a slight increase in flow and HP.  I make 
>the claim, it's not the PD in the RS2 it's the design of the hot side wheel.
>
>What are the tradeoffs?  Very little.  In fact your "*up to 5" degrees 
>more timing" could mean exactly no noticeable HP gain (or dare I say less 
>based on mods), and could be just 'none' with more "n" testing of 
>production RS2/7200comp turbos.  If we strictly use the PD argument, the 
>RS2 defies logical thinking:  It has better lowend than either the k26 #6 
>or the k24 (at 2250 rpm on the RS2 car) That's a good turbo with few 
>compromises, even reading your results.  I might respond that the 2672 
>cold side is ok, but there are a couple that have better output that can 
>be bolted to the hot side.  My claim, keep the hot side of the RS2 until 
>you reach a point where it's no longer capable of matching the needs of 
>the motor or the driving style.

Scott, the RS2 hot side is great, but assuming a totally free-flowing 
exhaust, it IS going to be the restriction in terms of exhaust backpressure 
from the hot side itself. Believe me or not, it makes no difference to me, 
I think I know how to tune a Motronic ECU (and I've long moved on to more 
challenging things like 1.8Ts, RS4s and such) and I know how to recognize 
it when boost is too high at the top end and the engine can't evacuate the 
exhaust gasses fast enough, and pinging/retarding starts to occur. I've 
discussed these theories with various people and we all agree except for 
you, sorry but I don't care if you don't, I can't spend my life arguing 
with you, I have an international business to run and a family to feed too...

Example of what I'm saying: if the hot side wasn't that big a restriction, 
why on earth did the Audi 90 IMSA run a K27 #13 hot side on its 750-840HP 
engine???? I guess a K26#8 could have been enough, or even a K27#7 from a 
SQ, but wouldn't it be because of the otherwise relatively high exhaust 
backpressure generated by a smaller hot side at these sort of power levels????

>
> >However, drop an RS2 turbocharger onto a audisport manifold or a tubular
> >manifold, you could easily argue the point.
>
> >>Hehe, yup, totally agreed, but a better manifold will also free up a 
> couple
> >>more HP from the same turbo.
>
>It could give "up to" more HP, it could give less. Define "better" first...
>
>For the 2226 I5, I kinda take a likin' to that RS2 turbo...

Don't get me wrong, it is a GREAT turbo, however if your goals are above 
370 crank HP, look elsewhere people, there's nothing for you here. Same 
applies to hot sides/turbines. If you're looking for 400+ crank HP, there's 
no way a K26#6 hot side can handle it without seriously compromising power 
at the top end. It's all about trade-offs anyway. But we've seen this on 
various high-HP 20vt engines in the UK mainly, where people would upgrade 
their exhausts/hot sides because I was thinking that we were having a 
backpressure issue, and we've seen gains as high as 30 to 40 HP just going 
up one size in the hot side and keeping the same boost curve/timing 
advance/fuel mixture at WOT. Free a couple more degrees of timing at the 
top end (up to 10 more in some cases), lean the mixture out some more 
because of the drop in EGTs (because of the increased timing) and voila, 
you have 50 more HP from the same cold side with no internal mods to the 
engine itself.


HTH,

Mihnea

www.mrc-developments.com


>
>Scott Justusson
>QSHIPQ Performance Tuning
>Chicago
>'91 v8
>'84 URQRS2 Project