RS2 turbocharger - turbo maps 101

Wed Jun 12 12:28:36 EDT 2002

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Great post!  A couple of comments to consider (inserted)

In a message dated 6/12/02 9:08:44 AM Central Daylight Time,
rpastore at writes:

>This is a great topic, so I hope we can keep the thread on track, and not
>veer off into a flame war.

Agreed.  There is a LOT to learn here on turbocharger theory and application,
as well as performance data.

>1) There is no doubt the rs2 has a boost onset that is later than the k24.
>The crossover point is around 3,000 rpm.  Above that, the rs2 has a huge
>advantage over the k24.  The k24 feels out of breath around 5,800 rpm, while
>the rs2 keeps pulling harder all the way to the rev limiter.

Be careful here Bob.  If you go to the two maps you will find that the RS2
narrowly edges the k24 in terms of max airflow.  The k24 maxes it's efficient
speed (accepted as 100-115k) sooner, which means heat soaking will occur
quickly.  I've watched it on the k24 within <1/4 mile.

>I think the question becomes, "Who cares which setup is stronger at low

A lot of folks do Bob.  But we'd have to look really hard at the turbo maps
to understand that the RS2 just might have many of the k24 advantages (more
to follow below)

>3) Scott's point on the compressor maps being almost identical at low flow
>is true, except that the k24 is turning a much higher rpm at an given flow

A REALLY tough argument to make Bob.  Looking the MAPS, take 1.5PR (7.3psi).
At the surge line, you have a 5000rpm differential. BUT, if you look at the
MAPS closer, you will see that the airflow is the same at the 65% CE plot at
1.5PR.  The turbo may spin slower, but it's output and efficiencies in the
low PR's mimics the k24 very closely.

Let's do some math here to put the above in perspective.  At 1950rpm the
following equation is used to figure cfm.

displacement (ci) x rpm x .5 x VE/1728

So, if we pick VE as a constant 90% on a 20vt motor (which we know it's not,
but let's go with it). Also for sake of argument, DR=1

136 x 1950 x .5 x 90/ 1728 = 69.06 cfm

Let's go to the 2.1PR (overboost) on the stock 20vt (AAN, ABY).

69.06 x 2.1 = 145.03 cfm

Go to the turbo map, follow the .07 (148cfm) line up to 2.1PR, we get the k24
65% CE @ 115,00rpm, and the RS2 with 63%CE @ 108,000rpm turbo speed.  Both
turbos are quite capable of producing the required air to meet the engine
requirements without going to the left of the surge line.  Note too, that big
ole tank "slower" spinning RS2 doesn't NEED to be faster.

My conclusion?  The RS2 might be slower, but in terms of output, that appears
to NOT be the problem.  For the TURBO to be a problem, you either have to be
left of the surge line OR way off in terms of CE.  Here we have neither.

>Finally, I've found that the right software is a HUGE variable in the way
>the car makes power and the way the turbo spools.  MTM 1+ does a real good
>>job of getting the k24 spinning early, but the typical rs2 offerings from
>Hoppen/IA/Tap etc do not do the same for the rs2. I do not see the rs2 as
>having much lag
>at all, but it certainly has a boost threshold that is several hundred rpm
>later than the stock k24.

Boost isn't everything.  It's engine design and airflow.  If the
heavier/slower RS2 can put up the same numbers with it's weight disadvantage,
the problem is elsewhere in the design equation.  I do agree with you that
the RS2 software could use some serious tweeking tho.

My .02

Scott Justusson
older and slower turbo duder

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