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Re: just for the hellofit
In a message dated 97-10-30 09:30:12 EST, you write:
<< Something that QShip posted has me a little perplexed, that is, about the
RS2 turbo v. the turbo mod Eric is running, with success--at least by my and
Randall's seat of the pants , unscientific findings [BTW Scott, my ass is
calibrated, but I have temporarily misplaced the numbers, ok?:) ] Anyway,
what has not been answered--or at least answered in a way that is
understandable to me is this: If you can get the fuel mixture right, and you
have the "right" intercooler, aren't there any number of turbo hybrids out
there that could, potentially be "better" than the RS2? I mean, enough fuel
+ dense air has always seemed to be the problem when adding hp modifications,
no? That is, staying away from untimely detonation. I guess I just don't
understand the fuss here about cfm, and how, based on a cfm number, people
are quick to shout "it can't work".
Help wanted here...still.
Very legit questions Bruce. I assure you, bttt X 10 (my record of drive to
drive on a turbo install became 1:33h) AND with the all the calculations. I
argue it can work better. Here's why (and more on just some rambling
thoughts will follow, for now we will address this specific issue). Let's
assume that Eric did EVERY concievable mod to the engine to increase it's VE.
Big valves, EH everything, coating the pistons and the runners. What do you
have? Well, bottom line: Better VE. Ok, but understand what that does to
the equation of
136 * 5500 * .5 * VE/1728 = baseline CFM, and then
baseline CFM * PR = boosted CFM
My argument, and challenge, is, that the real math is right in front of you
in this whole discussion, to have to go larger turbo than the RS2, you would
need to exceed the VE efficiency of a 136CI motor with a 4 valve crossflow
head. Let's look at that some more. How? Well, without physically touching
valves, you can get 4.5MM (that's B.O.D., I would never do it with a
hydraulic head, but another discussion, let's assume this), doesn't really
matter for this exercise, what and how you divide that intake to exhaust. My
argument (without math attached, someone please correct me) is that even with
increasing effective valve area (cam lift), you won't surpass a crossflow 4
valve head in VE. So, at max tweeks for a 2 VPC 136ci motor, you might equal
a rock stock 4 valve crossflow head (big B.O.D.) of the same displacement.
Surpassing it, I don't think anyone would or could argue that.
So, pick the number for VE, in fact I was happy to pick both. My original
post already GAVE eric the B.O.D. that his VE equals that of a 4 valve motor
@ 85%, and really since a 4 valve crossflow will always be higher in relation
to the stock MC, the numbers we choose are irrelevent here.
Back to the exercise. Whether we pick 480CFM or 520CFM, it really doesn't
matter, I am happy with anywhere in between. What this does is translates a
motor to a turbo MAP. A turbo MAP gives us Compressor efficiency, turbo rpm,
Pressure Ratio, and CFM. For education, CE of 72% is high, 60% is low, PR of
over 2.5 is high (for a street car, cuz you have a BMEP to hold you down),
turbo rpm over 120k is high, and CFM can be anything you want to target.
So you open the book of MAPS and you take a PR (try for the lowest at the
highest efficiency>least heat) and a CFM, and x/y that on a MAP. Now
remember, the turbo map is an area (oblong circles of efficiencies), so for
instance, 460CFM is 72% CE in a RS2 map at 2.5 bar, so is 490CFM. So an RS2
looks to be pretty good. Well, let's jump the turbo to a k27 MAP. Choosing
a 3.0L porsche turbo, for which this turbo was designed, we can see that
CI*PPO*.5*.80/1728 = baseline CFM
So, leave rpm's at the same as the MC, it isn't relevent anyhow, rule of
thumb is that PR should be max from 1/2 redline, so 5500 is safe here.
183 * 5500 * .5 * .80/1728 = 233 baseline cfm
Given 486CFM, let's go back and figure the PR, that way I can still earn my
bux doing it other ways. So this means
PR * 233 = 486cfm
486/233 = PR
PR = 2.08
Now, that means that a 3.0L porsche gets 486 CFM at 2.0bar PR. If PR = Po +
2.08 = Pb+14.5/14.5
Pb = 15.5
Ok, so without mapping anything, I can say, that a porsche turbo with 15.5
boost is getting the same turbo to make the same 486CFM eric is claiming AT
40% LESS BOOST. What does/should that tell ya? Well, the obvious answer is
that the CE is higher for the bigger motor. More specifically, if you assume
that porsche targets the 72% CE that the rest of engine manufacturers do to
turbos, then CE is really low when you put that same turbo (or just bigger,
see disclaimer) on a 136CI motor that needs 2.79PR to make the same flow. So
heat is really high, CE is low, and the surge line left of the map, really
until at least 1/2 redline or 3K rpm, but regardless, the CE is low thruout
the MAP. To correct that back on a 136 motor that makes for IC efficiency or
VE efficiency beyond magic or logic. Look at this:
136 * 5500 *.5 * 99.5/1728 = 215.35
215.35 * 2.08PR = 447CFM
So, a 99.5 VE won't even get you to the CFM of the porsche motor. The only
way to really do it is with CI baby, that's it.
So, this can tell us what exactly. Well, when you go choosing, be very
careful why you do what you do. Increasing VE doesn't mean that you need to
increase turbo size. In fact, for a 2.5PR, a RS2 is hard to beat. Can it be
done, I suppose so. But if you back up and accept that you won't beat the
20v head in VE, WHY?
So, to go beyond a RS2 in a 10v head is hard to do. In a 20v head, only if
you are going for a greater VE. IN either, the time to jump is when you
increase baseline CFM. A big effect is CI. Increasing CI really
exponentially increases PR vs CFM.
Another discussion could come from doing it better. By definition in this
exercise, Eric hasn't. No big deal, but the numbers TELL you things about
what and why you do what you do. I argue, by definition, he has the wrong
turbo doing the wrong thing on the wrong engine. And I don't have to give
away the farm to show it. Logic and or the numbers show something ain't
Can a hybrid work? You bet, Garrett KKK are done all the time and are great
mods, since the Garretts, are faster spooling and have larger map
efficiencies than most kkk's. Staying in the kkk line however, that would be
a REALLY tough argument to make, and I personally don't think you can on the
10vt motor. Given KKK's availability of RS2's of late, not sure it may not
become a project once again. Take a hybrid k26/27 (my suspicion)? Fine
except that the given PR is too high for the flow. I argue that a k26 hot
side with a k27 cold side given the PR is creating HEAT somewhere, that by
definition DECREASES CE.
My disclaimer, don't know, don't care what the actual turbo is, for example
purposes the above is clear, LARGER is not necessarily BETTER.
So, if it's fast now, I claim, "think of what it could be." Calibration of
your ass? Well, I'd like to see that number sir, in PR form. I think Dave,
Dave et.al. could get some airflow @ delta t's for you. :)
I'm going to need to pass the hat pretty soon, this isn't paying well guys:).
I do promise to declare the OT though, Jeff.
Scott Justusson, RS2 wannabe