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LAST EDITED ON 11-Dec-00 AT 07:12 AM (PST) LOL.
So who else is reading the digest here 
From the latest Talon Digest, the same current argument is going on. Rather than rehash or rephrase someone else's comments, read thispost - make sure you read Todd's comments at the end though -they are very important.
Date: Fri, 8 Dec 2000 10:06:01 -0600
From: Charles O'Connell <skipchip@kscable.com>
Subject: 16G with no more power than a 14B?
Message-ID: <#37>
I don't know how many times this has been said before but here we go...
The engine is a limited displacement type. It can only pull in a specific
amount of air at any given RPM and boost.
So in order to accurately describe how much air is being displaced the boost
is added to 14.5 then divided by 14.5 this value is multiplied by the
naturally aspirated displacement of the engine at a given rpm point. An
example is: A 2.0 L engine at 3000 RPM and 15 LB of boost displaces <(15 +[br />14.5) / 14.5] * 105 = 214 CFM. Therefore the turbo only needs to flow 214 CFM
@ 15 lbs. to keep up with the engine. So a 16G @ 15 lbs is not going to put
505 CFM into the engine at 3000 RPM, only 214 CFM.
To find what turbo can meet this requirement a compressor flow map can be a
good tool to show the possible capabilities of a turbo. In the example given
a T25 would do the job. In fact it can keep up with the engine air flow
requirements up to about 5500 RPM. This shows a 16G will only provide more
power at RPMs higher than 5500 RPM. One question remains: can it flow 15 lbs
at low RPMs to meet the engine requirements?
Remember that the turbo wants to flow 505 CFM @ 15 lbs, the question is can it
spin slowly enough at a high efficiency to flow only 214 CFM, if so what is
the lowest RPM it can hold 15 lbs. at? (ignoring the turbine section right
now)
Looking at the map will show that it is right on top of the surge line. So in
effect it can, but it's not desirable to tax the turbo in order to do it.
So you may be saying "my 2.0 L engine can boost to 15 lbs. at 3000 RPM with my
16G". Well yeah it probably can. If the engine efficiency is higher at that
RPM than my example engine then the flow is increased and the point moves
within the acceptable efficiency range.
There are other factors such as intercooler efficiency, ambient temperatures,
so on and so forth. The description I gave above is just an overview of the
basics. Any person who modifies their turbocharged engine should know this so
they don't get stuck with a turbo that doesn't perform the way they want. For
example if a fast street car is what you want then the Garret T3 60 (not
super) with the T28 turbine the best way to go. You get the highest sensible
amount of boost to run on pump gas all the way to redline and the boost comes
on early (before the super).
(Below, I take issue only with the 2nd and 3rd paragraph above.)
Your derating equation ignores temperature. In the ideal gas law, it
is as important a factor as pressure or volume. The efficiency of the turbo
directly relates to the temperature of the air it is pumping out, and
therefore how much air you can ultimately get into the cylinders. Your
equation might be right, but it also assumes the temperature output of the
16G at 214CFM is the same as the temperature output of the 14B at 214CFM.
I would think you'd have to come up with a similar derating factor for
outlet temperature. It would show that the 16G potentially could shove more
CFM into the cylinder than the 14B for the same pressure. It could do that
because the temperature is lower.
Also, assuming knock is not a factor, for equivalent CFMs and equivalent spark
timing and fuel, if the engine is consuming X CFM, it is going to make Y
horsepower, no matter what turbo you have on the engine. The problem is that
knock is indeed a factor, and a very big one at that. In general, the more
you advance your timing, the more horsepower you make. You have to back off
on timing as you increase boost because you also increase the tendency to
knock. It doesn't help that the air is probably getting hotter as well,
especially if you are beyond the most effecient pressure point on the turbo,
as the 14B is (I think, from my experience) at 15psi at 4000 RPM.
So, if you are at a point where the 14B is producing X psi and Y CFM,
the 16G will be producing 0.80X psi for the same Y CFM. This is a rough
estimate, as I don't have the compressor maps of either in front of me.
The 16G is producing the same amount of air at less boost. By PV=nRT,
the 16G is also producing much cooler air. This means you can run much
more timing for the same air consumption, resulting in more horsepower.
So you can see that there might indeed be an advantage in running a more
effecient turbo, even if you don't take full advantage of it.
I thought about trying to come up with a companion derating equation to your
(Pt+Pa)/Pa for temperature, but instead of that, I thought I'd give a real
world example. When the West Coast Caravan made the journey to Norwalk,
I was running my 20G with a FMIC. One of the other guys had a fairly
stock Talon. We had CB communications. As we went up this very long and
steep and constant grade, I pulled along the stock Talon and asked him to
match speed with me. Once we were stablized, I asked him to read me the
value on his non-stock boost gauge. He read 8psi. My reading was -2psi.
No, that isn't a misprint, if the turbo wasn't there, it would read -11psi
or so. Anyway, his 14B had to run 10psi higher than my 20G to make the
same power. Ultimately, his stock setup was less effecient than
my 20G setup. That was also reflected by an increase in my highway gas
mileage of about 2-4MPG after going to the 20G setup (of course, not all
due to the turbo).
In summary, I'm personally convinced that so long as you aren't too worried
about boost lag, you should strongly consider the turbo that best fits your
HP requirement without busting your budget. I've never personally cared
too much about boost lag as I find on my 90 AWD that first gear is so frickin
short that I'm out of it seemingly before I cross a normal two-lane quad-
stoplighted intersection. Plus my car has enough power to break parts on
launch as it is, so I don't care too much about the boost not coming on
at 2500. Once I'm out of first, the rest of the gears never see below 4000
anyway. Probably not the best autocross setup, though...
-talon mgr
MuRiX
97 Eclipse GS HRC Stage II
And a whole lot of other mods...
89 Accord LSi - yes it's mine
http://murix.home.icq.com/index.html
05 Mazda RX-8
06 Lotus Elise
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