Turbo Theory

by Mike Kojima

 

I came across this while surfing and I had to grab a copy to keep around.  Mike outlines all the critical issues with turbocharger sizing in this

short essay.  Although it's geared toward an SR20DET application, the concepts apply to virtually any turbocharged engine.


 

Turbo Theory, By Mike Kojima
Project Ultimate SE-R, Searl's 200SX SE-R

Here is a preview of Project Ultimate SE-R. I would like to thank Peter
Medina of F-MAX Fabrication and Clark Steppler of JWT for making this
project possible. I would also like to thank Rick Head at Turbonetics for
assembling this very custom turbo in such a timely manor.
Conventional wisdom has always been that in order to reduce turbo lag one
would have to run a small turbo. Big, powerful turbos were laggy and
unstreetable. When we started this project , we set out to show that a big
turbo can be better for reliability, power and still have a reasonable,
streetable amount of lag.
It has long been my contention that the tiny Garret T-25 turbo which is OEM
on the SR20DET and used by most listmembers when doing a turbo conversion or
when installing an SR20DET is much too small for extreme levels of
performance.
These turbos were spec'ed from Nissan to have very little lag. Quick
response at low OEM like boost levels is what the original design intent set
by Nissan was. If one is just interested in a stock (DET) level of
performance these small turbo are fine. If one wishes to compete with the
big boys, something more is needed!
If boost levels above 10 to 12 psi are intended to be used, the T-25 is
challenged. The Pulsar GTI-R's T-28 is only marginally better being good for
about 10 to 15 psi. Higher boost levels are possible with the T25, T28 but
damage to their thrust bearings and even engines can result if these turbos
are pushed higher than this. Running boost levels of over 15 psi are in the
range of diminishing returns on these turbos. Even if higher boost levels
are reached, the corresponding gains in hp will not correlate for the
reasons which we will list below.
Above these boost levels the exhaust side of the turbo is physically too
small to flow enough exhaust gases and the backpressure behind the turbine
starts to increase. When the boost is turned up past the point previously
mentioned, the back pressure in the exhaust manifold soars to over 50 psi.
When 50 or so psi is present in the exhaust manifold and only 10-15 psi in
the intake manifold, it is possible to get backflow of hot exhaust gasses
through the engine during the overlap period where both the intake and
exhaust valves are both open. Normally an engine depends on the inrush of
relatively cool intake air and fuel during the overlap period to internally
cool the engine's valves, piston tops and combustion chamber. Because of
this backflow or reversion, the engines internals start to get heat
saturated under high boost. When things get real hot they can cause the fuel
air mixture to auto ignite causing detonation
At 10 to 15 psi, the tiny T-25,T-28 compressor is zinging to the tune of
250,000 rpm plus. At this speed the air is being sonically whipped to a
froth so to speak and being beaten to death. This physical abuse of the air
raises the temperature to over 350 degrees F, further increasing the heat
load on the engine. Imagine your engine ingesting 350 degree air! Think how
much the CAI helps you guys with NA engines when it drops the inlet
temperature a mere 50 degrees!
All this heat accumulation results in three things, one the engine becomes
prone to damaging detonation, two, the engine cannot make much power because
of charge contamination by the reversion and three, a reduction in power
caused by the hot thin air being pumped in by the turbo.
On Searl's car we are doing some radical steps to reduce both the
backpressure induced reversion and intake charge heating. The turbo we are
using is a pretty extreme application that in theory should work well. It is
basically a Garret TO4, T3 hybrid with a few twists. The compressor is a
TO4E rather than the common TO4B that most people run. The TO4E is
remarkably efficient, being able to maintain close to 78% efficiency from 10
to 20 psi of boost pressure. I think the Miata supercharger boys are exalted
to declare around 60% efficiency at only 6-7psi of boost which I believe is
a good comparison in the potential difference in power that this turbo is
capable of producing over the supercharger. This means that the intake
charge will be about 100 degrees cooler than the T25 even before it even
hits the intercooler. The high efficiency also means it will take less shaft
horsepower to turn the compressor wheel. This allows a relatively
free-flowing turbine to be used because it will not have to recover as much
energy from the exhaust stream to spin the shaft and compressor.
On a side note, the TO4E originally came from a big diesel truck motor and
is a mid eighties design. The common TO4B was designed in the late 60's.
Thus the T04E has the benefit of being designed after at least ten years of
fluid dynamic research which shows in its better performance.
On the exhaust side is a T-3 turbine, but it is the biggest T-3 turbine
available, known as the stage III by Turbonetics. It is used on their all
out Buick Grand National turbo It was originally OEM for a Navistar Diesel
engine! This is a pretty free flowing turbine selected in our case mostly to
eliminate backpressure.
These parts were also chosen in part to reduce lag! The T04E wheel makes a
good amount of boost at 80,000 rpm, down quite a bit from the 250,000 the
T-25 spins at. So even though the T04E is quite a bit bigger and heavier, it
does not need to be spun up to such a high rpm to make big boost numbers.
The big wheel also starts to move air at a much lower shaft speed so the
boost onset rpm is kept low. Thus the lag will be kept reasonable even with
big honkin reciprocating parts.
The lower shaft speed of the TO4E results in a more gentile handling of the
air and thus less charge heating. A more technical explanation is that the
super fast spinning T-25 has the compressor wheel tips traveling at near
sonic speed. When the tips reach the speed of sound the air forms shock
waves inside the compressor and the turbo stops pumping. Operating at
conditions near this cavitation point are not so hot for efficiency which
creates more charge heating.
An exotic ball bearing center section was selected to eliminate the
traditional sleeve bearing's oil-induced viscous friction in the center
housing. The Nissan GTP car used this technology to reduce turbo lag and we
will use it here. The ball bearings should help the turbo spool 5-600 rpm
sooner.
I feel that the big gun turbo will start to produce boost by 2500 rpm, be
impressive at 3500 and get scary at 4500 rpm! This is about 1000 to 1500 rpm
later than the spool up point that is enjoyed by the ultra responsive turbos
run by the T25,T28 crew but is still quite streetable. The power of the big
gun will climb strongly until redline instead of surging hard to 5000 to
6000 rpm then falling off. The smaller turbos start to peter out in the 5000
to 6000 rpm zone because the backpressure starts to rise, leading to power
robbing reversion. That is one of the reasons why the Stillen car turbo car
was not as fast as what everyone had hoped it would be. This is not to speak
badly of Stillen as their design intent was to produce an ultimately
responsive, factory like, super-streetable turbo while maintaining a
reasonable price, a mass market turbo kit so to speak. I believe that they
effectively accomplished that goal. Our goal however is to push the envelope
of SR20 performance! Price was no object here as this turbocharger alone
will retail at about 1400 big ones.
By reducing reversion and intake air temperature we should be reducing the
chances of detonation which should allow us to run more boost on pump gas.
When Searl completes the low compression bottom end, I am figuring that at
least 17 psi should be possible on good old 92 octane pump pee. On stock
compression we are hoping for 15 psi with the aid of a sophisticated 3-D
mappable water injection system and 10-12 psi without it turned on.
Although Searl is not planning to drag race his car, nor is his car set up
for drag racing, if he did, with a drag racing suspension and slick set-up,
this combination could push his car into the 11's with about 420-450 hp to
the wheels@20-23 psi on race gas. Searl's main intent is to give those turbo
Porshe's hell at Willow springs, once we figure on how to get that hp to the
ground!
A traction control system is being sourced with an English firm made of ex
F-1 engineers being contacted. Stay tuned as we report the progress on this
ambitious R&D project.



What's been going on with the car?
Here's a little info on the car. The car is a 1995 Nissan 200SX SE-R. It has
a T3/T04E turbo on it with a huge 24" wide by 8" tall by 4.5" deep front
mount intercooler. The motor is completely stock internally with the
exception of the Jim Wolf Technology (JWT) 2nd generation cams. The head has
not been romoved and it still has the original head gasket. The stock
compression is 9.5 to 1 which is a little high for a turbo but not as bad as
a Honda B16 motor.It also has Aquamist water injection which is controlled
seemlessly with the JWT ECU. It is activated based on MAF sensor voltage
(it's set at a certain value or load on the car) and is activated when it
reaches that voltage. Then 120 milliseconds later, the JWT ECU switches to
another set of timing and fuel maps. Water injection alone will yield a loss
in horsepower, but with the timing and fuel map changes we are able to get
more power out of the car on 92 octane pump gas. The car also has an MSD 6A
ignition with a Nology Coil. Suspension wise the car is outfitted with
Stillen front and rear Strut Tower Braces, GAB adjustable Struts and Shocks,
and Ground Control Coil Overs. The car is also equiped with NuTech's front
3-way adjustable sway bar and (thicker than ST) rear bar. For wheels, the
car has light weight Enkei RP01s and Toyo T1-S tires. The sizes are 17 X 7.5
and 215/45-17. The exhaust is a custom mandrel bent 3" exhaust with a Random
Technology 3" cat.
June 1999 - I bought Searl's car from him. Big props to Searl for putting
the money up to begin the real emergence of Turbo SE-Rs with prototyping the
FMAX <http://www.f-max.com> Turbo Kit. Before I purchased this car I owned a
1993 Sentra SE-R that made 140hp to the wheels with very few mods. I had
that car for about 3 years. The turbo 200SX is my only car, and is daily
driven 60 miles a day to work and back.
July 1999 - I took the car out to Carlsbad Raceway here in North County San
Diego. For this trip to the track I had a test pipe made (unfortunately the
shop made a 2.5" test pipe for my 3" exhaust system which I found out
later?). I also bought some VP C-16 race gas. This gas is just awesome. Both
the Research and Motor Octane (RON and Mon respectively) ratings are 117
octane. Unfortunately this gas is leaded and would kill a catalytic
convertor. Thus the test pipe and also for more power. Another side effect
of using leaded fuel is that it will kill an O2 sensor. My check engine
light came on a little after this event. Although it later recovered. If you
plan on running race gas, buy yourself a spare O2 sensor. I ran 22 X 8 X 15
slicks for this run. These slicks are way to small for a car this powerful.
The best 60ft time of the day was a 2.0X. I took the car out on Saturday to
get some practice runs in and only ran 14 psi and left the timing at 15
degrees advance. I ran the car 4 times trying to get used to launching it.
It ran between 13.0 - 13.4 in the 1/4 mile for all runs. The car ran well
and I felt confident for Sunday. Then on Sunday the boost was set to 20psi
and the timing was set to 20 degrees advance. Remember though, that my ECU
has been completely reconfigured for this turbo, so 20 degrees timing on
this car is not the same as 20 degrees timing on a stock car. The car was
run 11 times at this boost level and attained a 12.3 @ 114MPH. Every one of
the 11 runs were in the 12s.