In a previous article (Designing a Factory Turbo Engine) we looked at how car
companies design and implement their turbocharger systems. In this interview, we
talk to a leading aftermarket turbo kit designer - David Alexander of Sydney's
Silverwater Automotive. David has been the main man behind some excellent
Hyundai Excel and Lantra turbo kits (go to the end of Lantra Ball of Fun for our drive report) as well
as a few one-offs.
Can Your Car's Engine be Turbocharged?
Pretty well any naturally aspirated petrol engine
from the past two or three decades can be turbocharged while maintaining good
reliability. According to David, the biggest drama is the compression ratio of
the latest engines.
"The Hyundai Excel that I turbocharged had 10.0:1
compression, the Lantra 10.3:1 and it's not uncommon to have 11.0:1 in some of
the more high performance atmo engines," says David. He adds that most of these
engines have an efficient combustion chamber design allowing them to tolerate
low boost levels.
"You can probably get away with turbocharging more
on small bore, long stroke engines. With a larger bore, it takes longer to
initiate combustion and you need to retard ignition timing under boost. You
start wasting a lot of energy when you have to do that type of thing."
"You definitely need to run an intercooler with
turbocharged engines having a static compression ratio above about 9.0:1 -
you're shooting yourself in the foot otherwise. Ideally, I wouldn't go about
turbocharging a standard type atmo engine without an intercooler."
Which Turbo to Use?
With the widespread availability of brand new
turbochargers, many people have no idea what sort of unit to choose.
David says: "I generally look at what the
manufacturers are doing as a guide. Without going into the complexity of
compressor maps, you should look at the turbos that manufacturers fit to certain
capacity engines and the power output that they work at.
"In the case of the
Hyundai Accent kit we used a Garrett T28 turbo based on what Nissan use on their
200SX - the SR20DET is a similar configuration and displacement engine with an
output similar to what we wanted to achieve. When choosing a turbo it's also
important to pick one that's readily available and there are parts for
At the present moment, the turbos most
widely available are suited to Nissan 4-bolt turbine patterns. VF and TD series
turbochargers are also available to suit the unusually shaped Subaru Liberty/WRX
"You don't want to run a turbocharger that's too
small," stresses David. "Exhaust gas temperature goes up along with combustion
temps. I think erring on the large side has advantages but, of course,
drivability suffers a little. Choosing a turbo is all about compromise."
Turbo Exhaust Manifold - How to Get One
"For the Hyundai Excel kit we designed a new
exhaust manifold and had it cast in iron," says David.
"Quality cast manifolds can support a lot of
weight and tend not to crack like some handmade manifolds. I think it's the best
approach. The cost involved in a cast iron manifold is mainly in the pattern
design. Depending on the complexity and nature of the manifold it costs about
AUD$5000 to have a pattern made."
(For another industry expert's view on turbocharger
exhaust manifolds, read Buncha Bananas)
Turbocharger Oil and Water Feeds and Return Pipes
All turbochargers require a supply of oil to
lubricate the center bearing assembly. Most modern turbochargers (after about
1986) use water-cooling to maintain a stable operating temperature, but some
turbochargers are simply air-cooled.
So where should you obtain a turbocharger oil feed
from and where should it be returned?
"The oil pressure switch is usually a perfect spot
to source the turbocharger's oil feed," says David. "When you're running a
ball-bearing turbo you might want to run a restrictor in the line from the oil
feed source, because you don't want to tap into a too great an oil supply.
"For the oil line to the turbo we normally use a
Teflon braided hose. You need to have some flexibility for expansion and we've
never had any failures with the Teflon braided stuff. An option would probably
be to use a hard pipe with a spiral winding.
"The oil from the turbo should drain back into the
sump above the oil line. It's good to have a baffle near the return fitting so
that oil doesn't splash back up or restrict oil flow out of the turbo. And make
sure that the hose you use is rated to the appropriate temperature."
As mentioned, most modern turbochargers have a
water-cooled core. Where should the water be sourced from and returned to?
"A lot of cars these days have heated throttle
bodies, which are a good place to tap into - you still run the water through the
throttle body but then into the turbo and back into the block.
"You've got to be careful tapping into the heater
hoses because if you switch the heater on you can change the direction or at
least reduce the amount of water flow through the turbocharger. This generally
shouldn't be a problem, though. I think the main reason for having water-cooling
is to maintain a stable core temperature when you shut the engine down," says
Changing the Engine Management to Suit Forced Induction
One you've got the turbocharger hardware sorted
out you will need to examine what needs to be done in the area of engine
management. Is it possible to retain the factory ECU?
"When you add a turbo you will overshoot the
tables in the ECU - they generally only have a small amount of scope above
standard," says David.
"In some cars you can squeeze in about 5 psi of boost and
only then you're at the limit of the airflow meter - cars like the V6 Holden
Commodore and some Mazdas, for example. The thing is, none of them were designed
for force induction so the fuel and timing will be less than ideal.
"There's no easy solution.
"For example, in the early Excel that we
turbocharged I ran a larger bodied airflow meter to measure the extra airflow
and modified the factory program to suit. I chose that approach because I have
the ability to do it and there are advantages in terms of starting and various
other areas. In the case of the twin-cam Excel with a standard MAP sensed
system, I ended up changing to a 2 Bar MAP sensor and extensively modifying the
program to run with that.
"A lot of people America simply use a rising rate
fuel pressure regulator in low boost applications and they seem to get away with
it - to an extent. But it's not a great approach from a tuner's point of view. With
this set-up, the engine usually drowns in fuel because fuel pressure is raised
exponentially and the ignition timing is usually left as it is.
"Of course, with higher fuel rail pressure the
less fuel flow your pump can deliver. You really need an upgrade pump as
"And there are considerations beyond mixtures and
timing. I've turbocharged a 1.8-litre Honda VTEC with a F-CON piggyback computer
- the fuel side of things was great, but the cam switchover stage was all wrong
with the forced induction.
"At the end of the day you should aim to maintain
a usable torque curve. If everything is set up properly I often find that the
turbocharged torque curve is fairly similar in shape to the base torque curve -
it's just lifted higher," says David.
Take this man's advice. He knows what he's talking
Silverwater Automotive Services
02 9748 1300