Step 6: Picking the Right Turbo
|In this step you're going to learn about the turbo. The science
of Turbos is enough to cover several books and then some. Please consider this a
simple introduction and if you would like to do further research refer to the
books and web links below.
Before we get started on the turbo, it would be helpful for you to have a
basic understandings of a couple things. First of all, turbochargers make
torque, not horsepower. Horsepower is a function of how much torque the engine
has at a given RPM (ie. It is speed related). In order to increase HP
without increasing torque, you will need to increase the RPM. Most of the wear,
tear, and abuse in a engine is going to come from increasing the RPM because of
a simple law of physics: Force increases with the square of the speed
increase. In simpler terms, as you double the speed of an object, it's force
increases fourfold. These are the forces that tend to tear an engine apart not
add power so be aware of that when you design and build your engine. They are
also the same forces that require you to spend the big bucks on the expensive
high RPM parts.
A much safer and cheaper way to make the car go faster is to increase it's
power output while staying in the same RPM range. This can only be done by
increasing torque. With a properly sized turbo you could double the torque of
the motor at a given RPM while only increasing the peak force on the engine 20%
or so. Yes it sounds far fetched but here's how it works:
Keep in mind that the pressure in your combustion chamber is a combination of
the how much pressure your piston created when it compressed the fuel mix and
the pressure from the burning mix. This fuel mix will burn in your combustion
chamber at a certain speed depending on mixture, pressure, and other factors,
but for simplicity sake just be aware that it does not burn instantly. In a
combustion engine the peak pressure is reached near the top of the stroke when
only a small portion of the fuel mix has burned. After that point the piston is
accelerating downward and the cylinder pressure drops off rapidly while the fuel
is still burning.
In a turbo engine under boost, you may have twice as much fuel mix in the
combustion chamber, but since it does not all burn at the same time this
additional pressure does not add much to the total cylinder pressure that would
have existed in a normally aspirated engine. Now as the piston is accelerating
downward there is more burning fuel in the combustion chamber and this burning
fuel mix pushes harder than a normally aspirated motor. This is where the real
power increase in a turbo takes place. At about 90 degree crank angle the
turbo engine's fuel mix is pushing on the piston 3 or 4 times harder than a
normally aspirated engine would push. The pushing pressure is still less than
the peak pressure which occurred near the start of combustion so it does not
create the "overload" to the engine that most people would expect.
|Ping, pong, knock, detonate:
When the turbo compresses air, the air gets hotter. Most of this heat is due to
a law of nature that says when you compress something it will get hotter. Some
additional heat is due to inefficiencies in the turbo itself. What's important
to understand is that the hotter intake temperature increases combustion chamber
temperature. The higher combustion chamber temperatures, higher pressures, and
higher compression speeds (RPMs) can lead to deadly detonation. Detonation can
be thought of as a spontaneous explosion in the cylinder, rather than the more
desirable even burn. It is for this reason that you must use good quality fuel
especially at higher boost pressures and never ever let the engine detonate. Fix
the source of the detonation immediately.
So you see that while the peak pressure in the cylinder has not doubled, the
average pressure pushing on the piston over the entire stroke has doubled. This
higher average pressure translates into more torque at the rear tires at the
In the above explanation I have attempted to summarize a fairly complex
topic. For a more in depth explanation I invite you to look in the "Maximum
Boost" book below which has a fairly good explanation of the entire
If you've read the previous sections, the above may also help you further
understand why the racers use low compression motors with turbos and why turbo
cams have different valve timing. In short, you want as much fuel/air mix as
possible in the combustion chamber and you want it to push on the piston harder
As an added bonus, a properly designed turbo car will be more drive-able at
low speeds than an equally fast non-turbo car with the same size engine.
Remember turbos like relatively docile cams. The low overlap turbo cam will
provide better low speed driving and more low speed torque than a high overlap
"race" cam. When you hit the gas and the turbo kicks in though .
. . watch out.
|One of the problems a lot of people
have with turbos is the dreaded Turbo Lag. How do you pick the right turbo for a
motor and and how do you minimize turbo lag?
|Lag: Some define turbo lag as a big
hole when coming off of idle. Others define it as when you
come into boost. With a carb suck-through setup, you will
have a big hole off of idle, and to get rid of it, you
usually have to be about 2500 RPMs. With fuel injection, there
is no hole, it just runs like a naturally aspirated engine until you get
to enough RPMs to build boost.
You will never have instant power off of
idle unless your running a blower. Even a good running naturally
aspirated engine will not pull good until it gets into the RPM range
where the cam can do it's magic. Same holds true for a turbo setup.
Until you get to the point where there is enough air going through it, it
cannot produce any boost. Do not confuse this with a stumble off
of idle. I think the lag that the carb boys talk about is
the lean-out stumble, or hole off of idle. Fuel injection
works just fine as if the turbo wasn't even there until
you get to the point where the turbo starts to push the engine,
then of course, the engine wakes up and the fun starts.
The point where the turbo comes in depends on a lot of things, cam,
compression ratio, design of intake and exhaust manifolds, and the turbo itself.
Different snail shell housings change the "Turbine Map" of when and
how it pushes air. The lower the AR number(.42, .48, .6, .8 etc.), the less CFM
it takes to get it going. The disadvantage of too small of an AR number is that
you will exceed the limits of what it can push at higher engine RPMS. You want
to match the output of the turbo to that of the cam profile of your engine. A
good example is say you have an engine that you don't want to go above 5500 RPM.
Well, for a VW 2276 with an Engle Turbo grind of their 120 cam, using the
Chrysler T-3 off of a 2.2 Liter Daytona, the engine will be into full boost by
about 2400 RPM (12 PSI) and by the time you get to a little above 5000 RPM, the
engine starts to quit pulling so hard, kind of like someone put a nail in the
tach. What we found out to be was the exhaust was starting to back up and
couldn't get out fast enough, to produce more on the intake. This is where the
"Pumping" losses take over. To correct this, say I now want my engine
to go to over 6000 RPM (providing that the rest of the engine stays together),
all I need to do is change the exhaust snail shell from an AR of .48 to .6. What
I just did was slide the turbine map upward. Now what happens is I don't make
full boost until about 3300 RPM but hang on all the way through 6000 RPM!
Pay attention to what kind of motor you want. From the example above you can
build a good play motor with good bottom end and loads of fun at the top, or, by
changing the cam, and other engine components to handle extreme RPMs, you can
build an 8000 RPM engine at the cost of loosing bottom end. So again, are you a
drag racer, or all around play car?
Use the IHI RHB52 or Ford Probe turbo for anything under 1835cc's. Use the Chrysler
T-3 for 1915cc's and bigger. On big engines or high RPM engines, you will have
to change the AR of the exhaust to a larger number so it will not exhaust lock
on you. Exhaust lock is when no more exhaust can get out, so no more boost can
be put in. These are things that the individual has to play with. If you just
start with these initial pieces, you will be happy. Then start modifying
and you will see what effects what and when.
|When you find a turbo at the junkyard,
what should you check before you pay for it?
|When you go to the the junkyard, do an overall assessment of
the turbo vehicles. Look at their mileage, pop the hoods and focus on the best
one. Check to make sure there is oil in the motor, and look for signs of obvious
engine damage. In my opinion, it's better to find a car with obvious body
damage because that will tell you why the car is likely in the junkyard.
Remember, the turbo charger was typically an afterthought to the auto
manufacturers so it will be stuck wherever they could find a little bit of
space. Before you bust your knuckles taking it off, pull off the air intake and
put your fingers on the shaft. It should rotate smoothly all the way around
without the blades rubbing on the housing. Now
move the turbo shaft side to side. A little tolerance is built into the oil
bearings so it should have slightly noticeable play. For sleeve type bearings,
look for less than .022" side play which is a noticeable wiggle, and less
than .008" end play which is not very noticeable. (A match cover is about
.015" thick to give you an idea) If there is too much
play, the turbo is either shot or needs a rebuild. Also check for signs of the blades rubbing on the
housings, chips and rough edges on the blades and check the turbo outlets for signs of oil leaking past the seals.
Disconnect the wastegate rod and check the operation of the wastegate
valve. Look for signs of major cracks on the wastegate port and check for a
good sealing valve. Small cracks are to be expected on the wastegate port but
if they are opening then scrap the unit or buy it for parts.
Pull off the oil lines and look for heavy deposits of charred oil in the
ports. Heavy deposits may indicate that the turbo has had a rough life.
Watercooled turbos will probably be in better condition.
The turbo will usually be tucked away in a tight space and the exhaust nuts
may be rusted tight. Now would be a good time to spray the nuts with penetrating
oil and look for other parts in the yard. When you remove the turbo, keep the
oil lines on because they will help keep the dirt out. Whether you use them is
up to you, however keep this in mind: That small line on the top of the turbo is
the life blood of the turbo. You do not want a cheap or worn line to fail.
Depending on the application, some turbos may have an exhaust outlet that
makes a hard bend toward the exhaust pipe. The housing I have seen are cast
steel not cast iron so they can be cut and re-welded if need be.
thinking about it, remember to take a look at the ports where the intake air
and exhaust flow through the turbo. If you see any roughness from casting
marks or other defects, polish them out before installing the turbo and get a
little more free performance.
Expect to pay between $35 and $100 for a used turbo at a junkyard.
|The Bypass valve is installed between the turbo and the
Butterfly valve. The purpose of the bypass valve is to release the pressure on the
output of the turbo when shifting. What happens is when you are racing up the
hill, and the turbo is putting out full boost, then you take your foot off the
pedal to shift gears, the air pressure can't go down into the engine, and the
turbo doesn't have any exhaust driving it anymore, so the air tries to go back
out the way it came in. Sometimes under high boost conditions it can actually
unscrew the nut holding on the compressor turbine wheel. Anyway it either stops
the turbo, makes it go backwards, or at least it slows it down. Now the turbo
has to start all over again to get back up to speed.
The fix is the bypass valve. It has a sensor hose connected below the
throttle plate. This valve is normally closed to the outside world. When a high
vacuum signal shows up, like when shifting gears, it pops open, and blows off
the extra boost. As soon as the vacuum signal goes away (like putting your foot
back on the gas), it closes and the turbo did not see any back pressure this
whole time, so it stays spooled up ready for action.
Turbocharger Vehicle Application Table
This list is by no means inclusive. Please note turbos such as T03 and RHB5
are "family" names. Within that family you will have somewhat similar
max air flows, but different boost characteristics. There is no best turbo for
any given engine because that depends on the driver, the engine, it's volumetric
efficiency, the vehicle, and your right
For each turbo that you may choose, you also have the option of different A/R
ratios, different wheel trims and different housings. These aftermarket options
will change the airflow and efficiency characteristics of the turbo to suite
your particular application. Generally speaking, the turbos that you find on the
typical consumer car are designed to provide moderate boost in the mid RPM
range. If you desire higher boost levels at the upper RPMs, then look for
a turbo off a vehicle that has a slightly larger engine.
Please send any updates or inaccuracies you
may have to: email@example.com
A good application chart with specific models of Turbos and their vehicles: latta.pdf.
||Chevy Sprint 1.0L
||Possibly good size for smaller motorcycle engine or a twin
|84-86 Ford Escort
Ford Laser, Capri
pre 91 Mazda 323, 626 or MX6, RX7
84-86 Mercury Lynx
84-87 Isuzu Impulse
85-90 Isuzu I-Mark
85-90 Chevy Spectrum 1.5L (RHB521)
||Generally found on 1.6 to 2L vehicles
One distributor has told me that the bearings are too small on these and
that Garrett Turbos are easier to get parts for.
Good for 73-208 HP
Turbine Wheels: VJ20 - 1.8L Mazda GTX
12-R - 1.6L Mazda
Subaru: 15-R or 20-R
RHB models have been replaced by RHF
*Late 80s T-Bird has a higher flow compressor wheel. Could Boost 2.3L to
13-15psi to redline
**Early WRX models had slightly larger RHB52
Diagrams and Crude
|89-91 Ford Probe
||84-86 Pontiac Sunbird GT 1.8L
88-90 Pontiac Sunbird GT 2.0L
1990+ Ford Fiesta 1.6L Engine 8.3:1 CR
|Over 1912 CC:
||New VW 1.91 TDI Bugs or can be bought aftermarket
Some 88-89 Volvo 740,760 others used TD05
Volvo 940 2.3L used TD04H-13C-6
850 T5 used TD04HL-15G-7
Volvo S70 and V70 used TD04HL-13G-7
smaller compressor and smaller shaft than Garrett T03,
Identifiable by bend in wastegate arm.
Flow at 15psi for different compressor wheels
and exhaust housings:
TDO4-9B-6CM2 / 265 CFM TDO5-12A-8CM2 / 320 CFM
TDO4-13G-5CM2 / 360 CFM TEO4-13C-6CM2 / 360 CFM TDO4L-13G-6CM2 / 360 CFM
TDO4L-15C-8.5CM2 / 390 CFM
Similar to TD04??
|88-93 Chrysler (Turbo 1) 2.2L and 2.5L
||After 1989, 2.5L turbos had larger exhaust outlet
Smaller and faster spooling than T03. Exhaust Flange interchangeable with
Uses small shaft and small bearings.
300ZX Twin Turbo
||1990 and later Volvo 740,760,940
(some of these also had the TD04), 95-xx 2nd Generation Mitsubishi Eclipse,
Talon (Manual), Nissan 88-89 300ZX
1990 and later Saab 9000T
89-90 Pontiac Grand Prix 3.1L V6
||good for 125-210bhp, maybe up to 250HP
|Garrett TD05H 14B
||90-94 Mitsubishi Eclipse, Talon, Laser
||Slightly larger than T25
|Pre 90' Saab 8 Valve 900 and 9000T
Some Saab owners will swap the compressor wheels with a 60
or super 60 trim
|Pre 90' Saab 16 Valve 900 and 9000T
|84-87 Chrysler 2.2L (Turbo1)
||84'-pneumatic dual port wastegate actuator
85'-87' computer controlled single port wastegate actuator.
used on Non-intercooled cars 7.5-10PSI boost stock
Thunderbird, Mustang GT
|85-86 Ford Thunderbird
(The compressor AR numbers may be reversed)
||84 Nissan 300ZX
||85-87 Nissan 300ZX 2960cc
||Mercury 85-86 Capri, Cougar
Merkur 85-88 XR4Ti
|.60 or .62??
Chrysler 2.2L (Turbo II) With Intercooler
86' Shelby Omni GLHS
||1986-1988 have weak wastegate actuators springs.
1989 switched to bigger exhaust outlet and better wastegate
Exhaust housing marked with M4 (some 1987-1988s) is less
prone to cracking than M3
|Garrett 25 Variable Nozzle (VNT)
|89 Chrysler Shelby CSX, 90 Le Baron GTC, Shadow ES, and Daytona
||Excellent idea but carbon on moveable vanes can
lead to sticking and overboost.
(very rare but Garrett may still have some new
DOHC engines (16 valve) Intercooled
Spirit RT, Daytona IROC RT
||Larger compressor wheel and housing
Stock engines rated at 225hp
pre 87 740,760
||Factory Volvo 240s had between 127 and 162HP
||Whether this one will work on a typical 2276
depends on the specific model, AR ratio and trim. No other data yet.
|Over 2500 cc:
|88-92 Isuzu MPR Truck
probably best for a 3 Liter
Crude Flow Maps