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Building Engines that Survive Under Boost
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There are two ways to approach the construction of a boosted engine. In this section, I’m going to
use supercharging and turbocharging as equivalent in terms of engine building. The differences in
terms of the build are very slight because to the engine a pressurized intake is a pressurized
intake. It just has to be built strong enough to handle the power level. If you are going to be adding
a modest-boost ‘charger (one which will produce a maximum of five to eight psi) to a stock engine,
you probably won’t need to make any significant changes to the engine before getting started. Most
of the Roots, centrifugal, and turbo kits are designed for this type of installation. Rather than pulling
the engine out, tearing it apart, and changing pistons, rings, bearings, cam, valvetrain, heads,
manifolds, ignition, and so on, you can buy a kit, bolt it on to the engine in an afternoon, and drive
away with a strong, usable 40-percent horsepower increase. If an intercooler is included in the kit,
even greater power gains can be achieved.
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As long as your stock engine is in
good condition and has a compression
ratio of 9:1 or less, the addition of a
supercharger in the 5- to 8-psi boost
range can really wake it up without
further modifications. Electronic fuel
injection makes supercharging stock
engines even easier — just hook a
blow-through supercharger to the
throttle body and go.
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In contrast with traditional, naturally aspirated performance modifications, a super/turbocharger
making in the neighborhood of six pounds of boost will greatly increase both horsepower and
torque in the midrange (2,000 to 4,500 rpm), where you can really feel it in a street-driven vehicle.
Of course, like naturally aspirated performance parts, the boost will continue to produce power right
on up the RPM scale. Keep in mind: an engine that uses higher-compression pistons and a
longer-duration cam to get the same horsepower increase as the supercharged stock engine will
have less power in the lower RPM. In addition to making less power down low, the hopped-up
naturally aspirated engine will run rougher and probably be less efficient at street and normal
highway speeds.
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The great thing about superchargers,
positive displacement or centrifugal,
and turbochargers is that they’re fully
adjustable. If you want more power,
just turn up the boost. This might mean
you’ll have to make commensurate
modifications to your engine, such as
cooling, ignition, or perhaps internal
upgrades to handle more cylinder
pressure.
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On the other hand, if you intend to bolt
a big blower on your engine and make
some real horsepower, then you had
better build the engine to handle it.
Several companies offer high-
performance crate engines with forged
pistons, heavy-duty head gaskets, and
upgraded valvetrains if you don’t want
to build your own.
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Since most sport-compact car engines come with compression ratios in the 8.5:1 to 10:1 range,
adding boost is a very practical, relatively quick, and simple way to achieve a healthy performance
increase. As long as you keep boost below approximately 8 psi, and you don’t rev the engine above
6,000 to 7,000 rpm (sometimes higher for certain models), you shouldn’t really have to make any
internal modifications to the engine. If anything, you might want to install slightly stiffer valvesprings
to help keep the intake valves on their seats (add valvesprings to the exhaust side for
turbocharged engines), and you might want to switch to a good super/turbo cam or cams. For
DOHC engines, a set of adjustable cam gears to dial in the valve timing for maximum power is also
good choice.
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Although stepping up to billet mains and
cross drilling the block would be preferable
for any high-performance engine, it’s
certainly not a necessity for most boosted
street applications. However, most new
engines are very well designed, as the main
bearing girdle of the new GM Vortec inline
engine shows. Engine parts selection
depends more on the state of the equipment
you’re starting with, the amount of boost you
intend to run, and the amount of horsepower
the engine will make, rather than the fact
that it is boosted.
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The decision on whether or not to use the stock
connecting rods on a boosted street engine has to
be based on the design of the stock rods. The
bottom end of this Ford Focus engine would
probably handle a little boost — though one of the
rods in this set is bent. The machinist at L&R
Automotive Supply in Sante Fe Springs, California,
didn’t know how it happened, but you bend rods
with cylinder pressure. The bolts break with
excessive RPM. For this engine, instead of
Magnafluxing, shot peening, polishing the beams,
and fitting ARP rod bolts, a set of Carrillo or Crower
rods or equivalent should be considered.
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Obviously, the above statement – that you can bolt a 5- to 8-psi blower system onto a stock engine
without making any other modifications – requires some qualification.
First, of course, we’re talking about an engine that’s in good condition to start with. But most
importantly, you need to figure out at what level the new combination will begin to ping or detonate.
This point will depend on the design of the combustion chamber in the particular engine, the
efficiency of the compressor, your driving habits, the weight of the car, the gearing, and the quality
of gasoline you tune to, among other things.
My point is that the life expectancy of the parts in any street-driven boosted engine these days
depends primarily on whether you can safeguard those parts from detonation. To be safe, you’ll
want to run the highest-octane pump gasoline available. You will have to either reprogram the stock
computer (this is currently the best choice) or use interface computers to enrich the air/fuel ratio
and tune the ignition (retard the ignition timing slightly). Perhaps you will have to add a water
injector, or some other add-on to stop detonation, because you certainly do not want to rattle the
pistons in a stock engine. But with adequate octane gasoline (91 or better) and proper air/fuel
ratios (12-12.5:1 at full throttle, full load), you will find that six to eight pounds of boost is very
livable and fun with a stock motor. Further, remember that in a street-driven supercharged vehicle,
the compressor very seldom actually gets into boost, and then usually only for short bursts. The
rest of the time the engine operates under no greater pressures than normal.
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If you have any doubts about running 5- to 8-psi boost on a stock low-compression engine, my
personal advice would be to go ahead and install the system on the car, being sure to add a boost
gauge so that you know exactly how many psi you’re making. If you experience knocking or pinging
under boost, and you’re already using the best gasoline available, the air/fuel ratio is right, and the
timing isn’t too advanced (you’ll have to experiment a little to get it right), then you need to back off
the boost or get a better tune.
If the boost is kept in the 5- to 8-psi range, I seriously doubt that you’ll have any problems. If, for
some reason, the engine can’t handle this boost, and you blow a head gasket, burn a valve, or
even break a piston land or ring, then you’ll have to pull the motor out and go through it. If you’re
the type of person who would rather be safe than sorry, it certainly wouldn’t hurt to blueprint any
engine that will see performance use. But with mild blower boost (below 8 psi), and low static
compression (9:1 or lower), the instances in which production parts will fail are the exception.
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If you fully intend to get your foot into the throttle now and then with boost levels greater than 8 psi,
you had better think about upgrading some of the components. Again, if you can keep your
charged motor out of detonation, you’re not going to have to worry a whole lot about parts damage.
Likewise, if you’re not going to abuse the engine, over-rev it, or race it every week, you don’t need
parts like a billet crank and mains, O-ringed heads, and so on. There is a middle ground. Many
tuners have bolted street blowers and turbo kits onto relatively stock engines without any problems
at all. But once you start making boost in the 10- to 15-pound range, or buzz the motor to 8,000
rpm or more, you are certainly in marginal territory with factory components – especially in terms of
pistons, rings, and head gaskets.
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Previous | Next
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This has been a sample page from
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Sport Compact Turbos & Blowers
by Joe Pettitt
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Lightweight and high-revving, sport compacts are today’s most
popular cars. They have developed a cult following among today’s
youth and are fueling a multi-million dollar industry in modification
parts and equipment.
While most owners of sport compacts can afford the simple bolt-
ons available, some owners want to take their modifications a step
further. There is intense competition to be the fastest, and quite
often the only way to win is to go to the next level – by installing a
supercharger/blower or turbocharger on your engine.
This book is an enthusiast’s guide to understanding and using
turbochargers and superchargers on sport compact cars. It
covers the basics of each system and compares their pros and
cons. Building and tuning small-displacement 4- and 6-cylinder
engines to maximize performance and reliability with forced
induction is also covered.
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Click below to view sample
pages from each chapter!
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Chap. 1 - Exotic or Practical
Chap. 2 - Supercharging
Chap. 3 - Roots Blowers
Chap. 4 - Centrifugal Blowers
Chap. 5 - Turbocharging
Chap. 6 - Turbos & Compacts
Chap. 7 - Tuning for Boost
Chap. 8 - Building Engines
Chap. 9 - History
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Softbound
8-1/2 x 11
128 pages
300 black & white photos
Item: SA89
Price: $18.95
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Click here to buy now!
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Other items you might be interested in
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Turbochargers
How to select and install the correct turbo for big or small
horsepower gains. Discusses turbocharger design, sizing,
matching, controls, carburetion, exhaust, ignition,
intercooling, marine and high altitude applications. The most
comprehensive book available. Turbo suppliers and kit
maker addresses are included. “Everything you could possibly
need to know about turbochargers for automotive applications
is in this book.
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Price:
$18.95
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Engine Management: Advanced Tuning
Engine Management: Advanced Tuning explains how the EFI system
determines engine operation and how to change the controlling
parameters to optimize actual engine performance. This book takes
engine-tuning techniques to the next level. It is a must-have for tuners
and a valuable resource for anyone who wants to make horsepower
with a fuel-injected, electronically controlled engine.
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Price:
$22.95
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Shipping is combined and discounted for multiple item purchases!
Buy more and save on shipping!
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MRE
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