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Tuning for Boost
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Compression Ratios and Supercharging
When matching superchargers to engines, the most important factor to be resolved is the engine’s
mechanical compression ratio. Since a supercharger can elevate the volume of air ingested by
each cylinder beyond that magical 100 percent VE figure – the compression ratio is much higher
than it was prior to supercharging – this new dynamic dictates the engine’s output and the life
expectancy of the engine’s suddenly higher stressed internal components.
There’s a significant difference in the calculated static compression ratio and the dynamic
compression ratio. When you pencil out a static compression ratio, you compare the volume of air
the piston’s movement will displace with the volume of air that will exist between the piston dome
and the combustion chamber surfaces at Top Dead Center (TDC). In supercharged applications,
however, the chamber volume will be included in the displaced volume, increasing the Bottom Dead
Center (BDC) displacement figure without an associated increase in the TDC volume. This
effectively increases the compression ratio, at least under boost.
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For an engine to run right, the perfect
amount of air and fuel need to ignite at
just the right time for things to work
out. When adding a supercharger or
turbocharger, every one of these
parameters needs to be adjusted
correctly – or you’ll break parts.
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To optimize a supercharged engine, reducing the static compression ratio will allow higher intake
manifold pressure (more boost) without wandering into problems with detonation. Unfortunately,
having too low a static ratio can affect drivability. If the vehicle will be expected to negotiate traffic
most of the time, the static compression ratio must suit those conditions and be high enough to
maintain adequate spark plug and combustion chamber temperatures for low-speed operation. This
usually requires a compression ratio above 8.0:1.
To allow higher boost levels, the static compression ratio can be set at approximately 7.5:1 with
pistons having shorter compression heights than stock. This ratio is useable on the street only if
the engine is exercised on a regular basis. Otherwise, you’ll be changing fouled spark plugs a lot
more often than you’ll care to.
The payoff, however, is awesome performance under boost, which can be raised to at least one
atmosphere (15 psi) without worrying about melting your mill. Most high-performance engines with
low static compression ratios will handle considerably more than a single atmosphere of boost, but
it takes some careful planning to control the spark and fuel demands.
The fuel requirement of a supercharged engine is not precisely related to the increased volume of
air under boost. Programming the fuel curve to reflect the increased airflow using the fuel
requirements of an unsupercharged engine will result in a very lean mixture under heavy boost.
The additional fuel required under heavy boost often calls for an auxiliary fuel pump to control the
combustion temperature in the chambers.
While the excess fuel in the engine will absorb large amounts of heat in the process of achieving
optimum fuel vaporization, if oxygen no longer remains in the chamber to support combustion, the
vaporized (and usually some raw) fuel will leave through the exhaust valve. But without what sounds
like a wasteful process, the engine would self-destruct under detonation.
Since the amount of air within the intake tract has been increased, the compression pressure will be
higher than normal. The ignition timing must usually be retarded to compensate for the quicker
burn time of the dense, rich air/fuel mixture. The entire concept of variable ignition timing is to begin
the combustion so that maximum cylinder pressure occurs slightly after the piston passes TDC.
Lean mixtures burn more slowly than rich mixtures do, and highly compressed, dense mixtures burn
very quickly. The proximity of the fuel molecules with the oxygen in these mixtures ensures fast
flame travel.
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Blower Cams
It’s a given that the more air/fuel (higher VE) you stuff into an engine’s combustion chambers,
the more exhaust gases will have to come out. Under naturally aspirated situations, the
exhaust lobe center on a conventional hydraulic, flat-tappet camshaft has enough duration for
the engine to be able to cleanse itself of the exhaust in-between the intake, compression,
ignition, and exhaust strokes. However, with a blower, dispensing this exhaust in an efficient
manner using the existing valvetrain components may prove somewhat problematic. To more
fully understand the dynamics of camshafts and street supercharging, we spoke with Chas
Knight, Domestic Valvetrain Product Manager from Crane Cams.
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Dual profile cams (blower cams)
feature a more aggressive exhaust
lobe separation of 114- to
116-degrees duration to allow more
time for the cylinder to cleanse itself
of exhaust gases. Other notable
effects include a reduction in
cylinder pressure and increased
torque and horsepower through
superior exhaust scavenging.
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“When it comes to supercharged street engines, we like to use dual-pattern camshafts wherever
possible. Traditionally, the dual-pattern camshaft provides the broadest power and torque band
with favorable idle characteristics and good performance. To put it in laymen’s terms, what you
have with the dual-pattern camshaft is a different profile on the intake lobe master and a different
profile on the exhaust lobe master. Traditionally, you have a slightly longer duration lobe (112 to
116 degrees) on the exhaust because of the excess heat and boost that’s generated by a
supercharger to allow the exhaust valve to stay open longer, remembering that most popular
American V-8 engines feature an exhaust port that doesn’t flow nearly as good as the intake, and is
governed by a smaller diameter exhaust valve. By extending the exhaust lobe duration slightly, it is
a very efficient way to increase exhaust port efficiency and relieve back pressure.”
What is the best design dual-pattern blower cam? Hydraulic or flat tappet? Roller or non-roller?
“A blower cam can be anything from a hydraulic flat tappet, hydraulic roller, a conventional flat-face
solid lifter, or a solid roller. You must remember as a rule, the average street supercharger
enthusiast isn’t big into high RPM. So, normally for these kinds of applications, you could use a
hydraulic roller camshaft. Of course, with a hydraulic roller camshaft, you can set and forget the
valve pre-load once instead of having to go back and readjust the lash intermittently like you would
have to do with a solid grind. I guess you could say that it’s a more user-friendly camshaft.
“The hydraulic roller cam profiles that are available also provide much more efficient valve motion
than you can obtain from a flat-face lifter. With the hydraulic roller cam, you get better idle, better
torque, and better horsepower!”
How would you go about selecting the ideal profile blower camshaft?
“This is where it starts to get a little more involved,” says Knight. “Normally when you go to a
supercharged application, chances are you’re going to be lowering your static compression ratio
because you don’t want to run the risk of constant detonation, or have to run exotic (and oftentimes
expensive, and illegal) racing fuel on the street. When lowering your compression ratio to prevent
detonation, you’ll also need to go to a milder grind camshaft with less duration on the intake,
because you’ll now be force-feeding the intake mixture into the engine mechanically. Instead of
depending on the vacuum pulses of the intake system to draw fuel into the combustion chambers,
you’re going to be doing it more positively allowing the supercharger itself to fill the cylinders or do
the work.”
Prior to delving into turbo cams, we posed a hypothetical, yet commonly asked, question. What kind
of camshaft would I use on an 8.5:1 compression, Vortech VS-1A-supercharged 5.0L at 6 psi and 5-
speed manual transmission?
“With an application like that we would use a hydraulic roller cam with 226 degrees (duration) at
0.050 inch on the intake, and 232 degrees (duration) at 0.050 inch on the exhaust, with something
like a 0.544 to 0.559 (inch) max valve lift. This type of cam would feature a lobe separation up to
114 degrees to reduce overlap so you won’t be blowing the intake charge right back out the
exhaust.”
What type of camshaft would you typically use in a turbocharged application?
“That’s another deal altogether. For a basic street-driven small-block Ford application, we would
probably recommend a single-pattern grind camshaft where the intake and exhaust would be the
same duration, or quite possibly what we call a reverse-pattern cam, where the duration on the
exhaust is actually shorter than the intake.”
Knight went on to explain that the turbocharger is a thermal pump. It lives on heat, and uses the
engine’s exhaust to drive itself.
“The more efficient the exhaust system, the more efficient the turbocharger becomes. What you
want to do is capture that heat (through a less aggressive, shorter exhaust lobe duration), and use
it to drive the turbocharger. With a single-pattern grind, you’ll spool up the turbo quicker. You’ll get
better throttle response. You’ll get better power, and less turbo lag.
“Again, your ‘average’ street turbo-equipped car will be turning 6,500 rpm or less, so a single-
pattern hydraulic roller cam will work just fine. Anytime you start getting over 6,500 rpm, you’re
probably going to be better off with a solid-lifter roller cam application.”
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Carburetors Versus Fuel Injection
In the long-running dispute over which is the best induction system, the results have generally been
favorable for EFI, and in particular sequential electronic fuel injection (SEFI). However, if you’re
fascinated by things like that (and if you’re reading this book, we guess you are), you might be
interested in why that is.
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Conventional wet street
supercharged systems – like
this Holley-equipped, BDS-
supercharged, Ford 351W –
have long used the traditional
600 to 650 cfm Holley center
squirters, which are available in
various states of preparation
through Holley Performance
Parts retailers.
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Conventional wet street supercharged systems – like this Holley-equipped, BDS-supercharged,
Ford 351W – have long used the traditional 600 to 650 cfm Holley center squirters, which are
available in various states of preparation through Holley Performance Parts retailers.
The first point for EFI is its amazingly quick response time. The second point isn’t as simple,
because the tenure of carburetion has been a long and successful one.
Let’s cover the importance of response time first. Think about how many events, in terms of fuel
delivery and ignition, occur every second while an engine runs. At 6,000 rpm, the crankshaft is
turning 100 times per second, the ignition system is charging and discharging 400 times per
second, and the fuel-injection system will be timing the pulse length for each of the injectors at the
same rate as the ignition system, varying the amount of fuel delivery at intervals of milliseconds to
optimize the mixture cylinder by cylinder. That’s a darned good trick, by any standard!
A well-tuned carburetor is a wonderfully precise fuel delivery system, but it cannot respond to the
requirements of an engine on a cylinder-to-cylinder basis. Instead, the carburetor is dealing with
the engine’s gross airflow for all eight cylinders. The carburetor responds to a generalized airflow
and fuel requirement, where an SEFI system can be capable of responding to each cylinder as it
comes up on the compression stroke.
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Other approaches to street
supercharger induction systems
include the use of a conventional “bug
catcher,” or a two- or four-hole fuel
injector housing like those used on the
old gasers or dragsters, along with a
dry supercharger and digitally
controlled electronic fuel injection.
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The fuel injection system is able to control fuel delivery so precisely because it can estimate the
amount of time between a cylinder firing and the resultant spent charge passing the oxygen sensor
in the exhaust system, for a given engine RPM. No carburetor is capable of that kind of interactive
calculation. Typically, fuel injectors are measured in pounds per hour, which is a measure of the
actual static flow, or output of the injector. For example, the SEFI system on your average stock 5.0
L Mustang V-8 uses a set of 19-lb/hr injectors. Ford engineers decided this was the optimum size to
provide the best performance and overall drivability, while meeting stringent emissions
requirements. Of course, once you start bolting on big-bore throttle bodies, blowers, etc., all those
parameters begin to change. To make more power, you need more fuel, hence, larger fuel injectors.
However, carburetion shouldn’t be dismissed entirely. After all, carburetors were the mainstay of the
internal combustion engine for close to a century prior to the introduction of EFI.
In one particular instance, however, the carburetor will continue to rule supreme. We’re talking
about the “carb-in-a-box” setup used on the Paxton-supercharged 289 Shelby GT-350 Mustangs.
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Previous | Next
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This has been a sample page from
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How to Build Supercharged and Turbocharged
Small Block Fords
by Bob McClurg
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The supercharger and turbocharger in their various forms and
applications have both been around for well over a century.
What makes them so popular? Looks, power, performance,
sound, and status. And how do they relate to, and improve
upon, the performance level of a small-block Ford pushrod V-
8 engine like a 289-302, a 351-Windsor, a Ford 351-
Cleveland, or even the latest generation 4.6L / 5.4L “modular”
small-block V-8 engines? That’s EXACTLY what this book is
all about!
While Ford dabbled in supercharging and turbocharging on
production cars all the way back in 1957 with the legendary
Thunderbird, and then again with Shelbys and over-the-
counter kits, and then again in the late ‘70s and early ‘80s
with turbocharging 4- cylinder applications in Mustangs the
real revolution in supercharging and turbocharging Ford
products has come through the aftermarket in more recent
times. The Fox Mustang, created in 1979, and the platform
that would eventually feature fuel injection in 1986, allowing
much more boost, created a genre of lightning-quick and
affordable performance cars.
Featuring legendary supercharger and turbocharger
manufacturers like Paxton, Vortech, Pro-Charger, Garret-
AirResearch and Power Dyne, as well as traditional Roots-
style systems, this book covers everything you need to know
about supercharging and turbocharging your small-block
Ford. Read the sample pages to learn more!
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Click below to view sample
pages from each chapter
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Chap. 1 - Considerations
Chap. 2 - Roots Superchargers
Chap. 3 - Centrifugal Blowers
Chap. 4 - Eaton / Magnuson
Chap. 5 - Twin-Screw Blowers
Chap. 6 - Tuning for Boost
Chap. 7 - Turbocharging
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8-1/2 x 11"
Soft bound.
128 pages.
Approximately 425 b/w photos
Item # SA95
Price: $Discontinued
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Click Here to buy now!
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This is a great book that anyone considering the
installation of a supercharger on a Ford should own!
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Other items you might be interested in
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Sport Compact Turbos & Blowers
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.
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Price:
$18.95
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How to Build Big-Inch Ford Small Blocks
Thoroughly explains how to build a stroker, with information that will
help you to better tailor your heads, cam, intake manifold, carburetor &
exhaust system to get the most of the extra cubes. Also included is a
complete guide to head and block castings so you can choose exactly
the right parts for your project.
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Price:
$18.95
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