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Carbureted Verses Fuel Injected Engines
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A typical four-barrel carburetor
as installed on a single plane
manifold. This arrangement is
best suited to high RPM
operation. (Nate Tovey)
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If you can tune a carburetor, you can tune EFI. Every single component of a carburetor has its
counterpart in the tables of modern EFI systems. Many die-hard racers swear by their trusty old
double pumpers and it’s tough to argue with results.
The reasoning is dead simple. Carburetors by design MUST deliver an amount of fuel directly
proportional to the velocity of air flowing through the venturis. Let’s start with the fuel side.
Obviously, a pump is required to transport fuel from the tank to the carburetor, but the side effect is
a lightly pressurized feed to the bowls. The bowls of a carburetor act much like a water tower. The
higher the level, the more pressure we get—even on a relatively small scale. If we introduce a hole
near the bottom, we get more pressure if the level above this hole is higher or if there is added
pressure on the liquid itself. Controlling the size of this hole (jet size) controls the flow rate of the
fuel once something begins to draw from the bowls.
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The venturis of a carburetor act exactly like the wings of an airplane. As the velocity of the air
increases in the smaller diameter of the venture, Bernoulli’s Law states that the pressure must
drop. This yields a low-pressure zone immediately outside the fuel discharge. This pressure
differential is what draws fuel from the bowls into the engine. Since both the venturis and the fuel
discharge are fixed in size, their flow ratios are also fixed. Very simple: more pressure drop equals
more fuel flow.
The biggest compromise here is that, other than at idle where the mixture screws have enough
authority to change things, we are locked to one flow ratio of air to fuel for the primary circuit and
one flow ratio for the secondary circuit, if so equipped.
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This means tuning a carburetor often boils down to a compromise of desired air/fuel ratios between
cruise, light throttle, and wide open throttle (WOT). With crude adjustments at best to control
transition and no real way to adjust for nonlinear performance across a wide RPM range, there is
room for improvement. Additionally, since actual fuel flow is proportional to air velocity in the venturi
rather than actual air mass flow, air/fuel ratios can change slightly with changes in ambient
conditions. The term “good enough” works for many racecars, but leaves a lot to be desired on a
daily driven car expecting good economy and emissions.
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The “dry” intake tract of a modern fuel-
injected engine shown from filter to throttle
body to manifold. Fuel is not added until just
before the air enters the cylinders, allowing
for more convoluted path design without
puddling. The long individual intake runners
can be seen here. The long intake path is
tuned to improve cylinder filling efficiency at
low engine speeds. (Nate Tovey)
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Fuel flows from the bowls, through the jets,
and out into the venturis of the carburetor.
The smaller area of the venturi creates a
low-pressure zone that draws fuel into the
air stream. (Nate Tovey)
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Timing
The trusty old distributor is a reliable, but crude method of controlling ignition lead. Assuming that
there is little play between the crankshaft and distributor, static ignition phasing is easily controlled
allowing for accurate timing of the spark event relative to piston position. However, automotive
engines are not static devices. Increases in engine speed mean less actual time between crank
angles, so ignition advance must increase just to maintain the same amount of burn time between
the first spark event and TDC.
On mechanical distributors, this increasing advance is accomplished with a set of rotating weights
secured by springs. Adjusting the mass of these weights and the tension of the springs gives a
crude adjustment to the phasing of the distributor’s shaft relative to the crankshaft, advancing
ignition timing at higher engine speeds.
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To compensate for engine load, many mechanical distributors incorporate a secondary
adjustment from a vacuum diaphragm. This diaphragm employs an actuator rod that also shifts
distributor shaft phasing relative to manifold pressure. Since lower loads exhibit increased
vacuum, timing is advanced to compensate and increase performance. As load increases, the
diaphragm returns to its static position, reducing timing to prevent detonation from excessive
spark lead. This allows maximum WOT timing to be easily set with the vacuum line removed and
better fuel economy and power at cruise and idle when vacuum is present. The drawback to the
vacuum advance is the lack of adjustment range. Combinations with larger camshafts or certain
intake manifold designs often render load adjustment based on the single vacuum source
unreliable. This often has the tuner resorting to removing the vacuum reference to keep ignition
advance more stable.
Ideally, there would be a way to accurately adjust the amount of spark advance versus speed
and actual engine load. EFI systems give the calibrator the flexibility to make timing adjustments
at more than one or two points.
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Ram tuning can work on both the
intake and exhaust side of an
engine. The tuned header on this
Toyota MR2 greatly improves
exhaust scavenging.
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Typical mechanical distributor with
vacuum advance. As vacuum
increases, the phase of the rotor is
adjusted by the arm on the right,
increasing ignition advance. (Nate
Tovey)
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Previous | Next
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This has been a sample page from
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Engine Management: Advanced Tuning
by Greg Banish
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As tools for tuning modern engines have become more powerful
and sophisticated in recent years, the need for in-depth
knowledge of engine management systems and tuning techniques
has grown. Tuning engines can be a mysterious art, as all
engines need a precise balance of fuel, air, and timing in order to
reach their true performance potential.
Engine Management: Advanced Tuning explains how the EFI
system determines engine operation and how the calibrator can
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 calibrators and a
valuable resource for anyone who wants to make horsepower with
a fuel-injected, electronically controlled engine.
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Click below to view sample
pages from each chapter
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Author Greg Banish is a calibration engineer with extensive
aftermarket performance calibration experience. With over a
thousand unique calibrations performed over five years, he has
worked with enthusiasts and OEMs alike to improve the
performance and driving behavior of a wide range of vehicles.
The book contains detailed equations, graphs, and illustrations.
Also included are valuable and practical examples, including real-
world examples based upon the author’s experience that will help
more advanced readers apply this new information to situations
that are commonly seen during calibration.
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1 - Introduction to EFI
2 - Basics of Fuel Injection
3 - Carbureted Engines
4 - EFI System Inputs
5 - Fuel Injectors
6 - EFI System Fuel Control
7 - Ignition Systems with EFI
8 - Data Logging
9 - EFI System Calibration
10 - Idle Calibration
11 - Tuning for More Power
12 - Fine Tuning EFI
13 - Tuning EFI with Blowers
14 - Tuning Ford EFI Systems
15 - Aftermarket EFI Systems
16 - INCA OEM Calibration
17 - External EFI Controllers
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8-1/2 x 11"
Soft bound
128 pages
200 color photos
Item # SA135
Price: $22.95
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Click here to buy now!
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How to Build High-Performance Ignition Systems
The complete guide to understanding automotive
ignition systems. Covers components, systems &
subsystems for street & race applications. This book will
help you understand how your car’s ignition works, and it
will help you choose the right components for your cars
performance needs, whether it’s a 1965 289 or a 2003
Cobra with a 4.6-liter modular motor.
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Price:
$
22.95
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Tune and Modify Engine Management Systems
Drawing on a wealth of knowledge and experience
engine control expert Jeff Hartman explains everything
from the basics of engine management to the building of
complicated project cars. This book is updated to address
the incredible developments in automotive fuel injection
technology from the past decade, including the multitude
of import cars that are the subject of so much hot rodding
today.
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Price:
$
27.95
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Turbo High Performance Turbocharger Systems
This book is the most detailed and up-to-date resource on
turbocharging. You'll learn how turbochargers work, how to
choose the right turbo or turbos for your engine by reading
flow maps, and how to tune your engine to run perfectly with
your turbo system. Uses more than 300 photos and technical
information to help you make more horsepower. It also
discusses the various components of a turbocharger and
explains how to decode turbocharger model numbers,
compressor maps, other specifications and includes a
complete step-by-step turbocharger tear-down and rebuild.
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Price:
$22.95
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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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Payment, Shipping & Sales
Tax: Iowa
residents must pay 7% sales tax. Items usually ship
within one
business day of receipt of payment! Standard shipping is a flat rate of
$4.95 to
anywhere in the United States with USPS Media Mail.
Priority Mail shipping is available for an
additional $3.00, or
$7.95 for shipping. Shipping is combined and discounted for multiple item
purchases as follows: first item regular shipping price, add
$1.95 for each additional item. For
purchases of 3 or more items
shipping is automatically upgraded to Priority for no additional
charge! We offer world wide shipping and ship to Canada and Mexico
with USPS Priority Mail
International for $14.95, and to most
locations in Europe, Australia, Asia, Japan and South
America for
$18.95. Satisfaction is Guaranteed. Our store has a NO HASSLE RETURN
POLICY
within 30 days of purchase.
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