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Throttle Body
101 - Class Is In Session
Click on a topic addressed in this article:
Q: What’s the matter with the throttle
body that came with my car and why should I install a larger one?
A: You shouldn’t, if
enhancing performance and increasing horsepower is not on your agenda.
Let’s start at the beginning. An engine can be defined as an air pump.
That is, it moves or pumps air, starting at the air inlet to the air filter
and basically ending at the tail pipes. The more air that can be moved
through the engine, the more horsepower potential it will have.
Everything that can be done to an engine to increase horsepower (other than
strengthening the rotating components on a race engine) is related to moving
additional quantities of air. This includes adding larger flowing cylinder
heads with larger valves, cylinder head porting, higher lift camshafts, more
efficient intake manifolds, higher flowing exhaust systems (you can’t get more
air in unless you get the old exhaust out), and………a larger throttle body (or
carburetor if the engine is non fuel injected).
Additional ways to move greater quantities of air may also include using a
larger displacement engine or increasing the displacement of the existing engine
(boring and stroking). Bigger engines can move more air than smaller
engines.
Or, if adding displacement is not feasible, spinning the engine to a higher RPM
can increase airflow.
The term most used to measure the quantity of air moving through an engine or
through it’s independent components, is CFM, or Cubic Feet per Minute.
We used the term “horsepower potential” above. That is because there is
one other element that is connected with increased CFM when discussing
horsepower, and that is gasoline. If you recall from your high school
physics class, “fuel” is actually made up of approximately one part gasoline to
ten to twelve parts of air. To increase horsepower significantly, the
engine must burn additional fuel, and because the primary ingredient in fuel is
air, we need to figure out how to move more air through the engine.
Is this all starting to make sense?
In the “old” days when everybody relied on carburetors, installing a larger
carburetor increased the CFM. Installing a larger CFM carburetor also
required an increase in the gasoline supply. This was usually done by
changing to larger jet sizes.
Today, things are a lot easier. With electronic fuel injection, the
on-board computer or “fuel management processor” handles this for us, and much
more efficiently than was ever possible with a carburetor. For instance,
all electronic fuel injection (EFI) engines have a series of sensors that notify
the computer as to what is happening inside the engine. There are specific
sensors that notify the computer if the engine is running too rich or too lean.
With this information, the computer controls the amount of gasoline injected
into the engine in order to maintain the correct air to gasoline ratio (not too
rich, not too lean) in a real time atmosphere.
So, if we add additional airflow potential to our engine in the form of a larger
throttle body, the sensors will notify the computer of the increased CFM and the
computer will then add additional gasoline into the engine. Simple, eh?
In a high performance street type application with an EFI vehicle, there is
generally little reason to modify the gasoline side of the “fuel” equation.
The stock computer will, in most cases, handle this for us. Most OEM
computer systems, fuel injectors, fuel pumps and fuel pressure regulators can
handle greater horsepower potential than the stock engine produced.
Our job in the quest for additional HP is to concentrate on the “airflow side”
of the equation. And that is where Accufab, Inc. comes in. We design
and manufacture one of the most important components in the airflow path, the
throttle body. The throttle body is the “door” that the air needs to get
through before it can get into the engine. Any air restriction at the
throttle body will invalidate almost anything else that you can do to increase
CFM. No additional airflow; no additional horsepower, it’s that simple.
Therefore, if you want to increase performance on your EFI vehicle, start by
installing a bigger throttle body.
Not all throttle bodies are alike (you knew that was coming, didn’t you?).
For instance, all stock OEM and most aftermarket throttle bodies are
manufactured using cast aluminum (or pot metal in some cases) to save money.
At Accufab, Inc., the cost of the product, while an important consideration, is
secondary to the quality of the component. We use aircraft quality, CNC
machined billet aluminum, polished to chrome like luster for our throttle
bodies. The Accufab throttle body is a work of art, and would look good on
your coffee table if it weren’t so effective on your engine.
In addition, we use the most expensive internal components, including sealed
roller bearings, heavy-duty throttle shafts and machining to tolerances way
above OEM specs. Nothing but the best, to insure a lifetime of trouble
free performance.
We know that our customers expect a high quality product, at an affordable
price, whether it’s for high performance street driving or all out racing.
An interesting side note regarding Accufab throttle bodies is that the 200 MPH
all out racecar is going to use the very same throttle body that anybody can
purchase off the shelf for their street machines. The race engine may
require a larger size throttle body (more CFM), but other than that, all
throttle bodies for the same applications are identical. We don’t
manufacture beefier or heavy-duty throttle bodies for a race environment and
cheaper ones for street use. Other than the different sizes available, all
of our throttle bodies are built to the same high tolerances, using the same CNC
billet aluminum, the same internal components, and the same attention to detail.
INSTALLATION
How tough is the Accufab throttle body to install? Not very. In most
cases, all you will need to do is to remove the old (or stock) throttle body and
replace it with the Accufab throttle body. Most of our throttle bodies are
designed to bolt to the original manifolds, in the original location using the
original bolts. No grinding, beating, filing, or other fabrication is
required with most applications. Typically, throttle bodies have a throttle position sensor (TPS)
bolted to them. Simply unbolt the sensor from the old throttle body and
bolt it on to the new one. You will need to insure that the TPS is
accurately adjusted to insure a good idle. This is easily done using a
Voltage Meter. You are looking to set the TPS between .98 and 1.0 volts
measured between the hot wire and the ground wire. Also, the throttle stop
on the throttle body has been pre-set at Accufab and does not require any
adjustment. It can’t get easier than this. Same with the throttle
cable. The Accufab throttle body is designed to accept the same OEM cable
mechanism in the same location.
FORD 1996-1998 COBRA INSTALLATION
The Accufab Ford Cobra set-up uses a large single blade throttle body instead of
the original “2-hole” throttle body that was installed at the factory.
Because the inlet transition from the throttle body into the stock intake
manifold was originally cast for two smaller holes rather than one large one, a
slight modification to the intake manifold will be required. The bridge or
wall between the original two holes will need to be relieved (cut out) slightly,
just enough to provide clearance for the larger, single throttle blade.
This is done by removing the several bolts that hold the throttle body adapter
to the intake manifold, removing it, and grinding or cutting the area between
the original two holes enough for the clearance required. Hardly rocket
science. There is no “wrong” way to do it, either.
WHAT
SIZE THROTTLE BODY SHOULD I USE?
Bigger than what came on the engine from the factory.
For instance, the popular 5.0 Ford Mustang typically was equipped with a 60MM
throttle body from the factory. This equates to a flow rating of 495 CFM.
The Accufab 5.0 Mustang replacement throttle body sizes start with 65MM.
The 65MM unit flows 664 CFM. This is a CFM flow increase of 34%. The
Accufab bolt on replacement throttle bodies are available with units up to 105MM
for the 5.0 Mustang. The 105MM unit will flow 1550 CFM, way more than
triple the stock flow rating. Of course, only a serious race engine could
even come close to benefiting from that amount of airflow.
A word of caution here regarding throttle body size. While the throttle
body is the most important component controlling CFM, it is by no means the only
component. In other words, if you install a large, high CFM throttle body
on an otherwise completely stock engine, don’t expect a giant increase in
horsepower. Simply put, if the engine is unable to make use of the
increased airflow because of other air restrictions in front of or behind the
throttle body, it won’t be able to produce much of an increase in horsepower.
In the case of a dead stock engine, the larger throttle body probably won’t
hurt performance, but it may not help it much, either.
Our tech staff will be happy to work with you in selecting the
throttle body size that will most benefit your specific engine combination and
driving requirements.
HOW
ABOUT MY FUEL INJECTORS? DO I NEED BIGGER ONES?
For most situations in normal high performance street applications, the factory
injectors will handle the additional horsepower created when you install a
larger throttle body.
Electronic fuel injectors are usually measured in pounds of gasoline per hour
(Lbs/hr) at maximum flow conditions or at 100% duty cycle. In most
applications, the flow rating or lbs/hr at 85% duty cycle is used. One
gallon of gasoline weighs 6.2 pounds.
A typical 5.0 Mustang is equipped from the factory with eight 19-lbs/hr fuel
injectors. Some Mustangs are equipped at the factory with 24 lbs/hr
injectors. If you calculate the gasoline flow from the 19-lbs/hr
injectors, at full duty cycle the eight fuel injectors would go through 24.5
gallons of gasoline in one hour. That’s a lot of gasoline. It would
take a very high horsepower engine to use that much gasoline, and remember,
that’s just about 1/10th of the total amount of actual fuel,
considering that you would need 10 to 12 times the amount of air to gasoline to
arrive at the correct fuel ratio. Engines don’t burn raw gasoline.
They burn fuel.
But, if you are contemplating the requirement for larger flowing fuel injectors
here are a few things to consider. First of all you must remember that we
are dealing with both volume and pressure when we discuss gasoline as it relates
to EFI, two distinctly different issues. The volume side of the equation
comes into play where the fuel pump is involved. The fuel pump (always
electric and typically mounted inside or very near the gasoline tank with EFI
applications) will need to move a greater volume of gasoline if we are going to
develop more horsepower. Most stock fuel pumps will handle this increased
volume requirements, at least up to a point. After that, an aftermarket
High Volume Fuel Pump will be required.
The second part of the equation is the pressure side. All (or most) EFI
systems have a return line built into the fuel line system. In essence,
the gasoline is picked up from the gas tank and driven forward to the engine
(and eventually to the fuel injectors) by the fuel pump. But before it
gets to the injectors, it passes through a device called the
Fuel Pressure Regulator (FPR). The FPR, by blocking the free flow of the
gasoline directly from the pump, raises the fuel pressure. It works like
the nozzle on your garden hose. With no nozzle to interrupt the flow, the
water gushes out with plenty of volume, but with little ability to create much
of a high-pressure stream. As soon as you install the nozzle on the hose,
the volume is held back but you can now shoot a stream of water quite a
distance.
The FPR raises the pressure by creating a restriction, just like the nozzle does
on the end of the hose. As it turns out, there is usually a greater volume
of gasoline going to the engine than it can use. This extra volume is
re-routed by the FPR back to the gas tank via the return line. To achieve
more fuel pressure to the injectors, the FPR creates a restriction on the return
line. It kind of acts likes a vice grip on the return line, if you will.
The less fuel allowed to return, the more the pressure builds up.
There are now some new vehicles where the fuel pressure is completely controlled
by the engine management system (the computer) and a return line is not used.
In situations like this, any major adjustment or change in fuel pressure from
the stock settings would need to be re-programmed into the computer via a
chip.
As you can see, the fuel pressure is a very important factor in determining the
fuel flow at the injectors. Because pressure rises as the square of the
flow through an orifice, to double the flow through an injector takes four times
the pressure. This is assuming that the injector can actually flow that
amount.
The stock factory FPR has a range or window of fuel pressure capabilities built
into it and adjusts itself automatically based on engine vacuum.
This is more than acceptable for most street type applications but if really
high horsepower is needed, such as for racing, a larger, stronger and more
adjustable FPR is usually required.
Did we mention that Accufab, Inc. also manufactures heavy duty, adjustable FPR’s
for many applications?
But, we initially were asking about fuel injector size, before all this other
stuff came up.
And wouldn’t you know, there is a formula for this. Here it is.
“Maximum engine output, times brake specific fuel consumption, at peak power
time, divided by the number of fuel injectors, equals the flow requirements per
injector.”
And you thought that this was going to be easy, didn’t you? Well, it
actually is.
Let’s simplify this equation first.
For Maximum engine output, lets substitute “horsepower”.
For Brake specific fuel consumption, lets substitute the figure “0.5” for
naturally aspirated engines.
For peak power time lets use the figure “1.175” which is a conversion factor for
the 85% duty cycle.
For the number of injectors, obviously it’s going to be “8” if the engine is a
V-8.
Let’s assume we are looking to produce 400 horsepower in street trim.
So, we take 400 x 0.5 x 1.175 divided by 8, which equals 29.37 lbs/hr.
When we round off the number it looks like 30 lbs/hr injectors would be needed
to support 400 HP. But, you should also remember that the engine won’t
always be making a “foot to the floor” 400 HP if it’s street driven (unless you
want to pay a ton in traffic tickets) so a slightly smaller injector size might
be a better overall choice. A 24-lbs/hr injector size might be more
appropriate for a 400 horsepower street driven vehicle.
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