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Tech
Information
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Common
Questions
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| I
changed the tire size on my car, what's my speed now? |
Your
actual mph = new tire diameter / old tire diameter * indicated mph.
The units on tire diameter don't matter, as long as you use the same units
for each.
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| What
is Brake Specific Fuel Consumption (BSFC)? |
BSFC
is a confusing name of a simple concept.
BSFC=pounds of fuel per hour / brake hp
This is a measure of the efficiency of your engine, and you want it
as low as possible. A lower number means your using less gas to make
the same horsepower. It's about 0.5 for a good running engine, turbos
are a little heavier on the gas, maybe 0.6 or so. Peak torque is where
this number should be best.
If you are picking a carb or throttle body out, you need to calculate
CFM (cubic feet per minute of air) for your engine.
CFM=rpm*displacement/3456
CFM is cubic feet per minute, rpm is revolutions per minute, and displacement
is engine displacement in cubic inches, 3456 takes car of the units.
But don't forget volumetric efficiency VE. Our engines can't fill the
cylinders fully with air, due to intake/exhaust compromises, so we need
to know this.
VE= actual CFM/calculated CFM * 100
Most engines have a VE of 80%, race engines approach 90-95%, turbos/superchargers
can go higher because they pump air in.
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| How
do I pick a carb or throttle body? |
To
pick a carb or throttle body, calculate your CFM, multiply it by your
estimated VE / 100, and pick a carb or TB that will flow this much. If
you are using one throttle body per cylinder, just take total engine displacement
and divide by number of cylinders, and use that displacement number in
the CFM calc. Don't go too big, get a carb or throttle body that can handle
the CFM at your peak RPM, anything much bigger will hurt your low end,
and won't gain you anything much on top. If I had to pick 2 that were
close, I'd go with the smaller one every time.
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| Do
I need bigger Fuel injectors? |
Maybe, but there are some tricks you can use. Let's start at the beginning.
Injector flow is measured in pounds per hour (lb/hr), or cubic centimeters
per minute (cc/min) at a specific fuel pressure, usually between 34-43
pounds. You need to know the pressure they were specified at, but to compare
to fuel injectors tested at different pressures just use this handy formula:
New flow rate = square root ( new pressure
/old pressure) * original flow.
( Just make sure the units are the same)
lb/hr flow = 10.5 * cc/min flow.
gallon/hr flow = lb/hr flow / 6
Now, how many injectors are you using, and what is your total horsepower
target?
horsepower per injector = total
hp / number of injectors
This is easy, remember BSFC above, usually about 0.5,
injector
flow needed per injector = horsepower per injector * BSFC * 0.8
Injector flow needed is in pounds/hour, horsepower in BHP, BSFC see above,
0.8 is injector duty cycle, if you run the injectors on more than 80%
of the time they may overheat.
I have found that this formula is conservative, most injectors can
be maxed out to 100% duty cycle for short bursts.
Another way to get more fuel flow without getting bigger injectors
is to raise the fuel pressure, remember New flow rate = square root
( new pressure /old pressure) * original flow, up the fuel pressure,
up the flow. 100 pounds pressure is pretty much the max. for injectors,
make sure your fuel pump can provide the extra pressure/flow. Any fuel
pump as you raise the pressure past 40 pounds or so the flow starts
to fall off. You can re-wire you fuel pump with some 10ga wire to help
juice it up a little, otherwise get a bigger one, and make sure you
get the flow vs. pressure data at the pressure you will be running..
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| More
tech info |
Oxygen
Sensors
Thrust
Horsepower
Conversions |
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1039
riverside street suite10 
