Join Date: Jan 2008
Location: Seattle, WA area
Re: Picking fuel injectors...
Danny - Here's the full scoop, and you'll be able to understand everything that goes into injector and system sizing if you want to understand it. Injector sizing is based on the maximum horsepower (HP) you need to feed, and the efficiency of the engine that's making it. The efficiency is generally measured in brake-specific fuel consumption (BSFC), and for a typical street/strip 347 will be in the neighborhood of .50 to .52 pounds of fuel per hour per HP. So, now you see how you can roughly figure how much fuel your engine will need, as 100 HP at .50 PPH would be (100hp x .50) = 50 pounds of fuel per hour. You can also use this figure for sizing your fuel pump and lines.
So, to turn this into injector size, we also consider other factors, such as the extra fuel when the engine is cold, extra fuel for acceleration enrichment (pump shot), and system degradation. We use a rule-of-thumb of 15% to compensate for these additional requirements. This buffer is called Duty Cycle (DC), and is the amount of time the injectors are ON (open) for normal full-power operation - leaving that extra buffer. So, we are looking for injectors that can supply full power at 85% DC. That leaves us with a simple formula:
(HP * BSFC) / (#Injectors * DC) = PPH per Injector
An example of this would be a 347, making 450 peak HP, with matched and efficient intake, heads, piston dish and exhaust for better BSFC of .50, and port injected with 8 injectors. The calculation would be:
(450 * .50) / (8 * .85) =
225 / 6.8 = 33.088
We can round that off to 33 pounds-per-hour per injector. To convert to CC/minute for injectors rated that way, it's roughly 10.5 cc per PPH, so:
33 * 10.5 = 346.5 CC/min
As we can alter fuel pressure to change flow rate, we could use 32 pound injectors with about 3 psi extra fuel pressure to get 33 lb/hour - if necessary. The extra margin with our 85% DC may be sufficient with 32 PPH injectors. Likewise, we could use 380cc or larger injectors and drop the pressure a bit.
For your bearings, use minimum to average stock clearances for street operation, and only larger (max stock) clearances if you are planning to rev really high. The extra clearance is to allow more oil to leak past the bearings for cooling with very high power or revs, or extended power modes like offshore boat racing. HTH
-=≡ If it was easy everyone would do it ≡=-
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