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Location:
Finland
Registered:
November 2002
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Re: What kinda injectors?
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Tue, 05 August 2003 15:21
 
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I used this rough formula...
7 * (Estimated)Power (kW)
cm3/min = -----------------------------
Number of injectors
And here's a clip from MegaSquirt manual...
(MegaManual Version 2.02 ©2003 Bowling & Grippo Page 45)
Injector Selection
In order to properly install your MegaSquirt, you need to select and install fuel system components appropriate for your engine. Most important is that you have fuel injectors that are the correct size. Injectors that are too large will make it difficult to tune the engine at idle and cruise. Injectors that are too small can starve the engine of fuel at full power, and
seriously damage your engine. To determine how big should your injectors should be, multiply estimated horsepower (HP) of your engine by the brake specific fuel consumption (BSFC)* and divide by the number of cylinders and the desired duty cycle and you'll get a rough estimate of injector size:
InjectorSize = (HorsePower * BSFC) / (#Cylinders * DutyCycle)
for example, a 135 horsepower gasoline fuelled 4 cylinder engine with 2 throttle body
injectors and 0.55 brake specific fuel consumption gives:
(135 HP * 0.55 lb/hr/HP) / (2*.85) = ~ 43.7 lb/hr
Injectors rated between 42 and 45 lb/hr would be okay in this case.
*BSFC is usually between 0.42 and 0.58 at maximum power. Normally aspirated engines with efficient combustion processes are at the lower end of the BSFC scale [~0.45], supercharged
engines tend to be towards the higher end [~0.55].
Or you can use the following chart to select injectors based on the total horsepower of your engine and the total number of injectors:
Number of Injectors
HP 1 2 4 5 6 8
100 59 (620) 29 (305) 15 (158) 12 (126) 10 (105) -
150 88 (924) 44 (462) 22 (231) 18 (189) 15 (158) 11 (116)
200 - 59(620) 29 (305) 24 (252) 20 (210) 15 (158)
250 - 74(777) 37 (389) 29 (305) 25 (263) 18 (189)
300 - 88 (924) 44 (462) 35 (368) 29 (305) 22 (231)
350 - - 51 (534) 41 (431) 34 (357) 26 (273)
400 - - 59 (620) 47 (494) 39 (410) 29 (305)
450 - - 66 (693) 53 (557) 44 (462) 33 (347)
500 - - 74 (777) 59 (620) 49 (515) 37 (389)
550 - - 81 (851) 65 (683) 54 (567) 40 (420)
600 - - 88 (924) 71 (746) 59 (620) 44 (462)
based on 0.50 BSFC and 85% duty cycle
Supercharged engines should add 10% to listed minimum injector size.
Minimum Injector Sizes in lb/hr (& cc/min) for Gasoline Engines.
Another way to select injectors is to take them from an engine that makes nearly the same power as your engine will [assuming the same number of injectors].
Injectors seem to be rated in either lbs/hour and cc/min. The accepted conversion factor between these depends somewhat on fuel density, which changes with formulation (i.e., by
season), but the generally used conversion for gasoline is:
1 lb/hr = 10.5 cc/min
Injectors frequently have identifying numbers stamped on them. You may be able to identify
your injectors by looking on:
[[url]=urlhttp://www.geocities.com/MotorCity/Pit/9975/dat aBySubject/Injectors.html]urlhttp://www.geocities.com/MotorCity/Pit/9975/dat aBySubject/Injectors.html[/url][/url]
or
http://www.telusplanet.net/%7Echic hm/tech/injectors.pdf
Injectors should not be used at much more than 80-85% duty cycle. However, injector rates are always specified at 100% duty cycle and some nominal pressure (usually 43.5 psi = 3
atmospheres). The manufacturer leaves it up to you to determine a system pressure and maximum duty cycle in order to compute the resulting flow.
[Updated on: Tue, 05 August 2003 15:26]
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