RBMZ28

Hey guys,anyone know the formula for proper injector sizing?thanks in advance.

------------------
87 IROC WITH 409(.060OVER400) MINIRAM,NOS(125HP)TRICK FLOW PORTED HEADS,C.C.CAM,FLOWTEK HEADERS,CUSTOM PROM,MODIFIED 700R TRANS,2400 B&M CONVERTER,3.73POSI REAREND.
LOOKING FOR THE 11 SEC. ZONE.

Willie

iTrader: (1)

Unfortunately, there is no such thing. Determining proper injector size is primarily a hit and miss issue. From the article, "Go With the Flow -- How to properly size an injector to match your engine's fuel requirements" by Ray T. Bohacz,

quote
One of the most common areas of confusion in the world of late-model performance is the proper sizing of the fuel injector and the corresponding issue of what fuel pressure to utilize. Whenever discussing this topic with racers of injected iron, you will uncover a multitude of injector size variations for the same engine configuration and just as many opinions about what is the hot ticket for the fuel pressure setting. This confusion and the lack of understanding have prompted HTP to once and for all set the record straight about injectors and fuel pressure. Since this article is going to reference only the relationship of fuel pressure vs. injector flow and the effect that BSFC has on the ability of the injector to support horsepower...."
unquote

At a BSFC of 0.47, a 19# injector will support 346.72 hp at 50 psig. If 22# injectors are used in the same application, fuel pressure must be reduced to less than 40 psig (not good) to sustain the same horsepower level.

The true method to determine whether your injectors are not large enough is to observe O2 sensor voltage at WOT. Chances are with 19# injectors (on a 305 or 22#ers on a 350), you will be able to sustain 850mV O2 readings without exceeding 80 - 85 percent injector duty cycle.

Although my case is not the norm because my duty cycle is in excess of 130 percent, I have been able to sustain O2 voltage in the 900mV range with my stock 19# injectors and still be able to run the times in my signature. Because my duty cycle is excessive, I will be going with larger injectors in the near future. This move is not to increase my performance (which larger injectors will not do), but to decrease my duty cycle.

------------------
Willie

Supercharged 1987 305 IROC-Z, Daily-Driver, Emissions-Legal.
Former Paxton (6-psig) with 50-hp nitrous: 12.043 @ 112.86 mph.
ATI D1SC (10-psig): 12.056 @ 116.62 mph.
All stats are altitude corrected for 3,100 feet using NHRA's Altitude Correction Table.

http://willie.camaro-firebird.org/

1987 "20th Anniversary Commemorative Edition" Z28 Convertible -- Super Chevy Show Class Winner, 1998.

RBMZ28

willie ,you don't think reducing the duty cycle would improve performance since being over 100% , hence lean/leaning out?



------------------
87 IROC WITH 409(.040 OVER400) MINIRAM,NOS(125HP)TRICK FLOW PORTED HEADS,C.C.CAM,FLOWTEK HEADERS,CUSTOM PROM,MODIFIED 700R TRANS,2400 B&M CONVERTER,3.73POSI REAREND.
LOOKING FOR THE 11 SEC. ZONE.

Willie

iTrader: (1)

No. That's because my O2 voltage is in the low 900mV range at WOT throughout my rpm range. That's on the rich side, but because I run a supercharger, I would rather be slightly on the rich side.

Physically, an injector cannot be open more than 100 percent. My ECM is commanding much more, but even when they are constantly open, I achieve a slightly rich air/fuel ratio. By installing larger injectors, I will have to reprogram my EPROM for a shorter duty cycle. BUT my engine will still see the same amount of fuel per given time interval. That means my duty cycle will be shorter, yet my air/fuel mixture will not be affected. Can you see why no performance would be gained?

Willie

87tpi420

so willie... are my 35# injectors too big for my 420? just curious what a 700 hp motor should have for injector size

------------------
87 z28 bowtie aluminum 420sbdry sump crower lt. wt. knife edged crank oliver lt wt rods brodix track 1's super ram intake accel dfi art carr 700 r4 .....other parts on the way ,soon to be finished!! couple more weeks!!!

[This message has been edited by 87tpi420 (edited July 18, 2001).]

Willie

iTrader: (1)

It depends on what your fuel pressure is. At a BSFC of 0.47 (a good value to use for a fuel injected engine), a 36# injector can support 720 horses at 60 psig. Unfortunately, my chart jumps from 36# to 55# injectors. A 55# injector can support 900 horses at 40 psig.

Willie