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impco FPR model J vs cobra & what is W.C.?

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fixitandy
Pennsylvania, United States

So I am mostly an electics guy, but I never shy away from ICs when they come my way. Nearly all the LPG powered ICs that I ever see have nissan H20 engines with impco model J FPRs, but last week I had a service call on a Mazda powered yale with a bad FPR, it was buzzing and freezing up. I went to replace it with a model J, but found that it had an identical looking cobra FPR. I noticed that the outlet pressure was .2 W.C. as opposed to the .08 W.C. of a model J. More than twice the pressure of a model J (but still not a lot). So I had my parts guy order a "correct" replacement FPR, and what I recieved (from total source) was a cobra with an outlet pressure of .08 W.C.! What could the possible difference be between a model J and cobra with the same outlet pressure? They look identical in every way. Also what does W.C. stand for? water column? why not use psi or bar?

The engine ran good enough with the .08 cobra when I installed it, but I wasted a lot of my customers time and money if the model J would have done the job just as well.

Posted 22 Apr 2009 02:27 PM

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mike_n
Alberta, Canada

Yep, it's water column.
They don't use psi or bar because the pressure is so low.

1.0" WC = 0.036127291827 psi

Posted 23 Apr 2009 06:00 AM

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justinm
New York, United States

if its frozen
its most likely a water problem unless that caused one of the diaphragms to rip

model J's are discontinued
all the primer buttons were removed from new units (lawsuits, ppl were blowing themselves up from holding that thing in)

helmar still sells model J's but it must be old stock

basically the cobra replaced the model J

there is another cobra that is for emissions trucks
becareful what you order

-------------------------
New York, New York its a heluva town ..you know that The Bronx is up ..and im Brooklyn down

Posted 24 Apr 2009 11:15 PM

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fixitandy
Pennsylvania, United States

there was not a coolant circulation problem on this engine, and model Js are still available, they just don't have the button anymore. I've got 2 of them on my truck now. mainly I wanted to know why the cobra has the same outlet pressure as the model J, but is a different model, and if there are different cobra models in the same housing since the stated outlet pressure on the failed cobra was more than twice that of the one that replaced it.

Posted 25 Apr 2009 02:46 PM

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jeremy_s
California, United States

WC is the nominal outlet pressure
You can get convertres with different secondary springs.

Standard Cobra=
Blue spring: -0.37 kPA (-1.5 w.c.)

Cobra I=
Orange spring: -0.12 kPa (-0.5 w.c.)

CobraII=
Red spring:  -0.05 kPa  (-0.2 w.c.)

Model Js have the same optional springs and pressure ratings, I usually just use Cobras.  There are only slight variations in the dimensions.  70.3mm vs 73mm overall depth, 117 mm vs. 118mm overall width.  These ratings and dimensions are from an IMPCO Catalog that I just happened to have sitting in front of me at my home computer.

If you are working on some carb systems there will be an additional vacuum port on the front cover of the Cobra Converter.  The computer determines the mixture requirements from the O2 sensor, and in some cases it combines a map sensor,  to energize the fuel control solenoid valve.  When energized it puts vacuum on the other side of the diaphram, hence the vacuum port, which essentially lowers fuel pressure and leans out the mixture.

Posted 27 Apr 2009 10:07 AM

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