Injector Sizing for High Altitude

Frank Z · Nov 17, 2006

Since I dropped off my block and assorted parts at the machine shop yesterday I thought it was time to do a litlte injector research,
I've read quite a few threads regarding Ford injectors, Accel injectors, different psi ratings, etc., but I haven't come accross any info regarding proper sizing based on lower air density at high altitude. My stroker is going into a DD that's driven at 6000' above sea level and our off-road trips routinely take us above 11,000 ft and higher.

Your thoughts.

Gerr · Nov 17, 2006

good question

xjbubba · Nov 17, 2006

The ECM automatically calibrates itself each time you turn on the ignition key. It uses the MAP sensor to indicate changes in atmospheric pressure. You don't need different injectors because of altitude. If you improve on the "volumetric efficiency" of your engine (better breathing through head work, bigger cam, exhaust, more cubic inches), then you could justify larger injectors to go along with a bigger throttle body.

CRASH · Nov 17, 2006

Will this engine have larger than stock displacement?

badron · Nov 17, 2006

The computer on the Cherokee is not perfect by any means. None of them are as fare as that goes.
Small changes in injectors have shown improvements under some condition. Small improvement at that.
But at 11K feet and at WOT when in the open loop mode you may very well see more HP with smaller injectors. This kind of thing is hard to call. Improvements tend to be very small if any, Any final determination must be done by time and money consuming testing.
I would start with the stock stuff get some baseline numbers and go from there.
(IMO) The BANG-PER-BUCK just ant there.

Frank Z · Nov 17, 2006

CRASH said:
Will this engine have larger than stock displacement?

Yes, I'm building a 4.6l stroker motor.

xjbubba · Nov 17, 2006

With the larger displacement, you'll probably need larger injectors; however, at higher altitudes, there's less oxygen, so less fuel would be required. If you haven't done so, you need to reSEARCH stroking the 4.0. There's probably a lot of little things to be addressed to get it to run right.

Dr. Dyno · Nov 18, 2006

xjbubba said:
The ECM automatically calibrates itself each time you turn on the ignition key. It uses the MAP sensor to indicate changes in atmospheric pressure. You don't need different injectors because of altitude. If you improve on the "volumetric efficiency" of your engine (better breathing through head work, bigger cam, exhaust, more cubic inches), then you could justify larger injectors to go along with a bigger throttle body.

x2.
You need about 15% more injector flow in a 4.6L stroker that has all the external bolt-on performance mods plus ported head/camshaft than you do in a bone stock 4.0.
Ford 24lb injectors are usually the ticket for '87-'95 Renix or OBD 1 strokers with a 39psi fuel pressure regulator and the same ones also work well in a '96+ OBD 2 stroker.

Frank Z · Nov 19, 2006

Thanks all.

old_man · Nov 19, 2006

I went with the 24# injectors on my 4.7L stroker with headers and large exhaust. Here at 5000ft I failed emissions. It was running too rich. I built a MAP adjuster and all is well. Personally I think if I were to do it again I would shoot for 21#.

Dr. Dyno · Nov 19, 2006

old_man said:
Personally I think if I were to do it again I would shoot for 21#.

The 21lb'ers would be too small if you came down to sea level. I'm running 24lb'ers in my 4.6 stroker with the fuelling optimized at a MAP adjuster setting of 5.10v.
The engine computer automatically shortens the injector pulse width when the MAP sensor sees a lower manifold pressure at higher elevations, so there's less fuel to go with the lower air density. There's no need to install smaller injectors just because you live at a higher elevation Tom.

Injector Sizing for High Altitude

Frank Z

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Gerr

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xjbubba

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CRASH

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badron

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Frank Z

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xjbubba

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Dr. Dyno

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Frank Z

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old_man

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Dr. Dyno

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