Thread: How to size fuel injectors?

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Carb guy here...

I understand that carbs can be sized pretty close using engine cubic inches and % of efficiency. That makes sense to me.

Is there a similar rule of thumb to size injectors?

How about an example of 383, profiler heads, 9.6 static comp, cam with 230 degrees at 50 on both intake and exhaust and .575 lift?
Then, how much larger injectors if blown with 6lbs boost later in engine life?

What happens if you put on much larger injectors than needed? Can the control use only the amount needed or is a squirt one size no matter what?

Thanks in advance for the teaching.

Originally Posted by Scooting

What happens if you put on much larger injectors than needed? Can the control use only the amount needed or is a squirt one size no matter what?

The problem is turn down - the ability of the injector to respond to a small demand. If your injectors are over sized the minimum amount that they can "squirt" will be more than you need at idle and low rpm range into mid-range. That's one of the benefits of having eight smaller injectors. They can each turn down to small volume, but together meet the need of WOT.

There are two types of Injectors-LOW IMPEDANCE and HIGH IMPEDANCE (think about Subwoofers-they can take more power if they are low impedance)-

Most LOW IMPEDANCE injectors are engineered to "SNAP" open with more current than a HIGH IMPEDANCE" injector. They snap open faster because of less resistance in the coil, hence they are more controllable. Generally, the more horsepower you have, the more you need the peak and hold injector (faster reacting opening and closing). The high impedance injector was designed to accomodate the OEM market for reliability and lower flow arena where absolute reaction time of the internals are not as critical. This is a cost savings for the OEMs without hurting the performance of the total fuel control package (ECU, injectors, etc.). The reason MOST people ask for the HIGH IMPEDANCE injectors is that they want to use them with their FACTORY ECU. If you have the choice, I would suggest the LOW IMPEDANCE injectors to optimize your injector linearity (FLOW PERFORMANCE) and overall tunability. IMHO, not all Injectors are created equal, though-

In my Car, I am running a large, low impedance Injector (to handle 800 hp+), but I wanted it to have good manners around town, so my Injectors are from Injector Dynamics (IMHO, the best there is right now):
Injector Dynamics - The standard for tuners and car builders worldwide.

They also have a "Library" where they discuss various topics: The Library - Injector Dynamics

"What happens if you put on much larger injectors than needed? Can the control use only the amount needed or is a squirt one size no matter what?"

Basically, that is what I am doing-using larger Injectors than needed (most of the time)-you control the Fuel by "pulsing" the Injector (in milliseconds) to get the desired Fuel rate-the trick is to "controlling" the fuel rate on high hp engines-

As far as what size of Injector to use, this may help:
https://www.rceng.com/technical.aspx...BumsfW2BlNBghf

hope this helps-

This is just what I was wanting to know about injectors. That RC website is the place to go. The online calculators are really setup well. Looks like I'm gonna need 40lb injectors )) thanks for getting this up there.
HotRodN

Agree on the good information given. Thanks. It has helped a bunch.

I never knew injectors cost so much so that was a bit of a surprise. I have an aftermarket manifold and will need to do fuel rails also. Aftermarket ones seem to be way overpriced for what you get. Again a surprise on my part. If I was more knowledgeable on this from the start, I am not sure I would go this way. Sure hope this works out good as far as drive-ability and power goes.