A question about fuel injector flow ratings

[bobc455]

I am overanalyzing here, and getting all confused. (Gee, never done that before)

Let's say that I have an 83# injector. Well if I reference the FP to boost and the fuel pressure drops at idle (let's say from 43# to 36#), do I now have a 74# injector? The ECM thinks it is an 83# injector, so are all the calculations messed up?

Or, when I set fuel pressure, should I set it to 43.5# over atmospheric pressure?

I'm trying to figure out if the injector size used in the ECM should change depending on whether you reference the fuel pressure to boost.

More importantly, I'm trying to figure out why my buddy TyrFryer has to have many of his VE values over 100 (like 120 or more).

Thanks,
-Bob Cunningham

 

[JDEstill]

If the fuel pressure regulator is referenced to manifold pressure (the way a TR typically is) then there should always be a constant pressure difference across the fuel injector. This will make its flow characteristics the same for any manifold pressure. Example - set the fuel pressure at 45 psi, vac line off. Put the vac line back on, then when the manifold is at 16" Hg (about -8 psig ) the fuel pressure drops to 37 psig, and you have 45 psig across the injector (37 - (-8) = 45). And at 20 psig boost the fuel pressure rises to 65 psig, and you still have 45 psig across the injector (65 - 20 = 45). So the injector always behaves like it is at 45 psi.

If the regulator is not referenced to manifold pressure, then the injector will get "bigger" or "smaller" as the manifold pressure changes. Extreme case, suppose a TR didn't have the vac line to the regulator hooked up. At 20 psi boost the fuel pressure would still be 45 psig, and the difference across the injector would be 45-20 = 25 psi. A 50 lb/hr injector (at the normal 45 psi difference) would become the equivalent of a 37 lb/hr injector at 20 boost, but then it would also be like having a 54 lb/hr injector at idle.

With the problem you are looking at, the effectively smaller injector would require more fuel (ie more pw) than what the FAST would give if the actual engine VE was being used. An abnormally high VE could be used to crutch that problem, make it think the air flow is higher than it is, and so have it give more pw than it otherwise would. And at idle the VEs would have to be lower than the "real" VE, to compensate for the "bigger" injectors.

John

 

[JDEstill]

And to answer your actual questions...


Let's say that I have an 83# injector. Well if I reference the FP to boost and the fuel pressure drops at idle (let's say from 43# to 36#), do I now have a 74# injector? The ECM thinks it is an 83# injector, so are all the calculations messed up?

Nope, since the manifold pressure drops just as much as the fuel pressure, the difference between the two is always the same. So the 83# size is always correct.



Or, when I set fuel pressure, should I set it to 43.5# over atmospheric pressure?

Always a good starting point.



I'm trying to figure out if the injector size used in the ECM should change depending on whether you reference the fuel pressure to boost.

If you don't have the fuel pressure referenced to manifold pressure, its like having a bunch of different injector sizes, and there isn't a single "right" injector size to enter into the FAST setup. With a n/a motor, the range of pressure difference is probably small enough to ignore this issue. With a forced induction engine, the range of pressure differences is big enough that it could cause some headaches.


John

 

[norbs]

Here's another good question, on lbs/ hour. The FAST system displays actual lbs/pr hr. If i drive it shows about 30 lbs/hr @75 mph. Based on a gal of gasoline weighs 6.5 lbs. i get a fuel comsumption of 4.62 gals, if i were to drive for 1 hour at 75 mph. That works out to 16 mpg. Do you think this is accurate ?Is this based on 43.5 psi at the injector used a a constant in the fast lb/s calculation?

 

[bobc455]

Re: Re: A question about fuel injector flow ratings

Originally posted by JDEstill


Or, when I set fuel pressure, should I set it to 43.5# over atmospheric pressure?

Always a good starting point.



John [/B]

So if I'm setting pressure when the engine is off, I should set pressure to 43.5+14.7=58.2?

That means injector flow ratings are when the injector is firing into a perfect vacuum? I would have thought the injector flow ratings were tested into atmospheric pressure.

-Bob

 

[Craig Smith]

This is the difference between absolute and gauge pressure. Since the gauge reads zero with the pump off and not 14.7, you should set static pressure to 43.5 psi. I don't know if this is as relevant with fuel pressure measurements as it is with air pressure measurements but that's the concept.

Let's say you were making 43.5 psi of boost. As far as pressure readings on a gauge are concerned, that's 3 bar of pressure. But with a MAP sensor, it requires a 4 bar sensor to read that pressure level because absolute measurements have zero pressure, not atmospheric pressure, as a reference point.

 

[bobc455]

Originally posted by Craig Smith
This is the difference between absolute and gauge pressure.

D'oh! That's where I goofed. Thanks for uncramping my brain.

-Bob C.

 

[bruce]

Set injector size.
Set fuel pressure at rated PSI.

Adjust tables as necessary.

Unless your doing the code end of things, the above should all be done in the code end of whatever your working on.

If your really into wanting to see the code work then you need to build an ecm bench and spend alot of time dinkin with things.

Worring about how the code runs, or entries on a table, won't make the car any faster.

WB, and some way to measure time, tells what you need to know as far as tuning.

Yes, I've spent wayyyy to much time worring about things that really don't matter when the bulb turns green.

Just make sure you don't exceed the permitted entry for any table.