Injector Size?

I built my engine with 19lb/hr injectors thinking that since I used the stock ford FI system they would be big enough. After all, they are about 1/3rd bigger than the stock 14lb/hr units.
Well, unless I have some other problem that I am not seeing, they aren't. I guess with the EGR and the smog pump out of the picture, and with a little porting and slightly oversized valves, and with maybe just a little bit of a cam and Chevy rockers, the damn thing is flowing too much for the injectors.
My original plan was to build the engine so that if I wanted to I could upgrade it without tearing it down all the way. I thought that with a single exhaust and the stock intake, it would limit horsepower and make a nice, efficient workhorse of a pickup truck engine. Then, I could throw a manifold and 4bbl on it, or maybe upgrade the FI somehow to make it fast if I ever wanted t do that.
Now, at about 2500 rpm the injectors can't keep up at WOT. So, I guess I need to figure out what injectors I should get. The problem is, that I don't know the horsepower, and I couldn't know the horsepower without being able to run it flat out even if I did have a dynamometer, and I can't get injectors that let me run it flat out until I know the horsepower.
So now I either start changing out injectors at $200/guess, or start asking you guys what you think.
So what do you think? Has anyone had to sort this out before? Here are some calcs I did that I would really appreciate it if someone would look over. I have certainly slipped a decimal or two in my day...

I calc my max HP that 19lb/hr injectors can support as 182.4
to support 250hp, I calc that 26.04lb/hr injectors would be needed.
Go up to 300hp, and it is 31.25lb/hr.

I thought I had plenty of room with these because of these two things, one of which was dumb, the other still seems strange to me. The dumb thing was that I looked at a chart somewhere and it said that 19lb/hr injnectors would support up to ~240hp. I didn't think about it at the time, but that was obviously for an eight cylinder engine with two more injectors to share the work. The thing that still seems strange to me is that Ford used 14lb/hr injectors. That means that my 19lb/hr units are some 35.7% larger. With no modifications to the plenum or to the exhaust, I really didn't think I would need to increase my injector size more than that!
Thanks in advance if you bothered to read this.
A.L.T.

I don't know where you got your math.

19 lbs/hr x 6 x 90% duty cycle / 0.5 lbs/hp-hr = 205.2 HP

Your 19# injectors would be big enough if they were running at 39 PSI. If you still have the stock 58 PSI back pressure regulator, they are actually flowing quite a bit more.

19 x (58/39)^0.5 = 23.1

and

23 lbs/hr x 6 x 90% duty cycle / 0.5 lbs/hp-hr = 248.4 HP

Whatever your problem is, it's not injector throughhput

Whats your symptom? You should be good to around 200 with those and it does not seem like you have done anything to that motor that should be too far out of that range. Can you get it to have the problem and log it?

The difference in the math is that I used an 80% duty cycle:
1/2 hp = lb/hr * no. of injectors * .8
1/2 hp = 19 * 4.8
hp = 182.4
is exactly what I was thinking...

I have been getting an erratic O2 sensor reading intermittently too. I am beginning to wonder if I don't have a weak injector. I had assumed that I had a problem with the O2 sensor because I would get erratic readings when I was idling and I had the engine running deliberately rich to test it. Now I am thinking that maybe five of the six cylinders were rich, and made the exhaust look and smell rich, but that one of the cylinders was running lean because of a bad injector and causing the leanburn reading on the O2 sensor.

The exact symptom is that the O2 sensor reads lean and the engine knocks if you get on it too hard. For obvious reasons, I don't like to make it happen and I regret that I don't have a good, clean log stored that shows the problem clearly.
I kept on throwing more fuel at the problem to fix it, but that didn't seem to help. Then I noticed that my pulse width was maxed out and thought "my injectors are too small, I am telling the ecu to put in more fuel, but since the injectors are maxed out it can't."

I guess the thing to do is pull the plugs and look to see if one is reading lean. Anyone else got any ideas?

PS - I just looked at my original post and it is a little misleading. I used the stock Ford hardware, but the computer is a megasquirt...

What fuel pressure are you running? If you still have the stock regulator, even at 80% you're good to 220HP. Also you probably have some parameter in the program wrong since the injector throughput is higher than the nominal 19 lbs/hr.

What fuel pump are you running?

What is your TOTAL timing advance at 2500 RPM?
What you're hearing might be spark knock due to too much timing rather than detonation due to an overly lean condition.

I thought about spark knock. I have the total advance limited to 20 degrees for the entire range of operation. I was worried about spark knock so I cut it back to 20 and figured I would advance it slowly. The distributor is locked - there is no mechanical or vacuum advance, and I verified the ECU thinks the timing is what it actually is with the good old analogue timing light.
The parameters in the program might be wrong I will check everything again, but they still eventually control the pulse width and I still monitor the pulse width - and my pulse width is maxing out at about 30 milliseconds for that RPM range. When it is maxed out, I still see a lean burn on the O2 sensor and get a knock.
I am running the stock regulator, I will check the fuel pressure a little more extensively. Maybe it is starving for fuel, but I don't think that could be it. There are too many other things that would go wrong if there was a problem getting fuel to the system... Unless the pressure isn't going up when the vacuum drops.

Are you batch or seqential and do you have some noid lights, I had almost the same thing and one batch was not firing the injectors and I would constantly go lean and it was hard to tell on the plugs since they were firing with no fuel, finally followed up noid lights with the suspicion of different exhaust temps using an infrared heat sensor gun, you may have a ground problem in one of the batches, those injectors are plenty big enough to at least 5000. If this is the case, watch your vacuum pressure, it will be lower than normal

I am batch fired. I alternate between 1,2,3 and 4,5,6. I don't have lights, but I think i would have bigger problems than what I have now if I was loosing a signal to three injectors. I have all my wiring soldered and everything is shunted rather than spliced. I am using a 12 gauge wire to all six injectors for my hot, and the grounds are sunk through the MSII. Noid lights would be kind of cool, but I am pretty fed up with the project at this point, and need to start using the truck to gather firewood...

I pulled out the plugs, and No. 3 was looking super lean. The deposits were so light it made it look like the plug had never been used because they were the color of the insulators. All the other cylinders were rich looking, but not too far away from normal. I was very careful when I put the engine together, but I wonder if some piece of rust or something from the boneyard fuel rail didn't find its way into the filter on the injector.

Anyone know how to clean and test a fuel injector? I honestly don't have that much experience with them...

You can swap it with the richest cylinder. Then see if the problem follows.

One other quick test is to jump the grounds to injectors and if it runs good, one bank is not firing.

Alfred Lord Tenniscourt":1t3562ck said:

I have been getting an erratic O2 sensor reading intermittently too. I am beginning to wonder if I don't have a weak injector.

Click to expand...

Is this O2 sensor a wideband or regular narrowband? A narrowband is little more than useless for doing actual fuel tuning. Also keep in mind that even a rich missfire will show as a lean condition on a lambda meter. A log file would be helpful too.