Hi all, Numerous times I've read here how these flow 24lb only under Ford's 39psi fuel system and higher under GM's fuel system at 43.5psi, whereas a few say they really are 24lb on a GM system. I'm using these injectors to feed my 305 TPI and had them flow checked and cleaned before installation (second hand injectors). At 3 bar pressure and 100% duty cycle they averaged 271cc. Converting these numbers, 3 bar pressure is 43.5psi and 271cc is 25.81lb and this final number seems consistent with the masses in this forum, i.e it's higher than 24lb. So the question is, when you read that a given injector is x number of lb, what duty cycle is that considered to be measured at? 100% duty cycle seems logical to me based on the numbers I got when I had my injectors flow tested. I also have figures for 50% duty cycle, but the flow seems to be less than half that achieved at 100%. Thoughts anyone?

If you are running a Maf system and looking for the injector constant number, it is a chicken or the egg first, kind of thing.

Your maf calibration is tied to the injector constant in the bin. What you wind up doing is setting the constant to your best guess at rated flow and then calibrate the maf tables to the maf sensor.

I have seen MAF sensors new, rebuilt, used, and thin film all take a major change in the injector constant or maf table calibration to get back to a 128 at cruise blm. I am talking about 23# clear up to 32# this is with the same car only changing the Maf, the order of air flow was lowest used, new, rebuilt, and then the thin film.

Air flows at WOT peak were 187, 215, 235, 255. This makes a 36% difference in your fuel flow at the same "actual air flow". Idle Blm's would change from 160 to 108, without changing the injector constant.

I am still debating which to use, but the 215 new maf needs a 24# IC to be at 128 and the 235 rebuild needs a 30#. That is a 10% difference in air flow and a 25% difference in fuel flow.

I run the 24# SVO at a 50# fuel pressure so both are off. The IC math says 25.5# to 27#?

So basically I need to pick an IC and then tweak the maf tables to be sensor specific as not to have to change all the rest of the tune AE, PE, and ect.

At warmer weather I will install the WB and see how far off the AFR is.

Thanks for the reply, it seems no one on this forum knows what duty cycle injectors are measured at judging by the number of replies. But as you say, the injector constant math seems to say 25.5 to 27lb, and since I end up with 25.81lb that seems to fit with the range.

If I set my IC to be 25.5lb then I can at least calibrate around that since I know what my injectors actually flow at.

How do you go about calibrating your MAF? I thought that I could data log while having a digital volt meter hooked directly up to the MAF and have the engine running in drive, set the throttle to exactly each voltage in the MAF tables in turn, and data log actual grams of air recorded. Then, change the grams/sec value in for each voltage in the tables to be what I record.

Sound like a good way of going about it, or do you know of a better way?

instead of a volt meter, you can get an ALDL cable from moates.net as long as you have a laptop with a usb port and do it that way.

Thanks for the idea, but I've already made a cable of my own, and the MAF voltage isn't part of the ALDL data stream. Be nice if it was though.

Watch your blm's at a normal cruise mph (55), If your lucky they will be 128. If the blm is lean (over 128) raise the grams/sec at that exact point (volts), if rich (under 128) lower grams/sec number. A lot of guys use %.

I adjusted the injector constant to get the most blm's at 128, then corrected all that were high or low, then readjusted the injector constant back to actual and moved all the maf values by the same percentage.

OK, sounds good. How do I factor this is with setting the timing tables as won't this also affect BLMs?

If your within the adjusting range of the ECM blm's between 108 and 160 the steady state running will be the same. 110 blm has the same injector pulse width as a 150, rolling the injector constant up and down will only change the blm not the pulse width.

If you hit the ends, then the pulse width will change and everything is affected. Keeping the injector constant close to the "actual" fuel flow keeps all the other tuning values close to "original".

I picture it as trying to set an object on a table. The more level the table is and the closer to the center you can place it, the more stable and tip resistant it is.

If your tune is like a "house of cards" rather then a brick, it will be subject to challenges.

Thanks for that info. I'm not sure though how that relates to my question about adjusting the timing table, if they do need changing at all. I suppose another way to think about it, under what modifications to one's engine do you need to consider altering the timing table?

If you have knock or ping causing the ECM to retard spark then it needs lowered.

Changing the air flow to match "actual" will change the LV8 and this will change spark advance, but not a big amount.

Get the fuel close then do the spark. You might have to tweek one and then the other, and then go back over them again.

SD systems are more prone to spark vs VE inter reactions.

Manufacturers rate the injectors at the maximum amount of fuel they can flow in a given amount of time. They run them on at 100% of the given amount of time (100% duty cycle) and with a set amount of fuel pressure which most likely will be the 43.5 psi.

Given information:

Ford fuel pressure: 39 psi
GM fuel pressure: 43.5 psi

Conversion (43.5 psi - 39 psi)/43.5 psi = .103448

.103448 x 24lbs/hr = 2.48275 lbs/hr (difference)

24 lbs/hr + 2.48275 lbs/hr = 26.48275 lbs/hr (adjusted for GM fuel pressure)

I can see how the injectors would flow differently with the difference in the fuel pressure.

Which do you think would be better, having my injector constant set to the calculated value that you've shown, or the measure value that I came up with? Numerically it's not much different, but in terms of an injection system it could be, but I don't know the answer.

I think you have an advantage that you had a shop actual flow bench the injectors.

Now having said that, there are two trains of thought on tweaking the fueling of the engine with the injector flow rate constant.

Some say to set it at what you know the injector constant flow rate to be and then adjust the tables to richen/lean out the mixture. Others say to globally adjust the injector flow rate and then go back and tweak the tables.

That decision is going to be up to you.

I started out by adjusting the flow rate globally and noticed that I started having little tuning issues pop up. Idle gremlins, spots where the tune was way off; its hard to say what may pop up. If you look at some other posts, long time tuners will have said the same thing. They even may have changed their minds about adjusting the flow rate to make a wide spectrum change.

IMHO, I think if you are a beginner its easy to adjust the injector rate and learn how to tweak the tables and modify the settings to get your feet wet. I think you begin to see how the changes you make begin to affect the performance and the quality of the tune.

You may or may not eventually realize that the better way would be to set the injector rate at the best known rate and then compensate with a constant spread across the tables globally to lean/richen the mixture or just make adjustments to specific tables. Its more work but may yield more quality in the end result.

I dont think there is a right answer, just knowing about whats going to happen if you choose either way of doing it is enough for you to make your own decision. Someone told me this before, almost in this exact wording and it didnt make sense to me, now its starting to sink in.

Lets start over. If the injectors flow 24 #/hr at 39 psi the conversion for the higher fuel pressure (43.5 psi) is such:

new flow = old flow * sqrt(new psi / old psi) = 25.3 #/hr = 24 * sqrt(43.5 / 39)

Not a lot of difference.

For the flow test that was done: 3 bar pressure is 43.5psi and 271cc is 25.81lb

The issue here is what is the weight of the fuel? This is required to convert from cc's to pounds. Back in the 70's and 80's fuel here in the US weighed 5.7 pounds per US gallon. Today it weighs between 5.8 and 6.1 pounds per US gallon.

So there can be quite a discrepancy in what the injector flows in a #/hr rating.

Your best bet is as others have posted, tweak the injector flow rate in the calibration to get the BLMs mostly in line. This would be for cruising through moderate throttle. Ignore the BLMs at idle and WOT.

Then adjust the MAF tables along with the injector short PW and battery compensation tables. Once this is in line you can then work on getting the SA squared away.

RBob.

Get an AFPR, bump down FP to 39 PSI at idle with vac line off, then use 24# IC in the .bin, then tune for stoich 14.13 (due to the crap E10 gas we have nowadays) and you'll be fine....

Bumping up or down the FP from what an injector is rated at will change spray patern (from what I've been told).

Thanks for all the feedback guys, much appreciated.

I've been studying a recent logging session where I was cruising at 60-63MPH, and for a part of it, the INT and BLM where sitting perfectly on 128 in cell 10 and my throttle was constant also. But, given the 128 BLM, I noticed the O2 sensor voltage fluctuated under 100mV - doesn't this indicate lean?

Otherwise, the BLM eventually came into line to very close on 128 at about 60MPH, coming down from 136. I really wonder if I should get myself a wideband setup, or just take the car to someone to it tuned right. Otherwise I can see that this could potentially take up a lot of my time and it's doing my head in already...

Pandin