Blackbird

I am a bit confused on this. I have 36# injectors in the car now. (98 formula) used to have the stock 28.6# injectors. The 36's are approx 26% bigger so I thought if I went the injector flow rate and multiplied the values there by 1.26 then I have the right injector sizing. Well I look at a pcm on a 98 done by superchips to compensate for 36# injectors and it looks like they icreased the numbers by 40% or multiplied the base numbers by 1.4

Where is my logic going wrong? Or are they kuku?

Thanks in advance,

Black LS1 T/A

Wouldn't it be best to look at what O2's are doing and increase or decrease the injector flow rate to get the O2's where they should be.

I'd start with where your aftermarket tuning already is and, if they are rich, bump it slightly down from there until the O2's were in line.

Better yet, if you could tune on a dyno, I'd shoot for the best A/F Ratio I could, monitoring the O2's, then check what the O2's do at the track for final tune.

Is this too simplistic a train of thought?

KenK

Black LS1,
You are thinking along the right path, but reading the O2's will only work at WOT, and the switching type O2's are not accurate there. You want to use them in closed loop What you should do for the injector sizing is to look at the Long Term Fuel trims. The PCM is always learning to correct the fuel when in closed loop. You want to adjust to get the LTFT to near 0

Ken

KenK

You are thinking along the right path, but reading the O2's will only work at WOT, and the switching type O2's are not accurate there. You want to use them in closed loop What you should do for the injector sizing is to look at the Long Term Fuel trims. The PCM is always learning to correct the fuel when in closed loop. You want to adjust to get the LTFT to near 0

Ken

Black LS1 T/A

Thanks, KenK

----

Try this link...

http://www.laroccas-performance.com/Calc/injector.htm

Perhaps the calculations will put you in the ballpark...

Brake Fuel Specific Consumption

Doc99SS

iTrader: (2)

Blackbird,

I think that you are going about it the correct way, increase the fuel constant by 126%, then look at your Lterm fuel trims.

If you have positive Ltrims, the PCM is adding more fuel becaue it detected a lean condition. If the Ltrims are negative, then the PCM is pulling out fuel because of a learned rich condition.

If you increase the fuel injector contant from its proper value, then the engine should run leaner, By contrast, decreasing the injector constant will tend to make the engine run richer.
At least this is the way it works on my OBDI system.

Try to get your Ltrims to be zero across the board or slightly negative. If the Ltrims are positive, the PCM will dump in this amount of fuel in during WOT up and above what the PE tables are adding.

XLR8NSS

iTrader: (4)

Hey Heath

Are those 36# injectors rated at our fuel pressure or a lower one? <img src="gr_images/icons/wink.gif" border="0">

Johnnie

Team ZR-1

Your logic is OK, just remember you have to dial in the values that are best for the overall makeup of your powertain and the mods it has.
I also used the straight math and that ended up too lean for my mods. I had to back down the values in injector volume table and now my LTFT is hoovering right around zero.
This is with using SVO 30 injectors which are replacing stock 26 lbs.

[quote]Originally posted by Blackbird:
<strong>
Where is my logic going wrong? Or are they kuku?

Thanks in advance,</strong><hr></blockquote>

[ February 22, 2002: Message edited by: Team ZR-1 ]</p>

Blackbird

[quote]Are those 36# injectors rated at our fuel pressure or a lower one?<hr></blockquote>

thats a good point I forgot about that. Our stock injectors are 28.6@58psi where as the 36'rs are 36@43.5psi I think it is???

so my math is flawed if that is correct?? the % difference should be higher.

yes or no?

Blackbird

well low and behold if I do the match that way I get what superchips got.

since the 36's look like 43's at my FP then 43/28.6 gives me a multiplier of 1.5

the car that superchips programed has a bit lower base FP thus the 1.4 number.

John@PACE

[quote] also used the straight math and that ended up too lean for my mods. I had to back down the vlaues in injector volume table and now my LTFT is hoovering right around zero.<hr></blockquote>

Lean verified by a Wideband?

Team ZR-1

No, from the long term fuel trim values after 1 hour of engine run time to assure those values are averaged out properly.

The problem is when switching injector sizes and the values used in the math.
I find that not all vendors use the supposted fuel pressure value of 43.5 lbs when flow testing a new injector to get its rating.
I find at times Ford uses 39.6, G.M 44.1 and then the 43.5 lbs, thus depending on which is used in the math changes what values to be used in injector volume table.

[quote]Originally posted by John@PACE:
<strong>

Lean verified by a Wideband?</strong><hr></blockquote>

Christian Millard

iTrader: (1)

The flow ratio is actually directly proportional to the square root of the pressure ratio. This might help:

Injector constant= (sqrt (new psi/rated psi) * flow lb/hr)

so 50# GM multec injectors are rated at 43.5 psi.
What are they delivering at 50 psi?

Flow = ( sqrt ( 50psi /43.5psi) * 50#)
flow = sqrt (1.149) * 50#
flow = 1.072 * 50#
flow = 53.605 lb/hr

Remember that exhaust leaks are the second worst tuning enemy (detonation is first).
Upstream or downstream exhaust leaks introduce fresh air to the O2 sensors, which cause a lean reading. This is the most common cause of poor fuel trimming.

A stethoscope hooked up to a vacuum hose is great to listen for leaks.
Pressurizing the exhaust (hooking low psi hose to the tail pipe) is the best way I've found to make sure there are no leaks. Exhaust shops never go into this much detail, too bad really.

Hope this helps,
-Christian

NicD

iTrader: (7)

[quote]Originally posted by KenK:
<strong>What you should do for the injector sizing is to look at the Long Term Fuel trims. The PCM is always learning to correct the fuel when in closed loop. You want to adjust to get the LTFT to near 0
Ken</strong><hr></blockquote>

Now should we use this method of adjusting fuel injector flow rates if we have stock injectors and are trying to get our LTFTs to 0 like a MAF translator does? If not, what do we use, MAF calibration?