Injector opening time questions...

[Ricky Trussell]

I posted this in another thread but thought it would get a better look on it's on.

What reason would you set your injector opening time other than
1.0mS ??

Say I have 42 lb. injectors with a VE at idle of 55 and a 1.0mS setting and I change my opening time to .5mS. Would I not be just leaning the engine out and have to change my VE table to higher numbers to get back to my A/F target??? looks like to me I would max out my A/F correction .

Yep, I'm a noob, but seeking a higher edumacation.

 

[Ricky Trussell]

Come on guys, all the knowledge on this board. Help a noob out!!

 

[Ricky Trussell]

tt

 

[Lonnie]

Typically you want it to be set to the actual opening time.

1mS is generally close, but if you notice that your VE table has large numbers (upper 70) in the idle portion of the map, your opening time may be too small.

On the other hand, very low numbers (30's) in this range generally signals the need for a shorter opening time.

This will have a dramatic effect on the idle quality/tunability as the % change of VE is greater with smaller numbers eg. changing the VE of 50 to 51 is 2% whereas VE of 30 vs. 31 3% change.

If you notice that dropping the VE by 1 number (idle/low speed only)has a dramatic change if fueling, you may not have enough opening time. I had 1 example where dropping from 60 to 59 changed the A/F approx 1.5 points due to the opening time being set to .8 on a slow set of 50# high impedence injectors. At the low end of injector output, the VE calculation can start to remove the base pulse width required to open the injector (due to too little of an opening time). When this happens, the fuel will virtually shut off with a small change in pulse width. If changing the VE value has little change, you may already have it too high. This error will be more noticible with larger injectors & bank to bank configurations than small injectors or sequential operation.

Please note that additional opening time has little effect on full throttle high RPM tuning .1mS of 20mS is roughly .5% (.055 A/F change at 11:1), but .1mS of 2mS at idle is 5% (.7 A/F change at 14:1)

Hope this sheds a little light on the subject.
There is by far more explanation required to fully explain this but it's a start.

 

[Ricky Trussell]

Thanks Lonnie.

 

[norbs]

That should be added to the c-com manual...

 

[Craig Smith]

<lookin around>

What manual?

Very well worded and helpful, I agree. I'm adding stuff to the manual right now so we can get this new version on the web and a clear, concise description of injector opening time and how it impacts tuning like the one Lonnie gave us would be a nice addition.

 

[Superskwrl]

Lonnie, Great info!!!
My car idles in the 80's ve it has 36's and a stock cam, what would cause the time to be too short??is lengthening it going to trick the computer? I would assume it would affect all the other parameters ie ae vs. tp etc

 

[Lonnie]

Typically stock cams may have relatively high VE numbers & this may be OK. For reference high impedence injectors are somewhat slower than low impedence ones.

You did not state what your injector time was set at so I'm not sure if there is a problem with your setup. Also if it is working fine, no need to worry. The opening time seems to be more critical when there are idle problems that seem impossible to tune out.

 

[fast4d]

my idle speed VEs are in the 80s as well. my injector opening time is set a the default of 1ms.

I am using 72# low impedance sieman's injectors.

 

[Craig Smith]

One thing that I have noticed is that manifolds with long runners like TPI manifolds have relatively high VE numbers at idle as well. This may be coincidence, but on all the stock-type engines with long-runner manifolds I have tuned, the VE numbers aren't much smaller than the WOT numbers. The cam profile may have a lot to do with too, like Lonnie said. However, from a quick glance, big runner length seems to be a common factor for me in the "high VE at idle" experience.

 

[Superskwrl]

Well fwiw my car has a stock camshaft i think somethin like 198-204 advertised,with a 1.7 rocker,1.6 is stock and it has a regular EFI lower but has a box upper so runners are fairly short, i dont think there even as long as a 4barrel intake.

Should i even be concerned the VE's are so high?
Should i try and change injector opening time to 2ms, or is that just taking from peter to pay paul?and only the #'s will appear different but the same amount of fuel etc will be used?

I have hi imp injectors,36's, it seems i need 50/55's will i be better off with a low imp injector? or hi imp?

 

[JrTuner]

Here are a couple of things to think about. What air fuel ratio are you trying to idle at? Are you idling it in closed loop? and if not what is the AFR set at on the table? because it does have an effect on VE even if you are not in closed loop. Another is make sure your injectors are IDed properly, if your fuel pressure is higher or lower than 43.5 psi then you have to reID the injectors because the flow rate has changed. Make sure your cubic inch displacement is IDed correctly also. I have also found that smaller, high impedance injectors usually require a minimum injector opening time more than 1.0ms sometimes as high as 2.0ms.

 

[mosk]

A followup question on fuel pressure

Originally posted by JrTuner
Another is make sure your injectors are IDed properly, if your fuel pressure is higher or lower than 43.5 psi then you have to reID the injectors because the flow rate has changed.

I have a FAST system. Reading through this thread, it occurs to me that my stock fuel pressure at idle is 38 psi (which is typical for Toyota). I have a 1:1 FMU, so at max boost (14 psi) I'll see a fuel pressure of 52 psi.

I can do the math to calculate the difference in flow rates between 43.5 and 38 and calculate the effect that has on injector flow rate, but because fuel pressure varies with boost, I'm not sure how to apply this info. I understand that this doesn't really matter too much in terms of results, because my VE and fuel tables are optimized for a/f and performance. But I am curious: when your stock fuel pressure is less than 43.5, do the experienced tuners scale down the value for injector size by the appropriate amount? Or do you toss out the idle number and just worry about max boost, or some average in between?

Just trying to learn...

Jeff

 

[Craig Smith]

Check the pressure with the pump on and the engine off. The regulator will also decrease fuel pressure when there is vacuum in the intake. It sounds like you are right on the money.

 

[ZBass28]

This is something I have been considering changing around.

Currently my injector opening time is set to 1.0 and my VE at idle is 59 or 60 (1000rpms) and aiming for a 13.7:1 A/F in open loop.

I am running 65# injectors on a fairly healthy cammed and supercharged 350 LT4. I've always thought that the VEs were a little high at idle, but the car idles and runs well and is dead on at hitting the 13.7 A/F that I have it set to.

Is it worth it for me to even bother to try and dunk those numbers since everything else works well? If so, where might be a good starting point to set the Inj. open time to drop those numbers a bit?

 

[Lonnie]

Tom,
Since my specialty is the LT1, I will tell you that 59-60 is a good number near idle unless you have a very big cam... I'll assume you do not since you are running a supercharger. My guess is that you probably have VE's in the 80's at cruise as well & upper 90's under boost?

My numbers are in this range as well on my Z28 & many of the other supercharged cars that I deal with.

If it runs well, do not worry about it. 1mS is what I would start with if using 65's. Opening time only becomes an issue if you have drivability or idle problems. With a bank to bank FAST this usually isn't a problem on a LT1 unless using larger than 72# on a 350 or 83# on a 383. At this point the opening time nears the idle pulse width, making control a problem. With a sequential box, this is easily corrected as each injector only fires every other revolution. No need to fix what isn't broke.

 

[ZBass28]

Thanks for the info Lonnie! Looks like I am doing alright then

Cam is what I would consider big, but not huge. Specs are 236/236 .560/.560 114 lsa

You are pretty much spot on with cruise and boost VEs. Light cruise is high 70's to low 80s and boost numbers range anywhere from 90's to low 100's at full boost (10-12 psi).

I have some very minor drivability issues, but I attribute those to poor AE adjustments that I need to work with this spring. Idle quality is rock solid when left in open loop, a little shaky/surge-prone when put into closed.

Thanks for the advice, I can cross that off my 'to-do' list

 

[Lonnie]

Typically, if your idle is erratic in closed loop, check idle timing trim. I've had some cars that hate idle timing correction & some that love it. Actually 2 identical motors with different size injectors were totally different.

On the LT1's sometimes you have to add a considerable AE fuel enrichment vs. TPS rate of change. Often starting the table out at higher than "0" (lower left corner) to get rid of a stumble at light off idle tip in. I have 2 that start out at .19.

Typically I try to run these cars at cruise with a 14.5 A/F ratio. Mileage is relatively good 16-18 on a 383. As you get leaner (above 13's), use caution as your AE tuning gets substantially more critical to prevent stumbles. Also your cam with 236@.050 may want more fuel due to the overlap. I typically stay under 230deg intake on a daily driver.

Feel free to send me your tune & I'll be glad to look at it for you.

 

[ZBass28]

Thanks for the offer Lonnie!

I'm currently doing a few things with the car while it's in storage for the winter, so when I fire it back up I'll put a file on disk and send it out to you.

I've pretty much got my cruise A/F at the same guidelines you were talking about there. My low idle cruise areas (1500-2000 rpms) I think I have around the 14.0-14.2 range to keep from surging, and then from there on up 14.5 is predominant in the light throttle areas.

Hopefully you may spot something in my AE's that is causing my quick throttle stumbles. I've tried a few variations and nothing seems to work, I am baffled. Admittedly though the AE sections of the FAST programming are the ones that I comprehend the least. The concepts I understand, it's applying them with good results that seems to be elusive