on a fully warmed engine i experience bucking in first gear and somewhat in second when engine is not under load. manual transmission(that does not help). i did the "search" and found a few suggestions. one possible ans was too lean in those cells that buck opccurs. my blms are in line and will keep them a bit on rich ride as i fell it lends better drivability. my timing is conservative and what i believe is a stock timing curve for my engine. i burned a chip OL and ran with it a couple days and found when cells changed i saw a significant change on my WB A/F. overall i was rich 12.3 to 13.3 on average. i went back to CL and corrected my VE as BLM's were 119-128 for most part when i last logged. VE table got messed up due to my changing FP to 20 lbs (from 15) due to lean at WOT(13.3).

another cause for bucking i hear is a very short PW on a 80 lb injector at 20 lbs FP. dont know if i can fix that? idle is currently at 750. maybe moving idle to 850 wil help? IAC at idle is zero. with the bucking i see more IAC counts and fluuctuation in A/F from 12.0/ to 16.0/1( i guess that is expected).

7747 $42

 

Ron,

Its interesting to see someone else experience similar issues with
the Xfire and using 80# injectors. And with a different manifold and TB setup. I have been trying for the better part of a year now to straighten the idle out while running 20psi. BTW, what BPC are you using? I can run C/L everywhere else except idle. There I am O/L. I tuned using my ZT-2 WB and BLMs/INT locked. When I unlock them, the AFR is still right at mid 14's to 15.0. So pretty good at part throttle with O2 sensor modifying fuel flow. My WOT AFR is in the mid 12's with high 80% DC.
But idle is when the fun starts. I have measured PW down in the 1.2ms range and so I am becoming more and more convinced that the PW is just too short to properly control AFR at that level. I would also suggest that the 7747 tables are too coarse to properly control fuel for the size cams you and I are running with faster valve timing events.
My best results so far have come from running O/L at idle and lowering SA and increasing VE in the idle areas. The VE increase gets the PW to a more manageable level. Runs a bit rich but I still have a surge when I try to hold raised idle under No Load.
As part of this process I have learned a lot more about the PID and IAC controls and that's helped.

 

my BPC is 80 or 82. at office not exactly sure but it is corrected for FP @ 20 lbs.

idle CL is 14.5 to 15.0. generally. occasionally (especially when engine very warm) will start to get happy and swings from 13.5 to 15.5 or so. i am certain this is cam/injector/higher FP related as the prior 210/216@.05 did not buck and had solid idle A/F. in fact the buck became worse when i upped FP from 13 to 16 to 20. i just loaded tunerpro RT so will be able to DL the BPW and confirm. i think i will need to run open loop and give up on CL in part or complete. i am using TunerCat. dont think i know how to allow OL only at idle and allow CL off idle. will need to look at TC to see if there is a enable-disable CL for RPM. i was able to successfully disable CL in entirety. stumble bog issue is so much better. some delay b4 the Ae elimiates it. i think i have a handle on that.

 

I've also had problems getting my idle where I'm happy with it. I'm running a 218/226 cam and 21 psi with 70 lb/hr injectors. My PW is around 1.6 ms at idle it seems pretty happy. My AFR hangs around 12.8 -13.2 which is a bit rich, but if I go any leaner my idle goes to crap and I get a nasty acrid smell out the exhaust, I'm guessing it's from a lean miss. As it is now with the richer AFR I have the best idle and least smell.

I've been running open loop all over for a while now. I tried to run just open loop at idle, but it never seemed to work. My blm's still moved around and my AFR still hung around 14.7 no matter what I had the idle afr tables set to. I think the open loop afr tables are only in affect when the entire ecm is in open loop IE: while warming up. Hopefully Rbob or some others can chime in here. I can't seem to find the qualifiers in the hack.

 

that may be the ans i was looking for. i can live with a rich idle. my ignition system can handle it. my mileage may suck further(17.4). thanks monte.

 

Ron/Monte,

My cam is a single pattern 269 Hyd Roller using a 108d LCA. And just as Ron described, I had a previous cam that was a 264/270 and 112d LCA that idled beautifully at 10.5psi. In fact, this current cam idled beautifully at 10.5 psi. The only problem was that WOT AFR was in the 14's.
So up went the FP to 20psi. I am using the O/L idle settings that you find in the 7.4L bin for 7747. My idle is in the mid 13's for AFR. When I decel and finally come to a stop, the motor will tend to hiccup and stumble and then finally find a steady idle, although it may have an intermittent stumble even at steady idle. Idle rpms are around 850-925. I have also set the IACs low for idle. Trailing throttle decel has a tendency to produce a stumble. I have disabled ASYNCH completely. Haven't checked PT BLMs but the AFR for that doesn't change with BLMs locked or unlocked. It cruises very nicely at high 14's to low 15's for AFR.
We can discuss changes to the PID controls if you'd like.

 

it is becoming clear why Holley rates the 670 cfm system at 275 max HP. the Holley 700 cfm 4 barrel/4 injector sequential opening butterflies seems to be a potentially better idea!

 

Ron,

Yep.
Or PFI. One other alternative I discussed with a friend was using the VAFPR capability of my Aeromotive regulator. After all, all the PFI systems use vacuum to bump up FP at WOT where its needed.
This way I could run 10.5 psi at idle and 20 at WOT. One issue
that I need to understand is "What BPC do you use?"
What PSI do you set it for?

 

i was thinking of that this lunch hour. i too have the aero with vacuum boost available. Ben on this and CF boards played with the vafpr and gave up. i tried it before the 7747 install and subsequent tuning. i guess it works OK in a marine TBI system where load is linear(that word again). i question how the vafpr could live with a 02 sensor and map sensor all trying to control the engine. maybe you need to lose the map and use only OL and TPS? i dont think i want to go that route.

 

I used to run 2 fuel pressure regulators on my TBI. One vacuum referenced, and one not. The none referenced one served to *clip* the range of the vacuum refernced one.

 

Grump: was that a "street" driven car?

 

Bruce,

I recall you doing that. Anyone ever come up with a valve that would pop at a certain vacuum allowing for WOT FP pressure. Or if someone could modify the source to allow for a variable BPC based on MAP. How does the cal on a TPI motor handle that? RBob??

 

Ths isn't what is happening. The PFI systems (3rd Gen) manifold reference in order to maintain the same pressure differential across the injector. The open end of a PFI injector is subject to manifold pressure. A TBI injector is not.

By referencing a PFI system fuel pressure to manifold pressure the injector flow rate is the same no matter the manifold pressure.

OTOH, LS1 fuel setups are dead headed and the PCM via the MAP sensor compensates for this differential.

RBob.

 

[QUOTE]Originally posted by RBob
Ths isn't what is happening. The PFI systems (3rd Gen) manifold reference in order to maintain the same pressure differential across the injector. The open end of a PFI injector is subject to manifold pressure. A TBI injector is not.

By referencing a PFI system fuel pressure to manifold pressure the injector flow rate is the same no matter the manifold pressure.

OTOH, LS1 fuel setups are dead headed and the PCM via the MAP sensor compensates for this differential.

[/QUOTE

RBob,

As always, thanks for clarifying. So the pressure change in the manifold is compensated for at the injector nozzle. So no need to change BPC then?

 

Correct, on a PFI system the vacuum referenced regulator is used to maintain the same fuel pressure as referenced to manifold pressure. This way the injector flow rate does not change. It is always the same.

I may revisit using a vacuum referenced fuel pressure regulator (FPR) on the TBI. The current code has a lookup of BPCs based on manifold vacuum (vacuum is referenced to barometric, which is different then the MAP value). When I first tried a vacuum referenced FPR I didn't have the BPC lookup table. It was ugly and didn't last long (ever see a VE table shaped like a bath tub?).

Two other areas of concern is a filter to lag the change of rate in the BPC. This is required to match the lag of the FPR. The code has this, would just need to tune it in.

The other area is AE. Currently AE is simply PW based. So any change in fuel pressure will mean a change in the volume of AE fuel. No reason another table based on vacuum (like the BPC table) couldn't take care of that.

I just haven't tried it again as the engine runs rather well as is.

RBob.

 

RBob,

I'd be happy with an On/Off type of switch for using the VAFPR.
Don't feel it needs to be something progressive.
I had no problem at idle or PT with an FP of 10.5psi. I only need 20psi when my foot is on the loud pedal.

 

Hmm, yes, it is a possibility. Can't connect the FPR port to manifold vacuum as it would then vary the fuel pressure.

Having the ECM control the switching of fuel pressure is easy enough. There is already an output that is activated whenever in PE mode (talking my custom TBI code here). In this case the ECM would also know that the fuel pressure was increased. It would also be able to switch between two different BPC values. One BPC for 10.5 psi and the other for 20 psi. That will all work.

Now, how to change the fuel pressure.

One way would be to use a regulated vacuum source. Say a small vacuum reservoir. Vacuum can be supplied by the engine, check-valved, and then regulated to a set value. This is used to change the FPR from the high of 20 psi, to 10.5 psi. ECM sees PE mode, activates a vacuum switch to release the vacuum on the FPR, boom, 20 psi.

Out of PE mode and the vacuum switch applies this regulated vacuum back to the FPR and we are back to 10.5 psi. Seems do-able to me. And not too complicated.

Another method could be to use two FPR's in parallel. One set to 10.5 psi and the other to 20 psi. Have an electric fuel shutoff valve on the 10.5 psi regulator. Activate the valve by the ECM PE mode enabled output. Boom, switches to 20 psi. Turn off the fuel valve and back to 10.5 psi.

RBob.

 

RBob,

How about linking the ECM control to a relay of some sort that would open or close a valve between a vacuum source and the regulator vacuum port. The canister purge or EGR relay comes to mind for instance. You could have a setting in the cal perhaps linked to PE mode based on TPS %. And if I recall, you've come up with a 3 table VE setup that would work nicely when the ECM goes into this enhanced performance mode.

 

Yep, that's the idea. Only I'd just have the ECM switch between two BPC's. One for low fuel pressure and another matching one for high fuel pressure. That way there is no need to tune another VE table.

RBob.

 

http://www.efitune.com/forum/index.p...=ST&f=17&t=98&

 

va454,

That's pretty awesome. Its also interesting that JPrevost was aware of this in 2002. Did anyone ever modify the 7747 code to incorporate this new table? Again, I'd be happy with an On/Off switch.

 

It dawned on me. For AE modify the AE PW via the BPC vs. Vacuum table value. Easy enough and it'll work. This would also compensate for changes in injector flow from different injectors or different fuel pressure. Change the BPC table to match the new injector flow rate and the AE changes with it.

RBob.

 

Umm, well, yea. Did it ages ago. Only problem is that the ECM needs to be converted to ROMLess first. Then again the code isn't even close to the original '747 $42 mask anymore, way, way better. . .

RBob.

 

There you go RBob teasing the rest of us with little tidbits.
I know that you and JPrevost have been using 20+psi on TBI. How are you able to control the PW at idle where the rest of us appear to be having a problem? Is it the custom code?

 

For the most part, yes, the custom code. The low RPM VE table is high granularity which makes a world of difference. Every 100 RPM by every 5 KPa (70 KPa and lower), then 80, 90, 100 KPa. Goes from 400 RPM to 2,000 RPM.

If I wanted a better idle I could enable the N-Alpha blend mode. This really works out nice. Used it for a while then went back to normal VE only. I really should use it again. The difference was night and day for idle and off idle response. Not to mention the traffic-crawl.

Another item that helps is the high speed Lockers data logging. Can't make the proper changes unless you have the data.

I also have the injector bias set to 778 usec. And the async fueling mode is disabled. DFCO is disabled. I also cheat and use closed loop for idle. Open loop once off idle for everything else.

AE fueling is added to the normal sync fuel (no longer is AE async fuel). The AE tables are also expanded along with having an AE modifier based on RPM. This all helps in getting the engine off idle and accelerating in a smooth fashion.

Yep, custom code is the only way to go. I've even re-written areas of code and corrected issues in my '92 Camaro running the '730.

RBob.

 

RBob,

Always a fountain of information. I have the Async disabled and the DFCO disabled. Is your Decel Enleanment enabled?
My injector bias is currently at 549usec. I'll try bumping that up to 778 again to effectively increase PW.
How is N-Alpha Mode enabled and how do you use it?
No doubt that the additional granularity of the VE tables is a big help. Right now I just can't seem to enable C/L idle without the motor going into spasms. C/L works just fine every where else.

 

Decel enleanment is enabled. It s important to prevent the AFR from going too rich on a throttle lift.

To enable N-Alpha mode an option bit is flipped. This functionality is only included with the custom code. How it works is that there is a 3D table that is TPS% and RPM based. The value in each cell is a pseudo MAP value. This MAP value is further modified by the current IAC steps which is compensated for RPM (this provides for the IAC airflow ).

Basically it is a model of the throttle body airflow. Included in this model is RPM, TPS%, and IAC steps. All together they provide the value of what the MAP will/should be. However, this modeled MAP value isn't bouncing all over the place. And, it doesn't need to be filtered like the real MAP sensor value. Too much filtering causes lag along with the problems of doing that.

This pseudo MAP term is then blended with the real MAP value. The result being used throughout the code for the various lookups (VE, SA, . . .). The amount of blend is controlled by a blend vs RPM table. At lower RPMs more pseudo MAP is used. As the RPM increases the final MAP term is finally all of the real MAP value.

RBob.