New Build - Wide disparity front / rear low trim only

[awdonly]

Just taking her out for a first run after complete rebuild. I've got idle at stock 650 rpm and she is running smooth with the ID1000 injectors (set for 650 as I'm running e85). My low long term trim figures are a bit odd - -9.5% rear and +5.0 front. I've never had that big of a front/rear spread previously. My mid trims are within 1% front to rear.

I'm going to try bumping idle up to about 800 or 850 RPM to see if thing start to balance out. Any other suggestions on cause/solution?

[Unlogic]

Low trims are very sensitive. For example a tiny exhaust or intake leak could cause the difference your seeing. This leak might be so tiny that as the flow increases their effect is no longer noticeable.

Over the years I've replaced injectors, turbos, pre-cat eliminators and all related gaskets on my car but I've always had difference of a few percent between the banks.

[Chris@Rvengeperformance]

The more I see these the more I see it is more the usual than the exception.



I recently installed dual widebands on my aem setup and the rear shows 13.8:1 and the front is 14.9:1



Its two different type of widebands, but I have the same new type coming so they are matched, but lots of 3si research shows 7% leaner front happens a lot at low loads.

[awdonly]

Thanks. With a 14% spread I can't get my trim to the desired +/- 5% to allow a reasonable range of trim adjustment. I found with my old setup (gas - 550s) that I only had 4% spread but also failed emissions at stock idle. Bumping it up to 850 got me through emissions with flying colors. I'm hoping the change will have a dramatic impact here. I'll report back.

[Chris@Rvengeperformance]

Thanks. With a 14% spread I can't get my trim to the desired +/- 5% to allow a reasonable range of trim adjustment. I found with my old setup (gas - 550s) that I only had 4% spread but also failed emissions at stock idle. Bumping it up to 850 got me through emissions with flying colors. I'm hoping the change will have a dramatic impact here. I'll report back.

It may be a bit of a derail, but I've been thinking about the way our banks seem to get more unbalanced as the car is modified. I think putting an aftermarket fuel system that should equalize the banks may actually make the symptoms worse. Maybe keeping the stock fuel loop restricts the fuel enough to the rear on a stock setup that overall it is more balanced (at low loads).

I think the thought has usually been the rear turbo should flow more due to the manifold design, but maybe there are greater factors? The downpipe design for instance may make the front have better flow at low loads. The turbo exhaust housings are not the same either, although I haven't studied one closely enough in a while to guess which would flow better.

Maybe somehow, as shitty as the front manifold looks, it actually flows better than the rear, lol. Maybe only at idle or very low loads.

[awdonly]

Some variables to eliminate because I didn't change them so aren't causing 10% delta: Oversized fuel rail loop connector; downpipe (3sx); turbo housings. New stuff that could be cause: Fuel system (new pump, filter and most plumbing to rails, AFPR, Injectors) ported lower plenum, stage 3 heads, ported exhaust manifolds.

[Chris@Rvengeperformance]

Some variables to eliminate because I didn't change them so aren't causing 10% delta: Oversized fuel rail loop connector; downpipe (3sx); turbo housings. New stuff that could be cause: Fuel system (new pump, filter and most plumbing to rails, AFPR, Injectors) ported lower plenum, stage 3 heads, ported exhaust manifolds.

I suspect an AFPR can cause this. I am running one too. I don't know what pump you had before, but if you were over runing the stock fpr then I think the front bank could see higher fuel pressure than the rear. When you installed an AFPR it "fixes" the pressure disparity and then you see more difference in the banks since the front is naturally lean at idle.

Yours does sound like a pretty big delta though. 1.5 times what I'm seeing right now. I will report back sometime this weekend when I get the matching wideband installed. Maybe I am full of crap.

[awdonly]

I doubt it's the AFPR - here's why. I was running a AEM on my old setup after my stock FPR went bad. I didn't have this huge front/rear low trim spread with that setup. What I didn't like was the AFPR leaked down pressure fairly quickly after shut down impacting starting. Later, I converted back to a new OEM FPR and it held pressure much longer after shutdown and was not overrun by my Supra pump (and didn't impact trim). The new Walbro 450LPH pump overruns the OEM FRP so back to the AEM and its relatively rapid leak down.

FWIW I have a relatively unusual fuel setup that uses the stock hi/low pump relay, but with a 2nd stock resistor in parallel with the factory resistor (with supporting large wiring all the way to the pump). This seems to work well with both my old Supra pump and my current Walbro 416 to slow/quiet the pump when in low mode but still has plenty of flow when needed.



The Supra never overran the stock FPR, but the Walbro sure does.

[Chris@Rvengeperformance]

The walbro 416 at full voltage at idle may even overrun the factory return line. I currently have base pressure set to 55 anyway, but I can't go lower than about 43 with vacuum connected on an aeromotive fpr.

[awdonly]

The resistor setup solves that issue. Out of curiosity are you running into the max supportable pressure on the Walbro at max boost? 20psi boost would be 75psi at the rail.

[Chris@Rvengeperformance]

The resistor setup solves that issue. Out of curiosity are you running into the max supportable pressure on the Walbro at max boost? 20psi boost would be 75psi at the rail.

The flow of the walbro 416 doesn't seem to drop off rapidly until about 80 PSI. Should be enough, but if its not I may need a boost a pump anyway, or I can always drop the base pressure to 50.

[murphys_law]

One thing I haven't seen mentioned is intake manifold design. On many other applications that is what causes the afr to differ cylinder to cylinder.

Maybe the ported lower intake has exaggerated this even more?


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[awdonly]

Played with it a bit tonight. Adjusting idle up to 850rpm had no impact - still 14% F/R trim spread. Increasing the idle barely moved the airflow numbers up from 45hz so I guess its not that big of a surprise. Moved idle back to stock and adjusted idle trim to +/- 7% for the moment
. Not a lot of movement before I run into the 12.5% limit, but its better than nothing. Mid trims remain at 1% F/R spread. I'm curious where at what airflow things level out, but I can't think of a good way to figure that out on my current setup (2 narrows in stock positions, 2 narrows in CA emission spots below pre-cats (gutted) and 1 wideband at join in the downpipe. If I can figure out where the spread stops I could make that the cutoff between low/mid trims and isolate the issue to the low trim.

[Chris@Rvengeperformance]

FYI, I switched to identical AEM X Series Wideband 30-0300's. The front and rear got closer together, but still about half an afr apart.

Adding 5% at idle to cylinders 1,3,5 got both sides 100% identical. I set it to 2% at redline and horizontally interpolated back to idle, so its about 3% at 2000 RPM and its basically balanced. It is 3-5% off in certain areas. It is hard to get it perfect because the aem individual cylinder trims are just rpm based and not load based, but its close enough that o2 feedback can dial it in. I may tinker more later, but it idles smoother than I ever remember.

So, the old wideband read a few percent richer, but not much. It does show the importance of having twin widebands that are the same and also keeping your sensors relatively the same age.

[awdonly]

I imagine the same applies to narrowbands in the Chrome setup.

[Chris@Rvengeperformance]

I imagine the same applies to narrowbands in the Chrome setup.

Yes, although I think narrowband's "aging" is a little different. They will slow over time and get lazy. I think the widebands do that to some degree, but the AFR range they can read also starts to narrow slowly over time. This may not be as much of a problem with the 4.9 sensors since they do not use the air reference chamber it can't get contaminated.

IMO anytime you do some tuning and your widebands are over a year old replace them. Narrowbands should probably be replaced every 50K on a performance application. I'm sure they are rated for 100K, but excessively rich mixtures and misfires can wear them faster. Leaded gas too, of course.

[awdonly]

Still having a 12 - 14% spread front to rear at idle and it wanders a lot so I'm bumping into the 12.5% limits on one side or the other. My current theory is that I have a leaking injector on the rich side. I think the easiest way to test is to swap the front/rear injectors and see if my rich bank moves.

[tom93vr4]

Have you degreed your cams? I think I saw that you installed stage 3 heads.

[awdonly]

I did.

[tom93vr4]

Had a chance to swap injectors yet?

For reference, I have the same problem - rear low trim is about 11 to 12 % lower than front. On mine, swapping injectors front/rear made no difference.

[awdonly]

I did swap and still have spread. Think spread is same f/r and not reversed, but will double check when I get home. No injector leakage. 10psi lower compression on cylinder 2 than other 5 might be cause in my case.

[tom93vr4]

I suppose that's a possibility, but not sure it makes sense. A 10psi lower on 1 cyl is probably 5 to 8 % (guessing 150 ish is normal), so that issue on only 1 of 3 rear cyl doesn't seem like it would make a 14% difference in idle fuel (and not affect anything higher).

Again for reference, I didn't measure compression but I did check leakdown. All were 48 +/- 0.5, although the rear bank average was 47.67 while the front bank was 48.33 - again, don't see how this small a difference could be the cause. I've been searching on and off for the cause of this on my car for over a year and so far not found the cause.

[awdonly]

Confirmed. Injector swap made no difference.

[murphys_law]

Were the heads CNC ported or hand ported? Could be a variation from hand porting?


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[awdonly]

Mine are Dynamic Racing's stage 3 heads. Not sure of their process.