AFRs go lean under boost only in certain gears: SR20det?

[KaminaSan]

Not sure if anyone can explain this.

After finally fixing the issues with my fully built SR, I've had this irregularity you could say.

Fully Built SR:
86.5mm Mahle pistons
Eagle Rods
1.5mm Cosworth headgasket
BC Springs Retainers

On this, I'm running an s14 t28 with stock ecu and stock injectors. STOCK 6-7 psi.

Just to be safe I have the fuel pressure turned up to 45 psi with the vacuum hose on.

Here is where the issue comes up.
In the lower gears, 1/2/3 My afrs are around 13.3:1 WOT from 3600-4000 rpm, then drop down to 11:1/ 12:1 after that all the way to redline, however in 4th/5th on the highway at the same RPMS, I drop down to 11.8:1 Immediately when I press the throttle all the way in?

Cruising AFR: 14.5-15.2
Slight throttle doesn't change it.

Here is the list of gears and what they do exactly. I do have a slight exhaust leak somewhere, just haven't pinpointed it.

1st Gear: WOT as soon as I press all the way down it goes to 11.1, then as boost builds it goes all the way up to 13/14, until right after 3800 rpm, then it goes back down to 11 all the way to redline.

2nd gear: WOT at 3,000 rpm, 7 psi it's 12:1 from 3k rpm until 3800 rpm, where it then goes to 13.3 or so until it hits 4200 RPM, and then it drops down to mid 12, and by 5k rpm it's low 11s.

3rd Gear: WOT at 3,000 RPM it's high 12:1, then as soon as it hits 3800-4200 rpm it's 13.1:1, and from 4200-redline it goes down to about 11.2.

4th Gear: WOT at 3500, mid 12s, and goes down to 11s at 4,000, 10s at 5,000.

5th gear: WOT at 3,000 RPM it's Low 12:1, 11.5:1 at 3800-4200, 10.5:1 at 5,000 and drops all the way to 9.6:1 towards 6k rpm.

Now I do have an exhaust leak that can be heard possibly between block and exhaust manifold, and another one possibly between turbo elbow and downpipe(DP is where my Wideband is).

My question is this: Why would I be going lean between 1st/2nd/3rd, only for about 400 rpm (3800-4200), yet when on the highway and in 4th/5th, I get good AFRS at the same rpms, even going SUPER rich (9.7:1) at 5-6k?

[Kingtal0n]

Not sure if anyone can explain this.

After finally fixing the issues with my fully built SR, I've had this irregularity you could say.

Fully Built SR:
86.5mm Mahle pistons
Eagle Rods
1.5mm Cosworth headgasket
BC Springs Retainers

On this, I'm running an s14 t28 with stock ecu and stock injectors. STOCK 6-7 psi.


My question is this: Why would I be going lean between 1st/2nd/3rd, only for about 400 rpm (3800-4200), yet when on the highway and in 4th/5th, I get good AFRS at the same rpms, even going SUPER rich (9.7:1) at 5-6k?

Sounds like either fuel pump or MAF sensor related- Or air inlet related, perhaps a leak somewhere after the maf before the turbo. Or of course, as you mentioned, the O2 sensor is being deregulated by the exhaust leak. Although this situation is less likely during full throttle performance.

Ah, one more thing. A misfire, or slight misfire, such as caused by too small of a plug gap, will leave unburnt oxygen(and fuel) in the exhaust manifold- giving a false lean reading. Before you start changing fuel pumps and maf sensors you might want to try a brand new set of OEM plugs. I commend you for utilizing the 7psi setting while doing this and noticing the inconsistencies. Once a stand-alone ECU is used, the tuner should be able to notice that the (new 700cc injectors) seem to be (80% duty cycle) underfueling the engine- in other words, a stand alone is a diagnostic tool for this situation, since dialing the mixture richer and richer without noticing any extra fuel would indicate that fuel pump issue I mentioned.

edit: also I want to mention this scenario is somewhat typical of OEM computer/maf/injector setups. typical as in, I have seen it happen before, In fact the car I am driving right now exhibits the same scenario- leanest during 1st and 2nd gear pulls around peak torque, with 3rd-5th being richer and "safer". Honestly if you try 3 different MAF sensors you will get three different A/F ratios, generally, but all will display the same general trend: leanest in the numerically lower gears, and always around peak torque. Try cleaning the maf hot-wire sometimes that makes the situation a little better also.

[KaminaSan]

Sounds like either fuel pump or MAF sensor related- Or air inlet related, perhaps a leak somewhere after the maf before the turbo. Or of course, as you mentioned, the O2 sensor is being deregulated by the exhaust leak. Although this situation is less likely during full throttle performance.


edit: also I want to mention this scenario is somewhat typical of OEM computer/maf/injector setups. typical as in, I have seen it happen before, In fact the car I am driving right now exhibits the same scenario- leanest during 1st and 2nd gear pulls around peak torque, with 3rd-5th being richer and "safer". Honestly if you try 3 different MAF sensors you will get three different A/F ratios, generally, but all will display the same general trend: leanest in the numerically lower gears, and always around peak torque. Try cleaning the maf hot-wire sometimes that makes the situation a little better also.

This is EXACTLY what I'm experiencing. As soon as I hit 3800 rpm or so in those gears, I feel the most 'pull' on the car, but that's also when it seems to lean out a bit.

I doubt its the pump, as I'm running higher fuel pressure,and have watched the pressure stay consistent. I put in a new walbro at the same time as the rebuild(400 miles ago)for safety. Just in case, I bought a deatchswerks fuel pump hardwire kit, and ill see if that changes it when it gets in.

Ill swap mafs with someone else, until my tune gets back from enthalpy, then ill be putting in 550s and a z32 maf.

Could me running higher fuel pressure be flipping the ecu out, making it try to lean it out so it isn't so rich?

I know I have a fully built motor, but do you think these low 13:1 afrs for a second in the lower gear/ rpm range will cause damage on 6 psi? I only drive when its cold out, and the plugs are a brownish color, not rich, not lean. The manifold doesn't glow or anything either.

[Kingtal0n]

I know I have a fully built motor, but do you think these low 13:1 afrs for a second in the lower gear/ rpm range will cause damage on 6 psi? I only drive when its cold out, and the plugs are a brownish color, not rich, not lean. The manifold doesn't glow or anything either.

Two things you have to consider here:
1. It doesnt matter if you have cast pistons or forged pistons, danger is danger! forged slugs just means that they are less likely to fail... which just means the engine will keep taking the abuse longer. Its still abuse, it will still eat up components and may reduce their life spans.

2. Is it truly dangerous? Are you really close enough to the threshold of excessive component wear and tear due to lean air fuel ratios? You are in the same boat as many others in this respect, and no one could tell you for exact 100% certain because all driving conditions change all the time. It may be fine run #1 #2 but #3 there might be just enough heat to do some damage.


Keep in mind the color of plugs is an overall indication of the conditions they experience overall, an engine running rich all the time that only goes lean for a split second will still display black plugs- even if that split second is all it takes to ruin the engine. I am sorry that I can not give you an exact answer but anybody that pretends to know what is happening inside your cylinders at Xrpm with Y* of ignition timing and Z air fuel ratio would be lying. Monitor the plugs for signs of detonation specifically, flakes of aluminum are a tell tale sign, often the deck of an SR20 will exhibit "pitting" and this aluminum usually finds its way onto the plugs as an indication that damage is occurring.

for safety sake you might also consider dialing back the timing at the CAS a degree or two. The factory does not give much advance to begin with but a degree might be all the difference it takes. Of course keep the straight 93 in the tank. If you are capable boost the octane a few points just to keep it on the safe side. An Air/fuel of 13:1 might be disastrous on 87 octane, doable on 93, and plenty safe on 110. See where I am going with that? And the fuel pump wiring might do the trick, as the sudden draw of current load as fuel pressure rises dramatically fast in a high gear is a likely culprit! keep us posted!

[rebornS14]

stock ecu with 1.5mm gasket, bigger pistons, more aggressive cams, and bigger turbo?

you need a tune. there's no way the cars gonna run safe as a stock setup when you change out the components. messes up everything.

[KaminaSan]

Ahh man, all of this is just nuts. Hopefully I haven't done any damage yet. Compression is 140 across the board. Sucks that California only has 91 octane. Guess I'll only be boosting on the highway.

No cams, stock s14 Turbo. 1.5mm headgasket lowers compression.

[steve shadows]

What EMS are you using?

This is the most important question, sorry if I missed it...and how was it tuned etc...

I may be able to help so email me or pm me, we'll see though

[Kingtal0n]

What EMS are you using?

This is the most important question, sorry if I missed it...and how was it tuned etc...

I may be able to help so email me or pm me, we'll see though

I was under the impression he is using an OEM computer. That is why he mentions a rom tune. Although, to the poster I would recommend a minimum that you invest $750 into a L-detro PFC.

My post comparison was based on the intuition that he is using the OEM computer. His OEM turbo/injectors/Maf suggested that, steevey

[KaminaSan]

What EMS are you using?

This is the most important question, sorry if I missed it...and how was it tuned etc...

I may be able to help so email me or pm me, we'll see though

I am in fact using the stock ecu, untuned. I understand this may not be ideal, but besides the bottom end internals,and headgasket everything on the engine is stock.

The engine is:
S13 sr20det
Stock s13 cams
B.c. springs and retainers/RAS
86.5mm mahle pistons
Eagle rods
ACL bearings
1.5mm headgasket(head and block were both decked)

Stock S14 bb Turbo at 6psi, STOCK 370cc injectors, Stock ECU(untuned).
Adjustable FPR at 45psi with vacuum hose attached.
I ran this solely for break in period, so as to determine any issues that might hinder me during tuning.

I originally had other plans for a bigger turbo, but now that I'm just using the s14 turbo, I figured my afrs would be fine, not going into the 13s. Especially with higher fuel pressure.

I can't honestly see spending 900 bucks on a powerFC and then 450 on a tune just for a s14 turbo...

[Kingtal0n]

I am in fact using the stock ecu, untuned. I understand this may not be ideal, but besides the bottom end internals,and headgasket everything on the engine is stock.

The engine is:
S13 sr20det
Stock s13 cams
B.c. springs and retainers/RAS
86.5mm mahle pistons
Eagle rods
ACL bearings
1.5mm headgasket(head and block were both decked)

Stock S14 bb Turbo at 6psi, STOCK 370cc injectors, Stock ECU(untuned).
Adjustable FPR at 45psi with vacuum hose attached.
I ran this solely for break in period, so as to determine any issues that might hinder me during tuning.

I originally had other plans for a bigger turbo, but now that I'm just using the s14 turbo, I figured my afrs would be fine, not going into the 13s. Especially with higher fuel pressure.

I can't honestly see spending 900 bucks on a powerFC and then 450 on a tune just for a s14 turbo...

Well, heres the deal. I could get you setup with a Power FC and tuned for about $1000 total using any Maf sensor. And this would give you the computer to run any turbocharger you could desire in the future (headroom).

Did you ever get the nice 12V source on the fuel pump?
and finally, I bet if you swapped those OEM cams out for some of longer duration that peaky torque hill should disappear- giving your engine headroom through that rpm range.
You could also try a few different SOHC KA maf sensors to see if that helps. I tried a few different ones and found one that went a bit richer than the others- enough so that I am around 12.6 in that region instead of 13.x

I am just throwing temporary bandaid solutions at you until you get a real computer/tune/sorted

[KaminaSan]

Well, heres the deal. I could get you setup with a Power FC and tuned for about $1000 total using any Maf sensor. And this would give you the computer to run any turbocharger you could desire in the future (headroom).

Did you ever get the nice 12V source on the fuel pump?
and finally, I bet if you swapped those OEM cams out for some of longer duration that peaky torque hill should disappear- giving your engine headroom through that rpm range.
You could also try a few different SOHC KA maf sensors to see if that helps. I tried a few different ones and found one that went a bit richer than the others- enough so that I am around 12.6 in that region instead of 13.x

I am just throwing temporary bandaid solutions at you until you get a real computer/tune/sorted

I am going to try a different sohc mafs, and while powerfc would be ideal, its still out of my price range right now. My fuel pump hardwire kit, and enthalpy tune should be here by Wednesday. Ill try the hardwire kit first, see if it makes a difference, then I'll move on to to installing my 550s/z32 maf.