Another case of blown ring-lands....(pics!)

[MarkEmark]

So I was talking to my machine shop the other day (engine is being rebuilt with 9:1 wiseco forged pistons, ferrea oversized valves, new upper/lower timing chains/guides, new valve springs, seals, retainers, re-shimmed, 3 angle competition valve job, new oil pump, new water pump, new ARP head/main studs, etc etc.) and they told me that when they went to take the pistons out of the block, they literally "fell right out..." meaning the the rings or the ring-lands (or both) weren't in that great of shape. When I went to the shop a couple of days ago, they gave me one of the pistons (from the #1 cylinder) to look at and I guess they weren't lying...They said it came out like this, but the cracked ring-lands were still in place and all the rings were still in place as well. Luckily, no scoring occured that wasn't taken care of by the .020 over-bore. But wow, I had no idea these things failed so easily...



These pistons had about 95,000 miles on them, about 2000 of which were turbocharged (at first about ~7 psi, then ~9-10 psi).

The engine was intercooled with a very large, efficient intercooler, and also had alcohol injection since day one. 93 octane was used all the time. The fuel system was a JWT ECU programmed for 370 cc SR fuel injectors that I had cleaned, flow-tested, and blue-printed by RC-Engineering. Stock MAFS, aeromotive adjustable fuel pressure regulator (turned down 2 psi at idle to compensate for the inherent richness of the JWT ECU). Timing was advanced 2 degrees at idle after logging the timing with my Blitz R-VIT II to see how ridiculously conservative JWT's timing maps were. To check out the thread where I posted the timing numbers, go here:

zerothread?id=79664

The injectors never exceeded an 89% duty cycle. The turbo was T3 super 60, .48 a/r turbine, .60 a/r compressor and put out 10 psi up until 5000 rpm, after which it only put out 7-8 psi to redline (~6750 RPM). The guy at the machine shop said it looked to him like there wasn't enough fuel...there WAS enough fuel, but too much air, I believe. My MAFS reached max voltage past 5000 rpm in every gear (5.11 volts), meaning at full boost when I exceeded 5000 RPM (which was actually RARELY done), the MAFS couldn't meter any more air (no more fuel), but more air was being ingested, causing a lean condition.

However, according to the narrow-band 02 read-out of the Blitz (which, of course, needs to be taken with a grain of salt), the A/F ratio never exceeded what I'd consider a safe 12.5:1, except at 1 point in 4th gear around 5000 rpm when it's 13:1 (but 5000 RPM in 4th gear is VERY fast, so again, I almost never went up there).

The car ran like a champ and felt extremely powerful right up to the point where I parked it and yanked the engine. I never once heard a ping, and it felt really, really fast. So unless these cracked ring-lands happened the last day I drove the car (I did beat the hell out of it, the first time I really did), then the car performed amazingly well given the lack of compression and destroyed ring-lands.

I'm just worried that this type of thing happened, at relatively low boost (if I average out the boost over the RPM range, it was never past 8.5 psi), with ample fuel and an ECU designed conservatively for boost...

When I get the engine back, I'm turning the boost down to 8 psi, breaking the engine in (not under boost) for at least 500 miles, and not going past 8 psi until I have the money for 50 lb fuel injectors and a Z32 or Cobra MAFS.

In your guys' opinion, what do you think caused it? The MAFS maxing out at 5000 rpm, even though I rarely exceeded 5000 rpm and the 02 sensor voltage suggested that I was running rich enough? The lowered fuel pressure maybe? The 2 degrees advanced timing to compensate for the very conservative JWT timing maps?

Just thought I'd share with you guys...these types of things happen at low boost, even when the car seems to run beautifully, has plenty of anti-detonation protection, and is tuned and monitored fairly well....

[:: orion ::]

Even if the MAF was maxed...the ECU would be seeing max load, and giving enough fuel.

Typically, if the MAF maxes out, it misfires due to a rich condition.

I'll bet you had a littl detonation...couple extra degrees of timing wouldn't do it (I'm at 22 BTDC and 12psi, 300rwhp...never a knock).

I'll bet the lowered fuel pressure...did you do that to 'trim' the fuel curve?

- Brian

[klattr1]

if you are gonna mess them up, do it good huh? haha

no telling what caused it. usually 10 psi is fine for stock motors for a long time.

[MarkEmark]

Even if the MAF was maxed...the ECU would be seeing max load, and giving enough fuel.

Typically, if the MAF maxes out, it misfires due to a rich condition.

I'll bet you had a littl detonation...couple extra degrees of timing wouldn't do it (I'm at 22 BTDC and 12psi, 300rwhp...never a knock).

I'll bet the lowered fuel pressure...did you do that to 'trim' the fuel curve?

- Brian

I was under the impression that if the ECU were maxed out at 5000 rpm, and therefore giving "maximum" fuel from 5000 rpm to 7000 rpm, it would only be giving the same amount of fuel at 5000 rpm when it just reached its max as it would at 6000 rpm and at 7000 rpm where the voltages were identical to those at 5000 rpm...but seeing as the MAFS voltage increases with RPM until its max at 5000 rpm, I was assuming that the airflow through the MAFS was increasing proportionately, meaning past 5000 rpm, say, at 6000 rpm, there was more air flowing through the MAFS and therefore being ingested into the engine than at 5000 rpm, and likewise, the turbo would be pushing more air into the engine, meaning all this extra air was not being metered because 5.11 volts is the max. So, the same amount of fuel would be supplied at 5000 rpm at 6000 rpm and at 7000 rpm, even though the engine IS ingesting more air past 5000 rpm, and therefore the lean condition occurs...

Am I wrong?

[DRIFTEADOR]

^thats what i thought too. if thats the case, most likely that was the cause. even though you didnt get past 5k often you said you beat on it the last night driving it. narrowband afr is really useless, gives a whole ~.20v of resolution from 14.7 to 10:1. and thats with a new calibrated sensor.

[:: orion ::]

Not excatly...airflow is plotted with TPS voltage to get LOAD...then load is plotted against RPM to determine the fuel curve.

Think of the X and Y axis...and a set table...lets say 16 X 16 for purposes of this discussion, may be more or less in a Nissan ECU. Airflow (MAF voltage) is our X, TPS voltage is our Y axis.

With max airflow, you're X axis is reading ALL the wya to the right, maxed out. Combined with TPS voltage (also maxed at WOT), you're at the far right, and top of the LOAD table. Max load the ECU can register...

NOW...

We take that max load (let's call it the X axis on the fuel table), and plot it against RPM (our fuel table Y axis)...and we still get proper fuel (most likely quite rich) for a given RPM.

...

So even if the MAF is maxed, it's just telling the ECU 100% load...which can still map for any given RPM, and it's gets enough (too much) fuel.

So, typically a maxed MAF will give you rich misfire...not too little fuel.

Hope that helps - Brian

[MarkEmark]

Not excatly...airflow is plotted with TPS voltage to get LOAD...then load is plotted against RPM to determine the fuel curve.

Think of the X and Y axis...and a set table...lets say 16 X 16 for purposes of this discussion, may be more or less in a Nissan ECU. Airflow (MAF voltage) is our X, TPS voltage is our Y axis.

With max airflow, you're X axis is reading ALL the wya to the right, maxed out. Combined with TPS voltage (also maxed at WOT), you're at the far right, and top of the LOAD table. Max load the ECU can register...

NOW...

We take that max load (let's call it the X axis on the fuel table), and plot it against RPM (our fuel table Y axis)...and we still get proper fuel (most likely quite rich) for a given RPM.

...

So even if the MAF is maxed, it's just telling the ECU 100% load...which can still map for any given RPM, and it's gets enough (too much) fuel.

So, typically a maxed MAF will give you rich misfire...not too little fuel.

Hope that helps - Brian

Ahh, I see. Yes, that helps...so those ringlands must not have blown from running even after the MAFS was maxed out (unless misfires typically cause a situation like this?)...the engine builder said there was NOT enough fuel (that is, too lean), which is what caused the rings to be destroyed. I wonder when the hell this happened then..because again, I never once heard a ping, and I never over-boosted, not that I was aware of anyway. If it's not from the timing, and it's not from the maxed out MAFS...then would it really be from the fuel pressure being turned down 2 psi at idle? I'm sort of worried...because according to the narrowband 02, it was never lean...but again, I guess narrowband 02's are all but worthless.

[WDRacing]

Carbon buildup can cause detonation as well. You get little hot spots that act like spark plugs all by themselves. Although I'm very surprised to see that much buildup with a motor that saw alcohol and water injection. Usually they are fairly clean. Good pics by the way.

WD

[DRIFTEADOR]

Not excatly...airflow is plotted with TPS voltage to get LOAD...then load is plotted against RPM to determine the fuel curve.

Think of the X and Y axis...and a set table...lets say 16 X 16 for purposes of this discussion, may be more or less in a Nissan ECU. Airflow (MAF voltage) is our X, TPS voltage is our Y axis.

With max airflow, you're X axis is reading ALL the wya to the right, maxed out. Combined with TPS voltage (also maxed at WOT), you're at the far right, and top of the LOAD table. Max load the ECU can register...

NOW...

We take that max load (let's call it the X axis on the fuel table), and plot it against RPM (our fuel table Y axis)...and we still get proper fuel (most likely quite rich) for a given RPM.

...

So even if the MAF is maxed, it's just telling the ECU 100% load...which can still map for any given RPM, and it's gets enough (too much) fuel.

So, typically a maxed MAF will give you rich misfire...not too little fuel.

Hope that helps - Brian

can anyone shed light on this?

i understand the ecu will give the max amount of fuel it was programmed to at the highest load/highest rpm but but i thought it wasn't programmed to give more fuel than the mafs could read since a stock, n/a motor would never outflow the sensor. for example, lets say the load scaling of a factory ecu is 0cfm-100cfm, and your turbo pushes 200cfm. how does the ecu compensate for the extra flow if its off the table?

[hysteria]

this kind of scares me because i am planning on running pretty much an identical setup to what you had that caused this... i guess i'll be going for a little larger injectors and an upgraded mafs now... there's some more money... always more money... thanks for the info.

[s13sr20chris]

1) that was a jwt na tune?2)i have an sr piston just like that. i think its from some sort of detonation.3)if your turbo is choking at high rpms then you will get really hot intake charge and that could cause the detonation(or help).4)you never dyno tuned with wideband?

really, i think the timing and a/f should be done on a dyno with wideband. if you are worried about your new motor then i would take some steps to keep it safe.

1)lose a little timing2)gain a little fuel pressure3)lean the idle electronically4)get some more maf5)get a turbo thats more efficient at 10psi at 7k rpm6)and tune her on a dyno with wideband

=thanks chris for spending all my money