Violent shuddering at low rpm and high load

[BLOZ UP]

Ok, I'm not entirely sure what I'm experiencing.

The problem I'm having is that at low RPMs, 2500-3500, and high load (boost), the car violently shudders. Pretty sure it's misfiring, as my datalogs are showing errant RPM readings. This only happens in 4th and especially in 5th gear, at 100% throttle, below 3500 RPM. Full throttle at higher RPMs results in normal ass hauling.

I'm seeing 2psi at 2500 RPM, 15psi at 3000, and 18psi at 4000.

I think it's spark blowout. I've encountered spark blowout before on VG30s, but only at high RPM and gapping my plugs down to .030" or so fixed it. I've tried as low as .025" in this case, and while it seems to improve the symptoms, it doesn't go away.

My next theory was that my worn out, inefficient engine was surging. Mapping out 5th gear pull datalogs over my compressor map, that doesn't seem likely (I'm using VE values from Megasquirt, which might be off, but are eons better than guessing).

 

So what gives?

[tooquick260]

I have seen some local guys running .16 to .19 to fix this same problem. 

 

Just recently dropped mine to .27, I am going to lower to .20-22 before I go to the dyno for the final upped boost tune

[Xnke]

So you're not used to the normal stuff that occurs when really lugging the engine hard?

 

That's the individual firing pulses smashing your rod bearings to bits when you load the cylinders with lots of fuel and lots of air and the oil pump is just barely keeping oil on the journals. It's a really excellent way to ruin your rod bearings.

[BLOZ UP]

Lugging to me is high gear at 1500 RPM on a steep hill and the car is barely accelerating, if at all. This is 4th and 5th gear pulls on flat ground, the same as any dyno tuner will do. Also, oil pressure is already being regulated at around 60psi by 3000 RPMs in the VG30.

 

The shuddering doesn't happen until right at 3000 RPMs. If I get on it at 3k instead of 2.5k, it doesn't happen since full boost happens at or after 3500 RPM.

 

That said, ruining the bearings on this engine wouldn't be the end of the world. It's just a tuning mule for it's replacement.

Edited October 29, 2013 by BLOZ UP

[tooquick260]

I would try the spark gap. Easiest way to see if it fixes

[BLOZ UP]

That's the individual firing pulses smashing your rod bearings to bits.

 

Rotational speed kills bearings, not power. The force on the bearing at 6000 RPM is exponentially higher than whatever force due to power on it at 3000 RPM.

 

I have about 18 degrees timing at 15psi going to pull it back a bunch and try an even smaller gap. Problem is at low gaps I get misfires at idle on #4.

Edited October 29, 2013 by BLOZ UP

[tooquick260]

Can you try to leave that gap on 4 a little larger than the rest.

 

I know my dad played with gap a lot running nitrous on his air cooled beetle. Something with compression checking and adjusting the shot and setting the spark gap. Now thats custom tuning stuff there. His wet system for nitrous could change the jet for the fuel or gas. Played with that and would alter size on each cylinder specially if one showed signs of leaking seats or rings. 

[BLOZ UP]

Yeah, I currently have it about 0.005" higher than the rest. I didn't go lower than .030", I'll try that and pull some timing as well if that doesn't help.

[Xnke]

Rod bearings fail for two reasons, both of them oil-film related. RPM's cause the suface speed of the bearing to shear the oil film, and high specific impulse on the rods causes the rod to literally push the oil film aside and make contact. The latter isn't such an issue on the VG because the oil pressure is higher, most L's are only pushing 20-25PSI at 2000RPM. This latter effect is how detonation kills rod bearings.

 

I assumed you were turning a turbo L28, my bad. 2000RPM is definitely in the lugging range there; even though most don't consider it so.

 

So, the worst of the problem is right about the time you get into "full" boost? Does it correlate with temperature at all, on a cold day is it worse than a hot day, IAT's, ect?

 

If you run a smaller gap, say your 0.035" instead of 0.044", does it get better or stay the same? Are you running projected tip spark plugs? They are harder to foul in my experience; but they are essentially the same as adding in 4* of timing. I am waiting on a set of non-projected tip plugs to try in my car under boost. (BPR6ES is a projected tip NGK, BR6ES would be non-projected NGK)

 

Maybe pull a little timing right around 3000RPM; does peak torque occur near that range? Might be pre-ignition more than spark blowout; even if you're not getting any detonation. If you're hitting full boost right near the torque peak you could need some more fuel and less timing in that area. You can get a good idea of your torque peak by looking at your fuel maps; generally maximum fuel consumption lines up with peak torque.

[BLOZ UP]

Rod bearings fail for two reasons, both of them oil-film related. RPM's cause the suface speed of the bearing to shear the oil film, and high specific impulse on the rods causes the rod to literally push the oil film aside and make contact. The latter isn't such an issue on the VG because the oil pressure is higher, most L's are only pushing 20-25PSI at 2000RPM. This latter effect is how detonation kills rod bearings.

 

I assumed you were turning a turbo L28, my bad. 2000RPM is definitely in the lugging range there; even though most don't consider it so.

 

So, the worst of the problem is right about the time you get into "full" boost? Does it correlate with temperature at all, on a cold day is it worse than a hot day, IAT's, ect?

 

If you run a smaller gap, say your 0.035" instead of 0.044", does it get better or stay the same? Are you running projected tip spark plugs? They are harder to foul in my experience; but they are essentially the same as adding in 4* of timing. I am waiting on a set of non-projected tip plugs to try in my car under boost. (BPR6ES is a projected tip NGK, BR6ES would be non-projected NGK)

 

Maybe pull a little timing right around 3000RPM; does peak torque occur near that range? Might be pre-ignition more than spark blowout; even if you're not getting any detonation. If you're hitting full boost right near the torque peak you could need some more fuel and less timing in that area. You can get a good idea of your torque peak by looking at your fuel maps; generally maximum fuel consumption lines up with peak torque.

 

Thanks for the reply considering I was a complete a-hole about it.

 

So, yeah, it's right when I get close to full boost. Setting my controller to 12 psi or lower and the problem doesn't happen. I want to say it gets better with the smaller gap, but it's hard to tell, and I typically let off as soon as I feel it. I don't hear any detonation, but I also never heard detonation when a boost controller broke on my old VG30 years ago and I ran 30psi on stock injectors--cracking a head straight down the middle. I didn't try going lower than 0.030", and I'm going to try that this weekend if I get a chance, leaving #4 a little larger. They are projected.

 

Temperature doesn't seem to affect it. It's been getting colder and colder out with not noticeable difference. IATs are pretty stable regardless. (18-20 degrees C)

 

I will try timing if the smaller gap doesn't help enough. Peak torque on these motors is typically around 4400 RPM. My last VG was at that, this one seems similar since it's requiring more fuel around there.

 

Reason I ask is that I've only experienced and heard about blow out at high RPMs, where I am experiencing this problem at low RPMs. Car runs fine as far as I can tell at high RPM.

Edited October 30, 2013 by BLOZ UP

[Xnke]

I'd say you could pull some timing around 3000RPM, add a little more fuel, and it will change for the better.

 

Bet 30PSI felt GREAT till it quit...

[BLOZ UP]

I'd say you could pull some timing around 3000RPM, add a little more fuel, and it will change for the better.

 

Bet 30PSI felt GREAT till it quit...

 

Heh. It happened on a run at the 1/4 mile strip. It felt real slow, it was running so lean. After the run it was knocking like crazy. Still got me home, steam coming out from the valve cover and all. I only realized what had happened after doing an "I'm mad at you car" burnout and noticing the boost going waaay past what I set it at. Pistons ringlands and the skirts of one disintegrated upon dissembly. Have that piston as my phone stand at work.

Edited October 30, 2013 by BLOZ UP

[BLOZ UP]

Ok, so the key was 0.030" on all cylinders except #4, which is at 0.040" (pretty much stock). Runs like a champ now. It feels wrong to have that much difference, but my engine is wrong in the first place--#4 is pretty low compression and doesn't fire at all at idle with anything smaller than 0.035", and seems to be the one causing the misfiring at boost at anything lower than 0.040"--although I didn't test it much to be sure.

 

I also pulled a couple degrees right at peak boost at 3000 RPM for good measure, and interpolated a few lines around it.

Edited November 12, 2013 by BLOZ UP