It's not often I come here with a serious puzzle, but...

A restriction in the intake to the turbo would show as a drop in boost and a spike in shaft speed. That's usually when the comp wheel decides to seperate from the shaft. or that's when the turbo decides to suck oil out of the center housing into the intake stream. Check for oil in the outlet of the turbo compressor cover. I've also see air filters collapse under high boost scenarios. Make sure your filter is supported effectively.

I'd double check your waste gate. I wonder if you have a faulty spring. Have you tested it ?

Fueled by Caffeine is not ignorant about turbo design and function.

In reply to Fueled by Caffeine:

Have you ever seen a wastegate deform a little due to exhaust temp? I've heard some postulation that it will happen, but never in action. I have seen some evidence that they tulip under some conditions- but that's much longer lasting than this.

I didn't note that before, as the boost is high for quite a while before it drops off. But at the point where boost drops quickly, it's been running high for a while, so getting nice and hot. At these conditions, it does seem as if a leak would cause a sudden drop off.

DaveEstey wrote: I don't think your hot side is big enough. Likely need a 1.2AR

Barring an intake restriction (can pre-turbo vacuum be monitored/logged?) I think it might be too big. Can't keep the turbo spinning on the downside of the VE curve. The fact that the boost is dropping below the wastegate spring threshold supports this - it can't be a controller issue.

So, just pulling from the fact that you only get this issue in 3-6 gear, and not when on the dyno (I assume the dyno pulls were in a gear higher than 3rd). So, I am assuming you only get this problem when actually driving the car, going 80-100 mph plus.

Is it an aerodynamic problem? Are you getting some sort of low pressure bubble or turbulence just above your air filter at high speed? Seems like the problem would continue after a shift and be speed related not rpm related, but the turbo might be able to make up for the lower intake pressure at lower rpm? Just a wag, but an idea certainly.

Maybe try some sort of ram air setup?

Another crude thought: It's dropping from 240kpa to 170kpa in the intake manifold. What is the pressure in the pre-throttle body ducting? Is this a pressure drop due to the throttle body? Unlikely but it's something to check and therefore rule out.

I really really want one of those dual boost gauges. Show pressure in the ducting and the intake manifold.

bgkast wrote: I think you nailed it with the wastegate being pushed open by exhaust back pressure. Is there a convenient place you can tap with a pressure gauge pre-turbine? I measured back pressure when I did my VGT Mercedes Diesel and it's shocking how much back pressure there can be. I bet the measurement will show it dropping at the same time as boost. Compare your peak reading to the area of the wastegate valve and spring force. Or you could just shim or swap the existing wastegate spring with a stiffer one and see if the problem goes away.

Yes but by switching to a 4-port wastegate controller, the ECU is now able to actively hold the wastegate closed. No longer is MAP pushing the wastegate open - it's now being used to hold it shut. As the plot shows, the ECU is indeed attempting to hold it shut, but fails.

Wastegate manufactures state that when specifying wastegate spring pressures, it assumes that exhaust back pressure is roughly the same as boost target pressure. If it's measured at 170 kpa, that's about right. If it's 220 kpa, that's high(er), but does that make it bad? I guess I can go ahead and measure it; I'm just not sure what I'm going to do with that information.

Fueled by Caffeine wrote: A restriction in the intake to the turbo would show as a drop in boost and a spike in shaft speed. That's usually when the comp wheel decides to seperate from the shaft. or that's when the turbo decides to suck oil out of the center housing into the intake stream. Check for oil in the outlet of the turbo compressor cover. I've also see air filters collapse under high boost scenarios. Make sure your filter is supported effectively. I'd double check your waste gate. I wonder if you have a faulty spring. Have you tested it ?

Agreed. Much like a blocked vacuum cleaner, the compressor wheel could spin up like crazy due to no load. Back when I first swapped in the larger turbo, there was in fact some oil found in the intake tract. However, removing the oil filler cap stopped it, which means that it was a problem with excess crankcase pressure due to higher boost. The engine's since been switched to a dry sump oil setup for that and other reasons, and it no longer leaks.

The air filter is a flat element type; if it collapsed it would be very apparent. The picture of the intake box and tube shows that the air only has about 6" to get to the turbo inlet. There is a 4" bellows-type silicone coupler, but only about 1" is unsupported, so collapsing seems unlikely. However, it's easy enough to remove the entire box and try a run, though I hate running the turbo without filtered air.

The wastegate is a Tial 38 mm water cooled unit, brand new, with a "7 psi" spring, also brand new. While it's possible it's defective, I don't understand how it could maintain proper pressure for so long and then go weak.

Knurled wrote: Another crude thought: It's dropping from 240kpa to 170kpa in the intake manifold. What is the pressure in the pre-throttle body ducting? Is this a pressure drop due to the throttle body? Unlikely but it's something to check and therefore rule out. I really really want one of those dual boost gauges. Show pressure in the ducting and the intake manifold.

No, I haven't measured it, at least not directly. However, a few people suggested moving the wastegate MAP source from the upstream side of the throttle body to the downstream side (intake manifold). It made no difference so I doubt it's too terribly different.

Wastegate should always be routed to the pre-manifold side of the throttle body. IMO.

If the throttle body is a restriction, I don't think it would make a difference where the wastegate is routed to, since the issue would be the same as running at part throttle.

Monitoring exhaust manifold pressure would be interesting to be sure. It's not difficult to rig something up if you have an O2 bung upstream of the turbo. Miata diff plugs thread in (as do RX-7 trans drain plugs, and '83-85 12A oil passage blockoff plugs...) so you drill and tap for 1/8NPT, install a 1/8NPT to 3/16" brake line adapter, run a foot or so of brake line out to a long length of vacuum hose, then stick a pressure gauge on the hose. No gases will flow through the hose so it won't melt. I've driven for hours with this rig in place with no issue, with the hose on a 3" piece of hardline.

Actual boost vs. backpressure will depend on Many Factors. I've see 3:1 with no ill effects... but the thing to watch for would be a change at the RPM where boost falls.

rcutclif wrote: So, just pulling from the fact that you only get this issue in 3-6 gear, and not when on the dyno (I assume the dyno pulls were in a gear higher than 3rd). So, I am assuming you only get this problem when actually driving the car, going 80-100 mph plus. Is it an aerodynamic problem? Are you getting some sort of low pressure bubble or turbulence just above your air filter at high speed? Seems like the problem would continue after a shift and be speed related not rpm related, but the turbo might be able to make up for the lower intake pressure at lower rpm? Just a wag, but an idea certainly. Maybe try some sort of ram air setup?

Yeah it seems like a stretch, but who knows. Right now the air inlet is a rear-facing scoop, mostly because air is swirling back toward the backside of the windscreen, and it keeps any rain water from entering.

About it not appearing on the dyno plot, yeah that's something to look into, in fact I might go back and have them do another run to find out. If it's not there, that means that yes, it's present on the road only. If it is there, it means either I set something wrong, or maybe something's broken since the tuning session.

If I had a 1.0 AR turbine housing, I'd switch it in a second just to try it, but at ~$550 it's a big $$$ hit if that isn't it. As said above, I'm not sure how much it would move the torque curve higher and possibly make it narrower. Sigh, what's that saying, "gotta pay to play?"

On my VGT setup back pressure was sometimes double boost the boost pressure.

bgkast wrote: On my VGT setup back pressure was sometimes double boost the boost pressure.

I'll use you as an example then :)

So you measured the backpressure... but what did you do with that information? Did you go with a larger AR housing to reduce it? If so, were you happy with the results, or did it make the engine lazy and peaky?

I used it to resize the diaphragm diameters of the vacuum and pressure actuators I was using to contol the turbo vanes. I was using two opposing actuators to move the vanes, one that opened them with pressure to limit boost (like a wastegate) and one that opened them with vaccum to increase efficiency and reduce back pressure at part "throttle" cruise. The overall effect was similar to your wastegate's double sided diaphragm with pressure acting on one side and vacuum on the other, but by using two actuators with different diaphragm sizes and spring rates i could adust the vacuum and pressure that opened the vanes independently.

What are the differences in 1st and 2nd gear that are no there in 3-5 at 7500+rpm. Let's list those out.

Load. It can't put all of that power down in 2nd.

Could it be on the fuel delivery side? Like cavitation/overheating pump?

bgkast wrote: Load. It can't put all of that power down in 2nd.

Define that. More fuel? More Air? What...

Let's all define it...

Here's what I'm getting at.. an engine at 7500 RPM should move the same ammount of air regardless of the gear selected.

Most of my experience is with internally wastegated compression ignition engines.. Not what we have here. I hate throttles plates.. they're a bummer.

AngryCorvair wrote: Fueled by Caffeine is not ignorant about turbo design and function.

Thanks, but my experience is from many years ago.

In reply to Fueled by Caffeine:

Not full load = not full Throttle, or slipping tires...so more air and fuel. I bet data logs of the phonomon in 2nd and 3rd would show that the engine is not at WOT in 2nd when it's not happening.

alfadriver wrote: In reply to Fueled by Caffeine: Have you ever seen a wastegate deform a little due to exhaust temp? I've heard some postulation that it will happen, but never in action. I have seen some evidence that they tulip under some conditions- but that's much longer lasting than this. I didn't note that before, as the boost is high for quite a while before it drops off. But at the point where boost drops quickly, it's been running high for a while, so getting nice and hot. At these conditions, it does seem as if a leak would cause a sudden drop off.

I must preface my response with some conditions. All of my time was spent with compression ignition engines with internally wastegated turbos. Conditions are a bit different in a spark ignition scenario. I've never seen a wastegate flapper deform, I have seen them break or get jammed by chucks of left over casting cleaning shot stuck inside exhaust manifolds. I could check around with some of my old buddies.

A couple more tidbits.

Fuel pressure is logged, including the pressure drop differential across the injectors. It should - and does - stay constant with load and rpm, so that rules out a fuel system issue. Also, as noted above - and had I thought it through - it's unlikely to be an ignition thing either. If the air/fuel mix wasn't lighting off in a cylinder, even intermittently, the AFR should go rich, and that's not seen in the logs. So while I have new ignition coils on the way that'll be swapped in, I no longer hold out much hope that they'll solve anything. They were $140...

I hate to also spend $550 on a turbine housing that might also do nothing. I just don't see how a small turbine housing can cause MAP to drop off like a light switch - gradually yes, suddenly, I'm not on board with. That makes me think that something else is going on.

We've about covered very subsystem in the engine, so it's likely that something I'm discounting isn't as I think, or one of my either-or conclusions has a third possibility - that's happened to me before (and is rather humbling).

Maybe the interim step is to put the car back on the dyno and see if the problem is replicated. Either way, they can measure exhaust back pressure, though as I keep saying, I don't know what I'll do with that info, but good data is always nice to have.

Fueled by Caffeine wrote: Here's what I'm getting at.. an engine at 7500 RPM should move the same ammount of air regardless of the gear selected.

Consider these two situations.

1. A car coasting down a hill at 60mph at 7500 rpm with the throttle shut.
2. The same car driving up the same hill at 60 mph at 7500 rpm.

Engine speed is the same in both cases, yet the amount of work it's doing is vastly different. The amount of fuel consumed and burned, the amount of exhaust gas passing through the turbo, and the amount of boost made will cause significantly different air flow rates.

kb58 wrote: If the air/fuel mix wasn't lighting off in a cylinder, even intermittently, the AFR should go rich, and that's not seen in the logs.

Lean, not rich. Oxygen sensors, both narrowband and wideband, measure oxygen content, and a misfire will make it read lean from all of the oxygen.

Either way, a misfire will still be visble by sensor monitoring, and you're not seeing it happen, so that is probably not it.

Maybe the interim step is to put the car back on the dyno and see if the problem is replicated. Either way, they can measure exhaust back pressure, though as I keep saying, I don't know what I'll do with that info, but good data is always nice to have.

Yep. If it does it in 3rd gear on the dyno, you know it's not some weird air induction effect.

Knurled wrote: Lean, not rich. Oxygen sensors, both narrowband and wideband, measure oxygen content, and a misfire will make it read lean from all of the oxygen.

Dang, absolutely correct.