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

kb58 wrote:

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.

An engine is an air pump.. at7500 rpm.. keeping boost the same and compression ratio the same, it moves the same ammount of gases.. The veloicty and temperature may change, but you cannot change the size of the cylinders.

Ah, keeping boost the same. Okay then.

So.. He's what I'm thinking.. You're too close to surge and the turbo is too big.

Granted I can't find an exact map.. but I used this one.

Back of the hand math shows a 2.5-2.7 pressure ratio at 20 psi a 2.4 is playing in the 20 - 30 lbs/min range

Am I crazy, been years since I did this...

Well you're not CRAZY but your math is off. Turbo surge happens on the left side of the tach, not the right. If it is surging at a 2.5 pressure ratio at 7500 then it won't be making boost at all below that.

I don't know nothing bout no turbo K-series but if it is making 400hp then it needs something like 50 lb/min airflow. I think. I'm working backwards from 400hp times .65lb/hp/hr (appx BSFC) divided by 60 (minutes) times 11 (air/fuel ratio) and then round number. Close enough for a guesstimate I think.

Knurled wrote: I don't know nothing bout no turbo K-series but if it is making 400hp then it needs something like 50 lb/min airflow. I think. I'm working backwards from 400hp times .65lb/hp/hr (appx BSFC) divided by 60 (minutes) times 11 (air/fuel ratio) and then round number. Close enough for a guesstimate I think.

Could be.. I used some generic numbers off a garrett calculator. BSFC could be off. https://turbobygarrett.com/turbobygarrett/boostadviser

Ohh yeah, you're right I'm a bit off..

I wish garrett had a map for one with the alternate exhaust housing.

Yeah I also failed to ever find a turbo map that uses the 0.82 AR housing.

A couple thoughts:

1. This is a twin scroll turbine housing. It has to be more restrictive than a non-twin scroll because it has the addition of a center wall on the flange. In other words, I suspect that a "0.82 AR" twin scroll housing is equivalent to something like a 0.7 AR regular housing.
2. Even if the above is true though, I have a hard time believing that a under-sized turbo is going to kick MAP in the nuts like that. Taper off, sure, but not this light switch thing I'm seeing.

Compressor map is compressor map, it doesn't care what the hotside is.

Likewise A/R is A/R no matter single or dual scroll. Area is area. Technically a dual scroll will have more flow resistance because it has more surface area but in the real world you can go a size or two larger with dual scroll if you have a properly designed exhaust manifold. Some of the rotary guys use 1.32 dual scroll housings to replace .91 single scroll... Benefit is you get the same spool but less exhaust restriction, lower exhaust restriction means more power, and less detonation due to reduced residuals.

Rotaries also have the exhaust umph to work with such huge hotsides

Knurled wrote: 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.

Missed this one. The problem is that it's a "chicken and egg" sort of thing. That is, at ~7500 rpm, MAP drops off, but MAP is so central to any tune that nearly everything is related to it either directly or indirectly. MAP drops, fuel pressure drops, fuel delivers drops, spark advance increases... because they should. The trick is figuring out what came first. If any of the variables that normally follow MAP is actually the culprit itself, it would in turn also drag everything down. The trick is determining which is the leading factor.

You note that the thing to watch for would be a change in exhaust back pressure at the rpm where boost falls... I already know that'll be seen since back pressure is a consequence of all the above factors. Since they're all dropping off, so to will back pressure, then I'm right back to where I am right now, not knowing "what's leading the parade."

This is a silly question, but what is the top end of the MAP sensor for pressure readings. Are you "freaking" the sensor out by running "too high" MAP...

What's up with your spark?

I've seen something like this due to one of three issues.

Fist is cold side leaks, build a pvc inlet and pressure tester from a cap and a compressor fitting, make sure to test with your throttle open and closed, also check crank case and head ventilation, shouldn't factor in if the engine is healthy but you never know...

Second is knock sensor, or in my case noise from the injector fly back circuit on our megasquirt leading to over rpm leading to massive timing regard, we found this after quite a bit of testing and logging...

Third is Watergate improperly set and too small for the volume of air leading to a bizarre surge, pressure drop roller coaster, get a manual boost controller, set it with your compressor at lower pressure and step up gradually. If it's consistent until higher pressures it could be the culprit...

As for fuel vs spark vs pressure...if the AFR is stable then it's not the fuel as in fuel pressure, and if the AFR is stable then spark isn't the issue because it would be maintaining AFR especially since this isn't happening in lower gears...leaving you with a boost leak...

We've run our 3 BAR map past its operating range and just filled in the targets based on nominal duty cycle and ARF targets and haven't seen anything weird but we only go over by 3psi

Could it be something to do with the valve timing at that RPM? Turbo cars don't need much overlap, NA cars do. You mentioned RSX cams. Could there be too much overlap at that rpm, and the intake charge is getting blown out the exhaust valves?

Fueled by Caffeine wrote: This is a silly question, but what is the top end of the MAP sensor for pressure readings. Are you "freaking" the sensor out by running "too high" MAP...

50 psi

Boost_Crazy wrote: Could it be something to do with the valve timing at that RPM? Turbo cars don't need much overlap, NA cars do. You mentioned RSX cams. Could there be too much overlap at that rpm, and the intake charge is getting blown out the exhaust valves?

I don't believe so. Other turbo Honda engines use RSX cams without issue. In fact, some have tried "turbo cams" and returned to the safety of the stock cams due to their conservative overlap.

Raze wrote: What's up with your spark? I've seen something like this due to one of three issues. Fist is cold side leaks, build a pvc inlet and pressure tester from a cap and a compressor fitting, make sure to test with your throttle open and closed, also check crank case and head ventilation, shouldn't factor in if the engine is healthy but you never know... Second is knock sensor, or in my case noise from the injector fly back circuit on our megasquirt leading to over rpm leading to massive timing regard, we found this after quite a bit of testing and logging... Third is Watergate improperly set and too small for the volume of air leading to a bizarre surge, pressure drop roller coaster, get a manual boost controller, set it with your compressor at lower pressure and step up gradually. If it's consistent until higher pressures it could be the culprit... As for fuel vs spark vs pressure...if the AFR is stable then it's not the fuel as in fuel pressure, and if the AFR is stable then spark isn't the issue because it would be maintaining AFR especially since this isn't happening in lower gears...leaving you with a boost leak... We've run our 3 BAR map past its operating range and just filled in the targets based on nominal duty cycle and ARF targets and haven't seen anything weird but we only go over by 3psi

I don't understand what you mean by "what's up with your spark?" Are you saying there's something wrong? Spark timing is set by a 2D table with rpm and MAP as inputs and the log files show that it's tracking properly based upon those inputs. As said before, MAP goes haywire and drags every related variable with it, and spark timing is one of them. Referring to the logger plot referenced in the first post in this thread, right at the vertical blue line where the anomaly occurs, spark timing is 16 degrees, which is correct for the given MAP and rpm. 0.3 seconds later it's 19 degrees and the lookup table shows that at a MAP of 136 kpa and an rpm of 7600, spark timing should be 19 degrees, and it is, so spark timing is correct per the inputs to the table.

About one second later, the throttle is closed and the engine's slowing with an rpm of 6400 rpm. At that time, MAP is 17 kpa, and for those values, the spark lookup table specifies 37 degrees, which is exactly what it is. That's why I'm asking what's meant by "what's up with your spark", since it's where it's supposed to be - based on the values it's given.

Checking the intake for leaks is slightly more complicated than that, since with the throttle open, the air's free to flow through the engine and out. That said, it will still be done.

Crankcase and head ventilation is handled very well by the dry sump system, drawing approximately 10" vacuum.

Knock detection has been disabled until this issue is solved.

Wastegate sizing is good, and in fact the boost control duty necessary to bleed off pressure is lower than most other engines due to the free-flowing header and wastegate design.

Regarding the manual boost controller: Even without any boost control - running solely on the wastegate spring - the anomaly is still present, so regardless how the wastegate is controlled, the problem still exists.

Lambda is stable right through the anomaly, which seems to rule out fuel and spark. Fuel pressure is read by the ECU, allowing it to maintain a constant fuel delivery by monitoring the fuel pressure - MAP delta.

Sorry if I'm a bit testy, it's been a long day and I thought I'd explained much of this. I know you guys are trying to help and I appreciate it. It's just that you can't lob in a "what's with your timing" without explaining what's behind it

I obviously still have a problem, so something I'm claiming is "fine", simply isn't, apparently.

I talked to a well-versed turbo buddy who thinks the turbine housing might be a bit small due to it being a twin-scroll setup. However, he said that an undersized turbo will only gradually choke off flow over several 1000 rpm, not shut it off over 0.2 seconds and 200 rpm.

Not sure I saw this but what is the wideband setup like? I have seen "slow" setups miss misfires entirely due to signal averaging. I have seen this behavior come from misfires quite a bit and always ignition related.

In reply to kb58:

One thing that may help- it would be nice if one was able to move the cursor themselves. May help the observations that MAP dropped first, and then everything else reacted.

Interesting observation that even run w/o boost control and on the spring- this phenomenon happens. Is it safe to say that on the spring, the max boost is much lower than when you have control over it?

So the only common thing that is happening is that you loose boost at about 7500rpm, if you are there long enough (3rd and up gears).

It's NOT

-Spark control- it's not going anywhere until MAP drops.

-Fuel pressure- no reason to think it's dropping off, especially if it happens at lower MAP's on the spring

-Fuel- assuming the UEGO is reading and recording fast enough- but it would have to be some odd aliasing to not pick up misfires a few seconds after the event.\

Wastegate due to pressure- if it WAS that, it would open long before that. Unless there's a big spike in exhaust back pressure.....

wait.

You've looked at the intake- is there anything in the exhaust that moves (other the turbo)? Seems unlikely, as that would manifest itself a little in the data, but wanted to put that in there.

Back to the not:

-ECU- you've changed it, plus since you've taken control of the wategate out, I would more say it's a mechanical problem.

All we have is that MAP drops off at 7500 rpm. Which suggests that it's a vibration issue more than an airflow issue- if you've seen it happen at lower MAPs with the wastegate on the spring only. How much is the engine moving? Anyone watch it while it was on the dyno?

Thanks Alfa...that was what I was getting at...chicken or egg...spark vs fuel vs airflow or in this case pressure...determining which is moving first based on the resolution of the log appears ambiguous, I wish the sample rate was higher, might give a clearer picture, but I feel like some physical testing is going to be necessary.

I would just like to say:

1) This thread is awesome

2) We need more threads like this on GRM, brings me back to the good ol' days on here

I have nothing further to contribute.

HiTempguy wrote: I would just like to say: 1) This thread is awesome 2) We need more threads like this on GRM, brings me back to the good ol' days on here I have nothing further to contribute.

Here here... It got me thinking about how much I miss the automotive business and engineering. I do miss it. Tech pays the bills for now, but boy I miss automotive.

Also, it reminded me how little I know about spark ignition engines.. :-)

In reply to Fueled by Caffeine:

Come back http://corporate.ford.com/careers.html?gnav=footer-aboutford

Paul_VR6 wrote: Not sure I saw this but what is the wideband setup like? I have seen "slow" setups miss misfires entirely due to signal averaging. I have seen this behavior come from misfires quite a bit and always ignition related.

A good point as well. At engine speeds that high, an inconsistent misfire may average out enough to not be noticeable.

Certainly, dropping a heat range colder on the plugs and closing the gap 5 thou is cheap enough to try. Going to 10-range NGKs on my RX-7 cured a top end miss (more like a "softness") that only occurred on long pulls. As a bonus the R55761A-10s I use are a quarter the price of the RX-7 specific plugs I used to run

The solution may be at hand:

I was contacted by a very knowledgable turbo engineer, who said not to write off the wastegate being pushed open by exhaust back pressure. He said that even using a 4-port controller - which allows using MAP to help hold the valve closed - may still not be sufficient. He also said that exhaust back pressures as high as 200 kpa above MAP is not unusual, and that's the problem.

A 7 lb* wastegate spring implies that it will result in a MAP of 7 psi above ambient, or about 22 psia, but that may well be less than the exhaust back pressure, which could be as high as 30 psi (200 kpa). In his own words, "there's some apples-to-oranges math due to the ambiguous wastegate spring rating, but you get the idea." His solution is to swap in a stronger wastegate spring, something more on the order of 10-14 psi.

Also, he did suggest possibly going with a larger AR turbine housing. He said that while spool-up would be delayed by about 300 rpm at the low end, the engine would pick up about 30 hp at the high end, and exhaust back pressure would drop, allowing a smaller wastegate spring to be used. Due to the cost however, I think I'll just try out the higher spring rate and see how it runs.

The real solution is an electronic wastegate that doesn't use MAP as the pressure source to control the valve. Some cars already use them.

*Wastegate spring manufacturer has to assume some level of exhaust gas pressure when coming up with their boost ratings. Tial assumes exhaust back pressure is equal to MAP.

About this "awesome thread"...

I see it as "be the change you want to see." I greatly enjoy open discussions that figure out problems. Not only is the person presenting the issue helped out, but everyone walks away smarter. Best of all are threads that have the solution at the end, not the typical threads that just die out, leaving everyone wondering what happened.

On some of the other forums I'm on, they're dying off because no one bothers to spell things out. Worst of all are threads like "My car runs like crap, why?", then they get pissed when no one answers, and they never post what the problem was. Just my contribution... sort of like peeing into the Grand Canyon sometimes, but if we all do it... okay, bad analogy, carry on! I'll change the wastegate spring because I'm pretty sure that he's right.

Oh, and lastly, I built up a turbo car in order to learn all about them, and boy, have I. What keeps amazing me is just how much more involved the system becomes with the addition of just one simple spinning part!

kb58 wrote: About this "awesome thread"... I see it as "be the change you want to see." I greatly enjoy open discussions that figure out problems. Not only is the person presenting the issue helped out, but everyone walks away smarter. Best of all are threads that have the solution at the end, not the typical threads that just die out, leaving everyone wondering what happened.

I try to always post a followup. Further, I try to avoid acronyms, cutesy community nicknames, misspelling critical words, and the like. Acronyms are usually not searchable, and cutesy nicknames are useless if you're not from whatever clique invented them. The largest problem I have in searching out solutions online is learning what people are talking about because they're too lazy to write things out.