LAracer

I just read on the FAQs at

http://www.autosportracetech.com/RX-7/rx7stuff.htm

that my boost gauge might read several psi (3-4psi by my calculations) higher because I live at 7000ft above see level. Is this because of the gauge or because the car gives more boost? i.e., does the ecu (and mine is stock) regulate the pressure to be 10psi above sea level or 10psi above the measured atmospheric pressure?

LAracer

No one else here lives in the mountains?

LAracer

Sorry but...Bump.

Mahjik

iTrader: (1)

Check this thread:

https://www.rx7club.com/forum/showth...boost+altitude

It has a good dicussion on boost and altitude.

As far as the gauge readings, I couldn't say for sure. But I do know that there are gauges that correct for things like that so I imagine that it is an issue:

http://www.rockfordracecraft.com/autometer.html

13brv3

From the FAQ:

Boost Gauge readings at above sea level?

Boost Gauge will read higher when above sea level. For example at sea level, should get around 12-13psi and at 4,500 Feet, read 14-15 psi. The Boost Gauge is referenced to the atmospheric pressure. Less atmospheric pressure, the more measured boost on the gauge. The ECU is compensates for the altitude by using the MAP sensor and an atmospheric sensor in the ECU, allowing for a constant amount of air volume. Yes, this means your turbos work harder at higher elevations.
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The error should be in the gauge. I would also expect your gauge to read higher than 0 when the engine is off, but I believe you mentioned in another thread that it does read 0. Unless someone calibrated it somehow, I can't see how that's possible. If the gauge does start out at something other than 0, I would read the difference between the starting value and the reading. In other words, if it starts at 2, and your read 12 on boost, then you have an actual boost of 10.

BTW- does anyone know where the atmospheric sensor is located?

Cheers,

LAracer

Thanks Mahjik. I feel stupid for not searching that forum now .

13brv3 (hello again): Shouldn't my gauge read *less* than 0 when the car is off because the ambient pressure here is lower than sea level (where the gauge was probably calibrated)?

I think I understand this now: The ECU tries to regulate the boost to 10psi relative to sea-level-atmospheric pressure; that is, 24psia (absolute pressure), since absolute pressure at sea level is 14psia. Thus, my gauge *should* start out at -3 to -4 psig (about 10-11psia) and max out at 10psig. This should make for a difference between local atmospheric and boost of about 13-14psi. So the turbos really do work harder.

I need to look at my gauge (with the car off) a little more closely. The vacuum side is a different scale (right?) than the pressure side. Maybe it is reading lower, but it just doesn't look as much. If the gauge somehow corrects for local atmospheric (doubtful), then I should not be surprised to see 13psi max boost.

LAracer

Just did a little checking, and the conversion between psi and inHg is about

1"Hg = 0.5psi (roughly)

So I should be seeing 6-8inHg with the car turned off. My gauge (PFS) reads up to 20inHg, so I'm sure I would have noticed 6 (still, I'll check again anyway ).

Thus, I think my gauge must give pressure readings with respect to the local atmospheric pressure. I guess I could contact PFS and ask...

Thanks for all the help guys.

13brv3

Hey, quit trying to confuse me

Oops, I think you're right.

BTW, shouldnt' you be out playing in the snow

RonKMiller

Unless you are running a really high quality gauge (which unfortunately is not the case among most automotive aftermarket gauges) I don't think you can really trust it to be all that accurate.

At best I would use it as an approximation, unless you have it calibrated against a laboratory standard. Where do you get it calibrated? (If it is even adjustable...)
You might have some luck with a local airplane repair station in a large city, but I doubt it. It would probably need to be sent out to a specialty repair facility, and then the cost of calibration would probably be more than the gauge is worth.

I've always thought the reason you see so many blown engines is that guys just are not getting accurate readings, and the difference between running on the edge and going over it is pretty slim.

LAracer

Well...if you belive the PFS website (and I'm not saying I do) the gauge is accurate to within 1psi. Not that great, but enough to where I should notice a difference at altitude...maybe(?).

Of course, the reason I care about this is that I am having boost problems (but that's a whole other thread, hehe). I see a max boost of 13psi, and I'd really like to know if that's equivalent to the normal 10psi or if there's something causing me to get more boost.

ZeroBanger

I had a post the other day, you can search my s/n. Nobody really cared to respond. I live in the bay area (sea level) drove to reno last weekend and at 6000 feet I pulled over and my power fc started idling my car crazy. turned the car off and waited 5 minutes. turned it back on, my vacuum reading from my power FC read -650 (i'm always at 480 to 490) yet my boost gauge showed my normal boost. when driving, my peak boost at full throttle was shown as .58 on my power FC, yet my boost gauge would show the correct 12 PSI. This behavior continued until I came home the next day. as soon as I hit sacramento my reading magically returned to normal. What I don't understand is why my boost gauge showed correct readings. I would guess the boost sensor cannot accurately detect boost at elevation?

RonKMiller

While I'm not a big fan of the man, I can say that I think his products are quite good although I could not find out what brand he is selling by looking at this site. The site does mention that his is "aircraft grade" or something to that effect in the PDF instructions file.

I think there are such things as compensated and non-compensated boost gauges. Obviously a compensated one would give you correct readings at any altitude. A non-compensated unit would fluctuate slightly (and probably undetected) from day to day due to variances in atmospheric pressure.

I have a Mitchell boost gauge (which is common in airplanes) installed in my car, and frankly I've never noticed any difference in boost levels from Tucson at 2,600 feet (where I live part of the year) to Vail, CO at 8,100 feet (where I live part of the year)

I notice huge differences due to ambient temps in max boost and quite frankly worry about it a little. I do run some synthetic 2 stroke in my tank for a little extra lube insurance, however that decreases the octane which can lead to - you guessed it - blown apex seals!

Damned turbo engines, but they sure are fun when they're running right.

KevinK2

The Mazda Rx-7 service highlights graphs the overboost fuel-cut curve in terms of absolute pressure (table at cirian site). This would suggest some degree of absolute pressure based boost control.

WG and PC solenoid meaurements suggest that altitude compensation would be more likely to occur after transition, since before transition the WG solenoid is near max vent cycle at sea level. This means stock FD that is say 12-9-10 at sea level, would be 12-X-13+ at 7k.

The dash boost gauge just tells how many psi above ambient, so zero is true regardless of elevation, as most have observed.

LAracer

I am now absolutely sure this statement is true, i.e. that boost gauges measure the DIFFERENCE between the manifold pressure and atmospheric pressure. Check out how a pressure gauge works:

http://www.fluidproducts.com/insideof.htm

The air from the manifold would be inside the Bourdon tube, while the outside of the tube is surrounded by ambient air. Thus, the deformation of the Bourdon tube (and thus the reading of the gauge) is dictated by the difference in those two pressures.

Thus, the problem comes down to how the car itself regulates boost pressure. KevinK2: I'm not completely sure I understand what you're saying. Are you saying that primary boost is regulated with respect to ambient and secondary with respect to absolute?

Can someone explain to me what, exactly, the mechanism is that controls boost at primary and secondary. I know that wg and pc are duty cycled, but how does the ecu know what to cycle them? Is it a closed loop control or open loop? if it is closed loop, then does the ecu have its own pressure gauge?

I'm so close to understanding this...

13brv3

Not to be difficult, but I don't think that's the way it works. The air thats surrounding the Bourdon tube is pushing equally in all directions. It isn't directly opposing the movement of the tube caused by pressure inside the tube. If there were a second Bourdon tube that opposed the first, that was open to atmosphere, maybe it would work as you suggest.

There are certainly two types of pressure readings, absolute, and differential. A boost gauge could be made either way, particularly if it's an electronic gauge. If it's electronic, look at the pressure transducer to see if there's a second port that's left open to sense ambient pressure. That would probably be the first clue. I would bet that most mechanical gauges are absolute, not differential. Also, on most mechanical gauges, you'll see a fat bar around the 0 mark. I believe this is meant to show that the needle will not be in the exact same place due to changes in atmospheric pressure.

BTW- an excellent example of an absolute pressure gauge is an altimeter. Wouldn't do to much good if it stayed on 0 all the time

Someone mentioned that the ECU has an atmospheric sensor, and the boost sensor. Of course the boost sensor is cleverly disguised as "boost sensor" on the firewall behind the intake elbow, but I have no idea where the atmospheric sensor would be, unless it's just build into the ECU box. Having two sensors, the ECU can use absolute or differential pressure as it pleases, though I certainly don't know how it's programmed to behave.

Interesting discussion.

Rusty

ZeroBanger

How bout this..

Which is accurate? I mean, if I am at high altitute and hit 13 psi on my power fc and 17 on my boost gauge, what psi am I really at? Thats the difference between a blown engine.

rynberg

The stock ecu does not control boost with a closed loop system. It simply opens the charge relief valve to vent the primary and pre-spool the secondary and then opens the wastegate to vent the primary+secondary. It opens these air relief devices the same way every time, regardless of actual boost achieved. This is why your boost increases when you add mods (or the temp drops), the charge relief and wastegate duty cycles have not changed. It is also why on many stock-intake cars, the boost drops to 7 psi at 6k rpm or so (like mine) -- the ecu is opening the wastegate (95% duty cycle) and is venting too much boost.

LAracer

How does a powerFC measure pressure? Same way as the ECU I guess? I honestly don't know. Can someone tell me?

ZeroBanger

I don't know the technical details, but it just reports what the ecu reads from the boost sensor or whatever.

13brv3

Rynberg, that was the clearest description I've seen written. It should go in a FAQ somewhere.

As for the PowerFC, it handles boost differently, though I haven't had the chance to play with mine enough to figure it all out. I do know that my boost used to drop to 7 or 8 at about 5500 rpm with the stock ECU, and it now stays at 10 all the way to the redline with the PFC.

Cheers,

KevinK2

The newbe links have lots of good info.

http://www.rx7turboturbo.com/robrobi...boost_note.htm

Note that high solenoid duty cycle (ave voltage) means more venting of the actuator pressure, and a more closed wg or precontol valve.

The stock boost control appeared passive during my measurements. If there is active control after transition, it is minimal and allows a lot of error. I suspect altitude compensation would increase the defaul venting curve.

Some PFC datalogit output shows the wg valve can 'spike' to high vent after transition if boost falls too low. PFC may have more active boost control.

LAracer

Thanks rynberg. (I didn't see your post before).

Since stock boost control is open loop, a change in air density (due to altitutde) would result in a boost pattern that is not 10-8-10, right?

I could really use some input from people living at altidude using the stock ECU. I'd like to know what boost pattern they get to see how different I should expect mine to be. There's just too much going on to be able to predict it I think.

KevinK2

This is not correct. Inside the curved tube, more surface area is available to straiten the curve with internal pressure. Same with external pressure only ... more area for decreasing the radius of curvature. for typical thin walled bourdon tube, put same internal and external pressure and the area effects cancel out, reads zero.

May help to draw a 180 degree bourdon tube, in 2D, to see the effect of the areas.

LAracer

How did I miss so many of these posts?!! I write too slow I guess

I think it is much more "natural" to make a differential pressure gauge (as opposed to an absolute one). As far as I can tell, the only ways to make an absolute gauge are to have an extra sensor to detect local atmospheric (which is then added to or subtracted from the differential measurement) or to make the gauge somehow contain a constant pressure for reference (or something???).

I'm learning alot

13brv3

After far too much reading about Bourdon tubes, I agree that it will zero correctly at altitude. It appears to act as a differential pressure device as far as atmospheric pressure is concerned. My final answer is that I think your 13 psi is 13 psi above outside pressure, which is probably 10 psi above sea level pressure. I'm thinking this won't require any more fuel, or be any more dangerous than seeing 10 psi at sea level. Of course, you realize how many times I've been wrong in this thread alone

The more I think about this, the happier I am with my decision not to put a turbo on my airplane engine

Cheers,