Boost at altitude

[longislandjim]

The supplemental turbo manual that came with our cars has a description of the boost gauge. It reads something like, the turbo electronic control system can maintain full boost at all altitudes. However, the boost gauge will read slightly lower boost at higher altitudes because the boost gauge is based on barometric pressure and boost.

 

My question is boost controlled from this calculated gauge pressure or from MAP? It would seem that if boost were controlled from MAP full boost would nearly generate as much power as at higher altitudes, assuming the turbo can still provide the boost. I say relatively because the turbo will consume slightly more power to make the same relative boost at the higher altitudes resulting is less net power.

 

If boost were controlled from the calculated gauge pressure (derived from the MAP sensor) the engine would certainly generate less power since the boost pressure would be less.

 

When is barometric pressure calculated? It seems it could only be calculated before the engine starts. If you were to start the engine at sea level then drive up a 5000-foot mountain the boost gauge would then read absolute pressure, not gauge pressure.

[z284pwr]

The stock boost gauge is out to lunch to begin with, its like a regular narrow band 02 sensor/gauge reading WOT readings....its a "I am guessing" number....

 

And no, boost is calculated by the boost control solenoid on the front of the intake manifold, which is plugged in to the computer, which then controled the wastegate by bleeding pressure out the BCS until the computer senses the desired pressure at which it applies it to the wastegate, or atleast it should, these computers are weird.

 

Barometric pressure is calculated when you turn the key on...or should, I have done most of my computer with cars with a Haltech, which gave you the ability to calibrate the MAP sensor for the proper baro reading by turn-key for x number of seconds then turning it back off, then it was calibrated, however, a GM one, I dunno? Jeff or some of the GM tuning guys can probably help you there

 

The altitude vs. pressure thing. High altitude has less dense air. So 12 psi at sea level will have more air density than that of 5000 feet, hence why it is safe to run *with a JeffM or another chip that allows more than 9 PSI before fuel cut-off* with the wastegate not hooked up or computer controlled at 5000 feet but not at sea level. Generally if you car has excellent spool and bottom end though your transmission won't be happy with you about it.

[Guest TurboSedan]

Barometric pressure is calculated when you turn the key on...or should, I have done most of my computer with cars with a Haltech, which gave you the ability to calibrate the MAP sensor for the proper baro reading by turn-key for x number of seconds then turning it back off, then it was calibrated, however, a GM one, I dunno? Jeff or some of the GM tuning guys can probably help you there

 

i'm under the impression the TGP cannot recalibrate for barometric pressure. i know the Turbo Dodges can though, but they have a MAP solenoid for this, right next to the MAP sensor. i could be wrong, i'm certainly no expert here, not even close! i would like to know for sure though....

[longislandjim]

Thanks for the response. I am not questioning the accuracy of the stock gauge as much as the statements in the turbo supplement manual, so the questions may only apply to the chip in a stock TGP.

 

Hopefully someone who has knowledge of the mathematical equation that eventually controls the boost control solenoid output from the ECU can help.

[z284pwr]

Thanks for the response. I am not questioning the accuracy of the stock gauge as much as the statements in the turbo supplement manual, so the questions may only apply to the chip in a stock TGP.

 

Hopefully someone who has knowledge of the mathematical equation that eventually controls the boost control solenoid output from the ECU can help.

 

I re-read your question, well, here is some more info that may help

My question is boost controlled from this calculated gauge pressure or from MAP? It would seem that if boost were controlled from MAP full boost would nearly generate as much power as at higher altitudes, assuming the turbo can still provide the boost. I say relatively because the turbo will consume slightly more power to make the same relative boost at the higher altitudes resulting is less net power.

 

Boost isn't controlled by either, it is controlled by the BCS which is read from the outlet of the compressor wheel, these boost pressures from the BCS are sent to the computer, and the computer uses it's math from there and TPS *Power Enrichment Mode* and *Boost Pressure Fuel/Spark control* as well as Knock and Spark Retard.

And no, it wouldn't allow more power at higher altitudes at the same pressure because of the air density. NHRA correction factor is somewhere around .4 sec at ~4500 feet for forced induction cars if I remember.

The T-25 is already nearing its max at 12psi and the air is just to heated to go any higher to really make much of a difference.

 

However like I said, at higher altitudes, you are safe running with no boost control at all, the trans won't like it, but it is safe to do

 

Hope this helps and as always, this is just basing off of what I'm assuming plus knowledge of the Haltech, anyone with corrections, feel free, I'm sure something is wrong

[longislandjim]

Assuming the T25 can make around 12 psi of boost as you state, than the stock TGP has enough headroom to keep 7.5 psi of absolute boost at most altitudes. 7.5 psi of absolute boost at altitude would read higher than 7.5 psi on a gauge at sea level.

 

The BCS does not read boost pressure, the MAP does. The BCS is told by the ECU to open and close an internal valve that modulates boost pressure. The BCS allows the ECU to finally control boost pressure by opening and closing the wastegate using boost pressure.

 

Since the MAP sensor is an absolute device by design it seems logical that boost control would not be affected by altitude. However since the boost gauge is using relative pressure that calculation must be there to use for boost control if the designers required it.

 

i'm under the impression the TGP cannot recalibrate for barometric pressure. i know the Turbo Dodges can though, but they have a MAP solenoid for this, right next to the MAP sensor. i could be wrong, i'm certainly no expert here, not even close! i would like to know for sure though....

 

Exactly. On some Dodge turbos a solenoid can switch the MAP to the atmosphere then back to take a barometric reading when the ECU sees fit on these cars.

[TurboGTU]

Most fords have two map sensors...EVEN 3! One on the manifold reading pressure, one open to atmosphere....and even one connected to the exhaust manifold!... :!: :?: .

My Ford turbo cupe has such a system..but uses a Vam meter..and a Map sensor open to atmosphere.

 

 

ON the TGP ECM...Barometric pressure is gathered only on "BOOT". From there...it adds corrections to fueling. Sea Level..it should read 2.5 volts on the map sensor. Im sure at 7000 feet it reads 2 or 1.5 volts. It will look up the VE tables and add baro corrections to those fueling tables. If you drove the Pikes and Peak ...the ECM would only get the baro reading from the start. BUT it will start to corect itself VIA the IAT sensor and Waste GAte DUty cycle as you climb up as the air density changes. The Duty cycle will increase as the ECM is trying to keep the same boost and the IAT sensor will read the increasing temp as if you try and compress thin air...to the same pressure as dense air..it will heat up more. Its self correcting.

The IAT sensor will do wonders all on its own. Its not just raw boost its calculating..its the Density ratio for fuel. ITS just too bad it cannot ajust boost for Density ratio. ONly RAW boost.

[longislandjim]

I am not sure I understand your definition of raw boost. The MAP reads absolute boost after the air has been already compressed or made denser at altitude. It seems that the ECU would then be using absolute boost when controlling the wastegate, not relative boost. The only way the ECM could figure out relative boost from what I am reading is from the IAT and wastegate duty cycle. If the ECU could derive the barometric pressure from these inputs it could use it to correct the absolute boost from the MAP to relative pressure. If the ECU knew the correct barometric pressure it could then make the boost gauge read accurate “gauge†pressure and use it for wastegate control.

 

It would also seem that if all this is true removing the wastegate from ECM control could cause problems with ideal fuel delivery ratios using a stock chip.

[z284pwr]

Okay, after going thru all these posts again, kinda off-topic, but on-topic at the same time? Why are you so worried about the stock boost gauge? Is it really that hard to get an aftermarket boost gauge?

 

Oh yeah, and you are right about your last paragraph, if you remove the wastegate from ECM control on a stock chip, it will mess up, boost fuel cut on the stock chip is what ~9 PSI, unplugged it will get what 12-14 PSI, so you would hit boost fuel cut yes.

 

So if the MAP sensor is what controls boost and fueling, why is the wastegate control plug on the outlet of the compressor, the reading at the compressor will be pre-intercooler, so should have a higher boost reading than that from what the MAP sensor would be reading post TB. So, which one controls how much fuel and which one controls wastegate opening? How does the computer determine which one to use to control what? The MAP sensor is used for fuel, that would mean the engine is running at 10-12 PSI and the turbo is "technically" pushing ~12-14 PSI right? What about a mechanical boost controller? Shouldn't it technically be the same way, it would open the wastegate that much sooner than if it were actually using the same reading as the MAP sensor, so by placing the MBC off a reading post intercooler you should technically gain that much more PSI and heat....is the added air worth the added heat from the 2 PSI?

[TurboGTU]

By RAW BOOST I mean GUAGE BOOST..not pressure ratio or desity ratio.. The ECM is programed to only boost certing PSI at certin RPM and Throttle points...

 

Heres the stock DESIRED boost settings for 100% TPS..or WOT.

RPM PSI

6400 7.69

5600 7.69

4800 7.57

4000 6.1

3200 4.52

2400 5.43

 

WE ALL know that 7.69 PSI at Sea level and with 60* temps at the manifold..would be a MUCH higher DENSITY RATIO than 7.69 PSI at 5800feet and 180*. NOW the ECM can ajust itself to fuel the engine for those desity ratios....because it takes in the IAT sensors temp...and what it got on boot based on BARO READING.

 

NOW if it were to boost acording to DENSITY RATIO... :shock: ... You'd be seeing 20PSI at the guage in Colorado summer days to keep the same density ratio here in the Texas coast line at 7.69psi in a winter night. :!:

 

And by Waste Gate Duty cycle..I mean..the ECM is saying..."HEY...50% duty cycle isn't working to keep 7 psi...SO I'll kick it up a notch to 51%..see if that will keep the boost."....and does so till the desired boost is reached. THe waste gate has no sensor...its just a air relay...kinda like a fuel injector.

 

SO here it how it works.

 

"BOOT" it gets 12psi for borometor reading.

It stacks that to ALL the fueling tables.

SO what now used to be 2.7 vacume is now 14.7 psi..OR no VACUME. (hence why boost guages read wrong...they are not ECM controlled, They grab the reading straight from the map sensor.)

So what used to be 4.9 PSI BOOST is now 7.6 PSI BOOST. And ect. (I should have used KPA as thats more explanitory).

 

MECHANICAL boost guages work perfect..because they DON"T use a isolated diapram like map sensors. Im sure even ELECrical boost guages have a similar BARO reading during boot and then ajust accordenly...if they don't..they need to be ajusted manualy.

 

IF YOU choke the IC pipe with a 1/2in pipe..the ECM would keep 100% Waste gate duty cycle (Waste gate acutator fully closed)...to keep the engine at 7.6psi. This means that the turbo pressure before the choke pipe would be about 15-30psi..POOR little bugger. . Hence why McLaren desighend the 7.6psi cap limit. You could boost 14psi (doing it yourself..not ECM..ecm would keep the 7.6 not matter what)on good elevation ,cold weather, but only 7.6 psi on a rocky mountin on a hot summer day. Any more and the turbo would go off its flow map (efficiancy) and go into surge (BAD..verrry BAD).

 

I hope I didn't confuse you any more. :oops: . So some of your points were there...but not all.

[z284pwr]

No confusion here....one question though, just curious...

 

What should a STOCK TGP wastegate duty cycle generally be at WOT with the its programed stock boost? ~7.6 *After the boost ramp maps have finished pulling boost off bottom end.

In comparison, what should one be with a programmed higher boost count *JeffM Chip, or any other that allows ~12PSI*?

[longislandjim]

I hope I didn't confuse you any more. . So some of your points were there...but not all.

 

Thanks for the excellent post. It seem the boost gauge on the dash gets its input directly from the MAP. This will lead it to be inaccurate at altitude just as the manual states.

 

I wasn’t trying to imply boost limits being controlled by density ratio completely, more so by absolute pressure.

 

I would assume during WOT, 5600 rpms, at sea level, when the MAP sees 7.69 psi + 14.62 psi, (50.3 kpa + 101 kps) or 151 kpa, the stock ECU would modulate the wastegate to keep this boost level.

 

Now at 5800 feet, say the atmospheric pressure is 11.82 psi or 81.8 kpa. The stock ECM still has the limit of 151 kpa programmed in the tables to maintain boost. Since the MAP sensor is absolute, to reach same limit of 151 kpa in the ECM tables the turbo would have to boost to (151 kpa – 81.8 kpa) or 69.2 kpa or 10 psi. However the stock boost gauge would not read lower in this case so the system must not work like this, but it would be nice if it did.

 

The only other way this could work is if the ECU uses the barometric pressure to offset the MAP sensors absolute reading. In this case the ECU would use the barometric offset factor subtracted from the 151 kpa limit to set the level the wastegate should be modulated at. The stock boost gauge would now read lower at full boost at altitude as the manual states.