I have a thought and i figured i would bounce it off the forum. I live at 5100' 12.2 baro or roughly 85 kpa. Thats a roughly 2.5# hickey just to get back to sea level. Im currently running 10# through my sbe Ls1, in reality is this engine only seeing what would be 7.5# of load/stress?
If you were ok with running 10# at sea level, in theory wouldnt you see the same load and stress at 12-13# here at altitude?

 

Are your power numbers SAE corrected? Boost numbers are almost irrelevant in this case.

 

NO because boost pressure comes with the attached rising IAT. Your starting pressure will influence how much boost is required to "get back to sea level" exactly as you suggested- but that additional boost will warm the air some above atmosphere temp so there is that price tag attached which works against the octane of the fuel.

So what you really want to do is, use methanol/water to ensure safe operation then use as much boost as you need to get the engine where you think it needs to be (you determine safe output of the motor, and the compressor determines the max ceiling for power)

 

Im on e85 with a fmic. I think my high iat was 97* at 10#

 

Yes numbers were corrected to 585/598.

 

What are you using to log boost? Most boost gauges hit "0" at around 100 kpa from my experience (I live at 79-80 kpa). So if you're running 10 lbs of boost on the gauge, it's technically 12-13 lbs of boost from atmospheric here.

 

We were using a phone to log registered boost vs rpm while on the dyno then going back into the log to verify pe/boost timing. I see boost at 85 kpa. I realize that if i went to sea level that this thing would make 13# or so if everything was left alone boost controller wise but would be lean as all hell fuel wise, but 10# at sea level isnt 10# at 5200'. I should have looked at the uncorrected hp numbers while we were on the dyno. That would have told me actual stress on the engine. If i made 585 corrected id be willing to bet its only 540ish actual. Which would mean theres room for safe hp to be made. Im not sure why it really matters, the thing rips the tires off above 5000 rpm anyway. More hp is just gonna make it worse.

 

My boost gauge is only about a 1/2#off zero, key off. Thats why we only ran to 9.5# gauge as we agreed to run to 10# or 600hp, whichever came first.

 

A boost gauge will still read 0 whether you are at sea level or 50000 feet because it references the atmosphere, whatever atmosphere you are in.

You would need to mount the gauge somewhere at sea level, then run a line to your engine at XXXXfeet to see the difference in pressure.

 

I understand that, my gauge from autometer is a half pound off indicated, verified with a regulator. But once again 10 at 5200' doesnot exhibit the same amount of force internally as 10# at sea level. A 10# manifold pressure at sea level will read around 160kpa. That same 10# gauge reading here will only net you 145kpa give or take.

 

10psi is 10psi, no matter where you go. I think you are confused with air density vs air pressure. The gauge checks atmosphere and that is baseline, say 0psi. If you are at 5000feet is still reads 0psi baseline.

Then if you add 1 or 10psi, it will still show that exact amount. If it does not, your gauge is out of calibration. pounds of force is still the same pounds of force, think of it like a wrench you turn, it doesnt matter if you are 5000feet or -3000 feet, 1lb per foot is still 1lb per foot, minus the negligible difference in gravity from being closer to the center of Earth.

I never tested it I am only speaking from chemistry/physics point of view. Maybe I am missing something. too lazy to search tho!

 

Yes ten psi over 12.2 is ten psi over 12.2, but ten psi over 14.7 is 2.5# more force in direct comparison(sp)

 

Basically what im getting at, is in theory i will have to run 2.5 more pounds of boost at altitude to meet the same uncorrected hp numbers as i would at sea level. Even though im running the extra 2.5# the engine should see the same stress as its putting out the same hp uncorrected.

 

think in terms of absolute pressure, which you started at but seem to have deviated from. Forget vacuum and boost. Think in terms of absolute manifold pressure.

I'm at sea level, and sometimes below. Pressure is typically around 101kpa absolute. You are at a higher elevation, where you say your pressure is at 85kpa absolute.

Say a 1bar map. It reads basically from 0kpa(perfect vacuum) to 101kpa(sea level). Or a 2 bar map. It reads from basically 0kpa to 202kpa.

Boost can be atmospheric referenced or not based on how you are reading it in.

If you are using the ECM and say a custom PID in efilive...most GM cars poll for the atmospheric pressure before they start as the pressure has equalized in the manifold with ambient conditions. Therefore you can calc boost by using a PID that subtracts barometric pressure from total manifold pressure. The difference being the positive pressure in the intake, relative to true atmospheric conditions.

You could also use a non-atmospheric referenced boost gauge(0=101kpa) and/or use a PID that subtracts a fixed 101kpa(or other) from the total manifold pressure, the difference being the positive pressure in the intake, relative to the '0' calibration of the measurement.

on to your question- or what I think is your question...

If you car is running turbo'd with a manifold referenced wastegate, it will make less 'boost' at sea level than at altitude. Why?

Say at sea level your wastegate cracks at 170kpa (10psi, relative to sea level) absolute. At altitude, your wastegate will still crack at 170kpa because the spring pressure hasn't changed. The back pressure pre turbo may change a little but lets assume it's a tiny amount. At altitude, your barometric pressure is 85kpa, so you have to make 85kpa of pressure to crack the wastegate or ~12psi above atmospheric.

so you make more 'boost' at altitude than at sea level, but why does your car make less power at altitude than at sea level?

to see how this effects performance, you have to think in terms of air mass moving through the engine. In both situations you have the same total manifold pressure, but in the high altitude you have to work harder for it. the turbo is just a centri pump hooked to a turbine. centri pumps are constant head machines. This means as the density of the incoming air goes up, so does the available output pressure at the same impeller RPM. Basically, centi pumps like to pump dense fluids(up to a point).

Usually to pump the less dense air at high altitude moves the impeller RPM higher to keep the same air mass being pumped and some compressor efficiency is lost and wasted to heat generation. Also, your output pressure is higher and your input pressure is lower, therefore it is taking more work for the turbo to work across the wider pressure gradient.

so in the end, its tough to say exactly how much more boost at altitude to stress the motor equally. You certainly need to run more than 2-2.5psi more to account for the losses being at altitude to make the same HP.

But a 800hp turbo motor at sea level is running easier than a 800hp turbo motor at elevation. In terms of stress, a 800hp at sea level could be equal to 750hp at elevation. Thats a tough call because of so many factors.

 

Were on the same page.

 

You're getting stuck in the PSIa vs PSIg confusion. MAP sensors read PSIa, or absolute. That's local atmosphere plus measured. Many add on gauges will read PSIg. That's measured absolute minus local atmosphere. If a supercharged car made 15 psig at sea level (29.7psia) and went to Denver, it would make 12.3psig (27psia).

An absolute gage at sea level reads 14.7psia nominal. In Denver it's 12psia. PSIg will read 0 in both locations.

 

Correct RS, your blower motor would be seeing less stress/less hp internally at high altitude, you would have to change pullies to build more boost to create the same hp as u would at sea level. I completely understand psia and psig. What im getting at is a motor at 10 psig at altitude sees less internal stress than a motor at 10 psig at sea level. So in order the be at that 10 psi sea level(which ive found to be pretty SBE safe from previous experience living at sea level), i should be ok to turn up the wick to 12# or so here at altitude. My 12psi kpa altitude reading will be inline with a 10 psi kpa reading from sea level and should make the same uncorrected hp in the process.

 

turbo Boost Doesnt care about Elevation... thats why its so good at all tracks

with teh key on, but the engine not running
when you are at sea level your Map sensor will read 100 kpa... or 0 boost
when yo get up to higher elevations is reads less....because there is less atmosphere
so lets say it reads 85 kpa where you are now.

on a naturally aspirated or a nitrous engine, it matters because you just dont have the same amount of air molecules that you do at sea level

with boost...you are compressing the air molecules...

on a Turbo, if you have a 10 psi wastegate... it will get to 10 psi no matter what elevation you are at
and 10 psi at sea level is the same as 10 psi at 5000 ft....it just doesnt care.
you do not subtract boost from yoru total boost becasue of elevation.
it does take more work to get to the same amount of boost because there arent as many molecules to compress...which might mean a little more heat, but it will still get there if the turbo/engine combo is capable

and unless your Boost gauge calibrates itself on power up to local atmospheric conditions...
it will show less than 0 at elevation....(and I think AEM is the only one that recalibrates at every power up)

 

Correct, 10 psig is 10 psig at any altitude but 10 psig isnt going to make the same hp here as it does at 0'. 13# or so will. The 13# high altitude engine and the 10# sea level engine will make reasonably vlose to the same hp, and thus be subject to the same internal stress.

 

Agree or disagree?