legacycentral bbs

So... I've been running 15-16 psig for a while now with my t-bird turbo. Most of that has been down in Orlando, FL (elevation ~60-100 ft), where atmospheric pressure is at or around 14.7 psia. In these conditions, the vac/boost gauge read around -18 in HG at idle with the A/C off.

Now that I've moved to Elko, NV (elevation 5000 ft), obviously the atmospheric pressure is lower here; my vac/boost gauge now reads -14 in HG at idle with the A/C off. I have not touched my MBC, which still does result in the same boost levels I was seeing in Orlando (15-16 psig).

Turbos operate on pressure ratios, correct? Meaning that my 15-16psig is now demanding that the turbo work harder? Instead of being approximately 2x the ambient air pressure, it's a bit higher. Meaning that the turbo is probably running out of it's efficiency range...?

Meaning I should turn my boost down? The way it feels makes me want to turn the boost up.. damn I need to get my shop going, I need more power!

You have it figured out. There is less air, so the turbo has to spin faster to get to the boost your MBC is set to. Keep in mind that there are a LOT of turbos in high elevation places like Colorado that do just fine.

I'd be interested to know if the efficiency range of a turbo goes up or down with changes in elevation.

I'm not sure the efficieny changes at all because you use absolute pressures to determine the pressure ratio, and ratio should be the same because your initial starting absolute pressure is lower.

I thought the pressure ratio was in reference to atmospheric pressure, though. Like, a PR of 2:1 at sea level (2*14.7psi) vs a PR of 2:1 up here (say 2*12psi or so). So to get my 15 PSIG (relative to atmospheric as well) I'm running at a higher PR, which is more than likely less efficient on the compressor map.

Damn. I guess I'll turn the boost down a bit

I just felt boost cut too, looks like my FCD might need some adjustment.

Yes....but it's minor.

For example....at sea level

Atmospheric pressure is 14.7 psia

Boost pressure is 15 psig

To find pressure ratio, get everything in psia

15 psig + atmosph (14.7) = 29.7 psia

PR = 29.7/14.7 = 2.02

Now for 5000 ft

Atmospheric pressure is 12.2 psia

Boost pressure is 15 psig

Again, get everything in terms of psia

15 psig + atmosph (12.2) = 27.2 psia

PR = 27.2/12.2 = 2.22

So, you've got 2.02 vs 2.22. Not exactly a huge difference IMO.

True, the numbers don't look too different. I guess if I had the compressor map for the t-bird I'd have already compared those two PR efficiencies. I'm just not sure where the edge of performance is with my turbo, and if I was already there.

At any rate, the FCD not doing it's job is a bigger problem that I should take care of first. Only 3 seconds at a time of 13+ PSI is not enough

Anyone in a high elevation (5000+ ft) running a stock turbo with raised boost levels, how much boost are you running? Just wondering.

I run a stock turbo (for now) at over a mile high. My boost is set at 12-13psi (peak).
I have hit fuel cut when I get much above 13psi, and it has only happened in the winter (in 4th or 5th gear).
I am pretty confident in my boost reading too, as I have had 2 gauges with the same readings.

TBE, AWIC, MBC, and other stuff.... (VF12 and a FCD in the basement, to be installed later this summer - post house move)

-chad

The t-bird turbo coupe was a 2.3 liter 4 turbo right? Pretty sure that was someting like a 45 or 50 trim t3, in which case at a PR of 2.2 you're still in the max efficiency island. Those turbos can run up to a PR of 2.4 or so and don't loose much efficency up to a PR of about 3. I wouldn't worry about the stress on the turbo so much, as long as you're intercooled and don't have detonation problems, you should be fine with turning up the boost a bit to gain back some power.

haha sammy you got me quoted!

oh yeah, my gauge reads -14inhg at idle up here too.

Word... thanks guys.

That was another side of this; now that we've determined that I'm not overstressing my turbo, the stock fuel system can handle me turning the boost up a lil' bit to compensate for the lack of atmospheric pressure... right? The fuel system isn't delivering as much as it was in FL at these boost levels, so it naturally could handle a bit more? Sweet.

Freestyle,
That is correct. There is less air in the charge at altitude, so you are now running richer than you were before.

One other thing to add. My touring wagon (also run at 5000ft) without a boost controller ran close to 11psi. This is because the atmospheric pressure sensor compensated so that the car still ran the same absolute boost pressure. This isn't effecting you because you have overridden the stock sensors with your MBC. And when I went up to Copper Mountain and broke 10,000 ft it went up close to another 1 psi...

Interesting - that is good to note. 16-17 psi is very fun on curvy flat clay roads!

Yeah, you should be able to turn up your boost to compensate. If you simplify (forgetting the turbo's efficiency), basically the engine and fuel see the absolute pressure, not the boost. So if atmospheric dropped by a few psi, turn up your boost almost the same amount, and you're kinda at the same place you started.

As discussed, that ignores that the turbo will be working harder. So, if you know you are walking off the efficiency island's beach, the intake charge might be hotter, so you might want to be a little more conservative with fueling, timing, or boost than this simplification would suggest, but it gives you the general idea.

This is one of the reasons you hear about guys running 20+ psi on a stock blocked wrx or 22t. If you look at their profiles you'll see that most are in Colorado/Utah/Nevada or something other low air density location.

I don't know what the compensation differences are in the winter with the air temps being colder making the air a little more dense.

Random Thought - I have a set of cheapy air horns that hardly work when the air temp gets below 40. However they scream above 70 deg. This makes me think that colder densor air is harder to compress and provide the same flow as a warmer less dense air.

Yeah, I never thought about that before; that's why I added my elevation to my location. Figured it could be useful there.

Well, even at 17psi I'm not seeing any EGTs above 1500*F or so. Of course, the EGT probe is in the exhaust path of the one cylinder with a burnt valve so I might not be getting a totally accurate reading there... but it sure do feel good.

I ran a shielded cable for my FCD and installed it in the cabin so I could adjust it as I drove, and finally was able to get fuel cut to stop happening again. Man it feels good to have more than 3 seconds of boost at a time!

On the air horns, isn't the air already compressed and you're just letting it out? That makes me think that the cheap ones just aren't pressurized very much. Interesting thought though.

No there is a compressor that compresses that takes ambient air and compresses it then feeds it to the horns.

In colder temps the air is already dense. To make the air horns work you need to compress the air enough so that once it reaches the horns its allowed to expand at a certain rate. I guess when its cold and the air is dense the compressor can't compress the air enough to create the same reaction during expansion.

I don't know that much of the same thinking applies to turbocharging since the point really isn't to get the compressed air to expand until its ignited in the cylider.