Does anyone know what kind of time and speed differences will occur when racing the same 2nd gen Mini on a track located near sea level versus located near Denver, CO? I’m told higher elevation will slow it down, but how much? And is it because of lower pressure or lower oxygen content, or both?

Will someone with experience please describe their results? Maybe reference a source of technical info?

 

http://www.wallaceracing.com/braking-hp.php

 

Thanx Dave, now to figure out which chart to use, then how to use it. The linked chart requires BHP, which I don't know, and returns how much BHP is lost.

A better one wants ET & MPH from an "elevated" strip, and a HP correction factor. This returns expected sea level ¼ mile results. This factor is probably in BHP, not WHP, but I'll try both for effect --- once I figure out the BHP.

Gives me something to do 'til someone shares their results.

 

Isn't the point of turbo cars that they don't lose power at elevation? Or does it still happen since the waste gate is relative to local atmospheric rather than absolute?

James

 

I'm guessing that the loss of power is due to lower oxygen content in a given volume of air, nothing to do with waste gate. At elevation, air density and pressure goes down, therefore less oxygen. Forced aspiration helps compensate, but with less oxygen to burn, the power just isn't there. A naturally aspirated engine will probably suffer even greater losses.

Using the charts available from DneprDave's link, my ¼ mile time would improve by about 0.9 seconds and the MPH will be about 6 MPH higher. Start with this chart "Convert Your ET to Sea-level MSA ET" from the long list of "ET - MPH - HP Calculators", and use the "Calculate HP with Correction" link to get the HP Correction factor. I plugged in the most likely ambient environment numbers for the time slips I have, to get my "adjusted" numbers. And I didn't need the BHP number --- these 2 calculations gave it to me with the other desired info.

Now, if you're already at or near sea level, this pair of calculations is useless to you. Being based in Northern NV, I'm really curious how my "Baddest Mini in Northern NV" compares to others on a ¼ mile strip. So far, in my few trips to the local strip, there have been no other Mini's. And any of the other local Mini club members interested in competition racing have all chosen auto-X, or SCCA competition (wisely) --- no interest in ¼ mile performance.

For all the money I've thrown at this car, my time slips are pretty disappointing --- 13.9ET and 104MPH is typical for me at 4000' --- certainly nothing to brag on. I feel a lot better after filling out these 2 calculations. Of course, there's nothing like an actual run while near the coast. Maybe some day ---

I'd still like to hear from someone thats run at 2 very different elevations. Calculations can't compare with time slips.

 

14PSI boost in the intake tract has the same volume of air anywhere. Our atmosphere is a pretty homogenous mix so there is always 21.5% oxygen.

The only difference I can see is that the waste gate is relative to atmospheric pressure where you are, so if you are 0.5PSI less than sea level (NFI what is actually is, just making up numbers as an example) then 14PSI of boost in your intake tract is the same as 13.5PSI of boost down at sea level.

James

 

Try explaining your oxygen theory to the people that climb Mt. Everest. An extreme example maybe, but the density just isn't there. And naturally-aspirated engines have similar losses --- no waste gate involved there.

FYI --- according to turbo compressor calculation charts, 4500' corresponds to 12.5PSI. Not sure what my track's 4000' is.

 

Still 20 whatever% oxygen in the mix. There is just less of everything because of the lowered density.

http://www.adlers.com.au/oxygen.php

James

 

Nice article about Mt. Everest oxygen. As you said, percentages are pretty much consistent with other components, regardless of altitude. So it appears that oxygen content is dependent on altitude / pressure, or if you prefer, density.

I've taken a pic of my boost gauge --- ignition on, engine off, at both my home (4500') and at a rest stop on one of the Sierra Nevada mountain passes (about 9000'). Poor quality pics, and the scale is in inches of mercury, but it shows where boost starts. Around home, when boosting at 30PSI, the gauge is starting at 5inHg (the equivalent of about -3PSI), so 30PSI is indicated as 27 on the gauge. Even tho the gauge shows 27, the turbo is actually providing 30PSI, and it's relative to ambient atmospheric pressure. My waste gate is controlled by an external manual boost controller, nothing to do with the OEM WGDC valve.

Regardless of the how and why, I'm still interested in actual ¼ mile results for the same Mini at significantly different elevations. Charts, graphs, and calculations can't compare to time slips.

 

Not drag racing I know, but the cars at the Mexican F1 Grand Prix yesterday were achieving higher top speeds than at any other track, which was put down to the lower air density at high elevation and therefore lower drag. They clearly weren't struggling for power (well, maybe the Red Bulls but that's not the track's fault...)

 

Higher elevations will change the characteristics of the turbocharger.