bldrz

hi everyone, icey2k1max and i have been dicussing the cf for fi at (specifically) altitude. we agree that the cf is too high, however i dont believe it is a fair assesment to do away with the cf all together. it seems to me altitude still plays a vital role in hp regardless of fi or not. example would be nhra drag cars(turbo) run about a second slower at bandimere speedway(5200 feet)than sea level. any input greatly appreciated. thanks jason

IceY2K1Max

iTrader: (1)

To answer the drag car question, it depends on compressor efficiency, since the turbo must spin faster to still hit X-psi and this adds additional heat to the air. However, if there is an IC this can be offset some.

My .02 is that the SAE correction on FI cars at extreme altitude or IC cars in extreme temp areas over compensates, so it would be more accurate to compare uncorrected numbers.

A correction for ambient temp, humidity, and a *SMALL* correction for loss/gain of turbo efficiency due to altitude would be IDEAL for FI cars, but until someone comes up with "Alex's FI correction factor", I'd stick with uncorrected.

AmarilloMike

Hi again Jason:

After I sent you the last PM I started looking at the forced induction forum and read the latest posts on your Greddy TT thread. That answered my question.

I have a degree in mechanical engineering and am an air-conditioning contractor. I work and live at 3550ft elevation. I agree with you that there has to be a correction for altitude.
From the Munter's Psychrometric calculator ( http://www.muntersamerica.com/WWW/U...key/KBSR-5SFC56) , i got the following specific gravities for air, all at 70 degrees farenheit, moisture content the same.

sea level 13.55 cubic feet per pound.
3650 elevation 15.48 cubic feet per pound.
5200 elevation 16.4 cubic feet per pound.

13.55/16.4 = 83% less dense air. I would assume that is the correciton factor you would use. It is the one I would use when sizing the fan for an air-conditioning/heating job design.

And fans (superchargers, turbos) move cfm. That is they move the same cubic feet per minute at 5200 feet as they do at sea level. It is just that a cubic foot of air at 5200 ft weighs less so there is less oxygen to burn with the gas, so less gas and power.
A fan at 5200 feet would move 83% of the pounds per hour of air that it moved at sea level.

So, to go back to the 5200 ft elevation wHP from a corrected sea level HP (temps and humidity of outside air the same) you would mutiply the wHP sea level figure by .83. So, if you had a rating at sea level of 573 wHP you would get 476 wHP at 5200 ft.

Didn't mean to write a thesis. Anyway, thanks for your help!

Mike

UsafaRice

Here's my take. I'd be inclined with the smart guy who actually knows stuff, but the way I look at it is this:

You are running your car at around 14 psi. 1 normal atmosphere is about 14.7 psi. At elevation, the dyno is adding an extra .24 atmospheres of correction in. But you should only get that "bonus" on the 1st part of the power made by ambient pressure. The 14 psi you're pushing in is really 14 psi and should not be corrected for. Since it just so happens that we're talking equal parts (psi vs corrected sea level pressure), I would use a correction factor of 1.12 instead here.

That gives about 540WHP instead of 597WHP. Your example about turbo cars being slower is also right- the ambient pressure is less plus a turbo has to spin up a little more to generate the same pressure level as seen at the wastegate.

IceY2K1Max

iTrader: (1)

That's assuming they have the same compressor speed at sea level vs. 5200ft, which is true for a SC but NOT for a TC. As stated before, the TC will spin faster to see the same psi at the WG, so the airflow will be higher.

However, the actual point is that SAE correction compensates for the "scientifically tested" gain/loss of power a NA engine experiences at whatever ambient conditions are vs. SAE standard conditions, ie 77F, 29.235in-Hg, 0ft, 0% humidity. A trip up a mountain to 5200ft in a NA car using SAE standard temp/humidity and a calculated air pressure of 24.13in-Hg says that a NA engine will only have 80% relative hp, ie it loses 20% of its rated power. An example of why a 350z runs 15.x 1/4-mile in Boulder, CO vs. 14.x near sea-level. This is why SAE correction is necessary. Take that same trip up the mountain with a SC/TC car and although you'll lose some performance, however it's nowhere near the 20% a NA car loses, hence the overexaggeration once the SAE correction factor adds on 20% vs. a more realistic say 5% to account for additonal heating of the intake charge for TC or say 10-15% for SC...just a guess though.

Using SAE correction at extremes will skew the dyno too drastically to be fairly compared to others around the country. For comparison, a 1.25 CF would be the SAE correction equivalent to seeing 328F at sea level, which isn't realistic.

Another skewed comparison would be to dyno an IC vs. non-IC engine and then SAE correct to 77F, since the IC engine already has a denser intake air charge.

Again, I'm not saying some kind of correction isn't necessary, just that until someone comes up with a better "ruler", you can't measure FI engines expecting apples-to-apples comparisons ESPECIALLY at extremes. So, why not just use uncorrected? Yes, it's somewhat unfair to high altitude cars, but I'd argue it's more valid then using some 1.2X CF.

AmarilloMike

Hi IceY2K1Max:

You wrote:

"As stated before, the TC will spin faster to see the same psi at the WG, so the airflow will be higher."

I missed this point in your earlier post. You may be right. Good point.

If we had been talking about a supercharger rather than a turbocharger I think the altitude correction would be right. I don't know the correction factors SAE uses and have not compared them to the ones I am suggesting. I have compared the air density at 5200 feet to the air density at sea level and come up with a 83% correction factor to go from sealevel to 5200 ft. To go from a 5200 ft uncorrected actual horsepower reading to sea level horsepower I believe that you would divide by .83 or multiply by 1.20. I repeat that a fan at 5200 feet would move 83% of the pound per hour of air that it moved at sea level. This is true for a supercharger, I am not sure about a turbocharger.

This is interesting, thanks!

Mike