[CW3SF]

With just a few bolt ons my car did 567/586 SAE (smoothing 5) at 5865ft altitude (pre-flex tune - 91 octane and factory tune). How different would the numbers be at sea level or near sea level if all other factors were the same (temp, DA humidity etc)? I guess what I’m asking is if there’s a formula for hp loss due to altitude on supercharged cars? I know they read lower at altitude, I’m just wondering if it can be determined how much.


Edit: I know full well that dynos are only a tool. And what matters is the after mod dyno results to measure gains. This question isnt about that. TIA

[Camaro1973]

Originally Posted by CW3SF

With just a few bolt ons my car did 567/586 SAE (smoothing 5) at 5865ft altitude (pre-flex tune - 91 octane and factory tune). How different would the numbers be at sea level or near sea level if all other factors were the same (temp, DA humidity etc)? I guess what I’m asking is if there’s a formula for hp loss due to altitude on supercharged cars? I know they read lower at altitude, I’m just wondering if it can be determined how much.


Edit: I know full well that dynos are only a tool. And what matters is the after mod dyno results to measure gains. This question isnt about that. TIA

It’s roughly 3% hp loss (generally) per 1k elevation bhp.

You could lose up to 96bhp being 5000ft above sea level.

[CW3SF]

Originally Posted by Camaro1973

It’s roughly 3% hp loss (generally) per 1k elevation bhp.

You could lose up to 96bhp being 5000ft above sea level.

Very interesting. That’s what I was looking for. I searched thru a bunch of forums on Google and found a bunch of very scientific discussion that I couldn’t follow LOL. 3% bhp per 1k is a lot easier to calculate.

Appreciate the input.

[gotswap]

you lose about 3PSI up here compared to sea level. What that equals in HP, no clue. Part of the correction factor is accounting for altitude so that is probably what you make down @ sea level. I think uncorrected I was right below 600WHP

[7LitreC5]

Most dyno operators give you the corrected to sea level numbers unless you specifically ask for uncorrected numbers.

Those numbers probably are what your car would make at sea level.

Originally Posted by CW3SF

With just a few bolt ons my car did 567/586 SAE (smoothing 5) at 5865ft altitude (pre-flex tune - 91 octane and factory tune). How different would the numbers be at sea level or near sea level if all other factors were the same (temp, DA humidity etc)? I guess what I’m asking is if there’s a formula for hp loss due to altitude on supercharged cars? I know they read lower at altitude, I’m just wondering if it can be determined how much.


Edit: I know full well that dynos are only a tool. And what matters is the after mod dyno results to measure gains. This question isnt about that. TIA

[JSH]

Your dyno # should be corrected for altitude. Supercharged and turbos do not lose as much as NA, which can lose 20-25% up here depending on DA.

[CW3SF]

I appreciate the explanations. Just moved here last summer so this altitude stuff is new to me.

[JSH]

Denver has one of the lowest DA I've ever seen tonight - 3582 ft.

25F
25.19 merc
15F dew point

[CW3SF]

Nice. This thread brought out all the Colorado peeps.

[CW3SF]

So just to revisit what was input from above and trying to make some sense of it.

567whp can’t be what it would make at level. Add back in ~12% drive train loss foe the M6 and that’s 635 crank hp. This car has intake, TB and exhaust so that doesn’t add up. The car came with 650.

The post indicating that cars make 3psi of boost here would make sense. The car with those mods, but down 3psi of boost should be in the 567whp range.

They dyno shop told me that all cars make less here SAE than they would at sea level. He just didn’t know what the formula was for computing the difference.

[gtfoxy]

Originally Posted by CW3SF

So just to revisit what was input from above and trying to make some sense of it.

567whp can’t be what it would make at level. Add back in ~12% drive train loss foe the M6 and that’s 635 crank hp. This car has intake, TB and exhaust so that doesn’t add up. The car came with 650.

The post indicating that cars make 3psi of boost here would make sense. The car with those mods, but down 3psi of boost should be in the 567whp range.

They dyno shop told me that all cars make less here SAE than they would at sea level. He just didn’t know what the formula was for computing the difference.

The formula is a bit complicated.

It is, as a result to behavior of gasses & gravitational force, not a linear chart, it’s more of a gradient curve.

I always used .5psi drop/ 1kft of elevation. That is a rough estimate.

As air density is also attributed to other variables it changes slightly from day to day or throughout a day.

Base calculations for sea-level are 101kpa/14.67psi, Denver is said to be 85kpa/12.32psi.

Some reading, very basic, on the subject.

https://flexbooks.ck12.org/cbook/ck-...altitude-ms-ps

& a calculator.

https://www.mide.com/air-pressure-at...ude-calculator

Add: to answer your question as it applies to HP at atmospheric pressure, assuming a adiabatic quotient of approximately 1.4 (non ideal gas) if you double atmospheric pressure HP would double. In reality there is always adiabatic & parasitic losses, as seen in people’s referring to HP/ lb of manifold pressure, so as a very rough estimate 7.5psi is approximately 50%. 3.2psi would be 25% less.

[JSH]

Originally Posted by CW3SF

So just to revisit what was input from above and trying to make some sense of it.

567whp can’t be what it would make at level. Add back in ~12% drive train loss foe the M6 and that’s 635 crank hp. This car has intake, TB and exhaust so that doesn’t add up. The car came with 650.

The post indicating that cars make 3psi of boost here would make sense. The car with those mods, but down 3psi of boost should be in the 567whp range.

They dyno shop told me that all cars make less here SAE than they would at sea level. He just didn’t know what the formula was for computing the difference.

Mine made 574 whp stock in Houston which seems right to me. Not sure which dyno it was.

So it looks to me that yours makes around stock hp adjusted to sea level.

[wwiiavfan]

I wonder, has anyone ever created a dyno "chamber", where you could dial in whatever altitude, temp, etc conditions you wanted?
That would be pretty cool.

[gtfoxy]

Originally Posted by wwiiavfan

I wonder, has anyone ever created a dyno "chamber", where you could dial in whatever altitude, temp, etc conditions you wanted?
That would be pretty cool.

Yess.

They exist primarily in the aerospace industry.

Example.

https://www.nap.edu/read/9088/chapter/6