altitude and compression/pressure

[Riptide]

OK nerds get out your pocket calcs and help a moron like me figure this out.

At 5000' of elevation you are down approximately 2.5psi pressure over sea level. I have a pulley which should make 11.5psi @ sea level on this car. Right now I am seeing 9psig (pressure at the gauge). 11.5-2.5=9. Right so the supercharger is compressing thinner air and therefore reading lower psig.

But the nasty effects from altitude aren't finished yet. Because the air is thinner not only is the supercharger not able to produce as much boost but the air being compressed by the engine itself is also thinner by the same amount. 2.5psi. So therefore if you are at 5000' of altitude you are in effect losing 2.5psig (boost) + 2.5psia (atmosphere) compression in the engine. For a total loss of 5psi.

Therefore if I want this car to make the same power as an identical one at sea level I need to pulley down for an additional 5psig.

And ofcourse there are efficiency losses from spinning the blower faster, etc. but we'll leave that out of the picture for now.

Making sense? Or have I lost my mind again?

 

[07 Boss]

Not an engineer or anything but shouldn't you be basing those numbers on percentages instead of hard numbers? Just saying if you are multiplying the atmospheric pressure don't you need to use a percentage or a ratio instead of just 2.5 psi?

 

[Bud]

Don't forget the air moving across the heat exchanger for your PD blower is less dense, removing less heat, losing power as well. Same thing for an A2A IC on a centri. So higher IAT's are probably to be expected. You can't discount the parasitic losses of spinning the blower faster as that is still extra stress on the engine that has to be accounted for when deciding on a safe level.

I'm not sure that accounting for the pressure drop twice is accurate, but I didn't try to put any math around it with my stuff. I always just ordered the smaller pulleys (69mm on my tvs and 3" for my 2.9) and went with it. I was ok if the stock motor popped though (luckily it never did).

 

[Chiron]

The pressure drop should just be a function of the ideal gas laws, it won't be exactly linear I believe.

 

[Riptide]

Yeah Bud that is sort of the potential triple whammy effect of altitude. Efficiency of everything goes down because you have to spin it faster to get the same effect vs. more dense air down low.

I realize it may not be a perfect linear drop (2.5+2.5) but I am trying to get my head around whether or not my basic understanding of this is correct. Do you suffer a loss in not just psig but also psia.

Or to pose the question from another angle. If I pulley down far enough to see 12.5psig @ 5000' would the car then be roughly equal in trap/whp to a car that is making 10psig at sea level?

I am on a 79mm pulley right now FWIW.

 

[Chiron]

It's ultimately a question of how many O2 molecules you have in the cylinders, right? Oxygen density at sea level is about 1.2 kg/m3 and about 1.0 at 5000ft. 17% difference, so 17% less O2 molecules in your combustion chamber. As long as you make 17% more pressure (2.465psi), that'll give you the same number of O2 molecules as you had at sea level.

I don't think there's any other loss to account for, but maybe someone smarter than me can come and learn us some science.

 

[ksack]

Because the air is thinner not only is the supercharger not able to produce as much boost but the air being compressed by the engine itself is also thinner by the same amount. 2.5psi. So therefore if you are at 5000' of altitude you are in effect losing 2.5psig (boost) + 2.5psia (atmosphere) compression in the engine. For a total loss of 5psi.

I think you are double counting. The air going into your cylinders is the compressed air from the blower so atmospheric pressure is irrelevant in terms of the loss in the system. Now I am curious as to whether the gauge is based off a calibrated sea level pressure or if it is an open system. If it was an open sytem, you would be getting double screwed for 1)less air to compress and 2)less atmosphere as the baseline for psig. For reference, I was running a 10.5lb pulley on my TVS and was seeing 9.1 psi at 5000ft (per the dyno).

 

[Riptide]

So:
Car A is at 5000' and shows 12.5psi at the boost gauge.
Car B is at 0' and shows 10.0psi at the boost gauge.

Both cars, all else being equal, would produce roughly the same horsepower. True?
Ignore efficiency losses, etc. for sake of comparison. Car A has to run a smaller pulley to achieve the same pressure at the boost gauge.

 

[CPRsm]

You don't count the loss twice. You have a pressure loss from thinner air, compared to the same blower speed at sea level. On top of that there is mass loss into the engine at the same boost level. So bring your boost up to the same level and it will still be down on power. But pressure drop is only a one time thing

Efficiency at altitude does not always drop. Mass entering the engine does. Plot some points on a turbo or blower map on an engine some time and you'll see the larger the engine is usually in the lower right of the effeciency map it will be. Stunting lb/min of air flow and increasing pressure is not always a bad thing. Sometimes it moves your plots right up into the meat of things. That is one of the reasons smaller engine can usually make more power on the same size blower, even more so w a turbo, when you include back pressure into the equation.

To make up for the deficit you would need to introduce more mass into the engine. Whether it takes 2 or 10 more pounds, the end result will be the same HP. As long as your unit has the head room, you can make just as much at altitude as sea level. Like bud mentioned, the HE may play a role in how the effeciency is held over time during abuse.

 

[Riptide]

Turbos are a whole other deal. They will always make the rated boost no matter the altitude. Well close to it anyway.

The reason I was counting the loss twice is because the same pulley not only produces less boost at higher altitude. But it isn't enough to simply pulley down once to bring up the gauge pressure to equal it. You need to add even more to get to the same whp/trap.

Hard for me to get my head around it I guess. I appreciate the attempts.

 

[Bud]

So:
Car A is at 5000' and shows 12.5psi at the boost gauge.
Car B is at 0' and shows 10.0psi at the boost gauge.

Both cars, all else being equal, would produce roughly the same horsepower. True?
Ignore efficiency losses, etc. for sake of comparison. Car A has to run a smaller pulley to achieve the same pressure at the boost gauge.

That is problem, you can say both may be the same mass of air (not even sure that is true), but definitely can't say they would produce roughly the same HP. The parasitic loss is real, as is belt slip on smaller pulleys on little 6 rib belts. Also, you have to compress the air at elevation more times than you would air at sea level to reach the same mass, which means the air is hotter even before it reaches the intercooler (I think, I just searched google on that so I'm an expert now).

Another thing to think about is launch and converter if you like to drag race. People rip off stupid 60' times at sea level because the extra power off boost and down low. Even PD cars up here don't 60' for shit, they need a converter. Of course a turbo car is way worse, my car would take 3.5 seconds or so to spool up here, sea level is almost instant on the trans brake.

As long as your unit has the head room, you can make just as much at altitude as sea level. .

What is headroom? We run all the boost all the time!

 

[Bud]

Turbos are a whole other deal. They will always make the rated boost no matter the altitude. Well close to it anyway.

The reason I was counting the loss twice is because the same pulley not only produces less boost at higher altitude. But it isn't enough to simply pulley down once to bring up the gauge pressure to equal it. You need to add even more to get to the same whp/trap.

Hard for me to get my head around it I guess. I appreciate the attempts.

As Dustin was mentioning, Turbos have efficiency islands they may sit at that complicate the whole thing even more. Turbos just like any other power adder though, same boost# or even accounting for relative boost on a gauge compared to sea level does not mean the same ET. A buddies turbo car lost .40 from Kentucky to back here, the turbos were being pushed hard though. If a person was running show car psi, it would probably have been much less of a drop than pushing them to their limit.

I think it is hard for everybody to wrap their head around, real world experience is better than guesses and attempts at math. Unfortunately not a lot of people doing back to back tests between two massive swings in elevation.

 

[Riptide]

Yeah Bud it's awful. Here in MT we race at yellowstone drag strip and during the summer density altitude can get over 7500' on the worst days. Not quite as bad as your area but not far behind.

I know what you meant about the additional issues that come with it I was just trying to keep the focus on one variable.

Now the question is what can be done. To me a 72mm (14psi @ 0') pulley seems like the best option. Should be close (minus the other issues mentioned above) to a 82mm 10psi sea level car.

And I agree I'll be happy with a 1.8 60' out of this car on the stock converter. The 1.5-1.6 people get down low won't be happening.

 

[CPRsm]

If you do that and head up in altitude, then Ur just fucked lol

Turbos are a whole other deal. They will always make the rated boost no matter the altitude. Well close to it anyway.

A compressor map is a compressor map. Doesn't matter what drive it's compressors
But even w the same boost level they will be down on mass as well, just like a pullied down blower.
It takes a bit to wrap your head around.

 

[Bud]

Yeah Bud it's awful. Here in MT we race at yellowstone drag strip and during the summer density altitude can get over 7500' on the worst days. Not quite as bad as your area but not far behind.

I know what you meant about the additional issues that come with it I was just trying to keep the focus on one variable.

Now the question is what can be done. To me a 72mm (14psi @ 0') pulley seems like the best option. Should be close (minus the other issues mentioned above) to a 82mm 10psi sea level car.

And I agree I'll be happy with a 1.8 60' out of this car on the stock converter. The 1.5-1.6 people get down low won't be happening.

Elevation is a cruel bitch. 7500' DA is stupid for sure. The 72mm may not be a bad plan. I ran the 69mm pulley on my tvs back in 2012 and with some 104 octane in the tank, and I think I was around 3800ish race weight my car ran an 11.2 or so. How sad is that. I think it did 60' in the 1.7 range though.

If you have e85 up there I'd suggest that, with enough fuel system of course. It is safer and can take a little more timing/heat. That is the one thing we have going for us here, suck air, but e85 pumps everywhere.

 

[Riptide]

But even w the same boost level they will be down on mass as well, just like a pullied down blower.
It takes a bit to wrap your head around.

I think I get it. Even at the same gauge pressures the sea level turbo car will make more power.

I know that I would need to pulley down if I go to sea level. Or risk a grenade. lol

Elevation is a cruel bitch. The 72mm may not be a bad plan. I ran the 69mm pulley on my tvs back in 2012 and with some 104 octane in the tank, and I think I was around 3800ish race weight my car ran an 11.2 or so. How sad is that. I think it did 60' in the 1.7 range though.

All I can get is 91 or super expensive MS109. There is no E85 here. I'm not sure 91 would be an issue even if I pulley the car down. Because we are dealing with such low oxygen density here and the goal is to have power output roughly equal to a 10psi sea level car. Not looking for more than that. Tune would still pull timing when IATs got up there.

I might have to replace an idler to go down to anything less than a 75mm pulley.

 

[Department Of Boost]

Move..................

 

[Bud]

Sharing my data from the dyno if this helps give an idea of what each pulley was doing. These are PSIG so they are relative to ambient pressure so the 2.5 down gig should be taken into account on these numbers:

EngSpd    75m    69mm
4000       9.2    11
4100       8.9    10.9
4200       8.7    10.8
4300       8.6    10.7
4400       8.5    10.6
4500       8.5    10.6
4600       8.6    10.8
4700       8.6    10.9
4800       8.7    11
4900       8.8    10.9
5000       8.8    11
5100       8.8    11
5200       8.9    11
5300       8.9    11
5400       8.8    11.1
5500       8.7    11.1
5600       8.8    11.1
5700       8.9    11.2
5800       8.9    11.4
5900       8.9    11.2
6000       8.9    11
6100       9.1    11.2
6200       9.4    11.4
6300       9.2    11.2
6400       9.1    11.2
6500       9.2    11.2
6600       9       11
6700       8.7    10.7

 

[Riptide]

Bud what was your approximate density altitude at the time you got those figures? TIA

Move..................

lol OK. Kinda ridiculous to quit a decent job, sell property, and then start over after moving cross country (pita) just so you can have a more efficient boosted car.

 

[Bud]

Bud what was your approximate density altitude at the time you got those figures? TIA


lol OK. Kinda ridiculous to quit a decent job, sell property, and then start over after moving cross country (pita) just so you can have a more efficient boosted car.

Not sure what the DA was, that was back in 2011 before I paid too much attention to things like that. It was in September though so wouldn't have been the worst we've seen I'm sure.