how much boost do we lose at this altitude again?

[damian9669]

I was just looking for a concrete number.Meaning say we see 10 psi at sea level.How much would 10 psi be up here? like 7?

[ryanman]

Well I had my GSX set at 15 lbs up here, I took it to sea level and it only jumped 1lb to 16 lbs.

[damian9669]

I forgot who told me that theres about a 3 to 3.5 psi difference.Im just trying to confirm or deny whether its true or not.

[ryanman]

Well I only got 1lb. I guess every car is different.

[12seccivy]

mine lost 3 psi when i came back here
I was at 800 ft with 13 psi, here with 10psi.
this is with a Jrsc on a B16A2

Like ryanman says,depends on the vehicle and probably how you are boosting..supecharger would have more variance than turbo

[davidm_sh]

For any car NA or FI we lost about 3.5psi up here in Colorado Springs at 6000ft asl. It is rouhgly 1.1psi for every 2000ft you go up. This loss comes from less atmospheric pressure.

Supercharger people also see a relative boost loss since they are effectily operating their superchargers at a fixed PR (same size pulley at sea level or up here). Meaning if you were to keep your turbo at the same PR you would also lose about 3psi relative boost as well. So for supercharger guys it is sort of a double whammy effect as I like to call it [heh]. THey lose 3.5 psi from atmospheric losses and about 3 psi relative so about 6.5psi total overall air pressure as compared to sea level.

Turbos though can run the same relative boost pressure and even run a bit more boost pressure with the help of a MBC/EBC, but at the expense of running higher pressure ratios and as a result running in a less effecient part of the compressor map, but of couse this all depends on the turbo you have.

[damian9669]

thank you for helping me out on this one.I figured it was about 3 to 3.5 psi loss.Quick question though.If my boost gauge registers 10 psi and the gauge is tapped right into the intake manifold does this mean that my engine sees 10 psi as well? or does this mean that the engine really sees about 6.5 psi to 7 psi?

[David]

isn't 10 psi up here less than 10psi at sea level though?

[damian9669]

i guess so.but then again im not sure.thats why im asking.because if im seeing 10 psi on the gauge and my engine is really only seeing 7 psi then there definatly room for my downpipe upgrade.hehe.

[ryanman]

What makes you think that only the manifold would be seeing 10psi and not the rest of the engine?

[davidm_sh]

isn't 10 psi up here less than 10psi at sea level though?

10 psi is 10psi. You are looking at a boost gauge which measures the relative boost pressure. It does not measure absolute manifold pressure which would take into account the atmospheric pressure losses.

So if your boost gauge says 10psi you are running 10psi. Either up here to at sea level. The difference being (3 - 3.5psi loss) that at sea level atmospheric pressure is ~14.7psi and up here at 6000ft it is 11.2psi.

So if you add up the total atmospheric (MAP sensor) and relative boost (normal boost gauge) pressure you will see the difference of 3 to 3.5psi.

[David]

I was thinking that since we have a lower atmo. pressure, we would have a lower total pressure with boost. Since the gauge reads 10psi over the atmo. presure, we would still have less air in the cylinder at full boost.

does that make since?

[davidm_sh]

I was thinking that since we have a lower atmo. pressure, we would have a lower total pressure with boost. Since the gauge reads 10psi over the atmo. presure, we would still have less air in the cylinder at full boost.

does that make since?

Exactly.

[HONDA GHANDI]

I lose 2lbs of boost up here. err, I gain 2psi at sealevel.

[john]

There is no loss in boost pressure at altitude. The turbo just has to work harder to make X psi than at sea level.

15psi is 15psi, whether it's at 10000 ft asl or sea level.

Unless your turbo is taxed / maxed out at altitude, then you will probably see a gain further down.

I took my DSM to Ohio a few yrs ago. I was running 23psi here and it ran 23psi there. It was much quicker & faster there (12.7 @ 108 vs 14.0 @ 100), but boost pressure did not change.


I was just looking for a concrete number.Meaning say we see 10 psi at sea level.How much would 10 psi be up here? like 7?

[damian9669]

thank you everyone for helping me out on this one.Thats why I asked this question since ive seen spikes as high as 14.5 psi on my boost gauge.Heres the weird part.Almost everyone on rx7club says that anything higher than 12 psi will pop the rotary.Recently ive been doing some work to the car and now the car rarely sees such high boost spikes.Maybe 11 to 12 psi at the most.But generally it makes 10 psi like its supposed to.All the information you guys have provided has helped me out more than you know.Since adding a downpipe to my car will also raise max boost and boost spiking I will have to get a electronic boost controller to keep the boost in check.I was thinking of getting the greddy profec b spec 2.Any thoughts on this unit?

[rmcdaniels]

Here's some reviews:

http://www.sportcompactcarweb.com/tech/0207scc_boostcontrol/

[Street_Kings]

I don't think that you actually loose boost here, at least the way you're thinking about. The reason why your boost gauge reads the same, is boost is governed by the wastegate, which is regulated by the WG spring and pressure in your charge pipe or wherever you tapped it to. Boost is considered as pressure above ambient air right? So at sea level, ambient air has a pressure of 14.7 PSI, where at 5280' ambient air pressure is only ~12.2 PSI. So at sea level, 10 PSI of boost would mean 14.7 ambient + 10 PSI = 24.7 PSI Absolute, where at 5280' would mean 12.2 ambient + 10 PSI = 22.2 absolute. So TOTAL cylinder pressure is ~2.5 PSI lower.
Also, turbo/superchargers are more efficient at higher altitudes, due to the fact that our thinner air is less dense and easier to compress.

[davidm_sh]

Also, turbo/superchargers are more efficient at higher altitudes, due to the fact that our thinner air is less dense and easier to compress.

Sorry but that statement is just wrong. To achieve the same relative boost pressure (boost pressure on top of atmospheric pressure) you have to run a higher PR to achieve it = spinning faster = less efficient.

That is why a supercharger will see aboug 2-3 psi less relative boost pressure since by its inherent pulley design it is always operating at a fixed PR.

[SleeperZ]

And the short answer is, we are about 2.5 psi less absolute pressure here at altitude. A turbo running constant boost pressure will be running 2.5 psi less (absolute) boost. A supercharger will run even less boost because it runs a constant pulley ratio - no boost controller.

That was short?

[Canuck]

My car shows about 12.5 because it is trying to keep the manifold @ 10 psi (in sea level terms)