SpinMonster

Does anyone know of a formula for drop in boost pressure by elevation?

I am at 6850 feet and see 12 PSI. What would I be at boost-wise at sea level? 14? 15?

Is it simply the same formula as the HP loss drop of 18-20% for this elevation?

Craigster05

Here is some info from another forum and thread (an STI guy in Colo near Pike's Peak...) maybe it will help, but at the very least I thought since it was from your back yard you'd find it interesting none the less:

Just for fun I will put some numbers to the Pikes Peak example:

Lets take a stock STi with the stock VF39 turbo.

1ST - calculate the absolute pressure/air that you are pushing through the engine:
-At sea level:
run 14.5 stock psi + 14.7psi (atmospheric pressure) = 29.2psi total.
-At 6000ft ~ the beginning of Pikes Peak hill climb:
run 14.5 stock psi + 11.4psi (atmospheric pressure) = 25.9psi total.
-At 14,380ft ~ the end of Pikes Peak hill climb:
going off the above example I have noticed about 1.1psi atmospheric drop for every 2000ft you go up (6000ft ~ 3.3psi pressure drop). So 14.5psi + 8psi (atmospheric pressure) = 22.5psi total

2ND - calculate the % of air loss you experience compared to sea level (aka roughly the amount of hp loss you will experience with a stock STi):
-at 6000ft 25.9 / 29.2 = 88% so you have 12% less pressure going into the engine
-at 14,380ft 22.5 / 29.2 = 77% so you have 23% less pressure going into the engine

NOW same comparison for a NA car in terms of how much % less air:
-at 6000ft 11.4 / 14.7 = 77% so you have 23% less pressure going into the engine.
-at 14,380ft 8 / 14.7 = 54% so you have 46% less pressure going into the engine.

3RD - NOW for the good part - calculate the PR you would have to run on the STi to get the same 14.5psi of RELATIVE boost at each elevation:
-at sea level - (14.5 + 14.7) / 14.7 = 1.98 PR
-at 6000ft - (14.5 + 11.4) / 11.4 = 2.27 PR
-at 14,380ft - (14.5 + 8) / 8 = 2.8 PR

Some interesting notes on the above example:
-Just to run the same relative boost pressures WHILE still experiencing a 23% loss in air/power you have to spin the turbo at a PR of 2.8 instead of 2.0 at sea level.
-The above example doesn't take into account less efficient affects of intercooling with less dense air.
-The above example doesn't take into account lower VEs from higher turbine speeds from higher PRs.
-The fact that you have vacuum in the intake tube between your air filter and the compressor wheel/inlet. The short of this is you have to add an extra 0.2-0.3 to your PR since your turbo doesn't even get to work with the above atmospheric pressures I just told you. (This is starting to get into the real world side of things [heh]).

Now you might say well just turn up the boost to compensate right? Well lets just see what that would do to the PR at 6000ft.

So we want to run the same pressure through the engine at 6000ft as we did at sea level right?
-Sea level total pressure = 29.2psi
-So at 6000ft 11.4psi (atmospheric) + x = 29.2 where x is the amount of boost your turbo has to run which would be 17.8 psi.

SO the PR for running 17.8 psi would be (17.8 + 11.4) / 11.4 = 2.56 PR compared to 1.98 PR at sea level.

PowerLabs

Spin,

Its not a percentage: Look at the ABSOLUTE pressure your blower is seeing; At sea level it is 14.7PSI and at 6850feet it is 11.4PSI*. The simplistic way of looking at this is that you will see approximately 3.3PSI less than you would at sea level.
In reality because it is taking in more rarefied air your supercharger is also working at a different point in its compressor map and so the actual supercharger efficiency will end up being lower. The intercooler is also fed lower density air, making intercooling efficiency lower too: you can end up seeing even more than a 3.3psi boost drop because of the combined effect of lower operating ambient pressure and lower system efficiency.

Oh yeah, on the plus side, your car also experiences less aerodynamic drag in this less dense air... So your theoretical top speed at any given power leven should be slightly higher



* Air pressure above sea level can be calculated as

p = 101325 (1 - 2.25577 10-5 h)5.25588 (1)

where

p = air pressure (Pa)

h = altitude above sea level (m)

glass slipper

Sam,
It took me a minute to recognize the above formula because the exponents aren't clearly defined, so I'm going to rewrite the formula in an easier to understand format:

p = 101325*(1 - 2.25577*(10^-5)*h)^5.25588
where "^" means "raised to the power of".

Skunkworks

That looks about right, but PR (boost) or pressure ratio needs to be factored in i.e 1.8 * 14.7 (sea level standard day) is different then 1.8 * 11.3 (absolute pressure at altitude).


Mike

glass slipper

Of course, but the formula above is only used to calculate the absolute air pressure in the atmosphere at any particular altitude (within a certain range). Post #2 covered PR and absolute air pressure relative to boost.

SpinMonster

Thanks for the input. Lots of good info.

I finally got a solution to belt slip and boost loss from the BOV.

I replaced the bosch with a vortek unit and the pulley was a simple fix. I tried to get a reichard but no one had them and the only one I found was a different size (bigger) and was 200 bucks.....so

I took out the dremel and made a jig up to cut the slots at equal spaces to each other and put it back on my car....I gained 1.25psi. Nearly 100 miles and nothing looks bad with the belt either.

There is a great deal of improvement in the part throttle transitions. This car is so much more fun to drive now. The BOV closes sooner. The belt isnt slipping. The MAF sensor allows better part throttle.

So sharp. You gotta love FI. I cant beleive I resisted. ECS should hire me as the new poster boy.

PowerLabs

Quoted for posteriority... I only told you 100 times that you needed to ditch that plastic BOSCH valve for a proper adjustable one and that would improve your trottle response

BTW, on the subject of belt slip... That's one of the biggest downsides of pullying down... You increase the load on the belt, but at the same time you reduce the belt surface area in contact with the pulley... The slotting sounds like a clever fix; Gatorback belts are pre-slotted and are supposed to be less likely to slip too. Personally I think if I wanted to run anything smaller than a 3.6 inch drive pulley I would go 8 rib.

glass slipper

You talking out your azz again???







I know you meant prosperity, but opportunities like this are difficult for me to resist...the devil made me do it.

PowerLabs

Jerk

SpinMonster

Wow, I list three things that improved my car and you (assume) the BOV improved throttle response. Way wrong.

For the record the BOV was replaced first weeks ago with little change in throttle response so you can unQuote for posteriority. What it did do was hold the higher boost level making more power. When I saw it wasnt holding boost with the new pulley and hearing they had issues over 10psi, I got a new one. My real complaint about the bosch was that it sounded like a chirping cricket and cheesy from day one.

The belt slip was the biggest culprit.

The cutting of the slots produced over a pound more boost but did its biggest improvement on sudden hits of the throttle--hence the throttle response. The belt slip issue is one of initial rpm change (transients). A belt wont slip if the rpm remains steady. Its when you ask for a sudden increase in RPM that it slips initially and thats the case at part throttle too just as with a clutch or tire. Once it gains traction, it holds it. I have many many hours on the slotted pulley and the belt looks fine. I was initally concerned that the sharp edges would kill the belt but its fine.

Anyone who thinks they dont have pulley slip is wrong. All sharp increases in RPM will have some slip just as a clutch slips initially going into gear or a tire gets a 1/4 rotation (chirps) on hard shifts. This slight slip manifests itself in a minor loss in belt traction and feels like a mushy throttle. People who swap to a reichard report the difference I speak of here locally here in CO where FI is the rule of the day.

As I have said already, the MAF's driveability improvement was also very apparent.

I agree with 8 rib set-ups but I seem to have fixed the belt slip issue with a 2 minute (ok its was 10 minutes) proceedure which was free as opposed to a 1200 dollar pulley set. If it helps anyone, here is a pic of the slotted pulley:

I tried getting a reichard but no one had them in stock in my size, or anything close, and 200 bucks made it worth trying this first.

A quickie DIY on how to do this is to take electical tape and put it on the pulley unstretched around the entire circumference and cut it exactly to size. Remove it from the pulley and put it on a ruler and stretch it up top the nearest inch. Make marks every inch and when you remove it from the ruler the marks will shrink to equal but less than one inch marks. Put the tape back on the pulley and the marks will be eqi-distant and act as a template for the disc cutter of your dremel tool. You can do it while its still on the blower with the belt removed if you mask off everything from the aluminum dust and then blow it all clean with an air hose.