Login


04122010, 11:48 PM  #23 
Custom User Title
4
Rep 105
Posts 
You're both right. The argument is stemming from the difference between 5.5psi "absolute" pressure (technically 20.2psi absolute pressure)and 5.5psi "relative" pressure (20.2psi absolute at sea level or 17.7psi absolute at 5000'). Both of you are making correct statements  you're just not using the same standard...

Appreciate
0

04122010, 11:51 PM  #24 
Moderating
223
Rep 2,025
Posts 
I though SC and altitude would want to know more about relative pressure since that's what influences the power gain? Hopefully my balloons and butt lube example illustrates it in laymen terms

Appreciate
0

04132010, 12:54 AM  #26 
Major
43
Rep 1,405
Posts 

Appreciate
0

04132010, 01:51 AM  #27 
Bulldog
321
Rep 3,359
Posts 
BobMG, which weighs more at 5,000 feet 100 pounds of feathers or 100 pounds of butt lube?
Ha, sorry had to ask!
__________________
Prev: Individual F80 M3  Fjord Blue/Silverstone(interior)  Fashion Grey(exterior)
GTS 
Appreciate
0

04132010, 03:14 AM  #28 
Captain
36
Rep 689
Posts 
Ok I see I emphasize the word QUANTITY and everyone keeps harping on about the ratio. Of course the ratio is the same. There is still 80/20 Nitrogen vs oxygen. But there's less O2 and less Nitrogen. You may end up climbing up to the top of Everest and you may end up getting 2 02 molecules and 8 Nitrogen molecules in each gasp of air (not actual numbers, I'm just trying to illustrate that the ratio stays the same).
And Eugene, the diving example of taking air at a location and compressing it and then taking it to a different location is invalid. We up here joke about bottling sea level air and taking it up to altitude. Although it's a joke, that's what you are taking about. Bottling oxygen rich air. The air at the second location is LESS DENSE. IT has less of of everything. Why do you think there's less drag at altitude. See how your car accelerates from 250 to 300km/h at altitude? Try that in the thick muggy coastal air and the car takes longer (with more power) to reach 300. The car is much more slippery at at altitude due to less resistance. So is there less air there for a given volume? No, the volume is the same. Thes is just less "stuff" in the air and everything is spread further apart. I have practical examples where you crank the presssure a couple psi to compensate (on my turbo cars), and you still make nowhere near the power you dyno at sealevel. And the car spools so much quicker at the coast and peaks so much later (with big turbos that are not outside their efficiency islands). 
Appreciate
0

04132010, 04:52 AM  #29  
Moderating
223
Rep 2,025
Posts 
Quote:


Appreciate
0

04132010, 04:55 AM  #30 
Moderating
223
Rep 2,025
Posts 

Appreciate
0

04132010, 05:01 AM  #32  
Moderating
223
Rep 2,025
Posts 
Quote:
Only problem is when you get some really big pieces, they jam up the nozzle, so I decant mine into a larger Tupperware for easier ejection without mess 

Appreciate
0

04132010, 02:47 PM  #33 
Brigadier General
75
Rep 4,150
Posts 
Well, I tried to find my aerodynamics and thermodynamics of jet and rocket engines textbook last night but couldn't find it. So, I couldn't find an answer to questions that I was looking for. (Don't ask me how long ago I had this text. Let's just say it was long enough for me to forget.)
What I was looking for, and still haven't found the answer yet, is exactly how the a centrifugal compressor works. What I mean is, is the boost, say 5 psi all the time, or a relative 5 psi. Let me see if I can explain. If at sea level you add 5 psi to the ambient pressure, then when the ambient pressure is 20% less at higher altitude do you get 5 psi over the 20% less, or do you get 80% of 5 psi, i.e. 4 psi of boost? I strongly suspect it is the latter. In any case, for a centrifugal compressor, you don't get full boost until it is moving fast, i.e. in the top RPM range of the engine for a supercharger. Unfortunately, the boost is not a simple linear function of the rpm of the compressor. It is an exponential relationship, probably with the exponent being a ratio of the heat capacities of air (1.4). But, at this point, I just don't know. So, with all of that said ... Increasing the speed of the compressor with a smaller pulley will get you more boost (up to a point). How much smaller do you need to compensate for the increased altitude I cannot say. It is not a matter of, ok, I lost 20% density from high altitude so I need to spin this sucker 20% faster. I will keep researching this.
__________________
I've been assimilated. 6MT/ZPP/18"/PS extended. 
Appreciate
0

04132010, 02:51 PM  #34 
Brigadier General
75
Rep 4,150
Posts 
No, no, no. You're doing it all wrong. You must use a proper vessel to decant into. And please, let it sit for at least a half hour before using it. Do some research man. Hmph, those backward Kazak hillbillies .
__________________
I've been assimilated. 6MT/ZPP/18"/PS extended. 
Appreciate
0

04132010, 04:09 PM  #35 
Moderating
223
Rep 2,025
Posts 
Gents I will be finished with my VT2 kit on Friday.
Instead of the 100mm pulley which boosts 5,5psi, Roman supplied a 95mm pulley good for 6,57psi at sea level. Based on the calculated loss of 0,17 bar or 2,46psi at 5000 ft, I would be safe to assume that my boost pressure will be between 4,04,5psi. If it's closer to 4psi after dyno, we may contemplate a 92mm Which is normally a 7,5psi pulley at sea level. 92mm is the smallest pulley you can use with the VT blowers without changing belts and software. Practical will be better than this speculation 
Appreciate
0

04132010, 04:21 PM  #36  
Brigadier General
75
Rep 4,150
Posts 
Quote:
Cheers.
__________________
I've been assimilated. 6MT/ZPP/18"/PS extended. 

Appreciate
0

04132010, 08:43 PM  #37  
Lieutenant
18
Rep 565
Posts 
Quote:
__________________
08 E90 335i AT RB Turbos NX Wet Kit  Chiplogic Flash or Procede V5 M3 Diff  AE Exhaust Forge Dv's  Helix Intercooler  AR DP's  P3 Boost Gauge  Vishnu Meth Stett Chargepipe and CAI
09 C63 

Appreciate
0

04152010, 02:05 AM  #38 
Moderating
223
Rep 2,025
Posts 

Appreciate
0

04152010, 02:07 AM  #39  
Moderating
223
Rep 2,025
Posts 
Quote:
Doesn't the F1 teams still do their high alt testing at Kyalami? 

Appreciate
0

04172010, 08:08 PM  #40 
Lieutenant
18
Rep 565
Posts 
Its been years they haven't done any testing here, I think the track is substandard at this stage. I am not sure if any F1 tracks are at altitude in this day and age. Altitude sucks though, my V10 M5 only makes 370whp up here
__________________
08 E90 335i AT RB Turbos NX Wet Kit  Chiplogic Flash or Procede V5 M3 Diff  AE Exhaust Forge Dv's  Helix Intercooler  AR DP's  P3 Boost Gauge  Vishnu Meth Stett Chargepipe and CAI
09 C63 
Appreciate
0

04202010, 10:56 PM  #42  
Private
0
Rep 58
Posts 
Quote:
Spot on. To reflect on my own experiences, I have a 2005 GMC Sierra that has been both Supercharged and Turbocharged through its lifetime. Both systems were built for 10lbs of boost at sea level of a 6.0l V8. When driving to Tahoe, CA (6k' above Sea level) from the SF Bay Area, the Procharger would drop down to about 7 lbs max of boost. When I replaced the Procharger with twin Garrett GT3071r's with no other changes to the motor, I still pull the same 10lbs of boost at Tahoe that I would see at Sea level. Rick
__________________
2011 E82 1M  AW  loaded
2008 E60 M5  Interlagos Blue  SMG  loaded 2008 E90 M3 6sp  Jerez Blk/Blk  sold 2003 E46 330i ZHP Sedan  Stock  sold 2005 GMC Sierra Denali  6.0l  Twin Garrett GT3071r's  smokes all the Beemers... 

Appreciate
0

Post Reply 
Bookmarks 
Thread Tools  Search this Thread 

