TurboTommy

This is not true.
If you set your MBC at 15 psi your wastegate will open at 15 psi above the ambient air pressure (whatever that happens to be), which will match the gauge pressure on an aftermarket boost gauge. If your absolute ambient pressure where you live is 11 psi, than the intake manifold will see 26 psi absolute pressure for the cylinders to draw from (not 29.7 psi). The system does in fact know the ambient pressure where it lives. It works on the principal of pressure differential (the fuel rail on our cars is another example).

ArthurPE

I'm no turbo expert, but I know a bit about centrifugal fans and pumps...same principles
the turbo will make less boost at altitude, consider:

a turbo or supercharged airplane will not make sea level power at altitude
it just loses LESS than a na plane

series turbos...let's say you have two 2:1 turbos in series...2:1 = Pout:Pin
turbo 1, 14.7 in, 29.4 out (14.7 gauge)
turbo 2, 29.4 in, 58.8 out...
if you loose the inlet pressure to turbo 2, say it goes to atm (turbo 1 fails, hose comes off, etc.)
then turbo 2, 14.7 in, 29.4 out (14.7 gauge)
so it stands to reason, if the atm P in to a single turbo is lower, so will it's output be lower...

with a fan if you have an unrestricted inlet, you get the fans rated output, say 10" WC
but if you restrict it with a damper or filter and get say 2" WC press drop your discharge side goes down to 8" WC...same as closing the throttle on the turbo inlet, all the pressure it was generating shifts to the suction side, and boost drops...

if you have less atm press due to altitude, the turbo must always use some of it's boost (work or energy) to over come the press drop over the filter, AFM, throttle, tubing, etc., since for a given air flow, the press drop will be the same, but since you started out with less, you will have less at the turbo inlet, hence less boost...
let's say the filter, throttle, etc. have a press drop of 2 psi at a given flow:
at 14.7 atm the turbo will see 12.7, and if limited to (atm + 15 psi by the controls) will need a ratio of
at 12.6 atm (5000 ft) it will see 10.6
R1 = (15 + 12.7)/12.7 ~ 2.18
R2 = (15 + 10.6)/10.6 ~ 2.42, 10% more...
so even if the controls are absolute, not differential, the turbo must spin faster to make the boost...losing efficiency...and heating the air more...and is equivilent to 18 psi boost at sea level

ArthurPE

bingo

95ONE

but.. all this info can really lose the perspective. And, I really dont feel like explaining why..lol

V2Rocket

you didnt do very well in reading comprehension in 4th grade did you?

by all means though, beat your head against the wall. the blood is just a sign you're doing a good job.

951_RS

That's right I forgot about pressure differential, my bad. There is always the same barometric pressure applied to each side of the valve, which is the reason for the second port, causing the set boost pressure to only be what you set it at over atmospheric pressure no matter the altitude. I have no idea why that didn't occur to me. The system I described would be a wastegate that only opened under a set absolute pressure that would automatically adapt for various altitudes in doing so.

Rogue_Ant

Huh?
No. Gauge pressure does not equal absolute pressure. 15psi of positive manifold pressure here is not the same as 15psi of positive manifold pressure at sea-level, period.
At sea-level there is roughly 14.7psi of atmosphere. If your boosting 15psi, then the absolute manifold pressure is 14.7 + 15 = 29.7
In Denver, there is roughly 11.7psi of atmosphere. Now still boosting the 'same' 15psi: 11.7 + 15 = 26.7

Big difference.


-Rogue

95ONE

in a blanket statement,I dis-agree.

95ONE

Rogue, in your statement, you make it seem like the turbo can't pressurize the manifold to (any psi) say 15psi like u described if the outside (atmosphere) pressure is less than 14.7. That is just incorrect. What our gauges are reading from the manifold,(after turbo) are the same pressures in Houston as it is in Colorado.
The difference,I think is what you're saying, is the pressure differential that the turbo has to work to get the manifold to 15psi.

For example.. the atmospheric pressure in Boulder is 12lbs vs 14.5 in Houston. (all other variables being the same like barometric press, humidity & temp) pressure difference is 2.5lbs.. meaning the turbo has to work a little more to pressurize 2.5more pounds in Colorado than it has to in Houston. If you take my car at 18psi, I would have to run 20.5psi to get the same 18psi manifold pressure. Using my compressor map, my turbo is actually just as efficient at 20.5psi as it is at 18psi compressing the same amount of air..(402lbs/min) = 76% So the difference would be just the actual temp difference of the pressures. (still going to be hotter air going into the manifold at elvation) And of course the additional lag to get there.

Also the same reason the turbo cars in JGTC have such a huge advantage over the N/A cars in the higher elevation. (Fuji) The turbo makes up the pressure difference, and the N/a can't.

DanaT

Come on 95ONE.

You keep teasing. Let us know what you thinking. It like taking the the hot girl to the prom and hoping the night will be good....but going home empty.....

You may not agree, but the reality is that vehicles are much slower in Denver than most other tracks. N/A, turbo, supercharged, they are all much slower.

So it is easy for me to compare uncorrected dyno numbers to Rogue and Sid, but my numbers mean nothing compared to most people on the list. Therefore to get an "idea" if the car is the ballpark, I need to apply a correction factor. The question really is what correction factor. It seems about 12% (from the calculations above) is about right. That puts RWHP about 285. I think that would be "reasonable" for how the car sits now.

Now, lets get really quirky. Many dyno shops add the CF to nitrous vehicles. I really, i think the only way to "correct" a nitrous vehicle is to make a run w/o nitrous, correct the base HP, and then run with nitrous. Then add the uncorrected nitrous gains to the corrected base gains.

Also, even though you may not believe in CF, most car magazines use correction when reporting acceleration figures.


NHRA uses the following correction factors.

http://www.nhra.net/tech_specs/altitude.html

They state that for turbo vehicles to use the 1/2 correction factor. So they recognize that there is a difference in speed, just not as much as N/A experiences.

-Dana

95ONE

Dana, I think you're dead on. But 21-30% correction factors are Straight Crap.

-I'm on my palm, so typing more than paragraph is immensely time consuming.

The NHRA website shows a 7.8% correction for Denver.. Then using half of that as they say for turbo is 3.9%.. So......

Rogue_Ant

Again, no. Manifold pressure is the pressure differential, it is not absolute pressure.

Pretty much all gauges read manifold pressure (the pressure differential), not absolute pressure. Can my turbo make the same absolute pressure as someone at sea-level, yes. However, it forces the turbo to work harder (about 3psi harder), lowering VE. Subsequently lowers HP/TQ.

If you don't believe me, I invite anyone to set their boost at sea-level, then come up to Denver and see for themselves.

Airplanes that use turbo normalizing to make up for the pressure loss at high altitudes, run a quit a bit of boost pressure just to get back to "0" (or sea-level). An airplane at 20,000ft requires nearly 8psi of positive manifold pressure to have the same absolute pressure as it would at sea-level w/ 0psi of boost.


-Rogue

Rogue_Ant

Lol, we've seen those kind of correction #s on cars @ nearly sea-level. At Denver when the DA is 8000ft most the time, I don't think that 21% correction is that far off.


-Rogue

ArthurPE

imo it's way off if it doesn't include drive line losses...

a NA vs FI cf is ~ a fator of 2 difference at 5000 ' ASL and 1 atm of boost (14.7 psi)

8 to 10% is probably about right in this case, the 23% would be good for a NA car...

car & driver test report http://www.caranddriver.com/var/ezfl...bb1496d3cf.pdf
upper right hand corner
CF (full/ 1/2)...full is for NA cars, 1/2 is for turbo cars...
they define this in their 'how do we test' article

here's the GTR http://www.caranddriver.com/var/ezfl...426d0f3a63.pdf
ref CF field: notice how ' 1/2 ' is circled?

951_RS

I'm pretty sure we've all been talking about to the wheel horsepower, I know his dyno runs showed whp. Aside from when using an engine dyno, I think crank horsepower is kind of useless to be honest.

I know what you're saying here, but it sounded like in your post you were saying the manifold only sees the amount of pressure that you set the mbc too. The manifold does see absolute pressure, the gauge is just calibrated in a way so as to be at 0 at whatever atmospheric pressure is in your area. So that you know when you're at atmospheric pressure or lower. When the gauge reads 0 you're basically running N/A.
The gauge and the wastgate both use pressure differential to function correctly. The gauge on your dash (however unusable it may be) is a good example of exactly what's going on in your manifold. Manifold pressure will always be absolute pressure, because all absolute pressure is, is the exact pressure inside your manifold.
The only thing that confuses it all, is we need a way to talk about how many additional psi the turbo is providing over atmospheric.
If you already knew that then I've pretty much typed all that for no reason, but oh well.