Dock

Right, on a warm day the ambient pressure in the intake is less than 14.7 psi. I believe the ECU knows this, and allows the turbos to make up the difference between ambient and 14.7 psi prior to their making the specific ECU's maximum boost (1.0 bar in the examples I've made).

Dock

It depends - if your mechanical gauge is reading the total boost out of the turbos without regard to what the ambient is in the system, then it should read higher than on a cold day. If the gauge reads total system pressure (ambient accounted for) then it shouldn't read any different than on a cold day.

Basal Skull

Thanx guys it's making sense to me now. Should not see low boost on hot days regardless of how the boost guage is calibrated.
The other thing I noticed on my mechanical boost guage vs digital is that the digital one (993tt) does not correspond linearly to the range 20mmhg to 14.5psi on the mechanical and often get 0.2-0.4 bar on the digital guage when mechanical guage reads around 0, and only really seem to correspond to the mechanical guage when at full boost (ie. mechanical guage reads 14-15 and digital reads 0.8 (993tt digital guage only reads to 0.8 max). I assume that the digital guage is reading is actually some calculation value compared to what the intake pressures are relative to a ie. NA engine / what the engine sees if it didn't have a turbo? and not really pressures/bar?

9Eleven

I took my car out for a drive today and the temp was 75 with low humidity. I felt a difference in torque. The boost level was the same (.9 to 1 bar spikes) but I felt a definite difference in acceleration. Colder air equals more horsepower, more torque? Not sure, but it felt like it.

1AS

Colder air is more power per explosion, which is more torque. More explosions per minute is more horsepower, so they are related. Both will be increased at 5250rpm, where they are equal. AS

Red rooster

Guys,
There seems to be a little confusion here. Ambient temperature and pressure are not directly related . Temperature is temperature and pressure is pressure.
I spent 20 years running a chassis dyno and inputed those numbers many times a day !!
For power correction , high temperatures/low pressure lose power and conversely low temperatures/high pressure gain power.
Of the two parameters pressure variation is far more significant as it is a direct law . Temperature is a square rooted effect.
Hope that helps.

Geoff

Dock

The difficulty is with relating boost pressure to air density. The ECU is concerned about air density as measured by the MAF, but we read boost in bar (bar can also be expressed in PSI).

On a standard day at sea level (59 degrees F) the atmospheric pressure is 14.7" Hg. In these conditions there is a specific air density. If just the temperature is increased to 89 degrees F, the density altitude increases to 2000 feet. As density altitude increases, the actual density of the air decreases and the performance of a N/A engine goes down. Assuming standard atmospheric lapse rates, the atmospheric pressure at 2000 feet is 13.66" Hg and the temperature is 52 degrees F. So on a day at seal level (14.7" Hg) where the temperature is 89 degrees, the car performs like it was at 2000 feet where the air density is the same as it is at 13.66" Hg and 52 degrees F. The ECU senses the reduction in air density and allows the turbos to produce more pressure to generate air that has the same density as air at 14.7 psi and 59 degrees.

Red rooster

Dock,
I cant guess where you got these altitude numbers from. Density is proportional to the square root of temp 2/ Temp1. The ambient pressure relationship is direct P2/P1.
It is pretty natural for human beings to understand temperature variation effects far more easily than ambient pressure as we have no way of relating to pressure until extreme conditions are met say on the top of Mount Everest !!
If you do the sums it becomes clear that normal ambient pressure variation from low to high pressure have a pretty big effect on density, around +,- 15%.
It all underlines that atmospheric conditions can have a lot more effect on engine power than might be first thought.

All the best

Geoff

rajtt

Volume and temperature for a closed system are related by Boyle's law: PV = nRT. Since volume is proportional to temperature, law of conservation of mass dictates that density is inversely proportional to temperature (rho ~ 1/T). As temperature increases, the air gets more rarefied and results in a proportional power loss as greater volume (flow) is required to obtain the same mass. MAFs measure air mass (as the name indicates) using a venturi meter and determine the amount of fuel required for a stoichimetric combustion.

Turbos work on a differential pressure basis: this means that they will essentially supply the same amount of pressure (employing wastegates), or boost if you will, regardless of ambient conditions. I live at 6000 feet, and my 996TT faithfully produces a steady 0.8 bar (with spikes to 0.9) exactly as it would at sea level. However, the atmospheric pressure at 6000 ft is only 0.8 bar, compared to 1.0 bar at sea level (ignoring temperature and humidity influences). Consequently, the net intake pressure generated by the turbos at my altitude is only ~1.6 bar, compared to 1.8 bar at sea level, which equates to a power loss of ~11%. Obviously, this is much better than the roughly 20% power loss that normally aspirated engines suffer. Thus, power loss is inevitable as temperature increases and inevitable as altitude increases. But higher the boost supplied by the turbos the lower the percentage power loss at any given altitude.

Dock

Density altitude calculator, and standard atmospheric temperature and pressure lapse rates.

Kevin

rajtt should be making his .8bar at 6000 ft, however his turbochargers are working 30-35% harder to make that boost over sea-level..

Red rooster

I have always used DIN70020 which is the German industry standard used to correct power measurements taking temperature and barometric pressure into account .
I guess that if it OK for Porsche then who am I to argue !!

All the best

Geoff