George D

Most turbo applications are set to read boost for absolute pressure, and compensate for density. I'm not going to argue with you here, as you are probably someone I'd enjoy spending time, as we have similar interest in cars.

Maybe it's my personal experience with my turbo cars, and how they are set up to read boost pressure. I used to have a S4 TT and here is a quote:

The audi computer has altitude correction built in, so at 5000 ft, the
boost is increased from .425 to .575 (yo, that's audi-eeez, so +/- .0250 is
allowed, took mean for all calculations)... So, with the guage reading .845,
add the .575 = 1.42 absolute... Translating this, the MC motor will be
99.64% HP efficient (and given the altitude pressure table variance of +/-
.0250 of mean, 100% is within spec) of sea level HP @ 5000ft.... All other
things be equal (caveat), there should be NO DIFFERENCE IN POWER LEVELS
BETWEEN YOUR CAR IN CO OR AT SEA LEVEL.

Here is a chart to verify your previous post. I get this, but my issue here is that my turbo is boosting harder in Greer than in Tucson, I can hear it, and my boost guage reads the same. I do have a TEC stand alone, and I know it has altitude correction built into it's software. Maybe my old 951 cars didn't have this, and I'm mistaken with my previous post.


Altitude Air pressure (in. Hg) Air pressure (psi) Loss as referenced to sea level
(psi)
Sea level 29.92 14.7 0
1000 ft 28.86 14.18 -0.52
2000 ft 27.82 13.67 -1.03
3000 ft 26.81 13.17 -1.53
4000 ft 25.84 12.7 -2
5000 ft 24.9 12.23 -2.47
6000 ft 23.98 11.78 -2.92
7000 ft 23.09 11.34 -3.36
8000 ft 22.23 10.92 -3.78
9000 ft 21.39 10.51 -4.19
10000 ft 20.58 10.11 -4.59

I'll verify my thoughts tomorrow with folks much better educated than me in the science of this subject tomorrow.

Thanks for making me think about what I may not know.

Kindest Regards,

George

Rogue_Ant

Here is where I don't think I have been able to clearly get my point across.

You are correct, the boost control system is different in your S4 than most 951s.
What kind of boost gauge do you use? If you use an absolute gauge, then your gauge will read "correctly". However, a normal (autometer, ect) type of gauge fail to account for the less starting air pressure.

So if I have a absolute boost control, like your TEC, and have a normal autometer gauge, and set the boost to 15psi at sea level then drive to Denver, the absolute boost controller is going to increase the boost pressure to compensate (your Audi quote verifies this). Now that you're in Denver, you will be running the same absolute pressure, but the autometer gauge will show ~3psi more boost then it did at sea level.

Also, the "NO DIFFERENCE IN POWER LEVELS BETWEEN YOUR CAR IN CO OR AT SEA LEVEL" is not quite true either. The simple fact that the turbo has to make up the lost pressure means more work is being done. And work isn't free; this can be seen by a reduction in VE.

piperporsche180944

The best I could manage here in Michigan with my old 1987 951 (Autothority Stage 2 Chips & Banjo Bolt, 245's) was a 14.1@103 with a 2.7s 60ft. I was either spinning the tires or bogging down and waiting for the turbo to spool every launch.

Im taking my 1988 951 out next Friday, it has a few goodies

LR MAF, 951 MAX Chips, Powerperfect, Super 61-1, 55# Injectors, 3.0 BAR FPR, Dual Port Wastegate, WBO2, Full 3" Exhaust, LSD etc....

Im aiming for 12's

toddk911

The 60ft of the 951 will always kill any 0-60 or 1/4 mile times. I think the fastest 60 fit I ever saw a 951 pull was 2.1 or 2.2.

Although that has always let me beat much faster 1/4 mile cars in simulated roll races.

"yea, its only a low 13 sec car at best" lol But then in a 20 or 40 roll I can stay with mid 12 sec cars

DanaT

The absolute / relative is a question that 951 people dont fully understand.

To make matters worse I am running the Wastegate off of a MAP sensor (which references to vacuum and therefore reads the same regardless of altitude). Logging the MAP sensor with the car off (i.e. atmospheric pressure, it is about 800mbar).

I have not set the turbo boost using the MAP yet. It has been set based upon an relative boost gauge. I am at about 16-17psi on that gauge.

I have to hook up the zeitronix to log boost a (from the MAP) and RPM. So far it is only hooked up for AFR. Need a little more time to get things done


So, I am still running the car conservative until I get things dialed in. For the first outings, it best not to crank boost up until the head gasket blows and then back it off one psi.....

-Dana

Rogue_Ant

What!!! Heresy!

blown 944

just to touch on the subject lightly.

What seems to be forgotten whenever this subject comes up is how dramatic the lack of pressure is on a turbo car at elevation. I'm sure everyone here has sat and watched the gauge and wait for it to start building boost, then once it hits 1 psi it takes off on about any turbo charger. There is no diffence up here in that regard it is just that it ,takes longer,uses more mechanical enery, creates more heat = power loss.

On a 951 this is more pronounced due to the turbo positioning. It is always an intereseting conversation.

Now since this subject has come up it brings me to why I didn't take my car up... playing with the anti lag too much and blowing out the turbo flange gasket :-(. Hoping it will negate most of the altitude issues

next week maybe.

toddk911

What is this magical thing you speak of??? Please tell us more.

MooreBoost

i know a guy that runs up pikes peak for the hill climb and he says he has a switch for higher boost when he gets to the top

George D

Yup, you can run more boost at altitude. Some filling stations at high elevations don't sell premium gas. You don't need as much octane at high elevations. I still keep my car set the same. Don't want any more EXPENSIVE issues.

DanaT

Ok.

SId, I must say thank you many times. Your showing up there and giving me advice saved a brand new turbo from ruin.

-Dana