I am sure half of you know this, but I will type this for those who havent. Its pretty basic and also can be used to tell if you are falling out of your systems efficiency.

There is a simple and reasonably accurate way to calcuate how much HP your car will make at various boost levels, assuming that you know how much HP it is making at its current boost level, also assuming that you stay within your turbo systems efficiency range, and also assuming you are on the same dyno, etc.

Its pretty basic, heres how it works, using my car as an example.

My car made 522rwhp at 11psi of boost. You want to convert that to manifold absolute pressure, which is easy. Atmospheric pressure at sea level is about 14.7 psi. So add the boost pressure to that, and we have 25.7 psi.

So I made 522rwhp at 25.7 absolute.

522 / 25.7 = 20.311rwhp per psi.

So I want to see what I would make at 14psi of boost.

14 psi boost + 14.7 atmospheric = 28.7 absolute pressure.

20.311rwhp X 28.7 psi pressure = 582rwhp.

How accurate was it? Look at my signature. About 2 percent. That 2 percent could be attributed to lower efficiency in my FI system. Or maybe I had too much timing retard for the next 3 psi. Either way, its a hell of a lot more accurate than converting Dyno Dynamics #'s to Dynojets!!!

 

So what breathing mods/work/exhaust do you have for breathing. That will play a big role in HP number as much as the turbo effieiency. Every thing from the the over all friction coefficient of your entire drive train to the air temprature and humidity effect your power out put, as we all might know by now know. But I have a tendency to get a little over technical.

This is corect only to a certain extent, the law of diminishing returns comes into play so until this is given a finite numeral this is a linear WAG ratio:

(H1/(A+F1)) = (H2/(A/F2)

If you watch the weather channel or have your own barometer you take to the track with you you can do the folowing easily: Barametric pressure X .491 = PSI(A). So the pressure altitude for sealevel in sunny clear sky average temps is 29.92 inHG multiply that by .491 and you have 14.69702. This is the standard psi reading for the atmosphere at sea level. For you guys in the mountains, again watch the weather chanel or you handy dandy barometer and factor out your realative PSI with the above .491 conversion.

If you want the current pressure altitude go to www.weather.com, If you want a WAG (wild @$$ guess) use this table:
Altitude (ft) Pressure

0,000 29.92
1,000 28.86
2,000 27.82
3,000 26.82
4,000 25.84
5,000 24.89
10,000 20.58

This will make the above equation a little more accurate and universal and help prevent over inflated estamated for you guys in the mountains.

Sorry for the long winded interjecture.

 

nice one. your addition doesnt really make it any more complicated at all once the person doing it figures out their custom algorithm.

as you said, of course there are 100 things that can have an effect. but you will find that its generally pretty dang close assuming alike circumstances.

 

Nice-good info, thanks for posting...

 

your right I forgot to put that in there

H1- Current HP number
H2- Desired HP Number
F1- Original Boost Level
F2- Desired Boost Level
A- Converted Pressure Altitude (Barometric Pressure X .491)

Its a simple ratio, you an use it to determine desired HP at a given pressure or ball park the pressure at a desired HP.

My car was 413.69 @ 7.9psi and currently I am running 10psi and I dont know the HP so the estimate at this current volumetric efficiency is 18.32WHP per psi. Im running 10psi now the estimated WHP is 452.14 "ish" WHP. according to all the figures.

EDIT: Yeah and thanks for posting this info, good stuff.
EDIT EDIT: You can make a real quick excel spread sheet that will calculate all this out. It will take like 15 minutes to get a rough one, nothing pretty. You could enter your HP current PSI and Pressure altitude from the local weather and it would convert it to either the desired boost to reach a specific HP goal. Or input the specific boost to determine your current HP number. Simple.

 

Anyone else think this might be "sticky" worthy?

 

Just checked out www.weather.com . If you custom set up your city it will give you real time barametric pressure readings from the nearest weather station to your ZIP code.

 

This whole thing can be tricky. By this math at 7psi a VG30DETT I dyno'd makes 12.75hp per psi. That means in theory at 10PSI it should be at 315rwhp and at 15psi it should be at 378rwhp, but it doesn't work out *close* to that, check out this chart:

http://tinyurl.com/bbcna
(the 3 runs were back-to-back during the same dyno session so all other variables were removed)

Yes those turbos are pretty small an their max efficiency is probably about 10PSI, but I think it goes to show you don't know until you strap it ont he dyno (which still doesnt always translate to real world speed - how would you like to have "500rwhp"and get beat by someone with "only" 400rwhp?).

Take your dyno number, subtract 10% and then when you beat the guy next to you he'll wonder what's up - just tell him your horses are bigger than his...

Dave Lum
www.datsuns.com

 

^ +1

 

Thanks for sharing.

 

Look in the first post he was talking about the diffrent variables and the efficiency range of the turbos. There are many other varriables that go into this equation. This is an over simplified version.

I gues a good variable to find is one you can plug into for the efficiency range of the turbos and the percent of degradation due to multiple dyno passes (heat sinking of the block).

The big thing we are looking for here isnt real world speed translation, its ballparking your tuning for your desired HP. If you want speed gut her like a fish, carbon every thing, a nice big meaty set of micky T's, and a nice 3 mile strait line. That will give you plenty of speed.

 

perhaps someone should give a quantitative value compared to angle of incident for your smile?


"oh. yea.. that looks to be around... 550 hp."

 

Sweet...

 

Ah, that puts this thread into better perspective. Sorry but I'm coming from a land of "tune for maximum HP a.k.a. pick the turbos that will match your power goals and then apply the other mods to help support that goal".

This time around I'm buying VQ stuff to....well that's a different topic.

Thanks for the clarification.

Dave Lum
www.datsuns.com