Living in Michigan and DD'ing and Evo I easily see a 20-30 degree difference in my 6am drive into work and my lunch break. My cars tuned to 24lbs and controlled with a hallman MBC. Is there anyway to predict how much the temperature actually affects the amount of boost I'm putting down? It gets old when I'm peaking to high in the morning or only making 19-20lbs in the afternoon.

 

For constant temp changes you may be better off getting ECU controlled boost. It can account for changes in air density as well as avoiding a spring which is affected by the same temperature changes. I run a MBC as well because I like having the ability to change it at the track as needed without a laptop... but I can certainly see the benefit of having ECU controlled boost.

 

Once I get a good feeling of its 70 out give it 2 cranks up I'll be good. I was hoping get a decent every 10 degrees is on way or the other is a pound of boost but I know its not that easy or even linear for that matter.

 

You need to calculate the pressure altitude and the density altitude. Knowing the density altitude then will enable you to see how much the outside air pressure has changed.

Detailed explanation:

Pressure Altitude is "standard altitude" corrected for nonstandard pressure. Standard (ISA) pressure and temperatures are 15*C and 29.92inHg. (15*C translates into 59*F btw).

SO: We would find out what the local altimeter setting is via wunderground.com or noaa.gov (none of this nancyboy weatherchannel stuff).

Today in Oxford, MI the altimeter is currently 29.85. So we subtract the standard altimeter from that and multiply by 1000 (this is simply to convert the decimal into a real usable number.

29.85 - 29.92 = -0.07
-0.07 * 1000 = -70

So then, you need to add or in this case, subtract 70ft from your current altitude. Oxford is about 1055 ft so...

1055ft - 70ft = 985ft.

So 985ft is the PRESSURE ALTITUDE. This does NOT take into account the temperature though. For that we need to calculate DENSITY altitude.

To calculate the DENSITY altitude the formula is:
Pressure Altitude + (100(Actual Temp - Standard Temp)) where temps are in Celsius. So for today in Oxford the equation would read...

985ft + (100(17.7 - 13.0497))

13.0497 is the standard temperature at 985ft. To find THAT you take the dry adiabatic lapse rate of losing 1.98*C per 1000ft of elevation. We usually just use 2*/1000ft in the plane, but since our altitude doesn't change much in a car, you can be SUPER ACCURATE (lol) by using the 1.98/1000ft.

So then... your DENSITY Altitude is 1,450.03ft RIGHT NOW. To figure out how much you need to alter your boost you would need to know the temp and pressures (density altitude) on the day it was tuned. Lets ASSUME that it was a standard day, and as such, your density altitude was actually 1055ft.

1450.03 - 1055 = 395.03ft

You'll notice that the STANDARD pressure at 1000ft is 14.2psi (which is about your altitude) and you'll notice that the STANDARD pressure at 1500ft is 13.9psi. So... after ALL of that, you need to increase your boost by .3psi.

It IS worth noting however... that you're very close to sea level, and the temperature and pressure is very close to standard, so there isn't too much of an effect. If it was 91*F and very high pressure though... say 30.75 or something... the change would be more drastic. (quick example)...

30.75-29.92 (*1000) = 830ft
1055 + 830 = 1885ft Pressure Altitude

1885 + (100(32.7778 - 15.2277)) = 3640.01 Density Altitude

3640.01 - 1055 = 2585ft difference.

Pressure @ 3500ft = 12.9psi
Pressure @ 1000ft = 14.2psi

So you'd need to increase your boost by 1.3 to stay at the same levels. And note, that it gets MUCH more extreme in places like Denver. This all sounds complex, but its really simple math. If anyone wants more details or has other questions, just ask.

 

Wow, nice answer. Now you have me thinking that I have a boost leak. Temps were in the 40s last week so I turned it down to where I was making 24psi and today its 67 and sunny and I'm only pulling 21. That sounds like to much to just be weather related.

 

Well one thing that none of that math takes into consideration is the effect on the metallurgy of the controller. We're talking about spring and ball valve stuff here, so the temperature will have an effect on those as well.

But when in doubt, do a boost leak test. If you're just turning the boost up and blowing it out somewhere... you're going to smoke your turbo.

 

Yup, the spring is affected by the temperature as well, so the density of the air is only part of the equation. During really cold winter temps I have to turn my MBC down like crazy because the spring freezes and holds far more boost than it would during summer temps.

 

Hey hey watch it with that stuff. You're gonna give some of us "simple minded" folks a migraine...hahaha

 

lolomg!

 

i live in upstate new york and all of my cars are turbo charged. i have boost gauges, of course, on each one. i notice no difference at all whether its summer or winter - and we have some cold *** winters here. that being said, when it gets TOO cold, my cars eat up gas and even though the power seems to be good (its too slippery in the winter time to accurately tell), i never see a fluctuation in gauged boost. my 240 still hits 22lbs and the altima still hits 12. i dont know if this is what youre looking for, but i dont see a difference in boost, just power.

 

Whether you see a change or not. It changes. Trust me... I'm a pilot.

 

oh i believe you - im a helicopter mechanic and we have to account for it too when we diagnose. my gauge is not showing a difference but i know that cold air is denser so it HAS to be affecting me somewhere.

 

Haha... I was just messing m8. Which boost controllers are you running? If the gauge doesn't show it that just means your boost controller is compensating appropriately.

 

youll like this - im running a hbbc. home brew boost controller. cost me 16 bucks to make and its FLAWLESS. holds boost better than my greddy ebc did and it doesnt spike or creep, believe it or not.

 

So awzum... you should write up a how to! Though in all honesty... it probably can't be that hard so long as you get the spring pressure right.