Does ambient temp physically affect boost?

[imported_admin]

I'm too tired to start thinking Boyle's Law and Charles' Law right now, but does ambient temperature affect boost? For the past month ever since it got cold out I've been seeing 10 psi at 5500 rpm even though I only saw 8 psi this summer. I thought it was just my head or a parasitic loss causing me to lose those other two pounds but I swear, it's the outside cold air doing it!

How? Any mechanical engineers in the house or do I have to dig up my dusty old physics textbooks?

[Theodore]

yes colder air makes more boost for example in the summer u run 10 lbs leave it there and winter u will see 15lbs just by air being colder

[Dominic]

Maybe not a 10-15psi boost summer/winter unless it gets below 0°F, but you will see a 1-2psi boost in colder weather due to the denser air :D So no, you aren't going nuts...

Another thing that is contributing to the supercharger boost going up is the colder belt. A cooler belt won't stretch as much and will pull harder on the pulley - less slip = higher efficiency [img]smile.gif[/img]

[12secondv6]

Or is stefan preparing for the v6 shootout???
[img]smile.gif[/img] [img]graemlins/burnout.gif[/img]

[imported_admin]

<blockquote>quote:</font><hr>Originally posted by Theodore:
yes colder air makes more boost for example in the summer u run 10 lbs leave it there and winter u will see 15lbs just by air being colder<hr></blockquote>

I'm asking for someone with a background in either engineering or physics that's stronger than mine to explain why...

I keep thinking ideal gas law:

PV = nRT

Pressure x Volume = Number of moles x Universal Gas Constant x Absolute Temperature

In the winter now I'm seeing 10 psi, I see 8 in summer. Volume is constant, gas constant is constant (duh), temperature is lower so number of moles (mass of air) increases. So in that sense the pressure should remain equal. Unless the decrease in temperature does not equal the increase in the number of moles, so the pressure increases as well.

Is this what exactly is happening?

[FirebirdGT]

<blockquote>quote:</font><hr>Originally posted by Stefan:
I keep thinking ideal gas law:

PV = nRT

Pressure x Volume = Number of moles x Universal Gas Constant x Absolute Temperature

In the winter now I'm seeing 10 psi, I see 8 in summer. Volume is constant, gas constant is constant (duh), temperature is lower so number of moles (mass of air) increases. So in that sense the pressure should remain equal. Unless the decrease in temperature does not equal the increase in the number of moles, so the pressure increases as well. <hr></blockquote>


The Ideal Gas Law is for a CLOSED system (fixed mass), your engine is a OPEN system (flow of mass).

Yes, you will make more boost with colder air, but there is no way I can put the Open system equations here and explain them.

[Dominic]

<blockquote>quote:</font><hr>Originally posted by FirebirdGT:



The Ideal Gas Law is for a CLOSED system (fixed mass), your engine is a OPEN system (flow of mass).

Yes, you will make more boost with colder air, but there is no way I can put the Open system equations here and explain them.<hr></blockquote>

Exactly...

[V6Turbo]

There's more to explain than simply the air is denser. If that was the only explaination, then he would push that denser air through at the same pressure, meaning he's breathing more VOLUME at the same pressure.

There's alot to get into here about gas law physics and heating under pressure, and would take a few pages to explain everything fully, so i'll be lazy and not. ;)

[imported_admin]

<blockquote>quote:</font><hr>Originally posted by V6Turbo:
There's more to explain than simply the air is denser. If that was the only explaination, then he would push that denser air through at the same pressure, meaning he's breathing more VOLUME at the same pressure.<hr></blockquote>

That's what I was thinking, but since I've never studied open system fluid dynamics I guess it'll suffice to say that cold weather = about 2# extra boost.

Cool, thanks.

[chas]

the supercharger will be more efficient since the air is denser. its not a matter of pv=nrt completely, but one of those hideous flow problems. i dont have my me text anymore so cant give your the reynolds numbers for air at various temperatures.

another factor might be (and this is pv=nrt) is that your engine/intake is about the same temp as during the summer months, but the supercharger has moved denser colder air into the intake which gets heated to about the same temp and must therefore be at a higher pressure.

[94-3.4]

I thought i would throw this in here. Up in Denver our supercharged cars run 9lb pulleys and only see an actual 7lbs. I am sure it has something to do with air density at high altitudes.