A 401 CJ
A 401 CJ Dork
8/23/19 11:08 a.m.

I might need to go back to school.  I’m bothered about why a roots blower tends to get cold and covered in condensation even on a hot day...at least at idle.

I’ve seen my next door neighbor’s pro-mod car do it.  He said the same thing that Cleetus says in this video at about 18:40.  That it’s the ethanol.  If so, how?  And I’m dubious of that anyway.  I tend to think it has something to do with the Joule-Thompson Effect but I might be wrong.  



edit: my googlefu has failed me.  Which is strange because I am normally ninja level.

aircooled MegaDork
8/23/19 11:12 a.m.

I will take a guess an say: since Roots type blowers do not compress internally (they just move air), that does not create heat/pressure internally, but sucking air through a venturi (carb) into the blower does lower pressure (and thus temp).

Downside the blower, well, that's where the heat/pressure starts.

Vigo MegaDork
8/23/19 11:18 a.m.

Most roots blowers have a throttle plate in front of them. An engine would normally make something like 20" vac at idle behind a throttle plate. Between the throttle and the lobes of a roots blower with no bypass valve it's actually going to be lower than that. Almost any moisture (or liquid fuel) in that air column is going to evaporate or boil and thus remove heat from whatever it was touching when it did that. Then those pieces fall below the 'dew point' temp of the ambient air outside of the supercharger and collect condensation from the air. This is my basic understanding of what's probably happening there. 

Streetwiseguy MegaDork
8/23/19 11:32 a.m.

If there is a carb on top, there is fuel changing state in the vacuum of the blower.  Liquid to gas change absorbs heat.

Javelin MegaDork
8/23/19 11:44 a.m.

Carb or fuel injection will make a difference, as will gas or ethanol. Dad's blower with carbs and gas does not condensate.


A 401 CJ
A 401 CJ Dork
8/24/19 3:26 p.m.

But why does it matter carbs or FI, pure gas or ethanol?

Javelin MegaDork
8/24/19 4:05 p.m.

In reply to A 401 CJ :

Latent heat and vaporization. Alcohol is hydroscopic, so it absorbs water from the air. Carbs will have the fuel before the blower (draw through) and most duel injection setups are after the blower (blow-through, so a dry blower case). It's all above my pay grade, I've just read some things and watched some videos. The Pro Mod guys do a great job explaining the really exotic stuff.

Jcamper Reader
8/26/19 10:31 a.m.
Streetwiseguy said:

If there is a carb on top, there is fuel changing state in the vacuum of the blower.  Liquid to gas change absorbs heat.

This is correct. Latent heat of either fusion or vaporization depending on which way you are changing state. Ethanol absorbs about double the heat compared to gasoline when evaporating. This is one reason it tends to be more resistant to pre-ignition compared to the same octane gasoline. Remember most of the fluid is still changing state inside the combustion chamber, not intake manifold, especially on injected stuff. The combustion chamber mixture is very non-homogenous, not like the pretty graphics you see in simplified explanations.


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