[A4] Need for Premium Gas in 1.8T quattro?

[Huw Powell]

>>BTW, turbos add volumetric efficiency to ICE's, not compression.
> 
> Huh?  A greater volume of air is forced into the cylinder. It is 
> compressed into the same squish area.
> That is higher compression, compared to NA, although the *ratio* 
> remains the same.

Right.  The "compression ratio" is unchanged.

A n/a engine is working with up to atmospheric pressure, and compresses 
it by it ratio to a higher density.  A supercharged engine compresses a 
higher pressure inlet charge by the same ratio.

Now, as Marc put it to me, supercharged engines *behave like* high 
compression ratio engines in that the inlet charge is at a higher end 
pressure in the cylinder than a normal compression n/a engine, and so 
have to be managed in similar ways - avoiding detonation, etc.

The semantic question here is whether "high compression" is being used 
as shorthand for "high compression ratio" or referrring to an end result 
pressure in the combustion chamber over some general figure (say, 200 
psi vs 100).  I think it is usually the former - ie, the 2.3 in my 90Q 
is a "high compression" engine, meaning it has a 10:1 *compression 
ratio*.  Although it generates a higher pressure result than a lower 
compression ratio n/a engine, it won't be as high as even a "low 
compression ratio" engine with supercharging.

-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/