# Re: S4 20v vs S2 10v

```>On Mon, 6 Feb 1995, Robert D. Houk wrote:
>
>> (I suspect it also means you can charge five times as much for a tune-up
>> since you have five times as many coils to tune, right?)
>
>for sure, but do coils get routinely replaced?  labor to replace
>spark plugs will probably be more..
>
>>    my understanding is that the 9.3 compression on the S4 is for

snip snip

>> Um, I don't think so. 7:1 compression of 2 bars is 14 bars, while 10:1
>> compression of 2 bars is 20 bars of pressure.
>
>ok, so "boost pressure" as we know is induction pressure then..
>
>> At 20:1 you can dispense with ignition altogether (you have a
>> diesel engine).
>
>hmm the S4 has a 9.3 compression and 2.15 bars would make it
>19.995!!
>
>i wonder how they keep this baby from blowing its brains?
>
>how about the 1987 formula 1 cars that ran 4 bars of boost..  i think
>???
>
>> Now if you had direct injection . . .
>
>how about sequential injection that the S4 uses?  how does that
>affect things vs. simple CIS?
>
Ok, let's see if I can get this straight:

starting with air entering the compressor, we need to know inlet T and P
and either isentropic compression efficiency or a delta T across the
compressor along with the (compressors) compression ratio.  From
this I can get the increased density before the intercooler, the intercooler
further increases the density of the air (a good thing for sure) with relatively
small losses. We should now have inlet P, T, and rho (density) for the cylinder

We know our compression ratio E (as defined (Vc + Vh)/Vc where Vh is cylinder
displacement and Vc is cylinder compression space)

so we use the equation Pc =PL * (E** n)   where n = 1.35 (polytropic exp of
real gases) to determine final compression pressure  where PL  is charging
pressure and Pc is compression pressure

so for a charge pressure of 2 bar and a compression ratio of 9.3 you get a
final compression pressure of 41 bar

or for the formula 1 with E=6.5 and Pc ~4 bar you get a final compression
pressure
of 50 bar

I pulled most of this stuff from the Bosch Automotive Handbook (mostly p339,
2nd ed)

so maybe this sheds a little light (and maybe not).  Comments?

:-)
jim h
>
>eliot
>
>

```