General engine questions: Compression ratio and Displacement

[jkarlsso@metabolex.com (Jonas K.)]

On 6/12/02 5:11 PM, "Dan Bubb" <jdbubb@ix.netcom.com> wrote:

> 
> The basics of what you are saying is, of course, correct. My statement said
> "a small impact". When I say this I'm thinking in terms of going from, say
> 8.5 to 10 CR, or from 10 to 11. Something we might actually do, as opposed
> to going from 8.5 to 14.

I was refering to the differences I've experienced comparing 8.5:1 vs 10.0:1
CR's. Both in my scirocco.

> The major differences between the JH and RD is 1) CR from 8.5 to 10, 2)
> intake with larger longer (better for low end torque) runners, 3) totally
> different exhaust manifold and downpipe, and 4) a cam with considerably more
> lift and duration (not to mention hydraulic).

Well, in my scirocco I had the original intake, had the dual outlet exhaust,
and solid head, and I can tell you that there was a remarkable difference in
the torque, especially at low RPM. Granted this is somewhat irrelevant,
since I don't have a before-after dyno run, so there are no hard numbers and
no % change to look at. But it is indicative of the effect of changing only
CR. Anecdotally, power increases significantly, and the only way to do this
(typically) is to increase the air flow of the motor.

> Perhaps the biggest impact of increasing CR is that it will generate
> stronger vacuum and pressure pulses (similar to a temperary syringe effect
> at TDC) in the inlet and exhaust tracts that could be used to increase
> volumetric efficiency although I don't really think VW took much advantage
> of this.

Bingo. That's exactly the case. VW didn't have to take advantage of it,
volumetric efficiency increased nonetheless. The syringe example was grossly
oversimplified, only an example of how starting with a lower volume will
generate a stronger vacuum as the piston is moved down.

Since we're on this topic, I'd like to expand. Part of the difficulty in
thinking about the internal combustion engine is that it operates over a
very wide RPM range. For CR it helps to pick a few different RPM points and
compare them. 

The first point will be sub-idle RPM. Like 100 RPM. In this range, the
filling of the cylinder happens so slowly that there is really no pressure
drop between the plenum and the cylinder as it fills. Thus the CR will not
make any difference in the amount of air that enters. If the piston sweeps
out 0.5l, then 0.5l of air will enter.

Second RPM point is say, 2000 RPM. At this point the piston moves so fast
that there is flow restriction due to the intake runners, valves, etc. Thus
there is pressure drop. The more "suction" that the piston can apply, the
more air will flow into the cylinder. This is because the RPM's are still
slow enough that the entire column of air sitting before the intake valve
comes to a complete stop (more or less) with each revolution. Thus the
higher CR engine will pump more air, and generate more power.

Lastly 6000 RPM. This is around where most motors make their peak power. At
this speed, everything happens so fast, that the column of air before the
intake valve really doesn't stop flowing. It is nearly continuously flowing,
determined by the swept volume of the engine. CR has less of an effect here.
Power output is limited by all the other flow characteristics of the system,
starting with the air filter, and ending with the muffler.

So, a grossly oversimplified statement is as follows. Improving max CFM that
the intake/exaust can flow will increase max airflow (max HP), but do little
for the amount of air pumped at lower RPM's when there is little restriction
posed by these components. At the lower RPM's the easiest way to increase
airflow for a N/A engine is to increase compression ratio.

Jonas