Re: Gasoline grade BTUs per gallon?


"BobG" <bobgardner@xxxxxxx> wrote in message
news:1148668713.049997.307900@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Lets say I have a 9:1 engine running 'regular' and a 10:1 engine
running 'hi test'. I'm running at half throttle, so the manifold
pressure is about 7psi. I get 63 psi and 70 psi at TDC on each engine.

Well, ok...(I assume you mean 7 psi absolute (around 15" vacuum (gage)))?

Seems that the same amt of air would be ingested on each

Well, lots of factors to that, but on the face of it, no
How are you obtaining that higher compression ratio?
Stroked....shaved....cam....pistons? (Short throws will actually capture
more air *at the same point of intake valve closure*).
An engine with 9:1 (static) will usually be considerably different that an
engine with 10:1 compression, even if they have the same displacement.
The "effective" or "dynamic" compression ratio is quite different than the
"advertised" ratio....there are a *lot* of factors to bear out the VP
(Volume/Pressure) index of a particular engine configuration.

so the stoic amt of fuel would be the same on each, but the hi compression
engine
develops more psi on the power stroke?

On the face of it, yes. "In real life", not necessarily.....and you'd
likely have to retard the ignition timing, and run higher octane fuel for
starters, but you can "fix" that, more in a bit.
There are far too many variables to give a flat "yes/no" answer....what
happens inside the cylinder is very dynamic, and changes with rpm.
A higher compression engine you generally do more things with (amongst
others) the cam, when the valve closes, duration, overlap, etc....
The amount of cylinder "fill" might be the same at idle, but much different
at higher rpms, depending on profile (some Harley engine builders are
obtaining 130% cylinder fill with naturally aspirated engines) One
example.....I might have a 10:1 compression engine with cam "A", intake
valve closing at 66 deg ABDC....effective compression ratio of 7.85:1 and a
VP of 269, same engine with cam "B", valve closing at 76deg
ABDC....effective compression ratio drops to 7.15:1, VP drops to 217....on
the face of it, a drop in cranking pressure of 10%, a drop in VP of
20%.......and lower octane fuel requirements.... but it's horsepower/torque
curve is going to be very different, especially at higher rpms....even
though the "advertised" compression ratios are the same.

So this is the same situation of
adding 7psi boost with a supercharger?

No.
Cylinder fill (mass air charge) will be considerably higher with 7psi boost,
even with the same cam profiles.

K. Jones


.

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