Hi:
I wanted to thank Tom Witt, Theo Smit, Bob Palmer and Tim Ronak for
responding to my question about compression ratio.
I'm sorry it took me so long to get this written. Things have been a
bit hectic lately, and your responses had me looking for more
information so I could understand what it was you were trying to tell
me. While my success in the latter is questionable :-) , the journey
was interesting.
Tom, you and Tim both mentioned *cylinder pressure* with regards to
it being potentially very high with a high CR and stock cam, and Tim
mentioned the Super Stock racers looking for a max of about 195 psi. I
had trouble finding any information regarding cranking cylinder
pressure, but that may have been due to how I phrased my search. What I
did find was an article in Chevy high performance that had similar
numbers: it suggested a max of 200 psi for a street engine based on pump
gas. I assume that the Super Stock guys Tim mentioned had access to
higher octane fuel, so I'm not sure what to make of the Chevy article
and whether the thing would grenade on 104 octane with a really high
cranking pressure.
In short, more questions and not so many answers.
With regards to Tims' and Toms' point about *cylinder pressure* and
cams, the Chevy article said that the intent of the long duration cam
with late-closing intake was to bleed off some of the low-speed
cylinder pressure to keep the engine from grenading itself on *pump*
octane fuel (this ties in with what Theo wrote). It seems to me that if
104 octane fuel is the only fuel used, then really high cranking
pressure wouldn't be as much of a problem and I could run the 'stock' cam.
Is that an incorrect assumption?
Interestingly, I found reference to Sunoco fuel that suggests that
their 100 octane will handle 12:1 compression, and something called
Circle Oil Company that suggests that their product, called VP Red with
105 octane will handle 13:1. The two don't exactly agree, do they? One
point compression difference, but 5 octane points difference!
More confusion, I guess.
Bob, you mentioned that boosting the CR wouldn't increase *thermal
efficiency* very much, and what I found certainly agreed with you: it
suggests that for a 12:1 CR (and for which it recommends 108 octane
[more confusion, in light of the last paragraph] ) I'd get about 35%
brake thermal efficiency at full throttle, up from 32% at a 9.0:1 CR. I
think your example showed about a 4% improvement--this other source
works out to about 3%--pretty darn close, and, as you suggested, not
very much improvement.
Your results also suggested that *power* also wouldn't increase by much
based on just an increase in compression ratio. I found a site that
agreed--suggesting that for every 1 point in compression increase,
expect a power increase of about 2%. That'd translate to about 6% power
increase for a bump from 9.0:1 to 12.0:1.
And, I found two different calculators by Bowling and Griffo that offer
seemingly contradictory results.
The first is based on a given peak horsepower. For a CR change like
I've outlined, there's only an 8% increase in power, which more or less
agrees with the previous site.
Their second calcuator is based on CR, displacement, and rpm (and
they say sometimes it works, sometimes it doesn't) and it suggests that
there'd be a power increase of about 30%. But--when I plugged in the
stock CR, displacement and rpm, their power output matched pretty
closely to the stock.
So, more contradictions.
Anyway, thanks again guys.
Best Regards
David Sosna
|