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10 Bolt BOP - What's Your Problem?

A Question of Octane

Lee from Alabama, via CarCraft.com: I noticed that Chevrolet recommends 92-octane gas for both its 454 engine with 8.75:1 compression and its 350 engine with a 10.0:1 compression ratio. I was always under the impression the high-compression engines needed higher-octane fuel. Why the same octane for both of these engines?

Jeff Smith: That's a great question, Lee. Static compression ratio is just one part of the much-more-important point of dynamic cylinder pressure. This is an area where the intake-valve closing point has a dramatic effect on the dynamics of cylinder pressure. One way to test for dynamic cylinder pressure is with a simple cranking compression test. Have you ever wondered why engines with different static compression ratios will have similar cranking pressures? Let's take an 11:1 big-block Chevy equipped with a long-duration camshaft as an example. Let's say the cam has something like 250 degrees at 0.050 inch of tappet lift. In a cranking cylinder-pressure test, the engine may only have 180 psi-yet this is an 11:1 motor. This big cam closes the intake valve later than a cam with shorter duration. The later the intake valve closes, the less air there is trapped in the cylinder. At high engine speeds of say 6,000 rpm, there is less time to fill the cylinder, so the longer the intake valve stays open, the more "time" there is to fill the cylinder. Conversely, if we take the same engine, change the cam to a really short, RV-style cam with a duration figure of 210 degrees at 0.050, and check the cranking cylinder pressure, it might be something like 220 or 230 psi-very high! This motor will detonate its brains out because the low-speed cylinder pressure is way too high. Roughly 190 psi of cranking pressure is about the limit for 92-octane pump gas unless the engine has an exceptionally good combustion-chamber design.

We looked up the engines in the GM Performance Parts (GMPP) catalog, and the small-block with 10:1 compression is the ZZ4 350 engine equipped with a 208-degrees at 0.050 hydraulic roller camshaft. The 454 engine with 8.75:1 compression actually uses a longer-duration 211 degrees at 0.050 roller cam, which probably closes the intake valve a little later than the small-block. GMPP is probably playing it safe by recommending 92 octane for the 454. Combustion-chamber design is also a part of this equation, and the ZZ4 engine enjoys a slightly better chamber design than the older big-block. Plus, the iron heads on the big-block tend to hold a little more heat that can make the big-block rattle. It's possible you could run 89 octane on the Rat in light-duty applications with that cam. It would come down to testing the combination to see how the engine reacts to the lower-octane fuel.

Not So Olds

John Griffin, Cour D'Alene, ID: With gas being so expensive, I am looking for a way to get big torque with good mileage. Is there a way to get close to 600 lb-ft of torque at reasonably low rpm and get good gas mileage? I am using an Art Carr TH2004R overdrive automatic.

I am building an Olds motor and have a choice of blocks-either the 425 or the 455. The 455 Olds came stock from the factory with 500 lb-ft of torque and the 425's came with 480. In 1968, the two-barrel 455 with small heads made 500 lb-ft at 2,400 rpm!

So, 40 years later, can I improve on this torque and keep the rpm in the range so the overdrive will give good gas mileage? I'd like to get high-12s in the quarter-mile with more than 17 mpg, but I'd settle for mid-13s and 15 mpg or so. I ran these motors in the '70s and managed 15 mpg-but gas was only 29 cents a gallon. Thanks for your help.

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