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Maximum Compression Ratio On Olds 455 - What's Your Problem?

Squeeze It
Daniel Stafford; Nicholasville, KY: I've looked all over the Internet and asked the opinion of everybody I can think of, but I can never seem to get a definitive answer to my question. What is the maximum compression ratio I can run on my 0.060-over Olds 455 on premium pump gas? This engine will reside in my '68 4-4-2 convertible and is sporting Edelbrock aluminum heads with a Mondello super competition port job, Probe aluminum pistons, and an Offy intake. Is there a straightforward answer or are there some variables that should be taken into consideration? I'm sure the setup and type of ignition system play a big part in determining the maximum compression ratio I can run without worries of detonation, overheating, and so on. Also, I have yet to decide what type of cam to install, as I'm sure cam type and timing play a role in this equation. The more power, the better. Does 10.0:1 or even 10.5:1 sound outlandish?

Jeff Smith: To answer the question directly, Daniel, a static compression ratio of 10.5:1 with a performance camshaft of around 230 degrees at 0.050 would be a great combination. At that compression, don't get too aggressive with the timing on pump gas-around 34 degrees should be about right. While everyone talks about horsepower, keep in mind that some engines are more torque oriented. The Olds engines were intended to make mega torque but not necessarily as much horsepower mainly because you don't spin these engines as high as others. This is due to many factors, not the least of which is that the 455 Olds uses a monstrously heavy rotating assembly that includes a 3.00-inch main journal crankshaft. Basically, these engines are intended to run at 5,500 rpm and below. You should not treat a 455 Olds like a small-block Chevy unless you spend big dollars for quality rotating assembly parts. In our Oct. '10 issue, we built a 455 Olds with Edelbrock heads with 10:1 compression and a mild 224/230 cam at 0.050-inch tappet lift and 0.485/0.490-inch lift that made 511 lb-ft at 3,700 and 443 hp at 5,100. I didn't check the cranking pressure, but my guess is that with 10:1 static compression, it will be around 195 psi. Let's dive a little deeper into the relationship of cam timing to compression.

As you have guessed, there are many variables that play into the selection of compression ratio and cam timing. I'll narrow this down to a few essential points. Cam timing has a very distinct effect on engine performance. A long-duration camshaft is intended to increase the amount of time the intake valve is open to help engine breathing at higher engine speeds. A longer-duration cam also means the intake closing (IC) point occurs much later than a shorter-duration intake lobe. If you think about how a four-stroke engine operates, the cylinder cannot begin to compress the air and fuel until after the intake valve closes.

Static compression ratio is defined as the ratio of the cylinder volume with the piston at the bottom of its travel (bottom dead center or BDC) compared with the volume of the cylinder with the piston at the top of its stroke (top dead center or TDC). Static compression ratio does not take into account when the intake valve opens or closes. So 10 times more volume at BDC than at TDC creates a 10:1 static compression ratio. Notice the use of the term static. Dynamically, the engine sees a much different situation. Since the cylinder cannot begin to create pressure until the intake valve closes, with a later-closing intake, less air and fuel will be captured in the cylinder, especially at low engine speeds. Plus, long-duration camshafts routinely allow the piston to push a certain amount of air (accompanied by a small amount of residual exhaust gas because of the earlier opening intake valve) back into the intake manifold at idle and low speeds. This residual exhaust gas is one reason an engine with a big cam idles so roughly.

When there is less air compressed at slow engine speeds, the engine torque is not as high as it could be. A short-duration camshaft makes more low-rpm power than a long-duration cam because the intake valve closes sooner, allowing the cylinder to squeeze more air and fuel. At high speeds, the longer-duration cam allows the engine more time to ingest air and fuel to make more power. All this means you need more static compression with a longer-duration camshaft to compensate for the later-closing intake valve. This can be evaluated by using cranking compression. A long-duration cam with more static compression will still deliver decent cranking compression of 180 to 190 psi, while a long-duration cam with low static compression will lose cranking pressure down to perhaps as little as 160 psi-depending on the size of the cam. A cranking cylinder pressure maximum of 200 to maybe 210 psi is the most you can expect and still avoid detonation on 91- to 93-octane pump gas. A further advantage of more compression is it will dramatically improve throttle response and crispness on the street, not to mention its minor contribution to better fuel mileage.

As far as your question about ignition timing, a good place to start is 34 degrees of total mechanical advance. If the engine rattles on good gas, just retard the timing 2 degrees until the detonation is gone. The best way to evaluate maximum timing is at the dragstrip by watching the trap speed. If you add timing and the trap speed increases, keep going until the trap speed falls off and then return to your best mph tune.

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