The Stovebolt Site Forums The Shop Area Engine Shop Need help to plan max compression

Originally posted by 50GMC253:
Replacing the 235 with a 261 bought the extra CCs. I’m using a reground 261 cam which provides more lift than the 235. My dual intake will provide ample fuel mix at the top end. Increasing the compression will increase HP across the board and boost economy at the same time.

I don't think that this is a valid assumtion. Increasing compression will increase BMEP across the board, but it doesn't necessarily correspond to an across-the-board increase in horsepower or economy.

In order for BMEP to be utilized efficiently, adjustments (and trade-offs) have to be made in ignition timing, valve timing, intake and exhaust flow and combustion chamber shape and temperature. If compression (and the resultant BMEP) is relatively low then effective flow into and out of the cylinder is less critical than it would be on a higher (again relatively) compression motor.

We're talking about getting the maximum amount of air-fuel mixture into the combustion chamber to be compressed and burned. And in general, with higher compression non-Diesel motors (Diesels are a whole nuther color of horse) the idea is to make power in the higher rpm bands of the motor's capability. This requires more adjustments of valve and ignition timing and intake/exhaust flow become extremely critical. The result is a race motor, which is less than optimal for driving on the street...let alone towing a boat to the lake.


My goal with these questions is to determine the maximum compression I can go with to reach my goals with pump gas. After determining the appropriate compression target I need to determine how much to shave off my head to obtain it.

Lotsa guys have doe the empirical research in this area. Conventional wisdom says that with all other factors being equal, 9.0 to 9.5:1 is about the practical maximum for street use and "regular unleaded" pump gas. My own experience has shown that at 9:1 I could run "regular" with no pinging. At 10:1, I had to run "premium." This was on a fairly healthy 355 V8 with Vortec heads...at 10:1 it felt like 350 to 375 hp...give or take a fudge factor or two. There have been some experiments with cam lobe separation, retarded timing and water injection to allow higher compression ratios to be used, but I'll stick with the old 9:1 rule of thumb.

Now: How much to mill? Good question. The only real way is to cc the head and use a bit of mathematics to calculate the desired combustion chamber size. Make sure you take into consideration deck height and piston configuration, too.


With a fixed load we know that fuel consumption improves with higher octane fuel due to increased power per pound of fuel mix. And, the higher the octane the higher the compression can be. That is why I am asking the economy questions; it might well be possible to pay more for premium, increase compression, gain a reserve power across the board, and actually improve or break even in the all important miles per dollar category.

Some quick calculations show that at the current prices for unleaded and premium in this area ($2.79 and $3.09) there's a 10 percent price differential. You would have to increase mileage by that amount just to break even on the cost of premium...without considering recovering the cost of the modifications to make this possible.

You may have an idea that could be be made to work. I won't say that it's impossible. But it might be difficult to make it worth your while to accomplish it. A final thought on this. I considered what the automobile manufacturers might think about it and the first thing that came to mind was the difficulty they'd have in selling the consuming public that it was cheaper to buy more expensive gas. Some folks might see the wisdom if it could be demonstrated, but the vast majority of consumers would just see that they had to pay more per gallon without reasoning any further.

Good luck with the concept.