Engine Building Part 2 - Power Principles

Building your own engine is a great way to learn about what's going on inside your favorite mill. Also, knowing that your mind went into the design and your hands were used for the assembly produces a great sense of accomplishment. If you happen to be a member of that crowd and want to start delving into the world of engine building, then this article (which will be continued next month) is aimed at you.

The foundation of every engine is the engine block, and a well-machined engine block isn't anything without some strength behind it. A strong engine block isn't anything without precision machine work. You have to have a block that's strong enough to hold the horsepower and rpm that you have planned, and you have to have the machine work to support the game plan. There are two choices when it comes to block options: factory and aftermarket. We don't have the time to go into each Ford engine family and discuss every single strength and weakness, but we can offer some basic rules on block strength though.

1. Don't always assume that a factory four-bolt-main block is stronger than a factory two-bolt-main block.

2. Don't always assume that a main girdle will give a block magical powers. Girdles can help with main cap walk on milder applications, but beyond that, they usually just hold the pieces together when the block explodes. We've seen 302 blocks split in half (literally) with the three middle main caps neatly tied together.

3. For factory 302s, we usually draw the horsepower line at around 450-500 hp. These blocks are really weak between the cam tunnel and main cap areas. As stated above, they will literally split right down the middle. They don't run well like that. The production 351W blocks will generally hang in there a little longer and we've seen them survive well in 550-600hp applications.

4. Watch out for cylinder wall thicknesses on the Cleveland blocks. It's not uncommon to do a sonic test and find some thrust wall thicknesses that would make you scared to bore the block 0.030-inch over. A sonic test any time you plan a Cleveland build is money well spent just to see where you stand.

5. The factory big-blocks (FEs and 385 series) will handle a decent amount of horsepower. The FEs can gain strength by adding a girdle (little more effective here since the blocks are skirted and most girdles attach to both the main caps and the oil pan rails) or by adding cross-bolted main caps. Even then, they are still susceptible to cracks in between the cam tunnel and the mains. The 385 series blocks (429s, 460s, and so on) will handle a very large amount of horsepower and it's not uncommon to see them as foundations for 700-800hp engine builds.

If you're planning a build and you're on the fence as to whether or not the block is capable of handling the stresses, it may be wise to go ahead and ante up the money for an aftermarket block. The horsepower numbers that we've quoted up above are not set in stone. We've seen 302 blocks split at 450 hp and seen others take 600 hp religiously. Your results may vary, so use your own discretion and judgment.