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.

The big-block guys aren't left out in the cold here either. For the 460 fans, Ford Racing has a couple of blocks available, with one being offered in a 4.600-inch bore size. With a 4.500-inch crankshaft, you can essentially squeak your way up to the 600-plus-cubic-inch- displacement range (4.625x4.500).

Genesis, Pond, and Shelby are your manufacturers for aftermarket FE blocks. Most of these are available with larger bore sizes as well, which allow displacements well into the 500ci region. Genesis and Robert Pond offer their blocks in either cast-iron or aluminum. You'll pay a higher premium for aftermarket blocks, but the up side is that you won't have to grit your teeth every time you step on the loud pedal. All of these blocks will hold a substantial amount of horsepower.

Once you've decided on the block requirements for your engine build, you will need to have it prepped and machined. If it's a factory block, then it would be beneficial to have the block checked out before you put a decent amount of money into machining it. Several key areas need to be examined first:

1. Obviously, the block needs to be inspected for apparent flaws: "windows" in the block, broken bellhousing bolt bosses, major cracks, and more.

2. The block bore needs to be measured. There's no use in taking a block to a machinist to have it bored if it's already been bored past the "comfort zone." For example, if you have a 351W block that's already at 0.060-inch overbore and it will need to be bored further to straighten the cylinders, then this would be a block that should be passed on. If you're concerned about how far you can go on a particular core, then the sonic test would be your best bet to get an accurate map.

3. Once you check those first two tasks off your list, then it's wise to have the block pressure tested and Magnafluxed. Pressure testing will ensure that there are no internal cracks or areas where the block can leak. Magnafluxing will take care of the areas that can crack but have no fluid pressure behind them (main caps, main journals, and so on).

After the block has passed all inspections, then it's time to have it machined. Let's go over what's deemed necessary for a "detailed" block machining process:

Bore & Hone: A block needs to be bored to get rid of any straightness issues in the bores, to get rid of any cylinder flaws, and to roughly prep the engine for the new pistons. I generally recommend to only bore the minimal amount needed. It's unnecessary to take 0.060-inch out of a cylinder wall, when a 0.020-inch cleanup would suffice. Adding bore doesn't add that much to the displacement of the engine. I've heard many guys boast that their engine block is "bored to capacity." This isn't really something to boast about, as they've only added 5-10 cubic inches and they've got extra thin cylinder walls now.

Following a bore, the cylinders have to be honed. Boring a block will not put the finish on a cylinder that is necessary for proper ring seal and wear. To hone a block, torque plates are bolted to the deck surface to simulate the clamping force of the cylinder heads and fasteners, and a series of honing stones are used to put a final size on the bore and to put a "cross-hatch" on the cylinders.

Square Decking: There is a difference between decking a block and square decking a block. When you deck a block, you basically remove some material to make sure the deck is flat and straight. When you square deck a block, you ensure that the deck surfaces are equally parallel to the crankshaft centerline. When this is accomplished, then each piston should nominally come up to the same deck height on each cylinder. If this is not accomplished, then you may find that your block can be up to 0.015- to 0.020-inch tall/short depending on the particular cylinder. How good would it be to have one piston at zero deck height and another piston 0.010-inch down in the hole?

As an added note here, when you square deck a block, you can specify where you want the pistons to be in relation to the deck. As my own general rule, I try to get the pistons around 0.005-inch below deck up to zero deck (where the pistons are basically flush with the deck surface). With a 0.040-inch gasket thickness, this will give a 0.040- to 0.045-inch quench distance. Any quality machine shop will have a fixture that will allow it to machine the block to this spec.

How do you determine what deck height you will need? Add your rod length, half your stroke, and your piston compression height together. Take, for example, a 347ci small-block. This is a pretty common combination and in this case we have:

5.400-inch rod length + (3.400-inch stroke/2) + 1.090-inch compression height = 8.190 inches

If we wanted the pistons to sit approximately 0.005-inch in the hole, then we would need a 8.195-inch deck height on the block.

You've probably already picked up on the fact that before you have your machine work done, you need to know (or have a really good idea on) which rotating assembly parts you're going to use.

Quality machine work is imperative for a quality engine build. Sure, you may be able to get away with running a brush hone through the cylinders, knocking some new cam bearings in, shooting some rattle-can paint at it and calling it a day. However, the more detail you place here, the more you will be rewarded; not only in assembly, but also in performance. This is the foundation and basis for the entire engine build. Money spent here is very well spent.

For the next installment of Power Principles, we'll concentrate on picking out cylinder heads, camshafts, and intake. The engine block is the foundation, but a well-designed and well-built engine relies on a clear, concise plan of attack, and a well-matched set of induction components that can move the air and fuel as efficiently as possible into and out of the cylinders.

Brent Lykins is the owner/operator of B2 Motorsports. (www.b2motorsportsllc.com)