Understanding Engine Measurements

Piston Math

Jake Amatisto Oct 29, 2013 0 Comment(s)
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As gearheads, we love to remember numbers. Many can probably rattle off all sorts of specs from various cars and engines. Cubic inches, rearend ratios, and camshaft lift specs are a lot of the common numbers that come up in car guy conversations everywhere. Although most, us included, get lazy and round up or down (for example, our "383" small-block actually displaces 384.5686 ci), for the most part, the hot rodding crowd is obsessed with numbers. But when we ordered a set of custom pistons recently we realized there are a lot of dimensions of the small- and big-block Chevy that we weren't that familiar with, at least not as exact as we should be. Deck heights, piston dome volume, head gasket volume figures, and the following are not all that solid in our memories, even the term used. Knowing the details of your engine is useful to have, even if you're not building it; and as Chevy fans, we have really only three engine types to keep track of, the LS and the small- and big-block families. Becoming familiar with important numbers in our hobby is something we feel helps it thrive. After all, knowledge is power.

Deck Height
The deck height of a block is the measurement from the center of the crank to the top of the cylinder head mounting surface. Common deck heights for small-block Chevys are 9.025-inch for the 302, 305, 327, and 350ci engines, however the aftermarket also offers a tall deck small-block Chevy block that measures 9.325 inches, which is a dimension that comes from the rare GM Rocket block design. The late-model small-blocks, such as the LS and LQ engines, have a 9.240-inch deck height, while big-block Chevy engines measure 9.800 inches pretty much across the board (except for some tall deck truck engines, such as the 427); however, tall 10.2-inch deck big-blocks are also available in the aftermarket and are used for larger cubic-inches engines, such as the massive 632 and larger engines used in drag racing. This measurement of the block will ultimately determine what rod length, stroke, and piston design you'll need for a target compression ratio.

Deck 2/10

Compression Ratio and Compression Height
The compression ratio is a more commonly remembered number in the gearhead community; it determines what fuel you can run and how much horsepower you can make. The ratio is a figure that comes from the volume of a cylinder with the crank at bottom dead center (BDC) compared to the cylinder at top dead center (TDC). Case in point, if you measured a small-block with a cylinder volume of 80 ci at BDC and a cylinder volume of 7.5 ci at TDC, you take those two numbers and divide them you get around 10.6:1 compression. The farther apart these numbers are, the more compression you will have. The higher the ratio, the more likely detonation will occur and, in turn, higher octane fuel is needed. We've seen that 10.5:1 seems to be the agreed amount for naturally aspirated engines on pump gas, however some brave souls choose to push it.

The compression height is the distance from the piston pin to the top of the piston and this is figured out once the builder knows a couple factors; the deck height, rod length, and the stroke length (see chart). The compression height of your pistons isn't the most important number to remember, but if you familiarize yourself with a general idea of what's considered tall and short compression heights, it makes you more informed when you're researching parts for your next build. For example, our 10.5:1 compression, 385ci SBC has flat-top pistons with a compression height of 1.125 inch.

Powerhouse Dial Bore Gauge And 3/10

How Compression Height is Calculated:
Compression height = block height - rod length - (0.5 × stroke)

Example:
block height = 11.685 inches
rod length = 7.500 inches
stroke = 5.500 inches

Compression height = block height - rod length - (0.5 × stroke)
Compression height = 11.685 - 7.500 - (0.5 × 5.500)
Compression height = 1.435 inches

Piston Dome Volume
There are three choices when it comes to piston types: domed, flat-top, and inverted dome. Generally, for pump fuel, the flat-top is an ideal choice, with the inverted dome being great for low-compression supercharged engines, and the domed piston being more for racing engines. We were recently stumped when we were asked how many cc's the dish was in the pistons we are running, and had to look up what we ordered from Mahle. This also inspired us to look up some other more common piston dome/dish cc's for our own knowledge. If you see a small-block piston with a -5cc dish, for example, it's likely a flat-top with two valve reliefs, whereas a -31cc is more of an inverted dome. Because of the arrangement of the valves, big-block pistons with a -3cc dish is a flat-top with a single valve relief.

Flat Top 4/10
Domed Pistons 5/10

An example of two domed pistons shows the volume differences in the domes. The piston on the left is a 8cc dome while the piston on the right has a slightly less raised dome, at 4 cc. These two pieces typically determine if you’ll be running pump gas or race gas in your engine.

Combustion Chamber and Head Gaskets
The combustion chamber volumes of the three Chevy engines can vary quite a bit depending on manufacturer, or, if you're dealing with stock heads, the year they were cast. Skilled engine builders check this by using a CC Kit like the one pictured from Powerhouse (PN POW351150), which is a 100cc x 2cc graduated cylinder and stand that can accurately determine volumes. Some sizes we've seen from the aftermarket are advertised from 64cc chambers up to 80 cc for some race heads, so it's important to measure this. To do so, you flip the cylinder head so the chamber faces up, then, once you grease up the area around the chamber, place a piece of clear plastic sheeting with a small hole over the chamber, completely covering it. Then, with the graduated cylinder filled with colored liquid, you open the valve and record how much fluid it takes to fill the void in the chamber.

Head gasket thicknesses can range anywhere from 0.012 to as thick as 0.080, but the common size for small-blocks is 0.040. This, too, must be factored into your piston math, so it's a good idea to measure this using the CC kit, keeping in mind that it will cinch down by 0.002.

Understanding Airflow
If you really want to get into the algebra that engine builders use to figure out all your engine's parameters, HP Books published an informative book called Engine Airflow by Harold Bettes that features every equation you could want. Whether you want to figure out your intake port's cross-sectional area, or you just want to have access to the basic equations, like figuring cubic inch and compression, this book can help you delve into all the numbers needed to assemble a high-performance engine correctly.

Engine Airflow Harold 6/10
Graduated Cylinder 7/10

Skilled engine builders use a graduated cylinder and stand to accurately measure the combustion chamber size of a cylinder head.

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