Data Driven — Getting Accurate Measurements For An Engine Calculator

Data Driven — Getting Accurate Measurements For An Engine Calculator

Let’s face it, these days, not many people actually know the formulas and equations to calculate various engine parameters that we use every day. Even if you do know the formulas by heart — and I know there are some of you who do — there are probably very few of you actually putting graphite to paper to work out the math.

Since you’re already using a calculator for the number crunching, it only makes sense to go a step further and use a calculator for the whole shebang. It reduces the chance for human error, and let’s face it, we’re far more fallible than a computer’s coding. To that end, Engine Parts Group, Inc. has put together a collection of various performance calculators which will help you crunch all kinds of data as it relates to your project.

Engine Pro has nine calculators available for you to use for all sorts of calculations. We’ll be focusing on the two engine-related ones, displacement and compression ratio.

Why Calculate At All?

Believe it or not, there are still people out there who don’t just listen to what the internet tells them will work. If you’re putting together a combination yourself, it’s a lot easier and cheaper to run the numbers on all of the parts before you buy. EnginePro’s sales manager, Roger Borer confirms, saying, “Any change during an engine build can impact performance. Switching to a .020-inch thinner head gasket will produce a 3.5-percent increase in compression ratio, or just over a quarter-point in a typical small-block Chevy build.”

The closer you get to pushing any limits, the more important the small details become. “Figuring the net effect of any changes by trial and error is time-consuming and expensive,” says Borer. “If you are looking for a fast and easy way to model changes, the calculators found on Engine Pro’s website is a perfect place to start. There are nine tools, ranging from compression ratio calculation to gear size selection. Key in your specifications and get the results instantly. You can modify any specification to easily determine the effect on your engine build.”

Since this is EngineLabs, we’re going to focus on the calculators which deal with the engine alone. We’re also not just going to tell you how to use them since, as Borer says, you just plug in the info. Rather, we’re going to talk about how exactly you get the information the calculators need. While just looking things up on the internet might be easy, there are too many variables to just blindly trust without measuring for yourself. Also, keep in mind that any calculations — using calculators or otherwise — can only be as accurate as the data you feed them. So taking proper measurements is key.

Calculating Displacement — Bore Size

If you are interested in doing the long-form calculations by hand, or just curious about how the calculator is coming up with its results, here’s an article by Jeff Smith explaining the math. While this calculator might seem exceedingly simple, as there are only three variables asked for, the devil, as they say, is in the details.

Sure, you can go online and look up the bore and stroke — the two key figures required in the calculation, other than the number of cylinders — but what if you get an unknown engine in? Or, what if you get one of unknown provenance? Sure that LS says 4.8/5.3 on the block, but which is it? Is it at the factory bore? In order to answer those questions, and get an accurate answer from the calculator, you have to be able to properly measure those two variables.

Calculating displacement might seem simple, but I’m not sure how many times we’ve seen someone make a mathematical error when making a quick “easy” calculation. Better to use the calculator and be sure.

First, let’s talk about the bore. Measuring the bore of the engine accurately can be a tricky proposition. You can’t just slap a pair of calipers on the inside of the cylinder and call it good. Like anything, there are levels of precision, which are often guided by one’s financial resources. However, any correct method requires some slightly specialized tools.

The less precise method involves using telescoping bore gauges. Sometimes called telescoping “snap” gauges or bore calipers, these are low tech gauges. With its spring-loaded arms set to slightly larger than the actual bore diameter, you rock the gauge over in the bore, which compresses the arms to the actual diameter of the bore. You then measure the length of those compressed arms.

While a set of telescoping gauges can be had for under $20 at everyone’s favorite discount tool store, there are multiple points in the process where you can induce error into the measuring process. If you don’t need extreme precision or are incredibly fastidious in your metrology practices, this method might work for you.

The other option, which is the standard at most professional shops, is to use a dial bore gauge. Now, where this method can get tricky is that the dial bore gauge doesn’t give you an absolute measurement, but rather a relative one. First, you need to take a rough measurement of the bore. You can do that with the telescoping gauges as mentioned above, or you can even just throw a caliper on the top of the bore since this is just a reference measurement.

Once you have a rough bore size, you transfer that to a standard. Usually, that is a micrometer set to the reference size, but we’ve also seen shops with dedicated bore-sized “masters,” which are precisely sized references. Either way, once you have a standard to reference, you can then zero out your dial bore gauge.

With our "quick and dirty" bore measurement showing approximately 3.783 inches of bore diameter, we set the micrometer to that measurement and zeroed out the dial bore indicator there.

Once zeroed, the dial bore gauge will give you a plus or minus number in relation to your zero point. If you’ve done everything right, your initial zero should be well within .050-inch one way or another, which is coincidentally one full sweep of the dial face. Rocking the dial bore gauge over in the bore, you will notice the reading “peak” — that number is your variance from the standard you zeroed on.

This method also has several points where user error can be introduced, but has far more benefits, like being able to rapidly measure several points within the cylinder to get an idea of its concentricity and cylindricity. Additionally, you only have to measure with a micrometer one time, and once the zero is set on the dial bore gauge, you can measure every cylinder in only a fraction of the time, and get a more precise result. The only real reason not to use this method is because of the initial cost of a dial bore gauge.

While measuring the bores, we found that they all measured +.003 inch on the gauge. So adding that .003 inch to a base of 3.783 inches gives us a bore size of 3.786 inches — or .006 over the factory bore, which means the engine has been rehoned before.

Calculating Displacement — Measuring Stroke

This one can be a little tricky, as not many people have physically measured stroke before. It’s just an assumed and accepted parameter for most people. Again, if you have an unknown engine you’re trying to measure, you’re going to have to physically measure its stroke to know for sure what crank is in it.

This is actually relatively simple to accomplish, assuming you have the proper tools. For this, you’ll need a simple deck bridge with caliper retaining screws (the two socket head screws on one side of the bridge) and a six-inch caliper capable of measuring depth. (You can accomplish this measurement without the deck bridge, but you’ll potentially have to factor in things like piston rock and deck clearance.)

First, you mount the deck bridge just like you would when checking TDC or piston rock, but then instead of a dial indicator, you mount the calipers and zero the depth probe on the piston crown at TDC. From there you spin the engine over 180 degrees and measure the depth of the piston. You don’t need a degree wheel or anything fancy, as your calipers will show you BDC in pretty short order. Read your caliper’s dial or display and there’s your stroke length.

Measuring stroke is actually quite a simple process. By using our ProForm deck bridge to secure the dial calipers, we can zero the depth probe on the piston at TDC and then use the same depth gauge to find BDC and measure the travel distance of the piston.

Calculating Compression Ratio

Calculating static compression ratio is another area where the better your information is, the better your calculation will be. The first two components are bore diameter and stroke length. You know those because you just measured them in the section above. The next critical variable is the volume of the combustion chamber. Sure you could look online and figure out what the head you have SHOULD be, but between manufacturing tolerances and potential machine work performed on an unknown engine, you really need to CC the heads to be sure.

The compression ratio calculator has quite a few sections for us to tackle. Some require direct measurement, and some require further calculations based on our measurements.

Calculating Compression Ratio — Measuring Chamber Volume

Measuring chamber volume requires patience and concentration to do right. First, you need to ensure you have a liquid-tight seal between your valves and the valve seats, otherwise, this won’t work. Next, you need to make sure the appropriate spark plug is threaded into the chamber. Then, you’ll lightly grease the acrylic plate with a hole in it that comes in your head CCing kit, and press it onto the deck of the cylinder head to form a seal.

Once that’s sealed, it’s time to fill your graduated cylinder. These can range from inexpensive plastic beakers to pricier laboratory-grade burettes. Regardless of which flavor of graduated container you use, being able to properly read the fluid level is key. To that end, food coloring is your friend, as adding a few drops to the water in your container can add significant contrast to the markings and allow for an easier, more precise reading.

After ensuring you properly record your starting level, you then fill the combustion chamber, through the hole drilled in the clear plate, making sure you don’t spill any as that will impact your measurement. Once the chamber is completely full, you record the level of the cylinder, and depending on how it’s marked, you either do the math (full volume minus remaining volume) or just note the new volume reading (if the scale is inverted). That’s your chamber volume.

When CC'ing the heads, there are several tricks. First, color the water (or whatever liquid you are using) to aid in visibility. For this test, we used powdered Crystal Light, since there was no food coloring available. An accurate starting point is key (left). As you can see we're starting at zero (the ProForm burette counts the amount you have let out, not the amount remaining in the burette). Once we filled the chamber completely, we checked the reading and found a final chamber volume of 66.4cc, which is 1.4cc higher than the spec sheet says the 799 head should have. We'll recheck after we clean up the deck and valves down the road.

Calculating Compression Ratio — Calculating Gasket Volume

This is a relatively simple one, but will rely on both a measured value and a supplied value. First, you need to know the bore of your gasket. Oftentimes, this is larger than the actual bore of the block. This can be measured with calipers or is sometimes supplied by the manufacturer. The next thing you need to know is the gasket’s compressed thickness. This will need to come from the manufacturer, as you can’t exert enough force with calipers or a micrometer to accurately measure that.

Then, you simply calculate for the volume of a right cylinder (πr²h) by multiplying half of the gasket bore by itself and then multiplying that by 3.1415. You then take that result and multiply it by the compressed thickness of the gasket, and your result will be in cubic inches. Since the calculator wants that measurement in cubic centimeters, not cubic inches, you then multiply your result by 16.387 to get the volume of the compressed gasket in CCs.

Calculating Compression Ratio — Measuring Deck Clearance

A key factor in compression ratio is deck clearance, or where the piston sits in relation to the surface of the block. There are two ways of measuring this. One is to get the piston to top dead center and then use the depth portion of a set of calipers to measure the height difference between the deck of the block and the crown of the piston.

Another slightly more accurate way would be to use a deck bridge and dial indicator. First, zero the indicator on the deck of the block, and then take a measurement from the piston crown. Then rock the piston in each direction, and record the numbers. Add them together and divide by two. This works especially well on pistons with a large dome or dish, and only limited access to the piston crown.

Checking deck clearance can be done with a simple caliper, or more accurately with a deck bridge and dial indicator.

Calculating Compression Ratio — Calculating Piston Dish/Dome

This is one area in which we’ll happily say that most readers should refer to the manufacturer’s specs rather than measuring themselves, due to the complexity of the setup required. The method is similar to CCing a head, but is much more difficult, as you have to create a cylinder of a specific volume, and then measure how much volume there actually is. A true flat top piston would be 0cc. For the Engine Pro calculator, a domed piston would be entered as a negative value, and a dished piston would be entered

With all of our measurements taken, we can enter them into the calculator. Our measured bore, stroke, and combustion chamber volume are entered, while we calculated our gasket volume by using the bore of the gasket and it’s compressed thickness to calculate volume. Our block has zero deck clearance, and the flat top pistons have no valve reliefs, making them a true 0cc crown. As you can see, those larger-than-spec chambers give up some compression over the OEM spec of 9.1:1

So with that, you have now actually measured all the parameters needed to figure out your compression ratio. Now you simply enter the numbers into the Engine Pro calculator and it will generate four numbers for you: swept volume of the cylinder, deck volume, TDC volume, and compression ratio. Also, now that you have all the actual measurements for your specific engine, when you are considering making changes to your combination, you have a solid base to work from.

If you are considering changing your compression ratio, you can play with things like head gasket thickness, or piston dome size, or combustion chamber size to get an accurate idea of what your new compression ratio will be. Take for our example, our 799 heads measured here. Since they measure slightly larger than “spec,” we can accurately address that in the build planning so we don’t end up with less compression than we want. Whether we deck the heads to reduce the volume of the combustion chamber, reduce the thickness of the head gasket, or increase the volume of the piston dome, we can run the numbers before ordering parts so that we are sure everything is right the first time.


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About the author

Greg Acosta

Greg has spent eighteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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