This chapter provides information that will aid in understanding essential dimensions and volumes that are considered when planning and executing an engine build.
This Tech Tip is From the Full Book, MODERN ENGINE BLUEPRINTING TECHNIQUES: A PRACTICAL GUIDE TO PRECISION ENGINE BUILDING. For a comprehensive guide on this entire subject you can visit this link:
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Bore and Stroke
In general, a long-stroke engine (an engine with a relatively long stroke in relation to bore diameter) revs slower but produces more torque at lower RPM. A short-stroke engine (short stroke in relation to a larger bore diameter) revs higher, and produces peak power at a higher RPM range.
A crankshaft’s stroke dimension is the total stroke of the crankshaft. This is measured from the rod pin’s BDC to TDC positions. When selecting a crankshaft, connecting rod, and piston combination, you use one half of the crankshaft’s published stroke dimension in your decision making. The distance from the centerline of the crank rod pin at TDC, plus rod length, plus piston compression distance is the length that must fit within the block’s available deck height dimension (the distance from the main bore centerline to the block’s cylinder head deck surface).
As an example, if you install a 4.000-inch-stroke crankshaft, the crank and connecting rods have a 6.125- inch length and the pistons have a compression distance of 1.115 inch. The block has been square-decked to a deck height of 9.234 inches and achieves 403.13 ci with 4.005-inch cylinder bores. With that stroke/rod/piston combination, the pistons protrude above the decks by .006 inch, which is more than compensated for by using cylinder head gaskets with a crushed thickness of about .045 inch.
The stroke package must fit within the block, so you must always consider the block’s deck height. LS factory blocks, as an example, are notorious for having unequal deck heights (high/ low side-to-side and/or front-to-rear). So before choosing your stroker combination it’s wise to first have the block decks surfaced in order to establish equal deck distance from the crank centerline. You can probably fudge this and assume that the decks are okay, but if you want absolute precision, correct (or at least carefully measure) the block deck height at all four corners (right-front, right-rear, left-front, and left-rear) before spending money on rods and pistons for a stroker combination.
Connecting Rod Length
When discussing the “length” of a connecting rod, it is not simply the total length. Instead, I am referring to the distance from the centerline of the crankshaft pin bore (the big end) to the centerline of the wrist pin bore (the small end). Precision rod-length specialty tools are designed to precisely measure this. However, if you want to perform a rough check of an existing rod, you can use a long caliper with a dial or a digital caliper with a range greater than the length of the rod.
Using either method, you measure from the bottom of the wrist pin bore (6 o’clock) to the top of the big-end bore (12 o’clock). Record this distance. Then measure the diameter of the wrist pin bore and the diameter of the big-end bore. Record these diameters. And plug the figures into this formula:
Rod Length = A + 1/2B + 1/2C
A = distance from bottom of the wrist pin bore to top of the big-end bore
B = diameter of wrist pin bore
C = diameter of big-end bore
Although this isn’t the most precise way to measure, it gives you a rough idea of your rod’s center-to-center length, for rod identification purposes. Trying to measure rod length with a ruler by guessing at the two bore diameter centers is a waste of time.
Today’s quality aftermarket connecting rod manufacturers (such as Scat, Lunati, Oliver, GRP, Crower, Callies, and more) produce rods at extremely tight tolerances for center-to-center length. It is very uncommon to find a set of rods that are not precisely at the specified length and all the rods in a set are not matched.
Although you can rely on performance rod dimensions in general, when blueprinting, you don’t want to assume anything. Measure each rod for length, small-end diameter, and big-end diameter. Knowing exactly what you have is better than guessing or assuming. If you’re using OEM rods, you must check all dimensions due to the greater potential for tolerance deviations.
Rod length has a direct relationship to engine performance characteristics. Granted, the rod length is part of the TDC dimension (from centerline of the crank rod journal at TDC to piston dome location relative to the block deck), but the rod length can be selected in combination with crank stroke and piston compression height in order to tailor the engine for certain performance characteristics. A shorter rod is slower at the BDC range, but faster at the TDC range. A longer rod is faster at the BDC range but slower at the TDC range.
Here’s an explanation from Stahl Headers: “With a longer rod, the intake stroke draws harder on the cylinder head from 90-degrees after top dead center (ATDC) to BDC. On the compression stroke, the piston travels faster from BDC to 90-degrees before top dead center (BTDC) with a longer rod; but travels slower from 90-degrees BTDC to TDC, which may change the ignition timing requirement. It is possible that a longer rod could have more cylinder pressure at 30-degree ATDC but less crankpin force at 70-degrees ATDC.”
On the power stroke, the piston is farther down the bore for any given rod/crank pin angle. At any crank angle from 20- to 75-degrees ATDC, less force is exerted on the crank pin with a longer rod. However, the piston is higher in the bore for any given crank angle from 90-degrees BTDC to 90-degrees ATDC, so cylinder pressure could be higher. A longer rod spends less time from 90-degrees BTDC to BDC, which allows less time for exhaust to escape on the power stroke and forces out more exhaust from BDC to 90-degrees BTDC. If the exhaust port is not efficient, a longer rod helps produce peak power.
In order to place the piston at or near the block deck on TDC, the rod and crank stroke combinations can include a shorter stroke crank with a longer rod or a longer stroke crank with a shorter rod.
A longer connecting rod provides a longer dwell time at the TDC range. This helps to extend the compression state by keeping the combustion chamber volume small, which is good for mid- to upper-RPM torque. A longer rod reduces the rod angle, which helps to reduce friction. Also, with a longer rod, you can run a shorter piston compression height (that means a lighter piston), which helps to gain RPM.
However, longer rods are less effi- cient at promoting volumetric efficiency at low engine speeds. The piston moves from TDC (downward) at a reduced rate, gaining its maximum speed at a later point of crank rotation. Longer duration camshaft profiles tend to reduce cylinder pressure during the closing period of the intake cycle. Longer intake manifold runners with slightly smaller port volumes may be needed. Longer rods also pose more of a clearance issue (camshaft, bottom of cylinders, and pan rails).
Shorter rods provide higher intake and exhaust speeds at lower engine RPM, which improves low-end torque (and promotes higher vacuum). Shorter rods increase piston speed as it travels from TDC on the power stroke, which increases chamber volume. This delays the point of maximum cylinder pressure, which is a good match for forced induction (supercharger, turbo, and nitrous injection). Shorter rods also allow more radical camshaft timing. However, since a shorter rod increases piston travel from TDC, at high RPM the piston can run away from the flame front faster, which can decrease total cylinder pressure.
In a nutshell, run a shorter rod for the street or whenever low-end torque is the priority and run a longer rod where you want peak torque to occur higher in the engine RPM band.
Keep in mind that some rod, crank stroke, and piston CD combinations don’t work or are impractical due to clearance constraints or unavailable piston compression heights.
Rod ratio refers to the relationship of the rod length to the crankshaft stroke. Theoretically, a higher rod ratio produces more torque at peak RPM, and a lower rod ratio produces more torque at lower RPM.
Depending on the type of engine being built, there is a target range for rod ratio. So, a higher rod ratio for racing and a lower rod ratio for street performance seem to make sense.
Here’s the formula for calculating rod ratio.
Rod Ratio = rod length ÷ crank stroke
For example, you have a rod length of 5.700 inches with a 3.000-inch stroke. Using the formula:
Rod Ratio = 5.700 ÷ 3.000
Piston Compression Height
Also referred to as piston compression distance (CD), this is the distance from the centerline of the piston’s wristpin bore to the piston deck. Performance piston manufacturers offer pistons in a variety of piston compression heights to match your crankshaft stroke, rod length, and block deck height.
Changing crankshaft stroke requires changing other parts of the rotating assembly. This includes altering the connecting rod length and the piston height.
Your point of reference is the block’s deck height. Depending on the nature of the specific build, you may want the piston at zero deck (piston deck flush with the block deck at TDC), or you may want the piston to be slightly below or above deck. Once you determine where you want the piston to be placed relative to the block deck, you can “work backward” to choose the best stroke, rod length, and piston height combination.
Before you begin, the block decks must be finish-machined. This means that you cannot assume that the block deck is at a specified height. The block, whether OEM or aftermarket, may have a block deck height that is taller than the specification. For example, even though an OEM block is specified at a block deck height of 9.240 inches, it may actually be 9.320 inches, or 9.248 inches, etc.
Most performance aftermarket blocks intentionally have extra material (thickness) at the decks, which allows you to cut the decks to the preferred dimension. Also, a new OEM or used block may have variances in deck height (the decks may be twisted, taller on one end, or differ from bank to bank, etc.). You must know exactly what the finished deck height is in order to obtain a useful deck height reference.
For illustration, let’s assume that you want a zero deck, and use a big-block Pontiac as an example. The stock stroke is 4.210 inches, stock rod length is 6.625 inches, stock piston CD is 1.480 inches, and the stock bore diameter is 4.151 inches. Use this formula for finding block deck height:
Block Deck Height =
1/2 stroke + rod length + piston CD
(4.210 ÷ 2) + 6.625 + 1.480
2.105 + 6.625 + 1.480
This block deck could be refinished in order to “square” the decks. That means the decks are flat, parallel to the main bore centerline, and the same distance from the main centerline to the decks from front to rear. This is done by reducing the deck height to 10.205 inches, which makes the pistons theoretically stick up out of the deck by .005 inch.
You also consider head gasket crushed thickness, cylinder head chamber design, valve diameters, and valve lift in order to determine valve clearance.
Compression Ratio (part 1)
Engine compression is a major factor in building horsepower and tailoring an engine for a specific range of fuel octane. This section explains which factors affect compression, and how compression ratio (CR) is determined.
Compression ratio is the volume of the cylinder with the piston at BDC compared to the piston at TDC. Or, thinking of it another way, it’s the relationship between the combined capacities of a cylinder and combustion chamber with the piston at BDC and the piston at TDC. A two-part formula is used to determine compression ratio using five related measurements: combustion chamber volume (C), piston dome volume (P), head gasket volume (G), deck height volume (D), and cylinder swept volume (V).
The first part of the formula is the BDC Factor, which is comprised of C, P, G, D, and V. The second part is the TDC Factor, comprised of C, P, and D. To get the compression ratio, divide the BDC Factor by the TDC Factor.
CR = BDC Factor ÷ TDC Factor
(C – P + G + D + V) ÷ (C – P + D)
As you can see, the only differences between the first and second half of the formula are the cylinder swept volume and the head gasket volume. This is the actual displacement of the cylinder, which is controlled by the bore diameter and the crank stroke.
Before we get to an example of determining an actual compression ratio, let’s examine the individual factors.
Combustion Chamber Volume
We don’t use a formula to calculate chamber volume; it is measured using a burette.
The first step is to install spark plugs and valves. Place a smear of lithium grease on the seats to aid in sealing the valves. With the head deck facing up, position a flat piece of plexiglas (about 6 inches square) on the deck, centered over the chamber. Drill a single 1/4-inch chamfered hole through the plexiglas near the edge of the chamber. Angle the head slightly so that the hole in the plexiglas plate allows air to escape as fluid is added.
Place the hole over the deepest area of the chamber side. Apply a light smear of lithium grease around the perimeter of the chamber and install the plexiglas plate by giving it a small twist motion to improve the seal. If grease enters the chamber, it displaces liquid and creates an incorrect measurement.
If the valve lips protrude beyond the deck, they hit the plexiglas plate and prevent a flat seal. If that’s the case, grind small reliefs in the bottom of the plate and seal them with grease.
Prepare a glass burette on a stand. Fill the burette with colored liquid (such as solvent or rubbing alcohol tinted with food coloring). Open the petcock and allow some fluid to drain, until the fluid sits exactly at the zero mark. The fluid creates a slight cup shape at the top because it’s trapped in this small-diameter glass tube. Use the bottom of the cup as your index at all times, instead of using the highest point of the fluid where it meets the sides of the glass. Make sure there are no trapped air bubbles in the fluid.
Place the burette over the chamber you’re measuring, and insert the burette’s tip through the hole in the plexiglas plate. Open the valve and begin to slowly fill the chamber. Check to make sure there are no leaks (at the spark plug, plexiglas plate, valves). Before the chamber is full, close the petcock so the fluid just drips into the hole and then shut the petcock completely when the fluid reaches the bottom of the hole in the plexiglas plate. Note the level of the fluid in the burette and write this down. This shows how many cubic inches (or cubic centimeters, depending on how your burette is graded) of fluid were required to fill the chamber.
Repeat the procedure for the rest of the head’s chambers.
If all chambers match (unlikely, unless they have been equalized via grinding or milling), this is your chamber volume figure. If they don’t match, you can take an average of the measured volumes (or you can slightly relieve the smaller chambers to match the largest chamber). Unless you’re building an allout killer race engine, taking an average reading is likely fine.
Piston Dome Volume
Even though some piston manufacturers provide dome volume numbers for their various piston part numbers, it’s still best to measure this yourself. In some cases the manufacturer may not have accounted for valve relief pockets. By the same token, if you have modified the piston domes by smoothing dome edges or changing the profile in any way, you must measure the dome volume yourself.
The procedure for finding piston dome volume begins with connecting rod, piston, and rings installed. Bring the piston to BDC, and coat the cylinder wall with grease. Raise the piston to exactly 1 inch BTDC. Using a sealed plexiglas plate with a small fill-hole on the deck, fill the cylinder area with fluid from a burette until the fluid touches the bottom of the fill-hole (tilt the engine slightly so air can escape from the fill-hole in the plexiglas plate). Record the volume of liquid required, which is Actual P.
After the cylinder area is filled with liquid, check the underside of the bore to make sure that the liquid is not leaking past the rings. If you find a leak, no matter how slight, your measurement is wrong, so you have to start over by resealing the cylinder with grease.
Next, calculate how much volume exists in theory (Theoretical P), for that size bore at 1 inch below deck. Here is the formula (in which Piston Dome Volume = P):
For instance, if the bore is 3.50 inches in diameter, the radius is 1.75 inches.
This is the volume if the piston were a true flat-top with no dome or reliefs. Compare this theoretical number to the actual volume you measured using the burette. If you had actually used, say, 151 ci to fill the space, the dome volume is the difference between the actual measurement and the theoretical volume, which in this case is 6.53 ci.
157.53 – 151 = 6.53
So the piston dome volume is 6.53 ci.
If the piston dome is flat, with no dish, but with valve notches, you can simply measure the volume of the notches instead of measuring the dome volume in the cylinder. The easiest way to do this is to clean the notch and fill it with a small piece of modeling clay.
Slide a sharp, flat ruler across the dome to cut the clay flush with the dome and carefully remove the clay without distorting it (use an Exacto-type blade or other small, fine instrument, and avoid gouging or mashing the clay). Drop the clay into a burette and notice how much volume the clay displaces.
If the clay caused the fluid to rise 2.5 ci, then the volume of that one notch is 2.5 ci. If the piston has four notches of the same size, the total notch volume is 2.5 x 4, or 10 ci. If the notches are of different sizes, take a measurement of each notch, one at a time, and add them together to find total volume used by the notches.
Head Gasket Volume
You don’t often need a procedure to determine compressed head gasket volume on your own. Many of today’s gasket manufacturers provide the volume on the spec sheet that is packaged with the head gaskets. But just in case, here is the formula (in which Head Gasket Volume = G):
For example, let’s say the gasket hole diameter measures 4.040 inches, which makes the gasket hole radius 2.02 inches, and the compressed gasket thickness is .015 inch. Therefore:
When determining the gasket hole diameter, be sure to measure the actual gasket hole. Do not use the cylinder bore diameter; the gasket hole is likely to be larger than the bore. Also, because many gasket holes are irregular in shape (not a perfect circle), you have to estimate, as best you can, the hole diameter. If the gasket manufacturer has provided the gasket volume, simply use that number instead of measuring it manually.
Deck Height Volume
If the deck height is zero, deck height volume is zero. If the deck height is positive (with the piston quench surface above the block at TDC), the deck height volume is is subtracted from the gasket volume when used in the formula to obtain final compression ratio.
Here is the formula (in which Deck Height Volume = D):
For example, the bore diameter is 3.50 inches, the radius is 1.75 inches, and the deck height is negative (piston quench surface is below the deck with the piston at TDC), at .014 inch. Using the formula:
Cylinder Swept Volume
Here is the formula for finding cylinder swept volume (in which Cylinder Swept Volume = V):
For example, the bore diameter is 3.5 inches, making its radius half that, or 1.75 inches, and the stroke is 3.30 inches. Using the formula:
Compression Ratio (part 2)
Now that you know how to compute the volume for all of the individual components you’ll be able to determine the compression ratio of your engine. Here’s an example where: C = 56.00 ci, P = 6.53 ci, G = 3.15 ci, D = 2.20 ci, and V = 519.85 ci:
CR = (56.00 – 6.53 + 3.15 + 2.20 + 519.85) ÷ (56.00 – 6.53 + 2.20)
574.57 ÷ 51.67
The actual compression ratio is 11.12:1
Written by Mike Mavrigian and Posted with Permission of CarTechBooks