The position between the head-bolt hole and the cylinder is the first place to look for st
The Math (500)
We usually start with a compression-ratio rule of thumb for building a naturally aspirated engine. For a 350-inch engine with modern aluminum heads and a cam at or bigger than 220 degrees of duration, you can run about a 10.0:1 compression ratio. That assumes the quench area is around 0.040 inch, and you will be using 92 octane or better pump gas. The quench area is the space between the flat portion of the piston and the flat portion of the head outside the combustion chamber. As the pistons reach top dead center (TDC), air and fuel are squished toward the combustion chamber increasing turbulence and reducing detonation. Modern head designs utilize a kidney-shaped combustion chamber to accomplish this.
The cam selection will also reduce an engine's proclivity to detonate. As the cam increases in duration, the closing point of the intake valve is moved later. If you take two cams, one with an intake closing point of 54 degrees after bottom dead center (ABDC) and one with a closing point of 60 degrees ABDC, the camshaft that closes the intake sooner (54 degrees in this case) will create more cylinder pressure than the later-closing, longer-duration camshaft and will be more prone to detonation given equal static compression ratios. The ideal cranking compression is around 180-200 psi-any more will ping on 92 octane. Also, given the same duration, increasing the LDA decreases the amount of overlap and also closes the intake later in crank degrees, reducing the engine's willingness to detonate and increasing peak power.
So, with theory in hand, we set out to create a short-block with a cam that will not encourage pinging on 92 octane with a 0.040 quench and 10.5:1 compression. Before ordering anything, we called JMS for the details on the engine. They said the crank needed to be turned 10/20, 0.010 inch on the rods and 0.020 inch on the mains, and checked for some runout with an inside micrometer to measure the bore just below the deck, again in the middle of the bore, and finally at the bottom. Machine shops will bore the engine to within 0.005 of the final bore size, then finish the last 0.005 with a hone. Our engine needed to be bored 0.030 to a final size of 4.155 inches. Knowing that, we mathed out the rod-and-piston combination.
The area between the steam hole and the water jacket is less critical. It didn't have any
Piston manufacturers usually provide a ballpark compression ratio based on cylinder-head combustion chamber sizes. Our cylinder heads are AFR 210s that we bought used with 67cc chambers. A quick search on the Internet netted us two different off-the-shelf piston combinations that estimated compression ratios between 10.0:1 and nearly 12.0:1. Compression ratio is determined by the ratio of the entire volume of the cylinder with the piston at bottom dead center (BDC) divided by the volume of the cylinder at TDC. We determined the actual compression ratio by adding the cylinder volume to the chamber volume, then dividing it by the cylinder volume. The cylinder volume is easy to calculate, it's simply the displacement formula without the multiple of eight (pi / 4 (0.7853) x 4.155 x 4.155 x 3.750 = 50.84). This gave us a cylinder volume in cubic inches that can be converted to centimeters by multiplying each measurement by the conversion factor of 2.54 and rerunning the formula (0.7853 x 10.550 x 10.550 x 9.525). That gave us 832.540 cc total cylinder volume. To get the chamber volume, we needed to simply add the 67cc chamber volume to the gasket and piston deck-height. The gasket was 0.039. Converted to centimeters it was 0.099. Then we converted to volume using the old formula 0.7853 x 10.55 x 10.55 x 0.099 = 8.653. To get the depth of the piston in the cylinder, we needed the block deck-height from the machine shop minus the total height of the piston-and-rod combination. The rod we selected is 5.70 inches, half of the stroke is 1.875, and the piston-compression height is 1.425, so the total height is 9.00 inches. We asked the machine shop for 9.005 deck height after machining, which should put our piston 0.005 in the hole. Converted to cubic centimeters, an additional 1.110 cc's were added to the chamber volume. Finally, the pistons had two 8cc valve reliefs.
The final math is cylinder volume plus total chamber volume divided by total chamber volume (832.540 + 92.763 / 92.763 = 9.97:1). This is virtually what we shot for and likely will not detonate. We shall see.