10. How do I find TDC on an assembled engine?
The easiest and most accurate way to find true TDC is to use a piston stop (like the ones offered by COMP Cams and Crane Cams) in cylinder number one along with a degree wheel on the crankshaft. Both work in the same fashion. Use your thumb over the spark plug hole to find the compression stroke then thread in the piston stop. Rotate the crankshaft clockwise until the piston hits the stop and record the number indicated on the degree wheel. Next, rotate the crankshaft counterclockwise until the piston hits the stop from the other direction and record the new number on the degree wheel. The midpoint of the two marks is what you want, so add the two points on the degree wheel together then divide the sum by two. Rotate the crankshaft to this number and you have TDC for cylinder number one.
Prior to the introduction of High Energy Ignition (HEI), more powerful coils, and capacitive ignition boxes like MSD, stock plug gaps on most muscle cars were in the .035 range. Higher energy coils, such as an MSD Blaster which yields roughly double the voltage and significantly more amperage per spark, allow opening up the gap, but there is a point of diminishing returns. Finding the optimum plug gap for your application is best determined by experimentation, because there are so many engine variables to consider. MSD, for example, recommends starting at the original gap spec then increasing by .005 to .010 followed by repeated testing and tuning.
So what’s a generally recommended safe gap for naturally aspirated hot street engines? Experienced racer and dyno operator Eric Weinreich at Dyno-Motive in Placentia, California, says, .44 is the gap that seems to work best for a wide range of naturally aspirated high-performance and race engines with strong ignition systems. That’s the gap we dyno test the majority of our engines at, including the one for Max Effort in this issue.
Late-model muscle cars are already equipped with high-energy ignition systems so stick with the manufacturer’s recommendation unless you’re adding forced induction; that’s an entirely different scenario that may require tightening up the gap to maintain spark.
12. What is the max compression ratio for pump gas?
The important thing is to make sure you’re thinking about the correct compression ratio, because it’s the dynamic compression ratio (DCR), not the often cited static compression ratio (SCR) that makes the difference. SCR is the swept volume of the cylinder using the full crank stroke (BDC to TDC). The swept volume for the DCR is determined by the position of the piston when the intake valve is closed. That’s important because compression doesn’t begin until the valve is closed and a great deal of compression can be bled off before then—that’s why the DCR number is always lower than the SCR.
The easiest way to figure out the intake closing point is to use the camshaft’s advertised duration number. Obviously this means that the Lobe Separation Angle (LSA) has an effect, as well as advancing or retarding the cam, since that alters the intake closing point. Now you know why cam degreeing wheels are so useful and one of an engine builder’s best tools to alter an engine’s characteristics; it can adjust the DCR an engine sees. That’s how some guys get away with outrageous-sounding SCR like 12:1 on the street while still running pump fuel, and why you’ll often see manufacturer’s SCR recommendations accompanying cams with lots of intake duration.
Figuring out where the piston is at valve close isn’t quite so easy since it requires a few potentially confusing equations to correctly measure the difference in effective rod length. The good news is that the folks at United Engine and Machine (makers of KB, Silv-O-Lite, and Icon pistons) have a very useful calculator on their website that will calculate both SCR and DCR for you. (Find it at www.KB-Silvolite.com.) As for the max recommended DCR on pump fuel, shoot for around 8.6:1.
13. Antifreeze mix: What kind, what ratio, and when is it time to change?
All antifreeze solutions are glycol based, either ethylene glycol or propylene glycol, with ethylene being the most common. As far as performance, they’re roughly equivalent since it’s the additives that really determine effectiveness. Neither works well in pure form in a cooling system, though.
Regardless of type, antifreeze has three main functions: heat transfer, corrosion resistance, and freeze/boil-over prevention. Water by itself is a great conductor of heat for cooling, but it offers zero protection for corrosion or freezing temperatures. Adding glycol actually proportionally decreases the ability of water to transfer heat since its heat capacity is about one half that of water, which is why running 100 percent is a bad idea. In the right ratio window of 40-70 percent, it’s not enough to be an issue in a properly functioning cooling system and adds the benefit of raising the boiling point into the 225-degree F range for a standard 50-50 ratio.
On the other end of the spectrum, water mixed with glycol dramatically lowers the freeze point. That same 40-70 percent window works here as well, though the maximum freeze protection for ethylene glycol is at 67 percent and resists freezing down to -84 degrees F (pure ethylene glycol freezes at 8 degrees F). Propylene has no freezing point; it only exhibits a phenomenon known as supercooling, but the same 40-60 percent window is typically recommended.
As for corrosion, all antifreeze types have a chemical package that’s designed to mitigate the varying issues presented by the standard mix of electrochemically incompatible metals found in engines. All of those inhibitors require water in the proper ratio to function, however. Speaking of water, deionized water or distilled water works best, but there’s no real issue with just using good tap water.
As far as lifespan, most antifreezes have a recommended interval for change (usually two to three years or 30,000 miles for the standard green stuff), but the only true way to know is to test it. The easiest way we’ve found is using special antifreeze test strips that react to the pH of the coolant and change color.