How much cylinder pressure can be run on pump gas? Commonly, a cranking compression of 180 psi is considered pretty good. We've seen engines built with attention to most of the factors discussed here run 200 psi successfully. Would you believe we are experimenting with pressures as high as 220 psi? It might work, using every trick in the book.
Quench It If You Can
The quench effect on engine efficiency has been well documented and researched since early in the last century. What is the quench effect, you ask? Simply put, it is designing in a close clearance between a substantial portion of the piston area and the bottom of the cylinder head when the piston is at top dead center. A closed-chamber head has a large flat area, the quench surface, over a substantial portion of the bore. It has been found that if the piston rises to within .050 inch or closer to the flat of the head, good things happen in the combustion process. The effects here are multifaceted. First, is the squish effect, wherein as the piston closes the gap in the quench portion of the head as it approaches TDC the combustible mix in this portion of the chamber is rapidly displaced, creating combustion-promoting turbulence, speeding the burn. In the compression process, the gasses in the chamber reach a very high temperature. As the propagating flame front expands, the pressure can get high enough to auto-ignite the end gas at the far side of the chamber. Since with a tight quench clearance, most of these end gasses are squeezed out near TDC, the chances of auto-ignition (detonation) are greatly reduced. The temperature of autoignition is approximately 1,375 F. Clearly, the cylinder head temperature is significantly cooler than the end gas temperature at or near autoignition levels. Due to the temperature differential, the thin layer of detonation-prone gasses at the extremities of the chamber are actually cooled by the proximity to the head, further diminishing the tendency to detonate. It is from this cooling effect that the term "quench" is derived.
An engine with an effective quench will be more detonation resistant, and it is typical for surprisingly substantial improvements in torque to result from the more efficient combustion. Most builders consider .040 inch or so to be an effective target for piston-to-quench-area clearance, a spec easily obtained with a closed-chamber head, a piston at zero deck, and a standard FelPro .039-inch compressed thickness gasket.
An engine's detonation point and power output can be raised with thermal barrier coatings applied to the cylinder heads. Coating the valves and ports reduces the heat transfer to the charge, while the chamber coating helps normalize the surface temperature distribution. This package of heads and pistons was fully prepped by Swain, a specialist in the field.
Mechanical Configuration
We've discussed how the cylinder heads at the top end of the engine can provide a substantial benefit to pump gas power, but what about the basic mechanical configuration of the bottom end? There are numerous theories and opinions, but in most instances, little empirical data. Testing at MIT clearly established a relationship between bore size and octane requirements, and the results indicated larger bores increased combustion time and temperature, increasing the tendency to detonate. However, that testing covered substantial variations in bore size, from 2.5 to 6 inches. We doubt there is significant enough an effect to warrant consideration in the range of bore sizes typical of high-performance street engines.
Another aspect of engine configuration that has gained a following is the use of shorter rods. Shorter rods will result in quicker piston motion in the vicinity of TDC. Proponents of the short rod theory claim there is an advantage to be had by the accelerated volume gain away from TDC as combustion propagates, though we wonder if there are losses due to accelerated pressure decay. We've seen no empirical data to quantify these theories. We do know that shorter rods increase internal friction, and cylinder wall loading. The bottom line is we wouldn't lose any sleep over trying to find an edge in unusual engine configurations.
These Indy Cylinder Head Hemi pistons received a thermal barrier coating on the crown, and a friction-reducing coating on the skirt. Thermal barrier coatings can improve power, since retaining heat in the combustion process means getting more efficiency from the charge in the cylinder.
Coatings Can Help
There are several types of coatings that have gained popularity in recent years, and it's for good reason: they work. From the standpoint of making the most power with pump gas, Thermal Barriers Coatings (TBCs) are right on target. TBCs are designed to reduce the transference of heat. That characteristic can be put to effective use inside an engine in a number of ways. Heat is energy and energy is power, so it follows that keeping heat in the combustion space has the potential for an enhancement in power. Aluminum is a very effective heat conductor, and also happens to define a large portion of an engine's combustion surface area. Barrier coatings applied to piston crowns have become popular in an effort to retain more of the combustion energy in producing power, rather than being lost through absorption through the piston. Similarly, TBCs are commonly used to coat the combustion chambers of aluminum heads, again to curb heat loss.
These are the rods and pistons from a normally aspirated 700-plus hp street/strip small-block Chevy that PHR tech contributor David Vizard built. Not one to leave power potential untapped, everything is coated.
Heat transfer is a two-way street, and sometimes a TBC is just as valuable in keeping heat out as it is in keeping heat in. Valves, having limited means to transfer heat via the seats and guides, tend to be among the hottest component inside an engine. Intake valves are cooled considerably by the air/fuel charge rushing past. While the valves are being cooled, the induction charge is being heated, and we've already discussed the negative effects of that. At the other side of the chamber, the exhaust valves get little relief from heat. Temperature builds in the exhaust valve, making it the hottest component in the combustion space. Exhaust valve heat has been found to be a major contributor to end-gas temperature rise, and the resultant detonation.
Thermal barrier coating can reduce the ingress of combustion heat into the valves, with the promise of cooler surface temperatures. Coating the combustion face of the valves has become popular to reduce the valve's operating temperatures, and our own experience has indicated a genuine reduction in the tendency for the engine to detonate. While valves, pistons, and chambers are the most common applications for TBCs, port surfaces and intake manifolds are among other areas where some builders are employing these coatings.
Valve Timing Variables
We all know that pressure in the cylinder plays a critical role in making power, but we also realize that too much pressure will contribute to the onset of detonation. The camshaft plays a vital role in determining just what operating pressure the engine will see. Obviously, if the camshaft keeps the valves open all the time, the pistons will go up and down and there won't be any compression happening at all. Given that fact, it's clear that the cam has a substantial effect on the cylinder pressure. So how does cam timing relate to detonation tolerance, power, and cylinder pressure? We'll examine some of the key points.From the standpoint of cylinder pressure, the most relevant event in cam timing is the intake valve closing point. The intake valve closes while the piston is on its way up on the compression stroke, and there isn't any compressing going on until the intake valve closes. Long-duration camshafts will open the intake valve earlier, and correspondingly close the intake valve later, which reduces the cylinder pressure at lower rpm. With long-duration camshafts, the compression ratio should be increased to compensate. Other aspects of the camshaft alter the intake valve closing point, and therefore have a direct affect on cylinder pressure. At a given duration level, a wider lobe separation angle will close the intake valve later, bleeding off some of the cylinder pressure. Similarly, when the installed centerline is altered, the cam timing will change. Advancing the camshaft will increase the pressure, while retarding it will close the intake valve later and reduce it.
So what is the way to go with camshaft events if maximum pump gas power is the goal? The answer here is too specific to the infinite variety of engine combinations possible to make a blanket recommendation. There is an effective way to experiment with different combinations, without the need to try a dozen cams. The trick is to use one of the many engine simulation software programs available, to model the effects of various cam profiles. With a program like Dynomation's Engine Analyzer Pro, any changes of cam timing events can be simulated, and the resultant changes to cylinder pressure are estimated with a good degree of accuracy.
Cam timing and compression ratio are integral to cylinder pressure. Computer modeling can predict the effects of a variety of cam events to zero-in on an effective combination.
The Compression Ratio
With all this talk about pressure and detonation, we've purposely left the topic of compression ratio for last. There is no doubt that compression ratios have been trending up, with OEMs now building engines with ratios on the same level as during the high-octane musclecar days of the late `60s. High compression serves up many benefits, helping to achieve high torque and power numbers, increasing fuel efficiency, and improving throttle response, while the reduced clearance volume also aids in cleaning up the idle. Modern OEM engines are pushing ever closer to the 11:1 mark, with Chevrolet's new LS7 Corvette plant actually at that level now.
Beginning decades ago, when available fuel octane first began to rapidly decline, the trend was to lower compression ratio to 9:1 or even less. These days much higher ratios are again becoming commonplace in high-performance builds. The improvements in technology, particularly in the areas discussed, allow higher compression ratio to be tolerated on pump gas. Building an engine with all of these tricks makes it possible.