Compression Ratio Tech - The Power Squeeze

At least 150hp of a Pro Stock engine is due to the use of ultra high compression ratios (CR) of 16:1--or more. 110 octane race fuels make this possible, but don't be fooled into thinking that the use of 93 octane pump fuel in any way diminishes the importance of the CR used. Understanding how the CR affects your street or race engine can easily net as much as a 50hp benefit.

Basics
Increasing the CR produces an increase in output throughout the rpm range. It is also worth an increase in fuel economy. If a longer duration cam is installed, raising the CR at the same time can be worth even greater dividends than these two moves considered separately. When the CR is raised, peak combustion pressures are increased. As a rough rule of thumb, cylinder pressures are about 100 times what the CR is so, from a 10:1 engine, we would expect to see about 1,000 psi of peak combustion pressure.

Cylinder pressures and output will increase as the CR is raised, but what is less obvious is that the increase in compression also increases the engines thermal efficiency. Thermal efficiency is a measure of how effectively the engine converts heat into mechanical power. To appreciate this it is better to consider the engines expansion ratio (ER). This is the opposite side of the coin to the CR and describes what is occurring as the piston moves down on the power stroke rather than what happens as it moves up on the compression stroke.

Take a look at Figure 1 and then let's go through the characteristic difference (computed taking into account typical heat losses) between a high-compression cylinder versus a low-compression one.

The drawings are of a cylinder with a 15:1 CR and the bottom pair a cylinder with 2:1. For a moment, let's imagine that both the 15:1 and the 2:1 cylinders start off at TDC with 1,000 psi. As the piston of each cylinder moves down the bore, the drop in pressure follows a distinctly different line. The 15:1 cylinder drops pressure much faster than its 2:1 counterpart because of its more rapid change in volume. It only has to go down the bore a short way for the original volume to have doubled, whereas the 2:1 cylinder must travel almost half way down the bore to double its original volume. At the bottom of the stroke the 15:1 cylinder has dropped down to about 25 psi above atmospheric whereas the 2:1 cylinder is still at some 260 psi. In simple terms, the high-compression cylinder, when the exhaust valve opens at BDC, is only dumping 2.5 percent of its original pressure whereas the 2:1 cylinder is dumping 26 percent.

From this you can see why a high-compression cylinder produces better power and fuel economy. It is not just because the charge is squeezed harder and the resulting combustion pressure goes up, but also because the higher expansion ratio allows more energy to be extracted from the original high-pressure charge.

Because the high compression cylinder makes its power much earlier on in the power stroke there are other implications we can take advantage of. The most obvious is that we can apply the earlier exhaust valve opening needed for higher rpm output without significantly impacting the engines low speed output. Using as much compression as circumstances will allow makes dual pattern cams work at their best.

CR Power Gains
The theoretical thermal efficiency (E) at any given CR can be predicted by the following formula:

Where E = the Otto cycle (thermal) efficiency, R the compression ratio and k the coefficient of adiabatic expansion for air, which is nominally 1.4. Rather than flogging through a lot of calculations, the quick reference chart (Figure 2) has been made up. To use this chart locate your original CR horizontally. Next, locate the new CR down the first column. Where the two intersect is the gains that can be expected. For instance if the CR is raised from 9:1 to 12:1 we find the two values intersect at the box with 7.7 in it. This is the percentage increase that can theoretically be had by raising the compression from a lower to a higher level.

Now anytime we use the word "theoretically," it usually implies that there is not much chance of achieving as much in practice. Here the good news is that because intake valves do not close at BDC, the gains computed in Figure 2 tend to be a little on the low side. A typical mildly modified 9:1 350 small-block Chevy would make about 360 lbs.-ft. of torque. Raising the compression to 12:1 would theoretically pump that up 387 lbs.-ft. However, unless other problems are encountered by virtue of the compression increase, such a gain would be toward the minimum end of the scale. To understand how more can be had, let's look at the effect the cam has on the situation. At lower rpm we find that the static CR is never realized because the intake valve is assumed to close exactly at BDC prior to the start of the compression stroke. This does not happen in reality.

Dynamic CR
At low rpm, there is little-to-no ramming of the cylinder from intake charge velocity. As the piston starts to move up the bore on the compression stroke prior to the intake closing, some of the induced air is pushed back into the intake manifold. This means the volumetric efficiency (breathing efficiency) and thus the effective displacement of the cylinder is well below 100 percent.

In other words a 10:1, 700cc cylinder may only pull in 600cc of air. This means the dynamic CR, at 8.7:1, has dropped well below the static CR of 10:1. The bigger the cam, the more this effect comes into play. Figure 3 shows what we are up against here. Let's consider what happens to the dynamic CR compared with the static when three cams, all on 108 LCA and 4 degrees advanced, are installed. If this cylinder had a 12:1 static CR then the dynamic CR for a 300 degree cam would drop into the bottom of the 8s, the 275 cam the mid 10s and the 250 profile the mid to low 11s. From this you can see that the situation for the 300-degree race cam is not good. However, there is one important aspect that plays into our hands when using big cams and it is this factor that often results in gains greater than predicted by the basis cycle efficiency figures shown in Figure 2. Bumping up the CR one point from a low ratio has a greater effect then bumping it up from an already high ratio. This means the bigger the cam the more responsive it is to a raise in CR, especially in the lower rpm range.

Compression Pressure
Assuming your engine has good ring and valve seal, a simple way to determine if your engine has enough compression for the cam being used is to check cylinder compression pressures. For engines of my own utilizing a near-zero leakage ring package and intended for use with 93 octane fuel, I tend to set 190 psi as a lower limit with a preferable 200 - 210 psi target. For every octane number less than 93, the compression pressure needs to be about 5 psi less to avoid detonation under normal circumstances.

Running High Ratios and Living
The factor that ultimately limits the amount of CR that can be used is detonation. The key to being able to utilize high compression ratios and have the engine survive is to understand what factors can accelerate its early onset. Here the temperature of the intake charge is probably the number one issue to address. In practice we find that for an engine on the edge of detonation, every 8-10 degrees F reduction in intake temperature is equivalent to adding one more octane number to the fuel.

Most stock engines of the post '70s emission era are on the edge of detonation and run with water temperatures in the 200-210 range. While this may be good for lower emissions, it is not good for power or staving off detonation. For a high-performance street machine running service station fuel, water temperatures of 170-180 deliver a couple percent more power and the ability to use about a quarter of a ratio higher for every 10 degrees reduction in water temperature. Your first move toward utilizing higher CRs then is to use a 170-180 degree thermostat.

The real goal we are trying to achieve with reduced water jacket temperatures is to reduce the temperature of the air entering the engine and ultimately the charge temperature just prior to ignition. The starting point on this quest is to route cold air to the carb/throttle body of the induction system. Cold air packages not only deliver a denser charge to the cylinder but also move detonation further away, opening up the door for more compression.

After we have succeeded in feeding colder air to the engine, we must take steps to keep it cool. One of the greatest sources of intake charge heating is the intake valve. Because of its significantly greater area, this can absorb more heat during the combustion and power stroke than the exhaust valve. The reason it does not get so hot is that it is cooled by the intake charge, which is just what we don't want. The very least you should do is to polish the chamber side of the intake valve to reflect heat but much better yet is to coat the valve with a thermal barrier to cut the amount of heat absorbed in the first instance.

If your thermal barrier budget extends to having the ports of the intake and cylinder head coated, then that's the way to go. Assuming fuel atomization has not been negatively impacted, my own testing shows the cooler intake on a typical hot street 350 small-block Chevy to be worth at least 11hp and that's before raising the CR to the new level allowed by the cooler charge.

Ultimately, an engine's output is limited by the temperature at which the exhaust valve head runs (Figure 4). The hotter it is, the more likely it will promote detonation and thus limit the CR or boost. Coating the exhaust valve helps reduce the exhaust valve temperature to stave off detonation until a higher output.

Equally as important as the exhaust valve is the exhaust port, especially on the common wall with the intake. Here, aluminum heads without water between the intake and exhaust valve conduct a great deal of heat from the exhaust side to the intake.

Pistons are also an area where thermal barrier coatings should be seriously considered. They don't necessarily help to increase the amount of compression that can be used but the coatings do give a substantial amount of protection against piston failure from overheating or detonation. To prove the value of an effective coating on the dyno, I ran a full 80-lb. bottle of nitrous through a street/strip big-block Chevy that ultimately made just shy of 1,600hp in one push. The pistons as well as the rest of the engine were in perfect shape at the subsequent tear down.

Combustion Chamber Dynamics
A cool charge may be the first step toward utilizing a higher CR, but what happens in the combustion chamber can make or break any such efforts. A prime factor here is never to loose sight of the fact that the faster the charge can be burned the higher the compression the cylinder will stand. Chamber cavities between the piston and the cylinder head between about .060-inch - .0120-inch appear most likely to be the site of detonation. Speeding up combustion mixture motion/agitation is vital. This means maximizing the quench action. On a small-block Chevy with a stock block height, a stock compression height piston is typically .025-inch down the bore. With a .040-inch gasket this makes the static quench clearance .065-inch, which is way too wide. By cutting the quench clearance the burn rate and quality improve to the point where the motor gains compression and is less likely to detonate even at the higher ratio involved.

So how closely can the pistons approach the head face? Although it comes under the heading of "don't do this at home" I have run the static piston/head clearance down to as little as .024-inch in a 350 with stock rods and close-fitting hypereutectic pistons. The pistons just kissed the head at about 7,000 rpm. As far as power is concerned, an associate of mine ran some tests in a nominally 450-horse 350 and found that each 10 thousandths of quench reduction was worth approximately 7hp. If you are building from scratch, make maximizing the quench your number one priority toward achieving compression and avoiding detonation.

Piston Crowns
Before buying pistons you need to understand that flat-top pistons and small, compact combustion chambers are the racer's most user-friendly choice--by a big margin. Always get all the compression conveniently possible by minimizing the quench clearance and cylinder head chamber volumes before considering domed pistons.

The domed piston may look like an easy short cut to more compression but it does not always deliver the hoped-for power increase. Although intended CR ratio may be achieved, the combustion process may be severely compromised by the presence of the dome. I have seen a piston dome that was a tad too high and cost well over 100hp. Unless you or your engine builder are prepared to spend time developing a piston crown form that works, stick to domes no more than about .001-inch high.

Ignition And Timing
PHR editor Johnny Hunkins also wanted the 'more advance is better' myth exposed--well here goes. Bear in mind that pressure rise on the compression side of the stroke is negative power. The only reasons the plug is fired before TDC is to compensate for ignition delay (which is up to 10 degrees longer for highly leaded fuels) and the fact that the charge is not, initially at least, burning as fast as we may like it to. The proof of a good combustion chamber is that it needs little advance to deliver maximum power. Ideally, if we could burn all the charge between about 5 degrees BTDC to about 15 after, there would be little if any gains left to be had in this department. That, however, is not the case for the most part.

One of the principle reasons a fast-burn chamber can tolerate more compression is that the advancing flame front has less time to radiate heat to the remaining unburned charge. As the compression ratio is increased, so is the speed of combustion. All other things being equal, a high-compression cylinder requires less advance to deliver optimal results but the higher pressures existing at the time of ignition make it harder to fire the plug. To counter this, an ignition system with more than enough capability should be used. Indeed, if a really aggressive system delivering a sufficiently high current and multiple spark is used, the burn rate can be accelerated, thus requiring less advance and opening the door for a little more compression yet.

Spark plugs can be a source of detonation because either the center or side electrode overheats. One of the advantages of using an overkill ignition system is that it allows the use of plugs with a cooler heat range than would normally be the case. Also watch out for side electrodes that are needlessly long. These can also overheat, especially when nitrous is used. Side electrodes should be cut back so they only just start to overlap the center electrode.

Conclusions
So if you follow all the rules here what is the payback? With current 93-octane fuel, it is possible to run as much as 11.2:1, but the safety margin is small. At this level you can be slave to the weather. If it's hot and humid, the water content suppresses detonation to about the same degree as the increase in detonation from the temperature increase. If it's plain hot, such as a 118-degree day in Tucson, that 11.2:1 and 93 octane won't make it. The answer here is to back off the CR half a point (not good for night time racing) or utilize a knock sensing ignition retard such as the sophisticated J&S unit.

COMPRESSION RATIO DEFINITION

The CR is the ratio of the volume above the piston at TDC (right) compared to the volume at BDC (left). An example would look like this--say the volume above the piston at BDC is 550cc with 500cc being the displacement volume (V) due to piston motion and 50cc the combustion space (C) remaining at TDC. When the contents of the cylinder at BDC are squeezed into the 50cc remaining at TDC, the charge occupies 1/11th of the space so the CR is 11:1. The formula for the CR is (V+C)/C. To find out what total combustion chamber cc's are required for the CR, you want subtract 1 from that ratio and divide the result into the displacement volume of the cylinder.

STATIC CR VERSUS CAM DURATION--HOW MUCH IS LOST?

Calculating the CR at the point of valve closure involves some fairly heavy math or buying a comprehensive engine-modeling program such as Performance Trends Engine Analyzer. To make life easier we can short cut this with some functional approximations. Ignoring the small effect of lobe centerline angles and assuming a typical rod/stroke ratio we can say that for cams up to around 250 degrees advertised duration the dynamic CR will be about 0.6 of a ratio less than the static. For 275 degrees of duration the dynamic CR will be about 1.5 ratios less than static, and for 300 degrees, about 4.1 ratios less. Using the chart below, we can more closely work out what the maximum static CR should be rather than guessing it. An example goes like this: Cam--265 degrees, fuel 93 octane and the thermostat 180. Find the 93-octane point on the left-hand vertical scale. Move horizontally to the 180-degree diagonal line then drop vertically to the base line. This indicates an 8.2:1 dynamic CR as what is needed. Add to this the CR loss due to a 265-degree cam, about 1.1, and it indicates a static CR of 9.3 as needed. This chart is conservative; get everything else right and you can run at least half a ratio higher than indicated here.

15 COMPRESSION BOOSTING MOVES

1. Feed cold air to the induction
2. Keep water as cool as possible (170 F or less)
3. Keep the air cool in the intake ports
4. Put a heat-reflective shine on the outside of the intake manifold
5. Minimize heat transfer through the common exhaust/intake port wall
6. Keep fuel temperatures down (cool can)
7. Run with plugs a little colder than the minimum required
8. Use an ignition system that is gross overkill
9. Utilize as large a spark plug gap as possible
10. Use no more ignition advance than is necessary
11. Maximize quench action
12. Minimize head chamber volume
13. Use flat-top pistons if possible
14. Minimize under-hood exhaust heat--use coated headers
15. Do not ram in but vent out hot air through hood vents