Discussing Compression Ratio And Pump Gas Compatibility

If you call yourself a gearhead, then you most likely appreciate horsepower. One way to make more power with a normally aspirated engine is to start with a strong compression ratio. In this story, we’ll touch on several points relating to compression and how you can make this squeeze work to your advantage.

Compression is one of the few areas in an engine where the “more is better” theory really holds true. The standard recommendation for street engines running on pump gas has always been to shoot for a 9.0:1 to perhaps 9.5:1 compression ratio. This is in order for the engine to safely work with pump gas, which for much of the country, is limited to 91-octane. While 9:1 is a safe number, maximizing compression is a great way to increase power while also improving fuel mileage, throttle response, and drivability. The generally accepted estimate is an improvement of three to four percent per full point of compression. This means that merely changing from a 9:1 to a 10:1 static compression ratio on a 400 hp small block would be worth as much as 16 horsepower.

compression ratio

Graphically, this is what detonation looks like on a pressure trace. The jagged edges are uncontrolled and extreme pressure spikes that tend to rattle the piston in the cylinder and cause engine damage.

Deterring Detonation

The biggest limiting factor when attempting to increase compression ratio is the threat of detonation. This is defined as uncontrolled combustion that occurs after the spark plug has fired. Think of the combustion process not as an explosion, but rather, more like a brush fire burning across a large field of dry grass.

In an engine, the spark plug begins the fire in one corner of the top of the piston that is our grassy prairie. There is one big difference, however. As combustion occurs, the cylinder pressure continues to rise — along with the temperature. At some point, if the octane rating of the fuel is insufficient, the end gases will light off on their own in a spontaneous mini-explosion in a part of the chamber where the end gases have collected. This creates a pressure spike that tends to vibrate the piston in the bore. This is what causes that all-too-common rattle or knocking sound.

Detonation is bad and should not be allowed to continue because it can break parts, damage combustion chambers, and wreck head gaskets. The simplest and easiest remedy is to add octane to the fuel, and we’ll make some suggestions toward the end of the story that are both affordable and work extremely well. But from a mechanical standpoint, there are also several steps the engine builder can make to add compression while also minimizing the chance of detonation.

Static Or Dynamic

When we talk about compression, this should be more accurately defined as static compression ratio. This is literally the ratio of the volume of the cylinder with the piston at the bottom compared to the volume with the piston at the top of its stroke. If we compute the volume of a stroker 6.0-liter LS engine with a 4.030-inch bore and a 4.00-inch stroke, this generates a volume of 51 cubic inches (ci) or 836 cubic centimeters (cc). If we then push the piston to the top of its stroke, in our particular case we’ve now squeezed that same volume almost exactly tenfold, creating a volume of only 5.1 ci or 83.6 cc for a compression ratio of 10.0:1. That’s the static compression ratio.

compression ratio

Here’s an example of what detonation can do to a piston. Those arrows point to broken ring lands caused by extreme detonation. This will instantly kill cylinder seal and you will know very quickly that the engine is hurt.

While this is a good comparator between engines, the reality is that engines actually function using a much lower ratio because the intake valve is still open as the piston moves upward from bottom dead center (BDC). The actual or dynamic compression ratio can only be calculated by knowing where the piston is when the intake valve closes. United Engine and Machine (UEM) offers a dynamic compression ratio calculator that inputs static compression ratio, stroke, and connecting rod length along with the intake closing number at 0.050-inch tappet lift plus 15 degrees. If your cam card offers intake closing at 0.006-inch (advertised duration) you can use this number (perhaps adding one degree to the listed number) and you’ll be very close.

compression ratio

A tight quench is achieved by minimizing the piston-to-head clearance to less than 0.045-inch for 4.00-inch bore wedge head engines. So, for an engine with a 0.003-inch piston-to-deck, adding a 0.041-inch gasket would produce a piston-to-head clearance of 0.044-inch. A tight quench area improves mixture motion and will actually enhance combustion efficiency. Avoid wide piston-to-head clearances of over 0.050-inch.

For that same stroker LS engine, we plugged in 10:1 static compression, a 6.125-inch rod length, a stroke of 4.00 inches, and a 0.050-inch intake closing number of 47 degrees plus 15 degrees. This equals 62 degrees. With these inputs, the UEM calculator offered 8.198 or 8.2:1 dynamic compression. The generally accepted conservative estimate is 8.0 to perhaps 8.5:1 dynamic compression ratio for 91 octane pump gas. This tends to be true for older, traditional engines with less effective combustion chambers. But for later model engines with better chambers, that could be improved to 9.0:1 dynamic.

The two most effective variables in that calculation are the static compression ratio and the intake closing point. To push this further, if we add 8 degrees to the intake valve closing point (70 degrees), this lowers the dynamic compression from 8.2:1 to 7.7:1. To resurrect the dynamic compression would require raising the static compression ratio to 10.67:1. This reveals the dramatic effect valve timing has on dynamic compression.

To further emphasize this concept, the worst combination would be a big cam with a very late intake closing point used in an engine with a low static compression ratio. As an example, imagine a 350 small block with an 8.2:1 static compression ratio, 300 degrees of advertised duration, and an intake closing of 58 degrees at 0.050-inch plus 15 degrees equals a 73-degree ABDC closing point. This combo degrades the dynamic compression to a pathetic 6.1:1. This reveals how dynamic compression ratio can help determine the relative strength or weakness of an engine combination before building the engine.

compression ratio

LS engines are a good example of modern chamber design. This is the chamber in our 6.0-liter engine using 225cc Trick Flow Specialties heads.

But there are multiple other factors besides just the static versus dynamic compression ratio. Chamber design is certainly a critical factor. Late-model engines enjoy much smaller and better-designed chambers that improve the combustion process. The advantage of a better chamber is it reduces the amount of ignition timing required to make the best power. Perhaps 30 years ago, it was not unusual to see a small block with a big cam and domed pistons require 38 to 42 degrees of total ignition timing to optimize power. Compare that to modern engines such as the GM LS with 10.5:1 static compression and a good cam that needs barely 30 degrees of timing to achieve the best power. The reduced timing requirement is an important indicator that the combustion space is far more efficient.

Timing Is Key

Of course, too much ignition timing can cause other problems. For modern engines, a three-dimensional timing map based on both load and RPM will go a long way toward controlling detonation. All engines can benefit from this more finite ignition control. As an example, we have spent some time tuning our friend Eric Rosendahl’s 468ci big-block Chevy after installing a Sniper EFI throttle body.

compression ratio

With just four simple inputs, this free United Engine & Machine calculator can determine an engine’s dynamic compression ratio. As you can see from these inputs, a 10.0:1 static-compression engine computes out to 8.2:1 dynamic compression, which is good yet slightly conservative.

After fine-tuning the air-fuel ratios, we then replaced the HEI distributor and vacuum advance canister with a Sniper distributor and used the software to control timing. We were able to add more timing at cruise yet remove timing at two critical part-throttle load points that had caused detonation when using the vacuum advance. This previously required us to disable the vacuum advance because we couldn’t tune around it. But with finite digital control of the timing curve, we were able to add more timing where the engine wanted it while also keeping the engine out of detonation at other points. That wasn’t possible with a simple distributor.

These same techniques can allow the smart tuner to increase the dynamic compression while minimizing the problems of detonation with 91-octane pump gas. Another area that is worthy of mention is that inlet air temperature has a major impact on detonation sensitivity. We learned this bit of information from now-retired Rockett Racing Brand fuel engineer Tim Wusz. He told us that years ago, the OE’s performed a major test that evaluated the relationship between inlet air temperature and detonation. They found that a 25-degree increase in inlet air temperature, say from 70 to 95 degrees, demanded a one-point increase in octane (from 90 to 91 for example) in order to prevent detonation. Stated another way, if you can reduce the inlet air temperature by 25 degrees, this will reduce the engine’s octane requirement by one full octane number – as with from 91 to 90.

Baby, It’s Cold Outside

This effect can be moderated by other atmospheric conditions. For example, high humidity levels tend to reduce octane sensitivity slightly since the additional water in the air finds its way into the combustion chamber. This can modify the tendency of detonation. Conversely, an increase in atmospheric pressure will increase cylinder pressure. This adds power but also tends to tax the limits of the existing fuel octane. An ideal situation for maximum power would be cool inlet air with mid-point humidity and high atmospheric pressure. This helps power but can also spike the cylinder pressure and perhaps lead to slight detonation.

Compression ratio

Extended nose spark plugs place the spark nearer to the center of the chamber and can help in the process of minimizing detonation.

It’s also important to reinforce the direct connection between the intake closing point and static compression ratio as the really critical factors relating to dynamic cylinder pressure. For example, we’ve investigated several COMP Cams performance hydraulic-roller cams we’ve used over the years and the majority of these cams check in with an intake closing at 0.006-inch tappet lift (advertised duration) at between 62 and 72 degrees ABDC. This may offer some help in determining a useful camshaft, remembering that a smaller number (like 62 degrees) will raise the dynamic compression while a larger number (later closing) will decrease it.

compression ratio

One quick way to improve octane might be to mix in a little E85 to create ethanol mixtures of between 20 and 30 percent (E20 to E30). Mixing ethanol in mixtures up to these levels will bump the R+M / 2 octane rating roughly two full numbers, taking a 91 premium to 93 octane. Of course, this will also require re-tuning the fuel delivery system.

It’s difficult to make any blanket statements in terms of combinations but we can share a couple of examples of dynamic compression ratio. For example, our friend’s 468ci big-block Chevy with ported, factory cast-iron oval port heads, a relatively conservative COMP hydraulic-roller camshaft (XR-282HR, 230/236 degrees at 0.050 duration) with 10.5:1 compression is a fairly responsive rat motor that runs just fine on 91 octane premium. The UEM calculator delivers an 8.2:1 dynamic compression ratio. As mentioned earlier, the engine did rattle in certain areas that forced us to retard the timing slightly. This leads us to believe that it is fairly close at 8.2:1 dynamic to the maximum amount of compression we can run in this engine with 91 octane fuel.

Head Of The Charge

Some may be concerned about the iron heads, as there is a concern with enthusiasts that iron heads are more detonation prone than aluminum heads. We actually performed a dyno test several years ago using a small-block Chevy to test this theory. The results revealed the aluminum heads made more power than the iron version with the same chamber size and shape. A single test is hardly conclusive, but it might be fair to say that older iron heads with poor chamber design would be less efficient and would contribute to detonation sensitivity.

compression ratio

Here’s another example of detonation. This head came off a 434ci small-block that had experienced mild detonation over an extended period of time. This engine was running on 91 octane pump gas with a mild hydraulic-roller cam and 11.0:1 static compression ratio. The small craters were created by detonation.

We also ran a 6.0-liter LS engine on the dyno using a Holley HP EFI control with 10.5:1 compression, a nice pair of Trick Flow Specialties 225cc cathedral port aluminum heads, a 3.62-inch stroke, 6.10-inch connecting rods, and a cam with an intake closing of 62 degrees ABDC. This package developed an impressive 8.54:1 dynamic compression. The engine also made well over 550 hp on the dyno on 91-octane pump gas. We’ve not had a chance to run this engine on the street as it is our drag strip test mule engine, but from all appearances, it will be more than happy with this combination on 91 octane pump gas.

There are certainly opportunities to run static compression ratios up to and including 10.5:1 compression when combined with a modern combustion chamber, cam timing, and proper engine tuning. Certainly, the OE’s are moving in this direction with the new GM LT1 direct injection engines now running an 11.5:1 static compression ratio. These engines also benefit from detonation sensors and millions of dollars of research and development. But the indications are that with the right combination of parts and cam timing that the days of settling for a 9.0:1 static compression ratio on a normally aspirated performance engine are rapidly falling out of favor.

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About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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