The single most important engine parameter for clean and efficient operation of carbureted and port injected gasoline engines is of course the compression ratio. With popular ideas about what the correct compression ratio to run changing rather frequently over the past hundred years modification of existing engines to change the compression ratio has been of great interest. How easy or difficult it is to change the compression ratio of an existing engine depends to a large extent on what type of valve train is used, and even more importantly how that valvetrain is driven.
Ways to Change Compression Ratios
Valvetrain Considerations
Piston Compression Height
Head Work
When and Why to Change Compression Ratios
The primary means of compression ratio modification in existing engines is to move the cylinder head up or down by using different thicknesses of head (or sometimes base) gaskets or to move the cylinder head down by surface milling either the cylinder head or the cylinder (cylinder block). Increasing the compression ratio by surface milling is often the easiest operation since multiple or alternate thickness gaskets then are not required. Ultimately though the actual moving of the cylinder head up or down is rather easy to accomplish, the difficulty comes in what is required to get the valvetrain to function with the new cylinder head location.
The compression ratio of an existing engine can also be modified in a number of other ways, the most common being the use of different compression height pistons. The compression height of a piston is the distance from the center of the piston (wrist) pin to the average height of the top of the piston. This is easy to measure on a flat top piston, but pistons with domes, dishes or other irregular features require more sophisticated measurement and calculation to arrive at the compression height. The compression ratio of an existing engine can also be changed with the use of longer or shorter connecting rods or with the use of a longer or shorter stroke length. The modification of the rod length or stroke length of an engine is normally an expensive means of compression ratio modification since it requires either new rods or a new crankshaft, both being main expensive parts of most engines.
The compression ratio of some engines can also be decreased by grinding material out of the combustion chamber area of the cylinder head. And it also should be noted that an overbore for cylinder repair slightly increases the compression ratio unless the compression height of the replacement overbore pistons is correspondingly smaller to compensate for the increase in bore diameter.
Since moving the cylinder head up or down is the primary means of compression ratio modification in existing engines valvetrain considerations normally are extremely significant. Of course the easiest valve train to adjust to a new cylinder head position is the side valve (flathead) valvetrain. Since flathead motors are so dirty, inefficient and generally difficult to get to work at all this valvetrain type can mostly be ignored. For more on why flatheads should be ignored see Flatheads for Never.
There are a few other valvetrain types which are also extremely easy to adjust to work with a modified cylinder head position. The easiest normally is the belt driven overhead cam engine. Belt drives for overhead camshafts normally use a substantial idler pulley with a rather large range of adjustment that can simply be set to a new position to accommodate a substantial range of cylinder head positions. This is true both for single overhead cam engines and dual overhead cam engines, the range of adjustment available has to do with the specific design and construction of the individual belt drive system in question.
Also normally very easy to adjust to modified cylinder head positions is the pushrod valvetrain. Small changes in cylinder head position can normally be accommodated simply by changing the adjustment of the valve adjusters. Larger changes in cylinder head position require different length push rods or some other modification to the valvetrain. Non-standard length lifters have sometimes been available, or custom rocker arms with different geometry can sometimes be used. The lightest and easiest to manufacture lifters end up having just one pushrod height, but heavier and taller lifters also certainly have been used over the years. Likewise with rocker arm geometry, there would be one ideal shape for a particular engine but alternate rocker arm geometry also can sometimes be made to work acceptably. Basically what it comes down to is that with anything more than small changes in cylinder head location alternate length pushrods would normally be required.
Chain driven overhead cam engines are usually much more difficult to get to work with a modified cylinder head location because the range of adjustment of slide type chain tensioners is necessarily quite small. If a sprocket type chain tensioner is used then a chain driven overhead cam engine can be as easy to adjust to different cylinder head positions as a belt driven overhead cam engine. Since most chain driven overhead cam engines have used slide type chain tensioners it is usually assumed that the location of the cylinder head on these engines cannot be changed more than extremely small amounts. Substantially lower cylinder head positions for increasing compression ratios can be accommodated with modified slide type chain tensioners, but this increased cam chain angularity is going to dramatically increase chain slider wear rates.
It almost goes without saying that a gear driven overhead cam engine cannot be adjusted to accommodate modified cylinder head locations unless replacement gear sets for different cylinder head positions are available. If the modification of compression ratios of a particular mass production gear driven over head cam engine were in large demand then alternate gear sets would tend to be available and rather inexpensive. An entirely custom gear set on the other hand would tend to be extremely expensive.
Substantial changes to the cylinder head location on "vee" type engines and some horizontally opposed engines can cause some problems with intake manifold alignment. The wider the angle of the vee the more severe these problems are likely to be with large changes in cylinder head position. For most rebuilds of the heavy and crude V8 and V6 automotive engines from the 20th century just some port matching work to the intake manifold and the intake ports on the heads was sufficient to compensate for any normal amount of head or block surface milling. New slightly higher or lower locations of intake manifolds and exhaust manifolds have the potential to cause slight alignment problems on any engine, but these small relocations generally are rather easy to deal with.
In large part because of valve train considerations, but also because pistons tend to need to be replaced before the rest of the engine, a popular means of compression ratio modification has been the use of non-standard compression height pistons. Slapping a new set of pistons in an engine is by far the easiest means at the disposal of most hobbyist mechanics for modifying compression ratios. This has been true to such an extent that pistons have often been advertised specifically as delivering a certain compression ratio. To get that advertised compression ratio it was of course necessary that the stock engine actually was the compression ratio listed in specifications and that nothing else on the engine was changed that would tend to change the compression ratio.
Slight changes in compression height to compensate for an engine re-build overbore would be no kind of a problem, but large changes in the compression height of pistons to deliver substantial modified compression ratios tend to be problematic. If the compression height of a piston is substantially increased then the piston gets heavier. Not only is there more material above the piston pin to deliver the higher compression height, but the higher compression height also tends to require longer piston skirts to deliver equal levels of piston stability in the cylinder. Increasing the compression height of pistons unavoidably makes them somewhat heavier, but there is also one specific manufacturing challenge that tends to make taller compression height pistons dramatically heavier. In the normal milling operations performed to finish a cast or forged piston there is no way to remove any material from between the piston pin and the piston crown. Increasing the compression height simply adds more "dead" material between the piston pin and the piston crown that does nothing but make the piston heavier. Removing some of this extraneous material certainly is possible, but it requires an additional milling operation with a stepped milling cutter.
Going the other way there are also reasons that reducing the compression height of a piston tends to be problematic. The main difficulty in reducing the compression height of a piston is that there is then less room for the rings between the piston crown and the piston pin. Going down from two compression rings and the oil scraper ring to just a single compression ring and the oil scraper ring frees up quite a bit of space for reducing the compression height, but an engine with just one compression ring tends to be somewhat less reliable over many thousands of hours of operation. Single compression ring engines certainly can work extremely well, but the failure of that one ring causes immediate and near total loss of cranking compression in that cylinder. With two compression rings the failure of one ring will cause that cylinder to wear more rapidly and eventually fail, but this is a gradual process that gives the operator much more time to plan a rebuild or replacement.
The rings can also be moved up closer to the piston crown to reduce the compression height of a piston, but this tends also to cause reliability problems in high output engines. Rings closer to the top of the piston crown get hotter and wear more rapidly, and in extreme cases burned lubricating oil can clog overheated rings and cause rapid loss of cranking compression in an engine that is run hard.
For a particular engine there is one ideal piston compression height, and substantial changes to the compression height tend to reduce longevity and reliability. The exception to this are the plethora of automotive engines that have been built over the decades with largely excessive piston compression heights. For these engines reductions in compression height yield dramatic improvements in performance, reliability and longevity. The irony here is that it is the older 8:1 and 9:1 automotive engines that tend to have the most excessively large piston compression heights where the new 10.5:1 port injected and 13:1 direct injected automotive engines tend to have lighter pistons with shorter compression heights for higher efficiency and higher performance. The old 8:1 engines can easily be brought down to 6:1 compression ratios for use with huge amounts of boost in forced induction applications, but are not always so easy to bring up to 10:1 or 11:1 for use with standard gasoline in normally aspirated applications. If at some time in the future it is necessary to bring some of the 13:1 direct injected automotive engines down to 11:1 to run on standard gasoline in late compression ignition mode there is not as much room to work with because they already have near ideal piston compression heights. These "backwards" problems with low compression ratio engines being easy to bring down to lower compression ratios and high compression ratio engines only having room to go to higher yet compression ratios accentuate the need for other means of compression ratio modification than simply changing out the pistons.
Slight reductions in compression ratios can often be accomplished simply by removing some material from the combustion chamber. A well designed engine that actually worked would not have much extra material that could be removed from the combustion chamber to lower the compression ratio, but most cylinder heads from the past hundred years have had crude unfinished designs that actually benefit from considerable material removal. It is particularly removing material from around the outsides of the valves to reduce shrouding that yields considerable improvements in flow. For most cylinder heads the amount of material available to be removed represents only rather small changes in compression ratios. Un-shrouding around the outsides of the valves may make dramatic improvements in the ability of the engine to flow well at elevated engine speeds, but tends only to lower the compression ratio by a modest amount.
Some engines can also benefit considerably from some material removal around the spark plug so that more fuel can burn in a short amount of time just after the spark plug fires. Again though this is only true when the original cylinder head design is a crude and unfinished shape that never worked particularly well in the first place.
Material can also sometimes be removed from piston crowns to reduce compression ratios. This is particularly significant since an engine that has a large amount of extra material on the piston crown will also benefit greatly from the reduction in piston weight that cutting the crown down will provide. To maintain thermal stability some extra material on the piston crown is nearly always a good idea, so many pistons really have only quite small amounts of extraneous material to remove from the crowns. Since most stock automotive pistons and aftermarket replacement pistons that have been built in recent decades are plenty strong and plenty thermally stable for forced induction there is just about always some small amount of material that can be removed from piston crowns for normally aspirated applications. The exceptions to this are generally unusually weak stock or stock type cast pistons that break easily despite bulky and very heavy designs. Since these unusually weak pistons generally have extremely thick crowns even they generally can be cut down somewhat without dramatically increasing the likelihood of structural failure.
The higher the original compression ratio of the engine the more significant these small amounts of material removal would tend to be. Reducing a 14:1 engine down to 12.8:1 requires only 83% as large a volume of material be removed as for reducing a 12:1 engine down to 11:1 even though both compression ratio reductions represent the same amount of change in the compression ratio. That is reducing a 14:1 engine down to 12.8:1 with no other changes would require the same amount of additional spark advance as reducing a 12:1 engine down to 11:1 with no other changes.
Again there is somewhat of a "backwards" mismatch here where older low compression ratio engines generally benefit from larger amounts of material removal from the combustion chamber where newer high compression ratio engines generally have better cylinder head designs that are both less in need of material removal to improve flow and also simply have less extra material between the combustion chamber and the cooling jacket.
The big question here is of course is as to what the temperature and pressure requirement of the fuel for late compression ignition is. This big question is further complicated because there are in fact two standard grades of gasoline that require different compression ratios. Obviously the first requirement is a meaningful rating system for the main combustion properties of gasoline. In the absence of a meaningful rating system it is nearly impossible to pick an acceptable compression ratio for a gasoline engine, but this does not remove the requirement of a reasonably good match between the compression ratio of the engine and the fuel that is to be used. For a very long time it was considered that about 10:1 or 11:1 was the correct compression ratio for standard reasonably fast flame front travel speed premium gasoline. How accurate this information was though is not clear. Opinions differed widely as to whether the correct compression ratio to run was 9.5:1 or 11.5:1, and the reality is that both 9.5:1 engines and 11.5:1 engines can run on the same gasoline. On the same reasonably fast flame front travel speed fuel the 11.5:1 engine will run a very small amount of spark advance down around five or fifteen degrees BTDC under a full load at 4,000RPM where the 9.5:1 engine will run a very large amount of spark advance up around 25 or 30 degrees BTDC under a full load at 4,000RPM. With such a large amount of spark advance the 9.5:1 engine will be loud, harsh and inefficient, but at least it will run and make some power. Down at five or ten degrees BTDC the 11.5:1 engine will run smoothly and efficiently with much better torque production and higher power output, but it will also tend to be sensitive to small changes in spark timing. For most mechanically controlled engines the ideal compression ratio would be considered to be somewhere in the middle around 10.5:1 or 11.0:1. Going down to 10:1 or 9.5:1 would only be desirable where the engine operated most of the time under radically reduced loads in full flame front travel mode and ease of tuning with a sloppy advance mechanism was required.
With a meaningful rating system a cheaper and more efficient grade of regular slower flame front travel speed fuel could also be sold, and this cheaper fuel would tend to require a higher compression ratio. To allow two grades of gasoline to be sold the rating system would at a bare minimum have to give separate estimates of two different combustion properties of the fuel. These two basic properties that need to be rated are the flame front travel speed of the fuel and the temperature and pressure capabilities of the fuel. Of course the energy density and maximum temperature of combustion potential of the fuel would also be of interest, but these properties are ancillary in that they do not require different compression ratios or even generally different advance curves for reasonably good engine performance. Of course absolute maximum performance from a mechanically controlled engine or simple electronically controlled engine would only be attained when an engine was designed for a specific fuel where all the combustion properties remained constant, the biggies though are temperature and pressure capabilities and flame front travel speed. To further reinforce this point it should be noted that a computer controlled engine can compensate for any combustion property of the fuel other than the temperature and pressure capabilities. Switching between high and low flame front travel speed fuels will however always result in dramatically different wide engine speed range and wide engine load range performance with anything short of a variable compression ratio engine. Since a computer controlled fully variable valve timing system or a computer controlled variable vane geometry turbocharger can deliver fully variable effective compression ratios even the temperature and pressure capabilities of the fuel would not necessarily need to remain constant. Such a computer controlled variable effective compression ratio engine would be able to run as well as possible under all reduced loads on essentially any flammable liquid, but maximum power output would only be attained on the highest pressure fuels.
Ultimately what the lack of a meaningful rating system does is just use more fuel. Even a fully variable effective compression ratio computer controlled engine will always have to run on the highest pressure fuels to deliver good performance and high power output. If these fully variable effective compression ratio computer controlled engines end up running much of the time on lower pressure fuels then they will have to be considerably oversized to get the job done. Likewise low compression ratio mechanically controlled engines could run on essentially any gasoline that does not foul the spark plugs, but power output will be extremely low in full flame front travel mode on higher pressure fuel requiring much larger, dirtier and less efficient engines to get the job done.
Since 10:1 and 10.5:1 engines have for so long been considered to not have excessively high compression ratios for any standard gasoline there seems to be no reason not to go up to at least this compression ratio range when possible. Any older 7:1, 8:1 or 9:1 engine could benefit from the higher 10:1 or 10.5:1 compression ratio, and it seems extremely unlikely that any lower pressure fuel will ever be commonly available. For more on why it seems unlikely that supper low pressure fuel for 7:1 and 8:1 engines will ever be available see General Trends in Combustion Fuel Properties. For most older mechanically controlled engines the only other change that might be required to go up to a 10:1 or 10.5:1 compression ratio would be stiffer advance springs in the distributor to deliver maximum advance up at 4,500 or 6,000RPM instead of the standard 3,000 or 3,500RPM. In some cases limiting the travel of the centrifical advance mechanism down to 10 or 15 degrees of crankshaft rotation of movement instead of the standard 20 or 25 degrees of crankshaft rotation of movement would be a better modification.
For simple electronically controlled fixed advance curves modification for higher compression ratios can be very difficult. The concession here is that these most basic fixed advance curve ignition modules are rather cheap, simple and reliable and widespread upgrades of mass production engines to 10:1 or 10.5:1 compression ratios would tend to be easy. One-off custom fixed advance curve ignition modules tend to be a bit expensive, but retrofitting thousands of the same model becomes much easier and cheaper.
If a higher pressure and slower flame front travel speed regular gasoline became available, with the required meaningful rating system to go along with it, then some existing 9:1, 10:1, 11:1 and 12:1 engines might be upgraded to use this cheaper and more efficient fuel. The difficulty though of course is that mechanically controlled engines or simple fixed advance curve electronically controlled engines are not well suited to running on slower flame front travel speed fuel. They can be made to do it, but tuning is tricky and performance is at best mediocre. Still though if cheaper slower flame front travel speed fuel for 14:1 or 15:1 engines became available there would likely be significant numbers of hobbyists and performance enthusiasts who would want to try to run the new fuel without computer assistance.
Because slower flame front travel speed fuels for 10:1 and 11:1 engines have sometimes been available over the decades there is in fact a body of knowledge about how to run them in mechanically controlled or simple electronically controlled engines.
Using 10 or 20% standard fast flame front travel speed premium gasoline as a pressure lowering additive in regular gasoline has not worked out very well in the absence of a meaningful rating system, but it is also something that could be made to work better with a meaningful rating system. It is not just the lack of a meaningful rating system that has thwarted attempts to sell a regular gasoline with premium gasoline blended in as a pressure lowering additive. There is also the simple fact that regular gasoline without any pressure lowering additives is going to run much better in the higher compression ratio engines than it is in lower compression ratio engines with the use of pressure lowering additives. The basic idea here is: Why give up the better flow capabilities and overall slightly better thermodynamic efficiency of a 14:1 or 15:1 engine in order to run slow flame front travel speed fuel in a 10:1 or 11:1 engine? Slow flame front travel speed regular gasoline has a significant performance handicap compared to faster flame front travel speed premium gasoline anyway, so why exacerbate the problem with the use of a pressure lowering additive?
Then there is also the problem with a non-homogeneous fuel not running quite as well at very high engine speeds. This is not usually a problem in gasoline engines that run at very slow 3,000 to 7,000RPM speeds, but up at some higher engine speed only a homogeneous fuel is going to deliver the highest possible engine performance and efficiency. Just how fast does a gasoline engine have to spin before non-homogeneous fuel becomes a problem? That is not entirely clear, but likely at least up in the 9,000 to 14,000RPM range. In a worst case scenario at very high engine speeds much of the heat energy in the pressure lowering additive is wasted because it has a lower temperature of combustion potential than the bulk of the fuel. It might be said that using standard premium gasoline as a pressure lowering additive in regular gasoline would only cause non-homogeneity fuel problems at mean piston speeds higher than where an engine could run efficiencly on 100% standard premium gasoline. Non-homogeneity fuel problems would of course show up at lower engine speeds in long stroke gasoline engines where the mean piston speed is getting too high for the temperature of combustion potential of the fuel. Four inch stroke gasoline engines are already running into excessive mean piston speeds down at around 6,000RPM, so non-homogeneous fuel would also show up as somewhat of a problem down at those still rather low engine speeds. A two inch stroke gasoline engine on the other hand just runs better and better up past 9,000RPM, so it is unlikely that non-homogeneous fuel would cause any sort of a problem at that engine speed.
Probably the biggest single problem with using 10 or 20% standard premium gasoline as a pressure lowering additive in regular gasoline would be that hobbyists or racers involved with engine development might notice a 10% reduction in efficiency at 6,000RPM in a four inch stroke high performance engine and conclude incorrectly that this 6,000RPM was the absolute maximum reasonable engine speed for the four inch stroke engine on regular gasoline when in fact 7,000 or perhaps even 8,000RPM operation would be possible on a homogeneous fuel. The real danger here would be that any higher maximum temperature of combustion potential of slower flame front travel speed regular gasoline over faster flame front travel speed premium gasoline would be overlooked.
For use only in mechanically controlled engines any advantage of slightly higher maximum temperature of combustion potential in regular gasoline is mostly insignificant because the slow flame front travel speed itself tends to much more severely limit maximum engine speed. With consistent fuel and a precision mechanical advance mechanism or a simple electronically controlled ignition system there would be some application for high compression ratio engines that could spin up to very high engine speeds on regular gasoline at least under a full load. And with sophisticated computerized engine management there seems no reason at all not to use the regular gasoline to it's highest potential in efficient fast spinning engines.
In the absence of a meaningful rating system for gasoline there is little reason to increase compression ratios beyond about 10.5:1 or 11:1 unless the intended fuel is specialty race gas. For racing it is of course necessary to increase the compression ratio of an engine up to where it will attain high efficiency and high power output on the race gas that is available. If all that is available is fast flame front travel speed race gas for 14:1 and 14.5:1 engines then it would not be possible to run less than about a 12.5:1 compression ratio without severe performance penalties. And even at that the 12.5:1 engine is going to be very loud and harsh with dramatically reduced torque production at all normal engine speeds below about 8,000RPM. Up at very high engine speeds the lower compression ratio will only moderately diminish peak power output, but the fact that the engine will be so much louder and harsher at lower engine speeds will result in shorter engine life and a higher likelihood of dramatic unexpected engine failure.
The point here is that there really is no choice but to run the same high compression ratio as everyone else, even on race gas. Some people over the years have tried using pump gas as a pressure lowering additive in race gas, and this certainly has sometimes worked. There are however sever problems with this practice. One of course is the potential for severe non-homogeneity of the fuel. If regular gasoline that already uses a small portion of standard premium gasoline as the pressure lowering additive is used as a pressure lowering additive in race gas then it is going to require some substantial quantity of that regular gasoline to be able to run an 11:1 compression ratio. If half pump gas is run with half race gas, as has sometimes been common, then using regular gasoline as the pressure lowering additive is going to dramatically reduce the flame front travel speed of the composite fuel. If premium fast flame front travel speed gasoline is instead used half and half with race gas then there is the potential for an extremely severe non-homogeneity problem having to do with the temperature of combustion potential of the fuel. Essentially any higher temperature of combustion potential of the race gas will be wasted because the mean piston speed will not be able to be substantially higher than for running 100% standard premium gasoline. The race gas is then just wasted, and the engine would run the same on 100% standard premium gasoline.
If a smaller portion of standard premium gasoline is used as a pressure lowering additive in race gas then better results are possible, but this requires a steady supply of premium pump gas. If the premium pump gas is replaced with regular mixed with premium or is temporarily replaced with race gas as has been happening lately then the situation is right back to the same reality that the only way to run race gas is up at the same very high compression ratio that everyone else is running.
The big problem is that some people really do want to race with 10:1 or 11:1 engines simply so that they will be cross compatible with standard premium pump gas. This is particularly true for dirt bike racers on a small budget who want to ride the same bike for practice on a regular basis but still need to be able to run the best race gas in competition. For a dirt bike that uses only a gallon or so of gasoline for a substantial ride the cost of race gas tends to seem insignificant, but actually maintaining a steady supply of race gas that is special ordered ground freight only in five gallon steel containers complicates dirt bike riding considerably.
There are other substantial advantages to sticking with a 10:1 or 11:1 engine for performance and racing applications. The big one is that the pressures involved can be much lower. If the correct fuel is used for a fast spinning 10:1 or 11:1 engine then the peak cylinder pressures are considerably lower than for a 14:1 engine. These lower peak cylinder pressures result in lower piston, cylinder and ring wear and can also help considerably with preventing rod and main bearing failure in svelte engines. If higher pressure race gas has to be run in the 10:1 or 11:1 engine with larger amounts of spark advance then not only is the engine going to be extremely loud, harsh and inefficient, but the peak cylinder pressures are also going to be just as high as for the 14:1 engine. The only way to derive any advantage from a lower 10:1 or 11:1 engine is to actually use lower pressure gasoline that works at those lower compression ratios.
To further complicate matters many forms of sanctioned racing place a blanket ban on any "power increasing additives" used in the fuel. It would be very hard to disqualify someone for mixing in 10 or 20% premium pump gas with the same race gas that everyone else is running. If on the other hand a racer with a 10:1 engine were to obtain standard premium gasoline that is currently unavailable at the pumps from some other source then large numbers of moderately ignorant racers and race officials could very easily conclude that the 10% additive was in fact a "power increasing additive". The argument would be very simple. "Your race engine makes much more power when you add 10% of this mysterious substance currently unavailable from any standard suppliers to the same race gas that everyone else is running, therefore that mysterious substance obviously is a banned power increasing additive". When the knowledgeable 10:1 racer argues that the mysterious substance is the exact same stuff that often has been sold as premium pump gas he is right, but he has a very difficult time proving he is right without specific knowledge about what compounds are found in the cheapest standard fast flame front travel speed premium gasoline and what general proportions they are found in. In essence it becomes a situation where reliably contesting a 10:1 or 11:1 engine in sanctioned racing events requires the services of both a chemist to supply the standard pump gas when it is not available and to prove that his synthesized specialty additive is exactly equivalent to standard premium pump gas and also a lawyer to argue the case against seemingly overwhelming contrary observational evidence.
Even if this is successfully accomplished it has the potential to cause large problems for sanctioned racing. If one racer has a system for reliably running a 10:1 or 11:1 engine on either race gas with a pressure lowering additive or 100% standard premium pump gas then there is the potential for a situation to develop where he has the only competitive machine if high pressure and fast flame front travel speed race gas suddenly becomes unavailable. A nightmare for every other race team that wants to run a single compression ratio.
Again the only real solution is a meaningful rating system for gasoline sold as motor vehicle fuel. With a meaningful rating system and consistent premium pump gas available then sanctioning bodies could impose "pump gas only" mandates, or at least institute pump gas racing classes. With a meaningful rating system and pump gas classes engine development and racing would then focus on the much more meaningful debate about the relative merits of fast flame front travel speed premium gasoline versus somewhat denser and perhaps somewhat hotter burning slow flame front travel speed regular gasoline. If it turned out that the slight density and maximum temperature of combustion potential advantages of slower flame front travel speed regular gasoline could not reliably win races against the faster flame front travel speed standard premium gasoline then there would be all the more reason to continue a two gasoline system where slightly more expensive fuel that could not produce quite as high peak power output levels was still considered desirable for some applications.
Those applications would be most forms of off-road racing, many hobbyist and performance enthusiast vehicles and a small but significant portion of "normal" gasoline engine users who would rather pay slightly more for slightly less fuel just to get their engines to sound better and run smoother over a wider range of engine speeds. The other option simply for sounding better and running smoother would of course be fully variable effective compression ratio computer controlled engines. Computer controlled fully variable valve timing or a computer controlled variable vane geometry turbocharger could deliver both high efficiency and a nice sound over a wide range of operating conditions from slow flame front travel speed fuel in a gasoline engine.
At this point really all that can be said is that without a meaningful rating system for the pertinent combustion properties of gasoline there is little reason to run any compression ratio other than about 10:1 to 11:1 for what has long been assumed to be the standard premium fast flame front travel speed gasoline. In the absence of a meaningful rating system the only reason to go up to higher compression ratios than about 11:1 is when the only fast flame front travel speed gasoline available persistently is a higher pressure race gas and reasonably good engine performance is required despite the large and significant rating system problems that have been plaguing the petroleum industry.