Long a highly contentious subject, the question of maximum compression ratios for carbureted or port injected engines is currently a hot topic. With extremely high compression ratio production engines being widely distributed there is new information fueling the fires of debate.
Port Injected Automobiles
Street Legal Motorcycles
Forced Induction on Port Injection
Gasoline Direct Injection
How Close is Close Enough?
For a period of time in the late 1950's and into the 1960's all of the V8 powered Buick, Cadillac, Lincoln, Mercury and Chrysler cars had 10:1 or 10.5:1 compression ratios while the six cylinder automotive engines and V8 light truck engines were down at about 8:1. Around the same time certain V8 engines, mostly big blocks, from Ford, Chevy and Dodge were offered with 11:1 or even as high as 11.25:1 compression ratios. This was a large deviation from the general trend of all automotive engines from the 1950's through the 1980's using 7.5:1 to 8.5:1 compression ratios.
Though the late 1980's and 1990's the compression ratios of automotive engines slowly climbed back up to about 9.5:1, and now just recently the standard compression ratio for port injected automotive engines has become 10.5:1. There are a few 2015 port injected models with as low as 9.5:1 compression ratios, but for the most part all port injected automotive engines are now 10.5 or 10.6:1. There are also a few very slightly higher compression ratio port injected automotive engines, such as the 11:1 Ford Fiesta, but for the most part 10.5:1 has become the standard compression ratio. Compared to other production engines though this now is looking like an extremely low compression ratio.
The entire 250cc four stroke class of dirt bikes is now at 13.5:1 using port injection, but these are closed course only bikes that are for the most part not even permitted on public lands riding areas. The exception to the 13.5:1 rule for the 250cc dirt bikes is the KTM 250SX-F which has been a 13.9:1 engine for several years and is rumored to be going up to a 14.4:1 compression ratio for the 2016 model year. As far as street legal motorcycles go port injection is universal, but compression ratios range from 9.5:1 for certain big heavy air cooled cruisers up to 13.6:1 for the 2015 Aprilia RSV4 RR superbike. The BMW S1000RR, the Kawasaki Ninja ZX-10R, the Suzuki GSX-R1000 and the Yamaha YZF-1R all have 12.9:1 and 13.0:1 compression ratios while at 12.3:1 the Honda CBR1000RR has the lowest compression ratio of the "big liter bikes".
It might be argued that these 200hp monsters would never be opened up on the street, and would only be run hard at the track using the same specialty race gas that the 13.5:1 250 class dirt bikes run. This argument does not however hold water, maximum compression ratios tend to be extremely universally applicable to port injected and carbureted engines. Just about any reasonably high performance normally aspirated engine will attain very close to 100% volumetric efficiency at some engine speed close to the point of maximum torque generation. The much milder 160hp KTM 1290 Supper Adventure also uses a 13.0:1 compression ratio and belts out 105 foot pounds of torque all the way down at 6750RPM.
As far as these big high performance motorcycles never being opened up on the street, that is just ridiculous. As overly excessive as 200hp or even 160hp is on a motorcycle there is always a big hill on a highway somewhere where accelerating wide open at rather high speed easily gobbles up all that power. Most port injected engines are also tuned so that somewhat reduced load operation is achieved with a leaner fuel mixture and the throttle opened far enough that maximum volumetric efficiency is attained. What this means is that running under any kind of a substantial load around the engine speed where peak torque is generated is going to fill the cylinders with a maximum amount of air, creating the highest compression pressure.
Smaller displacement street bikes typically have about 11:1 or 11.5:1 compression ratios, but the port injected KTM 350cc dual sport bikes are up at a 12.3:1 compression ratio. The 600cc baby supper bikes (Suzuki's GSX-R600, Kawasaki's Ninja ZX-6R and Yamaha's YZF-R6) share the 13:1 compression ratio with their bigger siblings.
Where the pump gas is really getting pushed hard though is with the turbocharged port injected automotive engines. Back in the 1980's turbocharged automotive engines generally had 7.0:1 or 8.0:1 compression ratios and ran at rather modest 5 to 7psi maximum boost levels for effective compression ratios of about 10.5 to 11.5:1. In recent years though the turbocharged port injected automotive engines have been using higher compression ratios and higher levels of boost for huge torque generation. The 10:1 port injected and turbocharged 2015 Ford Fiesta delivers 112 foot pounds of torque at 5000RPM from it's 1.0 liters of displacement. That level of torque from a 10:1 port injected engine represents an effective compression ratio of more than 13.5:1.
Other examples of high output port injected turbocharged automotive engines exist as well. In 2010 and 2011 the Cadillac SRX used a 2.8 liter port injected and turbocharged engine with a 9.5:1 compression ratio which delivered 295 foot pounds of torque from just 2,000RPM. This level of torque generation on a 9.5:1 port injected engine represents an effective compression ratio of more than 12:1. Considering the extremely low 2000RPM engine speed down to which this rather big torque is generated the effective compression ratio is likely well above 13:1.
For a really unbelievable sort of boost level on a port injected engine the 2013 Dodge Dart is noteworthy. The optional Fiat derived port injected and turbocharged 1.4 liter "MultiAir" engine has a 9.8:1 compression ratio and is rated at an unbelievable 160hp at 5,500RPM and 184 foot pounds of torque. That level of torque production on a 9.8:1 engine is up at an effective compression ratio of around 16:1, not the sort of thing that gasoline is expected to be able to do in a port injected engine.
Just what level of boost these engines are running and just what effective compression ratio it works out to is not entirely clear, these are just estimates based on what a high compression ratio normally aspirated engine can do. These numbers are loosely based on the 105 foot pounds of torque from the 1301cc KTM 1290 as well as the 2015 Yamaha YZF-R1 which delivers 82.9 foot pounds of torque from it's 998cc of displacement, but not until a blistering fast 11,500RPM engine speed. The 1000cc BMW S1000RR delivering 83 foot pounds of torque at 10,500RPM as well as the 1285cc Ducati Panigale 1299 S delivering 106.7 foot pounds of torque at 8,750RPM give a similar impression of what a 13:1 port injected engine can do.
If the traditional ideas of a maximum of 1.2 foot pounds of torque per cubic inch of displacement at about 4,500 to 6,000RPM for carbureted engines are used then the performance of these turbocharged port injected engines looks even more off the charts impossible. With this lower estimate of maximum torque generation for a normally aspirated engine the 10:1 compression ratio and one liter displacement Ford Fiesta engine would appear to need so much boost that the effective compression ratio would be up at 15:1 to deliver it's rated 112 foot pounds of torque.
If on the other hand the baseline normally aspirated port injected engine is considered to be the 2015 Aprilia RSV4 RR then the numbers come out somewhat differently. The 1000cc Aprilia is rated at 84.8 foot pounds of torque at 10,500RPM with it's 13.6:1 compression ratio. This somewhat unbelievable level of torque generation would indicate that the 10:1 one liter Ford Fiesta engine would need only enough boost to be running with an effective compression ratio of 13.2:1 to deliver it's rated 112 foot pounds of torque at 5,000RPM. The 2013 Dodge Dart on the other hand still appears to need so much boost that it runs with an effective compression ratio higher than 15:1 to deliver it's rated 184 foot pounds of torque.
There does sort of appear to be a pattern here, and that is the Italian engines running what appear to be higher compression ratios and unbelievable levels of torque production compared to other high performance port injected engines. The 13.6:1 Aprilia supper bike has a bit higher compression ratio than any other production street vehicle, and the Fiat 1.4 "MultiAir" engine appears to be rated to do the impossible no matter how it is looked at.
Unbelievable levels of torque production from normally aspirated engines have been delivered on pump gas before. By 2007 383 cubic inch stroker small block Chevys were routinly making well over 500 foot pounds of torque on pump gas with compression ratios as low as 10.5:1. The really big 520 and even 530 foot pounds torque maximums were however only atained over a narrow range of engine speeds right around 5,000PRM where tuned length intake runners, optimized port geometry and scavenging exhaust headers all worked together to deliver significantly higher than 100% volumetric efficiency.
Most of the recent turbocharged automotive engines are equipped with direct injection systems, and this changes the compression ratio debate significantly. If these direct injection gasoline engines operate in time of combustion injection mode then they are just diesel engines running on gasoline, and they could potentially run three atmospheres of boost and an 18:1 compression ratio like high output turbocharged diesel engines do.
If the gasoline is injected directly into the combustion chamber during the middle part of the compression stroke and is later ignited by the spark plug then effective compression ratio limits do still apply. There are however some reasons why a direct injected gasoline engine could get away with slightly higher levels of boost on the same 10:1 compression ratio than a port injected engine can. The most often quoted difference is that the fuel being injected in the middle to later part of the compression stroke is more effective at cooling the combustion chamber, and this allows more boost to be run without destructive full compression ignition taking place.
This small additional cooling effect of direct injection over port injection does not however account for the 12psi or more of boost that GDI engines are able to run with 10:1 compression ratios. The level of torque generation claimed on many of the 10:1 compression ratio direct injected and turbocharged automotive engines represents effective compression ratios higher than 16:1, higher than gasoline or really just about any possible combustion fuel could handle. Universal use of larger capacity intercoolers to cool the intake air on new turbocharged automotive engines is also often cited as a way that higher levels of boost are able to be run on 10:1 engines, again though this ends up accounting for just a small amount of additional boost that can safely be run on the same compression ratio. The rather small automotive intercoolers certainly are enormously effective at prolonging engine life when small bore engines are run continuously at high output for sustained 120mph autobahn cruising. Those small intercoolers are however not much good for preventing the intake air temperature from spiking up when a high boost engine is run at maximum output for accelerating from a low speed or up a steep hill. Once boost has come up to maximum it really only takes just a few seconds for the combustion chamber to heat up to it's maximum operating temperature. On most normally aspirated engines maximum combustion chamber temperature is attained within two or three seconds of full power acceleration.
Turbochargers are also typically much more efficient for medium high power output at the medium engine speeds encountered for sustained high speed cruising. Maximum power output at maximum engine speed typically overloads the small housing turbochargers, causing inefficient operation and excessive heating of the intake air. Again this means that the small automotive intercoolers can do a good job at mitigating the moderate heating of the intake air during sustained high speed cruising, but are ill suited to preventing intake air temperature spikes during brief heavy acceleration at lower vehicle speeds. Then there is of course the simple fact that higher levels of boost will tend to cause much higher levels of intake air heating.
Either the supposed 13:1 compression ratio limit for pump gas is totally bogus or these GDI engines are in fact running in time of combustion injection mode, and are therefore just low compression ratio diesel engines running on gasoline with spark plugs to assist in starting and idling.
Most normally aspirated direct injected gasoline engines are now at about 11.5:1, although a few such as the 2015 Mazda CX-5 have as high as 13:1 compression ratios. Around 2012 and 2013 some GDI models had as high as 13.5:1 compression ratios. The extra cooling of injecting the gasoline directly into the combustion chamber in the middle to late part of the compression stroke certainly can allow slightly higher maximum compression ratios than is possible with a port injected or carbureted engine running the same fuel, and there are a few other factors that would tend to allow sophisticated computer controlled engines to run slightly higher compression ratios as well.
A fully variable valve timing system can deliver a constant, say 80 or 85%, volumetric efficiency over a wide range of engine speeds. If the weight of the pistons and rods is correspondingly lighter to go along with this constant reduced volumetric efficiency then the compression ratio can be increase correspondingly. An engine that never attains more than 85% volumetric efficiency could run a 16% higher compression ratio than an engine that attains 100% volumetric efficiency at some engine speed. If 14:1 is the absolute maximum compression ratio for regular gasoline then an engine with a fully variable valve timing system that can deliver a constant 85% volumetric efficiency could potentially run as high as a 16.25:1 compression ratio. Similarly if the absolute maximum compression ratio for premium fast flame front travel speed pump gas is 11.5:1 then an engine with a fully variable valve timing system that never attains more than an 85% volumetric efficiency could potentially run as high as a 13.3:1 compression ratio on premium fast flame front travel speed pump gas. Aside from the substantial mechanical and control complexity of a fully variable valve timing system the only disadvantage of a constant 85% volumetric efficiency engine would be that it's displacement would have to be correspondingly larger (18% larger) to get the same maximum torque generation. This disadvantage of lower peak torque generation would however not necessarily mean that the engine would actually have to be sized 18% larger, as the constant 85% volumetric efficiency over a wide range of engine speeds would make for an engine with a broad power band and high peak power production up at maximum engine speed.
This same concept of an engine that maintains a constant and slightly reduced voumetic efficiency over a wide range of engine speeds has been around for a very long time. Back in the late 1950's and early 1960's some of the special 11:1 and 11.25:1 engines from General Motors were only available with special camshaft profiles to go allong with the unbelievably high comrpression ratios. The idea was that increaseing the durration of valve lift would reduce volumetric efficiency down around 4,000 to 5,000RPM, then even with the longer durration camshaft voumetric efficiency would remain slightly low from 5,000RPM up to the maximum engine speed simply because the two vavle per cylinder engines did not flow well at high engine speed. The problem with this concept was that any reasonable camshaft for an engine still ended up delivering nearly 100% volumetric efficiency at some engine speed, and anything that was done to try to prevent 100% volumetric efficiency from being attained just caused the engine to run over a more narrow range of engine speeds or worse lead to a reduction in peak power output. Computer controlled fully variable valve timing systems can however actually deliver this constant reduced volumetric efficiency.
It should also be kept in mind that the actual volumetric efficiency profile of a real engine programmed to be able to use the highest possible compression ratio would not be perfectly constant. Because heat transfer between the intake air and the cooling jacket decreases at higher engine speeds there would tend to be slight variations in the volumetric efficiency at different engine speeds. Varying the volumetric efficiency of an engine would also be a way to compensate for changes in altitude and the less significant changes in ambient temperature so that maximum performance could be delivered under all operating conditions.
Arguably the most significant reason that sophisticated computer controlled engines can run higher compression ratios than traditional mechanically controlled engines relates to throttle response and drivability. On mechanically controlled engines with little or no load dependant timing adjustment it was necessary to keep the compression ratio low enough that the engine could run in full flame front travel mode in order to support a range of light loads over the lower engine speed ranges. The reason that this was important was that if close to the maximum compression ratio was attempted the engine became harsh and unusable down at 2,000RPM and was only able to run at near maximum output up in the operating speed range of 4,000 to 8,000RPM. Backing off just a little bit from the maximum compression ratio would allow an engine to run fairly well under light to medium loads down at 2,000 to 4,000RPM but would still mostly only run under a full load within the normal operating speed range of 4,000 to 8,000RPM. This was the reason that automotive engines worked acceptably well at 9.5:1 or even 10.5:1, but the 11:1 and 11.25 engines were considered a bad idea back in the 1950's and 1960's. Just how close to the maximum compression ratio a mechanically controlled engine can work well depends first and foremost on the consistency of the fuel supply. If the fuel is always the same then engines can be designed, maintained and operated to run at their best. If the fuel is changing all the time then mechanically controlled engines will run poorly at any compression ratio, and there is little advantage in trying to get close to the maximum compression ratio. The application of the engine also plays a large role in how high of a compression ratio can be run in a mechanically controlled engine with a consistent fuel supply. Mechanically controlled engines in airplanes and boats which run under a mostly constant power versus engine speed curve are much easier to tune to run close to the maximum compression ratio. Likewise racing engines that can be tuned for a specific race track and for a certain driving style can run close to the maximum compression ratio with good results. For automotive use or off road racing the engine must be able to tolerate a wide range of speed and load combinations, and this generally means that even the best mechanically controlled engines end up needing to stay well back from the maximum compression ratio to work reliably.
Computer controlled engines don't have this limitation, and very close to the maximum compression ratio can be run with excellent results. Again though the consistency of the fuel supply does play a role in how close to the maximum compression ratio can be used. If the fuel supply is always constant then rather simple electronically controlled gasoline engines can run very close to the maximum compression ratio with excellent results. If the energy density and flame front travel speed of the fuel are changing all the time though then the engine management system has to be much more sophisticated to deliver good performance with a compression ratio close to maximum. It goes without saying that any engine, mechanically, electronically or computer controlled, requires that the temperature and pressure capabilities of the fuel are consistent if close to the maximum compression ratio is to be run. If the temperature and pressure capabilities of the fuel change from batch to batch then the compression ratio has to be substantially low to prevent destructive full compression ignition on the lowest pressure fuel that is encountered.
Just how close to the maximum compression ratio an engine needs to run also depends on it's application. For racing applications light load efficiency is not usually of much concern, and all that matters is efficient high power output over a range of engine speeds. For many forms of racing the efficiency of the engine at high output is significant both because high efficiency allows higher power output but also because carrying enough fuel for sustained maximum power output can be problematic. Because light load efficiency is not significant for racing the compression ratio of the engine only has to be close enough to the maximum compression ratio possible with the fuel being used so that full load power output is reasonably efficient. This means that running a spark timing of 10 or even 20 degrees BTDC on a four inch bore engine with fast flame front travel speed fuel works quite well. It is only when the spark timing gets up in the 25 to 40 degree range that maximum power output is severely compromised. Slower flame front travel speed fuels can deliver near peak power output up at 25 or even 35 degrees of advance, but because the range of speeds and loads over which the engine will run well is severely compromised slower flame front travel speed fuels are never desirable for racing.
For automotive use where light load performance is of great concern there is much more reason to try to get the compression ratio up as close as possible to the maximum compression ratio. Running a higher compression ratio so that the spark timing is 5 degrees ATDC under full torque might not deliver much higher peak output than running a spark timing of 15 degrees BTDC, but the higher compression ratio certainly does improve light load efficiency.
For any gasoline engine it is important to keep in mind that the amount of fuel burned in flame front travel mode goes up roughly with the square of elapsed time. What this means is that an engine running a spark timing of 15 degrees BTDC and a time of late compression ignition of 15 degrees ATDC will burn four times as much fuel from TDC to the time of late compression ignition than it will from the spark time until TDC. Getting a gasoline engine to run as well as possible requires that only a small portion of the total intake charge be burned in flame front travel mode before late compression ignition takes place. If a large portion of the fuel is burned in flame front travel mode before late compression ignition takes place then the engine will be loud, harsh, inefficient and torque production will suffer. Because the rate of fuel consumption in flame front travel mode increases dramatically as time elapses it is not so much the position of the crankshaft when the spark plug fires that is critical, but rather the position of the crankshaft 20 degrees after the spark plug fires that is of great significance. In the first 20 degrees of crankshaft rotation after the spark plug fires only a small amount of fuel is burned, but then once 20 degrees of crankshaft rotation has elapsed the amount of fuel being consumed by the expanding flame front is large and significant.
If the spark timing on a four inch bore engine is 20 degrees BTDC on fast flame front travel speed fuel then a rather large portion of the fuel is being consumed in flame front travel combustion. Because flame front travel combustion starts out slow though only a small amount of fuel is actually burned before the piston reaches top dead center. As the amount of fuel consumed by the flame front dramatically increases past top dead center the piston is already moving down and this large portion of the fuel that is consumed before late compression ignition takes place only makes the engine a bit harsher, louder and less efficient. If on the other hand the same fast flame front travel speed fuel is run in a lower compression ratio four inch bore engine so that a spark timing of 30 degrees BTDC is required then a large and significant amount of fuel is going to end up being burned between 10 degrees BTDC and TDC, and this fuel burned while the piston is still moving up is going to lead to extremely loud and harsh operation with torque production suffering immensely. If the compression ratio of the same four inch bore engine is increased so that a spark timing of 10 degrees BTDC can be run then only a small amount of fuel will be burned until the piston reaches 10 degrees ATDC, and the engine will run essentially as well as possible with the bulk of the fuel being burned in late compression ignition mode. The point here is that the difference between a spark timing of 30 degrees BTDC and 20 degrees BTDC is much larger and more significant than the difference between a spark timing of 20 degrees BTDC and a spark timing of 10 digress BTDC.
The change in compression ratio required to reduce the spark timing from 30 degrees BTDC to 20 degrees BTDC is a much larger change compared to the smaller change in compression ratio required to reduce the spark timing from 20 degrees BTDC to 10 degrees BTDC. Getting the spark timing down to less than about 20 or 25 degrees BTDC on fast flame front travel speed fuel is absolutely critical for smooth and efficient engine operation. The smaller increase in compression ratio required to get the spark timing down to 10 degrees BTDC is also beneficial, but much less significant. The additional very slight increase in compression ratio required to get the spark timing to be after top dead center under full torque yields only very small increases in performance and efficiency, and is only worth the substantial difficulty for computer controlled engines that run most of the time under radically reduced loads.