OT- High compression automobile engines...worth the extra fuel cost ?

Just to pick two average (i.e. not really high performance) examples... Mazda Miata and Mercedes SLK sports cars... I've wondered for years if it makes that much difference in the engine size/weight and acceleration potential for these to have high compression engines vs conventional "regular gas" engines ? In other words could the engines in these two be redesigned such that the size/weight would be just a tiny bit more.... or would the "regular gas" engines need to be larger and heavier enough to negatively effect the whole picture ?

Another issue, if one uses regular gas in modern vehicles that are supposed to use premium gas, the timing is automatically retarded to eliminate engine knocking. And damned if I can tell any difference in acceleration, mpg, or engine vibration using regular in one of these. But then one reads that long term there could be engine damage using regular in a vehicle meant for premium. But assuming no knocking, I never read exactly what this damage might be. Thoughts ?

If you calculate out the cost at a $.50/Gallon price differential the cost is negligible... I would have to think the type of person buying a convertible "sports" car wouldn't even consider this type of thing to even factor in to their budget.

My honest opinion the premium fuel requirement has more to do with the sports car experience than any engineering decision. Gives the midlife crisis guys something to talk about at their next round of golf... or on their internet forum...

Modern engines have knock sensors and adjust the ignition timing to suit the fuel. Premium fuel burns slower than regular so timing is optimized for the fuel. My Acura Integra GSR is supposed to run premium, it has been about 18 years since it has had any to no ill effects as it is about to roll over 300,000 miles any day now. still uses no oil and pulls hard to 8,000 RPM on a regular basis.

My brother had a jeep he used to commute in and it was cheaper for him to run super than regular. The extra MPG more than made up for the extra cost at pump. YMMV

These are marginal cases.
Mileage is dependent on your right foot more than most things.
Most cars have enough acceleration to get you into trouble.

Pick the car you like.
Enjoy it.
If you don’t like it, get a different car.

A high compression engine is no different than a low compression engine of the same design weight included. Most performance engines require premium fuel to meet the advertised performance numbers . Lower octane fuel will lower performance, that difference will only be measured by a stopwatch . Low octane fuel in most cases will not cause engine damage but there are some operating conditions where the ECM may not be able to stop damage to the engine if it is using low octane fuel . Most damage would be a result of preignition . Bill

SVE Performance said:

A high compression engine is no different than a low compression engine of the same design weight included. Most performance engines require premium fuel to meet the advertised performance numbers . Lower octane fuel will lower performance, that difference will only be measured by a stopwatch . Low octane fuel in most cases will not cause engine damage but there are some operating conditions where the ECM may not be able to stop damage to the engine if it is using low octane fuel . Most damage would be a result of preignition . Bill

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Agree. In some cases low octane fuel can really zap the power and mileage and this I think is more manufacturer driven than anything. But the pre-ignition is a serious issue and is worse in high compression engines such as most modern day vehicles. Almost all engines now are running a minimum of 10.5:1 and a wide majority are running 11.5-12.5:1 ratios. Low speed pre-ignition (LSPI) is an issue in these engines even on high octane fuel. A lot of development work is going into that realm currently.

If a modern engine is pinging it’s not fuel.
If it’s pinging, there’s something wrong with the car.

I thought the turbocharger changes the compression ratio.

digger doug said:

I thought the turbocharger changes the compression ratio.

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It changes the cylinder pressure, but mechanical compression ratios are (almost always) fixed by design of the engine performance goals. Piston and head surface profiles, injection type and control sophistication, as well as function of the ECU and sensors will determine how efficient and uniform the combustion is.

Valve timing plays a big role too, and the so-called "Miller Cycle" uses radical timing to improve the efficiency of a high mechanical compression engine to behave like a lower compression engine with a high expansion ratio - it's a pretty clever idea: Miller cycle - Wikipedia

Turbos will give higher cylinder pressures, mechanical compression used to be lowered to keep overall combustion forces in an acceptable range, but modern fuel injection and controls helps make high compression work fine too.

Higher compression means more efficient combustion which means better milage and more horsepower.To utilise the benefits of higher compression the fuel must have a higher self ignition temperature or it will pre ignite and damage the engine.

High octane fuel does not have any more btu"s than low octane in fact might have less.It burns slower and is more controllable and allows a higher compression without damage.

If your engine's compression ratio allows you to use low octane fuel without knock all through out it operating range adding high octane fuel will not give more mileage or hp,total bs.As a matter of fact using high octane in a low compression engine will usually result in less mileage.

This has changed somewhat in later engines with computer controls and more so with DI as the engines are capable of running higher comp with lower octane fuels.

Using low octane fuels in a hi comp may result in lower mileage because the timing will be continually retarding when knock occurs not because the fuel has any less energy.

The whole scenario is load dependant and that muddies the picture
A hi comp engine running light load ,hi manifold vac will have less RUNNING compression then a low com engine running high load low vac and the knock scenario fuel requirement is reversed.

Confusing enough for all kinds of myths and misinformation to be created. Diesels don't have this problem since the fuel they use not only has more energy but allows them to run wot all the time which gives maximum running compression loaded or not.

ratbldr427 said:

Confusing enough for all kinds of myths and misinformation to be created. Diesels don't have this problem since the fuel they use not only has more energy but allows them to run wot all the time which gives maximum running compression loaded or not.

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Yup, it can be confusing. A pedantic note: Diesels in general don't use or need a "throttle", as they're governed by fuel injected, rather than air allowed in. So WOT is sort of "built in" to diesel design.

Now, some diesel engines may still incorporate a throttle as a governor in case of "runaway", but I don't know of any specifics.

I have 2002 Subaru outback with 3.0 6 cylinder. says to use premium, but found best fuel mileage is with 1/2 tank of mid mixed with 1/2 of premium. Problem is fuel pump is not ethanol friendly and O-ring swelled and blew pump's assembly bottom metal cover off in fuel tank and lost fuel pressure and towed home last week. Went on internet found this is common Subaru problem..1 cheap O-ring = consumer with large repair bill...bad Subaru...Bad

fastone53w said:

I have 2002 Subaru outback with 3.0 6 cylinder. says to use premium, but found best fuel mileage is with 1/2 tank of mid mixed with 1/2 of premium. Problem is fuel pump is not ethanol friendly and O-ring swelled and blew pump's assembly bottom metal cover off in fuel tank and lost fuel pressure and towed home last week. Went on internet found this is common Subaru problem..1 cheap O-ring = consumer with large repair bill...bad Subaru...Bad

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Well, it's not quite fair to blame Subaru here, as the use of ethanol in regular pump gas didn't really take off until later in the decade:

"September 7, 2006 The Renewable Fuel Standard Program (RFS) is signed. This national renewable fuel program is designed to encourage the blending of renewable fuels (ethanol) into our nation's motor vehicle fuel. The nationwide Renewable Fuels Standard (RFS), will double the use of ethanol and biodiesel by 2012.

December 2007 Energy Independence and Security Act signed by Congress and the President, which requires the use of 15 billion gallons of renewable (ethanol) fuel by 2015. In 2007 about 6.5 billion gallons were produced.

2007-2008 Surge in individual states mandating the use of 10% ethanol E10 gasoline.
Rapid increase in documented engine problems (drivability, performance, parts damage), and lawsuits related to E10 blends of gasoline."

From Ethanol Fuel History.

ratbldr427 said:

Using low octane fuels in a hi comp may result in lower mileage because the timing will be continually retarding when knock occurs not because the fuel has any less energy.

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I am pretty sure this is what my brothers jeep was doing.
On a side note what about water injection to stop the knocking in motors that ping?

Rob F. said:

On a side note what about water injection to stop the knocking in motors that ping?

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This works when done right, but can be a major problem with badly setup systems or with poor control. And while you wouldn't have to worry about it, those in colder climates need to add some alcohol to the mix to prevent freezing.

Water/Methanol Injection Kit for Forced Induction Gasoline Engines | AEM (For super/turbo'd cars, but with the right parameters you can setup something for an N/A car.)

The 1986 Buick Grand National was the first US built car with a knock sensor. Since then most cars have them. Stuffing regular swill into a modern engine won't really harm much, but the power output (and mileage) may be reduced slightly because of it. Doing things like towing, climbing mountains, etc would make it a benefit to use high octane fuel. Back in the old days, putting the wrong fuel in could damage an engine. My '67 Thunderbird had warnings all over NOT to use regular fuel. Today, I don't think it matters much.

The advantages high compression has are greater flame propagation speed, a physically smaller space to spread the ignition flame and a sharper leading edge to the vacuum signal. The disadvantage of higher compression is a higher energy loss internally to drive the greater squeeze, excessive fuel/air heating that can exceed the flash point of the fuel causing detonation. Please note that I said detonation not pre-ignition. Pre-ignition is only one possible cause of detonation. Detonation is the collision of simultaneous flame fronts of which pre-ignition (glow-plugging) is only one. The more common cause is dieseling where the fuel flash point has been exceeded due to compression heating.

Further, the compression ratio is not the only cause of excessive compression heating. Pressurized intake (supercharging), higher than normal volumetric efficiency (engine lugging) and early intake valve closure in the compression cycle (cam timing) are also major contributors. Like everything else in life, engine design is a series of compromises.

To answer Milacron's query, it depends, but usually not because the safety margin built in by the manufacturer is sufficient cushion to prevent detonation damage. Remember there is no more energy in high octane fuel over regular fuel. There is a rather simple rule, if the ingested fuel is not burning before the piston changes direction odds are that it will leave the engine unburned. That said ignition timing is really important to efficiency, so if the ECU has to back off timing because it detects detonation, this will not help your mileage, but if you are paying attention, you will hear the knocking anyway.

Milland said:

Now, some diesel engines may still incorporate a throttle as a governor in case of "runaway", but I don't know of any specifics.

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Most Detroit Diesels (the 2 stroke variety) have the emergency shut down air door as standard equipment....whether or not it's kept in working condition
is another....

digger doug said:

Most Detroit Diesels (the 2 stroke variety) have the emergency shut down air door as standard equipment....whether or not it's kept in working condition
is another....

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Actually, it was an option on the Detroit series "A" motors. They were primarily used on unattended engines like generators, marine engines and pumps as a safety shutdown. However, a better example of air throttled diesel engines was the Mecedes taxis used in the 60's and 70's. These were throttled in order to maintain some semblance of stoichiometry.