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Old 01-20-2008, 02:33 PM   #1
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Fuel milage and Compression ratio

I've been reading a lot about this matter, although i havent found very much haha, or it could be that i dont know what im looking at, which is a good possiblity. anyway i see guys on here saying with the increased compression ratio they get better milage and stuff. does that mean emissions are decreased?

heres why im asking, my dad who grew up in the muscle car era, he had a 1969 SS el camino 396 said after 1970 the government stepped in and made all the car manufacturers to drop the compression ratios on the cars to help reduce emissions.

can someone shed some light on this? i dont get it lol.


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Old 01-20-2008, 02:38 PM   #2
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If im not mistaken the lower the compression ratio the less fuel it takes. The higher the compression the more fuel the motor uses
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Old 01-20-2008, 02:43 PM   #3
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now im really confused lol. if you saw the MPG @ 80mph thread, i PM'd illinipo and heres the pm he sent me in a reply.


What gave me the mileage increase was raising compression, which is a DIRECT way of raising your motor's thermal efficiency (what we know as gas mileage).
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Old 01-20-2008, 02:50 PM   #4
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^^that seems right to me, about increasing the efficiency. It's the same concept that makes diesel engines with their 18:1 ratio more fuel-efficient.
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Old 01-20-2008, 02:52 PM   #5
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Originally Posted by MattZX3 View Post
If im not mistaken the lower the compression ratio the less fuel it takes. The higher the compression the more fuel the motor uses
No, compression ratio has nothing to do with the amount of fuel that a motor uses- sorta. That's more determined by the displacement. Higher compression engines are generally more efficient. Race cars are generally high compression, and some fuels like diesels require high compression to combust.

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my dad said after 1970 the government stepped in and made all the car manufacturers to drop the compression ratios on the cars to help reduce emissions.
Older muscle cars had higher compression engines because 100+ octane leaded fuel was readily available at every pump. Compression ratios did drop after 1970 with the introduction of unleaded fuel, but only because of fuel. The engines were actually more inefficient than their predecessors- a quick look at hp numbers will give you a better idea. There were other factors as well like early EGR's, early catalytic converters, new carb designs- etc. All in the name of reducing America's lead output and pollution. According to a really crazy looking scientist I saw on Discovery channel who was comparing the current global climate change situation to lead in groundwater in the 70's- it's something we corrected rather handily.
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Old 01-20-2008, 03:01 PM   #6
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^^that seems right to me, about increasing the efficiency. It's the same concept that makes diesel engines with their 18:1 ratio more fuel-efficient.
That's not entirely it.

Diesel combusts as a result of the high compression and some heat- not a spark. It's a completely different critter than any gasoline engine.

You can see that the average SVT gets better fuel economy than the average Zetec. The SVT has higher compression than the Zetec. Duratecs also have higher compression than Zetecs- at least the D20 version does, although it's less of a viable comparison because the engines are different. Even the SVT has enough differences other than the piston style which would make changes to efficiency.
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Old 01-20-2008, 04:53 PM   #7
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My car, with 11.6:1 pistons (over the 9.7:1 ones) can easily get 31-32mpg on the highway doing the flow of traffic (70-75mph). So, I would say I haven't lost much, if any, due to compression.
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Old 01-20-2008, 06:36 PM   #8
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see this is what confuses me lol. like my cars milage drops horribly at certain speeds. im not too familiar with the actual miles/gallon since i live in canada, and i can never seem to find out whether or not the sticker on the car is measured in US gallons (3.8L) or Imperial gallons (4.45L) so ya...lol

like if i do 100km/h (60mph) the milage is AMAZING i'll get like 700kms to a tankful thats like 440 miles. I could probably squeeze out 800kms i've never really tested it, all i know is when i did a long roadtrip to my cottage and back i was getting 200kms at each 1/4 mark, so theres 4 of them, so im gonna guess 800kms before its empty.

However if i do 120km/h (which is what everyone does in the fast lane) thats like 75mph the milage drops horribly to like 500kms to the tankful. So im trying to figure out how i can get the same milage while doing what everyone else does in speed.
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Old 01-21-2008, 08:05 AM   #9
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To get better mpg's or kpg's, there isn't much you can do. Every engine sucks more gas, the faster you go (well at least at those speeds).
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Old 01-21-2008, 01:50 PM   #10
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Given the gearing and extra drag once you go over 70-75mph, there's no surprise that the gas mileage drops.
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