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My car consistently gets fabulous gas mileage on the highway. On trips, where I can cruise at 67 mph and use a whole tank full with only one stop, my 1996 Geo Prizm (Toyota Corolla clone) gets 51 mpg, consistently . That is approximately double what an average car on the American roads gets and even Corolla’s are generally considered to be in the 34 mpg range. 

My 96 Geo Prizm overlooking Lake Tahoe CA - ( at +38.8202 -120.0273 )

 I have wondered why this car gets such great mileage but barely passes the famously strict California smog tests. Actually, it does great on all the tests but one, the NOx test but on that one test it almost fails. I have suspected that high NOx was because it was running hot and at high-compression even though it never has shown hot on the temperature gauge or had pings when I accelerated. I do accelerate rather conservatively but cruise with the traffic in lane two on four lane roads, which is generally about 67 mph.
Earlier today I was asking my auto mechanic friend John if enlarging the combustion chamber with a thicker head gasket would make any difference in my NOx rating. He didn’t know for sure but because my car is so close to failing a percent or two might make the difference between pass and fail. So, I thought about some alternatives and chased down this emission chart put out by the Department of Energy and Climate Change. 

Generic air/fuel ratio combustion zone curves

 Assuming this chart is applicable to my standard 1600 cc gasoline engine it appears that my car’s engine is operating over on the right side near the pink zone. Perhaps a previous owner shaved the heads for some reason and didn’t install thicker head gaskets to compensate for the removed material which gives my car more compression and higher operating pressures and efficiencies. Generally for most people operating at overly high compression is bad because it puts more demand on the driver, of a 5-speed stick shift, not to over stress the engine by lugging it at slow rpm. That lugging might result in pinging and dieseling and blow out the engine’s cylinder walls. That was an engineering compromise that somehow, in my used car, got overridden. I like the great gas mileage but because it has come close to failing the NOx test I am trying to figure a cheap fix for picking up a percent of NOx improvement. I think I have one. Remove the spark plugs and put a second gasket on each one. That seems like a tiny amount but it is ten times bigger than the simple volume because where this counts most is when the piston is under full compression and that is about ten times atmospheric pressure. 

A 1600 cc engine has about 400 in each of its four cylinders and when that is compressed ten to one the volume we are actually concerned with is only 40 cc. Spacing the spark plug back with a gasket 2mm thick in a hole about 12mm wide gives 2 x 3.14 x 6 x 6 =224 cubic millimeters or .224 cc. That divided into the 40 cc of compressed fuel air mixture gives about 177 or about one half of a percent. So, if there were a linear relationship of compression to NOx in that situation then there would be about a half a percent improvement in NOx production. Since my car’s engine is within about a percent of failing a half a percent might make it into a pass. I said in the title it was a little bit of improvement but sometimes that is enough.