I read the high flow muffler thread and was motivated to start a test last weekend.
I filled my 87 Civic's tank and then I drove a little more than 200 miles this week with a crude mod that decreased the size of the tailpipe opening from 1.5 inches to about the size of a dime. Its actually a steel can that is clamped over the tailpipe with a radiator hose clamp (fuzzy pic is attached).
Mileage for this tank (with the restrictor mod) was 37.45. The last tank was 38.59 mpg.
The A-B B-A part of the test I drove was 120 miles (60 miles between A and B) at 60 mph. The other 80 miles or so were mixed city/highway driving.
The weather was colder this week than last week and I wasn't sure if the previous tank was completely full. I also did no EOC this tank.
Noise- about the same at idle, definitely quieter at 60 mph.
Driveablility- not noticeable around town, a little less power on long steep hills
Other differences- when pulling a long steep hill, I could see the engine temp coming up noticeably even when outside temps were cold. It usually stays within 1/3 and 1/2, on a few hills it went up to maybe 60%. The temp doesn't rise this much without the mod when driving the same hills in warmer weather.
(By the way, I know I was no where close to boiling over. I overheated the engine several months ago and recall that it happens at the edge of the red zone- or at about 95% up the gauge).
I could also tell that the cooling fan was running more even when in city traffic. This makes sense because probably more of the hot exhaust gasses were remaining in the engine.
I plan to test it for a few more tanks before making a judgment. Right now I feel the results are inconclusive as to whether it increased or decreased my mpg.
The next model I build to test this will be a spring loaded exhaust tip that will maintain a certain amount of backpressure regardless of engine load/rpm.
I'll probably take it off tomorrow and do a few tanks without it and then do the improved version.
How about that and just swap out springs on it to adjust the pressure
Very good idea there- I was about to fabricate something, but that will be the easy route.
I wasn't doing this as a warm up strategy (although that could be a benefit), I was more interested in it as a "forced EGR" device. I was hoping that more backpressure would cause more exhaust to remain in the cylinders so less air/fuel was drawn in. This would cause one to open the throttle more, which should reduce cruising vacuum losses.
BUT, on the negative side, it will increase pumping losses on the exhaust stroke and when I open the carb throttle plate more, the power valve may be opened so that my mixture gets richer- which would be bad news for FE...
Increasing Air and / or spark should increase mpg so long you do NOT 'test' your new found power. Increasing fuel otoh will not increase your mpg.
As for the air, it really doesn't matter whether you increase the in or the out flow, for best results enhance both.
A FE gauge should be standard equipment in every vehicle.