PeachParts Mercedes-Benz Forum - View Single Post

mpolli · #10 06-03-2008, 06:52 PM

Thank you for the interesting replies! We are all thinking the same way generally, but I now have come to a new conclusion. I will try to put all my responses together here:

*One disclaimer: Since we are getting into the issue of the throttle plate, this discussion (currently) will be limited to gasoline cars, since I don't think diesels have a throttle plate. In fact, I have heard that this fact is why Diesels are more efficient than gasoline engines, so maybe diesels would benefit from a cleaner air filter...

ForcedInduction: I think you are correct, there will be NO difference. You make an excellent point about the throttle plate, which I will get back to in a minute.

MS Fowler: Yes I agree on the carb. However the gov't and advertisers are pretty far behind if they are talking about carbs. I think the last car sold with a carb was the '89 Subaru Justy, nearly 20 years ago.

BobK: This is exactly what I WAS thinking, that there would be (small) losses due to the "sucking through the soda straw" idea, pumping losses.

dculkin: I think you are where I am now but saying it a little different than I will say it.

Swampyankee: Idleing losses are very significant. There is no "startup loss" in a warmed up fuel injection engine. My car when fully warmed up uses .33 GPH at idle. Even after a 30 min trip on the highway I can watch my trip MPG melt away as I idle at a stop light. But you have to weigh the savings against the wear and tear on the starter.

Here is my conclusion: I thought there would be "pumping losses" as Bobk describes. However ForcedInduction got me thinking about the throttle plate. The throttle plate acts as a resistance in series with the filter. The engine cannot tell one from the other, it just sees a total resistance. The throttle opens until the resistance is such that the amount of air you "request" (with the gas pedal) goes into the engine. If the filter is more clogged, then the throttle will open more, but the total resistance WILL BE THE SAME. The engine will have no way of "knowing" the difference. There will be NO IMPROVEMENT in MPG. The only effect of a dirty filter would be a limit on maximum possible horsepower. (and possibly letting dirt in I suppose), but NO MPG improvement as advertised.

Now since we probably don't all agree, and I myself am not POSITIVE, I propose the following tests:

1) At idle, with engine fully warmed up after a long trip I will record:

Engine temp
RPM
GPM fuel flow rate
HP

I will do this 10 different days with the old filter, then again with the new filter.

2) I will run a measured course on the highway at a fixed speed, measuring the total fuel consumed over a specific trip segment. I will do this with the old filter and then with the new, both on the same day when the air temp is stable and there is no rain.

Comments?