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Originally Posted by vex
I'm going to be frank here. I think the gains in MPG to be had are going to come in the transmission, differential, and wheels. Increasing the transmission and differential alone will yield a large percentage of the MPG increase. Thinner wheels (reducing rolling resistance) will also increase your gas savings quite largely.
The engine itself could save on MPG by cheating it a little bit. Mixing of various fuels (IE LPG) to decrease the required fuel from the normal fuel source, but that's just ignoring the other fuel costs.
For increasing engine efficiency look at the intake itself. If you lower the air intake via a restrictor you lower the required fuel. This of course comes at a cost of a less horsepower, but you aren't going for hp when you're thinking of MPG.
To regain the hp you can install a bypass which will allow the addition of more air when the TPS is increased beyond a certain rate. This should remedy the lack of power, but again at the cost of MPG. I personally think that's the best bet beyond running on one rotor. Additional saving could be had from ensuring the intake adheres to the hemholtz equation throughout the RPM band. Thereby increasing volumetric efficiency just by running the engine.
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I agree that it would be nice to get the revs down on the highway, and I wish that there was a good solution (transmission or differential) but I don't know of one. I am still hoping to get a set of 3.73:1 gears made for the rear differential, that change alone should drop 250 rpm on the interstate and increase manifold pressure to -37 kPA average (a significant improvement in thermal efficiency).
As to the intake idea, I already have a restrictor (my throttle plate) and it is the very element that is destroying my fuel economy. My bypass is activated when I open my throttle plates. Think about it this way: Based on rolldown analysis, it requires 12 Hp to go 60 mph in my RX7. At 60 mph, I am running 2400 rpm and -48kPA MAP. The engine is turning 40 revolutions/second - amounting to 1.3X40 or 52L of air per second being pumped against a 48 kPA head (plus exhaust backpressure - which I am ignoring for this calculation). Since pump horsepower is simply W = QP, I can convert the units to m3/s and N/m2 and I get 0.000052 m3/s * 48000 N/m2 = 2.5 kW = 3.5 Hp. This is just the flow work to pump air across the throttle plates, and it amounts to 30% as much power as is required to drive down the road. Intake restrictions (whatever they are) do not really help fuel economy.
To add to the above calculation, let me do some quick math on the thermal efficiency of my engine. at 2400 RPM, the engine puts 100 Lb-ft of torque to the wheels. This is with an injector on-time of 4.8 ms (primaries, per rotor) with no intake vacuum. At 48 kPA, I am running 2.6 ms injection time and putting 26 Lb-Ft of torque to the wheels. This is more than half the fuel and about 1/4 as much torque. (Efficiency of less than 50% WOT efficiency).
This is the big reason that I tried DOD, unfortunately, I have no valves to hold open, so I still do a lot of work moving the air through the second rotor while generating no power with that rotor. The increase in efficiency is the only reason that DOD makes sense.
As to the tires, I am running Bridgestone Ecopia 195/65/R15s (Low rolling resistance). They improved my 30 mph and below fuel economy substantially (about 10 mpg) over the 245/40/R17 Goodyear Eagles that I was running before, but at 70 mph, there is about 1 mpg difference (the overwhelming majority of the drag at 70 mph is aerodynamic). I am running dyno fluids in my transmission and differential, I picked up almost 1 mpg average by using synthetic in my NA drivetrain (so that is still on the table).