Awhile back I proposed that a Cobra replica could be a fun high mileage car. Well, that got flamed for bad aero. So, I'm stubborn. What would happen with an RX7 with a 302, with a MS controller setupfor max mileage. They have good aero, light weight, what do we think of this combo? I think mid 30's would be possible. A little NOS for fun. What say you?
I'd say go for it... I know 'vettes can get 28mpg. I've heard of 5.0 mustangs turning 30+ mpg.
Anything is possible with enough time and money. How much of each do you have put into this project? 
what's the problem with cobras? bad aero is only part of the equation - frontal area is the other part, and that number has to be pretty dang small on a cobra.
If you want good mileage from a 302 RX-7 you'll need to change the rear end to get better gearing.
Get really computer geek on it and have an economy tune that cuts out the injectors and spark on half the cylinders.
Then you still have all the pumping losses of an 8 cylinder, with less power than a 4 
If you really want to try that, get a 5.3 DOD Vortec motor from a GM truck. >300hp, Aluminum block, and the mechanicals are all already in place to shut off the valves on one side.
I'd say really low 30s is where you'll top out. Thats assuming you put in a 6 speed with uber-overdrive T56 style. RWD is not the most efficient drivetrain layout. Vettes get upper 20s becaue they have A) the super OD and B) are super slippery (way more-so than an RX7).
If I were aiming for good gas mileage and fast, but couldn't deal with FWD, MR2 is where I would be looking.
I'll challenge you and say that 30mpg isn't really "high mpg." Something around 50mpg and up is really high mpg.
ignorant wrote: I'll challenge you and say that 30mpg isn't really "high mpg." Something around 50mpg and up is really high mpg.
+1
Many fast STS cars will average over 30mpg, and get very close to or exceed 40mpg on the highway. But then I guess we need to know your definition of "fun".
is there a cheap high efficiency, high MPG non diesel RWD engine?
I need the room of a mid sized car. I like the Merkur, and I have a shell that needs an engine. I wonder if 35-40 MPG is possible?
BMW's Eta six maybe?
I'd say "Suzuki Cappucino" but I don't think a 600cc turbo 3-banger would be that happy pushing a Merkur around.
J
40 mpg is unlikely with the eta six. 30-32mpg is a reasonable expectation.
I'd be looking at seeing how to make a TDI RWD.... of course, you said non-diesel.
I'll bet a 3.8 buick and s10 trans would give great highway mileage.... I was always impressed at how fuel efficient those 'large' cars were.
40 mpg is unlikely with the eta six. 30-32mpg is a reasonable expectation.
I'd be looking at seeing how to make a TDI RWD.... of course, you said non-diesel.
I'll bet a 3.8 buick and s10 trans would give great highway mileage.... I was always impressed at how fuel efficient those 'large' cars were.
John Brown wrote: is there a cheap high efficiency, high MPG non diesel RWD engine?
No. Typical RWD is inherently inefficient when compared to a transverse setup. And the shell you are starting with is not terribly efficient either.
ProDarwin wrote:ignorant wrote: I'll challenge you and say that 30mpg isn't really "high mpg." Something around 50mpg and up is really high mpg.+1 Many fast STS cars will average over 30mpg, and get very close to or exceed 40mpg on the highway. But then I guess we need to know your definition of "fun".
+2
AE92 SR5: clean over 40MPG combined on the street with a few hypermiling techniques, and 25MPG on the track. It's not very fast in a straight line but once you get going, you rarely have to slow down for corners Mind you mine is the lighter and slightly more aerodynamic JDM model, and it's had some lightening and modification beyond that.
I have a hard time with this, because you have to keep your foot out of the throttle to get the mileage. I liked my miata because I could drive it hard everywhere I went. Though it wasn't "fast" it was sporty and still delivered ~30 mpg driven like each stoplight was a drag tree and practicing hitting the apex on the on-ramp.
I'd love something with over 200 hp, but the fuel mileage would absolutely go down because, with the power on tap, I'm going to have fun with it frequently...
Clem
ProDarwin wrote:John Brown wrote: is there a cheap high efficiency, high MPG non diesel RWD engine?No. Typical RWD is inherently inefficient when compared to a transverse setup. And the shell you are starting with is not terribly efficient either.
My thinking comes from the inheirant aero design of the body (Merkurs = great Bonneville salt flat cars) the car is going to go through a severe diet program including removing the sunroof assembly, modifying a Miata, E30 or Supra suspension to reduce weight on the suspension. Reducing the wiring harnesses. Target weight ~ 2150lbs
How good is the mileage from the BMW M20? Mazda MZR? Toyota? Nissan? Who has the MOST efficient big 4 or small 6?
I'd guess the most efficient small 6 would be the Mazda 323 1.8 V6, also the lowest displacement production V6.
LS1 RX-7. /thread
Most people swapping the 2.5L V6s into the 323s don't see a drop in MPG though. Of course they tend to drive them at 11.
You can't tell who has the most efficient engine unless you can find the BSFC curve for all of those engines (not likely). There are too many variables in the rest of the chassis, gearing, etc.
Your best bet is a small motor with lots of turbo and some serious tuning skills. Turbo Saturns are seeing high 30s mixed and 40mpg highway. And 350ish whp. But once again, transverse engine.
Outside of the above suggestion, spend your time and energy on finding the correct gearing, body aero mods, and weight reduction.
John Brown wrote: Who has the MOST efficient big 4 or small 6?
You might want to look at the Miller Cycle V6 out of a Mazda Millenia. I think those are unusually efficient.
Build yourself a Locost. 40 mpg and FTD.
Type Q wrote:John Brown wrote: Who has the MOST efficient big 4 or small 6?You might want to look at the Miller Cycle V6 out of a Mazda Millenia. I think those are unusually efficient.
The miller cycle engine helped (better mileage than the base model, with the addition of approx. 40 hp), but its still not terribly efficient. FWD and only rated at 28mpg highway. Its a pretty good concept, but nothing spectacular. The Atkinson cycle engine out of a Prius works on the same principles minus the boost... but makes crap for power, so is probably not desirable in this application.
The Prius uses the Atkinson cycle to get it's economy out of the same basic motor as in the xA, xB, Echo, and Yaris. If you didn't care for performance you could pull the 105hp motor out of an 1800lb Echo sedan, put it on as much of a diet as possible, and replace the motor (or at least cams and ECU) with the 75hp Prius ones. Still have better Hp/weight than a VW bug, but that's not saying a lot.
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