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And yet, re-engineering an existing platform in your garage is never going to work unless your "garage" is one of the R&D facilities of a Wayne Enterprises or Tony Stark.
MX5 NA is still cheapish here. No idea about MS Miata though. For a decent rust-less NA bone stock its 2-3 grand. The one I had I picked up from an older couple, it was their sons car. Cost me £550 Been sitting in their driveway for several years. I fixed it up in my garage over the course of a few months. Paintwork was in terrible shape, I bought a rotary buffer and various compounds and spent a couple days completely doing that. Fixed up the roof which the rear window was broken on. New rocket cover gasket and painted it. New coolant hoses. It had lowering springs and a racing beat exhaust on it too when I bought it. I was planning on turboing it but underneath it was rusted out pretty bad, it was really heading for the scrap yard since it was beyond repair. Its a shame because the interior and exterior were almost mint condition after I finished restoring it.
Spoiler
Any way, I don't really have an issue with constructive critics. But people who just get dismissive over ideas because in their opinion its stupid grind my gears. Everyone has a different situation and still wants to enjoy a nice car. For me, I chose the RX8 because its one of the very few RWD cars I can afford to insure because its technically a 4 door family car. My only other choice was a MR2 Turbo but those have plenty of issues on their own, not to mention a greater insurance and buying price.
From what I gather here, you can't do rotor deactivation without serious ECU and Mechanical modification.
The every other side misfire idea sounds like it might work, could be a cheap way to reduce mileage when cruising.
Hybrid is the best bet with motor assistance and possibly a bump in torque from that motor. But thats requires sinking a lot of money into the project.
I know Mazda are toying with running rotaries on hydrogen. I don't have a hydrogen station near me.
LPG is much cheaper and I have one locally but it doesn't last nearly as long. I don't want 50 miles per tank.
And of course there is always the old engine swap. Also a lot of money.
I think the RX8's MPG fate is sealed really.
Here in Canada, even the AT NA can go for CAN$5k. It's pretty expensive for what you get.
Yours actually a very good looking NA. I don't see one in such good exterior condition often.
Mazda actually made the RENESIS run on hydrogen. From what I have heard, though, the thermal efficiency is still lower compared to a piston engine. It's also not nearly as powerful as the 6-port, even in gasoline mode. They only came in automatics, too. Hydrogen in general is not feasible - this stuff is pretty hard to get on the Earth despite being the most abundant element in the universe.
But yeah, not much you can do with the gas mileage in this car. All the modification options are just too time and money-consuming.
Originally Posted by BigCajun
Isuzu V6, run it on 3 cylinders.
PM Kickerfox, he'll drop the knowledge on you.
Honda has been doing that for a while. The second iteration of VCM is actually pretty trouble-free from what I have seen.
Or you can be like Toyota and make the V6 engine switch between Atkinson and Otto cycles on demand. Atkinson cycle makes less power but is more efficient. It just keeps the intake valve open for a tiny bit during the compression stroke. I think Mazda also does it on some of their 4 bangers. I can't think of a way to translate the Atkinson cycle to a rotary engine, though.
I can't think of a way to translate the Atkinson cycle to a rotary engine, though.
Moar intake ports :D
But seriously, just extend a sliver of intake port all the way toward the compression side. Added bonus: the compression force charges the next chamber.
Moar intake ports :D
But seriously, just extend a sliver of intake port all the way toward the compression side. Added bonus: the compression force charges the next chamber.
... this may have downsides.
Well you have to have a way to seal that extended part of the intake port when you want the power and return to Otto cycle.
Well you have to have a way to seal that extended part of the intake port when you want the power and return to Otto cycle.
Thinking about it further, and taking a page from Brettus, you could totally do that with tube and actuated valve. Rather than extend the intake port itself, you extend a tunnel to the appropriate location on the compression side and insert an actuated valve. Intake air is washed with fuel, so it shouldn't gunk up too much. This could all be terrible... but it doesn't sound as terrible the more I think about it?
Regardless what you do, the only realistic thing you could do to improve highway fuel economy is to lower the cruising rpm. Fd Rx7 would run 3k at 75mph. I think Rx8 is over 4k due to gearing. My fd got 26mpg hwy.
On a NA car, it helps to have a lower cruising RPM to reduce the pumping loss. On an RX-8, however, it doesn't help too much. The S2 6AT has taller gear ratios and final drive compared to the 6MT, and that amounted to 1 MPG better on highways.
The reason why your FD gets better gas mileage on highways is because of the turbochargers. Turbochargers greatly reduce the pumping loss on the highways with the light boost they put out, sometimes to the point that cruising RPM doesn't really matter.
I have looked at the new Accord 2.0T gas mileage. The EPA rating shows that the 6MT actually gets slightly better gas mileage on highways than the 10AT, despite the fact that the 10AT has a much lower cruising RPM. The internal losses in the transmission itself likely plays a bigger role at that point.
I am sure people with turbo Renesis can get the same, if not better, gas mileage on highways compared to a REW.
Last edited by UnknownJinX; 09-19-2018 at 01:02 PM.
Regardless what you do, the only realistic thing you could do to improve highway fuel economy is to lower the cruising rpm. Fd Rx7 would run 3k at 75mph. I think Rx8 is over 4k due to gearing. My fd got 26mpg hwy.
Assuming lower rpm uses less fuel to produce the same power is (obviously) incorrect. Every engine has an efficiency peak and it is usually in the low rpm, high load part of the envelope. So 800 rpm and full throttle right? But that's not realistic because while it might be efficient, it doesn't produce enough power to move the car forward. So there is a sweet spot somewhere, but without engineering data I don't know where it is.
In my own testing, I can repeatably produce better fuel economy on long cruises at 4500 rpm than at 3500. Go figure.
75 MPH is 120 km/h, which is roughly 4k RPM on my S2 in 6th gear. Personally, I rarely cruise at that RPM due to the local speed limits. 120+ km/h in an 80 km/h zone is a good way to lose about $800 and your car for a week in BC. On the highways near me, I usually sit at around 60~65 MPH(96~104 km/h) if the traffic permits.
And if you are lugging the engine, that's worse for the engine's overall health.
Regardless, it doesn't really help OP because he wants to improve city gas mileage, where all these ideas go straight out of the windows.
Or you can be like Toyota and make the V6 engine switch between Atkinson and Otto cycles on demand. Atkinson cycle makes less power but is more efficient. It just keeps the intake valve open for a tiny bit during the compression stroke. I think Mazda also does it on some of their 4 bangers. I can't think of a way to translate the Atkinson cycle to a rotary engine, though.
What you describe is essentially what my patent idea was doing . Switching from Atkison to Otto on demand ....although I didn't realise this was happening at the time I did it. https://www.rx8club.com/rx-8-discuss...cation-266923/
The problem is that there is already no power with two rotors, so how is disabling one going to work?
Originally Posted by Surasonac
So one of the biggest issues at least for me is the ungodly fuel consumption on the RX8. However I recently read about how modern V8's utilize displacement on demand which disables cylinders when power isn't needed. This got me thinking about how this can be applied to the humble wankel.
I thought about posting here because there are a lot of people far more knowledgeable than me to tell me i'm wrong.
My idea is that by disabling say the rear rotor, you would improve your mileage significantly for city driving and poodling around. The oil injectors would still be lubricating the apex seals so no problem there. The only problem I can foresee is you are essentially turning the unused rotor into an air compressor which would have a parasitic effect on power. But I feel like it wouldn't be too much.
I'm going to unplug the fuel injectors for my rear rotor tomorrow and see how the car runs. If its all okay then i'll probably just hook a relay up so I can flick the rotor off whenever I want. I really only need the full 230bhp when hooning around.
Has anyone attempted this before? 30+mpg sounds better than 18 for sure. This is only really an issue for me since I go to college and daily my 8' so it cuts into the finances. Don't get me wrong... its totally worth it. But an improvement is always welcome.
The problem is that there is already no power with two rotors, so how is disabling one going to work?
It would work just fine if you could wangle it so one side was free wheeling . You need around 40hp for cruise at highway speed and there is over 80hp available at cruise rpms . If there wasn't quite enough you would just gear it to rev a little higher.
The thing to do as a test would be to take out the apex seals on one side and see what economy you then got at cruise .
This thread is a timesuck, imo.
Maybe I'm clueless, but it seems to me if there was a practical way to do it, someone would've done it in the 15 years of horrible MPG/KPG complaints.
Especially if there was money to be made.
Simplistic 'solutions' such as offered in the beginning would've been done years ago if it could work.
I would still be interested in the result of pulling the injector. Your ECU will go bonkers due to a variety of inputs but it should still drive.
All pulling an injector would do is cut fuel to one side and make the other side have to do more than double the work = a net loss in efficiency. The energy require to spin the rotor while it is still sucking air is significant.
This thread is a timesuck, imo.
Maybe I'm clueless, but it seems to me if there was a practical way to do it, someone would've done it in the 15 years of horrible MPG/KPG complaints.
Especially if there was money to be made.
Simplistic 'solutions' such as offered in the beginning would've been done years ago if it could work.
There are solutions that can be translated onto the rotary. For example, cylinder deactivation can be done by plugging up the ports of a rotor and shutting off the fuel injectors for that rotor.
The reason Mazda didn't go too far is probably because...
- It's a sports car. Crappier fuel economy could be tolerated a bit more.
- Most of us drive a manual trans, so most of these fuel saving technologies don't apply.
Originally Posted by wannawankel
I would still be interested in the result of pulling the injector. Your ECU will go bonkers due to a variety of inputs but it should still drive.
Like I mentioned early, you need to make some kind of valve to plug up the rotor for this to be effective. Otherwise, the other rotor just becomes a compressor like Brettus mentioned and that sucks up power as well.
On a NA car, it helps to have a lower cruising RPM to reduce the pumping loss. On an RX-8, however, it doesn't help too much. The S2 6AT has taller gear ratios and final drive compared to the 6MT, and that amounted to 1 MPG better on highways.
The reason why your FD gets better gas mileage on highways is because of the turbochargers. Turbochargers greatly reduce the pumping loss on the highways with the light boost they put out, sometimes to the point that cruising RPM doesn't really matter.
I have looked at the new Accord 2.0T gas mileage. The EPA rating shows that the 6MT actually gets slightly better gas mileage on highways than the 10AT, despite the fact that the 10AT has a much lower cruising RPM. The internal losses in the transmission itself likely plays a bigger role at that point.
I am sure people with turbo Renesis can get the same, if not better, gas mileage on highways compared to a REW.
Guess it's time to do some experimenting because I also owned a heavy 91 convertible NA Rx7 (3000lbs) and it too was geared just like my fd and got similar highway fuel economy so something isn't adding up. Seems either the side exhaust or the factory Renesis tune is effecting the economy if you're getting less economy at a lower rpm. At cruise, my FD is in vacuum so I don't see how the turbos are helping pumping looses. To spin any engine to a higher rpm naturally takes more fuel given the exact same A/F so you cant spin the engine higher with less fuel as injector duty will have to increase. Keep in mind S5 Rx7 also cruised at 3k at 75mph so I have no idea why Renesis cant benefit from lower rpms also.
How much fuel is injected is a function of power demand, NOT rpm. I know it's weird, but more rpm doesn't require more fuel. To produce 30hp at 3000rpm requires X amount of throttle, where at 4000rpm the same 30hp is produced with less throttle, therefore less air and less fuel per revolution. Obviously it's not a perfect correlation, the actual efficiency of producing power varies with rpm, but there is no hard/fast rule about what rpm is optimal for a given engine and given speed.
The side port everyone is hating on is designed the way it is to improve fuel economy in low regimes, like city driving. ItsI a compromise for sure, but should be a net improvement in fuel economy for a city dweller over a peripheral. How was your FC's economy in the city?
I think it'd be more accurate to say more RPM doesn't necessarily require more fuel. As RPM increases, pumping losses and/or volumetric efficiency may decrease (to a point), but parasitic losses will usually increase. That's why any engine gets its best fuel economy within a certain RPM band: high enough that it's breathing efficiently, but not so high that parasitic losses offset that.
Guess it's time to do some experimenting because I also owned a heavy 91 convertible NA Rx7 (3000lbs) and it too was geared just like my fd and got similar highway fuel economy so something isn't adding up. Seems either the side exhaust or the factory Renesis tune is effecting the economy if you're getting less economy at a lower rpm. At cruise, my FD is in vacuum so I don't see how the turbos are helping pumping looses. To spin any engine to a higher rpm naturally takes more fuel given the exact same A/F so you cant spin the engine higher with less fuel as injector duty will have to increase. Keep in mind S5 Rx7 also cruised at 3k at 75mph so I have no idea why Renesis cant benefit from lower rpms also.
Like mentioned, the side exhaust ports actually help with fuel economy and efficiency. The idea is that you are trying to sweep some of the unburnt fuel into the next cycle so it can be burnt again. It also eliminates the overlap where the exhaust port and intake port are open at the same time.
While a turbocharged car may still be at vacuum at cruise, it will definitely be less of a vacuum than an NA car. That light boost helps the engine suck in air.
Also, RX-8 is not the only car guilty of having too short of a final gear. Lots of other newer cars have this problem as well. The manufacturers assume us manual trans drivers are too lazy to downshift in order to pass people on the highway.
And lastly, changing the diff will cost quite a bit of money, to the point where it will take a long, long time before the fuel saving will offset it. And your acceleration in lower gears will suck even more.
09-01-2018, 09:20 AM
