Potential engine mod? Otto to Atkinson conversion

[hyperyaris]

Ah ok yes that is it thanks

[Christ]

Sure... of course you want to know about the insanely complex one that not very many people can actually build :P

[hyperyaris]

haha

[Allch Chcar]

Rotaries get low torque as it is, hence the usage of a supercharger for the Miller Cycle. If I had a RX-8 I'd yank that Rotary out and put a Duratec in it. With lower gearing the thing would actually get good fuel economy. The Rotary is more for redline racing. The weight of the Renesis is something like 240/270lbs, the 2.3l Duratec in the Ranger, B-Series, and Focus is 200lbs now. The older 2.3l Limas were 240lbs. Rotaries can put out 800-900hp in race trims. In comparison the 2.3l get 400-500hp. The Rotaries are great for racing given their power:weight advantage, and their design is simple, light, and freakishly reliable. The MPGs are terrible but they're competing with V8s not 4 bangers.

Honestly, Mazda should use them in the Miata too. It's a waste of money to have a special RWD Engine for both of their niche platform when they could just standardize the Rotary and use it in both of their Sports cars.

...At least we're talking about something interesting now.

[evolutionmovement]

The Miller Cycle engine has nothing to do with a rotary—the Mazda one in discussion was a V6. I'd hate to see a rotary in the Miata, unless it was an option only, as I like the idea of a sports car with a durable engine and the capability of getting decent mileage without using more oil than sleeve valves. V8 fuel efficiency with lawn mower torque and 2-stroke oil consumption in exchange for some smoothness never seemed like a good trade to me. Some Honda engines rev high, too, but use much less fuel and regularly run for 200k miles without a problem.

That and it's just ridiculous to have to have the entire engine spin around a fixed crankshaft, requiring the wheel to be bolted right to the engine block! (that's a joke about the original aircraft rotary engine)

[Allch Chcar]

Yeah, Mazda never did anything like that with the Rotary but that's what we were all thinking of when we saw Rotary. The biggest problem with premium cars having a 4 banger; Noise, Harshness, Vibration. Right? The Duratec can pull the same power with more torque but most people hate the way it sounds. Meanwhile the Miata has some dinky little one model 4 banger. Their lightweight sportscars go best with lightweight high revving engines. In that respect a 4 banger would work but the smoothness of the Rotary means the engine sounds better than a V-6 but weighs more like a 4 banger. The RX-8's biggest problem is the fuel mileage. They made up for the lack of torque with those axle ratios and 6 speeds with close ratio.

The Rotary IS a durable engine. I bet you saw all the consumer reports of engine failure though. I was looking through RX-8s for sale and couldn't find a single Rolling Chassis or engineless, but I did see alot with new engines :har:.

Anyone I think the Atkinson might be a cop out for building 3 cylinders. The 1.3 liter 4 banger used in the Festiva is the smallest 4 banger I've seen. The Volt is getting a 1.4 4 banger because they claimed it was better than a 1 liter 3 cylinder with turbo. With the ease of making 100hp out of a 1.4 I believe they were either fighting going with a 3 cylinder or trying to provide for cruising at 80mph.

Anyway there is some hope for the Atkinson cycle. A study on the effects of ethanol, E85 that is, in the Prius showed a big power difference. It's a bigger difference than most flexfuels... <.< >.>...it might make up for the range loss.

[Christ]

Originally Posted by Allch Chcar

Yeah, Mazda never did anything like that with the Rotary but that's what we were all thinking of when we saw Rotary. The biggest problem with premium cars having a 4 banger; Noise, Harshness, Vibration. Right? The Duratec can pull the same power with more torque but most people hate the way it sounds. Meanwhile the Miata has some dinky little one model 4 banger. Their lightweight sportscars go best with lightweight high revving engines. In that respect a 4 banger would work but the smoothness of the Rotary means the engine sounds better than a V-6 but weighs more like a 4 banger. The RX-8's biggest problem is the fuel mileage. They made up for the lack of torque with those axle ratios and 6 speeds with close ratio.

The Rotary IS a durable engine. I bet you saw all the consumer reports of engine failure though. I was looking through RX-8s for sale and couldn't find a single Rolling Chassis or engineless, but I did see alot with new engines :har:.

Anyone I think the Atkinson might be a cop out for building 3 cylinders. The 1.3 liter 4 banger used in the Festiva is the smallest 4 banger I've seen. The Volt is getting a 1.4 4 banger because they claimed it was better than a 1 liter 3 cylinder with turbo. With the ease of making 100hp out of a 1.4 I believe they were either fighting going with a 3 cylinder or trying to provide for cruising at 80mph.

Anyway there is some hope for the Atkinson cycle. A study on the effects of ethanol, E85 that is, in the Prius showed a big power difference. It's a bigger difference than most flexfuels... <.< >.>...it might make up for the range loss.

What about apex seal failures on the RX-7, Cosmo, and Commuter engines?

Or were you just referring to the Renesis engine?

[Allch Chcar]

Neither, the Apex seal is still a problem on the Renesis. I found Rx-8s with new engines remember? They've improved it even more since 2004 but it's not getting any high marks for used models even with the 100k warranty Mazda slapped on all the Rx-8s. I should have been more clear and said the design is good but the implementation is poor. RX-8s are low mileage vehicles and failure rates are noticeably higher.

With the Miata they always have the option of committing more towards Rotary development. Although the size may actually be the reason they've got a smaller 4 banger in use. It's always an option if they up size the engine any more, 2.3l is about on par with the Renesis.

[Staska]

Great thread, with great perspective, sadly to go out.

My two cents, as car tuner. Going to Atkinson cycle could be achieve without special cams. Look for Europeans with DOHC (double overhead camshafts), maximum compression ratio. When with adjusted intake camshaft timing (2-3 teeth on crankshaft pulley) and with less Ron gas like 92 instead of 98, you will get Atkinson cycle. It was tested accidentally on my past car, fitted different camshaft, was looking for correct timing. It had lost some of top rpm power, but got better mpg (don't remember exact figures, but it was noticeable, it was more about power those days, not economy)

It would be simple Diy mod. But remember - only to adjust intake camshaft timing.

[Angel And The Wolf]

To get a true Atkinson conversion, you will need to raise the compression ratio to 13:1 for the power stroke.

You can do this by shaving the heads, or the deck, or both, to reduce the TDC volume, but be advised that your new 10:1 Intake/Compression volume will be slightly less than your unmodified 10:1 Intake/Compression volume.

You really need VTEC so you can reprogram the controler, because the higher your RPM, the earlier your intake valves should close due to the less efficient cylander filling at higher RPM.

You will need to reprogram your Fuel Injectors too.

It can be done, but you won't get real Atkinson benefits simply by cam regrindng.

That will only give you lower compression and lower power.

You need the 13:1 Expansion/Exhaust, and the 10:1 Intake/Compression ratios

It may be cheaper to simply change out your final drive, so the engine can turn slower at a given speed.