March Chat Thread
Post Fiend
Joined: Dec 2007
Posts: 8,521
Likes: 16
From: Lost
Yeah, many people do that. Doesn't make it the right way to treat one, though.
The factory redline is 7K on the 79-85 models, 8K on the 86-95, and 9K on the RX8. They keep making the internals lighter, so they can rev higher without having apex seal chatter/bounce.
The "redline" RPM is just short of where the chatter can start to occur. If the engine has been built with special parts, they can withstand 10-12K without much issue. It's the seal chatter that usually leads to engine failure. Stock seals are made of steel, therefore tend to be a bit heavier than the carbon or ceramic "Race" seals. The heavier the seal, the lower the chatter RPM, it just a matter of physics.
The apex seals are held in place by metal springs, and those springs can only handle so much force before they cannot hold the seal tightly against the surface, so they bounce just like a tire will when the shock is bad. This bouncing starts putting dents in the housing surface, which then increases the bouncing. In turn, the dents get bigger, and it never stops getting worse, until the seal breaks. Then the engine is done.
The factory redline is 7K on the 79-85 models, 8K on the 86-95, and 9K on the RX8. They keep making the internals lighter, so they can rev higher without having apex seal chatter/bounce.
The "redline" RPM is just short of where the chatter can start to occur. If the engine has been built with special parts, they can withstand 10-12K without much issue. It's the seal chatter that usually leads to engine failure. Stock seals are made of steel, therefore tend to be a bit heavier than the carbon or ceramic "Race" seals. The heavier the seal, the lower the chatter RPM, it just a matter of physics.
The apex seals are held in place by metal springs, and those springs can only handle so much force before they cannot hold the seal tightly against the surface, so they bounce just like a tire will when the shock is bad. This bouncing starts putting dents in the housing surface, which then increases the bouncing. In turn, the dents get bigger, and it never stops getting worse, until the seal breaks. Then the engine is done.
Posting Guru
Joined: Feb 2008
Posts: 1,081
Likes: 0
From: Loganville, WI
That is crazy rpms. My truck when doing burnouts hardly reaches 3500rpms. The only thing I've ever owned that held more than that was a Saturn SC2, that would go to about 6000rpms, poor car couldn't take burnouts and broke the axle after me putting bigger tires on it in an attempt to get traction 
Oddly the only cars I've ever broken axles on are fwd cars. And my only display of power tickets were in a 93 Chrysler Lebaron sedan. Kinda funny that I never get in trouble when driving my pickups or land yachts, maybe with collector plates you can get away with more?
Oddly the only cars I've ever broken axles on are fwd cars. And my only display of power tickets were in a 93 Chrysler Lebaron sedan. Kinda funny that I never get in trouble when driving my pickups or land yachts, maybe with collector plates you can get away with more?
Posting Guru
Joined: Feb 2008
Posts: 1,081
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From: Loganville, WI
Hm maybe my 302 needs to be rebuilt, or I could make use of one of the 429's or 460's I have and eliminate the electrical nightmare that is my efi 302. But when it works it's nice getting 22mpg, vs. the 4-6mpg I'd mostly likely get with the 385 series, damn right foot.
Post Fiend
Joined: Dec 2007
Posts: 8,521
Likes: 16
From: Lost
Yeah, back in those days of sub-$1/gal gas, MPG wasn't much of an issue.......
I can get 22-26 MPG in the car, in spite of the fact that it's spinning nearly 4K @ 75.
One thing that most people have a hard time understanding about these unique little engines, is the rotors are turning 1/3 the RPM of the shaft. The shaft may be spinning 9K, but those ~9LB rotors are only doing 3K......
They're triangular for a reason.
I can get 22-26 MPG in the car, in spite of the fact that it's spinning nearly 4K @ 75.
One thing that most people have a hard time understanding about these unique little engines, is the rotors are turning 1/3 the RPM of the shaft. The shaft may be spinning 9K, but those ~9LB rotors are only doing 3K......
They're triangular for a reason.
Posting Guru
Joined: Feb 2008
Posts: 1,081
Likes: 0
From: Loganville, WI
Interesting how the rotary engines work. If I remember correctly aren't they a 5 stroke engine because the power and compression are seperate? hm so if you can get the shaft to go 6k then it would read 18k, that would be crazy. So at higher rpms would it float out because its moving so fast? Do they make them with more then one rotor? Just kinda curious what the potential for these engines could be. I wonder how long Mazda holds the patent on that engine design?
Post Fiend
Joined: Dec 2007
Posts: 8,521
Likes: 16
From: Lost
They're just a 4 cycle engine, like piston engines. They go thru the same intake, compression, combustion, and exhaust cycles. But, the rotor has 3 sides, so 3 of the 4 are always happening at the same time on a single rotor. They also make power on every revolution, like a 2 stroke. The cycles happen in different areas of the engine, unlike piston engines. The air is always moving inside.
Most Mazda rotaries are 2 rotor, though they sold some 3 rotor engines. In 1991 Mazda took the 24 Hrs of LeMans with a 4 rotor powered car, and the rotary was banned immediately afterwards.
Mazda holds or held numerous patents on their rotary design, but the original engine design was german. NSU owned the patent on the engine, and licensed it to many other companies, including GM and Mercedes, who built many prototypes, but never sold any to the public. Mazda is the only company that sold (and still sells) rotary powered cars/trucks in north america. Suzuki sold a 500cc single rotor powered motorcycle back in the early 70's. Those were scary. More power than the chassis and suspension were able to deal with.
Most Mazda rotaries are 2 rotor, though they sold some 3 rotor engines. In 1991 Mazda took the 24 Hrs of LeMans with a 4 rotor powered car, and the rotary was banned immediately afterwards.
Mazda holds or held numerous patents on their rotary design, but the original engine design was german. NSU owned the patent on the engine, and licensed it to many other companies, including GM and Mercedes, who built many prototypes, but never sold any to the public. Mazda is the only company that sold (and still sells) rotary powered cars/trucks in north america. Suzuki sold a 500cc single rotor powered motorcycle back in the early 70's. Those were scary. More power than the chassis and suspension were able to deal with.
Thread Starter
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Fleet Mechanic
Joined: May 2009
Posts: 1,971
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Rotary engines are back in le' mans.
Detroit '08 Preview: Mazda Furai breaks cover! — Autoblog
Detroit '08 Preview: Mazda Furai breaks cover! — Autoblog
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Post Fiend
Joined: Dec 2007
Posts: 8,521
Likes: 16
From: Lost
Minor problem with that graphic. The plugs don't fire at the same time. The lower (leading) fire first, then the upper (trailing) fire 10-20* later. On 86+ models, the leading plugs then fire again 180* later, which is still during the power cycle. That lasts 270* in a rotary........ (90* longer than any piston engine could possibly achieve)
Last edited by Rogue_Wulff; Mar 9, 2010 at 01:48 PM. Reason: typo.....
Post Fiend
Joined: Dec 2007
Posts: 8,521
Likes: 16
From: Lost
In theory, yes a rotary diesel would be possible. However, Mazda has just now been able to reach 10:1 CR in them. The air being moving makes it harder to get a full burn, so a recess is made into the rotor face so the flame can travel, rather than flame-out.
The other troublesome issue is the violent nature of diesel combustion. Detonation in a rotary is hard to make happen without high boost and radical ignition timing. But, when it does happen, apex seals tend to shatter, which totally eliminates any further detonation problems, as well as any chance for compression........
The other troublesome issue is the violent nature of diesel combustion. Detonation in a rotary is hard to make happen without high boost and radical ignition timing. But, when it does happen, apex seals tend to shatter, which totally eliminates any further detonation problems, as well as any chance for compression........
Modular motor junkie
Joined: May 2009
Posts: 6,190
Likes: 8
From: Texas
Thats funny, rep sent.
While I'm off to help a friend with a broke chevy.
Every time I see him he either tells me how much better it is than my ford or tells me its broke and he needs help to fix it.
While I'm off to help a friend with a broke chevy.
Every time I see him he either tells me how much better it is than my ford or tells me its broke and he needs help to fix it.