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Removing Carbon Deposits

Old 05-22-2006, 12:16 PM
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Removing Carbon Deposits

I have done the search and read the discussions on RENESIS carbon build up. I don't doubt that the engine should regularly be run at high RPMs. However, there is something I don't understand.

Why will revving to 9,000 RPM (momentarily) once a week clean out carbon build-up? Is 9K so abrasive that it cleans carbon deposited at lower RPMs (if so I would expect it to potentially damage the engine)? Is it the heat generated? What? I can even see it blowing out deposits on the exhaust port or in the exhaust manifold but not else where.

I can certainly see that an engine routinely run in Granny-mode (never over 4K RPM) would have carbon build up. I can also see that an engine that is routinely pushed (regularly revved between 6K and 9K) would stay clean or at least a lot cleaner. What I don't see is why extremely high revs for a fraction of a second once a week, or even daily, fixes anything.

Just puzzled ....

Last edited by msrecant; 05-22-2006 at 12:19 PM.
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Old 05-22-2006, 12:24 PM
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im confused as to yor question, but just curious, do u feel bad for revving it to 9k rpms? cuz if u do, this car is not for u my friend, ROTARIES R MEANT TO BE REVVED!

my 2 cents.
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Old 05-22-2006, 12:32 PM
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My explanation, it may not be correct, would be two things are happening at high rpms:
1. The pressure created within the chambers is increased causing some of the carbon to be forced out of the exhaust ports
2. The apex seals are exerting more pressure through centrifugal force which are in effect licking the housings clean.

As for damaging the engine at high rpms, as long as you are doing proper maintenance this isn't an issue. I haven't seen anything concrete, but I'd bet the Renesis can spin several RPM faster than 9K with no issues. Past ~13.5 rpms is where you'll start seeing trouble I'd think.
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Old 05-22-2006, 12:33 PM
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No, I don't feel bad about revving a rotary (I have had them since the 70s). I agree, the engine is designed for high revs, which is why I don't believe that abrasion is what gets rid of the deposits.

Everyone seems to agree that revving to 9K once a week somehow magically cleans away the carbon? I am just curious as the how the cleaning is actually accomplished.
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Old 05-22-2006, 12:39 PM
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Well, the apex seals may be removing some carbon, but this would be dependant on the seals be stronger than the deposits. I'm thinking they are, and if this is the case than the carbon deposits wouldn't be doing any harm to the seals.
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Old 05-22-2006, 01:33 PM
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I'd vote for the extra heat doing the job. The area of interest is the 1/4 to 1/2 inch section of the insulator tip on the spark plug.
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Old 05-22-2006, 09:30 PM
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no carbon buildups occur......basically ur car gives up HC or hydrocarbons also known as unburnt fuel, all vehicles have it, now u compare an engine that gets revved up compared to one thats cruised around all day n never gets on throttle...... the one that gets revved up is gonna have less HC simply becuz theres no unburnt fuel, from goin into the higher rpms it burns it all in the combustion process therefore eliminating the HC problem, whereas the engine that just cruises around can possibly build up carbon in the exhaust ports as well as in the combustion chamber which reduces effiency of the combustion.....WHOOO!!!! i think i hurt myself there i think i go need a beer..... lol
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Old 05-22-2006, 10:23 PM
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it is not a just a RENESIS carbon build up, it is from all rotary motors.
I think all the elements add up together, you just can't use heat or the force to break off the carbon deposite.
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Old 05-24-2006, 03:35 PM
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Anybody from old tried the water/steam clean idea?
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Old 05-24-2006, 04:34 PM
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Well, since everyone else is speculating, I will too.... <g>

I think "revving it to blow out the carbon" is mostly a load of bs. The term has been around a long time and people say it about piston engines as well.

I'd bet that just getting the engine up to temperature and running it for a good while is good enough. That said, I believe that the secondary ports open at 3450 rpm and above, and I think the aux. ports come open depending on load (forgive me if I have that backward - you get the point).

So, taking the revs over 3450 and some "spirited" acceleration to excercise the secondary, and aux. ports would be enough. i.e. a run down the highway. I really doubt there's any need to thrash it out to the redline all the time...

I have an stock '04 MT that's my daily driver. No starter update, no plug update (14k miles original plugs so far). Never flooded, no hard starts, even in winter (St. Louis Missouri). Runs great.

I have followed the short trip move procedure religiously and, my daily commute is about half highway driving. Even when I do put my foot in it (maybe once a week), I rarely run it much past 7k - can feel the power flattening out, so, what's the point?

JM2CW - I know I'll prolly get flamed, but I'd like to hear someone like RG or Zoom44 comment about this too....
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Old 05-24-2006, 04:53 PM
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Originally Posted by KeithL
Well, since everyone else is speculating, I will too....
.....

JM2CW - I know I'll prolly get flamed, but I'd like to hear someone like RG or Zoom44 comment about this too....
Yeah, I was hoping for more information and less speculation. No solid info yet.
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Old 05-24-2006, 04:57 PM
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Originally Posted by Nemesis8
Anybody from old tried the water/steam clean idea?
Been hearing about this for years but have never seen anyone actually do this, or spoke with anyone in person that has done it either.
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Old 05-24-2006, 05:00 PM
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I don't know I think that the revving it up to 6 RPM and holding it would be good enough for the carbon build up to break up and be cleaned..... I mean I have a friend who has an AT 8 and he normally drives his car from 4 to 7 RPM almost everyday, in regular mode, not manumatic, so I doubt he has any carbon build up or does it have to be in manumatic mode???... for people with MT's our RPM goes all across the board so I don't really think that this is an issue, but I just do it to be safe....
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Old 05-24-2006, 05:58 PM
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My understanding is the increase violence in the chamber helps shed loose carbon and blows it out the exhaust port. It is a combination of increased heat, pressure and centrifugal force that clears it out and burn it off. The higher the revs the more of all these forces are acting on the lose carbon. It has been confirmed several times on this board. A mechanic says run the car hard for an hour afterwards issues relating to carbon build up are reported cleared up by the owner.

I don't know about the steam cleaning but for a short time in the 70s water injection was a big thing. Supposedly it would cool the combustion chamber and an aid in the removal of carbon build up. I believe Saab even had it in their cars for a short time but found owners unwilling to keep refilling it.

Carbon build up can be very dangerous for a rotary engine for the following reason. When and engine is running and a piece of carbon build up flakes off it is blown out through the exhaust port and no harm is done. The problem occurs if the engines has not been driven to blow out the loose carbon and it breaks off at shutdown or start up. In a piston engine this is no big deal because there is no place for the carbon to get stuck but in a rotary engine the carbon flake can wedge between the rotor and the housing and seize the engine or jam open an apex seal. It is not uncommon to have to rip down a rotary with carbon lock.

Do as people say and red line the car from time to time what do you have top loose.
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Old 05-24-2006, 08:38 PM
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You hit the Nail on the Head

Originally Posted by KeithL
Well, since everyone else is speculating, I will too.... <g>

I think "revving it to blow out the carbon" is mostly a load of bs. The term has been around a long time and people say it about piston engines as well.

I'd bet that just getting the engine up to temperature and running it for a good while is good enough. That said, I believe that the secondary ports open at 3450 rpm and above, and I think the aux. ports come open depending on load (forgive me if I have that backward - you get the point).

So, taking the revs over 3450 and some "spirited" acceleration to excercise the secondary, and aux. ports would be enough. i.e. a run down the highway. I really doubt there's any need to thrash it out to the redline all the time...

I have an stock '04 MT that's my daily driver. No starter update, no plug update (14k miles original plugs so far). Never flooded, no hard starts, even in winter (St. Louis Missouri). Runs great.

I have followed the short trip move procedure religiously and, my daily commute is about half highway driving. Even when I do put my foot in it (maybe once a week), I rarely run it much past 7k - can feel the power flattening out, so, what's the point?

JM2CW - I know I'll prolly get flamed, but I'd like to hear someone like RG or Zoom44 comment about this too....
What you say more or less matches my 16 years with Mazda rotary engines.The real issue is that under certain conditions the rotary has to be run very rich,eg for smoothness when cold,when accelerating etc.Carbon 'sludge' forms under these conditions in the seal grooves and tends to make them stick.Then only way to avoid this is to get your motor up to normal temperature as quickly as possible and keep it there for as long as possible.
Highway cruising is the best for keeping your motor clean.
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Old 05-24-2006, 09:58 PM
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Originally Posted by KeithL
Well, since everyone else is speculating, I will too.... <g>

I think "revving it to blow out the carbon" is mostly a load of bs. The term has been around a long time and people say it about piston engines as well.
Are you kidding? Have you never been behind a car (piston or rotary, and not some old beater car) and they have jammed the gas and you see the black smoke coming from their exhaust, what do you think this is? This is the carbon that has accumulated in the combustion chambers and exhaust. It is well known that rotaries need to be revved to get this buildup out, ask the Automatic 8 drivers that have had their engines replaced becuase of carbon buildup. I bet they have not revved the engine like they should and that is where this comes from.
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Old 05-24-2006, 10:34 PM
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Originally Posted by Brice-RX8
Are you kidding? Have you never been behind a car (piston or rotary, and not some old beater car) and they have jammed the gas and you see the black smoke coming from their exhaust, what do you think this is? This is the carbon that has accumulated in the combustion chambers and exhaust. It is well known that rotaries need to be revved to get this buildup out, ask the Automatic 8 drivers that have had their engines replaced becuase of carbon buildup. I bet they have not revved the engine like they should and that is where this comes from.
Mostly it is improperly combusted fuel,it kinda makes black smoke ye know.
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Old 05-24-2006, 11:11 PM
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ever take your 8 to the track?? after a hard track day it will run smoother than ever on the way home. redline a day will keep the carbon away, well an extended redline.
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Old 05-24-2006, 11:35 PM
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Originally Posted by dillsrotary
ever take your 8 to the track?? after a hard track day it will run smoother than ever on the way home. redline a day will keep the carbon away, well an extended redline.
That sounds like a new sig. " A redline a day keeps the carbon away"
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Old 05-25-2006, 12:00 AM
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I havent redline my 8 for a while (maybe 2 months?) and for some reason its kinda like sluggish (somewhat, nothing major)

but I rev my 8 yesterday for maybe hmm 5 minutes? I dont know maybe just my *feeling*, my engine feels much smoother.

Just a thought.
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Old 05-25-2006, 12:05 AM
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This is fun, we tried to consume fuel for mileage and end up revving it to remove carbon. This car is love and hate in a package
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Old 05-25-2006, 07:16 AM
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What y'all are talking about is what some of us old farts call an "Italian Tune-up". You are basically thermally shocking the cylinder walls. The differing expansion co-efficient between carbon and the chamber wall causes the carbon to flake and blow out the exhaust. Works best on a warm day with minimal warm-up. I suspect a rotary has a bit more issues with this build up given its higher surface to volume ratio vs a cylindrical piston. ( BTW this is why the poor mileage)
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Old 10-27-2018, 05:35 PM
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Originally Posted by msrecant View Post
No, I don't feel bad about revving a rotary (I have had them since the 70s). I agree, the engine is designed for high revs, which is why I don't believe that abrasion is what gets rid of the deposits.

Everyone seems to agree that revving to 9K once a week somehow magically cleans away the carbon? I am just curious as the how the cleaning is actually accomplished.
hi i have a p2004 on my 2007 rx8 i need advice or if anyone know a rotary mechanic pls help my car has a check engine it wont pass the smog everything work good runs good is just dont pass the smog thanks
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Old 10-27-2018, 06:10 PM
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Google "RX8 p2004", that should give you plenty of threads to read. It's a common issue.
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