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Rotary Engine Warmup/cool Down


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why did i go for a rotary? or why does it flood?


i can answer both:


ill leave the second question up to a more experienced mind from rx8club.com. this is his response:


As with most of my posts this will probably turn into a book so before you read any farther just be warned that you should probably take all of your bathroom and food breaks now if you intend to read this! We get up to 20000 characters max per post so I will exercise my right to use them!


I see many people wonder why the rotary floods so easily and ask why it takes so long for it to unflood. Luckily I know what actually happens in the engine and now you will to.


First of all I'm going to explain through a real example of what can happen to your engine when flooded. When I say flooded I mean high water flooded not fuel flooded. I will get to that as well in chapter 2! 5 or 6 years ago my friend Chad and I met a very distraught 3rd gen RX-7 owner who had foolishly driven his car into high water. The water got into the intake and right into the engine. The engine died and would not restart for obvious reasons. The car sat in a parking lot for a couple of days until we could get there with a trailer to haul it back to my friend's place to work on it. Typically when water gets in your engine it is time for a new one. The first thing we did is to remove the spark plugs. Water poured out of them. Yep, I'd say it was flooded! The intercooler came out next and again more water was poured out. We pulled the intake manifold off and drained it. There was water sitting in the turbo manifold and the exhaust. It was getting impressive. When all was said and done we poured out a few gallons of water from the whole engine assembly!!! We turned the engine with a big ratchet and noticed that it still turned fine. We used a mirror and looked into both exhaust ports (this won't work on the Renesis) at the apex seals as the engine was spun over. They all looked fine. We didn't expect that. Rather then go all out and remove the engine for rebuild we made the decision to put the whole thing back together. If anyone here has ever worked on a 3rd gen RX-7 engine then you know what kind of a nightmare the vacuum lines are. We were afraid that we'd still have to undo them again later and 70+ vacuum lines on and off twice sucks! Anyways we got the car back together 100%. Tried to start the car. The engine just free spun. No compression pulses or anything. We installed new plugs since we first tried the old ones. Nothing. We poured a little atf (automatic transmission fluid) into the spark plug holes. I'm not taliking pints here just a couple of ounces each. I don't care what anyone says, it does NOT build compression. What the atf does do is to soften up the carbon deposits within the engine. We then cranked the engine over for a while and actually started to get a very small amount of compression. Knowing that you can start ANY rotary engine that isn't seized by pulling the car down the street in gear, we decided to do just that. BTW, I have actually started a rotary with a blown apex seal that scored up the housings and killed a bearing by pulling the car down the street in gear. Ran like crap but the point is that if you can't get the car started do this. OK back on track now. We pulled the car down the street behind Chad's truck in 2nd gear at about 15-20 mph or so. We kept trying to get it to start. Finally after about 2 miles of pulling the car tried to start. We started getting excited now! After a little more pulling the car did start but sounded like absolute crap. If the engine was shut off we couldn't get it restarted and would have to repull it. As another little side note, when atf burns in the engine you have the most impressive smoke machine out the exhaust the world has ever seen for about a half an hour! Words can't even begin to describe but we were damned sure the fire department would arrive. Once we got the car started the main goal was to keep it there. We noticed that the longer it was running the better it would run. Did this mean it would return to full power? We were just happy that it was running at all. We only left it this way for a few minutes. Now that we knew the engine could run it was important to get new fluid in the car. If the engine was in fact alright, we didn't want to mess it up. ALL of the fluids were changed, tranny, oil, antifreeze, brake fluid, etc. Hell we even topped up the windshield washer reservoir. After this was done we once again tried to start the car. Almost but not quite. It wanted to though. After a quick pull a few feet we got it going again. We let it idle for about 2 hours monitoring temperature and looking for leaks and noticed that it ran much better after this. We drove the car around the subdivision with several runs up to redline over the next 2 days. The engine felt good and started perfectly but the power wasn't there. Oops forgot to put a very important line from the turbo to intercooler back on! Re-installed it and holy crap that car ran good!!! A 3rd gen engine brought back from the brink of death and it was perfect!


OK if you're still with me now I'll get to the technical stuff and explain why this happened. On a piston engine, the piston rings have sharp edges. The rotary engine's apex seals are rounded on top. This is due to the fact that the apex seals rotate in relation to the rotor housing as the rotor moves around it's phases. A piston ring always has the same surface contact and consequently does not develop this rounded appearance. When water gets into a piston engine, it fills the combustion chamber. When the piston moves upward and the valves close, the water has nowhere to go. Remember that water can not be compressed only pressurized. At this point something has to give. The piston rings usually do and they allow ring parts and water to flow down into the engine crankcase and oil pan. Viola, one dead engine in need of a rebuild. A rotary does something very unique though. When water fills up the rotor housing and the port closes, a small amount of water is also under the rounded tip of the apex seals. Since water can not compress, the pressure exerted on the water is redirected at the apex seal. This results in the seal pushing upwards into its groove and the water passes by into the next chamber and so forth and so on until it finds a way to leave. The 3rd gen we had, had water sitting in the engine for a few days. The seals just stayed back in the grooves and the small amount of carbon around them ended up temporarily glueing them in their grooves. This was why even though the inside of our engine was dried out, we had no compression. This is what the atf was for and it was what weakened the carbon's hold on the apex seals. Now just because this engine was fine does not mean that every rotary that gets flooded will be OK. I'm sure the amount of sediment in the water had it's long term affects on that engine. The air filter can only do so much for such a large rushing volume! As with detontation, once may kill it or it may survive many. There is no telling but it should give you some comfort in your little engine's abilities.


Now as to why the engine floods so easily from fuel. It doesn't take hardly any amount of fluid to get a rotaries apex seals to push back slightly. The same thing is happening with fuel even in small amounts. A fuel injector may leak or there may be some residual fuel from the last run still hanging out in the engine. Maybe it was cranked for too long. Who knows? The point is that just enough fuel was present in the rotor housing that the apex seal was moved slightly into it's groove. It doesn' t take much to let all of the pressure escape into another chamber. What happens is that the fuel backs up along the back apex seal to that particular chamber. The small amount of pressure present from air compression has pushed up on the small amount of fuel under that particular apex seal and some of the pressure has bled back into the following chamber. It isn't much but when the engine is turning this slow, you need all the compression you can get. Ever tried to get a big mammoth low compression V-8 started? It sucks doesn't it! If the started could turn the engine twice as fast, we'd never hear of a flooded rotary at startup. When people flood their cars, the first thing they do is to keep cranking it like it is still going to start. You are only making it worse! As with my above story, you can almost always push start the car by popping the clucth. This usually does it but I've had my car flooded bad enough that we had to pull it. Interestingly enough when you flood the engine bad enough, as with the water you may also still have little to no compression for a little bit while cranking. It might just sound like free spinning. This is the sign of alot of fuel. The plugs also get fouled and this only makes matters worse. Don't go spend $40 for new ones. Just take them out, clean them and reinstall them. You shouldn't have to do this unless you really messed up though. The cause of flooding on the 1st and 2nd gen RX-7's was typically due to old leaky fuel injectors. They can't close all the way so the leftover pressure in the lines pushes the fuel in the lines into the engine and floods it. Another cause on all RX-7's was that the car was turned off before it was warmed up. Those cars run really rich when cold. Remember it only takes a very small amoutn of fuel to cause this. The cure was always to find a fuse that went to the fuel pump and remove it. On the 1st and 2nd gen RX-7's this was within reach of the driver under the dash. You'd just remove the fuse and crank the engine for and few seconds. This gets all the gas out of the engine and doesn't allow any more to enter. Reinsert fuse and ther car starts every time! The Renesis will prove to be a pain in the butt here as the fuel pump is controlled directly by the ecu. If the fuel pump does not run off of a fuse that sends power to the ecu then this should work just fine. My concern is that with the computer control of the car that a CEL will come on. Then again maybe you can't do this and I am typing for nothing!


Well after all of this you should now know exactly how and why a rotary floods so easily and what ways there are to get the car started again. Sadly I can't be completely sure why the car just randomly floods if you are doing everything properly but at least I can explain the internal forces at work when it does! Unfortunatly I also am not sure how to quickly get the car started other than pushing it or pulling it and popping the clutch. Also as a disclaimer, I can not guarantee that if you drive through flood waters that your engine will definitely survive. If you are careless enough to do this don't blame me for it should it not survive. Why couldn't I write this long on reports back in school? If you got this far I sincerely thank you for taking the time.


as to why i bought a rotary.... its prolly one of the only other engine in this world today that can spin as fast and hold as much power in such a smalll package. i love the way it sounds the way the car handles and the way the power is delivered. its just a crazy amazing engine and the rx7 is as great of a car. hope this answers ur question saint.

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Interesting writeup :clap:


I wasn't aware of the problem with vanished/ reduced compression which clearly makes starting hard. I have an old 4-stroke motorcycle running 5:1 comp ratio and a crazy high cylinder temperature, which can be a pain to start under certain conditions. However I've now, during 30 years experience with it, learned it who's in charge ;) A soundclip of this thumper is on its way.


Other than that; what I imagine is that a rotary is quite similar to a two-stroke engine when it comes to flooding (from fuel) tendencies. Part from the vanished compression of course. Anyone having lived with a two-stroke knows that they also tend to flood if handled any other way than correct especially if half- warm. And push starting might indeed do the trick!

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