ProtoConVert

I'm kind of looking for a technical answer... so didnt want to post it in the regular thread.

Do rotaries have to run as hot as they do?

dcfc3s

Yep. It's partly due to the very long combustion chamber, and the lack of valves in the exhaust port. They don't necessarily run hotter as far as water temps are concerned (they're actually on the mark there - look at how hot some German engines run!) but they do produce pretty darn hot exhaust gasses.

It's really not that big of a deal, IMHO.

Dale

TurboSE

so what makes you think it is running unusually hot? N/A rotaries don't run hotter than what you are usually used to.

ProtoConVert

It's because I've heard around this board figures of 1100 deg C operating temperatures of the RENESIS vs 8-900 C for piston engines... is this typically true? Also, where is this temperature taken, and what are some other temperatures at other components of the renesis?

neit_jnf

These are exhaust gas temperatures, and form what I've heard they are about right. The other components of the engine are in the same range as piston engines if you're meaning coolant temps and such.

ProtoConVert

So the exhaust gas temperature is what causes the current *notoriety* for warm trunks and cup holders?

neit_jnf

The engine is very far back and low in the engine bay, very close to the passenger compartment in a front-midship layout. And of course the exhaust system runs the lenght of the car. This can account for the warm central tunnel, the trunk is hot beacuse of the HUGE muffler that's under it.

donald121

I hope this is not a stupid question. So, is it a cold air intake and a better exhaust can help reduce the heat? Is it good for the car?

wakeech

...well, that's not quite it.

a "CAI" (meaning fat tube with big cone filter) is designed to move the point of air ingestion to an area of lower tempuratures in the car (closer to the ducts at the front). this won't help the engine get any cooler, it really spouts from turbo cars, where the tempurature of your ingested air is multiplied through the compression, so a little difference at the front end makes a huge difference after the turbo. for all-motor engines like the RX-8, the difference is much much less, and they're usually just gimmicky items if the motor is already fairly high performance. this isn't to say that you can't find power with a differently tuned induction system on an NA motor, but that most CAI's are just show-car junk.

the word "better" is hard to apply to anything automotive unless you have a specific goal in mind. if power is the main concern, then better means a system tuned to move high amounts of exhaust efficiently (which isn't to say that it will move lower amounts of exhaust as efficiently as one designed specifically to do it).
with the RX-8, it's got a muffler on the back end which is intended to appeal to those unaccustomed to rotaries, and who probably weren't RX-7 owners (who universally love it loud). this big ol' thing has a lot of metal to make it up, meaning that it'll conduct a whole lot of heat out of the exhaust gasses, and into whatever else that's around it (like, the cold trunk above it). you can solve this with a few ways, like a smaller thin-walled (less conductive)muffler, designing a heat sheild to hover above it, and lots of other stuff other people can think of for themselves :p

reducing heat in this instance isn't really BETTER for the car as it's not really a big problem for the car, it just makes using it a little more "colourful". the useless cupholders that no one uses would be perfect for coffee (or other hot beverage), but the warm trunk is a pain for those who want to carry things that can't get too warm (like RX-Nut with his icecream, etc). it's more for the user, not the vehicle.

ProtoConVert

another question i have is... suppose the engine heat is reduced by say... added fans or refrigeration i dunno. would that maliciously affect rotary operation?

wakeech

... that's not really a "yes or no" question. there are always optimums for operation, and the stock system will keep the engine there under most foreseeable driving conditions... again, it's not as if the engine is burning itself up uncontrollably, but that the heat of the exhaust is very high as it's not been dispersed by all the metal in the head of a piston engine (just flies right out the door, very unhindered).

donald121

Thank you, wakeech, your answer is very clear and knowledgeable. I have a better idea how's cai work now. :D

smoothSevenz

As opposed to a conventional piston engine?

Rotary engines dont run as hot as piston engines so im not sure where you got your info from.

Rotary engines run cooler, hence the reason you dont use synthetics in a rotary. In a conventional engine the heat gets high enough to burn all oil deposits, carbon, ect.


Since the rotary doesnt get as hot it wont burn away all oil residue therefore allowing carbon buildup(black sooty stuff) which can lead to many possible problems like carbon lockup, loss of power because seals dont flow freely around the edges of the housings, ect.

kidmarc

No, they don't, and the fault is with the fuel system - inappropriately applied.
Maliciously, no... However, depending on the purpose, the application may or may not warrant it. There are different methods for similar and different purposes.

At the present I don't have the time to go into the explanations but can do so this evening or later if you want.

Peace
marcus

Scotchee

You should read this: http://www.yawpower.com/jultech.html It's written by Paul Yaw - a member of our forum. Two things I never thought of before were (1) the inefficiencies of the shape of the combustion chamber in terms of having the fuel mixture burn evenly/completely especially due to heat loss to the metal surfaces and (2) the fact that the rotary flank / fuel mixture goes wizzing past the spark plugs as they try ignite the fuel evenly. I know it's obvious but I never thought of that particular difference between rotary and piston before. I can see why they would need two spark plugs to ignite everything evenly.

Btw, he's got other articles about the whole combustion process that you could read as well.

dcfc3s

OK, time to clear up some misconceptions...

First off, I would *love* to know where the idea that upgrading the intake "really doesn't make much power" came from. I think it came from a lot of Internet desk jockeys figuring that inhaling warm engine bay air would cancel out a big filter, blah blah blah. In the real world, on RX-7's, open intakes make a VERY big difference, especially on the top end of the powerband. K&N is claiming 16 horsepower for an intake on an RX-8 - that's a lotta power.

Now, the crazy cold air intakes that put the filter in the fenderwell and the like - I think that gives you an extra couple of ponies over just an intake, but not as big of a deal. Just using a more efficient filter and inlet path makes a HUGE difference.

Rotaries are very different, heat-wise, when compared to piston engines. A lot of the combustion heat is removed by the oil - oil cools the insides of the rotors. This is why every rotary-powered vehicle has a substantial oil cooler. Water temperatures are about the same as on a piston engine, though.

Exhaust gas temperatures, on the other hand, are VERY hot. Cheap aftermarket catalytic converters frequently melt down, cheap mufflers will totally blow themselves out...you get the idea.

As far as the excess heat around the transmission tunnel and trunk, that's primarily radiated from the main cat and the muffler. The solution to the problem is better heat shielding, which will probably be something Mazda does at some point. Or, you can get some Thermo-Tec blankets and install that around the high heat areas - that'll take care of it.

Dale

Werner

I do agree that most CAI's are junk, but that is not because cold air does not help, it is because they are not well done. If a CAI actually gets colder air into the intake system without compromising air flow, creating a bad resonance and so on and if an engine's fuel injection system has the ability to add extra fuel to a colder intake charge, the result has to be more power.

wakeech

i'm not saying things cannot be tuned differently to suit a higher performance application. if you modify the engine to the point where the entire unit is spec'd to make more power, an air ingestion system up to task is necessary: a "CAI" on a stock, natrually aspirated motor will make a negligable difference.
yes, 5 hp at 7000rpm is negligable (that's an increase of 3.75 ftlbs of torque). 16hp at 9000rpm would be ~9.3ftlbs, which is a far more impressive increase, but i'll believe it when i see it.

btw, what is the RX-8's stock filter media??

i never intended to suggest anything otherwise. i agree.

TurboSE

the exhaust temps of a rotary are hotter than piston motors due to the configuration/design of the engine. Quite a bit of energy is lost in the exhaust in the form of heat (30% iirc) rather than transferred to the crank. This is a drawback for n/a cars but is a great advantage for turbos.

ProtoConVert

what does iirc mean?

wakeech

If I Remember Correctly.

Icemastr

A lot of times the RX-8 is being compared to turbo rotaries such as the 2nd and 3rd gen RX-7's. Remember that the renesis is a fair bit different design from previous rotaries, and not being turbocharged is also another big difference. Cool air intakes on any car are going to increase horsepower, although probably 70% of the power gain on a Naturally aspirated car is going to be because of an open filter that allows the engine to breath more air easily. On a turbocharged car, air intake mods can make a more significant improvement because the turbo can "work" a bit easier, and the cooler the intake temps especially on a turbocharged car, the lower the combustion temps will be, the better it will be for the engine. Rotaries have had a tendency to generally create more heat and be more heat sensitive, as this was a big cause of detonation on turbocharged rotaries. I haven't read any evidence that states the new renesis runs significantly hotter than average piston engines, it seems a lot of what is said is based off of previous rotary engines.

rotarygod

I have alot of experience with non turbo 2nd gens as well as a 1st gen GSL-SE. A cone filter on the past n/a rotaries (fuel injected models) does practially nothing except fool the owners into thinking they are faster. The reason on these cars is due to a very restrictive air flow meter (afm) which the RX-8 does not have. The afm is more restrictive than the factory airbox and filter so gains are low if at all. On the other hand adding a cone filter on my little Honda Civic did add a little power. The length of the pipe the filter was on had the biggest effect though. Since that car makes practially no power to begin with, any gain is easy to feel. Since the RX-8 has a maf (mass air flow) sensor which flows much more than an afm, there may be some gains to be had with a filter. From K&N's past claims with their FIPK kits for other cars, cut their claims in half. At least this way the true numbers are consistent with their other claims on different cars.

The RX-8 utilizes Helmholtz resonant tuning in their airbox. Mazda engineers know good and well how their box flows and you can bet they have tried many different things. The biggest giveaway is the fact that the high power version opens up the shorter intake pipe into the airbox at 7250 rpm while the 4 port version doesn't. This gives them 2 different tunings since different rpm's have different air requirements. It isn't just for 'fresh' air. The 4 port engines power range is at the end of the road at this rpm so it doesn't require this variable fresh air duct. K&N may claim 16 hp gain but this doesn't tell us crap. That number may sell product to the uneducated but the smart people should ask how much more power does it make through the entire rpm range over the stock filter and does it lose power anywhere? This is what I want to know and is the reason why K&N should publicy release an actual dyno chart to back up their claims. Remember PEAK horsepower sells product. AVERAGE horsepower is faster.

To deal with the cold air/ram air thing, cold air gives 3 times the gains of ram air. Back when Racing Beat did land speed record runs at Bonneville they noted that ram air could add approx. 3 mph to the top speed of the car. Only 3 mph on a land speed car! Now see how much they would lose if they didn't have 'cold' air. At those speeds it does take alot of horsepower just to gain 1 more mph since drag rises exponentially with speed. They would have lost around 12 mph or so if the engine was breathing hot engine bay air. The example is relevant and does prove the importance of 'cold' air.

Past rotaries have benefitted most from a better flowing exhaust. The best gains come from headers and anyone with an older rotary (n/a) who doesn't have headers isn't getting the most out of their car. Period. Paul Yaw has found so far that different exhaust manifold combinations on the Renesis have proven "unresponsive". I would have guessed "less responsive" but this news is troubling. So far the biggest restiction in the exhaust is the beer keg sized muffler. Removing it is yielding 15- 20 hp gains.

It is still too early to understand what these new cars do and don't repond well to. Only time and a little patience will tell.