Last post on Mar 13, 2012 at 4:42 PM
You are in the Mazda RX-8
What is this discussion about?
Mazda RX-8, Coupe
#3357 of 3618 Re: Latest C&D [trispec]
Jun 07, 2006 (6:26 am)
I believe the RX-8 pre-2006 AT is derived from the Mazda mini-van and if so it should handle the extra torque (torque is the only thing you really have to worry about - the extra HP will heat up the transmission more but you can deal with that with a transmission cooler).
The 7500 RPM limit is due to the torque converter - it will fail if operated above that for extended periods. By fail, I mean explode.
My comment, (I know, unsolicited) on adding a turbo to the RX-8 is it's a "bad idea". I'll bet half the conversions will result in a destroyed engine within two years. To be done properly (read reliably) the compression must be lowered. This entails machine work on the rotors (or replacing them with different ones).
I passed on adding a supercharger to our RX-8.
#3358 of 3618 Re: Latest C&D [pathstar1]
Jun 07, 2006 (12:25 pm)
Wow, I didn't know the 4 speed RX-8 AT used the mini-van tranny. Paddle shifting too? The transmission oil cooler in the driver side grill doesn't cool the tranny, just the oil?
The extra turbo compression causes the apex seals to break down? Or do things just get so damn hot that the oil cooks? Or both?
Is it hopeless for the 13B rotary to be a brute force? It must be damn near impossible or they would have done it by now?
#3359 of 3618 Re: Latest C&D [trispec]
Jun 07, 2006 (5:53 pm)
You can add paddle shifters fairly easily to any computer controlled automatic transmission. They are just switches supplying control inputs to the computer.
The existing trans. oil cooler might have to be enlarged, or perhaps an electric fan added to it, to handle a larger HP load (more watts of heat to dissipate).
Compressing the intake charge has several effects:
1. It adds heat to the intake air and this can take the intake charge (once the fuel is added) too close to detonation making it unstable. You might think cooling it is not difficult (intercoolers), but I've not seen effective intake cooling on turbo or supercharged vehicles very often (my 3rd gen RX-7 is a perfect example, with intake air temps often far above 60C, even with 20C ambient temps).
2. It increases the amount of oxygen and fuel in the chambers putting the molecules much closer together and thereby increasing the chance of them combining before the spark creating detonation.
The result of all this is you increase the chance of detonation occuring. In a rotary where the apex seals are basically in direct line of sight to any explosions in the chamber, any detonation is death to the engine - it shatters the apex seals. In a piston engine the rings are hiding over the edge of the piston - shock waves hit the cylinder walls and only secondary shocks reach the rings through a tiny gap between the piston and the cylinder wall. Hence the detonation has to be really bad before it does damage, and that usually is a hole punched in the piston. Mild detonation puts "peening marks" in the piston - it looks like someone has taken the round end of a tiny ballpeen hammer to it - I've seen this too often on performance piston engines. A rotary wouldn't survive this "mild" detonation.
Finally, no, it isn't hopeless for the 13B renesis to produce a lot of HP - it just takes proper engineering. They (the engineers) must design the engine for forced induction, and they must target it for a specific pressure. The current engine is optimised for normal aspiration. It can be taken "over the edge" without pressurizing the intake (we've done it and been saved by the wonderful "detonation prevention" system built into the renesis computer). Pressurizing the intake of the stock engine is asking for serious trouble unless proper engineering is done to protect it.
What needs to be done? Look to previous rotary models for guidance - the second gen RX-7 turbo engines had lower compression. The third gen RX-7 had lower compression and some enhancements in the fuel injection system to ensure there was always "too much" fuel. The extra fuel acts as a coolant if the charge approaches detonation conditions.
The fact that Mazda haven't added a turbo is a hint - it probably isn't cost effective with the rotary. The third gen RX-7 was a classic example - see how few have survived and how short the average engine life is (typically less than 100,000 mi whereas the second gen normally aspirated engines go for well over 200,000 mi).
Perhaps we'll see a turbo or super charged rotary in the future. However, with fuel prices going the way they are, I'd be surprized.
#3360 of 3618 Re: Latest C&D [pathstar1]
Jun 08, 2006 (3:16 am)
Nice entry. Depth and breath of knowledge. Thanks.
So do the detonation protections in the Renesis 13B detect conditions prior to any detonation or would a single detonation event then trigger the shut down (check engine light and loss of power resulting).
I've been running various octanes of Shell gas, 89 and 93 with no problems. One time however, I tried a tank of 87, just to see how millage would go. But with that one tank of 87 there was a single event of a loudish popcorn/rattle that occurred just above 5000 RPM. The noise was much louder than the popcorn/rattle at low RPM noise I hear less and less often. The event did not cause a check engine light or loss of power, but I backed off the gas. Was that likely an episode of a detonation? Could that single event damage the Apex seals.
#3361 of 3618 Re: Latest C&D [trispec]
Jun 08, 2006 (6:17 am)
I'm beginning to suspect the automatic PCM (the engine control computer) is very different than the manual one. The manual cars' PCM will immediatly drastically retard the timing when it receives a knock input from the sensor (the sensor is basically a microphone bolted to the engine). If further knock inputs are seen, it will set the car into "limp mode" for about 15 min. I guess the thinking was the first knock events are mild, though they will "set up" the engine for destructive knock by increasing temps - so correct the situation before it gets out of hand. On the several times we triggered this protection scheme we didn't notice the popcorn noise, so it wasn't violent yet.
From your reports and reports of others it sounds like the automatic PCM doesn't have this effective knock prevention system. Perhaps this is what they are adding when they rebuild the PCM in California (as per reports from previous members).
I suspect in a normally aspirated engine, you would have to hold it in the knock event for at least 5 sec. before it could cause potentially fatal physical damage to the engine. This is the difference between normally aspirated knock and forced induction knock - there is so much more energy with forced induction often the first knock events or the beginning of the event is enough to destroy the engine. It's a bit funny, because with my RX-7 if you knock it cracks the apex seals, and the pieces subsequently are ejected through the turbos. Cost of new/rebuilt engine - $3000. Cost of new/rebuilt turbos - $4500. Thankfully I know of this from reports by others - I have kept my mixture rich enough to avoid knock! So "knowledge of when to richen the mixture - priceless" .
#3362 of 3618 Re: Latest C&D [pathstar1]
Jun 08, 2006 (8:37 am)
My dealer's mechanic can't heard the popcorn/rattle I hear clearly enough, so they can not do anything they said.
Is there a recall for the RX-8 AT in the works. I'm ready to get this solved. The car has 12500 miles on it. I don't want to risk engine damage. What should I do?
The one time with the 87 octane, RPM over 5000, the popcorn/rattle turned to a loud ball baring/rattle. But only that one time.
This is getting scary. My RX-8 AT's popcorn/rattle noise, which I notice mostly during low speed low RPM when I'm pressing back and forth on the gas peddle is mild and mostly not there when I listen with the windows up or down. If I'm driving beside a Jersey barrier or high curb, I can hear the popcorn/rattle clearly when it happens.
#3363 of 3618 Re: Latest C&D [trispec]
Jun 08, 2006 (5:58 pm)
Usually even without the knock prevention system when you experience knock you will notice more than just the noise. Sometimes an increase in power, sometimes a decrease - the knock upsets the engine physically. You may be hearing fuel burning exposively in the exhaust system. Generally this wouldn't cause damage. Does it happen when the engine is still cold or only when it's warm? If it doesn't happen with a cold engine it's probably fuel burning in the exhaust. This would make sense, as Mazda had to burn fuel in the cat. to keep it warm and working - one of the reasons the car gets such poor mileage. If it's really knock, it should be worse on days when the air is cold - cold air is denser and that can lean out the mixture a bit more making knock more likely.
#3364 of 3618 Re: Latest C&D [pathstar1]
Jun 09, 2006 (3:29 am)
Thanks Pathstar for the reply.
Wow, this is the first I've heard of "exhaust burning explosively" in the exhaust system. Would the auto tranny models exhibit more of this issue?
I've never noticed any kind of power surge or power decrease. Of course with the automatic tranny constantly shifting in Boston's traffic, it would be hard to notice small power fluctuations. Certainly nothing hugely out of the ordinary ever happens.
Yes, the gurgle/popcorn/rattle is less with a cold engine. Once the water temp gauge reaches vertical I can hear more gurgle/popcorn/rattle on the the first strong take off as I get on the first major road way.
As for cold ambient temperatures, the gurgle/popcorn/rattle seems the same.
As for millage, I get 14-15MPG in the city driving. And 23-25MPG on the highway.
#3365 of 3618 Re: Latest C&D [trispec]
Jun 09, 2006 (6:37 am)
There are many possibilities as to why you hear the sounds and others don't. I haven't driven an automatic, but they may be quieter than the manuals (the manual trans. tend to "whine" and the sycros sound like turbos spooling up from time to time), so both may make the sounds but only you can hear them. The PCM (engine computer) is different as the engine is different. It may be running a little richer. It's possible the lower RPM the auto. runs may have allowed some buildup of carbon in the exhaust. It could be acting as an ignition source for the excess fuel these cars are known to put out the exhaust (to heat the cat).
It sounds like you don't have detonation, which is not surprising, as if it were detonation I suspect you would have engine trouble by now. Keep an eye on it, make sure Mazda NA (not just the dealer) has recorded your complaint, just in case.
#3366 of 3618 Re: Latest C&D [pathstar1]
Jun 13, 2006 (2:36 pm)
I believe I've got an explanation for the single event of detonation that I thought I had back when there was a tank of 87 octane in my RX-8 AT. I think what I heard was the AC celliniod firing.
The other day, after weeks of rain, summer temps returned and I turned on the AC. I was running in manual shift mode and the RPM was up around 6000 when I shifted. A horrible cracking sound occurred. Like a single misfire/backfire. There was no loss of power, no check engine light. I think it was the AC. Does the AC disengage the compressor at high RPM.