I just thought it would be cool to get down to some of the subtle differences between the Renesis and the 13B engines. Everyone knows that the compression ratio is a little higher and that the Renesis has side exhaust ports but aside from that not too many people know much more. One thing I am going to leave out is the variable intake system.

The first thing I am going to get into is fuel economy as this seems to be a sore spot among owners. Hopefully you'll look at your engine a little differently after this. As we all know the Renesis makes much more power than any previous naturally aspirated rotary. The last naturally aspirated rotary of the same displacement was the 13B in the 89-91 RX-7. It made 160 hp. I don't care what everyone thinks their engine produces, even the low power engine can top 160!From a fuel consumption standpoint the Renesis is on average about 8% more fuel efficient than the 13B but as much as 15% in parts. Why then are RX-8 owners not getting any better mileage than the average RX-7 owner? 2 reasons. The first is that you have a 4.44:1 rear end ratio whereas the RX-7 had a 4.10:1. Lower yours to a 4.10 and you'll see a little improvement. The other reason is that the RX-8 is at least 300 lbs heavier (and in some cases depending on model comparison, much more) than the RX-7's were. Weight and gearing are holding you back from noticable improvements. You guys that complain now know what to do so do it or put up with it.

The side exhaust port led to many improvements (all of them!) over the 13B. The first of which is the fact that the Renesis no longer has any port overlap. This is the point in the rotation of the rotor where the intake and exhaust ports would simultaneously be opened at the same time. The Renesis does not have this but all of the 13B's did. The port overlap of the peripheral exhaust port rotary caused unstable combustion at low engine speeds and loads and this meant that the older rotaries had to run richer than the stoich a/f ratio in those ranges to run properly. This led to worse fuel economy than in the Renesis.

The side exhaust ports also did something else very important. It allowed the closing point to occur later, right around exhaust TDC. The exhaust closes at 3 degrees BTDC (Before Top Dead Center) to be exact. Having 2 of them per rotor made the total area 47% larger than the single peripheral exhaust port of the 13B. The 13B exhaust port closed later at 48 degrees ATDC (After TOP Dead Center)! There are 2 important things about this. The first of which is that there can now be changed intake port shapes to make them larger and still not have any port overlap. The second thing is that the 13B exhaust ports had a much longer expansion stroke. The exhaust ports opened much earlier on the old engines. Some RX-7 people seem to think that this means that it has much more time to get the exhaust out and it should do it under less total pressure. Wrong!!! A long expansion stroke is unfavorable in terms of thermal efficiency. Heat is energy that can work too and the Renesis uses it better. The proof of this was already mentioned in the fact that the Renesis has an 8-15% improvement in fuel consumption. People can argue this all they want but power levels also prove it. As some may have also noticed, this added heat from the Renesis is also murder to a cat. There are more cat failures on the RX-8 than there were on the RX-7. Now you know why!

On the intake side, the Renesis intake ports are 40% larger than those in the 13B. Again made possible by the side exhaust port layout removing port overlap. As an example of how this has happened, the primary ports of the Renesis open at 3 degrees ATDC while the 13B ports opened at 32 degrees ATDC. That's 29 degrees more of open time. The Renesis ports also close at 65 degrees ABDC (low power at 60 ABDC) while the 13B primaries closed at only 50 degrees ABDC. That's 15-20 degrees more on the closing side as well. Unlike a piston engine we can not keep total opening size the same if we change the timing. On a piston engine this would be done by simply changing the valve lift. The rotary has the piston equivalent of changing both lift and duration.

Opening the intake ports earlier had a big effect on a different aspect of the engine. That was the side seals. On the older rotaries, when people opened the intake ports too early, they risked both having a corner seal potentially fall into the intake port but also from having the side seal edge crash into the closing edge of the intake port. The Renesis has something very subtle to try to alleviate these issues. The side and corner seals are moved outward on the rotor by 2mm. I have seen people ask how hard it would be to modify a 13B rotor to work in a Renesis. This is one big issue but I'll get into the others.

A cutoff seal was also added to the Renesis rotor between the side seals and the outer most oil control ring to prevent any port overlap on the sides of the rotor. Without this ring, a small amount of exhaust gasses could actually get around the side and mix with the intake. So much for no overlap. The cutoff seal took care of this. The shape of the side seals is interesting as well. It is a wedge shape. This is to help remove any carbon from building up in the groove which would cause it to stick. Carbon is also the reason for the interesting shape of the Renesis exhaust ports as well as the overly large seal clearances in the Renesis over the 13B. And some of you thought carbon was related to synthetic oils! Shame on you! That was actually the reason why we haven't seen a side ported rotary until the Renesis. Back in the '70's Mazda did try the side exhaust port and back then they also found it to be superior. The problem was the carbon would cause seals to stick and break. You can't market that. They met the standards of the time with the peripheral exhaust port so that's what they stuck with.

The rotor shape has changed slightly as well too. It is very subtle but the 13B rotors are more "round" while the Renesis rotors are more of a true "triangle" shape. Hopefully you can figure that one out. Another very small difference and this one is real small is the rotor width. Yes there is a difference! The rotor housings are not any wider though so you don't get any added displacement. To minimize the hot gasses that could come around the sides of the rotors, the clearance on each side has been decreased an average of 18%! The 13B has more clearance than the Renesis does. Race engines add more clearance yet. This was always an issue at higher rpms over 8000 as there was the possibility that the rotors could actually come into contact with the side housings. You have a higher revving engine with less clearance! Why? Your rotors are lighter and better balanced, your bearings are better, and your eccentric shaft is lighter and stronger (more rigid). Don't sweat it.

What about compression? Well as we all know it went up from 9.0:1 on the previous RX-7 (turbo) and 9.7:1 on the last naturally aspirated rotary to 10.0:1 on the RX-8. Years ago Mazda found that there was no appreciable difference in power on a naturally aspirated rotary with compression ratios from 9.0:1 up to 11.0:1. Virtually no power difference. So why then would they do it? Emissions! Yes, emissions. I know that doesn't make much sense. Remember that although the engine has no port overlap, there is still a small space in the rotor face that some unburned air gets carried back into the intake side in. By raising the compression ratio, they made this area slightly smaller. This does a better job at minimizing the containment volume of the exhaust gasses at closing timing and reduces the need for exhaust gas recirculation. This results in improved combustion stability at low engine speeds and loads. I know many would like lower compression ratios for boosted applications but emissions comes first to Mazda. Boost is for you to work out!

I'm going to skip going over the intake system and the anti wet port jet air/fuel system.

On to apex seals... The old apex seals were 3 piece from the factory but 1 and 2 piece were available from the aftermarket and were even types that once appeared on much older engines. These seals had a total height of 8 mm. The Renesis apex seals are 4.5 mm tall and are 2 piece. That's it. A lighter apex seal can seal better with less spring pressure at higher rpms which improves efficiency and decreases wear. As a result of this decrease in wear, the Renesis does not need as much oil injection as the 13B did. The dual oil injector placement in the rotor housings also allows more even distribution of the injected oil in the engine. A downside to the lighter apex seal is that it is easier to succumb to flooding! The rounded shape of the seal tip allows gasoline to get under it. As it rotates and tries to compress the air, this added pressure will also be in the gasoline under the seal. This will exert pressure on the apex seal trying to push it into it's groove. If it does this, pressure will bleed from one chamber to the next which lowers compression. The fact that any fuel in the engine doesn't leave as easily out a side exhaust port as it does a peripheral one doesn't help matters either. That's the best explanation I can give any of you as to why your engine floods.

It's 3:30 am right now and I'm damn tired so I'll leave with one more thing and add more details to this tomorrow. Your corner seals have a coating on them which Mazda refers to as DLC. I'm too tired to go look it up right now. That makes them very hard. It has been reported that when used in Renesis rotors in older 13B engines, these seals absolutely will tear up a 13B housing. This means the Renesis housings must also have a coating on them which is much tougher. More on the engine later. I'm sure a few others who know some other small things will have contributed by then.

That's all for now.

 

Thanks for the excellent info!

Just curious, where did you get this info from?

Thanks,

Anthony

 

Thanks for your labor!. When you are rejuvenated can you go into the DLC. What is it composed of? Is it something Mazda developed or borrowed from another industry?

 

I understood that Mazda developed a technique for cutting micro fissures into the surface of the housings, in an effort to form a circumferential oil film, but I've never heard whether this technology made it to the final production Renesis.

Can you shed any light on the process and it's application?

S

 

I hate you RG ! I HATE YOU !

 

I await more rotary knowledge. RG is the best!

 

It's information that I've built up over the years from reading books, talking to people, studying actual parts, personal experience, as well as getting ahold of some good SAE papers. I'm just trying to put it all together in one spot. I'll slowly add more as I think of more.

 

RG and I have built a collection of SAE papers on the Rotary and Renesis engine. Alot of interesting info in there, that had I known a few years ago, would have answered some questions that have been forum debated until recently.

Having friends in Ph.D engineering programs is nice, I get access to the SAE database.

 

The Renesis Engine is truely a leap over the old 13B.

The reason most folks don't see the performance is that its having to lug around a heavier car and Mazda had to detune the engine for emissions reasons. In addition, it hasn't been as tuner friendly due to the ECU being such a bitch to work with.

Over time, as the Renesis becomes cheaper, 13B's will begin to fade away. IMO, there is simply nothing that a 13B does better than a Renesis for a street vehicle, other than cost less.

 

DLC stands for Diamond-Like Carbon. It is very hard but it is also very slippery. Nissan even uses this in their current 3.5L VQ35HR and 2.5L VQ25HR engines so it isn't only a Mazda coating.

 

awesome read, subscribes for future RENESIS enlightenment, ommmmmmmm

 

OK let's get on to the Jet Air/ Fuel Mixing system. This system is used primarily for decreased fuel consumption and emissions at idle speeds. This is located in the lower intake manifold and appears as small vacuum tubes inside the intake runners. They actually originate in the lower manifold but extend into the port runners in the engine housings. These tubes are nothing more than vacuum tubes which bypass the intake manifold. Air is sucked in these tubes at a slightly higher rate than the air in the manifold as there is a greater pressure drop in the manifold than there is in these bleed tubes. What these do is to create turbulence at the lower side of the intake runners right before they make the turn into the engine. Fuel is sprayed at this location in the housings and when the fuel comes into contact with this air, it atomizes better. Combined we call this the AWP or anti-wet port.

It sounds like a simple idea but how well could it possibly work? After all doesn't a small tube in the intake manifold mean less flow? Hydrocarbon emissions are lowered up to 30% at idle over the lack of this system so it is very effective at what it does. The added atomization also results in about a 7% decrease in fuel consumption at idle.

Just imagine how nice and slow and inefficient your RX-8's would be with the last nonturbo rotary! 160 hp at the crank, dirtier emissions and worse gas mileage. Gotta love the Renesis! It is superior to any rotary prior to it in every way.

 

Very nice, knowledge is power. Keep it coming.

I do have one questions. I had read some where that the turbo guys where speculating that one issue with turboing the RX-8 was the exhaust flow was restrictive. The RX-8 aving larger port openings would seem to make this a false statement but maybe there side placement is causing flow issues out of the combustion chamber. Any info you can add here?

 

Now let Mazda apply renesis magic to 12a's and bring back a newer cheaper better RX3.
How 'bout the mazda 3 with optional 12a renesis??

 

Logically you would think that a straighter exhaust path is better and should flow more. It should and the 13B exhaust ports probably do flow better if we are only going off of pure flow numbers each. However power isn't only determined by flow. (Don't tell the RX-7 people that!) Timing also plays a very important part as the old peripheral exhaust ports stayed open for too long which was inefficient use of heat. They also opened too early.

When the peripheral exhaust ports open, they are very sudden. It takes very little eccentric shaft rotation to open them fully compared to a side port. This means that the exhaust gasses leaving the engine do so much more abruptly. It is more of a shockwave. The rest of the port opening time isn't so much being used to expell any more exhaust gasses as most of them left right when the port opened. The total exhaust gas velocity peaks very early and then plateaus.

With the side exhaust port, the total port timing is much less. This isn't necessarily a bad thing though as the Renesis uses the total open time much better. The side ports open and close more slowly. In other words it takes mroe eccentric shaft rotation in order to fully reveal their total port area. This means that the gasses leaving these ports must speed up over a longer period of time. There is less of a shock wave type of pulse leaving them. Total flow is greater though as the gas velocity reaches a much higher peak speed than in the peripheral exhaust port. This helps leave a lower pressure in the combustion chamber which results in a cleaner chamber. There is always something left over though and even an engine running at 100% efficiency will still have 10% carried back to the next cycle.

The impact of the peripheral exhaust is what is so good for spinning a turbo. It isn't the total flow through the port itself as it pulses. The Renesis is more conventional and has exhaust pulses that more closely resemble those of a piston engine. It is still good for turbocharging though but the exhaust energy is very different.

Now it might sound like the impact is what we really want but remember that the max exhaust velocity out of the Renesis is greater than the 13B. There is also more heat in the system so the Renesis is still very good and capable at turning a turbo. It just does it in a different way. Mazda had to redesign their exhaust wheels on their turbos to take these sudden pulses into account. The blades were much more flat to absorb as much of the initial impact as it could as opposed to being more curved which favors the gentler exhaust profile of piston (and side port) engines.

 

RG's a friggin genius...

Score one for Houston...

Two for RG...

 

Nope, but I did stay at a Holiday Inn Express last night.

 

There is one thing I want to make very clear:

LONG LIVE THE RENESIS !!


Philip

 

Very Long

 

So in short the exhaust is more then adequate for turboing and given that the turbos being used are designed for piston engines they should work well with the RENESIS because of the similar exhaust signatures. This would lead me to believe that the main advantage that the RX-7 turbos had over the RENESIS for turboing is the lower compression ratio. I also wonder if the shared center exhaust port could be causing back pressure problems from one rotor to the next.

I hope these questions are still in the spirit of the original post, not meaning to hijack it but I find the comparisons of the two engines in the application very interesting.

 

The high compression is a issue for FI if your going to aim for boost levels above 1 bar (14psi). Not saying it isn't possible, but your tuning has to be very good if you want your engine to survive.

Lower compression gives you more margin for error, but I think people make it out to be a bigger deal than it needs to be for the majority of street FI applications. Our course there are people that will bitch b/c they want to build a 30psi 600HP monster engine that is reliable, but really how many people will really do that? 350 at the wheels will make this car a terror, and is easily achievable in a reliable package with proper engine management.

The center exhaust port shouldn't be causing back pressure issues between chambers since there is a divider between the two....I default to RG on this one as he knows more about that area.

 

For a turbo application it isn't an issue at all.

 

nice write rg!!!

thx.

beers

 

This following is the unfathomable part to me.

When I drive my 91 RX7 at 70mph in top gear (5th) the engine turns at 2800rpm.

When I drive my 03 RX8 at 70mph in top gear (6th) the engine turns at 3500rpm.

The Renesis has 20-30% (is that the right range?) more torque than the 13B and an extra gear, yet is running 25% faster at any given speed. That's not smart.

This is the only real problem I have with a great car, and it's a gear train complaint, not an engine complaint. I love the engine, especially the way it zzzzings at high revs...like a friendly chainsaw.

 

Mazmart sells a 4.10 rear end for the RX-8 if you are interested in changing it to be more comparable with the RX-7 gearing.