I was just twiddling around with an idea to swap my 12A with a Renesis, when I remembered some member's post in some distant thread... something like why can't you make a high compression 12A, or something like that... I think the thread was for someone wanting to fit a 20B or something like that...

Anyways, I was thinking about that and playing that little Flash Video of the Rotary engien from How Stuff works in my head when I thought, "well if compression is dictated mostly by the Rotor Recesses themselves. Why not modify the shape of the recesses or eliminate them completely..."

>.> of course, after about 10 minutes of trying to figure out how to do that... I realized the main reason we have two spark plugs and the shape of the rotor recess.

Anyways, enough random drivel... My question is; shouldn't it be possible to adjust the rotor recesses in such a way that there is still a properly shaped explosion, yet allowing for a shallower recess and possibly more compression?

I was thinking instead of the bathtub recess we see a lot, why not something like a shallow curve or droplet? Anyone have any ideas on this? Just something I'm sure we've thought of sometime, figured I'd just ask...

 

even if you do get the proper shape to raise compression, it would be very hard to balance the rotors

 

I don't quite understand what you mean by that? Even though the combustion occurs in only 2 of the 3 chambers. Wouldn't the recesses being cut equally on each side account for inequal balance?

Not much of a mathematics guys so explain it to me, because I am trying to figure how to apply an idea that I would prolly have little trouble to do on a piston engine with an engine i am not mechanically familar with.

 

It seems like its way too much effort than its worth. If you want high compression drop in a S5 N/A engine with a dual carb setup. You won't get much power out of a N/A 12A. But you will have 9.7:1 compression with a S5 N/A.

 

RB used to sell higher compression rotors back in the day. Then stopped selling them because "they didn't work".

If you REALLY want higher compression, buy ceramic apex seals and lots of spring pressure. Those seals are built to much higher tolerances and can take more spring pressure.

Did this in my 12 a j-bridge. Noticeable difference. The starter has to work a lot harder to turn the motor over and it kicks back real good when turning over by hand.

 

^ That sounds cool, hehe

 

Welll I wouldn't know how to even begin to guess what shape would be effective, so I wouldn't be modding my rotors unless I had a spare set...

But I was more curious if anyone else has thought of that or played around with the idea... It seems like you should be able to adjust the seals to hold a higher compression and modify the recess to create that pressure without altering the combuston dynamics too much... At least, that's what I'm thinking... but it would have to be a very specific shape and depth to accomodate all that...

and since the most geometry I can safely do is draw a trochoid and maybe measure half of it... I wouldn't know how to get that idea off the ground.

 

thought about it several times, but it seems like it would be close to impossible for the average person - even with decent mechanical skill - to account for all factors that would yield a viable rotor (all recesses equal in all dimesions for the pair).

 

Mazda uses a kind of copying machine don't they? I can't think of the name off the top of my head, but its the mechanical version of a copy machine, cutting things exactly the same from an original...

But you're right, I don't think an average person could figure out the proper curvature and shape to adjust the combstion and reduce recess...

But then again, who'd have though of a funny engine that works with a spinning triangle? Maybe this short conversation will spark some rotorhead's imagination and we'll see his 14:1 compersion rotary.

 

but wouldnt u build compression if u use a gsl-se (4 port na motor) with s5 rotors, wouldnt that build compression? i just thought i would...and if im wrong i apologise..

 

It should, if the sealing is good and the rotors follow the housing trochoid...
I was just thinking of modifying the rotor's design.. but your idea works too...

 

The S5 13B NA rotors are 9.7:1, and the 13B Renesis are 10:1. But yes, what you are asking about, adding material to the combustion pocket, can be and is done. They do "work," and it's not that hard to balance the rotors, which is good since OEM rotors aren't perfect to start with.

Vendors such as JHB Performance can increase your rotors' compression up to 11:1. Price list here: http://jhbperformance.com/services.php

 

S5 and later engines have CNC machined recesses for even compression ratios between the 3 faces of the rotor.The S5 N/A rotors are 9.7:1 which is a healthy bit of compression for a rotary,only a hair less than the Renesis......course,that doesnt do the 12A engine builder any good!
Welding cast iron is a tricky affair,I could understand why RB would stop offering built up,high comp rotors,just from a metalurgical standpoint.Then theres the issue of keeping that compression in the chamber,and actually using it.Modern,Renesis style seals would be a god idea.
Some good points are made in this post regarding the relatively high compression Renesis engine....
https://www.rx7club.com/1st-generation-specific-1979-1985-18/why-does-renesis-make-so-much-more-power-698558/

 

The shape of the depressions are different, but I don't see any reason that a 12A rotor couldn't be built up in the same manner as a 13B. They don't mention "welding".

 

For background, here is some technical info By Jim Mederer and Chris Ott:

In comparison to a recip, the intake charge (once it is inside the engine) actually travels a long, tortured path, as shown in the combustion cycle shown above. In a recip, the center of gravity of the intake charge only moves an inch or two as the piston moves back and forth between top dead center (TDC) and bottom dead center (BDC). In Mazda's rotary, the charge moves along way more like 20 inches-from intake to exhaust. One bad result is that there are a lot of square inches of surface through which to transfer heat, reducing thermal efficiency. However, here is the big point: The entire mass of the intake charge must pass through the narrow area between the rotor housing and the rotor as each rotor flank passes through TDC. This is made possible by the "rotor depression" which is cast into each flank of the rotor-if it weren't for that path, the partially burned mixture would never be able to squeeze through the narrow clearance between the rotor housing and rotor (usually around .010-.015 inch) at high rpm. There is a crude parallel with a recip that has a "pop-up" piston that tends to cut the combustion chamber in two at TDC. Some recips even cut a "fireslot" (notch) in the middle of the pop-up area to prevent it from stopping flame front propagation in the chamber. For this reason and others, the shape of the rotor depression is quite important. It also has a major influence on determining the compression ratio of the engine and, as all the "Internal Combustion Engine" textbooks point out, the compression ratio is a major determinant of the power and efficiency of any engine. Actually, this points out a weak point in the rotary - the maximum PRACTICAL compression ratio is not determined by detonation (as is common in recips) but by the ability of the burning charge to pass through the rotor depression! If the depression is too small, pressure builds up in the vicinity of the trailing spark plug causing NEGATIVE WORK! This can reduce power, overheat the trailing spark plug, and subsa ntially increase the heat dumped into the oil and water. Therefore, the shape of the rotor depression is a cut-and try balancing act to find the best compromise. Before we leave the subject of the rotor depression, one more point - The physical shape of the depression at its leading edge has a lot to do with the maximum usable leading ignition advance. You can understand this better if you set a late-model rotary at 35 degrees BTC, take out the #1 leading spark plug, and took into the spark plug hole (a mirror and light might be helpful). What you will see is the curved flank of the rotor rather tight up against the bottom of the spark plug hole. If the spark plug were to ignite at this point, the engine might misfire because the flame front might be snuffed out (quenched) when it hit the rotor surface.

If you now turn the engine to 20 degrees BTC, the way is open to burn into the mixture in the rotor depression. This is an important part of the reason why nearly all 1974 and later engines can run no more than 20 to 25 degrees ignition advance at high power (earlier USA model engines had a very long, shallow depression that allowed more advance). As I explained earlier, there are some parallels between rotaries and recips here - combustion chamber and piston top design are major concerns in recips - but there are some distinctive items to consider when working with rotaries.

The truth is, there isn't a lot that you can do to change the shape of the combustion depression, especially in 1989 and later engines with thin casting walls, but you can do some useful things. For one thing, you can ensure that the distance from the apex seal groove to the leading edge of the combustion depression is the same distance on all flanks of all rotors so that all will tolerate the same ignition timing (grind the leading edge of the depression as necessary).

 

Grinding and shaping the recess to promote better flame propagation and pressure delivery to the rotor face is a good idea and probably the extent of work the average tinkerer will be able to do since it only requires a grinder,time and a fair amount of knowledge/experience.

If you want to increase comp ratio a significant amount,I see little alternative than to weld up the recess to reduce its volume,then machine/grind it back into the shape that best delivers that pressure to the rotor.If shaping the recess alone will also affect the working comp ratio,(as stated in the above paragraphs) then that is another possibility.But you can only take away from the original shape.If you need to add material to obtain the desired final product,then welding is about the only option.

 

To add material to the metal rotor, we have several theoretical alternatives to welding. You could ask JHB what method they use. They state "The material is precision deposited" which I surmise is a spray deposition process. Privately cast Rotors to your compression specification, is another alternative. All this effective only to the 11:1 ratio.

 

Unfortunately,plasma spray (or whatever method they use), and custom cast rotors puts the entire process outside the reach of the average person......not to mention the potential cost,since whole replacement rotors are not nearly as common an item as aftermarket pistons.Id be interested to see how they stack up in price,tolerances and quality compared to the factory stuff.
We all know the general concensus when it comes to apex seals.......

 

i agree with Steve, welding would be the most likely method of building a higher compression recess, but it would still require some kind of precision (and accurately repeatable) process.

i suspect that there is a ceiling - before you hit the point of diminishing returns. given the 0.3 difference between the S5 6-port and the Renesis, i'd guess it might probably be in the 11.0 to 11.5 neighborhood. just guessing (with no data or evidence-based info). i'd be very interested in seeing what Mazda would choose for compression on any of the next generation rotaries.

interesting stuff though. i'd really like to learn more about any testing Mazda (or anyone else) has done on this subject.

 

Yes there is a ceiling, 11:1. The data you are looking for can be found in SAE papers.
There was also a thread about this on the other forum. It's called "hi comp rotor" and was started by "madaz matt" which is enough to search for it there, or send me a pm. (I don't know if it's allowed to post a link here, and you'd need to be logged in there anyway.) Be sure to read the very detailed post #16 by "Lynn E. Hanover", which includes his comment "A few minutes with a die grinder in the ports will get you more power than a year of compression experiments."

 

you got any links to the SAE papers?

 

The papers are sold on the SAE web site and available free in some libraries. Sorry I have been looking and haven't found the specific one with compression ratios, although there are numerous interesting Rotary papers. Rotarygod has a copy of it so perhaps if he reads this he will post. Otherwise, the rotary performance section here, and on the "other forum".

The SAE web site is http://www.sae.org but it is a little awkward to browse the papers (just their abstracts anyway).

 

I like the direction that this thread is going in, great to see so many minds thinking about this... It seems that compression is determined by the recess mainly... which brings me to a mysterious picture I saw a while ago... It was a rotor with a teardrop shaped recess in it... I don't remember where I saw it, but I think it was a Mazda racing rotor for one of their race cars. Don't know...

Also does anyone have any info or data on the 16C/16X Rotary Engine Mazda is making for the "Next-Gen RX-7?" If we could look at a little data on that, perhaps they have found a way to overcome the sealing issues that hold the compression at a capped 11:1?

 

I guess that would be a, "no one knows anything about the new 16C that culd benifit this topic?"

 

I am pretty sure mazda has there lips sealed on all of the juicy secrets on the new rotary