What effect does rotor beveling have on the CR of an engine?

 

Your removing material from the cumbustion chamber. Your static compression ratio of corse will go down. How much you ask? Well, how much material did/are you removing?


~Mike........

 

compression ratio is the difference between the maximum and minimum chamber volumes (on a piston its TDC/BDC). since you're increasing chamber volume, compression ratio will go down a little.

 

thats what I was thinking. Recently I got the chance to look an a motor that was built with S4 turbo rotors with bevels as large as they could be made. I know that a comprresion test is not nescisarily a good indication of the static CR of an engine, but it does provide some indication.

This motor with new 3MM NRS seals and new rotor housings has allmost exatly right under 90 PSI compression per face on a mazda compression tester.

 

Late closing of the intake port effects dynamic CR more so then early opening. 90 PSI for a aggressive port and 3mm seals is pretty good.

~Mike....................

 

Exactly!

 

look at it this way, the displacement of one rotor is 654cc. which means that in one stroke it moves 654cc of air. that should be roughly equal to the fully open volume of the cylinder. with a 9.0 cr, that means the fully closed volume is 72.7cc.

Lets say you really bevel the **** out of the rotor, and take off 10 cubic cm of material per face ( i dont think thats possible, but for discussion sake) this means that the open volume is now 664, and closed is now 82.7. new compression ratio: 8.03:1 so you would have lowered the compression a whole point. practically, i figure a really phased rotor would probably only lose about 2-5cc (call it 4), so that would be a final CR of 8.6:1, assuming you start with a 9.0 FD rotor.

pat

 

While not meant to chalenge any one in particular in this thread,
I find somewhat intruiguing all the new expert opinions on the subject of
" Rotor port phasing ", as if it were some late discovery on the Rotary scene.

Though in the purest and theoretical sence, there is an increase in absolute total compresable swept volume,....... In reality, the actual compresion of the Rotor is NOT altered at all.

I state this and welcome any difference of opinion, but the fact of the matter is that the Static CR of the Rotor is determined by the size and depth of the combustion chamber . ....NOT by space ( volume) contained in the flanks adjesent to the Rotor pockets.

The amount of time, and the duration of events ( Intake or Exhaust ) measured in degrees of crank shaft rotation is a function of Port geometry on a Rotary engine, or the size of the Cam in a receipricating engine, or in other words breathing capasity.
Volumetric Efficiency (VE) is determined or achieved by proper manifold design, taking into account Intake charge velosity, and exhaust escavange pulse waves.

It is a known fact that the Flame front in a Rotary engine never reaches the leadind edge of the working chamber.
Maximun Brake Mean Effective presure is achived at or about 70 to 80 crank angle degrees ATDC. Depending on chamber presure.
Furthermore, the flame propogation is consumed and extinguished early on the exhaust cycle by the cool walls of the side housings due to the extremly hi surface to volume ratio of the design.

Therefore depending on : Intake Restrictions and Pumping Loses, or aided by Scavange Assist and Dynamic Intake Resonance..........a 654cc chamber of a 13b can either compress 523.2 cc's of gas w/ a poor VE. of say 80% per revolution, or 784.8 cc's of gas with a good VE of say 120% when properly ported and with the Intake & Exhaust tumed to lengh.

By contrast, here is a simple example :
Take a 350 cu in. small block chevy w/ a 4" bore and a 3 + 1/2 " stroke.
Give it a 72cc chamber w/ a flat top piston.
It will yield a CR. of 10 to 1
If in doubt do the math.
Install a compression tester, and spin 4 or 5 revolutions to get a good compresion reading.
You will most probably see a reading somehere between 120 to 130 psi of compression.
Now swap the stock Cam shaft for a Full Roller, Hi Lift Solid Race Cam, and proceed to do the same compression test.
Guaranteed You will see a drop in cranking compression of over 20% to somewhere in the neighborhood of 85 to 95 psi.

Now the question is, did You change the 10:1 Flat top Pistons ???

The point is that on any engine, compresion builds up as the Rpm increases up untill VE begins to fall off, and then so does the power.
On a Rotary engine with no valves to block flow VE build up very rapidly.
The key to building compresion is to contain it without losing it to reversion that may occur as a function of poor "Port" timing, cross flow or back presure.
The alternative is to force the charge in ( hence increase compression ) with some form of boost.


Final point : All who have tryed the so called scallops, have reported substantial increases in performance.
E-production guys from average of 155 RWHP to 180 +
One guy in particular, cannot mention names made 216 RWHP in an EP 1st gen.
That's over 250 on a very reputable engine builder's Dyno in NC.
Turbo street guys on pump gas: 40 to 50 Hp gains at the same boost level.
Turbo Drag guys on Alcohol and Hi boost: 80 + RWHP gains on average.
One guy in the Up State area gained from 960 to over 1100 RWHP.
This is all documented and with testimonials

We developed this technology back in the 80's as a means to get around some class rules. later, we used this tech in our customer cars, many went on to class wins and championships, both in Road Racing and Drags.

A few years back, someone took a picture of our work and put it all over the Internet.
At that point Mazda themselves began using a variation of it on the Rx8's
Now everyone is talking about it, some positive, some negative but for the most part, without any data to back up any claims.

Bottom line, unless You've tryed it, then all one can do is speculate.

We can provide the service to either custome supplied Rotors, or we have a core exchange program for most series of Rotors.
All work is done in house and the lightening and scallops are CNC'd.
This insures identical amounts of material removed from each & every corner.
Other services provided are: Lightening, Clearencing, Balancing, Porting, and all general higher end engine build up services.

Any questions feel free to contact us.


CLR Motorsports Inc.
GT1-20b

 

Could not be explained any better than by one of the greatest and most respected rotary builders himself. This kind of experience and knowledge is not seen on this forum anymore so you guys better respect and feel honored that Carlos is willing and open minded enough to share his findings for everyone to benefit from.

 

thanks dude, its good to hear from the guy that has been doing it forever. just one minor point tho....the static CR IS altered, static CR has nothing to do with volumetric flow or engine speed or antyhing else, its just the geometric volume ratio. What you are saying is that the Dynamic CR does not change, and that is something you cant just easily figure out, you have to do and test, and youve done more of that than anyone else, so you know best. I will be doing this to my 20b when it gets its final build. I was gonna do it myself, but I really like the look of the milled ones you guys do, what kind of price tag does that carry (including balancing)?

Also, I might expect that phasing might cause a noticable increase in fuel mileage, have you had any reports on that?

thanks
Pat

 

wow carlos.thanx for taking the time out to type it out for those who couldnt understand the signifigance of your r&d on rotaries.you are a scholar and a gentleman!

p.s.that post made me want to find more books on engine theory.

 

So what effect does rotor scalloping/bevelling have on the seals and engine life on both turbo and NA rotary engines?