rpm math
 

I was just thinking today about how fast things are happening inside the engine of my rx8 at 9000 rpm. Then I compared it to the s2000, since it redlines at the same rpm. Now, I'm not sure if I'm right. So maybe you guys can tell me.

Honda S2000 9000 rpm 150 rps revolutions per second
combustes 75 times a second per cylinder

Mazda rx8 9000 rpm 150 rps revolutions per second
combustes 450 times per rotor


To me, something seems insane with these numbers. Please let me know.

" OooooK , for you people that don't seem to understand how this WORKS ! "

some great subtle inuendo in that sentence .........

that was a funny thread

If you use the search function you would find this has been covered back in 2003. Google helps too.

thanks for the info. hey, i was just wondering a couple more things. Is the cobb tuning device useful on a rx8 that has aem intake and rb exhaust? Or, is it for forced induction? Also, the mazdaspeed rx8 has 10 more horsepower but same engine is it a new pcm and if it is, can i buy the pcm and put it on my car and get 10 horsepower?

/\ we let you get away with that first question - now it's time to learn the search button ;)

i'll answer your questions anyway though
yes
yes
no

Oh dam, it's so much easier having some one else do the searching and reading. :icon_no2:

what mazdaspeed rx8??

he is talking about the JDM rx8 that everyone seems to think is a Mazdaspeed

there is no mazdaspeed rx8.

Actually the rotary would combust 150 times per rotor per revolution which would be 75 times a chamber a second which is the same as the S2000. Now if you really want to have some fun, compare the piston speed of an S2000 engine at 9000 rpm to an F1 engine at 18000 rpm!

Ok. But I thought rpm was calculated at the eccentric shaft. So, 9000 rpm would be 3000 rpm per rotor. Which is 50 a second. Where the s2000 is 9000 rpm would be 150 per second. And does that mean the valves on the s2000 are opened and closed 75xs a second. Because that is nuts. The F1 sounds ridiculous.

You should really watch that Youtube video again.

The rotor has three faces, or did you miss that part?

So 3000 rpm of the rotor is 9000 'combustes' as you call them, or 18000 sparks if you count them.

S

lol also at 1:05... make sure the volume is up so you can hear :)

OK I need to think about this a bit more as I may have confused myself. Looking at it again I think you may be correct. I did look at it initially really quickly so let's go ahead and work this out.

S2000 motor at 9000 rpm
9000 / 60 seconds = 150 engine crank revolutions per second.
The engine fires twice per revolution which is 300. Divide that by the number of pistons which is 4. Each piston fires 75 times per second so yes each valve does open and close 75 times per second.

Rotary at 9000 rpm
9000 / 60 seconds = 150 eccentric shaft revolutions.
Each rotor fires once per eccentric shaft revolution. There are 2 rotors which means the engine fires twice per revolution which is 300. Each engine fires the same number of times per revolution. It's the number of faces which makes it confusing. Each rotor has 3 faces so we would divide 300/6 (2 rotors x 3 faces = 6)which means each face fires 50 times so yes that's correct.

The really messed up part is that for any equal unit of time there are an equal number of engine combustion events between the two. Notice that at 9000 rpm that each engine fires 300 times in 1 second. It doesn't matter which one we are talking about. They are equal when it comes to time. However it does not take the same number of engine revolutions to fire every chamber in each engine. The S2000 engine or any other 4 stroke piston engine for that matter, will fire all cylinders in 2 crank revolutions. The rotary will fire all 6 faces in 3 eccentric revolutions so we'll always have a 2/3 ratio difference between engines when it comes to firing every chamber vs crank or eccentric rpm. Incidentally we can prove this with our information above. Take that 75 times per second firing of the S2000 engine and multiply it times 2/3. You'll get 50 which is what the rotary does per chamber in the same second.

Thank you rotary god. It's all cleared up now. I know this is off topic but, I am considering an access cobb device. I have a 04 rx8 in America. The car has an aem cold air intake waiting to be put on. I am debating between exhausts right now but I am leaning towards the Racing beat set up. I am aware that there is not a ton of power to be made naturally aspirated. But, I am curious what you would recommend. SR motorsports has been claiming significant power increases in their packages. Either way, I want to know is the access cobb worth the money for a mildly modified rx8, or is it for a turbo? And, what do you think of the SR motorsports's packages?

SR is full of BS

I'm not sure what everyone's final answer was for the firing frequency, but it's the same for an RX-8 and an S2000 at the same rpm.

Each rotor turns 1/3 of a revolution per shaft revolution. Three chambers, so there is one firing per rotor per shaft revolution. Two rotors, so there are two firings per revolution.

S2000 is a four stroke piston engine. Each piston fires every other crank revolution...half a firing per revolution. Four pistons, so a total of two firings per revolution.

The same. But the Renesis does it with fewer parts and more elegance.

Ken

That's correct. Each engine fires the same number of times per revolution. At 9000 rpm the S2000 fires all 4 of it's pistons 75 times in one second while the rotary fires all 6 of it's rotor faces 50 times per second. That's the confusing part.

that's important to keep in the conversation