ok, so i am trying to figure out exactly how port timing works...

it doesnt make much sense to me as to when the ports are supposed to "open" and "close".. here is my interpretation of when they open and close.

exhaust open:


exhaust close:


intake open:


intake close:


of course, these are all approximate, for illustration purposes only.. if im right, how does that equate to degrees before or after TDC and BDC? cuz the way im looking at it, "degrees" are wayyy off....

 

No, you got it.

Degrees are measured by eccentric shaft degrees. "BDC" is when the chamber is at its largest point (shaft lobe pointing straight up or down, depending on which face you're looking at) and "TDC" is when the chamber is at its smallest (shaft lobe pointed straight at the pinch)

The opening/closing time is always when the rotor starts to open/close it, either by the apex seal sliding over it or the rotor's side edge, not the side seal. Bridge ports can be either/or depending on if it's a relieved housing or not.

The funky thing to think about is that during the period around TDC, while the port may be open, flow is blocked off a lot by the rotor being in the way... Was a real eye-opener when I made my first peripheral port and was spinning the engine by hand and looking down the intake ports. Made me realize that trying to cut down on overlap to make a "nice" bridge/peripheral port isn't really important since there's so little flow there anyway, as compared to the flow gains you can get when the rotor is out of the way and air can rush in. My current bridgy is run down all the way to the port floor and I can't tell a drivability difference over the weeny little "low overlap" 35-38mm ports I used to make... but there's a definite POWER difference.

 

ok.. i just dont understand how port timing works then, cuz even with the port timing i WANT stuff was nowhere near close... i want the 74 spec intake and t2 spec exhaust, the exhaust looked close, but the intake was nowhere near, IMO.. i was just approximating degrees, but i know 30 degrees is a third of 90 degrees lol and it was taking like 90 degrees to "open" the intake... idk maybe ill just street port and go with it haha

 

Mind you that the opening time is relative to TDC (it's generally around 32 degrees ATDC, for everything) and closing time is relative to BDC (it's generally 50-60 degrees ABDC).

TDC intake opening is the rotor lobe pointed at the ports, BDC intake is the rotor lobe pointed DOWN, so going from TDC to BDC is 270 degrees of eccentric rotation. 270 minus 32 (since it opens AFTER TDC) and then plus, say, 50 (since it closes AFTER BDC) means about 290-odd degrees of rotation.

 

ok, wait huh? im kinda lost... i almost understand... maybe?

 

74 spec intake ports close just a little bit later than 81-85 12A spec. We're talking like 1/4" or so. Does that make it easier to visualize?

 

it does, but im trying to figure out how to find the degrees of opening and closing on the plates i have as i am really not sure what has been done to them before i had the motor.

 

The simple approach for the intake ports would be to print this out on the right scale and lay it on the irons. Just cut out the trochoid shape, you should be able to tell where to position it on the iron looking at the wear lines, then take a sharp pencil and scribble near the intake port edges. I haven't checked if the trochoid shape or the rotor positions and timings are 100% accurate, but it looks pretty well drawn so I guess it will be close.




If you really want accurate data you need one of those big timing wheels, assemble an iron with a stationairy gear, e-shaft, rotor and housing (use assembly lube or oil or something on mating surfaces), mount the timing wheel on the e-shaft, make a TDC and BDC mark and go from there. It's a little bit of work but will get you data that you can actually use and that will give you a much better idea of what different port timings will actually look like in terms or port size / shape

 

nothing to be ashamed of (unless you're an engineer by education and trade ) ... it's pretty complicated (maybe tedious is a better word) stuff and I have no qualms about telling you that I've built and ported more than just a few engines and this stuff still doesn't come second nature to me. the few times I break out my degree wheel and have to use it, I can literally spend hours working with it on ONE engine.

I don't know if you've seen THIS yet, but it may help you with the complete picture on how to measure timing using both reference points. I really don't know if there is a simpler comprehensive explanation out there that's readily available. if there is, I hope someone posts a link to it.

again, the important thing is to know where TDC and BDC are - these are your references. look at which reference is used for each of the 4 relevant events (IO, IC, EO, EC) and work on one port at a time. resist the urge to try and look at everything as you turn the shaft. use pen and paper as you work.

I wish I could offer a simple explanation, but as I said, it's still not second nature to me. I still have treat myself as a child whenever i'm working with my wheel.

on a totally different note though, if you're not going to be building engines and making your own port templates and such, then this stuff is really moot. in other words, if you're just porting this engine, building it and then that's it, then you're better off getting a set of mild templates (RB, Pineapple) and using them. much less brain-hurt.

 

i'm in the same boat, even with the wheel, its still a bit weird. i ended up picking one angle at a time, so like i'd find exhaust closing, reset and then exhaust opening.

 

wait, i think i got it. as referring to tdc in port timing, is the rotor flat against the side that the ports are on?? cuz tdc i was using was the other side, where the spark plugs are. ive been a piston guy for 21 years lol....

 

12A spec is 40 degrees
74 spec is 50 degrees

 

yeah thats closing right? i think opening is something like 32 on the 74

 

and john, that is awesome. just plain godly lol

 

correct.

TDC = smallest chamber volume with keyway at 9 o'clock

BDC = largest chamber volume (don't remember where the keyway is, but the apex sealing the next chamber is pointing directly at the bottom of the housing)

there's a diagram of a full cycle in the link I posted above.

 

so i was thinking about all this, as i always do when i cant figure anything out, and it hit me!

there are "technically" only 3 strokes in the rotary: intake, combustion, and exhaust.

intake and combustion both have their own respective TDC. intake port opening references the intake stroke's TDC. intake port closing references the intake stroke's BDC.

exhaust port opening references the combustion stroke's BDC. exhaust port timing references the INTAKE stroke's TDC.

i blew my own mind when i figured all this out lol... ive always had the "engineering mind" just not the plaque on the wall haha

 

not exactly ....

the rotary goes through the exact same strokes/cycle a piston engine does, the only exception is that it produces power with every one as opposed to every other one.

the rotor has three faces and is obviously capable of sitting with a compressed volume three separate times, but just like a piston engine (and the Highlander movies ), there is only ONE TDC.

in a piston engine TDC is referenced to piston #1 and likewise in a rotary it is referenced only to the front rotor. likewise, with OHC engines, you have to reference the cam(s)/valves to the TDC mark, in the rotary, TDC is only reference when the keyway is at 9 o'clock.

perhaps, i'm giving myself a bit too much credit, but I feel somewhat responsible for the confusion when I misspoke in your other thread. I apologize and I hope this makes better sense to you now.

 

if there is only 1 TDC, the link you posted is incorrect. it names a TDC for intake and combustion... only then did i put 2 and 2 together.

per the link above, port timing is referenced on intake's TDC, whereas ignition is referenced on combustion's TDC.

 

the link is correct and I got what you were saying.

what I was trying to get across (and this is the reason I made a point to say "not exactly" as opposed to wrong) was simply to be careful with thinking of them as "2" TDCs because there is only ONE. it is TDC that will synch your ignition events to the engine - whether it's distributor, CAS, reluctor wheel or whatever. that's why there is a distinguishing mark to separate the two (cam gear markings for them and the keyway for us) and allow you to find the TDC that matters.

the link takes you through one cycle and just like the reciprocating engine has the piston seeing TDC twice in one cycle, the rotary has each chamber of the rotor seeing TDC twice - it's just not in the same physical spot for us. so in this regard ONLY, there are two TDCs.

 

gotcha lol.. well at least now i know the difference between the "2" lol