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Naturally Aspirated Performance ForumDiscussion of naturally-aspirated rotary performance. No Power Adders, only pure rotary power!
From the "12A" to the "RENESIS" and beyond.
It also appears that this Group A main port opening is identical to a Racing Beat street port template, assuming that my Turbo II engine was street ported to an RB template. It matched exactly.
Ah, then the main port does open before the stock 32ATDC and drop the 1 side seal so that is why they had to scissor it.
Re-read what I wrote: The inner curve and the main port's opening line are identical to stock 12A.
I have never held any template of any sort in my hands, I have only ever ported by measuring and eyeballing. I merely assumed that my Turbo II was ported to RB templates because these Group A ports matched them exactly. They opened at stock port timing as well.
It was my understanding the RB 4 port streetport template did alter the opening line to the usual street port 24-25ABDC instead of 32ABDC.
Racing Beat "Street Port"
IO 25° ATDC
IC 60° ABDC
EO 84° BBDC
EC 48° ATDC
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Scissored describes when the side seal comes back onto the plate at the port closing line from the inside (oil seal track) to the outside (water seal).
If at any point a part of the side seal comes back onto the plate at the outside (water seal) portion of the closing line then the port is un-scissored or reverse scissored or incorrectly scissored.
This is a problem when you alter the port opening line which allows the outer end of the leading side seal droop into the port since the tip is no longer on the plate.
Mainly on the longer 2ndary port at low rpm high load where there is a lot of combustion pressure behind the seal and the seal is over the port for a longer time.
Even the big shop street port templates are incorrectly scissored because the radius on the outer top corner is too big so the very end of the side seal hits the radius and wears the side seal tip at an angle and eventually wears a groove on the plate.
They will tell you that is normal for a street port and they have been doing since the '70s, but if you just tighten up the radius for a more square outer corner the "normal" wear magically goes away...
The corner where the opening and closing lines meet was, now that I remember, something I addressed on that TII. I bought the engine as a bare keg that someone else had street ported, and I didn't like how that corner was rounded, forcing the trailing end of the side seal to go across it tip-first, so I squared the ports off with a "landing zone" with a small file.
The reason for the angled closing line has nothing at all to do with the side seal, though... if you think about what the goal of porting is and work backward from there, it makes perfect sense. The fact that it makes life easier on the side seal is simply a happy bonus.
I believe porting is to change the port timing and to a lesser affect gain airflow through the port.
Raising the closing line further at the outward (water jacket) side of the port than the inboard (oil seal) side of the port relative to the side seal crossing is not an improvement in flow or timing over raising the closing line on the inward (oil seal) side.
Especially the later the port closes.
On the 4 ports with late closing you get that poor transition from runner to that upper outer corner you have to port out (where the top of the port is well above the top of the runner casting) and then you have added a whole bunch of volume (and maybe some epoxy) to the port runner and intake velocity drops so lower rpm filling is poor.
As far as I can see, scissoring (the rotary kind!) is something you do just so your side seals last.
The shorter primary ports often get a different un-scissored shape at the same closing timing on a street port because side seal life is not a problem on them (the teardrop shape port).
I see the goal of porting as getting as much clean air as possible into the chamber. While a closing line that "slams" might be good for runner resonance, and may be necessary with carburetors for reasons I don't feel like pondering, it has a huge drawback. Bear in mind the Goal: Most air into the chamber, not most air through the port!
When the rotor is at its point of highest volume change per degree of rotation (IE the pull on the port is at its hardest) the port is aimed at the center of volume by turning through a channel. That's.... shitty. By having a sweeping closing line, the air will be turning over less of a channel and more of a flat curve, much happier for airflow *into the chamber*. This gets you more velocity in the port and velocity in the port is what keeps cramming more air into the chamber once past BDC. And in the case of a bridge port, when one chamber is closing the next chamber uses that velocity to start filling the next chamber.
Ever notice that MFR peripheral ports make more power than RB-style peripheral ports for the same port timing? The MFR ports are aimed better!
It has been a long time since I have seen a real MFR peripheral port housing, but what I remember is the runner was horizontal so the flow was pointed toward the back of the intake stroke at BDC.
The RB housings have the runner angled slightly down and their reasoning is that the air already in the motor is in a counter rotating swirl from the rotor so their port introduces air into the slipstream side of the swirl.
I did not know if/whether one style was known to make more power.
The MFR parts are good for about 300hp at 8500 and peak torque is around 190-195 ft-lb. RB engines need more RPM to make the same power, and a lot more port timing too.
From a machinist's point of view, I think RB did it that way because it was easier. No funky meet-up of the sleeve to the inner water jacket wall. Just machine a recess to seal all around the tube and not worry about poking it into the sleeve, or welding a steel tube (like some do).
IDK, but I bet most the difference between RB and MFR housings are the exhaust port runner.
RB exhaust is off the production car housing so the port runner is too small with the port sleeve in place for 300hp NA and too big with the sleeve removed for 300hp NA.
MFR exhaust port runner was sized for the 300hp NA application.
RB could have easily changed the intake tube angle if that was a "problem". Exhaust runner size- not as easy to fix.
the MFR housing has a SMALLER exhaust runner than the FC based housings. or rather the MFR housings have very little change in size from the port opening to the end of the casting.
The stock based RB P-port housings exhaust runner is kinda too small with the sleeve in there and kinda too big with the sleeve out.
The MFR has a smaller exhaust runner. You could make it bigger if you wanted, its just Aluminum.
-edit- oh, I see what you are saying now j9fd3s. You are saying even with the sleeve in the stock housing has the larger runner when compared to the MFR.
That may be so, but once you port the face big for NA power it is hard to transition that into the sleeve and get good flow numbers- so people pull the sleeve.
On the MFR housing you port the face to what you want and port transition to what you want and you can port the runner bigger if you think it needs it too since it is all Aluminum.
the FD opens at 71 BBDC, and the MFR opens at 73 BBDC.
the FD closes at 48 ATDC, the MFR is either 65 ATDC or 55 ATDC depending on when it was made. the 55 ATDC housing makes more power.
the FD is 8.45 cm3, the early MFR is 9.2 cm3 and the later port is 12.5 cm3. the later port is better. the later port is wider.
the FD exhaust sleeve ends up at approximately 48mm in diameter (its not quite round), and the MFR port is 43mm.
we we apply some maths;
the FD is a 32mm port with a 48mm runner (well it ends at 48mm), the early MFR housing is 34mm port and 43mm runner. the later MFR is a 40mm port with a 43mm runner.
its pretty clear that the FD port is actually fairly close to the MFR port, but the big difference is that the FD exhaust sleeve makes a funky transition, whilst the MFR port is actually the same ID as the header pipe.
in my simulation, it does seem that a 43mm exhaust is a little small at high rpm
Yep... the street exhaust ports have a huge area change in the port, probably as an attempt to reduce the amount of noise, since the rate of change is much greater than the 7 degree rule.
I wanted to make exhaust port inserts that kept the rate of change to a minimum but was advised not to since this makes the engine much louder.
my simulation engine is a 4 cylinder flathead with 654cc cylinders, as this is as close as it can get to a rotary.
it doesn't do the exhaust port runner, only the header pipe, but with a 43mm ID header exhaust gas velocity is 424 ft/sec@9000rpm, which is around supersonic. increasing the header to 50mm ID drops velocity to 330 ft/sec @9000rpm
the program wants to see between 250-300 ft/sec.
since it is a simulation though, i would not take these numbers as real, but as guidelines.
btw with a 46mm intake runner, intake velocity is 387 ft/sec @9000rpm which is on the high side of just right, this should give a stronger midrange without loosing up top.
We have a 96db limit. Haven't hit it, even when a hole blew open in the rear mid-muffler. (Ancient exhaust system, the .125" wall tubing is paper thin)
I've never had a meter in the car, but when the system is sealed, on the road the exhaust is drowned out by wind noise, tire noise, gear whine, and the rod-ends in the suspension rattling. It's still not very quiet in the car, because there is a LOT of wind noise, tire noise, etc.
11-01-16, 08:48 PM
