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Hello, I'll preface this with saying I know there are tons of threads on porting and the pros and cons of each. I've read as many as possible and have been learning a lot, but still cannot decide on what I would like to go with. I know eventually I'll have to man up and just pick one and go with it. I'm currently leaning towards a half bridge or an aggressive street.
Honestly, the main reason I am porting is for a nice brappy idle. Power is just an added benefit. I would go full bridge but I am concerned about keeping it "streetable." This is a weekend car and not a daily driver, but I do drive it to work maybe once a week in the summer. I still want to be able to cruise in high gear below 3k rpm. My main concern is the "bucking" at low rpm with a full bridge. I have never driven a ported rotary so I don't know what different ports drive like.
I know there is no right answer, but I am looking for more opinions and more knowledge on how the different ports actually drive on the road. I know that more overlap = more brap = less low end torque, but I am trying to find a common ground between streetability and a nice overlap.
TLDR: Whats the best port for a brappy idle without having to worry about "bucking" in low RPMs and cruising at <3k rpm.
Everyone will have an opinion - mine is for a nice street port. The others sound too extreme for what you are after. Harder to tune too.
That's my 2 cents.
Let me get this straight; you want advice on picking a port design because you want a certain SOUND... you don't care about torque band, HP peak, fuel economy, or idle quality, correct?
And you have the skillset and templates to port this engine without cutting into the water jacket, yes?
The low end and midrange power is incredible! I can drive it all day long in city traffic without exceeding 2500rpm. It was kind of a culture shock when I got my stock port 12A running and was like, I have to rev to 4k? This is weak!
The dyno is roughly 30-35% low, thanks to being a Mustang non-inertial dyno and running in 3rd gear to keep the driveshaft speed down at 10k. going by weight/trap speeds... calculates to 270hp or so, so that torque figure is about 170 at peak and about 130 ft-lb at 3000rpm, and it only gets better from there! And this was with an engine that was "tired" after 50k miles of driving and racing, the low end on a fresh engine is much better.
BUT
You cannot cruise. It took a lot of tune finagling, and an ITB type manifold, to be able to CRUISE at 60mph. At speeds much below that the engine is not under enough load to run smoothly, so you have to keep coasting in Neutral for a few seconds, shift back into gear and accelerate, coast again, etc. And driving in the city you HAVE to shift real early in order to keep the engine loaded enough to run smoothly. I blame driving bridge ports since 2010 for my wrecked left knee, and I am not being facetious. I have at least 80k of street driving a bridge port of some form or another.
It is a misconception that overlap kills low speed power. It actually increases it dramatically. What it kills is low load drivability. On the street you spend 99.9% of the time making 25hp or less and this is exactly what big ports are bad at.
Next issue is exhaust. High overlap engines do not tolerate any backpressure. The Racing Beat long primary exhaust system used to be a good balance of quiet enough to be sort of acceptable on the street and restriction-free enough for a bridge port to be happy (as happy as they can be, anyway) at part throttle. RB changed suppliers and the new exhaust is much, MUCH louder. And it is failure prone thanks to its stainless steel construction. And acceptable exhaust noise on the street has changed a lot, now if you can hear the engine over tire sing then you risk getting a noise ticket.
TLDR: Whats the best port for a brappy idle without having to worry about "bucking" in low RPMs and cruising at <3k rpm.
Thanks,
JP
Been driving my '81 because of course I have (has nothing to do with my Volvo's liter-a-month appetite for CHF11S, or its tires that are almost worn to air but I need winter tires and I don't want to soend $2000 for tires for the car in the space of a few months) and this made me think.
ALL rotaries buck. All of them. I have a '79 manifold on a stock port 12A, and at anything under 3000 and decelerating it buck buck buck bucks. That is why Mazda put the shutter valve in the carbureted models starting in '81, it eliminates the bucking by closing off half the engine and pumping air pump air into it. And why the EFI rotaries have a selective deceleration fuel cut, they cut one rotor long before the other one.
The more overlap, and the more restrictive the exhaust (the two are entwined) the less vacuum it takes to buck, until it actually still does it under light load.
If it is brapping it is bucking, period. If you want the sound and no drivability concerns, get the absolute heaviest flywheel you possibly can, it will help to a degree. Performance will suck, though.
A superr large street port will give u the same braps as a Bridgeport once u got it tuned to the right rpm on the ecu . Plus ur side seals wont get scrapped/damaged from the Bridgeport. Tell ur engine builder give me the biggest street port u can. Look at ir performance stage 3 street port. I swear you'll think it was a bridge port if they aint tell u it was a aggressive street port.
Super large street ports don't have nearly the same sound.
And super large street ports are the ports that are hard on side seals!! Opening them earlier causes the ends to be unsupported and they stub on the closing edge of the port. This is why bridge ports are more reliable, the bridge is kept low and this supports the side seal ends.
Super large street ports don't have nearly the same sound.
And super large street ports are the ports that are hard on side seals!! Opening them earlier causes the ends to be unsupported and they stub on the closing edge of the port. This is why bridge ports are more reliable, the bridge is kept low and this supports the side seal ends.
You are correct that large street ports can leave an unsupported side seal however the only ones that 'stub' on the closing edge are just poor/out of date ports. If you 'scissor' the closing edge by keeping it lower than the leading edge as well as giving it a sharp point the side seal will ramp up and clear before there is any way to blunt the trailing tip of the side seal.
To answer the OPs question though, 'brap' is essentially just a function of overlap. You'd be able to achieve it with a shittily late closing exhaust port and a tiny street port, or anywhere in between so you'll have to choose what you want for power etc and go from there. What I CAN tell you, however, is that the more overlap = the worse it is to drive almost 100% of the time.
As a footer, you can also achieve brap by having bonkers idle advance and fiddling with an ultimately poor tune but that that point is just a pose.
Anybody else notice that OP (JPsSparkYellow) hasn't been back to check on the status of his question? Maybe he just stopped by to troll a bit, and he got a few bites!
Anybody else notice that OP (JPsSparkYellow) hasn't been back to check on the status of his question? Maybe he just stopped by to troll a bit, and he got a few bites!
I'm still here Thanks for all the responses.
I think I will end up doing a larger street port, one that pulls the port down and advances the timing by a little bit. I'd like a little overlap but not too much to decrease the drivability by a lot. Anyone know of any street port templates that pulls down the intake port?
Its mind blowing, but as far as I can see no one currently makes a large streetport porting template that doesnt damage the leading sideseal tip!
In the past "Judge Ito" and "Extreme Rotaries" both offered large street port templates that were leading sideseal safe.
I have spoken to one of these big name engine builders with 30+ years experience that do their streetports wrong and they say streetports just wear the leading sideseal and the side housing at low rpm. When I mentioned how to easily fix that (square up the outer edge of the port for continuous scissor), they just admitted that yes, that will stop the wear.
If there is an arguement for allowing this sideseal and side housing damage/wear (such as making much more power) I havent heard it. It just seems to be builders saying this is how we did it for 30-50 years and we arent going to change now.
If you will be porting the engine I suggest you read up and make your own porting templates as I do.
1) Assemble your front housing/gear, rotor (with oil seals the 1st time you do this), rotor housing with the locating dowels and e-shaft on your engine stand or bucket.
2) Cut a piece of plexiglass to accomodate the e-shaft sticking through.
3) Mark the centers of the dowels and drill the plexi for a tight fit with the dowels.
4) Mark the outline of stock ports.
5) Put in two sideseal springs with a tip bent up where they would meet a common corner seal to just sit above the rotor slot for two side seals. And rotate the rotor backwards to scribe a line inside the plexi to show you the path of the sideseals tips.
6) Base your porting template on this with the knowledge you have gained reading about porting and what you see when rotating the rotor around (now without sideseal scribes in it).
Some tips-
1) Always keep the vertical sideseal tip on the sidehousing 100% of the time.
2) Allow the horizontal sideseal tip to drop into the port for early port timing AND make sure the horizontal sideseals tip does not hit the opening line of the port (outer most side) at any time but port closing.
3) Make sure on port closing the horizontal sideseal comes back onto the sidehousing continuously from the inside of the port that remained on the sidehousing surface to the outside of the port (this is called "scissor"). Do not let the sideseal tip come onto the sidehousing until all of the rest of the sideseal has (this is what all the Large Streetport templates get wrong by having a rounded outer top corner).
5) Dont take my word for any of this. Do your own experiments and figure this out.
Example, the horizontal and vertical sideswal tips tracks get closer together the later the port timimg. Maybe you can have a rounded outer corner on an earlier closing primary port and maintain horizontal sideseal "scissor".
General rules on porting-
1) Early port opening will increase power at all rpms.
2) Later port closing will increase power at higher rpms.
3) Any increases in port volume but especially port runner volume will hurt power at low rpms due to decrease in velocity (revisit rules 1 & 2)
My "Large" Turbo Streetport template (much earlier closing timing than Naturally Aspirated Large Streetport Timing).
. Note the lines scribed by horizontal (inner lines) and vertical (outer lines) sideseal tips.
How the port looks compared to stock port.
"Extreme Rotaries" porting template showing the ports squared up outer top corner.
08-02-22, 01:38 PM
Thanks for all the responses.
