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Old 03-11-10, 04:41 PM   #1
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NA and porting, let the secrets out...

I know what has worked for me in the past. But 99% of my port work has been done towards turbo. And in the past few months i have spoken to a few well known builders/racers about porting and alike. Some of them completely contradict eachother. But all are very successful. Leaving me, an up-and-comer, confused...

One builder told me that porting should be focused mainly on the intake because the exhaust ports from factory are plenty large enough. and you should never port towards the closing edge on the exhaust, and not is much needed towards the opening edge. Basically leaving them near factory sized.

Another told me that the main problem with making power is the exhaust ports can not be ported big enough!

TWO COMPLETELY DIFFERENT ANSWERS!!! both are very well respected...
So, for example... the first person is basically saying, barely clean up the exhaust ports but put a 2" PP??? sounds super wrong to me. The second person is saying, if we could only go larger on exhaust ports, we could have a 2" PP with the 4 ports on the plates as well.... makes more sence. But how much porting is too much porting. Wouldnt to much intake area remove the vacuum sucking action and basically lose all power? same for exhaust and back pressure?

Not to mention Square Vs. Round Vs. D shape Vs. MFR style.... On PP and exhaust ports.

Most of the big timers keep everything a secret. I dont understand why a small community such as the rotary world, keeps everything they do so hush hush. The piston community has the other 98% of the world working together and sharing tips. Our little world builds in the dark keeping rotarys behind and slowing our growth...

who wants to dive in on this?
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Old 03-11-10, 04:44 PM   #2
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Some of the fastest NA rotary cars in the world are semi pp/bridge motors. Proving that its a bad *** combo and in my personal opinion, the BESZT way to go for NA power. But if there was a way to make the exhaust ports even larger... would you lose power, or gain power?
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Old 03-11-10, 05:23 PM   #3
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too many questions. You talk about the piston community, You just asked "whats the perfect cam?"

How heavy *light* is the car? what rotors? what rotor housings? what Irons? what tranny? diff? Dyno queen peak power or wide autoX power band? how much street driving? what kind of idle can you live with? how quiet must it be? Carb or EFI? MPG? how much fab work do you wanna do on intake and exhaust? what flywheel? how long do you want it to last? How important is throttle response?

Edit, I put down 240rwhp with a custom shape low overlap BP, the exhaust still had its stock closing time and was not much bigger than the stock 13b-re

Edit 2, What most don't think about is that when you open up a port you lower intake / exhaust velocity, you can go too big. My intake on that engine used a primary runner that was 2/3 the area of the secondary to keep velocity up. It was also a dynamic intake with a long and short path, The long path was set for peak performance at 2800 rpm *5th gear at 75mph* and the other was 8000. My next engine will have another path in the middle and the same near stock exhaust port.
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Old 03-11-10, 09:53 PM   #4
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What's everyones views on porting the primary ports to the same size as the secondary ports even if it's a large street port,logistics aside,would it yield a higher H.P?
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Old 03-11-10, 10:54 PM   #5
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Edit 2, What most don't think about is that when you open up a port you lower intake / exhaust velocity, you can go too big. My intake on that engine used a primary runner that was 2/3 the area of the secondary to keep velocity up. It was also a dynamic intake with a long and short path, The long path was set for peak performance at 2800 rpm *5th gear at 75mph* and the other was 8000. My next engine will have another path in the middle and the same near stock exhaust port.
Did you have some sort of switch between these intake paths?
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Old 03-11-10, 10:59 PM   #6
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bumping for possible great justice.

I'm curious about this myself. I was going to install a TII in my 1990 GTU N/A but opted against it just to save money. With the money I was going to spend rebuilding the TII and installation and all that, Im going to use it to just buy other mods.

Ive been searching for a good N/A porting thread that would answer questions like What are the differences between the different types of ports out there? What upgrades should I be looking into to accommodate these ports? etc...but I must be looking in the wrong areas.
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Old 03-12-10, 01:30 AM   #7
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Quote:
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What's everyones views on porting the primary ports to the same size as the secondary ports even if it's a large street port,logistics aside,would it yield a higher H.P?
Again it falls to the questions in my first response. The thing is, If you have a bridge on the secondary and stock primary you will still have the overlap as if they where bridge unless you find some way to close the port at the rotor. Where the advantage of small primary ports lay is if you have the primaries and secondary on a separate plenum or throttle plate than the secondary and it is progressive, This way you can get a higher intake velocity on the primary throttle plates / ports then get a lot of air mass for high rpm. If you run ITB or single plenum then there is no advantage to running offset port size.

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Did you have some sort of switch between these intake paths?
Yes, sorry its so crude but... Blue is throttle body, black is plenum, red is intake valve *slide plate*, green is engine.
Click the image to open in full size.
Attached Thumbnails
NA and porting, let the secrets out...-intake-jpg  
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Old 03-12-10, 09:27 AM   #8
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As far as I'm concerned there are no secrets when it comes to porting as it is all pretty well understood by many. There are either those that will tell you what advantages or disadvantages different types have or there are those that have no clue how it works and don't want you to know it. That's their real secret! Just because someone tells you all they can doesn't mean they actually tell you what they have which is why I think "secrets" are a bit strange. You can give knowledge without giving away setup secrets. I personally feel that the biggest "secrets" are in tuning. This is where many give the least attention and many that do still don't know how to do certain things.

As far as porting goes, there is no one right answer. There are only answers that fit a particular driving style/need and even then no one correct way to do it but rather usable options. Engine setups also don't have to be complicated to work well. Many times in an attempt to get all they can, people will try to over think the problem which adds complexity and ultimately tuning difficulties. Start simple. Be realistic.

As an example to this, a year ago I bought a 90 FC off of a forum member. One of the auxiliary ports was stuck closed. Unfortunately it is going to take removing the manifold to get it freed up enough that the actuator can rotate it. I could turn it with pliers though. The quick solution was to wire them open which I am normally against. In my case it was temporary. I have driven around for the past year with it this way. I decided to fix it. I was hoping to turn it enough without removing the manifold that it would eventually free up enough. That didn't happen. I hooked it all back up to stock. Now only one of them opens above 4000 rpm while the other doesn't. Obviously this kills top end. Guess what? I'm happier driving it! Why? I drive 100 miles a day in this car. That's my work commute and much of it is on the freeway. I noticed that nearly all of my driving is below 4000 rpm with the occassional bump higher when I want to play around. In my case it is better to leave them closed. My drivability has gotten better and so has my mileage. I am still planning to fix it though.

Why am I telling you this? It goes back to being honest about what you want. Is this a race car? Is it a daily driver? Which is more important? No one, regardless of what they claim, has a race car that is streetable. They may have a race car they drive on the street though but if you are making sacrifices in order to make this happen, it's not streetable. A Cessna or an F1 car could physically roll down a street. Not streetable.

What I'm getting at is that it's possible to get completely opposite answers from different people. This could be based on what they do or even do not know. There are lots of people that think go as large as you can and rev the crap out of it. Then there are those that preach a lower peak power but with a wider powerband. Then it all changes with different uses such as street driving, autocross, road racing, drag racing, etc. Each one has different requirements.

There are certain port shapes that just don't work as good as others though no matter what the use. The reverse D exhaust port is one example. It doesn't mean people don't do it though. They just don't know any better. Basically when the bottom is flat but the top is round is what I mean by reverse. There are other things that can cause issues too but just remember that some people don't know any better and give an answer and some do know and give one. Even if they are both knowledgeable you may still get varying opinions.

This is a tough one to answer.
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Old 03-12-10, 11:40 PM   #9
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Yes, sorry its so crude but... Blue is throttle body, black is plenum, red is intake valve *slide plate*, green is engine.
Click the image to open in full size.


I built my 20b intake with the same philosophy. Long and short runners. Love the drawing and straight shot concept with the secondary runners! So how was the bottom end with the BP? Did you have transition/buckling problems with your controlled secondaries? I transition my secondary's at 3,200 rpms.
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Old 03-13-10, 05:15 PM   #10
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all ports had two runners each, the plenum has a chamber for primary and another for secondary, the throttle body was spread bore progressive. The long runners for primary ports are longer than the secondary by about 4"

It was not a "bridge port" as most use, it was designed to have as little overlap as possible with sacrificing as little port area as possible so I cant compare it to say the templets sold by mazdatrix. Low end was damn good but due to the extreme difference in tuning there was a sizable gap in the mid range. the key is a slow transition if it snaps open the ECU has no clue whats going on. the slide plate also helps give a smooth transition but they can bind on turbo cars.
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Old 03-13-10, 10:13 PM   #11
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I'm actually about to deliver two engines two be built into one and was agonizing over what porting to go with. After doing a little research here, and on nopistons, I came up with the most definitive answer I could. I'll never be sure. First I wanted a bridge, but figured with a 100 KM drive to work, and mostly city driving otherwise that would be impractical. I also autocross so any tight course would leave my engine gasping. After more reading I realized the best option for ME would be a medium streetport on the primaries and a wild streetport on the secondaries. My engine will be a 4-port 13B using SE housings and rotating assembly with my 12A plates. An RX-4 manifold and my Sterling Nikki with mechanical secondaries tied it all together to make it all clear. A medium streetport with my secondaries closed will allow me to consume only a little more fuel than I do now while cruising to work or puttering around the city, but still give me a decent bump in low end power. Crack the secondaries at 4K and the mid/high end gets a workout. This should provide me with a fairly broad powerband that will be useable on tight autocross courses. And if the craving hits for more power, I just have to open her up and bridge the secondaries to keep the streetability while adding top end.

As stated it's all about application. Think long and hard about what you actually use the car for and the answer will smack you right in the mouth.
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Old 03-14-10, 11:49 PM   #12
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I'm actually about to deliver two engines two be built into one and was agonizing over what porting to go with. After doing a little research here, and on nopistons, I came up with the most definitive answer I could. I'll never be sure. First I wanted a bridge, but figured with a 100 KM drive to work, and mostly city driving otherwise that would be impractical. I also autocross so any tight course would leave my engine gasping. After more reading I realized the best option for ME would be a medium streetport on the primaries and a wild streetport on the secondaries. My engine will be a 4-port 13B using SE housings and rotating assembly with my 12A plates. An RX-4 manifold and my Sterling Nikki with mechanical secondaries tied it all together to make it all clear. A medium streetport with my secondaries closed will allow me to consume only a little more fuel than I do now while cruising to work or puttering around the city, but still give me a decent bump in low end power. Crack the secondaries at 4K and the mid/high end gets a workout. This should provide me with a fairly broad powerband that will be useable on tight autocross courses. And if the craving hits for more power, I just have to open her up and bridge the secondaries to keep the streetability while adding top end.

As stated it's all about application. Think long and hard about what you actually use the car for and the answer will smack you right in the mouth.
Your thought pattern is correct only if you do not have the primary and secondary ports exposed to each other in the intake (like a stock 12a intake on the second rotor). Primary to primary and secondary to secondary is alright though.
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Old 03-15-10, 08:40 AM   #13
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True, I think most stock intakes have a passage just under the spacer between the primary and secondary on one of the rotors, I can't remember if it's front or rear though. I have to take a closer look at the 13B intake you sold me, I can't remember if it has one or not. While there will be some vacuum from the secondary ports, the secondary throttle plate on the Sterling should do a good job of closing it off.
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Old 03-15-10, 06:25 PM   #14
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On the bridge port engines i build i only port the exhaust to close later. Why? Becausse Mazda MFR PP housings the opening of the port is raised 4 or 5 mm and that will gain torque. MFR port is round and flows 160 cfm and my exhaust ports flows between 180-200 cfm, mainly becausse of it wider and rectangular shape. On the intake i first streetport it for maximum VE, that is as late closing as possible and make the bridgeport at an angle of around 15 to 20 deg. These 2 ports flows ca 100-110 cfm. My rallycross 13B bp makes 260 hp at 8000 rpm and 170 ft/lbs torque with a Weber 51 IDA and a FB dizzy with 2 MSD 6A boxes.

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Old 03-15-10, 08:32 PM   #15
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Your thought pattern is correct only if you do not have the primary and secondary ports exposed to each other in the intake (like a stock 12a intake on the second rotor). Primary to primary and secondary to secondary is alright though.
Thats what i said , you also want to tie the primarys to each other and the secondarys depend on what the aplication is but having a plenum on the primarys and not on the secondarys can make some odd things happen depending on the carb or ECU.

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True, I think most stock intakes have a passage just under the spacer between the primary and secondary on one of the rotors, I can't remember if it's front or rear though. I have to take a closer look at the 13B intake you sold me, I can't remember if it has one or not. While there will be some vacuum from the secondary ports, the secondary throttle plate on the Sterling should do a good job of closing it off.
just have a nipple on each plenum and use a T connector to tie them to the MAP or if carb just cut a inch square to connect the two plenums right at the carb.

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On the bridge port engines i build i only port the exhaust to close later.
I am against this 100% over lap for VE is WAY over rated, I vote radius the edge and done for NA, and never run with out sleeves. I also run tear drop shaped bridge's, Keep that over lap to a minimum.
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Old 03-15-10, 09:49 PM   #16
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I also run tear drop shaped bridge's
I've been thinking about this...

So, I'm imagining a teardrop bridge only at the top half, but having the main port move earlier, like a street port, at the bottom. Is there any issue with trying to lift the end of the side seal? Radius and be done?
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Old 03-15-10, 10:49 PM   #17
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I've been thinking about this...

So, I'm imagining a teardrop bridge only at the top half, but having the main port move earlier, like a street port, at the bottom. Is there any issue with trying to lift the end of the side seal? Radius and be done?
main port? as in primary or the big part of the secondary? the tear drop shape is to lower overlap. If you keep the bridge open all along the rotor housing you do not lower overlap at all, regardless if you have a bridge you want to keep the larger port out of the side seal area.

I say for the exhaust port just radius the edge, don't open it up much if at all. If you make it open early torque is hurt, late and you start getting overlap also hurting torque. The problem is, they are to big so they kill velocity, they flow enough exhaust for any NA except for PP's
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Old 03-15-10, 11:04 PM   #18
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I say for the exhaust port just radius the edge, don't open it up much if at all. If you make it open early torque is hurt, late and you start getting overlap also hurting torque. The problem is, they are to big so they kill velocity, they flow enough exhaust for any NA except for PP's

I haven't done anything with my exhaust ports yet. I'm trying to see my max potential on stock ports and (hopefully) proper scavenging with my exhaust. I hear the radius term all the time but have never asked exactly what that means. When you say radius, do you mean just rounding out the 2mm bevel to reduce turbulance?
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Old 03-15-10, 11:50 PM   #19
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When you say radius, do you mean just rounding out the 2mm bevel to reduce turbulance?
yep

Edit, I should add that in the past 6 or so years i have only used 13B-REs only for NA builds so I don't remember off the top of my head the size of any other exhaust port
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Old 03-16-10, 05:04 AM   #20
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Dig your sig, nillahcaz. lol
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VE [ (Displacement x Maximum RPMs) / 1728 ] + 15% = all the cfm your NA engine could ever use. Just do the math.
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Old 03-16-10, 12:10 PM   #21
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I am against this 100% over lap for VE is WAY over rated Keep that over lap to a minimum.
i agree. if you spin the rotors around on a partially assembled engine its pretty easy to see why.
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Old 03-16-10, 12:29 PM   #22
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Just because a radius is good doesn't mean a larger one is better. On the exhaust ports a 1/8" radius flows more than a 1/4" radius. Strange but true.
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Old 04-04-10, 07:44 PM   #23
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Why does Mazdatrix say no to exhaust porting on a 6 port iron?
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Old 04-04-10, 09:02 PM   #24
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So I did the half bridge thing to my Turbo II block. I learned a few things from it, not in any particular order.

First, bridge porting makes the Turbo II manifold even more restrictive, and the runners are too short. I spent a lot of time and head scratching converting a S4 N/A lower to four-port. (You port to air in a few places, need a LOT of epoxy filler, and it doesn't fully cover the center housing's air injection port)

With the S4 N/A manifold giving a more proper intake tract length, with MUCH nicer bends, and a larger primary plenum, it pulls like a freight train from 4000 to... I back out at 8500. The street port pulled from 5500 to 8000 just like a stock port 6-port or my street ported 12A. I had to pull out a lot of fuel in the sub-1200rpm range, and had to add a TON of fuel in the 2000-5000 range. I also added a little in the 7500-9000 part of the map but not very much. The light-switch 6000rpm power boost is gone, it's just seamless Big Fat Torque. Exactly what I was shooting for.

Ignition timing is even more critical. I run 20 degrees all-in right now, and for better idle and city cruise, I put one advance spring back in. 20 degrees is too much timing at idle, it makes manifold vacuum too high and that makes it run too roughly. I also found a ported vacuum source and connected it to the vacuum advance can to bring back the advance when it can be better utilized.

This, however, is THE most important thing: My car idles almost 100% on the secondary ports. The idea is, under light throttle and cruising on the highway, the vacuum in the secondary side of the manifold is still going to be relatively low, so it won't be pulling as much exhaust gases up during the overlap period.

It drives slightly BETTER than the street port (with TII manifold) did. It is getting slightly better fuel economy as well. The only real difference is having to slip the clutch a little more, and it "idles like a Top Fuel engine" at 1200rpm idle. No brap, just crackle.

I love it.

Now, the only problem, is that I'm cruising at 60KPA instead of 45-50KPA and there is more exhaust energy as a result. I'm looking into different muffler options. Under WOT or putting around in the city, it's no louder.
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Old 04-04-10, 11:17 PM   #25
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Gratz Peejay and welcome to the world of the bridge. I will need to try the idle on the secondary thing when i find a vert for what I'm looking to spend. My bridge ports are lower overlap already *comparatively* this may just top it off for MPG and size of power band. I also am going to tune the exhaust a little higher than my long intake runners as last time they where both the same and my there was a flat spot in power before the short runner efficient.
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Old 04-04-10, 11:17 PM
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