12a intake port dimensions
 

Hey guys I was wondering if anybody with a caliper can take measurements of the stock intake ports (Length and width). My engine is ported but wanted to know how much bigger the primaries mainly are bigger than stock.

Port sizes vary from year to year. I'm assuming you want the rotor side measurements even though the intake side is critical too.

When I took my motor apart and wasn't sure if it was ported, I bought a Racing Beat template and layed it on there. Keep in mind 3b plates in 74 were larger from the factory. But the pics should give a good idea.

https://www.rx7club.com/showthread.p...+%2Bstreetport

I actually meant from the intake manifold side, its for intake manifold porting purposes.

My Primaries on my car are about twice the size of what the gasket area is. I'll try to post up pictures of the outline from the squished gasket that is on the manifold right now.

To me it just seems like I might be loosing quite a few ponies from this.

most intakes do have primaries smaller than the actual ports. It's by design. Supposed to help with reversion.

I don't recall it being that much of a difference though. I know we're suppoe to leave about a 1mm lip around the whole manifold for that.

Here's the pictures of the imprint from the gasket. If you look at the primary ports, how far up and down should I be extending them? Also something that I just noted is if you look at the first picture, port on the left has an air opening hitting the engine, think i should weld a bead on there or something to close it up and then just get it down to the proper size?

https://www.rx7club.com/attachment.p...1&d=1269633153

https://www.rx7club.com/attachment.p...1&d=1269633153

https://www.rx7club.com/attachment.p...1&d=1269633153

https://www.rx7club.com/attachment.p...1&d=1269633153

You should be safe opening the primaries up to the point that the "overlap" is equivalent to what you have on the secondaries.

No, you won't.

https://www.rx7club.com/showthread.p...aw#post1044131

Tall intermediate runners came out in '76 along with the reverse runner manifold for 12A and 13B. Mazda kept the tall port intermediate runners but went back to regular flow manifolds. According to Paul Yaw, this was a good idea. I would agree. Plus it keeps backwards compatability if you happen to run across the odd reverse runner manifold in your travels, and wish to use it. :D

Thanks peejay! I stand corrected. As I originally stated I knew the purpose of the offset, but didn't realize the extent. I apologise. I knew port "matching" was bad. But was underthe assumption that you didn't need that large of a difference to accomplish the desired effect. Thanks for the link. I'm better informed now. ;)

Personally I would only match the secondaries.

I have seen some consensus that the best port mismatch is on the order of .060".

Clearly, the 12A manifold have a bit more mismatch than that :) The main issue with the manifold is that the port is trying to turn right there, and the port is shaped exactly wrong for turning in that direction.

Notice that the FD engine's intake ports are no longer parallel with the ground? They were trying to eliminate that turn as much as possible.

The 20B is even worse. The reason those runners look so tall is because you're seeing them sliced at a pretty steep angle. They're really not that large. Just look up inside the lower manifold before they begin to curve.

Not a great picture, but here you can see the center ports on one of my motors. Theyre actually almost too tall for my RB manifold to cover them. Im actually concerned about it. I have a RB Holley manifold that someone welded some extra material on to cover the area better, but it just seems sketchy. Its semi coincidental though someone did that, because the manifold/carb came off a different street ported motor. Had they not done that welding, I think it would leak right there for sure because the port would be taller than what the manifold covers.

~T.J.

http://i54.photobucket.com/albums/g8...l/DSC01340.jpg

Hmm some good food for the though there.

Here's a few shots of what I've done with the upper, the bottom one I haven't done too much but clean up the casting a bit. The 2 extra holes are for my 2 fuel injectors, I wanted them to shoot just a bit more towards the secondary side and I think I pretty much nailed it.

Peejay, why do you say putting a 1/4" radius up in the divider is better though? Its only pretty much at one point that it gets sharp at if it makes a difference.

https://www.rx7club.com/attachment.p...1&d=1269809915

https://www.rx7club.com/attachment.p...1&d=1269809915

https://www.rx7club.com/attachment.p...1&d=1269809915

https://www.rx7club.com/attachment.p...1&d=1269809915

https://www.rx7club.com/attachment.p...1&d=1269809915

Air flows out of a sharp edge just fine, but does not like flowing into one. (This is why a step out is beneficial - it discourages reverse airflow while still flowing forward)

Think of air horns for a carb - are they just straight pieces of tube or are they bellmouthed?

It's the same theory as porting on a multivalve head - the port divider is sharp edged in the exhaust side and rounded on the intake side.

Hmmm ... never thought of it like that, If I were to get those ports rounded again, what would I do that with?

My goal is to try and finish off this whole thing before next weekend, so I'm straped on time on what to really fully do. I'm definatly going to clean up the casting a bit more though.

Here's a thought though, If I roughen up the edges towards the bottom more as in to create more turbulence around the edges, would that help increase the velocity down there? I'm just thinking in the sense that turbulent air is represented as kinda static and most of the air would travel closer towards the center correct? So say I cut up some decent groves around there (kinda like what they do with water passages on the engine) do you think that would help?

Well a bit more updates on this, I took out those sharp edges as recommended. I just took some 60 grit sand paper cut it into 1/2" strips and worked it from one hole to the other to smooth out the top of that edge. Also I got around to getting a sanding drum and finishing the inside with 80 grit all around to give it a nice "rough" finish.

Hate to jump into your thread here, but I have a 12a IDF intake that the previous owner port-matched. What can I use safely to "rebuild" or build up the edges again? I've got JB weld, but havent used it in this manner before. These intakes are hard to come by, I'd like to have it "flow" right so I can put my IDF on and tune it once rebuilt.

One option, that may exist for Ray, is to hog out the runners on your engine if it's apart for a rebuild.

hey dj55b, I'm helping PercentSevenC do a weber DCOE boosted by an S5 turbo and it looks like someone attempted to smooth the insides of the RB upper and lower manifolds. They appear to have thankfully stayed away from the areas where you sharpened, and later rounded yours. Just an observation I made today before we dropped it back on the car for test firing after boost prepping the carb.

Alright,

So here's a few shots of the bungs installed in there and cleaned up from the inside more. I'll get pictures of the lower intake manifold later on.

https://www.rx7club.com/attachment.p...1&d=1270150224

https://www.rx7club.com/attachment.p...1&d=1270150224

https://www.rx7club.com/attachment.p...1&d=1270150224

https://www.rx7club.com/attachment.p...1&d=1270150224

https://www.rx7club.com/attachment.p...1&d=1270150224

https://www.rx7club.com/attachment.p...1&d=1270150224

Here's the final pictures of the intake before putting it on the car. I know i could of spent more time trying to further clean it up, but i needed a long bit and couldn't find one locally and i'm running out of time.

https://www.rx7club.com/attachment.p...1&d=1270406737

https://www.rx7club.com/attachment.p...1&d=1270406737

https://www.rx7club.com/attachment.p...1&d=1270406737

https://www.rx7club.com/attachment.p...1&d=1270406737

https://www.rx7club.com/attachment.p...1&d=1270406737

https://www.rx7club.com/attachment.p...1&d=1270406737