i'm playing with a really poorly prepped pro7 car, and when i took the carb off i see that they took out the shutter valve butterfly completely, which is not so good...

i went digging and ive got a bunch of intakes, which one is better and why? the engines a stock port, stock carb setup so i'm looking to maximise power in the 4000-8000rpm range...

1. 81-85 manifold without shuttervalve
2. 79-80 manifold
3. 71-73 rx3 intake manifold, this has the air injection right under the center ports, so its a 100% bolt on
4. 74-76? manifold, its a normal secondary manifold, no air injection under the center port like a 79, but i think its different

 

Hey Mike, are you trying to smog the car? I don't understand why is it not so good that they took out the butterfly? I also heard that at least between teh 81-85 and the 80-79, that the 79 is better because it's got bigger ports?

 

eventually i do need to smog it. i'm more worried about the airflow imbalance between the small runner, and the bigger runner without the butterfly

smog consists of air pump right to exhaust ports, and a new cat, it'll pass

 

I will lend them to you to try out. Roll down to SSX so I can hand it to you along with the SA lighter assembly I owe you.

 

If I recall correctly, vipernecious mentioned in one of his carb posts that you should take out the valve in there because one side is bigger than the other and mazda designed it to allow same flow with the valve in there, other wise your would be running leaner in the one.

 

Mike, I have all of those manifolds you mentioned, except for the '74-'76? It is similar but actually has the ACV air injection casting, and also channels like a '79-'80. It was a direct bolt on to my '83 12A and has lots of power. No need to add devcon or quicksteel to the ACV port in the intermediate plate.

Your '71-'73 manifold, while it does have the ACV casting, does not have channels. I'd highly recommend using a manifold with channels because it has helped the idle quality and power on every engine I've tried them on so far. Just on Monday I swapped 13B manifolds on my REPU from a stock non channeled '74-'75 to another '74-'75 which I cut channels in. The difference was night and day. Better idle, stronger primaries, stronger secondaries. Even part throttle was improved.

It sounds to me like your only option is to cut channels in your '71-'73 manifold. I like to go down about 15mm which matches the stock channels. I've also had success with an '81-'85 manifold cutting one channel after removing the shutter valve. https://www.rx7club.com/forum/showthread.php?t=637323

 

dj55b just for clarity
it is not recommended to take out the shutter valve. This port is bigger and was designed in size this way to match the airflow of the smaller port without the shutter valve.

if your cutting channels, see jeff20, this will change the ports entirely causing bigger openings so allowing both ports to be matched if you bench test them to check for balance. I suspect you'll get lower velocity air speed at near idle, which wont help any in this operating range.

hope this helps
regards
two79rx7's

 

channels huh? i'll have to look more close-like

 

Look at these pictures. They're all 13B but 12A channels are the same.

This was on my bro's '78 RX-4.


Notice the channels in the spacer? I wonder why Mazda did that.




Stock channels.


Left to right, we have '76 cosmo (with channels added), J-spec, '74, '78.


I had to reposition for the camera. '74, J-spec, '76 Cosmo, '78.


On the white REPU. The first setup. It sucked. No channels.


Second setup. I tried the channeld spacer. It was a small improvement.


Sorry no pic of the channeled manifold, but I went back to a standard 4 hole spacer. Big improvement.

I cut the channels as deep as these stock ones.


They don't look as nice as these, but work fine.


Here's the channels I cut in the Cosmo manifold. I need to test it next. They're not as deep due to how the runners are.


Here's a different angle.