Open plenum intake manifold?
 

Funny thing. Was looking at some of the stuff peejay played around with, and also saw references to an open plenum Nikki manifold that had been made here: http://www.nellump.net/peri/epi/firs...keOptions.html

I'm just wondering if anyone else had tried playing around with this or knew anything. I can't help but be curious, since we're looking at moving our race '7 into Production instead of IT...

Paul Yaw makes (made?) open plenum manifolds for E/Production. Claimed that the setup would make more power than the rules-permitted alternate carb: a 48IDA with 38mm venturis.

I haven't played around with it because I didn't have the time to recalibrate everything inside the carb, being that the car was not only my daily driver but I was hitting the dragstrip once or twice a week. I still like the idea, though.

Hmmmm. That's interesting. Might have to break out... ah... something capable of removing metal faster than a die grinder. Hum.

Just use a die grinder for hours and hours and hours and hours and hours and....

At least you can start with an '81-85 manifold... the degree of work involved isn't that much more work, and the manifolds are practically given away.

You will need to have replaceable air bleeds cause like peejay said, you will need to re calibrate the carb to run right.

The open plenum intakes I keep on hearing about, does the "open plenum" mean all 4 runners open to one big hole under the carb, or do they open to two big holes keeping the rotors seperated but allowing the primaries and secondaries to join into one?

The reason Iam posting this question is. I have tried having a open 2"spacer under a stock carb with stock intake on a street ported 13b and for me ran like shit. I never tried driving it, only reved and then took it off. I have always thought about opening the primaries and secondaries and keeping the rotors seperate, but had a bad taste in my mouth through my previous experience and didnt want to waste the time and money for nothing?

Nikkis arent designed to run on an open plenum. They are calibrated for a strong vacuum signal you get with seperated runners. Youd have to re cal the whole carb to get it to run. A near future project for me. ;)

So basically what you are saying is, I had the right idea, just didnt have the carb tuned for that application?

Supposedly. Look at what peejay has tried and written about. Makes perfect sense to me.

Paul Yaw has a picture on his website of a manifold that he hogged out so it would effectively be one big plenum. He admits though that it was too big and that the torque peak was shifted up to 8500 rpm!!! Way too high. That was for a Nikki carb. The plenum wasn't even that big. It just didn't work with that combo. On the Holley maifolds you can pick up some mid to top end power (at the expense of low end) by joining the corresponding ports together (primary to primary, secondary to secondary). This in essence gives you a dual plane manifold. The key is to not make the crossover between them too large.

There is much more to the way the carb behaves than just joining runners together. When the carb feeds a common plenum, you effectively have a larger carb in regards to cfm. This is a little hard to understand without seeing it in pictures. Just for the examples sake lets say that all 4 of our barrels flows 150 cfm a piece. One barrel feeding one runner means that the runner can receive no more than 150 cfm through the carb, duh. Now if we share a common plenum with 3 other barrels we have a total 4 barrels at 150 cfm a piece for a grand total of 600 cfm. Now before people go off and say that the rotaries ports are always open, which is for all intents and purposes true, remember that the intensity that air is traveling through the runners is in pulses and not a constant rate. When one runner or in the case of the rotary, 2 runners per rotor needs air, instead of only having 150 cfm (300 for 2 runners) of available air it now has 600 cfm from which to draw from. This obviously decreases low end power since there is less velocity through each barrel of the carb. At higher rpms this becomes irrelevant and power actually goes up since the fuel has more options on where to go instead of waiting for the next major pulse to direct it down a single runner. Instead all the fuel will tend to favor the strongest pulse and go where it is needed the most for power. Obviously we still come to a point where the car just can't flow enough air but this explains alot of why individual runner setups vs open plenum setups work the way they do. Plenum size and runner length still play very important parts as well. The dual plane plenum design is a good compromise of low and high end power so long as it is a dual between both chambers and not a dual plane that seperates the rotors like the the weber setups do. This in essence is still an individual runner system. Each setup has it goods and bads and each system can be made to work very well in a number of ways. This was just a generalized explanation of how things work the way they do. Sorry to ramble.

So, its kinda like one big reservoir of air/fuel mixture that can be drawn into the runners at any point in time?

~T.J.

Well said rg. :) If you look, the stock 79-80's are of the dual plane design you speak of. Compromises are necessary sometimes.

That was my take on it, but when I tried to put it to real world use, the open plenum ran like shit for me? With a stock carb.

How does a 1" spacer with the rotors seperate but the prims and sec open sound? Or does it need more room than 1"? For it to run good do you need to start opening the intake, to make the dual plane longer than an inch?

Tried the 1" open spacer and ran like shit!

this is the stuff i wanted to hear about. ie 1-2 inch 'spacer'

That is the open spacer that ran shitty, I use a 1" seperate runner spacer and seems to smooth the engine, similar to using msd blaster 2 coils.

When I first put the open spacer on, when you would open the throttle wide open the engine would bog and almost die, but if gradually applied gas it would run. Tried leaniong it out still no help, seemed like it didnt bog as bad, but a bog is a bog! Then when running it on the highway still ran like shit, so I took it off.

Did you ever mess with the accelerator pump?

No, just the jets. Do you think that would help? I dont know much about carbs, Iam trying to learn as I go?

How about opening up the Secondaries to each other, and the primaries being indivual, or is that something that wouldn't work on a First Gen like I'm thinking it would?
Do the secondaries open up seperately, or are they all at once?

On the intake manifold, the 2 inside runners are the primaries, and the two outside runners are the secondaries. That is why you can't join the secondaries together, cause they are on opposite sides of the manifold. If you can find a way then that would be something cool to try :)

It could help. The accelerator pump is designed to squirt out a little bit of gas when you floor it. If it shoots out too much gas, it bogs, if it doesn't shoot out enough, it bogs. When really changind the intake setup by joining all 4 barrels, then the AP's adjustment is probably thrown way off.

I don't have any real experience here in tuning carbs, I'm just going on others experiences.

noooooooo, right below the carb, arent they in a square?
like
PP
SS
or something like that?

I had thought about only having the sec open and leaving the prims seperate, but I dont feel like going through the work for minimal gains or no gains. I hate to work on my car then it run shittier than before and have to work on my car somemore to get it back to where I started. I just dont see how the flow can be smoother when it has to hit the top of the intake man, then find the runners to go down. If I had endless cash, it would sound like a good idea to open up the intake man and make that transition all open and smooth. While I was running the 1" open spacer, if you felt the intake under the carb, it would be real cold. To me that would mean, alot of the a/f was hitting the top of the intake and as the gas evaporated it cooled the intake. Which doesnt sound efficient to me?

I have my prims and secs opening up basically at the same time.

I dont have the resources or knowledge to play with the accelerator pump, yet! But is a good idea, I had the same thought.

I have 1984 rx-7, I am running a holly 390cfm carb on a homemade adapter with stock manifold. So the adapter
is 1 inch thick and resembles a open plenum but looks like a funnel. I get great performance with this setup.All the smog crap is removed, smooth throttle responce from idle to 9 grand. I also get 25 to 28 mpg on highway.lWally

Zoomie: Have you ever run it at the track? What kind of numbers are you getting in the 1/8th mile? What type of port and exhaust are you running?