84stock

iTrader: (5)

I did a search and did not get much. I have a 12a, 150,000km (=80,000 miles). RB air cleaner & header, no cats, soon to install apc muffler, nikki carb with a holley secondasy spring. I have 2 questions, what jets should I change to, I think I am running a little lean. Second, intake manifold porting, I know matching the intake to the engine is a "BAD" thing, but what about opening up the plated between the primaries and secondaries?? And removing the shutter valve (the hose is disconnected now). Also, the 79 manifold, will it bolt right up to my 84, I read somewhere about a leak problem, Thanks for any answers.

eddierotary

iTrader: (7)

as far i know matching the intake to the engine is nt a bad thing, it just raise verry little the powerbrand of the engine and take some verry little torque out of the engine but it give more power. the shutter valve if you have it disconected take it off completlly, you are basicly running with just 3 barrel only. and no the 79 intake does fit but doesnt cover a exhaust port that the 84 intake covers and you just end up with a loud exhaust leak. and if the APC muffler is glass packed, please dont get one or your car is gonna be verry loud after a while. get a magnaflow or a RB muffler. and for the carb, talk to sterling about it he can help you in that plus theres a sticky i thing about what you can do and what not

peejay

it *is* a bad thing because of the shape and position of the primary intake port. (The secondary is already "matched") It has to make a turn right there, opening the runner up just makes it have to turn and expand in area at the same time. Recipe for disaster.

Really the ports don't *need* to be opened up anyway... ever see how small the primary barrels are? Why does the port need to be bigger?

84stock

iTrader: (5)

I saw a pic once of a porting job done by yaw on a stock manifold. He opened it up to a full open plenum. What did this produce?? What about opening up the dividers between the primaries and secondaries in each chamber. Anyone doen this??

84stock

iTrader: (5)

still waiting for more responses, anyone??

peejay

It reduces the restriction of the stock carb by permitting each rotor to see all four barrels of the carb instead of just two.

Downside - besides the massive amount of time required to rework the manifold, it requires a complete recalibration of the carb. Stock-style, the airflow through the venturis is a series of sharp pulses. (This is known as the "vacuum signal" to the venturi boosters, which is what pulls fuel through from the float bowls. Absolutely nothing to do with manifold vacuum!) When you have a plenum, it turns into a less-strong, more constant flow of air. This weakens the vacuum signal, meaning it is comapratively harder for the airflow to pull fuel up through the boosters, especially at the top end. (If anyone chooses to dispute, I have proven this empirically, meaning I did the testing and I measured the results) Result is a very lean mixture as you rev up unless you rejet and more importantly resize the air bleeds, which allow air into the wells where the boosters get their fuel so that it can be emulsified. Weaker signal = harder to pull fuel up = less fuel is mixed with the air in the wells = less fuel goes through the boosters = lean mixture.

rotarydankus again

i under stand you exactly, its like my 455 buick, it has a .600 lift cam lots of lda, well over 300 on duration , in a 4000 pound gs it ran 11.40 at 127mph, that car screamed with a 900+cfm barry grant carb it absolutely howled! but to drive it and cruise main it had a 600 holley, small carb venturi+plus 4.5 inches vac at 1000 rpm in idle , with 900 velocity goes down an also does part throttle response, so i keep two carbs, can change it in about 6mins

81Rex6port13b

what do you guys think about port matching. i know with all the cars i have worked on. when i got a chance and had a nice hand grinder i could use i always matched the intake manifold ports to the gasket i was using. and when i head the head off (piston motors of course) i would just open up the ports to match the gasket there as well.

bigger opening more air.

i have not gasket matched a rotary but i have gasket matched my intake manifold to the carb.

Sterling

Carl has told me some suprising things regarding port matching that seem very counter-intuitive.
He should be enticed by the title of this thread...It's right up his ally.


Peejay is right on the money with that explaination. The pulsing is cause by the rotor sweep, but you have to think about it more like a violent vibration because of the speed at which it occurs.
The manifold milling (plenum) softens this pulse vibration and allows for higher inlet flow rate to be achieved.

I will have to restudy peejays website findings. A question about the specific gains of a plenum was posed to me by Jeff20b recently, and I was at a loss for a good answer. ('Course I sent him to peejay! )

peejay

Jeff20B and I seem to converse a lot about intake design.

Rx7carl

The stock gasket can be used as a template for port matching. It leaves enough material to kill the reversion wave. Peejay pretty much has it covered on the port shape. It sux. I been workin on ideas to soften the turns cause theres alot of flow seperation with the abrupt turns/shape changes, but not alot of success so far. Flow seperation chokes the ports and causes alot of turbulence, along with a big boundary layer. I may be reduced to making an entirely new manifold that would be much better airflow wise. If I could get them mass produced I might have some made.

10,000

When I was younger and dumber (not all that long ago) I port matched a '73 RX-3 intake manifold for my '85 12a and while I was expecting some decent gain I ended up losing all my bottom end but it would pull from 4k to 8k really effortlessly. This was with a Yaw Street carb. But now I am using A stock carb and manifold off an '82 and the torque and moreso the low speed/ low rev tractibility are worlds better for street driving. The yaw setup needed to be on a ported engine.

peejay

Do not do not do not port match the center ports. End of story. Don't even open them up a bit, they are too tall as it is! Why make a bad thing worse?