85rotarypower

I'm looking into making a custom intake manifold for my '82. Its got a 6 port 13B, but due to some malfunctioning ECU components, its going carbed. I'm very short on money, and am in no hurry to get this done, but need something cheaper than the RB holley manifold. Its simply just too expensive even to get one used.

Anyway, the flanges won't be a problem. My dad works on a wire edm, so he can cut out all the flanges I want for the cost of the steel. I'll be making this out of mild steel to keep costs down.

Ok, now to the hard part. I'm trying to decide if I'm going to use an "open" plenum design seen on many V8 intake manifolds, or if it would be better to have each port separate for each barrel of the carb. Which setup would be better for the street? Or will the open plenum even work? I need some help here, so if anyone has anything to contribute, please do. Any comments on the whole idea are welcome.

BTW, I'll be using a holley 650, probably not RB preped.

13B4port

the open design wouldn't work as weel because you would loose the volosity of air in the intake. I would recomend runners atleast 6-8 inches long off and then makes those into a one peice about 3 inches long and then a bracket for the carb. it would almost be like a hearder and collector in reverse. just remember that bigger is not always better.

Feds

As far as basics go:

A plenum will give you a wider power band, and make the car more streetable (in the traditional definition), at the expense of high end power, and possibly a little throttle response.

Long runners sacrifice low end torque for high end torque, will make for a more peaky setup, and have better throttle responce.

Start your research here:
https://www.rx7club.com/showthread.p...TOCK+PORT+AREA

85rotarypower

Thanks feds. What I meant by open plenum is exactly what 13B4port said. I guess I should have explained it a little better.

rotarygod

iTrader: (1)

If you keep an open plenum chamber, you will need a smaller carb. If you keep al the runners separated, you can use a larger carb. In an open plenum, each port "sees" all of the throttle plates at the same time. In an individual runner type of system each port only "sees" one plate at a time. I'm not sure if I'm getting the idea across effectively. There is some backwards info here. The larger plenum will give you less low end power but more top end potential. (The opposite is true for Helmholtz resonant tuning) An individual runner system will give you better low end. These are just generalizations though as there are more variables to it than just this. Don't expect to just be able to throw the same carb on both types of manifolds and have them both work equally well. The tuning requirements are very different between them. Each manifold type can be made very streetable so don't think you won't have a good running car with one type but not the other.

The RB manifold is actually quite cheap but probably the best flowing design out there! It will be hard to beat on both accounts. The way their system could be improved is to use vacuum actuated secondaries rather than mechanical secondaries. This would smooth out the drivability characteristics a little.

As far as a design goes, runner size should be kept as small as possible. You want this for high velocity. Don't make them any larger than the runner size in the engine. If this is a 6 port engine, make sure the area stays the same. You might be tempted to just oval out a 2" pipe to make it feed the outer ports but this is a much larger area. Stick to a 1.5"-1.625" runner for the secondaries and a 1.25" for the primaries. I'd make the plenum completely removable so you can replace it with larger or smaller ones or potentially add runner length if need be. You'd have the ultimate in versitile manifold this way. Theory is good for a general rule as far as getting you close but there is no replacement for direct experimentation and testing. I have learned a few other things since I started that thread but have just chosen not to update it anymore as there will always be those who can't accept how things actually work. I don't argue. I just don't help anymore. It's easier.

85rotarypower

Hmmm, that interesting. I would have thought things would be the opposite, but it makes sence how the smaller runner size makes more power. The engine is ported, so its going to be a little different. Unfortunately, I don't know the port timing since I bought the engine this way. I'm going to have to take an educated guess on the port timing. Its most likely the same as a RB streetport, mabey a little different.

I'm not too worried about low end torque and power since this will not be a daily driver, just a weekend car. I'll see about that interchagability though. It sounds like a very good idea. Thanks to everyone thats helped so far. I'm gonna have to read that thread fully.

Kim

I made a manifold just like the one you want to do.
Its used on my racecar and works well, it aint supernice, but I chose the cheap way too as we will be racing a PPort next year.

grantmac

What did you make it out of? Looks cast almost.
Grant

Rx-7Doctor

iTrader: (13)

Personally i would get a lower manifold set up for a side draft and get a used 44phh mikuni.
Makes alot more Hp than the holley and alot more responsive. If your 13b is not ported the 650 holley is a overkill also. Just the Doc's humble opinion.

85rotarypower

I agree, the 44phh is probably a great choice for a nonported 6-port, but since my engine is streetported, I think the 44phh is too small. The weber 51 IDA from RB would probably work fine, but these carbs are too expensive to get new, and I've never seen one for sale used.

Going by what size carb RB sells for a stockport 6-port 13b (600 cfm holley), my guess is that a 650 cfm carb would be needed.

Kim

Ha ha your screen must suck...

Seriously its just made from regular steeltubing, took quite some time to make but it was free.

grantmac

Yeah my screen does suck. You did a very good job though. I'm considering the same sort of thing to mount a set of 46mm throttlebodies onto my 6-port and ditch the restrictive lower manifold.
Grant

85rotarypower

Ok, if I understand the equation in that thead properly, then by my calculations, port timing doesn't have too much effect on the runner length. After going through a few calculations, having the primary ports close 12* later than stock only changes the runner length by 1/2". This really isn't much, and when you account for the fact that RPM has a much greater effect on runner length, then you would only move the power peak up 200-250 rpm, which is hardly noticeable.

This is where my problem arrises though. I don't have the information on my ports. I have no idea what porting templates were used, if any, and who did them. The guy that sold me the parts car the engine came in didn't know anything about it. He was just selling it for a friend that actually owned the car. Any suggestions?

MarkPerez

a 650 cfm carb. is over kill even jetted down. the best is a side draft, by design. the companys have done the math and developed the parts. remember it's a 6 port not a 4 port you don't include the 5th and 6th ports you'll loose what torque you may be used too. these engines don't have much torque because the engine/e-shaft is very short and other reasons. port timing is important but don't get side tracked with that. an open pluenum will work best with a progresive 4 V. IE primerys, secondaries
not all four V's dumping at the same time. That and a 3 inch open 'riser/spacer' under the carb base plate and a jetted 600 cfm holley would do well. IMO