So I will be building a manifold for my 383, I know I want a plenum volume of around 210 in3. Its been awhile since ive done the math and I cant figure out what length runner to use, Im not looking to make top end power so the runner will be longer, I would like to make power from 1500-6700 rpm range. every time I do the math I come up with an 11" runner what am I doing wrong? what should it be?

 

this might i mean might just help

http://www.team-integra.net/sections...?ArticleID=471

 

Ill check it out, I was starting ti wonder if I would ever get a reply.

 

You want a taper in the sidewalls

 

Tapering the runners increases top end, that's not what I'm looking for.

 

I suggest that you get professional help to do your intake manifold. Much of what you have posted is questionable.

FWIW, some of the references in this thread are not all that good either.

If you want to make power 1500-6700 there are some OEM and aftermarket choices, especially for cathedral port heads, that work quite well. Why are you tyring to reinvent the "wheel"?

Good luck whichever route you take.


Jon

 

I would just like to make one for a lsx motor, this is not my first sheet metal manifold, its just been awhile. The last one I made was for a turbo application on a 4 cylinder. Im not trying to reinvent the wheel just want to do it for fun. plus I don't think it will cost me anything with the materials I have laying around, I might need some more injector bungs machined but I might have enough.

 

Sorry I ragged on you. I thought you wanted to make power rather than just have some fun fabricating.


Jon

 

There can be a hell of a lot of satisfaction in achieving both.

 

I just love welding aluminum! and manifolds are fun and always look cool when your done. It doesn't matter if you do a stainless turbo manifold or an aluminium intake manifold, when you show it to others and they know you did it your car will always be Bad ***! I know its not a manifold, or a lsx car but here are some of the latest things Ive been fabing up on my track car. [IMG][/IMG] [IMG][/IMG] [IMG][/IMG] my newer inter cooler/oil cooler set up.

 

Absolutely! However the design has to be correct to make the power. That's where most folks fail.

To corrupt Gusteau's book title: "Anyone Can Weld"

FWIW: 10 bonus points if you know where that ^^^ came from.


Jon

 

11" is gonna be towards the upper RPM. It all depends on which pulse you are trying to catch. A longer runner will grab a stronger pulse, which also makes the manifold work in a narrower RPM range. Also remember when calculating runner legnth the measurement is from the valveseat to the plenum in the center of the runner. So 11" may be slightly long if you want it to lay down at 6700 RPMs. Someone correct me if im wrong. Its been awhile since ive messed with the intake math.

 

With a headport C/L length about 5.4 inches, plus an 11.0 inch intake manifold runner I get peaks at 7400, 5500, 4300, 3500.

If the intake runner is about 12.7 in. long I get peaks at 6700, 5000, 3900, 3100.

If you drop the intake runner to 8.0 in, you drop another harmonic. I get peaks at 6700, 5200, 4200.


Jon

 

Now I get more useful info, Im looking for the calculation. I cant find my text book on it right now its buried in the garage somewhere. Are you saying the head port C/L is 5.4 or are you just saying if it is 5.4?

 

You should measure your heads and decide what intake centerline length you want to use. Cathedral port LS heads have a considerable length difference from the long side to the short side. Some folks take the average as the C/L length.

Jon

 

Ok this is one of the reasons why I want to make a manifold. If you buy one it will be standardized for the 347ci with stock heads I would assume, and not account for any changes in ci or head manufacturer.

 

I don't know where that came from, but I'm guessing it applies handily to my shitty welding "skills".

 

See post #6:

Jon

 

Would you mind going over how you arrived at those numbers?

My understanding is that you'd need the cam's intake duration to put a specific RPM and runner length together.
I don't recall cam specs being mentioned...

 

Cam specs come last, not first. Well, that's not always what you hear, is it?

FWIW,"Anyone can weld" was not a reference to the quality of anyone's welding. It just means that anyone can learn to weld, but not everyone takes the time and study to understand how engines work and how to successfully design systems for them.

Making a beautiful looking intake manifold which has the wrong dimensions for making power is a lot like polishing a turd. It may look good, but in the end it's still a turd.


Jon