Intake Manifold on the brain

[sjbsuperman1425]

Well, my friend and I picked up an intake manifold for us RB25 last weekend, and the guy we picked it up from was talking about how if i wanted a new intake manifold, to cut the flange of an eBay SR20 manifold, and well the flange for the CA onto the SR ebay manifold.

Obviously the runner spacing is off and the IACV sensor and what not wouldn't match up, but I have since been pondering, what does it really take to make a manifold?

A lot of these manifolds i see are basically a few peices of sheet aluminum welded together with some runners, a flange, and some vacuum ports and IACV devices. i've been thinking of trying my own shot at my own custom intake manifold and seeing what happens but i wanted to see if anyone had a few tips or info about flow they could offer such as runner length, velocity stacks, and manifold shape and possibly how much it effects flow to each runner. I'm going to do some reading on this over the weekend.

Thanks guys.

[float_6969]

There is A LOT that goes into intake manifold design. It's not something you're gonna slap together in an afternoon and it's gonna work well. There are several interdependent relationships between plenum volume, port length, port diameter, port placement in the plenum, and plenum shape. Trying to build one without a flow bench to make sure that the ports are flowing evenly could be catastrophic. If the best flowing port moves 15% more air than the worst flowing port, you could easily damage the engine and never know it unless you've got widebands or EGT's on each cylinder.

[sjbsuperman1425]

So basically "F" that.

[float_6969]

Yea, I should have just said that to start with, LOL!!!

[mdb4879]

I found this stuff a while back. It's really for N/A motors, but I think it's a start for you.

The following is from this thread: http://www.240sxforums.com/forums/ka24d ... guide.html

"Determining IM Runner Length:
The general rule is that you should begin with a runner length of 178mm for a 10,000 rpm peak torque location, from the intake opening to the plenum chamber. You add 43mm to the runner length for every 1000 rpm that you want the peak torque to occur before the 10,000 rpm.

So, for instance, if peak torque should occur at 4,500 rpm the total runner length should be 178mm + (5.5 x 43mm) = 415mm.

Our Intake Runners are aprrox. ~430mm

Determining IM Runner Diameter:
You can calculate the ideal runner diameter by the equation:

SQRT [ (target rpm for peak torque x Displacement x VE) / (V x 18.5) ]

SQRT = square root

VE = Volumetric Efficiency in %

V is the velocity of the air flow in the IM plenum for resonance (usually estimated at 180 ft/sec max.)
Displacement in Liters (2.4)

ex.

So if we want peak torque at 4500 rpm at 65% VE for an KA, VE = 0.65

SQRT [(4500x 2.4L x 0.65 )/ (180 x 18.5)] = [(4500 x 2.4 x 0.65)/(3330)]

= 1.45 in. or 36.9 mm is the ideal runner diameter. (1 in = 25.4 mm)


Stock Intake Runner Dia. = 44.4 mm or 1.74 in (Ideal for 6500rpm)

Determining IM Plenum Size:
The plenum volume is critical on N/A engines, and a basic rule of thumb is: The smaller the plenum, the lower the rpm range, and bigger means higher rpm.

Determining TB Size:
The best way to find out if your TB is too small for your IM plenum is to determine what the intake manifold absolute pressure (MAP) sensor is reading (in the plenum) when you are at full throttle ( or wide open throttle (WOT) ) while the car is accelerating using a datalogger. The MAP should be equal to, or close to, atmospheric pressure. If it isn't or there is a MAP drop at WOT, then your TB is still too small.

Determining Intake Diameter Size:
The ideal diameter for an intake is when the intake has 25% more cross-sectional area than the TB's bore cross-sectional area . Your TB diameter (overbored or not) dictates your intake diameter.

ex. 60/25.4 = 2.4/100 = 0.024 x 25 = 0.6
add the 25% (0.6) to the original diameter(2.4)
2.4 + 0.6 = 3" or 76mm

This is what I came up with:
60mm Bore TB
76mm Ideal Intake Diameter

62mm Bore TB
76mm Ideal Intake Diameter

65mm Bore TB
82mm Ideal Intake Diameter

Determining Intake Length:
A suggested starting point for the length of a tube with peak torque at 6000 rpm is 13 in.

You add 1.7 in. for every 1000 rpm that you want to move the peak torque below 6000.

Or subtract 1.7 in. for every 1000 rpm you want to move the peak torque above 6000.

ex.
Peak Trq. we want 4500
6000 - 4500 = 1500
we want 1500 so 1.7 x 1.5 = 2.6
13 + 2.6 = 15.6 in is our Ideal Intake Length"

ITB Sizing:

http://www.texasnissans.com/tx/forums/a ... -2307.html

[float_6969]

And all of that just gets you close to the diameters and lengths of the runners and plenum size. Plenum SHAPE has a HUGE impact on cylinder-to-cylinder air flows. If you're building a "typical" IM, you want it to taper towards the rear (the runner furthest away from the TB), but if it's tapered too much, there isn't enough clearance around that intake port and it will starve, too little taper and the rear runner will starve. Also, I've READ (no personal experience) that the idea IM has no flat surfaces except for the plenum floor and ceiling, but most custom sheet metal IM's don't follow this rule and they all seem to flow fine, so I don't know if there is any truth to that.

[sjbsuperman1425]

I just had the thought because most CA manifolds are hella-expensive and it would be kind of nice to have one as to clean up the manifold area a bit. but thats too much math for me and i just dont have the tools. a good idea, but this guys a broke f*** LOL

Ryan, I think i read on an old post somewhere you bought a kit from summit, and port matched your intake manifolds, head, and exhaust manifold correct? (can't remember the thread)

When doing port matching, how far into the manifold/head do you go, or are you just opening up the ports themselves?

[float_6969]

http://www.summitracing.com/parts/sai-260001
This is what I used. I've actually done two heads with it. The 8 port (before the oil filter failure ruined it) and the 4 port I currently have.

As for port matching, all you're wanting to do is to make sure that the head and sub-manifold and upper-manifold all match up. To do this, you simply use the gasket, mark it's ID on the head/manifolds and remove material to match. It doesn't usually take very much. Once you've removed the material, you need to blend what you've removed into the rest of the port. The sub manifold will end up being done completely. The head will basically be done completely as well. The upper, I just got as far into it as I could reach.

If you start this, you'll notice the exhaust ports are especially in bad shape. The casting is off from where the valve seats sit, and there is a lot of room to reshape the bowls of the exhaust ports. If you have the valves out, you'll see what I'm talking about.

Then when you polish, you want to leave the intake side a little rough in the head where the fuel touches so it will help stay atomized. The rest of the intake (sub-manifold and upper manifold) and the exhaust can be polished mirror smooth. The kit includes the parts needed for polishing as well.

[tommey]

There are ALOT (i would say most) of people running random "s***" intake manifolds without any problems.
It works and might or might not give any gains.
I see alot of homemade manifolds giving good results also.
There is alot you have to do wrong to make a really bad intake manifold on a turbo engine.

Of course there are benefits of making a good one, but you should not loose sleep because you have a ebay intake manifold.
when it comes to high boost, aparently a good designed plenum are more important.
The typical china design manifolds tend to favor air to certain cylinders under high boost pressures.

[chalander]

Since we are speaking of custom intake manifolds I figured I would share the one built by my brother-in-law for his sr20, it's made of carbon fiber and some material of metal.

[float_6969]

Interesting. How much do you know about the process? He obviously vacuformed it. What did he use for the internal shape? How did he remove it afterwards?

[chalander]

I don't know much about how it's made, I'll ask him the next time I get the chance. He has a bachelors degree in composites.

-
Update, he responded to me via text message with this, "Its actually pretty trick, and based on chemistry principles. Also known as gasoline and styrene...Basically I built a styrafoam mold, did a wet layup over it, cured it and melted the stryefoam out with gas. Garage style."

[float_6969]

Yea, I started doing some research after I posted that and figured out what he probably did.