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Old 01-14-2003, 02:57 PM
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Post Fabricating intake manifold.

I'm trying to get a ram intake for my 350 olds v8 and the only ones I can find are in the $1000-$3000 range. Does anyone here have any expirience in fabricating a tubing intake manifold? I can't imagine it being incredibly hard. Thanks...

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Old 01-14-2003, 03:02 PM
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I am building one for my slant six. I am useing exhaust pipe bent up for me at the muffler shop. A liitle welding and bingo!!! done!
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Old 01-14-2003, 03:16 PM
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The hardest part is machining the flanges that have to match up with the intake ports. Once those are done you can get some tubing and start cutting and welding. Hint: bolt the flanges to the heads and work from there, that makes it harder to goof up the angles.
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Old 01-14-2003, 07:45 PM
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I purchase all my intakes at the exhaust shop. Those guys can bend some pipes!
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Old 01-14-2003, 08:33 PM
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Gemini, a while back willys36 taught me how to make a custom intake and if you e-mail me I have the discussion and directions saved. I'd be happy to send them to you. It very helpful. I got about halfway done so far, keep getting distracted from other prts of the prodject not cooperating. Its life, but one day I'll finish it.

HK
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Old 01-14-2003, 09:09 PM
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Hey thanks guys... At first it sounded like a good idea, then hopeless, and now I'm all excited all over again!!! LOL
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Old 01-14-2003, 09:10 PM
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"Ol' Yeller", a 500 Caddy that came out of a '76 hearse. This motor really is being brought back from the dead. It suffered 2 rod bearings failing, welding the front 2 rods to the crank journal. This is the shortblock going back together, after crank machining, resizing 2 rods, and line boring/block cleanup.


The tubing thingie on the towel is the start of my dual Quad tunnel ram intake for a Caddy 472-500 motor. I made the flanges, bent the pipes, will be fabricating the plenum next. Even had to make up a small tool from square tubing to re-size the 2" round to a tall,skinny rectangle to match the intake port size. I will build the plenum into a divided setup, the front carb will feed the front 4 cylinders, the rear the other 4. I plan to use Buick 252 CI Q-jets, they will be jetted right for one of each to feed half the 500. Straight throttle linkage to keep it simple.

Not too hard if you have the time. I have about 10 hours fabricating time in the intake flanges, another 2 hours in the end rail pieces, and even with the use of the shop's exhaust bender, another 2 hours in bending/forming the runners. I anticipate another 5-6 hours in fabricating the plenum for the top.

Doc
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Old 01-14-2003, 09:15 PM
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DrChop, How exactly did you square the round tubing?
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Old 01-15-2003, 01:36 PM
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Another question.... I'm not very expirienced in aluminum welding. Is steel ok to use for a first try?
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Old 01-15-2003, 01:37 PM
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Steel works great.
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Old 01-15-2003, 01:48 PM
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I used a little chunk of square tubing, and sliced v's into the straight sides from one open end almost to the other end. I then hammered them shut and this made a chunk of 2 tubing into a wedge shape. I then cut the tubing on the opposite sides (that were not cut previously) with a couple larger V notches so when welded back together, the tubing took on a distinct box or rectangle section from the big end. Mig together the cuts and grind fairly smooth, does'nt have to be perfect. It works by allowing the curved corners of the tapered chunk to re-shape the tubing from round cross section to a rectangle. I was able to form the required shape merely by hammering the tapered tool into the end, but the hydraulic ram attachment on the shop's exhaust pipe bender did a much nicer job.

Here's a pic of the manifold, the tool, and the scrap practice piece of 2" tubing...


And here's another of the tool and the tubing. Note the tool is shaped to slip inside the 2" tubing, and that the corners of the tool do all the actual shaping, so the roughly-ground welds do not matter...


I also used box tubing to make up a set of log-style exhaust manifolds, when I could not find any to fit the tight confines of the '54's engine bay. Top exhaust outlet is 2.25" tubing...

All of the intake and exhaust manifolds are steel, at least 16 Ga. The flanges are fabbed from 1/8" x 3" strip stock from the local home-improvement place. Look for 'structural' or 'weldable' steel in the bins...
Doc

[ January 15, 2003: Message edited by: DrChop ]</p>
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Old 01-15-2003, 01:57 PM
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[quote]Originally posted by 4 Jaw Chuck:


<strong>I purchase all my intakes at the exhaust shop. Those guys can bend some pipes! </strong><hr></blockquote>

Well what I like about it is that it cuts down on exhaust reversion flow so much...
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Old 01-15-2003, 02:04 PM
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hey drchop, cool intake, but that is about the worst flowing exhaust ive ever seen, no scavenging going on there at all,.. anyway Cool intake dude..
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Old 01-15-2003, 02:17 PM
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The whole idea behind the tubing exhaust manifolds was not for ultimate power, but to be able to fit SOMETHING in there between the exhaust ports and the upper control arms/steering box that would carry exhaust back to the mufflers. The whole system is nice and quiet, no leaks. With a 500 Caddy in a '54 Ford, I have more than enough low-end grunt to get it down the road with decent acceleration, and I have taken the car over 100MPH, so they must flow at least as well as the stock manifolds did...

When I have more free time/money I CAN fabricate up a set of tight-tube headers. This was just the easiest way to get quiet exhaust on the car so I can drive it.

And you'd be surprised how many people doing Caddy swaps have told me I came up with an idea that would allow THEM to do the swap they had in mind. Sometimes factory stuff does'nt work, and headers are too hard to build and are unnecessary. What I did could be adapted to work with other engines, and shows any swap is possible if some creativity and some sacrifices can be made.

FWIW, on my Caddy 500-powered '28 4x4, I plan to make up ANOTHER set of these log manifolds, and use them in place of a Chevy flange with a set of Speedway's "Limefire" BBC headers. The box tubing section will have the ports going thru both sides of the tubing, and when the end of the header is corked off, the exhaust will go thru an outlet on the end of the tubing that follows the toeboards down and under the body to a set of quiet mufflers.

Doc
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Old 01-16-2003, 08:45 AM
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here is an article on calculating the runner length.

hope it helps:

Induction Waves

Lets first look at what happens in the manifold to better understand how to use it to our advantage. When an engine is running, there are high and low pressure waves moving in the manifold caused by the inertia of the air and the opening and closing of the valves. The idea of port tuning is to have a high pressure wave approach the intake valve before it closes and/or just as it opens, forcing in a little more intake charge.

Presure Wave Causes

The most commonly known cause of a pressure wave is the piston as it moves down the bore. On the intake stroke, the piston makes a negative pressure wave that travels form the piston to the intake tract. Once that negative pressure wave reaches the plenum area, it is reflected as a positive pressure wave. That positive pressure wave travels back toward the cylinder. If it reaches the intake valve just before it closes, it will force a little more air in the cylinder. The second, less realized, cause of pressure wave is the exhaust. If you have a good exhaust system that scavenges well, during the overlap period there will be an negative pressure wave as the exhaust is scavenging and pulling in fresh intake charge. The same thing happens, it travels up the intake and is reflected at the plenum area as a positive pressure wave. If the length is correct for the rpm range, the positive pressure will be at the valve just prior to it's closing and help better fill the cylinder. This will also help by reducing reversion with long duration cams. To get the benefits form this you need a well tuned exhaust system (another tech article). The third and most complex cause of pressure waves is when the intake valve closes, any velocity left in the intake port column of air will make high pressure at the back of the valve. This high pressure wave travels toward the open end of the intake tract and is reflected and inverted as a low pressure wave. When this low pressure wave reaches the intake valve, it is closed and the negative wave is reflected (it is not inverted due to the valve being closed), once again it reaches the open end of the intake tract and is inverted and reflected back toward the intake valve. This time the valve should just be opening (if the port is tuned to the rpm range) and the high pressure wave can help.

Pressure Wave Speed (V)

The pressure waves travel at the speed of sound. In hot intake air it will be about 1250 - 1300 ft. per second. Engine rpm does not effect the speed of the pressure waves and this is why induction wave tuning only works in a narrow rpm range.

Combined Effects

On a well tuned intake set up there will be a high pressure wave at the intake valve as it's opening, at the same time the engine is in it's overlap period (both valves open). If the exhaust is tuned to the same rpm range as the intake, there will be low pressure in the exhaust (due to scavenging) at the same time. Since the intake port near the valve is higher than atmospheric pressure and the cylinder is a great deal lower, the air will start to fill the cylinder quickly. The higher pressure area will quickly drop in pressure as the piston travels down the bore, this creates the low pressure wave that travels away from the cylinder. Just as this starts to happen, the piston starts moving down the bore creating another negative pressure wave, so there is actually two negative pressure waves, one right after another. In a well tuned intake system there can be as high as 10 psi of air pressure at the intake valve due to these pressure waves. So you can see that it can have a very large influence on the volumetric efficiency of the engine.

Reflective Value (RV)

Getting an optimum runner length may be hard to do due to engine compartment space and/or the engine configuration. A small cammed engine operating at lower rpm will need a long runner length, so instead of trying to fit such long runners under the hood, you can just tune the system to make used of the second or third set of pressure waves and make the system much shorter.

Intake Runner Length (L)

Knowing that the pressure waves (positive or negative) must travel 4 times back and forth from the time that the intake valves closes to the time when it opens and the speed of the pressure waves, we can now figure out the optimum intake runner length for a given rpm and tube diameter. We must take into account the intake duration, but you want the pressure waves to arrive before the valve closes and after it opens (air wont pass though a closed valve). To do this you must subtract some duration, typically you take off 20-30 from the advertised duration. 30 works well for most higher rpm solid cammed drag motors. So the Formula to figure effective cam duration (ECD) will be:

ECD = 720 - (Adv. duration - 30)

For a race cam with 305 of intake duration it will look like this:

ECD = 720 - (305 - 30)

The ECD of that cam would be 445

The formula for optimum intake runner length (L) is:

L = ((ECD 0.25 V 2) (rpm RV)) - D

Where:
ECD = Effective Cam Duration
RV = Reflective Value
D = Runner Diameter

If our engine with the 305 race cam needed to be tuned to 7000 rpm using the second set of pressure waves (RV = 2) and had a 1.5" diameter intake runner the optimum runner length formula would look like this:

L = ((445 0.25 1300 2) (7000 2)) - 0.75

So 19.91 inches would be the optimum runner length.

Intake Port Area

Unlike intake runner length which effects power over a narrow rpm range, the size (area) of the runner will effect power over the entire rpm range. If the port is too small it will restrict top-end flow and flow, and if it's too large velocity will be reduced and it will hurt low-end power. The larger the port is, the less strength the pressure waves will have. Since the intake valve is the most restrictive part of the intake system, the intake runners should be sized according to how well air can flow through the valve area. Most decent heads will have an equivalent flow through the valve area as a unrestricted port of about 80% of the valve area, this is if the camshaft it matched to the heads. In other words a 2.02" valve, which has a 3.2 square inch valve area, in a decent flowing head will flow the same air as a open port with about 2.56 square inches of area (80% of 3.2). So the port area should be about 2.56 square inches just prior to the valve (this is in the head port). Some well ported race heads may have an actual flow of an area up to 85%, but for the most part it is around 78-80%.

Intake Port Taper

To further help fill the cylinder, it helps to have a high velocity at the back of the valve. To do this the intake port can be tapered. To be effective, there should be between 1.7 and 2.5% increase in intake runner area per inch of runner, which represents a 1-1.5 degree taper. For an example, lets say you're looking for a 2% increase per inch taper on the 2.02" valve we discussed earlier. We already came up with a port area of 2.56 square inches at just before the valve. Now lets say the total runner is 10 inches from the valve to the plenum and we're looking for a 2% per inch taper. This turns out to be a total of 3.12 square inches where the port meets the plenum. As you get near the 2.5% per inch taper point, you are pretty much at the limit of helping air flow. A larger taper will only hurt signal strength at the carb.
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