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[ Step 1: Fuel Tank ] [ Step 2: Flanges and Plenum ] [ Step 3: Runners and Rails ] [ Step 4: Plumbing and Testing ] [ Step 5: Computer Wiring and Tuning ]

Step 2: 

Make fan shroud reinforcement plate, intake flanges, and plenum parts

I initially planned to use this engine. It is a stock 1971 bus engine with an AS41 case. Upon initial inspection, it looks like the engine had a loose spark plug and overheated that cylinder head. The crank bearings have been pounded into the case and it's pretty much a boat anchor at this point or maybe a future grenade motor.

I will be using this new tin on my old engine. I'd like to mount the oil filter on the tin at a future point to keep the lines short and to make it easier to pull the engine. 

The stock tin is very flimsy and the oil filter is pretty heavy. The first solution I'm going to try is to make an aluminum sheet that will cover the fan shroud. I'm hoping that the aluminum I have is enough to distribute the weight over a large enough area.

First I cut the heater vents off because they won't be used anymore. I made a cardboard template that covers the old vent holes and wraps around the side and top of the ran shroud then I cut it out of aluminum sheet.

The markings show where I was hoping to put the coil and oil filter. I decided to hold off on permanent mounting the filter at this point until I finish the manifolds and mount a power steering pump. The internal fins will dictate where the oil filter can be bolted.  Thanks to Wes, I now know where the fins are located inside the fan shroud so placement should be easier. Cut up fan shroud

Intake System:

The stock VW intake design leaves little to be desired if you want to increase the airflow. I decided to make a custom manifold rather than using the original VW parts. In retrospect, I should have just made some aluminum bungs and had them welded onto the stock casting. That would have been real quick and easy and I'd be driving the EFI engine right now if I did that. 

Initially I was planning on a different design intake that would have longer runners into a 4-1 collector. That will now be reserved for the second design when I have my intercooler in hand.. For the time being I've decided to use something like Tim's design which is pretty easy to make and will provide ample air flow. 

This is going on a woods buggy and the injector mounting point is a major concern for me when it comes to trees, branches and rocks. I don't want to catch a fuel line on a limb and have high pressure gasoline pumping all over the place. Because of that I will place the injectors in between the runners and the fan shroud. The optimal position would be to have the injectors spraying on the valve, but this position is close enough for me.

Here's a glimpse at an almost finished product so you can get an idea of where this is going.

Three things to point out: 

1. The intake rails were designed to be as close to the fan shroud as possible and still allow the injectors to fit between them. I have seen some people who will cut into the shroud to fit the injectors. The point where they cut into then has to be blocked off so the air can't escape. If you pull the fan out you will see where it can be capped inside. I chose to keep the shroud stock and make the angle of the runners come off the head so that it can clear the exhaust tubes and allow room for the injectors.

2. On my frame design, the throttle valve sticks slightly above the rear engine cage. I minimized the distance by sitting the plenum 1/4" above the shroud. In the future the engine cage will be redesigned to better protect the plenum.

3. There are no flexible joints on the intake. I initially considered that this may be a problem but after a little use it appears as though the thin wall tubing has enough flex to compensate for the engine expanding when hot. I did allow about 3" of room near the plenum for a joint in case this manifold ever went on a stroker engine.

Intake Flanges:

Below is a paper template of the VW intake flange. Click image to open the PDF file to print it out.

Note: There is some more room available for the flange on a stock head. On the next version, I will make the flanges a little bigger all the way around. 

From the paper template I made a steel template for my $25 flame cutter and burned off 4 intake flange blanks. The holes are on the small side. The reason is so that I can later die grind the holes to match the head and intake runner angle.

Here's the flange on the head. Note the alignment pin that I drilled into the head to center the flange. This pin is on an old head and is just a cut down bolt. I think when I do my good heads I will drill and tap the head to screw a stud into it.

HELP: If anyone has access to a CNC plasma cutter or laser cutter please contact me so we can get a good source for flanges. tkirkwood@dune-buggy.com

The runners come off the head at about a 19 degree angle. This is a fairly steep angle for a stock head and you'll need to do some grinding/porting work to match things up. I made some short stubs that were cut to the 19 degrees. A test fit will show that they are a little to wide for the heads. I ovaled them a little with a hammer and then tacked them in position.

Now die grind the flange to meet that angle. Some port work on the head also needs to be done to straighten the flow lines. That process is spelled out pretty well in "How to Hot Rod Volkswagen Engines"

Plenum:

I'm going to use a 5" diameter plenum that is 7 inches long. 

Here's a picture of me bringing my plenum home. Yes I got a bunch of strange looks and yes, it's a diesel tail pipe...but sometime you just gotta use what ya got ;-)

It's generally said that the plenum should be about 50% to 100% of the engine size to provide an adequate air capacity. I believe that really depends on how much volume is in the runner. On my particular setup, I could have gone much smaller on the plenum because each runners can already hold enough air for it's cylinder. (That's just my opinion and it's certainly up for debate)

I chose the plenum size because that's what I had available to me and the extra space inside the plenum between the runners will aid air flow and allow the runners to be more staggered.  The larger plenum will decrease throttle response slightly because of the extra air capacity. 

Runner Spacing:

The runner holes are going to be staggered as much as possible in the plenum. The reasoning is that at higher RPMs, runners that are opposite each other will tend to steal air from each other. This has been documented on certain styles of V8 manifolds.

The runners will be staggered like the picture below. That will better separate  the air flow from the adjacent firing cylinders. 

Plenum Holes:

Make two lines on the plenum 180 degrees apart. Label one side left and the other right. Draw an arrow to the rear. On the left side measure 3" and 5.5" from the rear. On the right side measure 1.75 and 4.25 from the rear. 

Drill the holes with a hole saw.

Plenum Flanges:

This template is for a mid 80s Escort Throttle Body.

The plates were cut with a torch and cleaned up with a grinder. I used 3/8 plate for the throttle body flange and thinner plate for the other side.

Holes are tapped in flange for 5/16 bolts. The port was opened with a die grinder and smoothed for better airflow. I made the port about 1mm larger than the throttle body to account for any misalignment.

The parts store didn't have a gasket for this throttle body so I cut one from gasket material.

Temporary Distraction #398 - Waterproof dipstick 

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