Dune-Buggy.com - Fuel Injection Intake

Keep in Mind: You make HP depending on how much expandable air the motor is flowing. The Fuel Injection does nothing more than supply the necessary fuel for that air. That's why the intake manifold is so important.

The Options: There's 3 routes you can take for the intake manifold. The easiest way is to use an aftermarket manifold casting designed for fuel injectors. There two main types. One has a stock appearance and will use a stock center manifold. With this one you could easily modify the center manifold to accept a butterfly. The other type of manifold casting is something that was initially designed to accept a dual carburetor. With this one you would have to build the center plenum and mount the butterfly on that plenum.

The second route is to add injectors to the existing VW manifold ends. This is cheaper but requires a little more work.

The third route is to build a custom manifold like the one pictured below. This custom manifold is made with steel electrical conduit (1 1/4"ID) and bent with an electricians conduit bender. Most commercial electricians will have a conduit bender. The manifold flanges are cut from a stock steel VW dual port manifold.

Tim's Manifold Tech:
Intake manifold design should be a copy of what the new cars are doing. With lots of experimenting, we found some horsepower hiding in there. All the new go-fast cars are taking advantage of a natural phoneme called "Inertial Supercharging." First, a carbureted engine at best, can only be about 85% volumetric efficient. This is because it has to restrict the air flow, to create a low pressure area, to draw in the fuel.

Definition: Reversion Pulse: Did you ever take off your air cleaner and see a mist of sputtering fuel mixture right above the carb? That's the reversion pulse pushing air and fuel out the intake.

The reversion pulse occurs when the intake valve opens and the left over combustion pressure pushes spent gases into the intake. How much depends on the amount of valve overlap, how restrictive the exhaust system is, and at what RPM you are at.

The newer fuel injected manifolds work like this. The reversion pulse that comes when the intake valve opens, runs backward up the runner and bounces off the plenum. At the right RPMs, it will start to resonate. This pulse then goes back down the runner pushing up to 130% more air in front of it! The air speed gets up to about .8 Mach. This is why if everything is equal, fuel injection will make more horsepower than carbs. The higher air speed in the runners gives you better cylinder filling at the lower speeds which gives you better bottom end. Arrange the runners in the plenum so that the reversion pulse bounces off a hard surface. You don't want the pulse to blow down the opposite hole.

Intake Sizing: The size of the runners should be the same as the intake port, and the volume of the runner should be about 100% of what that cylinder displaces. The length of the runners "Tunes" the resonance at which point this effect will boost the intake. The longer the runner, the lower the RPM for the effect. The plenum size is usually 100% of the total engine displacement.

Volume = 3.14 * (radius) * (radius) * length

The intakes usually end up being about 23 inch long runners, 1 1/2 inches in diameter, connecting into a 5 inch diameter plenum that is about 7 1/2 inches long. This is about 100% of the cylinder volume for the runners and the plenum is about equal to the total engine displacement for a 2276cc engine. The runners usually come off the plenum at a 90 degree angle with the air valve mounted on the end (just like the new cars).

Air Valve/Butterfly basics: Note that the new cars use only one air valve. This is to balance the engine. Great for us dune buggy guys because no more throttle linkage to mess with! Usually a 2 to 2 1/2 inch Butterfly works great on 2276cc engines and larger, and for 1600cc to 1915cc a 1 5/8 to 1 3/4 butterfly works. Make sure you get a single throttle plate style, not a progressive.

If the air valve is too big, you don't control the air flow. For example, a smaller displacement engine will only flow X amount of air. If the engine is flowing all the air it can by the time the throttle is 1/2 open, then as you increase the throttle, there is no air flow change into the engine. The electronics needs to have a linear air flow to do the calculations correctly. Again, bigger is not always better. For turbo applications, don't go much bigger than the size of the turbo output.

Air Flow Rating Confusion Explained: First you have to compare apples to apples. On a carb, for example, the old two-barrel Holly was rated at 500 CFM. What is not readily known is that 2 BBL carbs are flowed at 7 inches of water column differential. That is to say that if you were to push enough air into the top of carb to where it would raise a tube of water 7 inches, you would then have 500 CFM going out the bottom. Now for 4 BBL carbs, they change the rules and only use 3 inches of water. For fuel injection, the standard is 1.5 inches of water.

Tech: Another basic is to make 100 horsepower, you need 140 CFM of air. The rule of thumb for naturally aspirated fuel injection, at a standard 1.5 inches of water column, is that for every square inch of butterfly, you will flow 140 CFM. To calculate the air flow for a 2" butterfly, you need to calculate the area first. Area = pi X radius squared. So, 1/2 of 2" is 1 inch, squared is still 1, times pi (3.14) = 3.14 square inches of butterfly area. This means that it will flow enough air for approximately 440 CFM, or 314 horsepower. Now remember this is approximate, and there are losses, but this is where you have to be real and ask yourself just how much horsepower is a VW really going to make?

Injector Mounting: The injectors should be placed so they spray directly into the intake port. Try to aim them so they hit the back of the valve. The easy way to mount the injector in the manifold is to get a 3/8" pipe thread to 5/8" compression fitting. The bottom of the injector is 5/8 and this fitting holds it securely and seals it. Drill and tap a 3/8" pipe thread into the manifold or better yet weld it in. You will have to figure out the most suitable location for your particular setup.

The above photo shows fuel and methanol injectors.

Step 2: The Intake Manifold

Below are some photos showing some other intake setups.

Further Reading: