Heavy Breather

Cost-effective EFI intake mods

By Adrian Cuesta

Click on pics to view larger images

You'd like more grunt for your EFI car? Got less than $30 to spend? Read on!

Nowadays when every car has got the 'EFI' badge on either the engine or bootlid, it's kind of hard to see where you even begin to get more power. A chip? A new exhaust? A filter? And when there are heaps of workshops offering instant power increases and all wanting to hoist your money out of your back pocket, where do you start? And how much is it going to cost?

Okay, I've got some really good news for you. It'll take an afternoon, cost you under thirty bucks, and you'll get extra real horsepower happening at the wheels.


When the car manufacturer designed the air intake to an engine, they knew what they were doing - it's just that their priorities weren't necessarily your's. They wanted to reduce to a minimum how often you need to change the filter element. They also wanted to subdue to the volume of a knat's fart how loud the induction noise was. Basically, their engineers were trying to satisfy a whole lot of wants.

You? You just want Max Grunt.

Getting the most power comes from giving the intake air the easiest path. You want to have as much air as possible get all the way to the intake valves, so that it can be mixed with fuel and then be set on fire. But any blockage that's in the way slows down that flow, meaning that less air gets to your engine. Another way of putting that is to say that your engine gets smaller when it can't breathe all the air that you'd expect it to. A 5 litre engine with a 10 per cent flow blockage breathes like a 4.5 litre donk. A 3 litre engine like a 2.7 - you get the idea.

Now heaps of people will tell you that a new filter element will fix the prob. You know - "10 more horsepower from our drop-in filter". What they DON'T tell you is that there was a 10 horsepower gain all right - but that was on an 800hp engine! Or, the power gain was around 1 per cent. One percent is four fifths of stuff-all, basically. What you want to see are gains of four or five percent....

Big Breaths

Okay we know where you're coming from. You're saying - what a crock of shit. He reckons he can get great power gains - but where's the evidence? Here, that's where!

We put a Australian Holden Commodore 5 litre HSV Maloo ute on the dyno. The 5 litre donk was breathing just through the standard over-radiator HSV intake, and power on the Dyno Dynamics dyno peaked at 125kW (168hp) at the wheels. Next, the HSV intake was blocked off - the sole source of engine air being a duct disappearing thru the inner guard and made from 3-inch PVC plastic stormwater pipe and fittings. Power immediately jumped to a peak of 128kW (171hp) - a gain of three horsepower at the wheels or nearly 2 per cent!

But we're not finished yet. Next up, both the new intake AND the standard HSV intake were opened up together. Power rose to 131kW (176hp) - no less than a 8hp gain at the wheels over standard, or 5 per cent. Put it another way, you're looking at 10-11 horsepower at the flywheel!

How Do You Do It?

What you're after is the free-est flowing intake in front of the factory airfilter that you can get. Factory airfilter? What about that replacement drop-in element? Well, if you actually measure the flow restriction of the standard factory filter, you'll find that it's pretty bloody good. Yes, yes, I know that's against all of the pub talk of your mates, but it's damn hard to skull a schooner and measure pressure drops at the same time..... What does generally cause the restriction is the intake to the airbox, not the filter in the box. In fact, in the Maloo example above, a slightly dirty factory filter was used during testing!

So, you want 3-inch or 4-inch duct that connects the world outside of the engine bay with the airbox. Ignore those people that tell you a 2-inch or 2.5 inch duct will do. You want the biggest duct possible that can be fitted in. Some people use convoluted tubing which can be easily bent into the right shape, but you'll find that all of the ridges on the inside of the tube tend to slow things up. No, what you want is a smooth-faced duct with gentle curves and bends.

Shoot it With Your Heat Gun

There's a couple of ways of getting a duct that looks right and flows well. The first way is to head down to your local hardware store and pick up a heap of 75mm (3 inch) plastic stormwater bends and pipe. If you get the right mix of 15? , 45? and 90? bends, you'll find that you can soon put together a duct that works well. You'll probably need to cut a hole in the inner guard underneath the airbox to fit the duct through. Some people back off a mile when that's said (what? - drill a hole through the sheet metal of my new car?) but if you're careful, the hole won't even be visible, let alone affect re-sale.

At the airbox end you can either cut a new hole in the right portion of the 'box to take the new duct, or adapt the duct to meet with the factory airbox opening. This can be easily done if you use a "gutter adaptor", which turns the round pipe into rectangular plumbing. If it doesn't quite match, file the airbox hole until everything is a neat fit. Airboxes are made from strong, hard plastic, so this can be done fairly easily.

Another - really nice - way of doing it is to form the whole new duct out of one piece of plastic PVC pipe, using a heat gun to soften it so that you can bend it into the right shape. Use either 3-inch or 4-inch pipe - both available from your local hardware store. You need to be really careful that the pipe doesn't close up on any bends and that the inside surface stays smooth. But with care, a really good intake duct can be formed. Plus, if you stuff up you can always start again - the pipe is dirt cheap. At the atmosphere end you can even flare a bell-mouth into the pipe by heating and then rolling the lip into shape with a spark-plug socket or pushing the pipe down over a funnel. Doing this definitely improves flow.

When you've finished either type of duct, paint it black with a spray can.

The Final Word

As you saw with the Holden Maloo, if you can use two good quality inlet ducts, so much the better. However, you want only cool air from outside of the engine bay being breathed - that means if the factory duct picks up air from directly over the exhaust manifold, block it off. With a new duct you'll probably need to change your filter a bit more often, but the increased grunt will sure make it worth it.

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