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Modifying the VL Turbo Intake

Measuring and eliminating the nasties in the intake system of my VL turbo...

By Michael Knowling

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This article was first published in 2001.
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The first area we're going to tackle on our stock-as-a-rock 5-speed VL turbo is the air intake system. But rather than just go out and slap on a K&N pod filter, we thought it'd make more sense to check out exactly where the VL was losing the most airflow. To do this, we knocked-up a simple manometer and stuck it into various airflow points along the length of the intake. And - once again - the humble manometer proved to be a deadly accurate device (despite its looks!)

(The pressure drop measuring techniques used in this article were extensively covered in our Eliminating Negative Boost series - Part 1.)

However, there's one additional important factor to consider when you're looking at the intake system: intake air temperature. As you have probably heard by now, the cooler (and therefore denser) the intake air is, the more power you're able to get (potentially, that is). In our case, we measured intake air temps in the bottom of the airbox with a Dick-Smith LCD temperature display.

Standard VL-T Intake Layout

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Let's look at what it is we're dealing with here...

Starting at the atmosphere end, the VL-T sucks air through a push-in plastic snorkel that feeds from directly behind the left-hand headlight. Flow then leads into the bottom of a straight-edged plastic airbox, which - just in case you didn't know - uses the same filter element as V6/V8 Commodores right up to the VS. Inside the top of the airbox is a small metal bell-mouth, which serves to aid flow into the mouth of a 65mm ID Bosch hot-wire airflow meter (AFM). Like many other hot-wire type meters, this unit is also equipped with two mesh screens at both ends. After being metered, intake air then zips through a partly convoluted turbo pipe (which contains a wire spiral to prevent it collapsing) and - finally - into the compressor side of the turbocharger.

And that's the VL's complete intake system - simple, eh?

The Measurements

Now we get down to looking at what parts of the stock intake are responsible for the most restriction. Here is the list of the individual pressure drops that our trusty manometer measured - and, to keep things simple, we've quoted peak restrictions of each section of the intake only. Note that these figures were obtained in second gear at full load and 5600 rpm (peak power revs).

Individual Item Peak restriction
(inches of water)
Intake snorkel 3.5
Bottom half of airbox 4
Air filter element 1
Top half of airbox (with bellmouth) 2
Airflow meter (AFM) 14.5
Convoluted flexible pipe 4.5
Total intake restriction (measured at PCV junction immediately before the turbo) 29.5

Of course, these measured flows are only half of the intake story. The other major issue is intake air temperature - something we discovered was quite high in the VL. Using the LCD temp display, we measured the air temperatures in the bottom of the airbox hovering at around 30 degrees C on a 15 degree night. Hmm, not too bad really. However, this soon climbed to around 50 degrees C when there was little engine bay airflow (ie the car was stationary or moving slowly). Eeek! (Note that ambient air temperatures remained consistent during our testing.)

So in standard form we have:

Peak restriction: 29.5 inches of water
Peak intake temperature: 50 degrees C
0-100 km/h performance: 8.0 seconds

Now - having thoroughly probed intake restrictions and temperatures - we can get stuck into opening the gates to release some more RB turbo horsies...


Phase 1

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Wanting to see what kind of improvements could be made just pulling bits off, we removed both of the AFM's mesh screens and also the intake snorkel. (We decided to take the protective screens off because we'd previously learnt that they're responsible for a considerable amount of restriction across the AFM. See "Eliminating Negative Boost - Part 4"). Amazingly, this 5-minute operation resulted in a total 9.5 inches of water improvement - with 6 of those coming from pulling the AFM screens. So already - without even really trying - we'd removed 32% of the total intake restriction without spending a cent. Yee-ha! And notice that there wasn't much point in worrying about the standard Holden air filter, coz it was only causing an inch (about 3%) of the total airflow restriction anyway. Stuff-all, to put it another way...

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However, we did pick up one negative side effect in removing the push-in snorkel - an annoying resonant drone coming from within the airbox upon deceleration. Oh well, it was a great start though...

Peak restriction: 20 inches of water (down from 29.5 in standard form)
0-100 km/h performance: 7.7 seconds (down from 8.0 in standard form)
Total cost so far: Zero!

Phase 2

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Carrying over from our inquisitive Phase 1, we decided to see how much more we could get by turning our attention to the factory airbox. After all, there was still a substantial 7 inches of water restriction lurking before the AFM. But before we set off chopping and changing, we first looked at how much potential there was for improvement. Thanks to the wonderful manometer, we had learnt that the lower section of the airbox (still without its snorkel, but together with the filter element) now caused a pressure drop of 5 inches of water. The top section (with its bell-mouth) drew another 2 inches.

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Interestingly - during our testing - we observed that as soon as the top section of the box was removed, the car's idle quality took a real nose-dive. The AFM was obviously being upset by the new (turbulent?) airflow patterns past its sensitive hot-wire element. No big surprises though - this was a phenomena that we've often heard about in the past (including instances where the complete airbox has been ditched and a pod-type replacement filter installed instead). So, given this - and the fact that the top section already flowed comparatively well - we had reason enough to leave the upper half of the 'box unchanged. In fact it was lucky enough to get past our modifications completely unscathed!

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But what we did decide to modify was the bottom section of the airbox - which, on its own, was responsible for a 4 inches of water pressure drop. The filter element - which drew only 1 inch - was absolutely fine. Looking closely, the box's 53.5 square cm (8.3 square inch) intake opening certainly appeared to be responsible for the majority of the flow losses. Plus, this was also the perfect opportunity to change the intake air pick-up point to sourcing air from a much cooler area (rather than from within the hot engine bay).

At last, it was time to start chopping!

Given the cross-sectional area of the inlet to the factory 'box, there was no point in fitting "the usual" 3-inch cold-air duct. The resulting increase in area was next to zero. Instead, we went for the biggest diameter PVC pipe we could physically fit into the lower side of the box - 100mm storm pipe.

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But then came the task of squeezing it in. This was done by drilling a circle of holes into the bottom of the plastic box, and then pushing the middle bit out. A file was then used to smooth the daggy edges 'till it looked sweet. After that, there sure wasn't much left of the bottom of the box! This same process was then applied to the car's metal inner guard, where the duct was to pass through. Again, we drilled a series of holes, broke out the middle bit (this time, with a bit more heavy-hitting force!) and filed the edge smooth. Paint was then applied to the bare metal edge to seal it, and a ring of plastic edging was run around the hole to make a nice tight seal when the big duct was pushed through.

Now came the pipework.

It looked like the 100mm diameter ducting would have tight clearance against the front left tyre - especially when the steering was turned to right full-lock and the suspension was bounced up and down! However, this was overcome by using a 45-degree bend directly before the airbox (which kept the yet-to-come straight length of pipe sufficiently up and out of the way of the tyre travel). And - when that cut-to-length piece of straight pipe was added - the 45-degree bend actually put the air pick-up in the ideal area. Perfect.

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So why didn't we go for a full forward-facing duct to get some ram-air benefit?

Well, that's because we didn't want any high-speed flicked-up stones, locusts or whatever to get the chance to clog up, or - even worse - punch a hole through the airfilter. With the airflow meter screens removed, this'd be absolutely catastrophic! Certainly, engine safety was our bigger concern in this case. And you might note that we also located the air pick-up point a long way forward of the front tyre. This prevents sucking up dust that the wheel kicks up when you're travelling down dirt roads or over road construction patches. Oh, and water cannot enter our strategically located intake either - unless we try driving the car through a river crossing!

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In order to get a little more flow into our new airbox duct, we opted to slightly flare the leading end of the new intake duct. This was done using a heat gun to soften the end of the plastic pipe, which was then quickly pushed down over the tapered end of - wait for it! - a ceramic flower pot. Surprisingly, this practice worked very well, giving a nice, gradual flare.

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So now - with the flared straight pipe and 45 degree bend already ready for installation - we gave them a quick lick of black paint, glued them together and hard-mounted both ends with some small brackets we made from aluminium strip off-cuts. These were attached to the big pipe using bolts with low-profile heads (to keep in-pipe turbulence down). The very final task was to seal the 45 degree bend to the bottom side of the airbox by sticking some foam rubber around the outside of the bend.

Now it was time for the moment of truth - the testing!

With our new 100mm flared intake in place, we took off down the road to see what improvement we'd achieved. And it soon looked like our effort had paid off. Restriction pre-air filter was now only 1 inch of water (down from 4 inches) and intake air temps had gone down to within 5 degrees of ambient temperature (ie 20 degrees C in this case). Yesss.

Peak restriction: 17 inches of water (down from 29.5 in standard form)
Peak Intake temperature: 20 degrees C (down from 50 degrees)
0-100 km/h performance: 7.6 seconds (down from 8.0 in standard form)
Total cost so far: $15

Phase 3

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Now - having improved airflow right through to the beginning of the AFM - it was time to look at the substantial 4.5 inches of water restriction hiding in the pipe between the 'meter and the turbo. It's hard to believe, but this innocent looking flexible turbo pipe (the flexibility isolates engine movement and make accessing the air filter easier) was the cause of 26% of the restriction that remained.

Time to get to work.

Firstly, by sticking the manometer into a small tapping half way along the pipe, we discovered that nearly all of the restriction was actually being developed across the short length of convoluted flex. Interestingly, the majority of the airflow impediment had nothing to do with the pipe's sus-looking 90 degree bend or wire support spiral. So, next up, we had to see if we could get rid of the convoluted section while still keeping some "give" between the engine and AFM. Luckily, it turns out that the VL also runs another highly effective flexible hose down by the entry to the turbo compressor - so it did look like we could get away with ditching the flex pipe. But we still preferred our replacement pipe to be not completely rigid. Some flexibility was better than none...

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The answer - low and behold - was found lying in a torn cardboard box at a major local import wrecker. Adelaide Japanese Dismantlers had a box of rubber pipes and ducts that had been ripped off various engines and - within a minute of rummaging through - we'd found the perfect candidate. A ripple-free rubber pipe with a 90-degree bend. It even came with a PCV junction and a (plugged) 25mm fitting - which would be ideal for a closed-loop blow-off valve further down the track. Things don't get any more "in your lap" than this!

Oh, and - even better - the cost for this li'l treasure was 15 bucks!

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Back in the driveway, the new pipe was the right diameter and slotted into place quite well. The only modification required was to cut about 20mm off one end, so that it would align correctly between the AFM and turbo elbow. The VL's half-inch PCV hose also pushed straight into the fitting in the side of the pipe. Then, with the new duct firmly clamped into place, we decided to see how much the pipe flexed when the engine was revved hard (though not under load). No problems. The flexible hose at the mouth of the turbo was doing its thing, and the new pipe had more than enough 'give' in it - especially after it warmed up and became softer.

So, now, out came the manometer for the final pressure drop test. The improvement - another 4 inches of water slashed from the restriction tally! Take that.

Peak restriction: 13 inches of water (down from 29.5 in standard form)
Peak Intake temperature: 20 degrees C (down from 50 degrees)
0-100 km/h performance: 7.45 seconds (down from 8.0 in standard form)
Total cost so far: $30

Mission Achieved!

With only simple DIY mods totalling a miserly $30, we've reduced by more than half the factory intake restriction, given a major reduction in intake temps and - accordingly - picked up around half a second in the 0-100 dash (with two people on-board). Now that's pretty flamin' good, don't ya reckon?!

On the road, the car's throttle response and torque throughout the entire rev range has also been boosted dramatically - you can even hear the turbo starting to spool up earlier. In addition, the big RB engine's willingness to venture to the redline has also been improved out-of-sight. The standard car's high intake temps previously experienced in stop-go traffic have also been totally eliminated - there's now plenty of response and urge, no matter how long you sit there.

And - again - all for only $30!

Why Didn’t they Build it Like That in the First Place?

The engineers at the OE manufacturer mostly know what they're doing! This is an often-overlooked point when making car modifications, but there are good reasons for how the original VL intake is organised. The mesh screens on the airflow meter protect against hot wire damage, and the air intake located behind the headlight reduces dust intake and prevents water entering the engine during creek crossings. So the other side of the coin is that making mods has trade-offs. In this case, the airfilter will need to be changed more frequently, the engine is a little less protected against the ingestion of rocks, and care is needed when handling the airflow meter.

Julian Edgar

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