Pipe Dreams
Story Craig Parker (STREET MACHINE)

It didn't take a rocket scientist to realise the Gen III (LS1, call it what you like) fitted to the Aussie V8 Commodore had a fair bit of grunt up its sleeve. After all, the exact same engine was rated at a far healthier 256kW when slotted under the bonnet of a C5 Corvette. As such, a number of Aussie companies have been burning the midnight oil researching ways to recover this latent power – with many reporting a tremendous amount of success.

With our VU FOR U project in full swing, we decided to explore a few of the better-known offerings. First cab off the rank is an exhaust upgrade. D.P.E (03 9747 0747) is very proud of its complete header, catalytic converter and tailpipe package, so we decided to give it a burl. Besides the full system, they offer a number of options but feel there’s too many deficiencies in the original system and recommend replacing the lot for maximum grunt growth. We took their advice and opted for the full 2.5-inch dual system, which includes their gorgeous stainless steel headers, high-flow cats and dual in-and-out rear muffler.

Best of all, the D.P.E system can be mail ordered ($2300 landed on your door step) and fitted at home in the driveway in around four hours. Alternatively, the less enthusiastic can have one of the many D.P.E dealers supply-and-fit the system. Along with the five-year warranty and healthy increase in horsepower, D.P.E also claims improved fuel economy through superior air/fuel ratio. More power and better fuel economy? Bring it on!

Step No. 01

While it might seem over cautious, always disconnect the battery before beginning any mechanical work on an ECU-controlled car. Even an accidental arc on the starter motor terminal is enough to shoot a damaging electrical spike into the ECU.

Step No. 02

While a workshop hoist will make the job considerably easier, driving the car up onto a decent set of car ramps will get the nose high enough in the air to make fitting the system at home in the driveway heaps easier.

Step No. 03

Out with the old. After removing the Ninja Turtle engine cover, unbolt the old manifolds and yank ’em out. Holden uses a very resilient thread-locking compound so the bolts are tight to undo all the way to the very last thread.

Step No. 04

Holden’s plug-lead retainers are a fair mongrel to get off; it’s a bleeding-finger, long-nose-pliers job. To gain access to the passenger-side manifold you’ll also need to unbolt the coil pack and lay it up on top of the inlet manifold.

Step No. 05

Thanks to the IRS – rather than a solid rear axle – removing the old system is a snack. Undo a few bolts, and pop off these clips holding the rubber fasteners in place and it’ll almost fall out all by itself.

Step No. 06

Ready for collection by the local metal recycler. Unlike a seasoned system where everything has pretty much welded itself together, the VU’s youth meant all the bolts came undone without too much persuasion

Step No. 07

The passenger-side header goes in from the top and is a pretty simple operation. Screw all the bolts into the soft aluminium head a few turns by hand before tightening – otherwise you might be rethreading with a Helicoil.

Step No. 08

A leaky exhaust system can mean a groggy and drowsy driver – very bad. Di Filippo recommends applying a small bead of special heat-resistant silicon to ensure header flanges and flange joints are a 100 per cent leak-proof.

Step No. 09

Things are a bit squeezy on the driver’s side, necessitating a two-piece header. As with the rest of the system, the mail-order kit contains with all the necessary bolts, nuts and gaskets. Plus a detailed instruction sheet.

Step No. 10

To improve access on the driver’s side it’s necessary to remove the support bracket/panel between the chassis and the firewall along with disconnecting the steering shaft from the rack – turn the steering wheel hard right before completing this step.

Step No. 11

Up, up and away. Get the header flange bolts in a couple of turns then drop the single header pipe in from the top. Get all the bolts started then tighten up the header flange bolts before nipping up the four flange joint bolts.

Step No. 12

After experiencing leakage problems with three-bolt header flanges, Di Filippo now uses 10mm-thick, four-bolt flanges for the collector/cat joints. Note the twin O2 sensors sticking into the pipes, they mount-up in exactly the same position as standard so everything is nice and legal.

Step No. 13

As can be clearly seen, the exit pipe on the original catalytic converter is flattened to less than an 1 1/8 inch of cross sectional area. The new Di Filippo 2.5-inch stainless jobbies are far less restrictive.

Step No. 14

Up to the 2.5-inch high-flow cats the system is all stainless. From there, D.P.E relies on thick-wall 2.5-inch mild-steel tubing and mufflers for the rest of the system. Full stainless can be specially ordered.

Step No. 15

The twin pipes into the rear muffler are the only slip joints in the whole system. This allows adjustment of the dual outlet pipes to ensure that they’re nice and level.

Step No. 16

D.P.E offers a number of tailpipe options, including these neat, rolled-edge chrome tips, so have a chat to them regarding your preference. We chose this dual outlet which allows the system to remain a true dual system all the way through.


Even ignoring the new deep, throaty growl, you didn’t need a dyno to tell you the new exhaust system had made a huge improvement – you could now haze the tyres by hard-shifting into third. In fact, there was enough of an increase to find the limits of the factory clutch. But for all the number junkies, we decided to dyno the car before (black line) and after (purple line) for some hard numbers. As you can see, even with a mildly slipping clutch, there was a solid improvement all the way from the bottom of the rev range through to redline. Verdict? Worth every penny and all Gen III owners should rush out and buy one. One word of advice, you’d better budget a few extra dollars for a beefier clutch and start buying shares in a tyre company!











Music to Your Ears

There’s nothing sweeter than the sound of an angry engine doing its work. This is why it’s aggravating when television programmes supposed to be all about speed and performance dub music over the images of cars. They seem to have missed the point that hot engines are music to the ears of any serious petrolhead. Darren Di Filippo is a serious petrolhead (he owns and drives a Top Fueller and Nitro Funny Car and built the world first nitro Doorslammer) and makes his living creating exhaust systems that improve both the performance and sound of whatever cars they’re attached to. His opinions on exhausts are some of the most controversial in the performance industry, which is a clear invitation to publish them as far as we’re concerned.

Rock the Boat

Probably the most antagonistic of all his views is that tri-Y extractors simply don’t work. “It amazes me that in 2008 people still ring up and ask for tri-Y headers”, he says. The first thing he asks them is, “Don’t you want your car to go that well?” He suggests that the only benefit of the tri-Y design is that it’s easy to make. “All the interruptions and joins actually impede gas flow and it has no scavenging abilities at all. The tri-Y was made as a cheap replacement for broken manifolds on old cars.” Rounding out his view of this pipe arrangement with a bit of hyperbole Darren suggests that the name tri-Y comes from the fact that someone tried to make a set of four into one pipes but couldn’t and wondered Y.

Some will say that Di Filippo takes this position because his company only manufactures four into one systems. But consider that he could easily make tri-Y pipes if he wanted to, and says it’s actually easier and cheaper. We suggested that there are many very respectable engine builders out there who disagree with him and insist that the tri-Y design most definitely does work. What’s more, the opinions of many of these people are backed by solid dyno and track testing. His response was that his opinion is also backed by extensive dyno and track testing. Certainly, much of it is with V8 Supercar teams but a great deal is also with more modestly powered engines traditionally said to be suited to tri-Y headers.

Darren does however acknowledge that the design of a four into one system must be correct for it to work. Back when he started the company and began making four into one pipes, he says they discovered just how difficult it is to get the design right. Still, perseverance paid off and when people started bolting them on they were impressed with the instant power gains they got. “Within two years they were being sold everywhere. Since then, we’ve found it hard to keep up with demand.”

Yes, but...

Despite all this up-beat conversation about the company’s four into one designs we couldn’t leave alone the fact that we know people who’ve bolted tri-Y headers onto engines on dyno’s and seen gains over four into one systems on stock engines at low revs. “So, what about that, when the numbers definitely show that tri-Y headers do work in some situations?” we asked. Darren’s answer was that in these cases the four into one designs replaced were probably not appropriate for the engines in question. He pointed out that a number of things have to be correct. Primary length and collector design are certainly important but the most critical aspect is pipe diameter. He feels that in an engine that made gains on swapping from a four into one system to a tri-Y, one or more of these factors was wrong for the engine in the first place.

Despite the critical importance of primary pipe diameter, Darren says that specifying a range of sizes against certain engine displacements in a table isn’t really the thing to do. Building the optimum system calls for consideration of whether the engine is blown, turbo’d (obviously), nitrous assisted, or normally aspirated. The overall combination including induction system, cam, porting, valve sizes and the like are also vital. And then there’s the rest of the driveline including manual or automatic transmissions, diff ratios, wheel sizes and tyres. Building the right system for a particular car/application comes down to experience and good judgement.

There are however, a few core concepts used in D.P.E systems.  Most headers are in the 28-inch to 32-inch range and come together at 14 degrees to form a merge collector that’s between about eight inches to twelve inches long. As we said, the final dimensions are determined by experience. One thing we can quantify, though, is the method for determining the ideal collector length on a strip car. Darren says run the car down the strip a couple of times. Then, look inside the collector and you’ll see a grey ring has formed. Trimming the collector back to this point results in the ideal length. An accurately made collector of the appropriate length causes the gasses to speed up and aid scavenging. This alone is worth around four or five horsepower.

Let's see, who else...

Now, just in case we haven’t upset enough people Darren’s next statement might do the trick - stepped headers don’t work in most situations. His rationale for this opinion is that gas shrinks as it cools. When exhaust gas leaves the cylinder it’s about as hot as it’s ever going to get. As it travels along the exhaust system it cools and consequently reduces in volume. At about fifteen inches away from the head, which is around the point at which the headers are stepped up, the volume has reduced enough to warrant a decrease in diameter to keep gas speeds high. A controversial view for sure but I do know of one recent test in Sydney in which this theory worked.

It’s true that the steps in such headers were actually for controlling reversion pulses rather than for gas flow control but even so, Darren suggests that it doesn’t really work except in the narrowest range of drag racing circumstances. In other applications reducing the diameter at this point creates more torque earlier, which is useful in a circuit car. Again, Darren feels that anyone who’s made more general power with stepped headers as opposed to four into one pipes probably had the primary lengths and diameters wrong in the beginning.

Darren says people also misunderstand stainless steel. Most systems are made from 316 or 304 grade material. This is good for polishing but bad for exhaust headers. These grades expand tremendously when hot and if, in that condition, they get splashed with water from a puddle they immediately undergo local shrinkage and crack. This is bad. D.P.E uses 409 grade stainless because it doesn’t expand anywhere near as much and doesn’t suffer that problem. The thing is, 409 stainless is somewhat magnetic. People have been known to check it with a magnet, find that it sticks, think they’ve been ripped off and start swinging. What makes it seem even worse is that 409 can also develop surface rust. Although it doesn’t rust through like mild steel people still feel that they’ve been cheated. Ironically 409 lasts about ten times longer than the other grades, which is why OE manufacturers use it and can include the exhaust system in a five year warranty.

X-pipes are an essential feature of modern exhaust systems but Darren insists that many of them are made very badly. The join, he says, should be nothing more than a hole about the size of a twenty cent piece. X-junctions in which half the pipe is cut away over six inches are a disaster because they mix the gas flows from either side of the engine and then separate it once more. This slows down the gas and causes droning inside the car. Keeping the hole small evens out the pulsations without mixing the gasses. Done right, Darren suggests this modification is worth around 40Nm. The H-pipe does a similar thing but again, it allows gas transfer from side to side and causes bad interference in a similar way to a big junction in an X-join.

We haven’t upset any muffler manufacturers yet. Let’s rectify the situation. Darren suggests that some companies come up with gimmicks presented as new and revolutionary designs yet the basics hold true. He says, “I’ve been in the exhaust game for nineteen years and my father has been in it for thirty-five years. We test everything we can get our hands on and we’ve never seen anything that flows better than a straight-through, stainless wrapped, fibre packed, perforated tube muffler. That’s why we make them.” He went on to say that sometimes they’ll create a chamber within the muffler but that they’re moving away from that idea and replacing it with an internal Y-branch. “Don’t get caught paying big dollars for flash names, fancy styles and all the hoo-ha.”

Stick a Cork in it?

There are a number of exhaust system myths about which Darren has something to say. One, which thankfully seems to be a touch less common than it has been in the past, is that engines need back pressure in the exhaust to run properly. Some people even suggest that it improves torque. Darren says this is incorrect and we have to say, we agree. In fact, in all the time I’ve been in the industry no one who believes this has ever been able to provide a logical explanation for why it might be so. It just isn’t right. There are a number of theories about why people believe this but one of them could be that reducing the diameter of the primaries can improve torque. However it’s not increased back pressure from the restrictiveness of the small pipes that does so, it’s the increased gas speed through the reduced diameter.

Gas speed is one of the most important things for getting a great note out of the rear of the car according to Darren. “If it’s done properly there will be no drone in the car at all. The XY Falcon got it right when the headers went into a 2¼-inch dual system up to the diff and then into 2-inch tailpipes.” He continued, “It was a mistake at first because they couldn’t fit 2¼-inch pipes over the axle tubes with the required clearances so they dropped the pipe diameter down to 2” to solve the clearance problem. Voila the legend was born. When you hear one going down the road you just have to stop and listen because it’s so tough.” Di Filippo Engineering commonly uses this practice of dropping the diameter of the rear pipe over the rear axle or drive shafts of an IRS vehicle to maintain gas speed and create a nice note into the bargain.

So, that’s a bit of a summary of exhaust systems according to D.P.E. You may disagree with what Darren has to say, which doesn’t worry him all that much as you might have guessed. Or, you may feel as we did; that it’s admirable for someone to hang his opinions out there for everyone to criticize. Darren wanted to make clear that he’s not presenting himself and his company as the ultimate reference for all things exhausting. “Aside from the production exhausts and specific testing, we do around fifty custom jobs each year. These include drag cars, circuit cars, boats, trucks, bikes, etc. With that many custom jobs passing through the shop each year we just get a good idea of what works and what doesn’t.” The company website is DPE.NET.AU or you can call 03 9747 0747.

Hitting the Right Note

Fender and Gibson guitars sound different as Holdens and Fords sound different. But it’s difficult to quantify because so many things affect the sounds of guitars and cars. Obviously, straight-through mufflers like those shown give a much more direct sound than a heavily baffled interference designs. Stainless steel systems can sound a little ‘tinnier’ than mild steel systems. X-pipes and H-pipes give a smoother note because the pulses from each side of an engine are evened out.

It’s not hard to pick a difference between tri-Y and four into one systems. The four into 1 will have the v8 pulse in the collector when done right. I used to go to the drags with a friend who could always pick which engines were going to grenade. He’d be the only one. Not even the team would know but he’d say “That won’t last” and sure enough – bang. He never could quantify what he was hearing, though. “I don’t know, it just didn’t sound right” he’d say. Getting the tune right results in a smoother exhaust note. This is why older carburetted engines often sounded incredibly rough at idle. Many liked the sound but it was actually just an engine running badly and   waiting to get open up .

Although bigger cams are obviously louder they can disguise the basic characteristics of an engine. Increased duration makes for a ‘lumpier’ sound. As you’d expect, the way fuel burns makes a difference and methanol creates a smoother note than petrol. Header diameter doesn’t seem to make a great deal of difference but where the tailpipe dumps does. If it discharges onto the ground it tends to make a deeper noise, particularly if it does so in front of the axles. Side discharge systems sound different on either side of the car because you’re only hearing from one bank. Lastly, EFI sounds smoother than a carburettor simply because an EFI engine is likely to be tuned more accurately as the air fuel ratio is 100%.