When talk turns to extractors and custom turbo manifolds, many an enthusiast starts to get a bit uncomfortable. Cold air intakes and cat-back exhausts are simple enough to speak authoritatively about, but the design and fabrication of those pipes coming outa each exhaust port is a whole lot tougher to understand...
For this article we tapped into the mind of Mark Marchesan of Adelaide's Exhaust Technology. Over the years, Mark has fabricated countless manifolds and, as he says, it's an art form that takes a fair bit of trial and error.
"There are a couple of reasons why there's a need for custom manifolds," he said.
"First, there are the people that want to fit a big turbo and external 'gate and a custom manifold is needed to hook it all up. Second, the factory cast iron exhaust manifolds on some engines tend to crack.
"Nissan SR20 turbos - in the S13 and 14, maybe not the S15 - Cordia turbos and some of the 4G Mitsubishis and Evos always crack. I think it's when you run more boost and the exhaust temp goes up that you run into trouble. Another reason they crack is they get disturbed when they're hot - like if you've had an 'off' or somebody's given you a shunt.
"The exhaust also has to be really well supported and braced and fitted with flex joints. The only problem with flex is ground clearance on some cars."
Mark commented there is often an aftermarket manifold available off-the-shelf to suit many applications - unlike a few years ago, a custom manifold isn't the only solution.
"I think the custom stuff is dying these days," said Mark. "I really only do the occasional 1JZ, rotary and the odd RB manifold. The prefabricated ones have an ease-of-fitment advantage and, therefore, a cost advantage and they give an instant result with minimal variation. The products are also usually proven, with a linear torque band and all the things you expect from a production part.
"They can vary in quality, though.
"A lot of the manifolds from Asian counties are made from 304 and 316 grade in 2mm, but I've seen some 1.6mm gear - that's trash. The 1.6mm pipes can work okay on Rexies, because the manifold is so long and it seems to be more stable in its temperature band. Other than on a Rex, though, I don't think 1.6 stainless would last too long.
"The only way I can get durability out of my custom manifolds is to make them from 3mm steam pipe. The steam pipe manifolds always outlive the stainless ones. Weight can be an issue when you're fitting a big turbo and 'gate, and I don't know how you'd go with stainless - not without a lot
of bracing anyway.
"I've seen some of my Mazda rotary stuff coming back more than five years after a manifold was built for them. With stainless, depending on conditions, I reckon they would only last up to about 2 years - they normally crack where there's a merging of pipes or sudden change of gauge.
"The polished stainless is great to put on display, though.
"The flanges for the stainless stuff can generally be fairly light - maybe 12mm or more, just so long as you can still fasten the studs. With the steam pipe manifolds, though, I use 16mm flanges - you can just lay heat into them and they're fine.
"The thick flanges also give the opportunity for future machining as well - the stainless ones wouldn't last long enough that the flange would ever need machining.
"There are a lot of principles involved in extractor design, but - at the end of the day - there's only limited space in a given chassis. It's important that the primaries are between 6 to 12 inches long and the pipes merge smoothly - collectors can vary a lot
"The steam pipe bends come in a number of different radiuses - the tightest one you can get is a 3A radius, which I think is about 3-inches in overall donut diameter. Once you've got the flanges done, it's just a matter of cutting the sections of bend you need and welding it all together."
Another important step Mark mentioned was to keep the oxygen sensors(s) the same distance away from the exhaust port as factory - this will minimise the chance of drivability problems.
"If you have some skills you can make a manifold yourself - they're really dead-easy so long as you have all the right parts and equipment. The only problems you can have are the studs - a lot of the Mitsubishi 4Gs are notorious for busting studs and they're a pain to get out.
"We can supply a do-it-yourself package with all the necessary parts included; all it needs is communication with us to work out what you need. I sent a supply package to the Eastern states the other day with all the bends, pre-cut flanges, nuts and everything that's needed and that was about $320, including a bit of over-the-phone guidance.
"I've seen guys sticky-tape their manifold layout together and take it down to get welded - that's not a bad idea.
"Of course, doing a lot of it yourself makes the end cost of a manifold a lot cheaper. As I said, we sent off a supply package for $320. For us to build a custom manifold, you're starting at about $780 for a four cylinder with an internal wastegate. It goes up to about $1250 for a four-cylinder with an external 'gate and, of course, it's more again for 6s and 8s.
"In terms of power, I've seen some of our customer's dyno figures - they've run the same top-end power with 2 or 3 psi less boost. So a new manifold can be quite an efficient upgrade. One thing I'd advise is to make sure you build it right to suit any future mods."
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