Choosing a turbo manifold to hang that snail on (Part I of II)

So you’ve already chosen what turbo you wish to use on your particular application; now comes a decision with just as many choices--selecting the turbo manifold. And there certainly is a wide variety to choose from, both in brand, with manufacturers seemingly springing up overnight, and in price, ranging from the $64 sold-on-eBay Speed Racer Performance manifold, to the $1,500 Full Race tubular, equal length manifold that is so masterfully welded and artistic looking that you may not want to bolt it to your greasy engine!

As with turbo selection, what type of manifold you choose is contingent on what goal you have for your particular turbo application. And while you’re sort of stuck on needing to pay at least $500 for a good turbo (remember, you get what you pay for!), the manifold is an area of your turbo kit where you can either conserve your money and only drop $200 or so, or spend it liberally on a manifold that costs six times as much. As was just said about turbos, you get what you pay for in manifolds as well. Set aside at least $300 if you’re looking to have a durable, well-made log style manifold, or $500 if you prefer a tubular, more equal length manifold.

Because the turbo manifold is responsible for directing all the exhaust gas from each cylinder into a single turbo inlet, the type you choose can have a huge effect on the engine’s powerband and performance, much more so than the type of exhaust header(s) one chooses to use on his naturally aspirated engine. Next to the turbo, the manifold is the second most-essential ingredient to any turbo kit, and the one you choose will dictate the physical dimensions of fabricating the rest of your kit (e.g., where the downpipe, intercooler and turbo piping will be), so research the manifold thoroughly before purchasing one.

Otherwise, if you buy a particular manifold and plumb the turbo based on where it sits on this manifold, and then decide you want a different one, you'll need to re-do a lot of labor-intensive work. Turbos often have very tight clearances in the engine bay of a previously naturally aspirated car, so even if the turbo is only moved an inch on the new manifold, you might still need to re-do the whole downpipe and (external) wastegate, for example.

Internal vs. External Wastegate

The first decision you need to make when it comes to picking the right manifold is whether your setup will use an internal or an external wastegate. Both types of wastegates regulate the amount of boost the turbo will produce. The internal wastegate, you might have guessed, is an internal component of the actual turbo.

In a turbo with an internal wastegate, a hose pressurized by the turbo system is attached to an actuator (this hose usually gets its source from the intercooler piping right before the throttle body, or from the intake manifold). When a certain boost pressure is reached by turbo based on how the actuator is adjusted, a diaphragm-type actuator pushes an adjustable rod that opens up a valve inside of the turbo. This valve covers a port (about 20mm in diameter) that leads from the turbine inlet to the turbine’s outlet (see picture below). Therefore, exhaust gases that would otherwise enter the turbine of the turbo and cause more pressure are now expelled when this valve is open. This exhaust gas then joins the rest of the gas exiting the turbine outlet, and both gases flow through the downpipe and through the rest of the car’s exhaust system.

(courtesy http://www.lotusespritturbo.com/Garrett_AiResearch_T3_Turbo_Charger.htm)

Turbos with an internal wastegate are far easier to plumb, that is, there is nothing additional to plumb because it's part of the turbo. You can also fine-tune the the boost level at which the wastegate valve opens by either shortening or lengthening the adjustable rod. Most basic turbo kits available for your engine, as well as most cars turbocharged from the factory, will likely use an internal wastegate; it's simply cheaper and easier to avoid the external wastegate hassle, and at lower boost levels, performance is not hindered by an internal wastegate.

If your turbo came with an internal wastegate but you want to go external, don’t worry. You simply need to unbolt and remove the internal wastegate assembly (usually 5 bolts), and then use a flange (part number TK049) for your downpipe that will bolt up to the turbine outlet while covering this hole. Conversely, if your tubo does not have an internal wastegate but you don’t want to go external, a complete internal wastegate assembly can be had for about $100 on eBay.

However, internal wastegates have their limitations. If the turbo is producing high enough boost (15+ psi) and flowing a sufficient amount of air, there is the possibility that even when the wastegate valve opens, all of the pressure isn’t successfully released because the volume of air flowing into the turbine at that pressure simply overwhelms the small port leading to the outlet; it isn’t big enough to flow the amount of exhaust gas necessary to relieve pressure. Because the pressure isn’t released, a phenomenon known as “boost creep” often results, and the boost pressure produced by the turbo actually increases, which can be dangerous or even catastrophic to your engine if it increases enough.

If you’re worried about boost creep and plan on running high boost, or are looking to make the most power in the most efficient manner possible (e.g., for drag racing), you’ll want to use an external wastegate. In an external wastegate setup, there is a cylinder that has a valve, an inlet, an outlet, a spring, and a vacuum fitting that receives a boost pressure signal from somewhere after the turbo compressor.

This wastegate is, you guessed it, outside of the turbo, usually on the manifold close to where the turbo inlet bolts up. An external wastegate valve does the same thing as the valve found in an internal wastegate: when a certain boost pressure is reached, the spring is compressed and the valve opens; excess exhaust gas that would otherwise enter the turbine exits into a small tube (usually 38mm) that is either vented to the atmosphere or welded to the downpipe. There is almost no possibility for boost creep in an external wastegate configuration because the valve and the hole it closes are considerably bigger and flow much more than is possible by an internal wastegate.

External Wastegate Manifolds

Thankfully for those new to turbos, it’s easy to tell which manifolds are made for internally wastegated turbos and which ones aren’t: the manifolds that have only one large flange (for the turbine inlet to bolt to) in addition to the flange that bolts to the engine’s head, will require the use of an internal wastegate.

Manifolds that have both the large flange for the turbo and another, smaller, diamond-shaped flange near the big one need an external wastegate to bolt up to that smaller flange. So make sure you can view detailed pictures of the manifold before buying.

Now that you know which way you’re going in regards to the wastegate, you still have another decision to make. Read Part II of this article to learn about the two basic types of turbo manifolds and the benefits and drawbacks of each.