RSS
Home
Choosing a turbo manifold: log-style or tubular? (Part II of II)
Print
Email
RSS
Subscribe
Part I: internal vs. external wastegates
Of the plethora of manifolds available, each falls into one of two categories based on its construction: log or tubular. And just like the turbo and wastegate, whether you choose a log or a tubular-style manifold depends on what your power and performance goals are.
Log Manifold
A log manifold looks just how it sounds: clunky and ugly. It is about as basic as a turbo manifold can get. The design is rather rudimentary and not terribly high-flowing; in fact, the same design would probably hurt a non-turbocharged engine’s performance more than it helped.
Most log manifolds, due to their stubby runners and smallish collector, do not employ external wastegates; there simply isn’t enough room to place the inlet of the wastegate efficiently, and you don’t want only one runner doing all the work of relieving the boost pressure, as is often the case with a log manifold employing an external wastegate. In general, the best and most efficient location for the wastegate is as close to the turbine inlet flange as possible so that each runner has an equal chance to expel excess pressure. Stay away from manifolds that have the wastegate flange welded on one runner or far away from the turbine inlet flange. See the picture below for an example of a log manifold with bad placement of the external wastegate:
Basic log manifold (this one's made out of stainless steel, not mild steel)
The other reason most log manifolds don't use external wastegates is that if someone is going to spend the extra money and go through the extra steps of plumbing an external wastegate, chances are they also have the extra money to buy a better, tubular manifold.
Log manifolds do have their benefits though: they’re cheap, they’re easy to make, they’re durable (especially if cast and not welded), and they allow the turbo to spool very quickly due to its stubby primaries (an exhaust primary is the first tube through which exhaust gas from the cylinder flows; there is one for each cylinder) and overall smaller volume. If you go with a log manifold, make sure that the flange that bolts to the head is cut between each runner. The resulting spaces between the flanges of each runner will allow them to expand and contract independently as they’re heated up and cooled down, preventing the chance of cracking. Log manifolds are usually made of thick, mild steel--again, there's no point in using expensive and hard-to-work-with stainless steel if the main selling point of a log manifold is price.
Tubular Manifolds
Full Race Equal Length Tubular Turbo Manifold
A turbo manifold is usually called tubular when it's any design other than log-style. They generally have much longer exhaust primaries with multiple bends. While tubular manifolds can be made from inexpensive mild steel, the best are made out of stainless steel, which is thinner walled than mild steel but lighter and stronger nonetheless. Stainless steel manifold will never rust or tarnish. The amount of fabrication, welding, and research and development involved in producing a tubular manifold makes it considerably more expensive than a log manifold.
Additionally, because of the longer primaries and various bends, tubular manifolds can end up having a lot of welds, which coupled with the length of the tubes, makes the manifold less stiff than a smaller log style. If the steel is stainless (which, while stronger than mild steel, is more prone to cracking under repeated heating and cooling cycles), it is especially important to make sure to cut (if not already), in between each primary, the flange that connects the manifold to the block. Doing so will allow the steel movement to expand and contract as it heats and cools.
There is much more theory behind the development of tubular manifolds, and as is the case with all bench racing, the theory is subject to debate. Nevertheless, most agree with Garrett Turbochargers when it comes to manifold design. The following list of what to do when it comes to making a tubular manifold comes straight from their website:
-Maximize the radius of the bends that make up the exhaust primaries to maintain energy (tighter turns = slower exhaust gas)
-Make the exhaust primaries equal length to balance exhaust reversion across all cylinders. (Exhaust reversion is when the exhaust gas flows back toward the exhaust valve instead of away)
-Avoid rapid area changes to maintain energy and exhaust flow to the turbine
-At the collector, introduce flow from all runners (a "runner" is a segment of the exhaust tubing that comes after the primary) at a narrow angle to minimize "turning" of the flow in the collector
-For better boost response, minimize the exhaust volume between the cylinders' exhaust ports and the turbine inlet
-For best power, tuned primary lengths can be used
Tubular manifolds are more expensive than log manifolds, but they do have their benefits. The design is far more efficient and high flowing, and while 350 whp can easily be produced using a log manifold, if you are looking to run a lot of boost and plan on making a lot of power (400+ whp), a tubular manifold will get you to your goal quicker. And while the longer tubes might increase the time it takes the turbo to spool up, a tubular manifold will never restrict flow (and therefore power) to the turbo in the upper RPM ranges. In fact, if you have the money, you can have the quick spool time of a log manifold with the high flowing characteristics of a tubular manifold: just shell out around $1200-$1500 for a truly equal length turbo manifold (wherein each primary and runner is the same length as the others).
So now we have the turbo, the manifold, and the wastegate. Next we need to plumb the turbo, so be sure to check back for the next article!
Useful Links:
Related Articles:
Add a Comment
.jpg)
