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There are a lot of common misconceptions between welded and cast turbo manifolds. This article will help you understand some of the differences between the ones we offer, and which style will better suit your build.
First we are going to cover the difference between the Turblown Cast Rx7 turbo manifolds, and run of the mill cast iron turbo manifolds . Most cast manifolds on the market only serve one purpose; to make a turbo fit, and to do it as inexpensively as possible. This really doesn't cater well to the enthusiast who is looking for maximum performance.
One will notice that most cast turbo manifolds weigh a lot. This is because they have a higher carbon content. This carbon content also conducts heat more efficiently, basically turning the manifold into an oven in your engine bay. This is part of the reason why the OEM twin turbo FD engine bays are so hot. It has been theorized that this is part of the reason the FD's lose coolant seals much earlier than previous models.
The carbon content is also the reason why these run of the mill cast iron turbo parts rust easily, and over a very long term duration do crack. It is to be noted that cast iron turbo manifolds are stronger than welded manifolds. Even a properly welded turbo manifold, will fail much sooner than a cast iron manifold. This is just the nature of casting vs welding.
The Turblown Cast Rx7 turbo manifolds are made from 347SS. The SS stands for stainless steel. Cast iron has about 20X as much carbon as stainless steel. The Turblown Cast manifolds actually weigh even less than their welded counterparts of the same design. This is possible because there is no excess filler rod on top of and inside the runners & flanges.
Generally most EWG cast manifolds over look waste-gate performance. There is usually just a single waste-gate of too small a size, and placed very poorly. The below manifold is notorious for this when a larger than 3" exhaust is used on a single turbo FC3S. The stock twins are even worse, and always require porting when using any sort of full exhaust system. Poor waste-gate placement leads to boost creep. This is generally only a problem for pump gas only cars, or those want to run too large of a turbo at low boost pressures etc.
Proper EWG placement is shown below courtesy of Turbosmart.
The Turblown Cast EWG ( external wastegate) turbo manifold has ideal waste-gate placement for proper boost control. The runners are right in the direction of flow, and the twin Turbosmart comp40 wastegates offer plenty of surface area.
In addition to overlooking waste-gate performance, they also overlook boost response and overall power output. Generally there are a lot of tight bends, which is the enemy of a turbo system. These sharp turns absorb a lot of heat & pulse energy that could be used to increase shaft speeds. How well the turbo manifolds transition from the exhaust ports to the turbo manifold, and from the turbo manifold to turbo flanges also makes a difference. As you can see below the Turblown Investment cast manifolds are designed around this criteria. Obviously some bends are necessary, but these are limited.
Single turbo setups do not have sequential options like the OEM twins, so you generally fit a small to medium frame turbo, and its response is "ok" on a standard cast manifold. Not great, but not terrible. If you try and fit a larger frame turbo onto these manifolds, your power-band will suffer considerably, especially if its dated turbo technology like a To4R/Z. The last To4R setup we tuned made 22psi at 5k rpms for example. Anything bigger than a To4Z usually don't fit either. So you are limited in turbo selections, and therefore overall power levels.
About 12 years ago we did our first cast vs tubular manifold dyno comparison, and the results were pretty stunning. The car picked up 800 rpms in boost response, and 40rwhp on the top end at 15 psi. We attributed these gains from the more free flowing design( more gentle bends & fully divided system).
Most cast iron turbo manifolds are not fully divided. Most are undivided, and use an open volute. Some do have fully divided main runners, but use a single wastegate. While this is better, twin wastegates seperate the pulses completely leading to even better boost response. We touch on that in this article dedicated to the Turblown Cast EWG FD turbo manifolds.
So we've covered that the run of the mill cast iron turbo manifolds for the most part are only better than a proper welded unit in longevity. In all other performance areas they are lacking;
This is where the Turblown's Cast Manifolds bring together the durability of a cast manifold, and the performance of a welded manifold. Since an investment cast process is used inconjuction with 347SS these are even more reliable than a cast iron turbo manifold. They are the most durable option on the market for your Rx7. 347SS is much stronger at elevated temperatures compared to both cast iron, and 304SS thats most commonly found in welded turbo manifolds.
More importantly these investment cast manifolds perform even better than their welded counterparts. The investment process leaves the insides perfectly smooth, & each and every transition is perfect smooth. These out-flow the welded turbo manifolds. The turbo manifold industry has been slowly shifting towards billet collectors, think of this like an entire billet manifold.
Furthermore performance is increased drastically by having a tapered manifold design. All of the Turblown Manifolds feature this, but the EWG version takes this to the extreme. The manifold gradually tapers all the way from the engine flange to the T4 flange. From a 2" ID down to a 1.682" ID. Since developing this manifold, you will notice people are already trying to copy it. Well its not possible to replicate this unless you cast it; the main entrance runner is larger than 2" pipe, and the taper gradual, not abrupt. 
As we mentioned previously you can see the least amount of bends are used.
So why aren't investment cast manifolds more common if they are the end all be all of turbo manifolds? It is simple most companies don't have the resources or demand to make them. Stay tuned as we release back to back dyno results with the new Turblown Cast EWG turbo manifold vs the HKS Cast FD. Same dyno, same tuner, same day, with full datalogs available..
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