Wastegate/Boost Control Issues. Poor Manifold Design


I have written this to hopefully educate some people on how and why turbo/wastegate combos need to be set up properly.
We have seen way too often, people come in with boost control issues. They have been on the dyno several times, changed wastegate springs, changed wastegates etc chasing this problem, when 99 times out of 100 it comes down to poor turbo manifold/wastegate design/execution. Most of them have well made turbo manifolds with a nice collector, flat flanges, ported nicely etc. Then they have a putrid dirty hole bored into the side of the collector, usually only covering 2 pipes, and have a 90° pipe welded in!

Now you don’t have to be a rocket scientist to see that this won’t work efficiently for many reasons.
Exhaust gases, like most gases or fluids, will take the easiest path. When an exhaust manifold has super high back pressures ( i will touch on this a little later ), the gases will go out the wastegate, due to atmosphere being lower pressure than what is in the manifold. But, put a large rear housing on one of these manifolds, and everything changes. The turbo is trying to get everything it can, so will take any and all of the exhaust it can get. Why would the exhaust go out the wastegate when there is a nice big turbo there, in direct stream of where the gases are going. In this situation, they simply don’t go out the wastegate. You could have a 7 psi spring in there, and still see 25+ psi with boost climbing, and the wastegate 100% open. Yes we have seen it. Ask most tuners, they have all seen it. Then usually the only fix is to either put the gate on the rear housing, or start again. $$$$$ Either way, its a new wastegate dump and possibly new dump pipe if you start again.

Wastegates need flow to work, not rely on shitty high back pressures to force the gases out. Turbosmart have a good read on their website about this, and have quite a few pictures also. Think of the gases hitting a Y, with some going to the turbo, and some to the gate, then when it opens, the exhaust actually goes out, to control boost.

These 3 pictures are straight from turbosmarts website.

Now, the ideal situation in an exhaust manifold, is a lower pressure than in the inlet manifold. Yes it is very hard to achieve, with most of the time there being a lot of compromises in turbo housing sizing due to lag, packaging etc.., especially for a street car. Race cars are easier to achieve this. When a manifold has super high pressures ( higher than inlet ), when your valves (inlet/exhaust) are open, gases try to ‘reverse’, as fluids/gases will always try to equal out. This is a tuners nightmare, along with them chasing tuning issues, they are chasing boost issues also. Heat is also worse when you have stupidly high backpressures.

When you see a manifold with a 90° gate supply and a 60mm wastegate feed on a T3 manifold, you have to ask yourself why ? because they are trying to compensate the shit wastegate feed/position with a large wastegate. I have seen 2 put on there and still struggle !!! Spend a little extra getting a decent manifold made to your application and you will suffer less pain in the long term. Dump pipe, wastegate dump and Turbo costs the same. You will spend extra $$$ chasing tune ups, heat proofing everything in sight etc…

Everyone seems to shop around for the cheapest manifold, then spend extra $$$ doing things twice, or chasing simple problems that can be eliminated with a decent turbo manifold. As in one with good flow to the wastegate, not at 90°.

This shows how a wastegate feed should be. Gases hitting the wastegate tube, whether they want to or not.

While we are talking wastegates, puting a tiny feed to a large gate, why ? If you have a 50mm gate, give a tube/pipe of 50mm inside diameter. If there is 2 pipes feeding it, work out the area of the 2 smaller ones, and added together they should at least add up to the same or be slightly larger than the one they are supplying. Yes we have seen it. The following pictures give you an idea of this, this has 2 pipes, with an internal diameter of 35.08mm each, feeding a 60mm gate. Each one of these has an area of 966.51mm². times 2 = total area of 1933.02mm². 60mm gate has an area of 2827.43mm². Anyway, check it out for yourself.


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