I'm building a twin turbo boss 429 . I do not like the placement of my wastgates. I am considering moving them to the back of the manifold. But I am worried they will not work as well there. They are both 50mm. Turbos are 78mm this will be a low boost setup. 10# maybe 14#

 

Placement is a huge factor, especially on low boost. What i'm picturing is a bad idea, but pics would help.

 

I would love to upload pics but can't seem to figure out how to do that from my phone.

 

Where are they located currently?

 

I don't see that working all that well, pics would definitely help to figure out where you want the gates, if you want the gates at the manifolds I would put them just after the exhaust pipes bolt to them, just Y off the existing pipe and you'll be able to control the boost a lot better than what I'm thinking you'd be trying to do on the manifold.

 

Unless you have a ton of back pressure, the exhaust isn’t going to want to exit at the rear very easily. Maybe dual 60mm gates would be able to let enough out but it’s totally going opposite form where your flow needs to go.

 

Here are some pics. I made what I already had work. Thanks for the input. Had this car 18 years. Decided to go big in the build. Been collecting parts for a years. Should be driving soon.

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I'm not going to say it will work or not, more experienced people can chime in for that.

But... I think the gates off the back of the log manifold would give them almost the same priority as the turbo at the front. I'd try it

Edit: Just wanted to add this car is awesome!

 

Priority to gate means it is easier for the gas to go out the gate than the turbo. The best place for a gate is somewhere like a bend where the exhaust gets a straight show out rather than having to make the turn. A turn is a restriction to flow. You can have a massive gate, but if its still easier for the gas to flow out the turbine that's what it will do.

 

Pressure is a scalar, it has no direction or vector associated with it. So at first glance it looks like it doesn't matter where we place something like a gate or turbo, since pressure drives flow and pressure has no direction it should work fine.

However, upon closer inspection, The mass flow of an engine in/out certainly carries kinetic and momentum energy, which DOES associate to a vector and direction. It can become turbulent, it can experience shear forces, it forms a boundary layer- there are many individual fluid dynamics associated with a flow rate to consider, especially under heat and pressure and large flow rates.

to put this simply, the direction of the flow is very important. You want the mass flow to be as non-turbulent as possible, and you want it all moving in the same direction very smoothly. Therefore, you want the wastegate to be in a position where the exhaust flow can maintain it's current moment and kinetic energy, only redirecting it slightly, and not have to stop and reverse direction.

I believe if you install the gates at the rear of the manifolds, it will cause a lot of turbulence and create a very unwelcome environment for the exhaust flow characteristic, causing a lot of annoying do-over type of fabrication.

 

So I decided to leave the gates where they were at. I rebuilt my downpipes and found a new routing for dump pipes. But one thing I wanted to throw out there. Look at the external eastgate mounting for alot of the import cars. It is directly on the manifd and sometimes in an awkward position. I am tempted to build a test motor just to try multiple locations. I enjoy reading theories but at the end of the day, that is all most of them are. At one point. Pluto was a planet. Then it wasn't then it was. I do agree that to get the most out of a wastegate there is a finite way, I am just curious if it could work on the rear if you over sized it. After all you are just creating a boost leak.

 

I've seen it done multiple ways as well. I've seen a 4-cylinder with the gate attached to a single cylinder runner.
It will "WORK" fine. But there is a price. To see the price, you need a dynometer and back to back comparisons.

The turbulent environment created by tight cast OEM manifolds for example would in theory be far more accepting of an alternative wastegate positioning since it is already basically guaranteed to be turbulent. However the pressure drop near specific cylinders or disruption to the direction of overall flow may still take its toll.

The one thing I know for sure is that if the exhaust gas can all flow freely in the same direction that will cost the least amount of energy, and yield the best results.

Wastegates are not boost leaks btw. A boost leak is a serious problem for a turbo car, it will blow the engine up pretty fast due to high EGT if enough boost leaks out. Every air molecule that leaks out of the intercooler plumbing will create additional back pressure and raise the gas temperature of the turbine, causing engine failure if there is enough of a leak. Seen it happen ALOT because people rarely boost leak test their plumbing. Before I tune a car I always do three things:
1. boost leak test = pressure test through the compressor to intake manifold should hold pressure like a balloon
2. compression test to ensure healthy cylinders before tuning
3. timing test to make sure the timing is accurate