External Wastegate on Stock Twins. For those who said it can't be done
 

Ever since I maxed out the BNR's I've been looking for new ways to squeeze more power out of the twins. One of the problems I was running into was being able to hold high boost to redline, with the boost peaking at 21psi and trailing off to 18.5psi by redline. Me and another member made some conclusions about what could be causing this. One, restriction in the inlets to the turbine housings (especially the rear), two, Excessive manifold pressure is overcoming the factory wastegate and is effectively holding it open. The other member ported the turbine housings but still had the same issue with boost falling off. With the external gate I can now hold 21psi to redline.

Many users on this site have talked about the idea of running a external wastegate on the twins to overcome the common boost creep problem many owners have had with the twins. There are many posts with people asking about running a external with them usually being shot down and someone claiming "it can't be done". Well I'm here to tell you it can and how to do it.

The answer is in the pre-spool access door on the backside of the turbos. It is positioned so that with heavy porting it will have direct exhaust flow from the rear rotor and is in the path of the bleed off passage for the front rotor. This of course means that you will have to run non-sequential to do this.

1. Remove the pre-spool access door and cut a large hole in it. Plasma cutter worked best for me.

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2. Weld a 90degree piece of 1.25" schedule 10 stainless pipe to the hole you just cut. Test fit exactly where you want to weld this beforehand. It's a very tight fit, measure twice and weld once.

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This is what it should look like mounted to the turbos in the bay.

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Next step in involves the stock exhaust manifold. In order to maximize flow to the wastegate a large section has to be cut out of the manifold. I knew someone with a Bridge Cutter that was able to cut out the section. The stock flapper has already been removed in this photo.

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With the exhaust manifold ported the same needs to be done to the rear turbine housing. In this picture you can see the rear turbine housing to the left and the amount of material that is in front of the pre-spool hole.

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This is what the rear turbine housing should look like after porting and is heavily ported compared to stock. This part took me about 3-4 hours and is definitely the hardest part of this job. This is also a good time to weld the stock wastegate flapper closed and fill the holes made by removing the pre-spool flapper and flapper in the exhaust manifold.

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After porting is complete the exhaust will have a direct passage to the wastegate and much better flow to the rear turbine housing.

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Mild porting can also be done to the front turbine housings inlet. Turbos assembled.

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Throw it all together now and you have a external gate on the twins! Here are the pics of the gate from above and below the car.

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I know I probably left out some important parts so feel free to ask questions.

Very cool. Do you have a picture of the twins fully assembled with the wastegate prior to install?

damn that's allot of work but great idea i hope it work for you.

Sorry I don't.

How's the sound with that dumped gate?

Is there any improved spool with the large wategate as opposed to running the stock gates or is the difference unnoticable?

very nice

Very innovative. Certainly is A LOT of work though. Interested to see some numbers

Another verifiable rotory gury, for sure. Amazing amount of work, even more amazing thought put into the effort. Good job, sir.

I think it's always been a case of "possible, but not without a lot of work". Kudos to pulling it off!

Is there any way you could route the wastegate's output back into the downpipe? Screamer pipes are fun for a day or two, that's just STUPID damn loud. Wonder if there's enough space around there to do it.

Have you driven the car with this setup? Any results?

Dale

Nice, interested to see some numbers.

I see a lot of work to add an external WG to the secondary turbo. What did you do with the internal WG and actualtor on the primary turbo? I can see the flapper arm in the install pics, did you end up reinstall the stock actuator? Weld the WG shut? Did you merge the front and rear exhaust flow some how so that they are booth controlled by the external WG?

Unless I am misreading something, I do not see how this is going to control boost on the primary turbo since it has an independent exhaust runner that is not going to be vented by the Tial WG.

Not trying to be critical, just a little confused by the write up.

Banzai, In the above pic the stock manifold is cut increasing the exhaust flow to the external wastegate. If you look sideways through the manifold you can see the exhaust ports are connected even when the manifold is not modified.

The stock actuator was removed and the stock wastegate was welded shut by simply welding the stock wastegate flapper arm in the closed position.

As for the front and rear exhaust flows, you have to remember the stock wastegate port is only in the front turbo and there is a passage in the stock exhaust manifold that links the front and rear rotors. From the factory the rear turbo bleeds off extra exhaust through the manifold and into the front turbo which contains that factory wastegate. My system works in the same way, but instead of the rear turbo bleeding extra exhaust to the front it now works in reverse, with the front turbo sending excess exhaust to the rear. This was my greatest concern when I decided to do this, as I'm reversing the excess flow inside the exhaust manifold.

I have tested the wastegate and it works flawlessly! Boost builds well and it seems to spool a little bit faster then before. The big advantage of this is that I'm now able to hold high boost (21psi) all the way to redline instead of it falling off to 18psi. When I get the time to tune I plan to turn the boost up even higher to see what these BNR's can truly do.

The wastegate with the screamer pipe is defiantly loud and just sounds nasty when it kicks on. There is room to route the exhaust back into the factory downpipe if I end up getting sick of it though.

Any idea what your EGT's are?

No. Down the road I'll install a dual EGT probe, but I'm in no hurry to rip everything back off the car.

Way cool and awesome job! It's too bad you cannot externally gate it while keeping them sequential though. What made you decide to keep the BNRs and attempt an external gate over say a modest sized single turbo? It just seems like an awful lot of work on the twins where it can be resolved with a nice single.

There's nothing wrong with being different though :)

Matt

Cool good job, that will be nice to keep the boost up and not falling off like it always did.

I decided on BNR's because I always liked how non-sequential twins deliver their power very progressively. I've been in many single turbo cars and while they're fast many of them deliver the power like a off/on switch and just roast the tires as soon as the power comes on. I made 452whp before this mod and I got full traction at the high end of second gear and for all of third.

I wanted to install the external gate because the stock internal gate was causing boost to drop off from 21psi to 18psi by redline. This external gate will allow me to hold 21psi all the way redline as well as run more boost if I want.

I also love a challenge :)

this is superb, never thought some one would pull the trigger and do this.. im eagerly awaiting to see how it does with the tune and how the BNR's hold up..

great work.

-Jaime

The BNR's are capable of making 450 plus. At 19 psi we made 432 rwhp with the sequential set up with a hot engine. We were making 13.5 rwhp per lb of boost. If we ran up to 21.5 lbs we would have been right at 470 rwhp. It will be very interesting to see what these turbo's are capable of at higher boost levels. Good job as usual and thanks for the pics...

Here's a video of the car on the dyno

Nothing but a big congratts here!! Great work on bringing the twins to another level!!

Wow, she sounds healthy!

care to share a graph?

Maybe I am silly but I am failing to see why you couldn't stay sequential????
I mean all you did really was port out a hole that was already there and blow a hole in a plate yes?

I don't have a graph from this run but I do have a graph from when I made 470whp 400tq on a dynapak two weeks ago. Just for clarification the prior BNR non-sequential record was set on a Dynapak at 425whp. On this graph/run I had ignition breakup at high RPM due to me getting greedy with spark plug gap. I regaped my plugs before I made the run you see in the video above which solved the problem as you can see.

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I chose to go non-sequential because the flapper doors limit the maximum power that can be made on the twins. The whole point of this build is to see how much power can be made using the stock frame turbos and using every exploit possible to extract that power.

As for "all you did really was port out a hole that was already there and blow a hole in a plate yes?" I think this doesn't give credence to the amount of time, work, and planning that went into doing this. This mod is something that has been talked about on this forum for over ten years with no one finding out a way to do it. Without this mod you cannot hold more then 18psi to redline and if you're running a wide open 3" exhaust you will also be dealing with boost creep problems at around 15psi.

If you don't understand the work that was done above then either you didn't read what I wrote, or you haven't been around the RX-7 world long enough do understand the problems that this fixes and the potential for more power it brings.