Overboosting like a mofo
 

I just got my new setup all finished and did some road testing this week. Full details on my build are here:

https://www.miataturbo.net/build-threads-57/winter-turbo-build-custom-equal-length-tial-v-band-goodness-61959/

The short list is:

GT2860RS with Tial .64 A/R v-band housing
Custom equal length tubey mani with a real nice collector
3" turbo back
Tial MVS 38mm wastegate right off the collector with a separated dump tube that merges back into the exhaust ~24" downstream
Link ECU, Greddy Profec B
BEGi Stage 2 intercooler with custom piping

The problem I have is that even with a 0.6 bar (8.7 psi) spring in the wastegate and the boost controller disconnected, it will creep all the way past 18psi above 6000 rpm (at which point I let off or my fuel cut kicks in). I don't understand, I thought my boost control would be rock solid with a 38mm gate mounted right on the collector! I am using a port ~6" after the compressor outlet for my boost reference, so pressure drop across the IC isn't a factor.

The turbo is really responsive, I hit 10 psi by 3000 rpm on a 4th gear pull which is pretty good for a GT28RS on a totally stock 1.8

Anything I can try??? Or am I going to have to go to a .86 housing and lose some response? :(

OK I just tried taking the spring out of the wastegate completely, and it still hits 17+ psi at only 6000 rpm. I can definitely tell the gate is wide open because it doesn't make any boost at all until I get past ~4500.

I would estimate that it would hit 20+ psi if I kept my foot on it until redline. What the hell am I gonna do? Go to a 44mm gate? Or will a .86 housing take care of it? I can't believe I am having this problem with a 38mm gate ported right off the collector and a GT2860RS!!!

Pics of Mani/ wg setup?

https://www.miataturbo.net/attachmen...ine=1323064304
https://www.miataturbo.net/attachmen...ine=1323064304

from his build thread

Yeah that looks nice. Either way the exhaust path thru the WG is creating too much restriction and/or the turbine path is too free flowing. Don't think larger turbine A/R will help, if anything, make things slightly worse.

Only thing I notice is the wastegate tube coming off the collector is a pretty sharp 90 degree+ angle/turn... and the 24" piping, just may be too much of a restriction with respect to the rest of the exhaust. Larger WG, shorter WG tubing would help. That's all I can think of really, neither exactly an easy solution. Verify the Wastegate plunger is opening all the way? That's all I can think of...

How is your port timing? Exhaust port advanced a tooth can wreak havok.

Not sure, I haven't inspected the timing belt and cam gears in a long time...? Is that a common problem?

Actually I have read the exact opposite from some Honda and Mitsubishi guys who claim that switching to a larger A/R cured their boost creep. I see your point though, you would think that opening up the flow path through the turbine would exacerbate the problem, but I think the difference comes from lowering the exhaust gas velocity where it is hitting the turbine because of the larger A/R.

The .86 A/R housing would be the easiest thing to try, but if I see a dramatic loss of spool I might consider a 44mm wastegate and 1.75" or 2" tubing diameter. 10 psi at 3000 rpm just feels so responsive I would hate to lose 500 rpm :(

I think your wastegate position and exhaust flow path is the problem. At this point you might just want to reduce the exhaust size coming off of the turbo.

Ordered a .86 housing last night.

If that doesn't work, I will think about switching back to the .64 housing and going with a 44mm gate. If that doesn't work, I will either drive it off a cliff or build the motor and run ALL OF IT.

Hey guys -

I wanted to bump this and see if anyone has any other suggestions. I am running a .86 A/R turbine housing now and it helped a lot, but I still sometimes see the boost creeping toward 17-18 psi regardless of the boost controller setting.

Since it is winter time here in MI, I am pondering some chopping and welding. Part of me wants to run a 44mm wastegate, the other part of me wants to just try shortening the dump tube and merge it back into the exhaust further up stream. No part of me wants to relocate the wastegate completely, since I would be left with a gaping hole in one side of the collector and would have to create another gaping hole in a different part of the collector.

Ideas? Is a 44mm gate and larger (1.75" instead of 1.5") dump tube a stupid idea?

I really can't imagine how the size of the EWG is the culprit here. I haven't seen any turbo setup smaller than 600whp worth overwhelm a 38mm ewg. Unfortunately my vote is on the placement/design of it than the actual size of the opening.

On my manifolds I run a 1.5" schedule 10 wastegate elbow. (that looks like 1.25", which is fine-but the area exposed to the innards of the collector is less.)

What I would do (personally) is cut the wastegate elbow off the manifold and put a 1.5" elbow on there.

When putting the 1.5" elbow on, justify it all the way down to the turbo flange. Yours is mounted pretty high right now and that limits the flow you would see from the opposite side of the collector.

In this case, the flow restriction is the manifold side of things and not the wastegate itself or the dump tube. Im willing to bet that you would overboost with the wastegate completely removed.

I'd try to get the gate as close to the collector as possible and attempt to eliminate the elbow altogether. I realize this is done a whole lot easier than it is said, but yeah...

The elbow isn't a problem. You would have an elbow after the gate anyways for the dump tube to make up for the elbow on the manifold.


The way he has it is good for keeping radiant heat away from the compressor side of the turbo, a nice amount of space to keep vac lines from melting and makes it really easy to change the WG spring without removing the WG from the manifold- not usually a concern, but its a nice thought.

Looking at the picture of the WG port hole, im almost 100% its because you have it too far away from the turbo.

Do this with a 1.5" pipe -Easiest solution without hacking it up and removing all of the beauty you built.

You want the hole to go right up to the turbo as close as you can while still getting the clamp for the turbo on and off. On my manifolds I usually cut the port so that I cut out all of the weld at the closest point.

Have you done enough testing to be sure that your EWG isn't faulty? It seems that it is overwhelmed at some point, is it the same rpm every time? Have you verified it is opening all the way when under pressure? If it was only partially opening, it would act like a smaller WG.

I recently read a document that modeled collectors and EWG direction/placement and it found that within the collector the pressure is nearly uniform after the merge in the collector, I couldn't imagine that 3-4mm would increase flow that much.

Yep. I would remove the spring and physically open it fully then see if you still had a problem. You shouldn't make major changes without isolating the variables.

Thanks everyone. I have tried removing the spring from the wastegate completely, and it didn't help. The gate can be moved freely by hand without the spring.

What really surprises me more than anything is that this same turbo, on my previous ETD long tube top mount manifold could hold ~12 psi all the way to redline. The crazy part about that is the ETD manifold mounted the EWG on the #4 runner close to the cylinder head! So you would think that my custom manifold, with the EWG basically right on the collector, would have no problem keeping the boost in check. I think the reasons can be summarized as;

1 - 2.5" downpipe on old setup versus 3" on new (same 3" exhaust though)
2 - .64 T3 style turbine housing on old setup versus Tial housing on new (tried both .64 and .86, the .86 keeps it down to about 18 psi but still too high)
3 - Much shorter dump tube on old setup
4 - Better flowing manifold design on new setup
5 - Poor EWG porting of flow path on new manifold - I should have radiused the edges and blended them into the wastegate elbow more.

I'm sure it wouldn't hurt. But it would require a lot of rework (the dump tube would need to change, the coolant lines, etc.). Personally I am starting to think my best bet would be to shorten the dump tube dramatically - like half the length it is now. It would actually be less work to do that than it would be to mess with the manifold since the EWG orientation wouldn't change. While I am at it I will see if I can break the edges on the inside of the manifold and create a smoother flow path into the elbow that leads to the wastegate.