Miata Turbo Forum - Boost cars, acquire cats.

Miata Turbo Forum - Boost cars, acquire cats. (https://www.miataturbo.net/)
-   DIY Turbo Discussion (https://www.miataturbo.net/diy-turbo-discussion-14/)
-   -   Boost Creepin' (https://www.miataturbo.net/diy-turbo-discussion-14/boost-creepin-82084/)

pdexta 11-30-2014 06:35 PM

Boost Creepin'
4 Attachment(s)
I've been having boost creep issues since turboing my current car. I wanted to run my setup by you guys and see if anyone had suggestions, because I'm pretty stumped.

gtx2860 turbo
tial mvs 38mm wastegate
atp external wastegate turbine housing
cast log manifold
wastegate plumbed into downpipe
exhaust is 3" to a resonator and a turndown before the axle



I started with a 4psi wastegate spring, the turbo spools, the wastegate opens, then it gradually creeps up the 200kpa overboost protection a little before redline. By that point I'm pushing my stock motor and RX8 injectors to the limit so I dropped overboost to 190kpa.

I've tried changing the source of the wastegate and gone to a bigger vacuum line which did nothing. I then put in a 7psi wastegate spring, which does the same thing, just with boost creeping up a little quicker.


It almost seems like the wastegate just can't flow enough air. The turbo is really efficient, the exhaust has basically no back pressure; but it still seems crazy to think a MVS can't flow enough air for a 200hp car.

If you guys have suggestions of thing to try or potential issues please let me know.

curly 11-30-2014 06:49 PM

You can solve it with timing believe it or not. Jared's car:


Had some pretty bad creep, but after we told the knock sensor to STFU, and messed with the timing and ebc, it all but disappeared.

Talk to a competent tuner, and on a real dyno.

pdexta 11-30-2014 07:10 PM

Thanks for the tip, I'll read up on the thread. Timing is still pretty close to the basemap right now. Since I was running low on fuel I didn't want to add timing and end up going lean up top. That'd be awesome if that's the problem. I've got some bigger injectors on the way, once they're here I'll add a few degrees and see how it does.

nitrodann 11-30-2014 08:31 PM

The boost creep you have looks beneficial to me?

sixshooter 11-30-2014 09:04 PM

Is the wastegate physically opening fully?

pdexta 11-30-2014 09:07 PM


Originally Posted by nitrodann (Post 1186117)
The boost creep you have looks beneficial to me?

Running 150kpa that creeps to 190kpa by 5800rpms before you run out of injector and have to shift to the next gear really sucks. It's be significantly quicker/faster/more responsive/enjoyable/reliable/safer if I could just run 180kpa all the way to redline like it should be able to.

pdexta 11-30-2014 09:12 PM


Originally Posted by sixshooter (Post 1186123)
Is the wastegate physically opening fully?

When I pulled it off to swap the spring I looked it over and compressed it, nothing looked unusual but I don't really know what to look for. Is there something I should check or any way to tell if it's opening all the way?

FWIW, it worked perfect on my old set up. But that was a slightly bigger turbo that ended up having a torn up turbine wheel so I assume it took a lot more to get it to spool.

matthewdesigns 11-30-2014 09:43 PM


Originally Posted by pdexta
Is there something I should check or any way to tell if it's opening all the way?

Remove all springs and lines from the wastegate and it'll essentially be like tying an internal gate open.

Savington 11-30-2014 10:12 PM

I really dislike your wastegate setup. I'd like to see the EWG sit directly on the turbine housing, and have the dump tube incorporate the 90* bend currently being made by the IWG. The 90* pipe means you have a bunch of pressure loss in the pipe leading to the EWG, which inhibits wastegate function.

sixshooter 12-01-2014 07:04 AM

^ I was initially wondering about that as well. That is a relatively tight bend. I seem to recall from fluid dynamics that every 90 degrees gives roughly the same amount of drag as 10 feet of straight pipe of the same diameter. That's a rough figure but gives you a good idea of the importance.

Braineack 12-01-2014 08:20 AM

How exactly is it plumbed & controlled right now?

That thing makes really awful low-end torque.

pdexta 12-01-2014 12:28 PM

The wastegate is plumbed directly to the intake manifold (no mbc/ebc for now). I know it's better to run pre throttle body but it should be fine for a wot pull.

I think the low lowend torque is primarily a result of basemap timing + weak wastegate spring. I'm sure it'd look a lot better if I could properly control boost.

I've been meaning to pull the spring out completely and see what happens, I guess that's my next test. Thanks for the advise on the wastegate, I hadn't considered the 90 to be a problem. I'll have to check fitment with the hood and if there's room to mount it directly on the turbine housing it should be pretty easy to have it redone.

sixshooter 12-01-2014 12:37 PM


Originally Posted by pdexta (Post 1186219)
The wastegate is plumbed directly to the intake manifold (no mbc/ebc for now). I know it's better to run pre throttle body but it should be fine for a wot pull.

Wait, wat?

I might have found part of the problem.

Braineack 12-01-2014 12:51 PM

and lower port only?

does this EWG have more than one lower port? if so they need to be blocked.

pdexta 12-01-2014 01:22 PM

Yes, it does have 3 lower ports; 2 are blocked and the 3rd sees the boost source. The top ports are left open. I'll double check the blocked lower ports to make sure they're tight, it'd be awesome if that was the problem.

Braineack 12-01-2014 03:06 PM

you might have a better job preventing creep by using a pre-intercooler source.

But it seems that something is preventing it from venting enough. Unsure why. Could be the 90 before the EWG like savington mentioned.

What source are you using on the IM. Coming off the brake booster might be a smarter idea. Actually, I'd test that one, by temporaily removing it from the booster jsut to see. But make sure you have plenty of stopping room after a pull to redline :P

pdexta 12-07-2014 02:52 PM

1 Attachment(s)
Figured I'd update this. I'm still having the problem. I've tried several wastegate sources off the intake manifold that didn't make a difference.

I love the pre-intercooler wastegate source idea. I ordered some aluminum bungs just waiting for them to arrive so I can get them welded in, I didn't want to screw up trying to tap the pipe myself.

I bought some ID1000 injectors, so there's no lack of fuel now. I added timing and saw no change. Then switched to E85, added more timing, and the boost curve is basically identical. The turbine housing flange is 44mm and my wastegate is 38mm so it's not as simple as moving the wastegate to avoid that 90 so I'm hoping the pre-intercooler source + adding a EBC will allow me to tune around the crappy wastegate setup.

I ran my 93 E10 datalog vs E85 datalog through virtual dyno to compare.


nitrodann 12-07-2014 03:11 PM

I'd have to see your torque curve but your boost curve looks a lot like high hp stock engine/5 speed boost curves should.

I am unaware of your setups particulars though.

pdexta 12-07-2014 05:17 PM

1 Attachment(s)
Maybe the virtual dyno log isn't doing the boost curve justice. Here's the same pull from megalogviewer.


Every other turbo miata I've had built boost until the wastegate opened, then held that pressure to redline. I don't feel like this is normal for any car, and certainly not something you'd want on a stock motor car.

Braineack 12-07-2014 06:01 PM

Would there be any way to weld the ewg off the manifold and rework the outlet?

All times are GMT -4. The time now is 05:41 AM.