Too Much of a Good Thing... - Miata Turbo Forum - Boost cars, acquire cats.

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Old 02-09-2008, 01:16 AM   #1
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Default Too Much of a Good Thing...

So like, I want to revisit this overboosting problem... With the wastegate actuator unhooked, I still am hitting the MS's overboost cut at 205kpa (~15 psi) when I rev it pack like 6k.

This displeases me.

I have ridiculously conservative timing, as low as 4 degrees total advance. Is there a chance that this is making the turbo over spin?
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Old 02-09-2008, 01:24 AM   #2
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wait, you are running the same turbo as me and a 3" open exhaust and you are questioning boost creep?

mine was making 15psi at 5500rpm with the wastegate braced completely open until we modified the wastegate opening making it slightly bigger and a smoother exit.

and with the timing so retarded you could be igniting the fuel mixture too late allowing the exploding mixture right out the exhaust valves to spool the turbo
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Old 02-09-2008, 05:45 AM   #3
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That's what I was asking, so yeah, you have the same issue then? I'm kinda glad to hear it - more importantly, porting your wastegate helped?? How hard was it to do, do you have any pics? I want to do this!!
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Old 02-09-2008, 11:30 AM   #4
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I just used a dremel type tool and smoothed it out on my old rx7 turbo. It was easy and one of the best things I did to that car, other than starting with a shell.
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Old 02-09-2008, 11:45 AM   #5
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Massive timing retard can cause overboost- it dumps "unburnt" fuel into the exhaust manifold and the combustion is completing there. You're throwing the spark so late that combustion can't complete before the exhaust valves open, so you're fuel is getting shoved out the "door". Have you looked at your EGTs? I bet they're through the roof pre turbine.
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Old 02-09-2008, 11:54 AM   #6
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Thats called antilag rob ;-) its a good thing. (robs probably right)
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Old 02-09-2008, 11:57 AM   #7
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Quote:
Originally Posted by AbeFM View Post
That's what I was asking, so yeah, you have the same issue then? I'm kinda glad to hear it - more importantly, porting your wastegate helped?? How hard was it to do, do you have any pics? I want to do this!!
Here are some pics from an old thread. No before shots though.
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Old 02-09-2008, 08:07 PM   #8
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How much would you say you took off, I mean, how much bigger is the door?

It seems I'm RIGHT in the same boat, 205 or so kpa at 5200 rpm. I sure wish I could get it to stop losing sync. That's another issue.

At least I got it to sit there, I redid the wastegate a bit, per the drawings I made a couple months back:



This lets the wastegate open more, a lot more, for a given throw. But the hole is too small... It's such a pain pulling the turbo but maybe I just have to. Anyway there's an oil leak I want to look into.

<edit> I wasn't sure if timing only got you boost at essentially idle, or if there was anything "real" behind it. Could it actually feed a real sink there?
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Old 02-09-2008, 09:52 PM   #9
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There is not given measurement for porting the wastegate, just port it the max amount so that the flapper still has a face to mate against. Then taper the outlet for improved flow transition.
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Old 02-11-2008, 03:45 AM   #10
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Ok, so, the door/flapper measured 33mm, the hole was 25. I bored it out to ~29mm till I was worried about it not being closed or burning up.

First off, you guys who do this with a dremel are insane, how long does it take you, like 16 hours? :-) Anyway, after some serious die-grinder action, I had it between 29-30 mm all the way around, given that there's some slop in where the door goes when it closed this is like 1-2mm of closing surface.

First it seemed like I could keep it undr 10, then 11 psi when I revved it out. I haven't checked the logs yet, but my roomate who was riding shotgun said he saw it hit 13 psi. Well, ok, fine. So it's under control (this was a medium-cold night) but just barely so. I guess I could do a BIT more.

While I was in there, I 1) fixed my broken oil return extension (the source of my oil leak) and 2) put anti-sieze on my nordlocks (all the old ones were fused together, so I guess that's why they didn't work)...

Then I got good and ******* drunk, watched a movie about black vampire rappers. Can you tell?

Pictures soon to follow. The thing to keep in mind, with the set up I have, even with a pretty open wastegate, and the flapper opening a bt over 90*, I still barely can keep boost under control. In the old days where I thought 12 psi was a lot, I couldn't do it.

Now the car is really running, it seems, so some tuning and it might actually be fast again.
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Old 02-11-2008, 10:18 AM   #11
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my before was 15psi @ ~5500rpm with the door propped and bracket completely open. 17 with it hooked up.
my after number was 12psi to redline with the wastegate door operational. now i use the EBC to bring it up to 14.

did you also smooth out the passageway to the wastegate? i'm sure that must have helped a lot too. too hard to get pics down in there. Tim did all the work so he might chime in and give you some better info.

you should also at least go back to the DIY base timing map. it's seemingly much more agressive than what you have toned yours down to. my map is still up if you wanna use that. i know its called "aggressive" but some have said they've run more aggressive numbers with luck.
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Old 02-11-2008, 02:00 PM   #12
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I'm also running 91 octane, not 94 or whatever, and it's california gas, which several folks have said isn't even as good as the number it's "rated" at. However, yeah, I was going to reload the maps I had from before... Before I pulled out buckets of timing trying to keep my fake knock from happening. I will be switching soon.


Virgin Wastegate, unhooked, flapper door 33 mm (beveled, and with a mm or two of slop)


After



"after", with the lever adapter mod.

It holds in warm weather to 10 psi or so, but at colder ambients it definitely goes higher. As long as I don't mind running 14 psi, I guess it's fine... :-( Realistically, I could do about 12 psi. I think my wastegate thing needs a bit of work still.

Pics At:
http://abefm.smugmug.com/gallery/432...mdPb#253372670
More to follow
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Old 02-11-2008, 02:18 PM   #13
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Any reason for not doing it properly with an external gate?

I have to say, I've never heard so many people trying to get LESS boost lol.

Fred.
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Old 02-11-2008, 02:22 PM   #14
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Fm manifold? It seems really hard to add more stuff in there, though I'll admit I was tempted. I was kinda hoping when I pulled my turbo off that it would be junk and I could get a new manifold, bigger turbo, and external wastegate.

But the gt28R seems a very good fit for this motor, so why would I go to bigger?

I could swear it used to spool faster, but my tuning is WAY off right now.
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Old 02-11-2008, 03:15 PM   #15
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Quote:
Originally Posted by rotaryjunky View Post
I just used a dremel type tool and smoothed it out on my old rx7 turbo. It was easy and one of the best things I did to that car, other than starting with a shell.
+1. did this on my old FC's. everyone "ported" their OE wastegate cuz w/ a 3" RB exhaust, it'd spike up to 15+ psi and blew the **** out of the stock 13BT...
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Old 02-11-2008, 06:24 PM   #16
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unless you use your EBC to bring the boost level up it should be spooling slower now. your wastegate opens slightly under boost even before your max boost pressure. so now with the bigger opening it's letting more air bypass the turbo. add to that your wastegate opener mod that's making it open even more than stock with less actuator travel
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Old 02-11-2008, 07:27 PM   #17
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You're fighting the spring, there's some amount of "pre-load" to keep it from opening. But you're right, I suppose. With more travel, you can add a little more preload, but for a given torque on the arm, there's more surface area on the door as well, getting it to open sooner.

Kinda unfortunate. I guess I need a wastegate for my wastegate. This whole thing reeks of the same logic as putting in 160* thermostat because the 180 overheats. The bigger hole it good, but a honest to god external is the way to go.
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