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Old 01-07-2016, 04:09 PM   #1
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Default LOWERING boost pressure

I've searched the forum and there's lots of info on raising boost pressure, but how about getting 3-4 psi out of a SR20 T25?

I know I'll get some flak for doing things a bit backwards, but I'd like to start with low enough boost where I won't need an intercooler, clutch or diff (It's a 1.6) right off the bat to get things sorted before having to upgrade.

Seems to me that you'd also get a great tune if you dialed in 4 psi, then all the cells for 6, 8, etc.

Is it possible that the stock 7psi is mild enough?
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Old 01-07-2016, 04:12 PM   #2
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Less wastegate preload. It still might creep.
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Old 01-07-2016, 04:12 PM   #3
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You would need a lower pressure waste gate. I know on my 1.8 with the stock wastegate and a intercooler I was running around 5psi on a stock exhaust. Honestly you need to get to the intercooler, clutch, and diff much sooner then later.
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Old 01-07-2016, 04:13 PM   #4
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You would just find the lowest rated wastegate for it. Possible you may get boost creep but I think others just tune 'up to' the creep level and just hold it all the way across the rpm band with ebc.

Last edited by Girz0r; 01-07-2016 at 04:14 PM. Reason: dayum, I post slow
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Old 01-07-2016, 04:39 PM   #5
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Thanks guys,
I'd hoped I could just adjust the rod to get it to open way early, but I don't know if the actuator opens linearly vs pressure or at a certain psi. Looks like there's an actuator on eBay that goes from 4-25psi. That's from adjusting preload, so I might be able to get down low with the stock actuator or worst case, buy a $40 actuator for a $100 turbo.
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Old 01-07-2016, 04:43 PM   #6
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It's linear to a point, then not. You're problem is going to be setting preload loose enough that it doesn't boost, but enough preload that it doesn't blow open on it's own trying to make any boost.

Ebay will lie right to your face, doesn't mean it's necessarily junk, but be careful. For whatever it's worth I think my china can is good to around 4-6 psi on a completely different turbo...
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Old 01-07-2016, 04:49 PM   #7
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We managed to get 3-4psi on my friend's chinacharger by tying the waste gate open. The only thing I can thing to do that is easy to lower boost is to somehow mount an adjustable spring trying to pull the waste gate open against the actuator. That would let you adjust pressure down. It may blow open from exhaust gasses though.
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Old 01-07-2016, 04:53 PM   #8
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Power curve of a rotrex, turbo with wastegate wired open. 3-4 psi max at 7200, whoopie.

Your friends car's wastegate is terrible and he should feel bad.
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Old 01-07-2016, 04:53 PM   #9
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Quote:
Originally Posted by deezums View Post
Your friends car's wastegate is terrible and he should feel bad.
Yup
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Old 01-07-2016, 05:01 PM   #10
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Quote:
Originally Posted by x_25 View Post
We managed to get 3-4psi on my friend's chinacharger by tying the waste gate open. The only thing I can thing to do that is easy to lower boost is to somehow mount an adjustable spring trying to pull the waste gate open against the actuator. That would let you adjust pressure down. It may blow open from exhaust gasses though.
Oh, I like this idea. It makes me wonder if the exhaust pressure on the waste gate affects the opening point much. Maybe porting the WG would cause more pressure on the arm reducing preload while reducing creep. Does that sound like as much of a crackpot theory to you guys as it does to me?
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Old 01-07-2016, 05:02 PM   #11
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Quote:
Originally Posted by deezums View Post
Power curve of a rotrex, turbo with wastegate wired open. 3-4 psi max at 7200, whoopie.

Your friends car's wastegate is terrible and he should feel bad.
Oh yeah, spool was terible. We tied it open since he had just bought it and it was trying to make 15psi at redline on stock injectors with an inop wideband. Once we got the wideband working and could see what the car was doing we reconected the actuator and set the preload lightly until we can get new injectors and such in there.

We also move the wga reference back to the conmpressor outlet rather than after the throttle body. Net result is ~4-5psi from 3500rpm and the tapering up from 5k to redline where it makes 7-8psi.

Also, OP, this is on a bone stock 1.6, stock injectors, stock clutch and stock dif. They seem to be holding up well, but my friend has yet to put anything but allseasons on it in the 4k miles he has owned it. So the tires break free before we run into trouble with anything else it seems.

Get and intercooler and you can likely get away with 6-8psi (set preload lightly, grab your waste gate reference before the intercooler so that it doesn't compensate for the pressure drop across it. Read the "don't need ebc" sticky to see why that is) and you should be ok if you have a modecum of mechanical sympathy.
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Old 01-07-2016, 05:04 PM   #12
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Quote:
Originally Posted by squeegee View Post
Oh, I like this idea. It makes me wonder if the exhaust pressure on the waste gate affects the opening point much. Maybe porting the WG would cause more pressure on the arm reducing preload while reducing creep. Does that sound like as much of a crackpot theory to you guys as it does to me?
No, that would work the other way. Porting it would expose the waste gate to more pressure, so it would be more likely to blow open.
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Old 01-07-2016, 05:09 PM   #13
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Is it on a bone stock 1.6 ecu as well?
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Old 01-07-2016, 05:13 PM   #14
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Quote:
Originally Posted by Girz0r View Post
Is it on a bone stock 1.6 ecu as well?
Voodoo II box and MSD for spark. It will eventually be getting a megasquirt. Can't ask for much on a car in good mechanical shape for $1500 that doesn't actually try to blow up and runs well.
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Old 01-07-2016, 05:15 PM   #15
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Op, What ecu do you plan to run?
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Old 01-07-2016, 05:23 PM   #16
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Quote:
Originally Posted by Girz0r View Post
Op, What ecu do you plan to run?
The next step is a MSPNP. I figure I'll tune NA for a while and step up to bigger injectors before bolting on a turbo.


The pressure drop on an intercooler might be the ticket. I guess an eBay intercooler wouldn't be too much more than doing the intake piping sans-cooler, right? Ironically I could probably run 7psi fine with that setup
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Old 01-07-2016, 05:24 PM   #17
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https://www.miataturbo.net/diy-turbo...tercool-83180/

If only there was a helpful sticky about ebay intercooling.
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Old 01-07-2016, 05:24 PM   #18
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Quote:
Originally Posted by squeegee View Post
The next step is a MSPNP. I figure I'll tune NA for a while and step up to bigger injectors before bolting on a turbo.


The pressure drop on an intercooler might be the ticket. I guess an eBay intercooler wouldn't be too much more than doing the intake piping sans-cooler, right? Ironically I could probably run 7psi fine with that setup
Corect answeres.
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Old 01-07-2016, 05:33 PM   #19
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Quote:
Originally Posted by aidandj View Post
https://www.miataturbo.net/diy-turbo...tercool-83180/

If only there was a helpful sticky about ebay intercooling.
Thanks for writing that up, I'll totally use that when the time comes.

Since I really want to spread out the costs, maybe I should intercool before I go turbo
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Old 01-07-2016, 05:47 PM   #20
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Quote:
Originally Posted by squeegee View Post
The next step is a MSPNP. I figure I'll tune NA for a while and step up to bigger injectors before bolting on a turbo.


The pressure drop on an intercooler might be the ticket. I guess an eBay intercooler wouldn't be too much more than doing the intake piping sans-cooler, right? Ironically I could probably run 7psi fine with that setup
Wait, your car is naturally aspirated, you can't afford an intercooler, but you think you are good to boost it?

Save up, do it right from the get go.
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boost control, low boost, low pressure, psi adjustment, t25

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