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Old 11-03-2003, 10:35 AM   #1
2manycarz
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Default technical discussion of $2 boost mod

I asked Mike Shields of SPD to comment on the so-called $2 boost modification. Here is his well thought out response:

The size of this restrictor is very sensitive, changes are measured in .1mm increments. Aside from setting a boost calibration for the waste gate solenoid duty cycle, the other purpose of this "restrictor" is to match the flow rate to the boost controller so it does not overshoot the boost requested by the ECU. This is common to all modern turbo setups so the line size of hoses is not so critical. Instead, the restrictor is in line to stabilize the boost control. If you close down the restrictor too much it can raise the boost momentarily because the waste gate does not open enough for the level the ECU wants to see. Then the ECU, seeing the over boost, bleeds more air, which causes an under boost, and then bleeds less air, causing an over boost again until the system settles down to steady stage. Since in the first two gears the engine is normally out accelerating the turbo boost rise this creates a momentary higher power condition that will catch people's attention. The trick is to not be so greedy. A change of 1.5 mm that you say is suggested is excessive. I would think a .2 or .3mm change would bring up the power without causing what is called hysterisis or a hunting of the boost values.

The correct way to go about this is to re-flash the ECU to a desired boost level, usually change the timing curve (the smog restricted timing curve change is a noticeable improvement in throttle response and turbo spool) and re-adjust the waste gate control solenoid threshold and sensitivity, which also greatly improves turbo spool. Depending on the degree to which the boost is raised it may become necessary to change the size of the control line orifice. But, changing the orifice size itself too much only creates boost spikes that cause momentary higher boost as the system now has over-shoot of the intended boost target, or if restricted excessively can cause the boost solenoid to go to 100% duty cycle and a permanent over boost condition. Not exactly how I would approach the desire for more power.

It can be done, if the change in size is not too great from the stock setting. I wrote an article in 1989 describing this modification to the Toyota 3S-GTE turbo motor in the Celica GT-Four. We changed the orifice .3mm and took the power from 190hp to 220hp, which was just right for our 91 octane gas, in fact the motor ran even better on 93 octane. We did have some problem with overheating as the ECU pulled the timing back too far when it was hot ambient temperatures to prevent detonation. The newer ECU's are smarter than this, but you still have to be careful not to leave the waste gate duty cycle at too high a percentage or it can fail. The reason we did it this way is we did not have any away to adjust the factory ECU on the Toyota at the time. Today we have full access to the ECU programming tools for the WRX and this changes what we do with the motor. .

I have not done any testing in this area of changing the restrictor size alone, but that is because we have seen so much improvement from modified fuel curves and timing curves that adjusting the boost alone these days is not considered the way to go. We get much better results in power, reliability and response by just redoing all the values for all three, or in the case of the STI four (cam timing) map targets and their inter-relationship 'Q' values. What most owners do not realize is that the Subaru ECU is a state machine that writes its own maps based on rules the engineers give it through programming. The targets and their rule relationships (The "Q" values) are a slightly complex set of programming rules that are done a certain way on the stock engine mostly to meet emissions and fuel economy regulations. You can now imagine that just turning up the boost by changing the orifice size is only part of the picture. There are limitations to timing because of NOx restrictions, limitations to fuel/air ratios and timing because of emissions and limitations on boost control for part throttle fuel economy EPA loop tests that can be replaced with proven full power timing targets, minimum full power fuel mixtures and higher and more responsive boost regulation targets when we re-Write the ROM code.

We will have a ROM kit for the Forester in the spring time. We have a couple of other project in ahead of this program at the moment. If you are interested in a properly engineered solution please let me know.

Sincerely,
==m==
Mike Shields
SPD Tuning Service
Redondo Beach CA
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Old 11-03-2003, 01:07 PM   #2
Frank Zuccarini
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Default Thanks for the info............

Very interesting, indeed.........................Frank
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Old 11-03-2003, 01:17 PM   #3
mo_boost
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Now that makes sense. I knew there was some VOODOO behind this mod. Anyway thanks for taking the time to research it.

P-Rod

Last edited by mo_boost; 11-03-2003 at 05:34 PM.
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Old 11-03-2003, 02:00 PM   #4
jimqpublic
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What I didn't understand and still don't, is that Mike's discussion and one from an Australian magazine I read online is that they are talking about a SMALLER orifice.

The WRX and XT "$2 boost mod" involve a LARGER orifice. This lets the factory boost controller and wastegate see more boost sooner, rather than later. I don't see what good this would do.
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Old 11-03-2003, 02:26 PM   #5
zzyzx
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I agree.

That said, a larger size in theory should let the whole system react more quickly since there is less physical restriction of the pressure change.

That said, I believe this is in line with what people notice from this - that there is boost sooner, but not necessarily higher boost at any point.

- Steve
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Old 11-03-2003, 05:30 PM   #6
2manycarz
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Default He's saying just a very small increase

You may be missing the point of Mike's analysis. What I think he said is that going from 1/16 to 2/16 may be too much. He suggested that just a .2mm to .3mm increase over the stock size would be more advisable. This also explained why the attempt to use a 3/16 didn't work well at all.

I think that this would have to be a custom restrictor, because the available sizes all seem to be in 1/16 increments, which is much more than he suggested.

I greatly appreciate the time he took to examine the issues.

2
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Old 11-03-2003, 05:32 PM   #7
mo_boost
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The restrictor that Mike is speaking of, is the one coming off of the compressor housing. Decreasing the orfice size at that point, is effectively the same as increasing the orfice size at the restrictor before the boost solenoid.
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Old 11-04-2003, 12:26 AM   #8
Peaty
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Quote:
I think that this would have to be a custom restrictor, because the available sizes all seem to be in 1/16 increments, which is much more than he suggested.
I'm going to have a few made w/ a ~0.25mm increase over the stock 1/16" restrictor and I'll report back. The trick is finding the proper drill bit size to make the hole

I have some 0.189" (4.8mm) OD 304 Stainless rod that I will have a 1.85mm (0.073") hole drilled into. I checked around and we have the bit I need at the shop at work I'm not sure what the optimum length for it will be so I'll have to make a few various ones.
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Old 11-04-2003, 10:15 AM   #9
2manycarz
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Default Sounds like a good approach... more

Peaty, it sounds like you are doing the right thing in taking a conservative approach. Have you considered getting a spare factory plastic restrictor and increasing the bore .025 mm in that (for obvious reasons). Surely the IC and turbo system can tolerate a 1 - 2 lb. increase in boost easily. The conservative ratings on the XT still smell like an arbitrary positioning issue vs. fully optimized performance.

I heard from Trey at Cobb, and it sounds like they have a pretty good handle on the re-designed exhaust to accomodate the diff protector and/or factory hitch, but it will be sometime after SEMA.

I am thinking about entering the XT in the annual BMW ice-cross in January. It was a hoot last year in my Mini S.

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Old 11-04-2003, 10:28 AM   #10
Peaty
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Quote:
Have you considered getting a spare factory plastic restrictor and increasing the bore .025 mm in that (for obvious reasons).
Yes eventually that's what I'll most likely end up doing. First I want to see if I get good results. These materials I have on hand and can give it a go pretty quickly. Sometimes I've found drilling plastic isn't all that easy though.
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Old 11-10-2003, 11:32 PM   #11
Peaty
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Well I made my own, I have some pics here:

http://www.subaruforester.com/forum/viewtopic.php?t=869

at the bottom. I'll give it a go in the AM.

Peaty
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Old 11-11-2003, 01:32 PM   #12
Peaty
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Well it's still raining here and because of that I drive considerably more conservatively than normal. However, it seems that the boost now comes on quicker but I can't say that it's any higher than before. I didn't do any high RPM runs to see if MAX boost has been increased beyond what I normally see.

I was thinking I should countersink the holes I made to make a smoother transition into the orifice. Humm
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Old 10-05-2005, 09:33 PM   #13
disque71
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so thumbs up or down on the 3/16th mod or anything similar to it?
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Old 12-18-2006, 08:11 PM   #14
splicer4u2
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Default help on 3/16 mod!!!!

2002 wrx....just did a 3/16 mod along with a boost gauge install and oil pressure gauge. The boost gauge now sweeps to 20 psi ang hits fuel cuttoff. the oly thing i did different is replace the hose from the solenoid to the new 3/16 connector. It runs like a bat outta hell. What would cause this and how do I fix???thanks
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Old 12-23-2006, 11:05 PM   #15
Absubtle
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I use nitrous jets for a restriction. I put it between the turbo's compresser pressure source and the factory boost solenoid. I remove the factory hose and add my own w/ my nitrous jet (size depending on what the car responds to... usually a .028-.032), installed in the hose. Now I only do this when it is strapped to our dyno, at work. I do not recommend doing so, in an uncontrolled enviroment. I.e, in your garage and then go drive it.
Basically, I only do this, when the customer is too poor and/or cheap to buy a boost controller, even a mechanical one for less than $80.
It still keeps it under the threshold of over-boost, but usually adds over 20hp. Air/fuel ratio's are always overly-rich (as always, even stock), w/ the higher boost.

Absubtle
CO
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