So, theoretically, what would happen

What would happen if I rigged the wastegate in a turbo car to open manually and stay that way. In theory the exhaust gasses would pass through and boost pressure would be minimized. This could allow the car to operate, almost, as if it were NA and hopefully increase fuel economy.

I guess the answer depends on how the engine changes timing and fuel trims... if those tables are based on boost pressure this could work right? Right? I know I’m not the first to think of this yet I don’t see it anywhere. What am I missing? 

Short answer, yes it would act like NA. Most wastegates fail to open to prevent runaway.

How the ECU would react? That's anyone's guess. I've seen them where they react just fine, just down on power, and when it complains endlessly.

When I blew up the turbo on the MS3 it just ran like an NA engine. It was mostly complaining because of all the oil getting thrown into the  intake and cat but there was no CEL.

I'm not sure there's much to be gained in mileage though.

Is this an NA motor with a turbo added, or a factory turbo car? 

If factory turboed, it will likely be way underpowered compared to an NA motor of the same size, especially an older turbo car with a low compression ratio. I've had the clip that only actuator arm on the wastegate come loose before, and the car would barely get out of it's own way. I'd be surprised if it was making 100hp (Galant VR4.) Even when not "in boost," you would be surprised how much airflow the turbo contributes even at low throttle opening and RPM. 

I don't think you would gain much in the way of MPG's either, Vs. driving easy with the turbo in the mix. It's not the turbo that hurts mileage, it's using the extra power that turbo allows that hurts mileage, for the most part. And the lower compression ratio, which you aren't changing anyway. 

I have a stock 2008 Mazdaspeed 3 so the question would be in regards to that. 

The0retical, that almost tells me all I need to know. At least enough to continue pursuing the idea. 

The mileage increase might not be huge but I find that where I live I spend a lot of time in or near boost at freeway speeds to just get up the hills around here. Throw a bike rack and a mountain bike on top and the problem gets worse. Gas is going up and I’m trying to find ways to make it easier to stomach. I babied the car on a road trip last weekend (no racks) and got almost 30mpg. Throw racks and a bike on and I’m well below 20. My BIL has a MZ3 hatch and while it’s not a mileage star it is better and the power is decent. I know the turbo would kill power in NA mode due to back pressure and such but I thought it might be a fun project to try anyway.

I don’t actually have a plan on how to implement this yet. I need to learn more about wastegates. 

Edit: I meant to add that my reasoning behind this is that the MS3 is known for DUMPING fuel when on boost. If I can get larger throttle openings before getting into boost it might be enough to make it up the hills or make a reasonable pass and not lose all of my fuel out of the tail pipe. 

In reply to spandak :

Aerodynamics are a bitch - I don't think you're going to see a ton of MPG gained doing so. Can you get the MTB inside? I can fit a pair of mtn bikes inside on my MS3 (and nothing else). If you want consistent MPG - driving slower is the main winner.

I've read - but not sure I agree - that a turbo gas engine can allow better mileage. Diesel engines get better mileage mostly because they don't have a throttle plate kicking efficiency in the nuts. The thinking is that if boost is set to overcome the throttle plate restriction in a gas engine, its efficiency will be boosted.

You sound like you're in position to test this: set the wastegate so that boost is zero (meaning MAP = ambient pressure), record the mileage, then for fun, reduce it further and see if mileage increases more or drops off.

Hmm, I guess I could try this experiment myself, but it's easier to have you do it for me :)

Just thinking about how the VE table looks in my MS, I'm not sure that it would help mileage.  Yeah, in theory, your ECU should be mapped properly to send the right amount of fuel for whatever amount of air is coming through the intake up to whatever boost level it's tuned for.  You might not normally run in that range so the tune might be a bit rough if it's MS, but my assumption would be that an OEM would have sort of worked all that out and you'd be okay with a stock ECU.

But....  if, in order to do as your right foot commands, boost builds up and the ECU moves into that part of the map, what happens when it can't build boost?  I would guess that the ECU is going to do whatever it needs to do to make the power that you're requesting which means you're pushing the pedal further which feeds more air which requires more fuel.  Because there's no boost there's less pressure in the cylinder with lower IATs which should mean less heat in the chamber which should mean you can run leaner than you'd typically want to under boost, but despite a lower ratio what would the volume of fuel be?  Are you pushing 300cc of fuel per minute when under boost at a 12.5:1 AFR but 350cc of fuel per minute under vacuum at a 14.7:1 AFR?  I think that would be a really interesting experiment to try, but my guess is that you're going to wind up throwing more fuel through the injectors without boost trying to maintain the same power levels.

At one time, I remember a discussion about if it was more fuel efficient to pull away from a stroplight by flooring it to get to 55mph as fast as possible and then letting off or to give as little throttle as possible to ever-slowly increase speed to 55mph and I thought the solution was that it burned less fuel overall to get to speed as quickly as possible, but I'm not sure how scientific that was.

Perhaps reducing the request for power to the ECU would be more effective?  You could effectively reduce the amount of boost generated by reducing the amount that the throttle butterfly is opened.

I don't think you're going to see any increase in mileage and you may get a low boost code which could (depending on how the ECU is setup) put it in an engine protection mode which would reduce mileage even further. 

There's a million other ways to save gas beside playing with boost.  That said, playing with boost settings is fun and that makes it worthwhile to investigate.  Looking forward to your observations.

I drive faster than my wife and get better fuel economy than she does. 

How you drive has the most effect on your fuel economy.  Accelerate from a stop moderately, then use the minimum amount of throttle necessary to maintain speed.  Accelerate lightly down hills and allow yourself to drop a few mph going up the other side. 

Use cruise on flat roads.  Even if I painstakingly maintain a set speed manually, cruise returns a better MPG over that same segment.  

Don't slow down unnecessarily and look as far ahead as possible to maximize coast if you are going need to slow or stop.  

Driving as fast as I can while returning good MPG is the fun game I play to stave off boredom.  (I drive a lot for work.)  I like to try to beat my trip E/T and economy over a 4hr drive.  

Some light aero mods would do much better than purposefully trying to make you turbo car run, without boost. 

In reply to spandak :

The other thing you may want to consider is having the car retuned. The Mazdaspeed3 is pretty well known at this point, the ECU is easily manipulated, and they're well known for being extremely conservative from the factory.

Working with a tuner you might be able to lean the tables out a bit to fix things or completely remap it to only build boost above a certain throttle position threshold or limit it at lower demand positions. That way you're not relying on the ECU to make the adjustments and risking it reacting in an unknown way. Pulling the restricter pill from the turbo and remapping the ECU provides a lot more control over boost on that platform. Not only can you probably tune for extra economy, the modification is reversible with a few button pushes when you want to rip around with it.

I'm pretty heavily modified (3" intake, larger turbo, and all sorts of other stuff) and I regularly return 33mpg on the highway, even with the Sport Comp 2's it wears most of the year. I can give you the contact info for an engineer at Stratified Tuning if you want to run some questions by them.

I would love to tune the car but it’s stock because I live in LA and I would really rather not mess with CARB stuff. I bought an AEM cai because it’s carb approved and that’s as far as I can go. I know everyone knows a guy who will pass it but no thanks. Not my thing. 

Tyler: I already do those things. On longer drives I play the game where I try to maximize economy but maintain a high average speed. The MS3 is terrible at this game. I could easily get 30 out of my E36 and I got 33 out of my wife’s old focus doing above legal speeds. The Mazda plummets to 25mpg doing this. 

Aero mods could help. I’ve been thinking about a rear undertray and changing the front air dam. I should get a fairing for my roof rack too. I could probably get the bike inside, I haven’t checked. But that’s not as fun! I’d rather tinker.

Thanks for the input guys. I still think it’s an interesting idea and I’ll update this if I end up trying it out. 

Make a rear diffuser, add a splitter up front, take the roof rack off when you aren't using, etc. 

In reply to spandak :

Well you don't really have to modify the hard parts.

Simply reflashing the ECU for economy, then flash back to stock and do a drive cycle to reset the monitors before emissions would avoid a lot of those issues as the reflash isn't permanent. There's a lot of bottom end power loss when the wastegate is full open. If you limit it to partially closed and adjust the fuel map for specific circumstances, it'll improve the drivability a lot.

There's basically no evidence of the flash even if you hit a checkpoint since you're not disabling any emissions controls. All the random checkpoints I got stopped at in CA were visual only, no one tailpiped me.

Barring that aero would be the next step for when you have the bikes mounted up.