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Old 07-30-2008, 12:24 AM   #1 (permalink)
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turbo engines: getting to speed

Anyone know what's the most efficient for why to accelerate with a boosted engine? I've read some claims that for n/a cars, it's best to go to 75% or even WOT at low rpm (shifting between 2000-2500 rpm) to get up to speed. Does anyone have links to actual tests that verify that? Or is gradual accel more efficient? What about with a turbo?


My car:

I have an '08 subaru spec b which has si-drive, which complicates things. In I mode, it limits boost to about 9psi, fwir and limits the throttle position to 40% (that's input, i believe, not the actual throttle plate). I think its ecu uses a torque target - it might go WOT with low boost, then ramp up the boost, rather than use a constant boost target across throttle positions and use the throttle to control engine torque (which would be less efficient and cause a lot of intake recirc thru the compressor). I need to do some research on that.

I'm not sure using I mode and flooring it (shifting at 2500rpm) is better than doing the same in S mode which will give 100% open throttle and maybe slightly more boost. Of course, you loose some turbine speed between shifts, so holding gears longer may be more efficient also.


Basically, I'm trying to make my city driving more efficient.

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Old 07-30-2008, 12:38 AM   #2 (permalink)
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Pull the wastegate spring so it doesn't boost? Sorry, I don't know much about turbos and that's my best guess. Someone smarter will chime in.

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Old 07-30-2008, 01:19 AM   #3 (permalink)
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I would imagine is similar to a n/a engine, just with more power and low end torque. Most stock turbo cars come with pretty small turbos so they spool pretty low in the rpm range, resulting in good low end grunt.

I guess the only REAL way to know is to try it yourself and see.
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Old 07-30-2008, 09:56 AM   #4 (permalink)
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I accelerate pretty much the same on the elantra and TDI. For the TDI, I just make sure I don't boost too much. I try to accelerate at no more than 50% of max boost, so I stay below 10 psi, and rev it around the max torque peak, which happens to be the lowest bsfc point and is conveniently situated at 1750 rpm.
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Old 07-30-2008, 12:13 PM   #5 (permalink)
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The problem with turbo gas motors is they have low compression(unlike a TDI). The subarus have about 8.5:1 compression. That means when the engine is pulling vacuum, all that expensive high octane you put in it isn't being utilized.

Max torque is somewhere above 3000rpm.

Turbos help overcome pumping losses by scavenging exhaust heat. A little boost makes up for the low compression and intake restrictions. Too much boost and the ecu goes into open loop mode and dumps extra fuel into the engine(10.5:1 in some cases). The subarus go into open loop mode at 60% throttle, ~4200rpm, or certain load ranges. It varies a bit depending on model. There's also no knock detection below ~2000 rpm so don't lug the engine too much.

The current internet theory is low rpm, highest gear, and slight boost, slight throttle. No one has tested to see what psi works best.

A retune might help you out as well. Subarus run really rich from the factory and have some odd blips in the timing map. It can be a pretty cheap upgrade, though I don't know if the romraider guys have cracked the 08 ecu yet. You could even set the close loop air/fuel target to burn leaner than stock.
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Old 07-31-2008, 12:43 PM   #6 (permalink)
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If you're trying to improve city mpg with a stick shift then you might want to look into a blow-off valve. This dumps manifold boost during shifts to reduce the load on the turbo and keep it spooled when you're building boost with the throttle closed and cut off exhaust flow to the turbine. This should help you get boost up faster after a shift at low engine speeds and should make the car more drivable. With your low compression ratio you need to keep it in some amount of boost to improve your thermal efficiency. Driving technique should help as would a scangauge; this setup should respond well to some P&G assuming you can keep it out of open-loop enrichment.
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Old 07-31-2008, 12:52 PM   #7 (permalink)
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I'd be very surprised if a turbo gasser didn't have a bov.
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Old 07-31-2008, 02:14 PM   #8 (permalink)
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I have been experimenting with my '96 F250 Powerstroke. Apples to Oranges here. Allegedly, the less boost pressure, the better fuel economy. Personally, I'm watching my EGT gauge and keeping it lower than 600 degrees on acceleration from a stop. This sorta emulates what I am doing with the 99 Chevy Metro 3/5 n/a.

Pulse and Glide? My F250 is an automatic. So I have tested the "glide" (in neutral) to see how long it takes to glide from 55 to 40. Then I do the acceleration such that it is about 1/2 the distance to get to 55 as it does gliding back down to 40. Example, if gliding 55-40 is 20 seconds, then acceleration is 10 seconds. This gives me about 5 psi boost, where max boost is 23 psi. Experimenting in progress...

There prolly ain't too many turbo gassers around, so you'll likely be in new ground on your results. Experiment A LOT and hopefully you have a scanguage or similiar to check your results.
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Old 07-31-2008, 02:30 PM   #9 (permalink)
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Quote:
Originally Posted by 99metro View Post
...There prolly ain't too many turbo gassers around, so you'll likely be in new ground on your results...
The Starion is - don't have much data, though...
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Old 07-31-2008, 02:43 PM   #10 (permalink)
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Subarus(and most modern turbo gas engines) have bypass valves instead of blow-off valves. Replacing the BPV with a BOV on a subaru without changing the MAF locatoin results in wasted fuel between shift.

Blow Off Valve FAQ: Read if you are thinking of buying one! - NASIOC

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