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.

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.

Welcome to ecomodder!

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.

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.

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.

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.

I'd be very surprised if a turbo gasser didn't have a bov.

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.

The Starion is - don't have much data, though...

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

Subies and most of your factory turbos come with a BOV not the best ones but they do have them. When it comes to the BOV the best setup is for you to have the BOV recirculate into the turbo. Turbo's need air to spool by keeping the air in the system it helps to keep the turbo spooling and that equals less load on it.

As for me my turbo is fully spooled at 3K RPMs and unless I'm in a hurry I shift before I hit any boost. That way the ECU stays out of the closed loop and isn't dumping extra gas in.

I found the best way to keep the Mazdaspeed6 (AWD here too)from getting poor FE is letting my wife drive. :)

But seriously what she does is pretty much keep it out of full boost, our car will spool up quite quickly.. all speed6's were manuals, and she basically will grandmaw the car to speed, and shifting at low RPM.
Try that keep it from full spool and drive that Spec-b (great cars by the way) like a grand-maw, until you get to speed.

I might be the opposite of what everyone else says, but here is my take. My worst tank came from babying it. Yes, you will use more fuel if you hit boost, and you will use alot more fuel if you go WOT. I usually go about 50% or so up to speed. IF you have a scangauge, find out when you go to "dump fuel" mode, and avoid that. Also, it might be a good idea to look at water/meth injection as a secondary fuel. It will keep your gas usage lower, and you can tune much leaner with it. Set it to come on early, and you should reduce gas usage when part-throttle boosting it. I will be installing one on my turbo move 660cc. 14 psi, and it's a dog out of boost, and probably isn't very efficient out of boost either. A big exhaust, DP, TP, and other flow mods should help you, as well as something to keep your IC cool. hooked up a sprayer on my IC for the hot days to cool it down. Heat below and insulation above a heatsink isn't the best way to cool things.

twinscroll

Check out what this guy has done with his Turbo Laser.
How To Save Gas - DSM Forums

Stay out of boost. Do what ever it takes to keep the engine from making boost. More PSI in the intake manifold the more gasoline the computer is going to inject into the engine.
I've owned an 81 Volvo 242 Turbo, 86 744 Turbo, and I'm running a 87 745 Turbo (362K) as my daily driver. Turbo gasoline engines are not the best way to increase gas mileage.
Turbo's on Diesel's are great way to increase power and fuel economy.

Thanks for the precision!

Turbos can be more efficient than NA gasoline engines because the compressed air can be intercooled, unlike piston compression. See miller and atkinson cycles.

I'm been driving in I mode, which I believe keeps it closed loop, even at with the pedal to the floor. I've gotten 28.5mpg in suburban driving. Moderate throttle and shifting at 2 krpm makes a big difference. I also have gotten better at choosing the right gear to downshift to, so that I can slow when coming to reds with the injectors off until just before complete stop.

The Miller cycle :thumbup:

Keeping it right at 0 or -1 is probably the best as most ecus really start dumping the fuel the second the MAP shows a positive. It's pretty agressive acceleration much more than most n/a's so you can coast a while. With the extra TQ we can shift quite early and not be bothered too much in driveability even with a 4 cylinder. Driving like that in a N/A 4 cylinder especially from the early 90s or earlier would shake it apart, and ping like crazy.

I'm no expert; but I did pull 32~33 mpg last summer in a mkiii supra turbo auto.

I boosted 2~3 psi in an attempt to use some exhaust energy; I'm sure it varies vehicle to vehicle. If you have a wide band o2 or Economy instrumentation you can attempt to make sense out of it.