Turbo Boost and FE

[GasSavers_Gollum]

Well, it's not captureing "free" energy, just wasted energy. The lower the friction on the bearings though, the lower the power required to drive it, and thus better spool.

I think that if you can get the turbo to be making 0psi manifold vacuum/boost under cruise conditions I could see the turbo improving FE. But it of course all comes down to the computer tune, and how hard it's working to reach 14.7 in close loop, or how lean it's running in open loop.

Some of the increase in some turbo setups could also be from more lean conditions due to an increase of air without as much, if any, increase in fuel.

I can completely agree that in a well tuned turbo setup it shuold be just as FE cruising the highway, but more efficient is still hard for me to swallow. I'd like to see some well document testing, but why shuoldn't I be the one to do so? Maybe if my next job pays well enough I'll go ahead and do it.

[GasSavers_Hal9000]

I plan do eventually do a cheap turbo setup on my honda (the "don't care" car!), but you'll have to wait until I have the time to source the parts.

As far as evidence goes, I'm afraid it's probably all anecdotal at this point since every turbo install seems to be for power these days and not for FE. Maybe we can break some new ground here....

Of course the computer tune will affect the way any car (turbo or NA) runs. I have a friend with a ZC powered crx that's getting 13 mpg. Not exactly stellar, and it's all due to his efi. I don't think I could get my FE in my CRX that low if I tried.

[GasSavers_Gollum]

Holy crap, 13mpg in a CRX? Even when a buddie's B16A CRX had a massive vacuum leak bringing idle up to 3500 rpm he still got over 20mpg.

[thisisntjared]

of course bigger injectors are going to allow the driver to burn more fuel. that is the problem with turbo setups, it can allow for BOTH better and worse fuel economy depending on how its driven. i have a feeling the reason everyone is skeptical is because it can dramatically cut your fuel economy in half with either a bad tune or a heavy foot.

forced induction is also artificial displacement. it does allow you to burn a lot more for a lot more power.

[GasSavers_Hal9000]

Originally Posted by thisisntjared

of course bigger injectors are going to allow the driver to burn more fuel. that is the problem with turbo setups, it can allow for BOTH better and worse fuel economy depending on how its driven. i have a feeling the reason everyone is skeptical is because it can dramatically cut your fuel economy in half with either a bad tune or a heavy foot.

forced induction is also artificial displacement. it does allow you to burn a lot more for a lot more power.

Amen to that! I think I've mentioned before that half of the equation is having a driver with the restraint to drive like a grandma instead of playing with any extra power that the system produces. To that end, when/if I do my system, I'll be limiting it to between 3-5psi max, and using as few of the blingy aftermarket parts that look fast and sound loud. If I forget that there's a turbo there, it'll be easier to ignore it and just drive normally.

[itjstagame]

Nice thread. I can't believe people are arguing over whether turbos increase efficiency or not, that's crazy. Of course I don't have any direct quotes or numbers either but they help with exhaust scavenging and improve thermal efficiency of the engine.

Everyone I know with a turbo gets quite a bit better MPG than the same car in NA form. Of course when they're at the track or driving as fast they can they can get anywhere from 5-15MPG. And then there are STis which seem to always get 15MPG for no reason.

I was trying to find a quote I thought I read about turbos boosting overall engine efficiency to 50% but I think I found the source and it was about thermal efficiency of 2 stroke turbo diesels being 50%. Which btw, are awesome engines and I really think we need them on smaller scales for auto use. I mean a 500cc to 1L should be more than enough power for a car and get quite a bit increase in MPG over a standard diesel.

Anyway, it's true that turbos can be compared to increasing the displacement of an engine and require more fuel, but it's really kind of a displacement on demand, I think the best application would be a 700-1000cc engine that gets awesome fuel ecomony at cruise (neutral pressure), but can take a hit to fuel economy and spool the turbo up to 'act' like a 2L engine when acceleration is needed. Then you would have better FE than a 1.5-2L but just as much power when you needed it.

Good luck in your project, someday I'll build my 2stroke turbo diesel engine out of my spare 550cc mcycle engine and throw it into my CRX. But not having a machine shop or money that time will be never.

[itjstagame]

Originally Posted by BEEF

I can see where there is an advantage to having a turbo but I can pretty safely say that your turbo car will probably never see what my car is getting (35.7 average) especially when you throw all wheel drive in the mix.

a turbo diesel will get realy good mileage and I guess in theory you could get better FE out of a turbocharged gas engine if it were smaller displacement.

I remember reading about chryslers slingshot. it was a concept car kind of sporty and tremendously small (by todays standards anyway) it had a turbo charged 3-cyl. it was a concept car that died. that was before the fuel price explosion though. back in $2 days.

Excuse me? How do you explain this fellow gassavers setup? http://www.gassavers.org/garage/view/207
Also I had a friend with a 2nd gen DSM and even larger turbo than Laser there and he average 33MPG all the time.

Plus all my friends with Saab turbos can easily get 30-35MPG. I really wish I had a turbo or diesel (or both) car.

[dkjones96]

Originally Posted by Gollum

Holy crap, 13mpg in a CRX? Even when a buddie's B16A CRX had a massive vacuum leak bringing idle up to 3500 rpm he still got over 20mpg.

No kidding!

A couple of my worst two tanks in the tracker was a 18.6 mpg city tank where i just beat that car flooring it to the redline at every light and hitting the brakes often. One of my worst highway tanks was 21.7 when I went from El Paso to San Antonio and had the engine literally wide open the entire trip with the transmission locked in overdrive for a top speed around 90 mph(speed limit was 80 btw).

[GasSavers_Gollum]

Let's clarify some things though itjstagame -

Engine Efficiency doesn't necesarly mean FE.

Volumetic Efficiency doesn't necessarly mean FE either.

Turbo's thrive on two things, heat, and pressure. If you don't have pressure in the exhaust manifold, it won't be making power. If the heat of the air exiting the engine is cold (as far as engines go) then it won't be as efficient either. The very NATURE of why a turbo works inhibits exhaust scavenging under boost. Now, under nutral boost or vacuum, the story is unclear and might be very dependant on a case by case basis.

I know about 50+ guys online with the same turbo engine I have in my 75' Z, the L28ET. It's a 2.8 liter inline 6 with a T04B turbo. Kinda old, but gets the job done. There's guys with modern ball bearing turbos, holsets, and of course plenty of people running the stock turbo. Almost ALL of these guys with aftermarket EFI are getting right around 30mpg. There's even a guy in Georgia putting over 500hp to the wheels on this stone age engine design who STILL gets 30mpg with SDS efi, not the most tunable EFI system out there, but it works.

What are the best NA engines out there getting in the same cars? 28-32 MPG, same exact range. Some of the guys getting 30+ are carbed!!!

Maybe a turbo CAN be more efficient, but it's the minority with most setups, and it's not just driver related. Look at EPA ratings on turbo vs NA cars. On the highway the turbo might be close, but not over the NA counterpart.

What I'm getting at, is that it's extremely hard to ever call a turbo setup a "mpg improvement mod". You'll NEVER make the money back in any increase. What you're REALLY doing is adding "the most economical way to gain power". Which I'm ALL about.

Turbos ARE a displacement on demand, and that's what makes them so different than superchargers which have a fixed curve based on RPM as to how much power they're adding.

Oh, and just because a turbo increases thermal efficiency doesn't mean better FE. Thermal Efficiency is about how much power an engine is making for the given heat generated by the engine. This has nothing to do with fuel usage under a cruise or around town condition.

[dkjones96]

I agree with you Gollum but I think the gains as far as using a turbo for fuel economy are situational.

A normal driver gets a turbo slapped on and keeps driving normally and they probably won't see a difference. Your typical lead foot will see a decrease. However, a person doing P&G driving will most likely see an increase as would a person that is careful about slowdowns but is hard on the gas up to speed.

The point of P&G is to use the engine only when it is most thermally efficient. Using the engine when it is most efficient all the time would yield a lower fuel economy but strategic use can mean better overall FE numbers. Under boost an engine will be more thermally efficient during pulses so I can see that giving you better economy. Most any other situation? Forget it.