Turbo (forced induction) and gas mileage

[Matt Timion]

Will using a turbo setup help with fuel economy? There has been some discussion of it recently, and I'm curious to know what you all think.

It might be good to discuss the fuel economy benefits of doing this and the physics behind it. I wouldn't be against installing a turbo setup on my car if it can make a difference at all.

What do you all think?

[diamondlarry]

The biggest problem I have with mods/equipment that improve power is that I'm tempted to use it.;-) I'm getting better about that the last few years though.:-) I firmly believe that if you increase power witout using the extra power you can get better mileage. A good example of this is the old 4-barrel carbs. The primaries were smaller than a 2-barrel and they got better mileage; IF you kept your foot out of the secondaries. So I think a turbo could have mileage benefits if used responsibly.

[MetroMPG]

from http://www.fuelsaving.info/chipping.htm:

Adding 10 or 20% more torque / power to a turbocharged engine (petrol or diesel) is relatively simple, and there is no question that these upgrades do approximately what they say. As a result, the engine may be able to cope with a particular driving situation in a gear one higher than is normal (say fourth rather than third). This means the engine runs more slowly, and so there is less power wasted in friction. The principle of using very "tall" gearing for better economy is well understood and indeed can be seen in many of today's 6-speed vehicles.

Generally the engine's combustion efficiency will be worse at full power, since both petrol and diesel engines "over-fuel" to some extent at full load and so some fuel is wasted. But the friction reduction effect will normally more than compensate for this and so overall the economy will be better.

he's actually talking about "chipping" an existing turbocharged engine for higher output. sounds like if you add more power (i.e. turbo), and it permits you to upshift at a lower rpm, then you'll be saving fuel.

unfortunately, it's not clear whether it will help or hurt you in top gear, since you're going to be doing the same rpm for a given speed (assuming a manual transmission) before and after the upgrade, so are no longer benefitting from friction reduction.

i've been very impressed with the http://fuelsaving.info site. written by a british auto engineer who examines fuel saving devices with a very level-headed, not to mention educated & qualified point of view. he de-bunks many too-good-to-be-true mpg gadgets and additives apparently out of sheer irritation with the pseudoscience surrounding them.

highly worth a visit if you haven't been there before (i know you've linked to him a couple of times already matt).

[Matt Timion]

I love that site. I first started reading it when I was reading about the Khaos Super Turbo Charger, a device which has even fooled a few government agencies.

Anyway, about the turbo.

Assume I drive on the freeway at 65mph (which is true). When I go on road trips, I maintain a certain speed. I do not pay attention to RPM as much as I do speed. If this is the case, wouldn't a turbo actually benefit me? If I maintain a constant 65-70mph with my cruise control won't the turbo actually lower the RPMs enough to increase efficiency?

SVOBoy and I were also talking about how removing the intercooler from the equation will also essentially make a warm air intake out of the turbo, killing two birds with one stone.

[SVOboy]

I'm not the authority on turbocharing, just starting to learn really, but from what I understand a turbo basically turns waste heat into something useful to the engine, therefore increasing efficiency of the engine as a whole, and if you don't use the efficiency for power, you can use it for gas mileage, there's so much to read on turbos, I'll post up some links soon.

[Compaq888]

having a turbo will require to look after all kinds of crap. It will be a huge headache.

P.S. Don't mean to crap on your thread.

[SVOboy]

Running a small, non-intercooled turbo will not be so much hassle. It'll be like a stock turbo that only runs 4 or 5 psi, and you never get crap from them, you just need to make sure you do it right.

[MetroMPG]

matt -

i remembered reading about a turbo conversion that improved mileage. the car that was modified was a first generation prius (like it needs its mileage improved). the results: hwy fuel economy improved by 3.6%

the prius conversion is presented in 5 parts (search "turbo prius" using their site search to find them all). note - it's free preview, but paid registration is required to access the full articles.

part5:

http://autospeed.drive.com.au/cms/A_2664/article.html

i've paid to have access to the articles on this site. it's not expensive, it's "lifetime", and there are some good ones in there. i paid to gain access to a series on "modifying underbody air flow" that improved mileage - also on the same prius, before they turbo'd it - by 10% through the addition of a smooth tray under the front of the car. good stuff.

(btw, i'm not affiliated with or compensated by the site.)

[MetroMPG]

was thinking a bit more about this...

one big reason a turbo would work to increase efficiency on a prius is because of its transmission - it's continuously variable.

that fits in nicely with the pwr/efficiency reasoning from http://fuelsaving.info - namely that an increase in power can be beneficial if it permits you to run in a higher gear for a given road speed. because of its CVT, the turbo'd prius would run at lower rpm for ALL road speeds (unless you stuck your foot in it of course).

yet another reason for my prius envy. as much as i love to row my own gears, i can never be as efficient as a CVT.

[Matt Timion]

Originally Posted by MetroMPG

was thinking a bit more about this...

one big reason a turbo would work to increase efficiency on a prius is because of its transmission - it's continuously variable.

that fits in nicely with the pwr/efficiency reasoning from http://fuelsaving.info - namely that an increase in power can be beneficial if it permits you to run in a higher gear for a given road speed. because of its CVT, the turbo'd prius would run at lower rpm for ALL road speeds (unless you stuck your foot in it of course).

yet another reason for my prius envy. as much as i love to row my own gears, i can never be as efficient as a CVT.

So in theory with a long geared transmission like I'm planning on putting in my civic, a turbo would actually be good. I could cruise around town in 5th gear with the turbo and essentially save lots of fuel.

A friend of mine told me that if I installed a turbo I'd have to get it tuned though. Is this true?