That's one of the ideas for the tracker. The stock engine with stock compression and everything but a turbo slapped on. The turbo should give me a little more grunt so I don't have to get out of overdrive on hills and the car won't need to keep the TC unlocked all the time to keep 75mph. Then again, I've already got a solution for the second one.

And what is that? I want to know!! (I actually want a way to lock my TC earlier at like 30-35mph)

I agree. The VW turbo 1.8 and 2.0 have something like 10.2:1 compression from the factory. They work well for getting good power and good FE.

I'm following the same trend with my other DSM. I'm just finishing the rebuild on it and I upped the compression from 8.3 to 9.0 I also have a 1.6 head which has smaller combustion chambers for more compression and slightly more quench area. If I can't run the same ammount of boost on it, I won't be sad since I was able to run over 30 psi (gage stopped at 30).

hmm, it appears that there are some opinions that are founded on incomplete evidence in this thread and i wish i saw this thread earlier.

if there are two motors that are identical aside from a turbo, the turbo COULD get better fuel economy or it could get much worse fuel economy depending on the driver. a turbo is alway spooling except when the wastegate opens and air is not sent to the turbo. this means that even though a motor may not have the turbo up to producing max psi, it is still spooling that turbo and feeding the motor more air than it would prior to having the turbo.

now how does that help fuel economy? more air means more gas, but it also means more power and at a lower rpm. somebody else had mentioned it earlier but i will say it again: this helps with pumping losses. the topic is similar to that of WOT shifting.

of course if your driving up into boost and not shifting earlier enough for fuel economy, you will naturally be getting much worse mpg.

essentially forced induction broadens the possibilities for FE

Could you clarify this statement? Something doesn't seem quite right.

What he means is that the turbo impellers are always spinning and producing some level of positive pressure in the intake system. As long as exhaust if flowing through the turbo housing, the impellers just about have to be turning.

When people talk about a turbo "spooling up" they're describing one of several things because the term has too much of a catchall meaning. The most important meaning for this discussion is when the turbo comes into it's efficient operating range and begins producing large enough quantities of pressure to affect performance... But even when a turbo isn't spooled up (like when cruising on the highway for example) it's still idling so to speak. When this is happening, the turbo only produces a small amount of pressure. That won't affect performance substantially, but what it does do is alleviate the need to the engine to suck air into the engine on it's own (like a vacuum pump). The turbo pushes the air in instead. This can make a difference in the VE and FE of the engine.

At part throttle where it is idling the turbo isn't pressing any air into the engine. You've still got manifold vacuum like any other engine at part throttle for it to work against. By the time you've opened the throttle enough that the throttle plate isn't an issue that turbo(if sized properly) should be well into creating more than just atmospheric levels of boost.

As for the TC fix, my TCC Experiment thread in the Experiment forum describes what i've come up with. It isn't installed yet but should be this weekend.

ah my bad

when a motor is running the turbo is always spooling except when the wastegate opens(and of course when there is 0 throttle).

when the wastegate opens, not all the air is not sent to the turbo because the turbo is fully spooled. what i mean by this is that all exhaust gases are pushing the turbo until it is either fully spooled, or the exhaust gases cannot cause it to accelerate any more. in the later case, where the wastegate is still shut, the motor is still using the exhaust gases to help the motor breathe. its still not positive pressure but yea, hal pretty much already said this.

If you have an electronically controlled wastegate that you can program, you can program it to be wide open under cruise conditions, but this will cause severe vacuum, and might put stress on the turbo's bearings. I'm not sure it's exactly optimal.

In my experience turbos DO seem to get slightly better gas mileage than the same NA engines. I have yet to actually figure out a testable, or believable reason why. But it's not much in highway conditions, maybe 2-5%. One of the large factors in alot of engines, is that turbo version of motors run lower compression, allowing for a leaner cruise, but also turbo engines usually have different cam profiles, which affects everything.

why would you ever want to do that?:thumbdown:

A turbo moving air at part throttle cruise is just sucking harder against a now-further closed throttle to maintain the same HP from the engine. If you're cruising at 55, it takes a given amount of power, no more no less. Now you just have a turbo blocking your exhaust while sucking harder against a closed throttle, all to put the exact same amount of air into the engine at the same pressure (vacuum).

Now, if your turbo produces any boost at all down at the lower RPMs, maybe pulse and glide will be benefited (assuming you're not boosting so much as to be enrichening the fuel excessively)

why would it be sucking harder? sure you need a given amount of power to maintain a given speed but the amount of fuel it takes to make that power varies when you consider other factors especially forced induction.

dont you think that a stronger vacuum would be created in the exhaust since the turbo allows absolutely no reversion?? thats right, when the exhaust valves open for a given cylinder there can be a strong vacuum in the exhaust with a turbine there that has been maintaining an airflow. all motor designs make huge amounts of power through this by tuning the header, but there is a problem, its tuned for a specific flow rate. there is a lot that i didnt get into, but its still a known fact: under light acceleration and cruising a turbo can help fuel economy.

This is pretty simple stuff guys. A Turbo not creating boost (most part throttle/low load situations) is an OBSTRUCTION to airflow, not necesarely helping.

It takes energy to move the turbine, and it's not giving you any real energy back. Opening the waste gate when you're not under much load would bypass the turbo and allow it to sit at a standstill.

BUT, you'd now have the intake side choking the manifold. So it's kind of a loose loose situation. The best option would be to also have an eletronic bypass solonied so air could be routed past the turbo on the intake side as well, effectively making it an NA engine.

read my post, its not simple, and to you, i guess its counter intuitive.

I'll have to see if I still have the book that explains the phenomenon... If I can find it I'll quote it for you; the author did a better job explaining it than I can.

First of all, the energy that turbochargers use to operate is waste energy (so it's free energy). You don't subtract the power that a turbocharger takes to run from the engine HP. If you were to take an NA engine and bolt a properly sized turbocharger and exhaust onto it but not connect the intake to the engine, you wouldn't lose any power at all. Sure, if you put an undersized turbo on, it will restrict the exhaust and the head temp will spike, but I personally don't make decisions based on what happens if you do things wrong.

Second, like I mentioned, the turbo never stops spinning completely. Even if it's not producing positive pressure, it's alleviating some or all of the negative pressure created in the intake manifold.

The newer generations of turbos by holset and schwitzer have amazingly wide operating ranges and they'll start producing boost at much lower rpms than older turbos did. That usually means that they'll spin faster even in light throttle circumstances (which is what helps the FE crowd out) than older turbos.

You're right that the difference in FE is minor and that it is only under certain conditions (I think I pointed that out at the beginning of the thread). But look at my car as an example. NA, I'm running nearly 50 mpg on the highway and I've yet to make any aero mods or engine mods to improve the FE or MPG. I'm expecting to get deep into the 50 mpg range or better as an NA car. I don't know about you, but I'll take a 5% improvement of 50 mph andan increase in useable hp (for when I need it only!) any day.

I don't care what you guys say, turbos take power. They will ALWAYS create SOME restriction in the exhaust, unless we're talking about a setup that's design for very high RPM. This restirction will create backpressure and heat.

It's just like how adding a Catalyric Converter will decrease power and FE in most cars. Excess back pressure created for no reasons usually hurts a well designed exhaust. Adding a turbo to most cars is about the worse thing you can do for your exhaust step, but this is offset by the fact that most turbo specific camshafts have ZERO camshaft overlap between the intake and exhaust. This means that there wouldn't be any scavenging effects anyways.


Let's all keep in mind that most turbo setups on unmodified NA engines have LOWER FE, not higher. It's usually turbo varients from manufactures that actually get comparable highway miles, if not a little better. Usually these setups have comepletely different fuel tables, ECUs, sensors, and camshafts.

So a turbo car has no manifold vacuum?

Have you ever driven a car with a turbo that is too big for it? The Honda guys tend to do that. An engine that is turbocharged is gutless when you are experiencing 'turbo lag', there is zero scavenging and the engine is working against a restriction(the turbine) which leads to very, very poor performance. If putting a turbo on an engine used nothing but waste energy and didn't affect off-boost performance then turbo lag wouldn't be as drastic as it is.

And it isn't in comparison to its on-boost performance that it's so gutless, it is against its NA counterpart that it is gutless. I've seen people lose races in turbo cars because it took too long to get boost,they were even the same exact engines and both automatics.

No, you don't subtract the power used to drive the turbo because that is already subtracted when measuring an engine's overall output. If you were to take that same amount of boost and apply it to the same engine without the turbine load in the exhaust it would make way more power. Turbos aren't a source of 'free' energy, they require work to be driven and said work comes from the piston moving upwards on the exhaust stroke. Like a supercharger gets its work from a belt off the crankshaft.

Now, I'm not saying that a turbocharged car can't be more volumetricly efficient than its non-turbo counterpart. Under heavy load and boost the larger volume in the combustion chamber means less percentage of heat can be lost to the walls of the cylinder and combustion chamber so more energy can be exerted on the piston moving downwards, the boost also increases the dynamic compression ratio of the engine. In some situations, the boost can keep the engine from needing a downshift to keep up which would increase revs and friction losses otherwise.

do you know what anti reversion is??? you are wrong, there is a reason car manufacturers dont do what you recommend

dkjones, you are at least providing good counterpoints and you are not being closed minded. you are worthy of debate.

anyway, turbo sizing is a different argument. yes i will concede, a turbo that doesnt fully spool until 5000rpm is going to take power away from 1.5-2.5k rpm which is the low end of your fuel economy range, even on a higher revving honda. so with that sort of application, you would lose fuel economy because the work that has to be done is just too much for the motor, much like having an exhaust that is too large for an all motor car, there is just too much volume to make an impact. usually only cars that drag race run this kind of setup with any sort of success, cars that road race dont have as much success, and if your auto-xing it would be better if you stayed naturally aspirated.

the argument still stands, although with the requisite that the turbo be properly sized for daily driving and fuel economy can broaden the possibilities of a motor on both ends of the FE spectrum depending on the driver. this applies with both static situations(cruising on flat roads) or dynamic(hills and traffic). whether you are saving or not depends on the driver.

do you know what anti reversion is???

I don't care what religion you are, I just want good fuel economy.

yup. for those that dont, google still works.

man, where is ben? he could probably give a better explanation on why appropriately sized turbos CAN be good for fuel economy than i could.

They did that on Top Gear a while ago during one of their challenges... Jerremy put an oversized turbo in a Mitsubishi (I think) and the car had awful overheating problems.

Ok, so now we're going to talk about reversion?

I'd argue that a well designed, and tuned exhaust on a NA engine will be far more benificial than the anti-reversion effects of a baffle (turbo in thise case).

It's better to have no energy wave reflection at all, then to try to control it. Let the pipe diameter control the max torque range for the exhaust, and keep bends minimal. Run a straight through muffler to keep the system open, and not reflecting waves back up the exhaust system.

Just because you have an "open" exhaust system doesn't mean it won't produce low end efficiency. Size the pipes right and the velocity will still be there.

I remember that, it was a starion.

The data they gave out was very vauge though, and the true reason for the overheating was very unclear. Was it due to a turbo running outside it's efficeny, or under? Was it due to poor exhaust turbine sizing, or simply lack of an intercooler? Turbo sizing is a complicated equation, and just picking any ol' turbo can get you in trouble quick for many different reasons.

My apologies if I sound terse here, I'm at work and have to be quick.

Gollum, I think that you'll find that an apples to apples comparison between a turbo specific exhaust system and an NA specific exhaust system will surprise you in terms of backpressure. Yes turbos add some backpressure, but so do normal mufflers. Turbo exhausts are designed to be as low restriction as possible after the turbo since that's necessary for the unit to function properly. In a good turbo exhaust, the turbo itself is the ONLY major restrictions.

Regarding catalytic converters since it came up. Newer generation catalytic converters are much less restrictive than older ones used to be. I read one article that said the increase in backpressure through a good cat is equivalent to less than a 1/4" restriction in the exhaust system.

dkjones, sure turbo engines can still run manifold vacuum. The question is when does the vacuum disappear and either become neutral or switch to a positive pressure situation? Ideally, you want that to happen at the same approximate throttle position you'd find yourself at during cruise. That's the whole point.

In terms of lag... 95% of the DIY and aftermarket turbo systems I've ever seen have had oversize turbochargers, intercoolers, intake piping, or all three. Of course they'll have lag. I'm not going to concern myself with problems associated with design flaws that wouldn't be incorporated into a system that I'd build for myself though. A properly sized turbo system should have little or no turbo lag under any circumstances. Doing it right involves quite a bit of planning and mathematics, but it's not difficult as much as it is labor intensive.

That was a very good post, and I can agree with all of it. Though here's a question I'll pose -

If a turbo is going to be right at the brink of negative to positive pressure, then will it run out of steam in the higher RPM? The turbo planned for a FE build up would have to have a VERY wide map to work without a significantly large intercooler. Most of the map calculations I'm comming up with to create pressure at cruise RPMS are dangerously close to the surge limit on the turbo. I'd really like to see what you'd recommend for a given setup.

Oh, and mufflers don't create much of any backpressure when they're straight through ;) There's some pretty decent straight through mufflers now days that work well enough for street vehicles not to be too loud and buzzy.

I completely agree about most turbo setups out there. A T3 on a honda is just way too big for most street cars. My 2.8 liter inline 6 does just fine with the stock T3 up to around 270hp, then a T3/T4 50 trim will go from 250 to 400hp easily.

But I have to be going around a 100mph cruise to get my turbo to positive pressure. I know with a modern ball bearing turbo that wouldn't be the case, but how much different would it really be?

EXACTLY! now if you apply how reversion (or anti reversion) works with the entire system of a turbo it has the ability to make the motor more efficient since it has the ability to be effective over a larger range of flow rates and oscillations in exhaust pressure. because the turbo isnt just a baffle, it is a turbine that carries momentum, it actually INCREASES the fluctuation of high and low pressures as the valves open and close between the valves and turbo. there is a lot more that goes into it, but this gives you a general idea of the possibilities and shows that its more than just a restriction.:thumbup: :thumbup:

I know a turbo isn't just restriction, but it's never moving under it's own power. The only instance I could imagine a turbo carrying gasses in a benifictial way would be under spool down.

Now, energy wave reversion... hmm. It would all come down to the manifold design. I'm getting where you're going with this, but I think the cost of building X amount of manifolds to get the turbo just far enough away for cruising RPM wil be difficult. And turbo sizing is extremely complicated, as you've got a very small window here. You don't want the turbo to just give you power when you put your foot into it, you want it to slowly ease on the positive pressure under cruise conditions. Not a simple task. What engine/turbo setup are you considering for a candidate?

I don't understand how this could help FE. The energy isn't free; it's not recovered waste energy, the engine has to push the exhaust out to turn the turbo, right?

I haven't gone as far as selecting a turbo yet since I'm still on the fence about actually turbocharging my mpg car, but it's an '88 crx with a D15. I'll probably end up with a very small turbocharger. But one of my other cars is a cosworth/mercedes that I'm building for bonneville. It's got a 2.3L 16V engine that I'm pairing up with a hybrid Holset HX 35/40 turbo. This setup should yield roughly 400-450 whp, although with some lag.

I agree with you about the stright through mufflers, most of the newer ones are pretty good. But NA exhausts and even some factory turbo exhausts are usually longer and have more bends than what I would build for a turbocharged engine. In general terms though, even the restriction of exhaust gasses through a turbocharger can be a good thing. A big problem that many garage tuners create for themselves by adding high flow exhausts to NA cars is overscavenging the head. That'll kill your FE and low end torque. Most street engines work best with some moderate level of backpressure unless you're only worried about top speed and high rpms. Zero backpressure is typically a bad thing except for a very few special conditions.

Many older turbos might run out of gas at high rpm, but I'd say that careful selection of even an older turbo could alleviate the problem. The first time I heard about this phenomenon was close to 20 years ago, and turbo design has changed quite a bit in the intervening years. Also, keep in mind, that the boost produced isn't usually linear. It really ramps up steeply in the upper flow ranges in most cases. What I'm talking about is not necessarily having the turbo beginning to spool up and produce real boost at low throttle settings. It's just kind of spinning lazily along and just barely producing pressure.

For turbos with good wide ranges, I'd personally start by looking at the Holset HX series units that have a "power ring" or "map enhancement ring". Basically it's a slit in the intake housing that allows air to be pulled in from the side partway down the inducer. This isn't state of the art technology, but it's new enough that you won't find it on the old t3's and t4's. I can't explain how it works, but it widens the useful gasflow range that the turbo works at substantially. This is basically why the honda guys have been able to get away with running HX 35's off dodge cummins trucks with any level of success. Schwitzer and I think the newer GT series Garrets also carry the same technology. What it will allow you to do is get away with a larger turbo that won't surge at high rpm, but will still spool at a relatively low rpm. Of course, header design, wastegate placement (be sure to go w/ external wastegates since the internal ones usually affect turbo efficiency and increase the rpm that the turbo begins to spool up at by a few hundred rpm) and many other factors can really affect things. I can't think of a way that IC size would affect things except to change the volume of the intake (more volume takes longer to pressurize => lag) and temperature. But those are more performance that FE issues in my opinion.

a turbo manifold design is not as complex as an all motor header design. are there large areas for improvement? of course! but even some cars(specifically integras and civics) have seen a small increase in highway fuel economy by adding the turbo kit alone, even with the grossly inefficient log manifolds. most of the time you can figure things out as far as turbo sizing with an online calculator.

My argument isnt that it works every time. My argument is that its very possible and that it isnt rocket science.

cow: a turbo does recover wasted energy: heat. it uses the pressure and heat to spin the turbo because the volume of air going into the motor is much more than the volume coming out. as gollum and i have discussed, the turbo aids in anti-reversion in cruising conditions. there is a vacuum formed in the manifold behind the exhaust valve from both the exhausts velocity AND the turbos momentum. its important to note that the turbo will NOT allow the exhaust gasses back into this vacuum. this not only helps exhaust gasses exit the cylinder, this effect is so strong that it is possible in BOTH NA and turbo setups that this vacuum can actually suck the piston back up the cylinder

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.

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.

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.

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.

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.

Excuse me? How do you explain this fellow gassavers setup? https://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.

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).

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.

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.