Shifting with a Turbo
 

My MX-6 is a 2.2L Turbo. I have been working on my hypermiling technique, and have been taking it very easy. I have been shifting at low rpms and using fifth gear in city driving. One thing I have noticed as i am never really boosting. Doesn't a turbo aid in economy? Do i need to shift at higher rpms in order to get the benefit of the turbo? Not sure how many other eco-drivers here have turbos, but maybe someone with a turbo-diesel has observed ideal shift points?

A turbocharger allows you to downsize your engine while retaining the power of a larger engine. This also allows increased engine loads while cruising which is the main benefit of the smaller engine.

In your case, you already have a fair size engine for the size of your car. The turbo is just a power adder to an adequate engine. I'd do as you have been and just keep rpms low and stay out of the boost.

I guess this is what i get for trying to boost economy out of a performance car!

On to the aero-mods...

your turbo is still providing positive pressure, even though your manifold pressure isn't going positive.

This is commonly misunderstood about turbos. You're "boosting" even when you're not in positive pressure ranges.

Because it's already got a vacuum?

I'll try to answer this, but I'm not entirely sure that I'm understanding what you're asking here.

The idea with a turbo is that even when the manifold is under vacuum, the exact same circumstances (normally) without a turbo would put the manifold under higher vacuum.

Thusly, adding a turbo to ANY engine will decrease the vacuum necessary to induce fresh air to the pistons, because as long as the engine is under some load, the turbo is starting to spool.

To quantify this effect, remove the intake piping from the turbo and pipe it into a cap with a pressure gauge, and pre-load the engine at part throttle w/ the brakes engaged. You'll see positive boost pressure on the turbo's cold side outlet.

The reason most people don't recognize this as boost is b/c the boost gauge is still in vacuum. The reality is that the engine is just requiring more CFM than the turbo can produce under that specific load at that specific RPM, but the turbine is still moving more air than the engine could on it's own, so the result is something like:
(Engine under partial load @ -14in/hg + 2psi = -9.92 in/hg.)
Versus engine under partial load without turbo @ 14in/hg + 0 PSI = 14in/hg)

(This is the engine with the turbo)
This is the engine without the turbo.

Christ, you seem to have some great knowledge on turbos. What differences might i see if i adjusted my boost higher or lower?

I know how they work, but I don't know everything about them by a long shot.

If I had to answer, I'd tell you that adjusting your boost level only compensates for max boost conditions, which you're never hitting anyway if you're eco-driving, unless your turbo engine is properly sized for your vehicle (obviously, it's not, else you wouldn't have been asking about boosting to begin with.)

This would mean that even if you adjust your max boost to something like 2-3 PSI, you're not actually going to hit that at all anyway, so it won't really affect anything, except the (hopefully) rare occasion where you floor it for fun.

Turbos are evil little things for mpgs. See, I just cant help but get into it at least once or twice a tank.... so I'll never actually get the best mileage I could possibly get out of my car.

I have been being pretty good about it. Trying to keep it nice and low. Maybe i will allow myself a "turbo tank" every once and a while...

A boost controller could help you stay out of boost when you don't want it and dial it back in if and when you want/need it.

You can only dial down as low as your wastegate actuator is set up for. On my tbird, thats 10-12 psi. On the shadow its 4-5 psi and on my capri its around 7 psi.

An adjustable actuator would let you do as people are describing above, you could drop the actuator setting to allow no boost or less than the stock actuator is set up for and use a boost controller along with it for easier adjustment when you want it.

whirlygigs aren't always bad!

VW twin charger 1.4 (160hp version) produces less CO2 and more MPG on combined than the NA version!(80hp) As the name suggests not only does it have a Turobcharger its also got a supercharger. You can get a 122hp and 140hp version in the Golf, there is even going to be a 1.2l 105hp version in the Skoda yeti and I'm guessin new polo!

Dr Strangelove has been hard at work!!!

The 1.4L twincharged engine is the right idea, but I'd really like to see it in 0.7L format, with 60HP, in a Polo.

You can also give more leverage to the wastegate actuator, or put a prescribed-weight spring on the other side of the actuator that will net you whatever boost level you're happy with.

IOW - If you want 3 PSI max, and your wastegate won't open until like 10 PSI, get a 7lb spring and put it on the other side of the wastegate actuator arm. I don't actually think the math is quite that simple, but someone else suggested it, and I'm posting as a proxy.

Christ is quite knowledgable about turbos but no one has touched on the basic problem: that is, any dealer-equipped model has too large a turbo for mpg applications.

I have over 2 million miles on properly-sized turbocharged engines. What are they on? Big trucks.

But those turbochargers are huge, you say.

No, they're not.

If you look at it in terms of power to weight, they are very small.

This is the Ecomodder Forum. Let's talk about ecomodding.

Any turbocharged engine has forged pistons and lower compression, if it has been properly assembled. All that is needed is to install a much smaller turbocharger with a properly adjusted waste gate and fuel delivery for excellent mpg gains. Your top end horsepower will fall off significantly but your midrange torque will delight you.

I love my turbo Del Sol.:)

My car never came from the factory turbo charged. I built my own turbo system.

As of right now my car gets over "10mpg city EPA" better then a stock
Del Sol, and closer to "15 mpg EPA" better freeway.

My BOV is open all the time until it sees boost. Then the valve closes.
So I never have any pre T/B positive pressure when the manifold is in vacuum.

At 5" hg or less you can here the BOV letting air out as Christ said. But on my engine it acts just like a N/A engine until you lay into it.

Now a adjustable BOV will only work with a speed density system. Mass Air systems won't like a open to atmosphere BOV.

You are both thinking of petrol engines, whereas the O/p has a diesel so no intake vacuum (due to no throttle) and probably the same CR as a N/A diesel engine.

Ptero is correct about a smaller turbo creating greater boost at low revs but will ultimately restrict airflow at higher outputs.

I have found the best results for economy on a diesel without an instantanious on board fuel computer is to use an EGT gauge..high temps = high fuel usage.

This might be further improved with a boost gauge as well, as it may be better to run a higher gear under boost and higher EGT than to run in a lower gear.
It's really interesting seeing what affects the EGT, for example blocking the EGR valve made about a 50 degrees C reduction, lagging the exhaust manifold brought this back up again which improved turbo pressure and response.

I'll ad that Any gas car that is turbo-ed, and is computer controlled, will ad extra fuel to keep the car from leaning out and blowing.

With diesels, you need to add boost according to your fuel input, and gas cars add fuel according to their boost. They seem to work on different concepts.

I've seen examples of turbo diesels consistently getting better mileage then N/A diesels. My mechanic dropped a 1.9L TDi from a 2001 golf into his 1990 jetta, added bigger injectors, a bigger turbo, and turned up the fuel pump. The car spews black smoke all over the place, is it pretty fast for a diesel, but he can still get over 50 mpgs on a car cruise that included a few little races on the highway with turbo-ed gas cars and vr6's (tdi won), and Got waaay better mileage then I did with my stock 2005 tdi-pd, driving normally.

My friend has a tdi beetle. He was happy with his mileage but decided to chip the car, and add bigger injectors to get better performance out of it. He doesn't care about mileage. However, By upping the boost and injectors, he now gets way better mileage, and a much faster car. The same guy also put a Tdi in a corrado and modified it for performance and power. He ended up with a car that can get over 700km to half a tank from normal to slightly aggressive driving.

Gas cars sorta get a lot worse mileage as you mod them for power. diesels, in the experience of seeing my friends mod their cars, get better. :thumbup:

Not all of them.

If you have a management system and have your turbo map correctly you should be able to improve your FE and crush your N/A performance by a mile.:cool:

Allot depends on torque, lower in the rpm band the better for mpg. Something most 4 bangers don't have hence the need for EOC and PNG. I can shift at 1500 rpms in my C5 Corvette where the SV650 it's nothing below 2K rpms with light throttle.

Add a turbo to a stock 4 cylinder and you'll see more torque at lower rpms which increases mpg in most instances. If you shifted at the same 3K rpms after adding a turbo you might not even match NA.

Depending on turbo ecu, if you stay out of positive pressure or below 100 kpa, the car should be still maintaining 14.7 or stoich which would be best for mpg.

I see you have added pistons to you Sol, change in compession?

Norm

Yes I have change the rods and pistons from stock. The compression now is 7.5:1

I know a lot of people will say you need to get the compression up for better efficiency but in my case with my own combustion chamber design it doesn't work right. I have tested this with an engine with a compression of 9.8:1 and the FE wasn't there.

There are major advantages to a low compression engine for FE purposes if one chooses to think outside the box;)

Unmentioned so far is the benefit of heat. N/A cars build warm air for a reason. We should get better MPG in summer because of the heat.
Compressing air creates heat, and the heat aids in atomization of the fuel for better economy.