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Old 08-21-2008, 12:01 PM   #51
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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)
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Old 08-21-2008, 12:09 PM   #52
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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.
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Old 08-21-2008, 01:12 PM   #53
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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.
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Old 08-21-2008, 01:19 PM   #54
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Quote:
Originally Posted by Gollum View Post
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.
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Old 08-21-2008, 03:02 PM   #55
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Quote:
Originally Posted by Gollum View Post
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.
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.
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Old 08-21-2008, 03:17 PM   #56
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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.
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Old 08-21-2008, 03:29 PM   #57
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Originally Posted by Hal9000 View Post
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.
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.
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Old 08-22-2008, 08:00 AM   #58
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Quote:
Originally Posted by Gollum View Post
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.
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.
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Old 08-22-2008, 08:08 AM   #59
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do you know what anti reversion is???

I don't care what religion you are, I just want good fuel economy.
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Old 08-22-2008, 08:17 AM   #60
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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.
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