Reading on Ryan's site and here, It seems that the 3800sc can avarage about 30mpg on the hwy and the NA version can get up to 37 mpg.

Just wondering what effect a small turbo on a 3800NA would be gas milage wise if the turbo were enough to get the HP up to around 250.

Also, if you swap the heads and intakes from a NA to a SC bottom end, would the compression still be the same as a stock NA?


Steve

Edit: I can't spell milage lol

[This message has been edited by JumpStart (edited 10-17-2011).]

I'm not an expert but turbo motors usually have reduced fuel economy than a N/A motor, although with a jump in power. The reduction, as I understand it, is because of two things:

1. The cylinders have to pump harder to get the exhaust out since the turbo basically acts as a restriction of flow
2. Because the intake charge is hotter than a normally aspirated one, the compression ratio has to be reduced to prevent detonation, thus lowering combustion efficiency

In principle a turbocharger increases the efficiency of a piston motor, using energy from the exhaust heat to help overcome pumping losses in the intake system. However, since turbos increase power and since most people do a turbo to increase power, the temptation to use that power more aggressively generally results in a loss of gas mileage. In other words, if you turbo a car and successfully resist planting your foot in the throttle at every opportunity you would see a decrease in fuel used to travel a given distance.

The way I understand a turbo you use more fuel to accommodate the more forced air entering the cylinder. I'm not sure how you could get better than stock when adding a turbo yourself.

quote Originally posted by JazzMan: In other words, if you turbo a car and successfully resist planting your foot in the throttle at every opportunity you would see a decrease in fuel used to travel a given distance.

So a turbo could actually increase your gas mileage a bit if, like you said, you keep your foot out of it. I had also read on ZZP that ratio rockers increase your HP and gas mileage. Now I am starting to wonder just how much mileage you could get with a small turbo and ratio rockers? If you could squeeze out 3-5 more MPG, this would get you the same as the 2.5 with 150+ more HP.

Something nice to think about. Thanks for the insight.

Steve

quote Originally posted by JumpStart: So a turbo could actually increase your gas mileage a bit if, like you said, you keep your foot out of it. I had also read on ZZP that ratio rockers increase your HP and gas mileage. Now I am starting to wonder just how much mileage you could get with a small turbo and ratio rockers? If you could squeeze out 3-5 more MPG, this would get you the same as the 2.5 with 150+ more HP. Something nice to think about. Thanks for the insight. Steve

Think of it this way. When you breath through a straw you're having to expend muscle energy to suck the air in past the restriction that is the straw. The throttle body is also a restriction, that's why there's vacuum behind it when the engine is running. When the valves open and the piston goes down, it has to suck air in past the throttle body and it takes the energy of the other pistons to get it to do that. A turbo pumps air past the throttle body restriction, which makes it easier for the piston to go down, ergo less energy wasted. The exhaust restriction turns out to not be an issue. As the exhaust goes past the blades the pressure drops, cooling the exhaust gases. That cooling represents energy transferred to the turbo blades, meaning it's a net gain in efficiency to spin the blades with exhaust energy.

Turbos get better mpg as it is more efficient in making power, such as climbing hills etc.
but on avg they get worse, well, just because boost is SOOO much fun.

------------------
"DRIVE IT LIKE A FIERO"
'84 Fiero, engine to be determined
'87Blue GT 3.4L Swap Completed!!!!!!!! Boosted!!!!!!!
^^^^ Now in the Construction Zone^^^^
Las Vegas Fiero Club Parts/Sales/Service/Club
Fiero Road Club Of Northern Nevada

[This message has been edited by sardonyx247 (edited 10-18-2011).]

I have some tricky tuning on my car.. but honestly I get the same MPGs with E85 and my 600+whp turbo setup as I did when I had a stock supercharged setup and 93 octane.

VW and Volvo have been using Turbo's to get power and mileage for years. If you keep your foot out of it then it will get good mileage.

quote Originally posted by JumpStart: Also, if you swap the heads and intakes from a NA to a SC bottom end, would the compression still be the same as a stock NA?

Still dont know about this. It would be already setup for boost but if the CR is too high, you could have KR.

A plain Jane NA 3800 with mild boost would be fine. You would have trouble running 20 psi with NA compression and 87 octane, but for 250 hp you would be running hardly any boost.

Turbos increase efficiency but note that efficiency is power out / power in. You'll be putting more "power in" (gasoline flow) than stock but will be getting more "power out" per unit in than stock so efficiency goes up.

It's not about keeping your foot off it -- engines have a most efficient operating point usually at high load, low speed.


The way manufacturers can gain efficiency from using a turbo is to say... Instead of having a Fiero 2.5 N/A they have a Fiero 1.6 Turbo. The 1.6T makes more power than the 2.5NA and since it's a smaller engine you will be loading it up more (causing a more efficient engine because it's running closer to its max torque more often). That's how the manufacturers use a turbo to increase MPGs on a turboed car vs that same car with an N/A engine.

Take the new Chevy Cruze for example.
The base engine is 1.8L NA. It gets 25/36 MPG (city/hwy) and makes 138/123 (hp/ft-lb)
The optional engine is the 1.4 Turbo and it gets 26/38. It makes 138/148 (hp/ft-lb)

So they downsize the engine by over 20% in displacement, put on a turbo and end up with an engine that makes equal power, a little more torque and gets a bump in fuel economy.

Their eco model has the same powertrain (1.4T) but has other bells and whistles to achieve 28/42 mpg.

^ I don't think that's completely true.

If you had two engines, a 3800 N/A and a 3800 turbo, the N/A would get superior fuel efficiency because of the reasons I gave in the first post (freer flowing exhaust and higher compression ratio). Turbo engines only increase apparent fuel economy when they replace larger displacement N/A engines, such as Ford's new Ecoboost 3.5 replacing V8 truck engines.

quote Originally posted by masospaghetti: ^ I don't think that's completely true. If you had two engines, a 3800 N/A and a 3800 turbo, the N/A would get superior fuel efficiency because of the reasons I gave in the first post (freer flowing exhaust and higher compression ratio). Turbo engines only increase apparent fuel economy when they replace larger displacement N/A engines, such as Ford's new Ecoboost 3.5 replacing V8 truck engines.

That's in line with what i was trying to say.. maybe it came out wrong? Or maybe your " ^ " points to someone else's post?

quote Originally posted by masospaghetti: ... N/A would get superior fuel efficiency because of ... freer flowing exhaust and higher compression ratio.

Higher compression ratio ... yes.
Freer flowing exhaust ... no, not with a properly-designed turbo exhaust system.

The turbine section acts to suppress exhaust noise, which means that a less restrictive muffler can be fitted without an increase in exhaust noise. At low to moderate throttle settings the turbine section adds almost no exhaust back pressure, because the compressor section is just spinning freely in the near-vacuum downstream of the throttle body.

Most turboed engines have a lower compression ratio to provide improved detonation margin. Supercharged engines, turbo or mechanical, have a lower detonation margin than normally-aspirated engines due to the intake air under boost being both higher pressure and hotter than normally-aspirated. Lower compression ratio carries with it the penalty of lower thermal efficiency in any heat engine. (This hold true for both piston engines and gas turbines.)

In practice, most supercharged engines also richen the fuel mixture substantially (more than N/A) when under boost, to cool the intake charge somewhat and further improve the detonation margin. Just cruising down the highway, though, it's the lower compression ratio that is the primary killer of fuel efficiency ... well, that and the tendency to actually use the extra power available.

Beyond that, I think JazzMan explained it all pretty well.

[This message has been edited by Marvin McInnis (edited 10-18-2011).]

I think the general rule of thumb is that forced induction increases BSFC, which of course reduces fuel economy. The efficiency increase from running an engine hyperbaric is mechanical efficiency, not fuel efficiency. They are two completely different things.

Isn't fuel consumption determined by how much air you take in? I always thought that the more air you shove into the cylinder the more fuel you would need to create the most efficient combustion.

Both sides are kind of right. As long as everything is balanced out well, a turbo engine should almost in all cases be more efficient than a NA engine:

"The objective of a turbocharger, just as that of a supercharger, is to improve an engine's volumetric efficiency by increasing the intake density. The compressor draws in ambient air and compresses it before it enters into the intake manifold at increased pressure. This results in a greater mass of air entering the cylinders on each intake stroke. The power needed to spin the centrifugal compressor is derived from the high pressure and temperature of the engine's exhaust gases. The turbine converts the engine exhaust's potential pressure energy and kinetic velocity energy into rotational power, which is in turn used to drive the compressor.

A turbocharger may also be used to increase fuel efficiency without any attempt to increase power. It does this by recovering waste energy in the exhaust and feeding it back into the engine intake. By using this otherwise wasted energy to increase the mass of air, it becomes easier to ensure that all fuel is burned before being vented at the start of the exhaust stage. The increased temperature from the higher pressure gives a higher Carnot efficiency."

Clearly it's not as simple as just bolting on a turbo and you boost MPG. There's a lot of tuning to make sure you're getting peak power and torque where you need it. However, assuming you're not changing the fuel input, using a turbo should increase engine efficiency in any engine where all of the fuel isn't burned completely. Even in a "perfect burn" engine, a turbo would give more air and burn more fuel, and that increases the power per cycle leading to a reduced engine RPM and typically increase MPG.

Any increase in exhaust port back pressure would be offset by increased intake pressure.

The buick 3800 is one of the best engines to come from G.M with out the turbo or supercharger ,,,,5 star
this engine is a bit heavy for the Fiero ,but with a custom head port job & good exhaust plumbing it delivers 4 cylinder economy & great performance . ..this is the engine to have for a performance buff looking up the road at gas shortages or the enthusiast who wants a 4 speed automatic
hooked up to some power..I am a minority of one preaching for the non turbo, non supercharged 3.8
the supercharged version will come very close to the MPG the NA version gets ,, the turbo will use more fuel
A good port job & exhaust will equal the performance of the other options,with much better MPG,,smoother power
Road test of buicks in the 90s show a very slight supercharger MPG penalty
Buick park ave 3.8 V6=19 city/27 hiwy
Buick park ave 3.8 supercharge=19 city/26 hwy
the 1993 Park ave. is a 3500 to 3620 pound car
this engine can get better MPG using a vacuum gage if you do not have
fancy ECM,,,hi speed 2.73 4 speed auto trans or mpg 5 speed

[This message has been edited by uhlanstan (edited 10-19-2011).]

A quote from "Maximum Boost" by Corky Bell

quote Will the turbocharger hurt my mileage? Yes. The turbo, when installed as an aftermarket item on a spark-ignition engine, is not an economizer and cannot be construed as such. There is no engineering basis for making such claims. If you are led into purchasing a turbo under the premise of improving your fuel mileage, be sure to get a written guarantee. When not operating under boost, a turbocharger is a small system restriction. This restriction causes a small loss in volumetric efficiency. Volumetric efficiency and fuel economy are definitely tied together. If your driving habits are about the same as most, your mileage will drop about 10% city and 5% highway. No miracles here.

It's been my experience that aftermarket turbos CAN improve mileage on the highway. Why? Because you use less throttle to achieve the same speed. Of course, it all depends on what speed you like to cruise at & where the engine makes power.
~ Paul
aka "Tha Driver"

Custom Fiberglass Parts

quote Originally posted by Tha Driver: It's been my experience that aftermarket turbos CAN improve mileage on the highway. Why? Because you use less throttle to achieve the same speed. Of course, it all depends on what speed you like to cruise at & where the engine makes power.

Adding a turbo doesn't give you a lower engine speed on the highway. That is determined by tire size and gear ratios in the transmission. If you're rolling down the highway at 2800 RPM without a turbo, you'd be rolling at 2800 RPM with one, to go the same speed assuming you didn't change tire size or transmissions. Only in the case where the turbo is pushing some air into the engine still, but so little that the ECM doesn't add more fuel, will you get slightly better MPG (maybe 1-2), because the engine is running slightly leaner. But there is a fairly narrow margin where that happens on modern (obd2) engines with stock programming. On the obd1 engines, there's a slightly larger margin, but still not a whole lot.

quote Originally posted by dobey: Adding a turbo doesn't give you a lower engine speed on the highway. That is determined by tire size and gear ratios in the transmission. If you're rolling down the highway at 2800 RPM without a turbo, you'd be rolling at 2800 RPM with one, to go the same speed assuming you didn't change tire size or transmissions. Only in the case where the turbo is pushing some air into the engine still, but so little that the ECM doesn't add more fuel, will you get slightly better MPG (maybe 1-2), because the engine is running slightly leaner. But there is a fairly narrow margin where that happens on modern (obd2) engines with stock programming. On the obd1 engines, there's a slightly larger margin, but still not a whole lot.

Right: you're getting the same amount of power using less fuel - therefore better mileage. At what speed & how much better all depends on the tune. My statement may be more true of carb'ed engines; they're the ones I've had the most experience with.
~ Paul
aka "Tha Driver"

Custom Fiberglass Parts

quote Originally posted by Tha Driver: It's been my experience that aftermarket turbos CAN improve mileage on the highway. Why? Because you use less throttle to achieve the same speed. Of course, it all depends on what speed you like to cruise at & where the engine makes power. ~ Paul aka "Tha Driver" Custom Fiberglass Parts

You are correct, and this is one of the theories that encourages high compression ratios. More combustion energy released = lower throttle input to maintain the same speed which equals higher efficiency and therefore greater fuel economy.

Here is a short detailed article with real world test results clearly showing a turbo version of a naturally aspirated engine producing better fuel economy. My high compression motor did 33 mpg hwy erroring on the low side and it had a near dead cylinder at the time. On the other hand, put your foot into a turbo motor and it will drink fuel several times faster than it saves it.

This is a good read, you'll appreciate it.

http://autospeed.com/cms/A_109931/article.html

quote Originally posted by Joseph Upson: You are correct, and this is one of the theories that encourages high compression ratios. More combustion energy released = lower throttle input to maintain the same speed which equals higher efficiency and therefore greater fuel economy. Here is a short detailed article with real world test results clearly showing a turbo version of a naturally aspirated engine producing better fuel economy. My high compression motor did 33 mpg hwy erroring on the low side and it had a near dead cylinder at the time. On the other hand, put your foot into a turbo motor and it will drink fuel several times faster than it saves it. This is a good read, you'll appreciate it. http://autospeed.com/cms/A_109931/article.html

These are always misleading, even if they are using "real world data" in their article. The turbo is not the only difference between the two engines. Nor is the compression ratio. What are the differences in the camshafts? What about ignition and fuel timing? Volume of fuel injected?

I'm not saying it's not possible to get better fuel economy with a turbo on the same size engine, but there's a whole lot more to it than simply bolting on a turbo system. I have yet to see anyone who has written such an article, or is advertising better economy with a turbo, offer a full explanation of all changes involved to get there.

quote Originally posted by Tha Driver: It's been my experience that aftermarket turbos CAN improve mileage on the highway. Why? Because you use less throttle to achieve the same speed. Of course, it all depends on what speed you like to cruise at & where the engine makes power. ~ Paul aka "Tha Driver" Custom Fiberglass Parts

I don't see how taking an engine and running it at lower throttle opening with a lower compression ratio is going to improve fuel economy, since volumetric efficiency drops at low throttle openings. You are making the engine work harder just to breathe. Therefore, a N/A motor will have higher volumetric efficiency AND combustion efficiency. The only benefit of running a turbo is a increase in specific output.

Basically, you're saying a 300 hp, turbo 3800 could run at 10% throttle (for example) instead of 20% in a N/A motor. However, even if combustion efficiency is the same for both engines, the same amount of fuel is needed to mix with the air to generate the same amount of power. So the turbo at cruise loads isn't gaining you anything except making the engine run at lower throttle opening. And then there's the reduced compression ratio that comes with it.

If this is flawed logic, someone please explain it to me.

[This message has been edited by masospaghetti (edited 10-19-2011).]

quote Originally posted by masospaghetti: I don't see how taking an engine and running it at lower throttle opening with a lower compression ratio is going to improve fuel economy, since volumetric efficiency drops at low throttle openings. You are making the engine work harder just to breathe. Therefore, a N/A motor will have higher volumetric efficiency AND combustion efficiency. The only benefit of running a turbo is a increase in specific output. Basically, you're saying a 300 hp, turbo 3800 could run at 10% throttle (for example) instead of 20% in a N/A motor. However, even if combustion efficiency is the same for both engines, the same amount of fuel is needed to mix with the air to generate the same amount of power. So the turbo at cruise loads isn't gaining you anything except making the engine run at lower throttle opening. And then there's the reduced compression ratio that comes with it. If this is flawed logic, someone please explain it to me.

What I'm saying is that if you turbo a NA engine with no other mods, it doesn't have to work as hard at cruising speed because the turbo is forcing air in as opposed to the engine having to pull the air in. This results in better fuel economy.
~ Paul
aka "Tha Driver"

Custom Fiberglass Parts

quote Originally posted by Tha Driver: What I'm saying is that if you turbo a NA engine with no other mods, it doesn't have to work as hard at cruising speed because the turbo is forcing air in as opposed to the engine having to pull the air in. This results in better fuel economy.

If you bolt a turbo on an N/A engine with no other mods, at best you will get the same fuel economy as you did without the turbo. Most likely, you are going to get worse fuel economy though, because there hasn't been any tuning done.

quote Originally posted by Tha Driver: What I'm saying is that if you turbo a NA engine with no other mods, it doesn't have to work as hard at cruising speed because the turbo is forcing air in as opposed to the engine having to pull the air in. This results in better fuel economy. ~ Paul aka "Tha Driver" Custom Fiberglass Parts

At cruising speeds turbos typically freewheel and don't produce any boost whatsoever. On the highway at steady speeds, your vacuum /boost gauge will read the same on an N/A or turbocharged engine. It is only when you open the throttle that boost is produced and the extra power comes on. I would then say that adding a turbo will add some exhaust restriction (and weight) and therefore result in a slight decrease in mileage. Maybe .5 to 1 MPG less than before. My turbo 3.4L lost 1 MPG Hwy mileage after I added the turbo.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, ZZP Intercooler, 3.4" Pulley, N* TB, LS1 MAF, Flotech Exhaust Autolite 104's Custom CAI 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

quote Originally posted by dobey: These are always misleading, even if they are using "real world data" in their article. The turbo is not the only difference between the two engines. Nor is the compression ratio. What are the differences in the camshafts? What about ignition and fuel timing? Volume of fuel injected? I'm not saying it's not possible to get better fuel economy with a turbo on the same size engine, but there's a whole lot more to it than simply bolting on a turbo system. I have yet to see anyone who has written such an article, or is advertising better economy with a turbo, offer a full explanation of all changes involved to get there.

I can't agree that articles like this are always misleading especially considering there doesn't appear to be any kind of sponsorship behind the article. The parameters you question; ignition, fueling, compression actually favor the naturally aspirated engine by nature given that in a turbo motor timing is usually reduced, along with compression and the camshaft the same or a little milder.
They compared two production motors from the same car that are the same size and share many of the same parts. I think what's most important to standardize the comparison is the mention of recovery of some of the thermal energy wasted in the exhaust system.

I run two EGT sensors one about 2 inches ahead of the turbine inlet and another about 8" from the outlet and there is about a 200 deg difference in temperature. The laws of thermodynamics don't change as a result of the turbo being added. So that 200 deg of missing heat did some work spinning the turbine helping the engine breath a little easier down low, not all of it but certainly some of it. Higher rpm and greater throttle input consumes more fuel. Their tests showed a turbo engine of equal size and smaller producing more torque at a much lower rpm than the naturally aspirated engine and that it results in fewer gear changes which usually saves fuel.

There's also this quote,

"In the Australian government test cycle, the naturally aspirated Skoda 2.0 FSI has a fuel consumption of 8.5 litres/100km while the turbo 2.0 FSI has a tested economy of 8.1 litres/100km! That’s right, the car with more power is also more economical! The CO2 emissions are also as you’d by now expect – 203 grams/kilometre for the naturally aspirated engine and 193 for the turbo."

Maybe the turbo car has better gearing, but if that's the case, why wouldn't they use it on the atmo motor if it were capable of equal economy results were they to do so? Usually more power results in taller gears because the torque is available to make use of them none the less producing better fuel economy.

quote Originally posted by masospaghetti: I don't see how taking an engine and running it at lower throttle opening with a lower compression ratio is going to improve fuel economy, since volumetric efficiency drops at low throttle openings. You are making the engine work harder just to breathe. Therefore, a N/A motor will have higher volumetric efficiency AND combustion efficiency. The only benefit of running a turbo is a increase in specific output. Basically, you're saying a 300 hp, turbo 3800 could run at 10% throttle (for example) instead of 20% in a N/A motor. However, even if combustion efficiency is the same for both engines, the same amount of fuel is needed to mix with the air to generate the same amount of power. So the turbo at cruise loads isn't gaining you anything except making the engine run at lower throttle opening. And then there's the reduced compression ratio that comes with it. If this is flawed logic, someone please explain it to me.

That's the point of enhancement, lower throttle input to achieve the same goal. With additional air/throttle you need more fuel to maintain proper afr. If you let in less air you use less fuel. If you spin lower rpm particularly around town because your powerplant can turn say 4th or 5th gear at 35 mph, you burn less fuel, that's why often you find smaller motors in smaller cars not doing much better in many of the same circumstances as full size cars and larger displacement motors.

quote Originally posted by dobey: If you bolt a turbo on an N/A engine with no other mods, at best you will get the same fuel economy as you did without the turbo. Most likely, you are going to get worse fuel economy though, because there hasn't been any tuning done.

I can understand the difficulty in accepting the idea, but most individuals that install aftermarket turbos are not doing so with the purpose of gaining fuel enconomy and rarely drive them in a manner that will net such. In my experience the first time I hit 28 mpg in my Fiero was after a tune up and new tires were installed. The engine was a 3100 with iron heads bolted to it (low compression, below 8:1 I understand) turbocharged turning a 4 speed muncie and not the econo box. That's great compared to a stock 2.8L many of which weren't getting that kind of mileage naturally aspirated with the 5 speed, and better compression. A bump in compression alone might have increased that 28 mpg by much more. That's a larger displacement, less efficient motor with a turbo hanging off of it.

I'm not an expert on the subject, I'm just speaking form what I've read along with what I've experienced. My stroked motor was 4.2L rounded and as I stated earlier it got 33 mpg hwy with a near dead cylinder turbocharged. Was it the turbo, the 6th gear, the high compression? I don't know for sure but I certainly don't blame the turbo for a loss in fuel economy under those circumstances with one cylinder 100 psi lower in compression than the other 5.

[This message has been edited by Joseph Upson (edited 10-19-2011).]

quote Originally posted by dobey: If you bolt a turbo on an N/A engine with no other mods, at best you will get the same fuel economy as you did without the turbo. Most likely, you are going to get worse fuel economy though, because there hasn't been any tuning done.

All I'm saying is that in my experience the cars that we've turboed got better gas mileage at cruising speeds than they did before the turbo was added. As I said before, they were carbed & it was decades ago - but I would think that with ALL the talk about how much better fuel injection is than a carb you should be able to tune it to do the same.
~ Paul
aka "Tha Driver"

Custom Fiberglass Parts

quote Originally posted by Tha Driver: All I'm saying is that in my experience the cars that we've turboed got better gas mileage at cruising speeds than they did before the turbo was added. As I said before, they were carbed & it was decades ago - but I would think that with ALL the talk about how much better fuel injection is than a carb you should be able to tune it to do the same.

A modern engine can certainly be tuned to get better fuel economy. On the carb cars did you change to smaller jets? Even if not, the way a carb works versus the way modern fuel injection works, is a completely different beast. Because the carb is entirely mechanical, at any given throttle position, it will always be dumping the same amount of fuel into the intake, assuming that the fuel pressure holds and doesn't get too high or too low. There are basically three ways to change amount of fuel going into the engine with a carb; throttle position, jet size, and fuel pressure. In a current model vehicle, there are significantly more variables which affect the amount of fuel which goes into an engine.

An engine can certainly be built and tuned to get better MPG with turbo, than without, but it's not as simple as just slapping on a turbo, which many of these conversations try to make it appear to be. It's just not that simple. If it were, then one of the "electric turbocharger" things would actually improve highway MPG, since it doesn't actually take that much power to create the very tiny increase in pressure that the turbo is making while cruising down the highway in 6th gear.

To maintain proper AFR, the throttle position itself doesn't really matter. Even if the throttle is open less to get the same mass of air from the slight pressure increase, the ECM should add more fuel based on the MAF, MAP, and O2 sensors' readings. If you're adding more air, and not more fuel, then you're just leaning out the mixture, which can be done just as well without a turbo.

quote That's the point of enhancement, lower throttle input to achieve the same goal. With additional air/throttle you need more fuel to maintain proper afr. If you let in less air you use less fuel. If you spin lower rpm particularly around town because your powerplant can turn say 4th or 5th gear at 35 mph, you burn less fuel, that's why often you find smaller motors in smaller cars not doing much better in many of the same circumstances as full size cars and larger displacement motors.

I am not disagreeing with this fact at all, this is the the key benefit of turbo motors...the higher specific output allows them to replace larger displacement engines. I understand anecdotally that turbos seem to help fuel economy, however, the math just doesn't seem to add up.

quote What I'm saying is that if you turbo a NA engine with no other mods, it doesn't have to work as hard at cruising speed because the turbo is forcing air in as opposed to the engine having to pull the air in. This results in better fuel economy.

But the turbo engine has to work harder to push exhaust gases out. Any energy that is helping force air in came out of the mechanical energy in the exhaust stream.

quote Maybe the turbo car has better gearing, but if that's the case, why wouldn't they use it on the atmo motor if it were capable of equal economy results were they to do so? Usually more power results in taller gears because the torque is available to make use of them none the less producing better fuel economy.

I totally agree with this as well, but I was talking strictly two motors, one turbo and one N/A, with no differences in gearing or displacement.

re: dobeys reference to the electric superchargers. From what I've read, they actually restrict the air at higher rpms. It always seemed to me that turbo economy was based on replacing a larger engine with a smaller one without giving up the power. Since a car that is just cruising doesn't need that big engine to maintain speed, only to accelerate. I kind of wish that I had completed my swap without the turbo first so that I could have compared. Cartuning says that they have reports of an extra 2-3mpg with the turbo, but I don't understand how since I still have the same size engine, not a smaller one.

Doesn't the statement about the engine not working as hard because the turbo is forcing air into the engine backfire on itself because you need more work to compress the now more dense air mixture? (talking about low speed operation)

quote Originally posted by masospaghetti: I totally agree with this as well, but I was talking strictly two motors, one turbo and one N/A, with no differences in gearing or displacement.

With no difference in the gearing the turbo is all that's left to account for the government test showing the turbo motor consuming less fuel. A turbo motor only works harder to move exhaust flow under load with a properly sized turbo. This is where the boost pressure to exhaust pressure ratio comes into play, 1 to 1 is assumed ideal. Many of the do it your self turbo installers have noticed that often at idle the turbo compressor does not spin much if at all. If the back pressure was as high as you are assuming it is that would not be the case. There is a threshold to exceed for turbine exhaust restriction to be significant.

You also have to offset turbine exhaust pressure with the typical exhaust system back pressure and the exhaust system pressure is usually negated and replaced with what in some cases is much less pressure with the turbo installed when followed by a high flow system. There are also dual signal wastegates to contend with which allow a good bit of exhaust gasses to bypass the turbine housing altogether when used because until boost is approached they are open by manifold vacuum.

At cruise speed the exhaust back pressure is probably insignificant and the compressor is likely moving enough air to exceed the need so that lighter throttle input is needed to maintain cruise speed. That amounts to a bump in pumping efficiency due to utilizing hot exhaust gases that otherwise would be pumped out the exhaust and not used at all. Thermodynamics at work, recovering some of the lost energy to do more work.

quote Originally posted by dratts: re: dobeys reference to the electric superchargers. From what I've read, they actually restrict the air at higher rpms. It always seemed to me that turbo economy was based on replacing a larger engine with a smaller one without giving up the power. Since a car that is just cruising doesn't need that big engine to maintain speed, only to accelerate. I kind of wish that I had completed my swap without the turbo first so that I could have compared. Cartuning says that they have reports of an extra 2-3mpg with the turbo, but I don't understand how since I still have the same size engine, not a smaller one.

For the electric superchargers, I am mostly talking about cruise speed, not high RPMs. At cruise speeds (iow, where you may get some very small amount of boost from a normal turbo), an electric motor running the compressor blades at the same speed should basically be just as efficient as the normal turbo, because the compressor is barely turning.

That 2-3 MPG is not becaue of the turbo. It is because of other associated changes, and it is probably primarily noticed in city driving. The big factor here is probably the shift kit, if those drivers who report such increase in MPG aren't changing their driving style from having the turbo. The transmission is probably shifting up into top gear faster, so the engine is spending less time over 2500 RPM where a lot more fuel gets used.

quote Originally posted by KaijuSenso: Doesn't the statement about the engine not working as hard because the turbo is forcing air into the engine backfire on itself because you need more work to compress the now more dense air mixture? (talking about low speed operation)

That denser mixture also releases more energy, some of which can be siphoned off to do more work. Think of the heat being put back into the system as a form of insulation. For example, the thicker the insulation layer surrounding a refrigerator or home, the less energy you have to expend maintaining its temperature. This is why higher operating temperatures to a degree actually help with fuel economy, it helps to keep combustion pressures up for greater force application to the piston. There is no other reason for auto manufactureres GM in particular to allow coolant temps to reach 240 deg.

If you've engaged in tuning a turbo car, you'll also notice that hot exhaust gases at idle will spool the turbo up at the same idle rpm where the compressor hardly moves at all when temps are much cooler, due to the difference in expansion of the gases.