Turbo Boost and FE
 

Hello. I was reasoning with myself last night (as any reasonable gassaver would do =P) If I had a car, with a 2.0L engine and a turbo. If I stayed out of boost, my gas consumption would be one of a 2.0L engine without a turbo correct? and only if I boosted then my FE would flush down the drain?

Thanks!

To varying degrees, yeah. And it's definately better to have a 2L Turbo than a 3L non-turbo. But just to further complicate things for you, under some circumstances a little bit of boost can increase efficiency. ;)

Yeah, I'm a relative newcomer to the FE world, but I've been a performance nut for years. One of the early arguments turbochargers was that if you drive conservatively, you should be able to get slightly better gas mileage. The idea as I remember it is that even when not under boost, the turbo is spinning and it'll just barely overcome the parasitic drag of the fuel/air charge and any pumping inefficiencies of the engine.

Folks with the 2.0t VW GTI report wildly varying MPG numbers, from 12 to 45 MPG without any hypermiling tactics. So, I'd say the answer to your question is yes.

My turbo really doesn't do much until I'm over 2300 rpm's. If I'm driving for FE I rarely get my RPM's much over that. When you have boost you must have RPM's so you will, by nature of the beast, be getting poor FE. The best part of a turbo, I have the acceleration of a small V-8 when I need it and the FE of a 4 cyl.

thats good news to hear! but now I'm wondering why do people complain about bad mileage on a gt-four. i'm planning to get one, but they run about 13L/100km mixed driving.. its a 2.0L turbo.
I'm guessing the 4wd also contributes to bad FE?

I can see where there is an advantage to having a turbo but I can pretty safely say that your turbo car will probably never see what my car is getting (35.7 average) especially when you throw all wheel drive in the mix.

a turbo diesel will get realy good mileage and I guess in theory you could get better FE out of a turbocharged gas engine if it were smaller displacement.

I remember reading about chryslers slingshot. it was a concept car kind of sporty and tremendously small (by todays standards anyway) it had a turbo charged 3-cyl. it was a concept car that died. that was before the fuel price explosion though. back in $2 days.

heck. i can't even get your mileage with my 88 camry 1.8L.
anyways. getting off topic..

so if i say i have 2 identical cars, except one has a turbo and one does not. Who'll get the better FE if driven identically?

I'm guessing at low rpm's the turbo and high rpm's the non turbo?

If the engines are identical, the non-turbo one will probably do better. The whole reason turbos are associated with fuel economy is because you can use a smaller engine.

If you have a 2.0L non turbo and a 2.0L turbo, the non will always get better fuel mileage. It comes down to compression ratio. A Turbo has a 8.5 to 8.7 to 1 compression. The NA on the other hand may have a 9.5 to 10.5 to 1 compression ratio.
More compression more HP at all throttle openings when boost isn't a factor. At any normal cruising speed the NA's throttle will be opened at a lower setting.
That 2.0L NA may make 150 peak HP where a Turbo 2.0L may make 200 peak hp. At 30 percent throttle the NA makes 90HP where the Turbo makes only 75HP. The Turbo only has an advantage in HP when boost is factored in.

I'd say it totally depends on the driver. The downside of turbochargers from an FE standpoint is that it makes it much more tempting to have fun... More fun to go zipping away from lights, accelerate around slow traffic, or just get on the gas to hear the sound of the turbo or the beautiful exhaust note. If you're like 99% of the population, even if the turbo engine would do better when driven conservatively, you'll end up giving in to temptation and driving harder than with the NA car (ruining your mileage in the process). The human factor is probably the biggie here.

The only way I can see to solve that problem would be to limit the amount of boost the turbo produces (thus limiting the power on tap) to a very low number.

Exactly.

It lets a small engine provide a drivable car. I used to have a VW Jetta TDI (1.9L turbo diesel). Operating correctly, it had about 95hp and about 140lb-ft of torque. It drove very nicely, good acceleration, capable of flying on the highway (at least 110MPH... um, no officer, I never tried that, I swear).

For a while, I had a fault that disabled the turbo. The car was a slug without it. It was drivable, but felt like a late 70's econobox. Definitely not the image VW was going for. Putting a turbo on increases the power more than it decreases the efficiency, so a small (say, 2.0L) engine with a turbo that performs like a 3.5L engine but uses less fuel. I wish they would bring back ideas like the old Chevy Sprint... it came (optionally) with a 1.0L Turbocharged engine that got nearly 50MPG and was still fun to drive.

EDIT: I'm not necessarily saying that the Chevy Sprint turbo was a fantastic car, but I think that a modern econobox (Honda Fit, Toyota Yaris) with a modern very small turbocharged engine (say 1.2L) could be really awesome.

EDIT 2: Wow, two posts between mine and the one I saw as most recent. I must be a real slow-poke today. Oh yeah, I had to answer the phone while I was writing. :o

I happen to drive a car that provides a great comparison; 1990 Plymouth Laser, which is the same as the Mitsubishi Eclipse and Eagle Talon.
From 1990 through 1994 they made this car with a turbo 2.0L and non turbo 2.0. There were only a few differences between the NT and turbo versions;
NT had slightly milder cams and different compression (9.0:1 vs 7.8:1).
Here's the gov't rating for the 1990 Laser turbo.
https://fueleconomy.gov/feg/calculato...lumn=1&id=6573
19mpg city, 26mpg highway, 22 combined.
and here's the gov't rating for the 1990 Laser 2.0 NT.
https://fueleconomy.gov/feg/calculato...lumn=1&id=6574
The two cars were also very similar in weight with the NT being slightly lighter at 2650 lbs. NT came with 135HP and 125 ft.lbs at 5k rpm, while the turbo model has 195HP and 203 ft.lbs at 3k rpm. Notice the low rpm torque advantage?
A significant difference also shows up in their transmission gearing.
vfaq.com/mods/Trannies.html
The NT was given lower gearing and lower final drive.
My theory is that the NT needed lower gearing because it didn't make much torque down low, so gearing had to make up for it. The turbo car makes excellent torque at low rpm ( similar to VW's turbo 1.8 and turbo 2.0 engines, which have their torque peak at 1800 rpm.)
which makes it easy to shift early and saves gas.

My friend has a turbo supra (1987) and has seen over 35mpg highway on more than one occasion and his car is pretty worked over. Will make nearly 380 hp at the rear wheels (I've witnessed it make 371 and now has more tuning done).

I just bought a 96 Miata w/ a DIY turbo install halfway done, when I finish the install and begin driving it, I'll have something to report of my own as far as turbo and FE is concerned.

I think the note about the higher compression engine making more hp at any given throttle setting (with boost factored out) is a bust because to my knowledge, there isn't a single turbo production car on the road that uses the SAME engine as its non-turbo counterpart. Like DRW has said, the NT to Turbo models had variations in cams, as well as timing, fuel injectors, and in many cases valves per cylinder. It's almost illogical to think two engines could be identical aside from a turbo added to one because it's just not that simple.

My Miata for example, yes, the engine was originally non turbo, and now a turbo has been added, but the exhaust system is totally different now, the injectors will be nearly double in cc's, and MUCH tuning will need to be done.

another interesting experiment would be to take a stock engine with pretty good FE and shave the heads to see if there is any merit to the whole increased compression ratio getting better FE.

I understand that it isn't that easy to do. you could also just get a thinner head gasket to prove/disprove it. I know that guys have been doing that for years for more power. I think if you had a dyno sheet of before and after, you could determine if there would be FE gains by the torque slope. if the torque went up (especially in the lower rpms) then you would see FE gains.

like I said though, it wouldn't be cheap to do ether and possibly not worth the gains depending on how much they are

Increasing compression should help- The 1984-87 CRX 1.5 HF engines had a few more tenths higher compression ratio than the non-HF 1.5 engines made during the same years.

The only issue with shaving the head on an OHC engine is that you end up with slightly retarded valve timing from. The gains from increased compression ratio would probably outweigh any losses from retarding valve timing a tiny amount.

Valve timing is an easy item to fix. Adjustable timing gears are a dime a dozen nowadays. I'd even go so far as to suggest that it would be a good addition to a stock engine to really tune things for max FE. Most of the time, manufacturers adjust their engines to meet parameters other than FE and power. Noise, emissions (especially cold start emissions), and driveability are usually the most important factors. I've seen examples of otherwise stock engines that gain more than 30 hp just through adjustment of the valve timing.

Being a mechanic and working on small engines in the 65 to 240 hp range,Watercraft.Higher compression will give better throttle response,a bit more acceleration and actually slightly less to speed on two identical engines.The only problem with this formula is it is two stroke engines I am talking about.I would think the optimal car would be an electric driven, powered by a small turbo diesel running at its optimum rpm and power output, charging the battery ,and being off when not charging or running at the set rpm when running, regardless if car is stopped , going uphill or down hill.>Marvin

Same basic rules apply for 4 stroke engines. Higher C:R > higher output, better throttle response, (better FE as well), and better volumetric efficiency. The only real hitch is (like I mentioned in an earlier post) the human factor. It's much harder to resist using HP you have than HP that you don't have. That alone makes it hard to quantify the benefits of increasing C:R in my opinion and even if the engine is more efficient, if the driver doesn't have the restraint to continue driving conservatively, or doesn't realize that they're driving more aggressively (just as common in my experience), then any benefits will be lost and then some.

I think I've shown that restraint is possible.
My car is putting out around 275HP and I've driven multiple tanks in a row without going WOT even once. Yes it's boring, but so is my commute.
Stomping on the 'GO!' pedal gives instant gratification, but restraint yeilds long term rewards.

In the 80's and early 90's Chrysler put a 2.2 turbo and NT (or NA, naturally aspirated) in evrything. I had an Omni that was an NA 2.2 and it ran good but it was a slug. I had an 86 Daytona turbo Z with the 2.2 and a turbo, it was fairly quick. I now have a 92 Daytona with the intercooled DOHC turbo and it's pretty quick. Chrysler put that 2.2 in freakin everything. Good motor, poochy without some help.

mrjetskey,

VW is making a diesel-electric hybrid car that is supposed to get 70 or so MPG. it works on a similar concept that you were describing (I think). they are putting it in a golf or a rabbit. not sure if it will make it to the states. emissions suck (laws I mean). I think that is the plan though, for it to come to the states.

We had several K cars with the 2.2 liter engines. Those were fine, but the 88 caravan with the 2.2 in it was a complete dog. Dad had that thing worn out at 70,000 miles. Mom had an 88 LeBaron that had the larger 2.6 (I'm pretty sure it was a 2.6, it was bigger than the standard K car engine). That car would move, and it wasn't even a turbo.

-Jay

The 2.6 was a Mitsubishi engine.

Turbo would be a great way to go with a smaller displacement engine. A perfect example is the Dodge (Mercedes) Sprinter Van.

In a small car I would like to see a 1-1.5 liter diesel with some of Gayle Banks magic for power, while you could get great economy when loads were small.


regards
gary

banks did a diesel dakota that broke a land speed record for diesel trucks.

he does have magic when it comes to diesels.

Yeah, that truck went 222 mph, and got 23.6 mpg. . .PULLING A TRAILER.
https://www.autobloggreen.com/2007/05...e-performance/

thats nuts. totally insane. but from what i read it seems like you need a diesel to actually get more FE from a turbo.. I still dont seem to understand why a celica gt4 would take as much as gas my honda odessy...

I like the hard hitting diesels.

I went to a dyno day with my dodge (gas engine) and they had a bunch of diesel trucks there. the heavy hitter pushed 809hp and 1416ft/lbs. his motor was far from stock. he was running twin turbos at somewhere around 80 psi of boost. converted it over to studs for the heads as he blew out the head bolts before. also he was using water injection to keep the things cool.

there was probably a lot more than that done to it but that is what I know. I am sure he was using forged pistons and other heavy duty inards to handle the pressure. he was having problems with his tires rippling on the dyno as well even though he was at max sidewall pressure as well.

he sent me a video clip of him running a qtr against a mach-1 mustang. he beat it and ran an 11.5 in the qtr. not bad for a 3 ton truck.

2 things to consider between a N/A and turbo motor. One is negative, the other is positive.....

1. exhaust restriction. The manifold is normally very short, creating restriction, plus the restriction in the body of the turbo, the restriction of the propeller, and any addition angles and turns that the exhaust must overcome to exit the into the exhaust system, plus the lost velocity that was used to turn the turbo hot side. Negative
2. Heat. Compressing air raises the temperature, hence the use in performance applications of an "Intercooler" (aftercooler), to remove heat. The additional heat on a turbo application, regardless of the the boost pressure, helps to atomize the fuel better, for a more efficient combustion efficiency. This is a built in hot air intake, so to speak. Positive

Ultimately, The heat from the turbo should help to overcome the restriction of exhaust, but it is a tight battle.

I have driven a turbo charger car for years also a 2 litre turbo, I thought I knew how to get the best FE but I didn't, I always assumed the turbo had to be in its efficiency range which on my engine would be 4.5k rpms, bad for FE obviously.

I honestly believe I have figured it out now, I am gentle in 1st and 2nd gear and I open the throttle a fair amount, not WOT but not chocking the intake either, in 3rd gear I look at my vacuum gauge and make sure that its at 0 or atmospheric pressure, the engine speed is low and the turbo begins to spin at 1.5k but I change gear before or on 2000rpms as this is the most efficient range in my fuel table, so I basically keep my throttle open but the engine speed below 2000rpms, what ever the turbo can do in that range has helped my FE because I got 160 miles from 3.7 UK gallons or 17.1 litres of fuel, when before I was getting 80 miles from the same fuel!

Keep the engine speed down but get the turbo working is the trick, if you can lean your fuel system I recommended that as well, turbo cars run extra rich under boost conditions, I hope to experiment with this idea this week.

My wife have a Hyundai Sonata 1992 with a 4g63(mitsubishi 2.0l) and 3 speed automatic FWD 3 speed + overdrive + lock-up.

My Galant VR4 also have a 4G63 but Turbo and 3 speed automatic AWD + overdrive witout lock-up.

Both car have similar FE but The Galant made a bit better with about 5MPG more. But It is Turbo without lockup and it's AWD. So if the galant was FWD with a lock-up, it will do better FE. Both car weight almost the same.

But if I dont run at low trottle FE will rise up, full trottle @ 25 psi of boost, i'm about 375HP at the wheel! :D Not good for FE :D

In 4g63, the compression ration of turbo and non-turbo engine is different.

One thing that is being missed here I think, but was covered on the ecomodder site, is fueling.When in boost, the fuel pressure will rise. The effective compression ratio will also rise.

As as anecdotal aside, several turbo mustang guys have noted increased MPG from their turbo bolt-ons.

I think a 9.0 CR with a low compresion turbo, or a turbo with some meth or water added would be the best of both worlds.

For me. 43 psi base pressure and 68 psi of fuel @ 25 psi of boost.

For shure compression ratio is different from a NT vs Turbo 4G63.

1G 4G63 NT: 9.0:1 (192 psi compression)
1G 4G63 T: 7.8:1 (164 psi compression)
2G 4G63 T:8.5:1 (168 psi compression)
Galant 92+ 4G63 NT: 9.8:1 (220 psi compression)

I'd love a 1 liter turbo 3 cyl.

talk about the best of both worlds. gas mileage without boost and enough tail to get out of the way with boost.

That's the rub, how much fun vs. how much FE?
Certainly a turbocharged engine is the right way to push both ends of the envelope. But there's still a limit to how much power you can get and still keep good FE. I'd like to see what a 3 cylinder 1 liter engine could do with direct injection and compound turbocharging. Maybe add an electric motor to make it a hybrid, and setup the engine so it can run as a generator when needed.
What would you guess? Maybe 75mpg and 250HP?

DRW, explain me how you get 46 to 53 mpg with your big Laser?

You almost make the FE of my VX with the set-up of my Galant that make 19 mpg...

No offense, but your galant's aero looks like this:
https://www.giveyourdreamswings.com/S...lyingBrick.gif

:)

It's also an automatic, does that torque converter lock up at cruise? Or does your high stall converter slip constantly? That would annihilate your fuel economy.

:D

This car is modify to drag, fnot for FE.
For shure I know that my AWD, Automatic, High stall and no lock-up don't help my FE, but I am not at my first DSM.

I had a 92 NT, 90T, 91T, 95T, Elantra swap 4g63T.
Even the Elantra(1000 pound less than the DSM) can't do best than 35mpg and it was MT FWD.

Haha, it's magic!:eek:
It's all in my gaslog. Essentially I have a long boring commute, which gives me plenty of time to think of new ways to squeeze more mpg out of the car. The car is fwd with a taller 5th gear and an eprom ecu with extensive mods. I've been on the dsm-ecu list for several years, and they released a full commented disassembly which makes it easier to mod almost everything in the ecu. Not to rain on anyone's parade, but even the famous AEM EMS would be a step backwards compared to the level of control I have with this ecu.
I also drive really slow. :rolleyes:

There is no reason why a turbo car can't have the same compression ratio as a stock non-turbo car if you tune it carefully.

I'm willing to bet that you could add a turbo to a N/A engine, keep the stock compression ratio, and tune it with a standalone for a pretty significant gain in both FE and HP.