rx8wannahave

OK, some of you are going to through tomatoes at me or give me the old “If you care about fuel economy why…etc etc, tired tired etc etc”

We all agree that our 8’s are not Civics nor should they be. We all agree that some people are getting real bad fuel economy already with their 8’s and some are hitting the EPA numbers without a problem. But I wanted to understand SC/Turbo’s a little better and I don’t get why it would mess with your EPA…other than…

SC

It acts like your AC does so when it’s on (a SC is on all the time) it takes away some power 5HP I hear but gives you another 50-100HP. Since when you use your AC it makes the engine work harder I understand why the fuel economy with a SC would be worse…but how much worse are we talking about? 1mpg for 75HP is a deal in my book. But 5mpg (with a car that has trouble hitting 18) would make it unappealing.

Turbo

It uses the exhaust gas to spin the turbine which in turn sucks in air and forces into the engine. With a turbo how would it mess with fuel economy? It is making the engine work harder since it creates more back pressure or blocks somewhat the exhaust pressure???

Forgive my SC/Turbo ignorance but I wanted to know how they hurt fuel economy since they add more air to the air/fuel mix making a car make more HP…more efficient. So, how does this work?

Thanks for your input!

Just trying to gain some Turbo/SC wisdom.

Red Devil

I see on this board many people commenting about forced induction and single vs. twin/sequential turbo, "you don't want twin turbo/sequential, your gas mileage will be horrible." I question such logic myself as a single turbo can or will force more air into the engine than the sequential set-up of the FD.

My current opinion is that whether SC or Turbo, the volume and density of the charge entering the engine is what is important, in these respects, choose your poison - turbo or SC.

Someone else will see this that can give you a better explanation than I, or will be able to correct me.

DreamWarrior

Under normal driving I've heard that you may get better MPG because the engine is running more efficiently. At WOT or when boosting hard, however, your MPG will be lower because you need more fuel to get more power.

Basically, if you're using all the power that you gain by the turbo (i.e. mashing the pedal) expect your MPG to be less than what it is now (you don't get more power for free). However, at similar power levels the FI engine may get better MPG because it is running more efficiently.

I could be off, but I've had at least one friend put a Turbo on his car and get better MPG and this was his explanation why; to me it makes sense.

dmp

I think you're about right, Warrior...

With the turbo system on my since-sold V6 Mazda, I could drive all day, and not enter boost - could drive up to about 70?mph in 5th gear before I'd get to positive manifold pressure. Until one gets into boost, they aren't typically increasing their fuel consumption. Up until the boost threshold, think of a turbo/wastegate as nothing more than a rather signifigant exhaust restriction.



The first tank of gas for my Probe went by at about a 12mpg clip....On some travels, I'd managed close to 30mpg.

rgordon1979

I am not too familiar with superchargers, but I am pretty sure you do not get into boost until you get into the throttle a good bit. If this is the case, mileage will stay at about the same level until you get into boost.

The A/F ratio must remain at a safe level during boost, as the manifold pressure (volume of air entering the engine) increases the fuel system must compensate for that increase to keep the A/F ratio within a safe range, thus injecting more fuel. So long as you do not stay in the positive all the time gas mileage will be fine.

rotarygod

Your gas mileage will never get better by adding forced induction. First off, more air requires more fuel. When you are under boost, you are using more fuel. This much is obvious. When you aren't under boost, you aren't. However, every type of forced induction system is a restriction to the engine in some fashion. For superchargers, they take power off the crank. With a bypass valve installed, they don't require much power to spin and don't do too much to hurt your economy. A turbo is an exhaust restriction. Typically you will also lose any tuning that a nice exhaust header will give you on a nonturbo car which isn't much on an RX-8. However, because the turbo is a restriction, you will make less power when no under boost than the engine would with no turbo. If you add a large restriction in your exhaust now, dont' expect gas mileage to improve. It won't. People wrongly think that turbos are using free energy to make power. Nothing's free. There's always a tradeoff.

If you drove normally and never got into boost, your mileage would be very comparable to stock but maybe just slightly lower. There's no point in having forced induction though if this is how you drive. Usually your mileage goes down considerably because you have too much fun flooring it everywhere. The worst I've ever gotten in a turbo RX-7 was 10 mpg but that was really driving hard for the entire tank. Normal for that car was about 18-19. Typically it might go down maybe 1-2 mpg if you don't get on it too often.

The EPA isn't too fond of turbo or supercharger kits because their logic is that more air in is more emissions out. If an engine met a certain emissions standards normally, it might not after the mods. It really depends. It's different if a manufacturer brings a boosted engine to them for initial testing. It would meet it's standards as equipped. You have to submit for testing to get approval for forced induction kits to try to make them legal. This takes alot of time and is very expensive. This alone is why most companies just tell you it is off road use only. You can thank all the liberal treehuggers for this crap.

Photic

Very nice link I came across yesterday, they even mention fuel consumption.
http://en.wikipedia.org/wiki/Superchargers

rx8wannahave

WOW fella's....that was great, teach me more...LOL

With all these fuel economy questions you guys must think I work for the EPA, ha ha. I just wonder because we complain about our fuel economy now so if a Turbo/SC droped those numbers down by lets say...5mpg n stuff, that would be a little painful.

rx8wannahave

I LOVE THIS WEBSITE!!!!!!! Learning sooo much...Thanks everyone!

Rotarian_SC

All you would need is a positive displacement supercharger and an intake/compression stage port overlap to create a Miller Cycle version of the Renesis which would get better gas mileage because the engine would get all the compression for less work and only take off a small amount of power for the supercharger .

rotarygod

Mazda tried and abandoned the Miller cycle rotary idea in the early '90's. They aren't using it on any piston engine cars either. It was only on the Millenia S. Mileage actually wasn't that impressive.

Richard Paul

iTrader: (5)

IMO the miller cycle thing was a stroke job.
So was the May combustion chamber.
I can't beleive that those were picked up by car companys.

Why can't I be that good of a salesman?

Rotarian_SC

That was something I thought of not too long ago. I would have to think this has been tried before but not used for some reason or another. It would complement the torque well down low, possibly give better gas mileage if done very well. Where theory detaches from reality, I don't know. My technical and practical knowledge are both pretty limited, but do you know why it didn't work? Mazda also tried and abandoned the porting location on the 13B in the 90s (well at least in the US, 2002 technically I think) :p. The Miller Cycle would be something interesting to see Hymee try, but it might require R&D funding beyond what he has.

rx8wannahave

Rotarian SC, ohhh...I need to learn more about the Miller Cycle. Thanks...

Sputnik

This is basically right, but it all depends on on driving style, location, and other things.

Around town, and even on freeways around town, you actually spend very little of your time under acceleration. A huge majority of your time is spent idling or at "cruise" (very light engine load that doesn't take much more effort than idling), even if you're a leadfoot. The engines for most cars are powerful enough nowadays that the engines are at "cruise" even at most freeway speeds. Cadillac, even with their heavy cars, is once again looking into ways to only run their V8s on 4 cylinders when idling or cruising the freeway to improve efficiency.

When at idle or cruise, FI isn't going to help. SCs become a restriction in that case. While one can include a bypass valve to improve things, it still doesn't offer as good of airflow as with a stock intake system. Especially with something like the stock RX8 intake system that would be completely reworked for any FI. Turbos become a restriction in BOTH the intake and exhaust. There are intake bypass valves for turbos as with SCs, so it helps, but there's no bypassing the exhaust of the turbo. And as with the SC, the intake path won't be as efficient as if it was NA, and the exhaust won't be either. And to get the benefit of an SC or turbo, you have to install intake and exhaust systems that flow much more air. Even cases where you add bigger exhaust and intake systems to increase airflow in an NA car, those systems will actually be less efficient at idle and cruise, and will cause your overall mileage to suffer.

But, when your engine is under constant load, then FI can improve mileage. Not always, and with differing results, but it can. The compressor has to be running efficiently (not too slow, not too fast) for the amount of airflow for it to be able to work. Since SCs are locked into the rpm, they don't always work well for that amount of flow in the middle of the rpm band, but they will occasionally help. For turbos, it depends on sizing (and other things). If you have a big-honkin' turbo, for big power, that normally won't work either. But if you have a medium or small turbo, it can work well, and make a big difference in mileage. For example, I had a turbo MR2 (2.0L, 2900 lbs) with a slightly larger than stock turbo, but even at 70 mph in Colorado (some altitude), the engine would be cruising, and the bypass would engage (you could hear it engage and disengage, and you can see what's going on with the boost guage), so the turbo was hurting my mileage, not helping it. Not only that, but I made the same cross-country trip before and after I removed the bypass valve (in the process of other mods), and my mileage decreased by about 5 mpg. The small Miata engine is under some load at freeway speeds (I also had a turbo Miata (1.8L, 2500lbs) with a small turbo), and adding the turbo improved highway mileage by at least 5 mpg, and alot of other turbo Miata owners have reported similar results with highway mileage.

In an extreme case, truck and train engines are obviously under load most of the time, and they benefit greatly from turbos. There was a time when there was a major trade-off between power/efficiency and maintenance/reliability, but now the technology is such that the power/efficiency significantly outweighs maintenance/reliability issues. To meet upcoming EPA requirements, engineers are looking to pump more and more boost in their engines, including trying compound turbocharging.

Another thing to consider comparing mileage in forced induction engines in general are engine designs. For example, the Pontiac Grand Prix has NA and SC versions of the same basic V6 engine. For the SC version, they decrease compression (to safely handle the boost), increase size of intake and exhaust systems (slightly), and have different cams for more flow (IIRC). Even though the SC has a bypass valve, those modifications hurt mileage at idle/cruise, even at freeway speeds.

In short, around town, it is very rare that FI will improve mileage. On the highway, it can, but not always.

---jps

rx8wannahave

Thanks Spuntnik...that was extensive and a great read.

I have alot of learning to do that is for sure.