Boy probably has a rotary....

 

You're ignoring 1 very important thing. That is, the Turbo has mass. Mass that is moving. It takes energy to move that mass. Energy from fuel. Energy that otherwise wouldn't be used to spin a Turbine, but instead move the car. Understand?

I think you're confusing the turbo being spun by the exhaust with free energy. It is not. Turbo's reclaim SOME waste exhaust energy by converting exhaust heat to mechanical energy. But, it is also using exhaust gases (read: pressure) to spin the turbine. That pressure comes off the piston. This is where Turbo lag comes from. It is also a reason why the exhaust manifold (turbo manifold) pressures are usually much higher than the compressor pressures. A Turbo CAN be more efficient that a Supercharged motor, but neither will be as efficient as an NA motor of the same size and power output. That's just physics.

Oh AND that ignores the change (read: richer) in AFR needed for boosted motors to prevent them from blowing up. Something about combating heat from compression and preventing detonation......


Actually engine size DOES play a significant role in vehicle fuel economy; to big and it's poor too small and it's poor. But, I understand your logic. It takes X amount of power to move Y weight at Z speed, every single time. It doesn't matter where it gets that power from, just that that's what it takes.....

BUT, it does matter where it gets that power from. Because, if that engine is too small it'll be working harder and get poor MPG. If the engine is too big, it'll make more power than needed and that extra power has to go some where (out the exhaust).

The reason an LSx spins slower is because it doesn't have to spin faster (it can). It spins that slow because it is making the power needed at that RPM to maintain the vehicle at the desired speed. It has little to do with rotational mass of the engine's components and more to do with BMEP. The LSx is not in it's peak BSFC range, but if it were it would be making WAAAYYYY more power than needed (although it'll be making that power at a high rate of thermal efficiency) and most of that energy would be sent out the tail pipe. Drop the RPM to a less friendly BSFC range, but where a lot more of the power made is actually being used and tada! higher MPG's.

This exact same principle is applied to ALL cars. Smaller engines make less power, but that can be advantageous to vehicle FE. Bring the RPM up to a higher BSFC (or even peak!) and with that engine making less power, you'll have a high percentage of peak BSFC being consumed to actually moving the vehicle at a desired speed. But, this limits passing power. Add boost. Now you have passing power when needed and almost the same FE as that engine with out it (it'll still drop even out of boost, but it's marginal).

A little something something for you to ponder.....

Quote:

Originally Posted by Tony Schultz, Vice President of Honeywell Turbo Technologies

With a 54.5-mpg standard looming, car companies have stood that approach on its head. Instead of adding a turbo to get more power out of the same engine, they are adding a turbo to get the same amount of power out of a smaller engine. The turbo itself doesn’t save gas, but using the smaller engine does.

The additional rotational mass of the AWD system on the Subie certainly isn't massive. To put that in perspective, a Tahoe 4x4 vs a Tahoe 2x4 differs by a whooping 1 MPG Highway, no difference in the City and immeasurable Combined. What does that tell you? Those vehicles have significantly more massive 4X4 components, stuff that can literally twist that Subie in half, but they cost 1 MPG highway...... And furthermore, the Corvette has much larger drivetrain components, this basically nulls out any AWD vs RWD tomfullery in your argument. The best you could possibly ask for is 1 MPG highway, and that s a stretch.

The Subie has a .32 Cd whereas the Vette is (reportedly) .37Cd to .47Cd. We'll use the lower of the two, give the Subie a Bone. If we calculated the frontal area vs the Cd of each car and compare them, the Subie has 6% less aerodynamic resistance than the Vette.

So we've discussed the rotational components and found the differences to be negligible. We also found the Subie to have the advantage in aero dynamics (a big big thing when it comes to highway FE). And the weight difference between the two vehicles is about the same. Yet, the one with the NA, Bigger, more powerful, faster engine gets 6, SIX, MPG better FE...... What's the last major piece of the the FE puzzle? The powertrain. More specifically, the engine and gear ratio for said engine (remember what I was just saying about BSFC, BMEP, and all that jazz?). How do I know this? Easy, the Impreza 2.0L (non-Turbo) get's 5MPG better City and 7 MPG HWY better than the STI with the smaller 2.0T; but to make it worse, the STi gets a 6 speed where the Impreza is stuck with only 5.....

Man, none of these real world results are working in your favor huh? The only time FE is increased in a vehicle that adds a Turbo (for FE, not for go fast) is when the Turbo engine is smaller than the NA one in the same vehicle. And even then, it doesn't always work out to improved FE; rated or real world.


Radical examples are what's killing you. Try using something more realistic, and well, real. Not some theoretical hearsay of a thesis. The real world isn't agreeing with what you are saying. Something about classical physics.......

 

Surprised no one has touched on efficiency.

That, and turbos run on heat, not pressure. Since we are in theory land and all.

 

Man I've been missing some gold in here .

 

Actually, they run on both. I can apply heat to a turbo via torch all day long, Nothing will spin though.......

And efficiency is being discussed.

 

Yes, obviously you are correct.

The KE is a major component of total enthaply, but the temperature delta plays a larger role. The larger delta you have will correspond with a larger power generation from the turbine. If you are assuming a fixed PR, raising the heat of the inlet of the turbine will raise the efficiency/total enthalpy loss, which means more power. So you can get a desired power level with less of a required PR drop with an increased inlet temp and increased reduction of enthalpy. Basically, you can operate with less back pressure and achieve the same power output. Heat > KE

But, its friday after 5pm, so who knows.

 

Sounds ghey, just get a bigger cam duhhhh.

 

Agreed. There are different inputs to achieve the same output. In the same sense that drives the Cylinder down, it does with spinning the turbo. Heat generates expansion, resulting in pressure. The turbo though, kind of does both. It uses some of that remaining heat expansion (that is still expanding) to spin the turbo, but in it's process it also puts some of that force (which goes in all directions) back on the cylinder that's being pushed against it. At lower RPM it's very noticeable because the turbine isn't really spinning yet; turbo lag. At high RPM it's negligible at best.

No matter what though, you need pressure on the Turbine. Whether it results in a greater Temperature Delta or greater Pressure Delta is another story. It is not possible to result in a Pressure Delta of zero though. Same goes for Temperature. They both go together, but not necessarily equally. If you drop pressure, you drop temps and vice versa.

But, that ties into the efficiency of it all. In theory a Turbocharged engine should have a better BSFC than a comparable NA motor. They don't. In fact, in general, Turbocharged engines usually have a BSFC between .6 to .65 (lb/hp/hr), while supercharged engines are between .55 to .6, and NA comes in the best with .45 to .5. There's a bunch of reasons for this; AFR's to combat detonation and lower compression ratios for example. DI will make huge head ways into this though, as it will allow leaner AFR's even with boost with less detonation risk which also allows higher SCR's. But, we're talking Gen 2 LT1's not Gen 5.

On a side note, I'd like a Gen 5 LT1.....

 

Oh for the love of all things holy, are you two ladies going to get into it now?

 

LOLTech

 

 

Pics of killer combo?

 

Boost pressure is just extra atmosphere crammed into the cylinder. You cant say that cramming it in with a head/cam is any different than cramming it in with a compressor. Its the same air, there is nothing special about boosted air that specifically causes bottom ends to fail.

 

[QUOTE=bufmatmuslepants;19137341]


This isn't always true. It depends on the size of the exhaust. A large turbine can have the same effect as a large manifold- it can reduce Exhaust gas velocity(EGV) and cause a delay to cylinder fill vs rpm, giving you a naturally aspirated form of "lag". Large exhaust systems are notorious for this (oversized) feature. We seek to provide our engines with the right size exhaust for the application. If the turbine is sized correctly, Exhaust gas pressure(EGP) will not be escalated to the point that it hurts cylinder fill, and if the camshaft timing is compensated towards it, there will be even less effect. We are throwing away that exhaust; any benefit gained from it (without increasing pressure) is a plus. You can gain from it on an N/A engine by correctly sizing the plumbing so EGV ramps up quickly with minimal increase to EGP.

Not if we wire open the wastegate. The beauty of a turbocharger is we have full control over the exhaust gas pathway (you should have fabricated it like this) such that the engine can be run with, or without the interference of the turbine. Yeah it does add weight though. I wasn't implying you could add a turbo to a car with 0% downside. But 100~lbs of plumbing, if sized correctly, can be worth more than 10 horsepower, even without a turbo. Its all in the fab/design.



Air -> horsepower -> Xlbs moves per Y minutes Z distance. It doesn't matter how big the (horse) engine is, they dont ask for horse weight or breed when calculating horsepower. When you say "work harder" what you mean is, the engine is at a higher VE and therefore requires a richer air fuel ratio to keep cylinder T and EGT down. So what you are really looking at is an octane problem. Put both engines on E85 and run them again at identical Air/fuel ratios, both will consume nearly the exact same airflow... still finding that the smaller engine is getting worse fuel economy and using more air? Its a rotating mass problem now, the smaller engine is running a higher RPM. There is nothing about engine size besides its weight and rotating parts weight (and any other necessary heavier rotating part downstream) that should affect economy, all else being equal.



So you add weight and MPG goes down. Yes, pretty normal.


this is personal "how I use a car" stuff. personally, I use a car for daily driving. Everything I do is based around getting max mileage and max safe power. Thats pretty much it.




So this is why I offered to post some setups and provide data. I am familiar with a 2800lb vehicle that provides 320-380rwhp (about the same power weight as your 400/3300) with an all stock internals engine (similar playing field). The car and engine go 200,000 miles, 93 octane performance, etc... For cost, if you want re-sale value, you spend 10k~ on the whole car with a pre-existing 60,000 miles (used engine) and you drive it for 150,000 miles then sell it back for 10k~ again. This is the platform to beat because it offers better fuel economy (30mpg) easier to work on (RWD with split sides, one side for intake, one side for exhaust) and it maintains value for 10+ years.





A turbo doesnt need to go whooosh. Every street setup I daily has a recirculated bypass and plenty of shields/blankets to keep noise down. You can hardly hear it until its too late. The whooosh is just for beginners.

 

Exhaust gas is being thrown away. It wont help you move the car at all, unless you take advantage of it somehow. Naturally aspirated engines can do this by creative plumbing that enhanced cylinder fill by taking advantage of helmholtz resonance tuning and by maintaining EGV through a useful range. An effective racing top tier design changes shape as the engine needs more room to breath. Some turbochargers incorporate this kind of technology, and we can add it to our naturally aspirated engines with a mechanical device and some planning (but technology is not to the point of being rote and mundane "ebay" whored yet).



Like I pointed out already, you can disable the turbine so exhaust gas bypasses it completely. Turbochargers are "on the fly" devices, you just haven't gotten used to thinking about them like that yet. There is no reason having one should reduce economy, and if it does, it needs tuning and proper implementation. Exhaust gas is being trashed; any benefit we can see from using it is a, well, benefit.



This is an octane problem, a fuel problem. Your gripe that boosted engines need more fuel for "preventing them from exploding" is actually against the fuel. Reaction rates at higher temperatures and higher pressures speed up, increasing the likely hood of having a catastrophic cylinder pressure spike, or several in a row. Your options are A: use better fuel (you should be if you are really racing anyways) B: turn the boost down to an acceptable level C: Reconfigure the engine/plumbing to assist with the temperature exchange responsible for slowing the reaction rates i.e. add methanol/water injection to reduce EGT (combustion chamber temps). What we do NOT want is to blame the turbocharger for the problem.



You are throwing around a few non conventional uses of terms. BSFC is best at peak torque; We do not use peak torque area during a cruise. There is no way to "shoot for" a best BSFC while cruising. The term does not correlate to better fuel economy for cruising the way it does for fuel usage at maximum power. BMEP on the other hand is a fundamental of the engines original design, as all engines are tested as models by engineers, they all have a specific rod/stroke ratio, piston ring positioning, an intended application for the model that is provided. One engine might be a little better at using 14 degrees of cylinder pressure after top dead center than another of the same exact displacement at slightly lower RPM because of these design limitations; none of them have drastic qualities or negatives because all engines (well, most of them) in mass production are modeled to fall within a "safe range" where air mass calculations are somewhat standardized for those models (we can almost always say 50lb/min of air is worth 500 horsepower, etc...)




A theory is nice, but I have hundreds of data logs from many engines of vastly different displacements and designs, and all of them have one thing in common. In every scenario where an engine was able to cruise on the highway with more than 8" of vacuum, a reduction to rotating speed always gives back an increase to fuel economy, even if BSFC gets worse, and the tire got heavier (and larger). Both small and large displacement engines have a minimum RPM where they are comfortable idling, and that is an OIL system issue. You want a minimum rotation speed due to lubrication concerns. The term "cruise" is also a misnomer, sometimes we cruise at 120mph. Once you pass 70mph the rules all change because wind resistance and vehicle design come strongly into the playing field. I like to keep my cruise comparisons in the 65MPH range for accuracy, because vehicles are substantially different in shape and weight.




I bet if you measure the weights of both the 2x4 and 4x4 drivetrain, they would be nearly identical. Otherwise, the efficiency is poor to begin with and so adding drivetrain weight is negligible due to an already inefficient design.


The corvette drivetrain uses mass very sparingly. The engineers chose only the lightest, strongest parts, that get the job done for 200k miles or so or whenever they need maintenance as per a schedule. The vette drivetrain is much easier to turn than any 4x4 system; The subaru design is much stronger, as the traction of all four tires is a much greater threat to breaking something. The extra capacity comes in the form of weight somewhere, I guarantee it.


I've been saying the drivetrain this entire time. The gear ratio is only responsible for bringing down the final cruise RPM, where I know that a lower final rpm always gives fuel economy improvement, until engine vacuum goes too low per fuel quality constraints (or if it happens that rotational speed is too slow for the oil system). So why does the Impreza 2.0L Non turbo get better economy than the STI? Hmm. Let me ask first, have you tested it? Are you sure that it does? Because if so, the obvious answer is compression ratio. Higher compression ratios will always give back more power per unit of mass of air. Compression ratio is the only thing a naturally aspirated engine has going for it, so manufacturers tend to give them as much as possible for the anticipated fuel quality / driving conditions. Also, I would think there is some measure of "driving style" factor, where they assume people with the STI will be boosting it some percentage of the time. I Guarantee if the both cars weigh the same, had all the same parts, same compression ratio, etc... the turbo engine would give better economy. Unless there was some serious obstruction in the air path of the turbo version.

 

This is conventional wisdom, a list of facts. But lets ask WHY instead of WHAT.

WHY do turbocharge engines have worse BSFC than they should? You throw out a reason: AFR to combat detonation. Well, what if I told you that instead of injecting extra fuel (and hurting BSFC) I will inject pure 100% distilled water instead? Now my BSFC IS actually better, and I used 0% extra fuel to get it there. My point is this: It isn't the turbocharger causing the reduction to fuel economy or BSFC... It is the fuel quality and to some extent the design of the engine and injection system. Point your finger at the right culprit, the true menace.

Lets look at this another way. Imagine I had an electric supercharger that can flow any amount of air I want. It runs on batteries, and lets pretend batteries are free. How much boost can I use?

Well, that depends on the fuel quality and engine design. It does NOT depend on the compressor because A: I know what temperatures to expect and have compensated for them with respect to my fuel quality and B: I have adjusted my engine's plumbing and parts with respect to the anticipated fuel quality and power output. Notice none of these things includes plans for the compressor! The compressor is simply acting in place of the atmosphere. Whether it spins or not is up to me (remember I can bypass my wastegate on a real turbine). A slick design would allow me to not only dial UP air mass (boost pressure) but also would allow me to dial it DOWN (below atmospheric pressure) for a variety of reasons/conditions this would be ideal to have full control.

Its like you are all blaming the turbo for these problems, carrying on about how they reduce efficiency and hurt engine ringlands and cause explosions when all along it was the fuel quality and engine design holding you back. Look at the parts that break: parts that are inside the engine? Then why are you blaming the turbo? A turbo is a plus, a benefit, a little extra weight around the belt that can double or triple engine output if conditions permit. If the engine fails because of it, it isn't the turbo's fault! If atmospheric pressure doubles overnight, are you all going to wake up and blame the atmosphere when your engines all explode? That is bass ackwards thinking.


It is worth mentioning that as you ascend levels of performance (actual engineering and testing with trial/error V.S. easy proven cookie cutter) each individual engine//train/chassis characteristic is added to our model for our own cars for our benefit. In other words, use existing knowledge to build reliable platforms, for example everyone knows that the 7MGTE has a weak head gasket, and that stock 10-bolt rear ends are not ideal for a slick type tire, and LT1 pistons are not good for boost. You might take this for granted but it is actually custom knowledge embedded in our model of how autos work. As we become more detailed we know which tensioner and guide will run the engine 250,000 miles+ without maintenance, one less problem area on a reliable engine thanks to previous testing (it became cookie cutter reliable). Now turn up the performance as high as you can, give me a number. Whats the most you can make, the best power to weight ratio, for the least amount of cash with the most reliable engine, set to a "beginner" level mods list (zero mods cars) at 500rwhp+. Think of a dodo bird and handgun, a dangerous level of power on a tiny budget is as easy as injecting it. Any argument against nitrous is actually only against the engine, it isn't nitrous's fault if we use it wrong or use too much. There is a line and you simply need to walk along the line and know how to not cross it.

 

The 2x4 vs 4x4 Tahoe isn't fair either, it has a part time transfer case and isn't turning the front driveshaft all the time. A 6.0 vortecmax Silverado 2wd or SS vs a Denali 6.0 with a full time case is better comparison for a wrx, the Denali with a full time case sucks gas, like the old np203 vs np241 cases, that's why they ditched the np203.

And another factor is fuel economy of a small engine vs a large engine is the rpm they have to run at cruise, the most efficient rpm is around 1600-2000rpm, below that a lot of heat energy in a cylinder is transferred to the coolant before it can be used for rotational energy, above that it gets pissed out the exhaust. The bigger engines that can pull at 1600-2000rpm on the highway are in the most efficient range for an internal combustion engine, and is a reason manufacturers like 190-200 degree T stats but we as performance enthusiasts like 160 degree stats, but you bleed too much heat to coolant for efficiency while cruising.

 

The dumb continues in today's episode of loltech.

Please tell me how a wide open 60mm wastegate is enough to cause no backpressure what so ever with a 350ci motor?

 

Wow, you are missing the whole point, on a NA motor, it is not "Cramming" in atmosphere, it it ingesting, sucking as much as it can, now with a compressor, now your are "cramming" forcing in more air than what it can do on its own, so take the heads and cam motor, tune it to max performance, now add forced induction and cram in more air and no failure will occur?, Dang, I should of never built my motor, i should of left it stock and just throw a turbo on it, put as many lbs of boost I wanted and would of been good to go.

 

VE is VE, you cannot add more flow through the motor. A turbo changes air density, not mass flow.