A Little Dyno Time Today
#82
How is the wastegate setting have anything to do with engine vacuum during cruise? It does not. I am strictly saying, during a cruise situation, turbochargers reduce the energy requirement of breathing. If you drive a turbo car and get 30mpg, then disconnect the turbine so it no longer spins (or slowly), economy will decrease as the piston works harder to fill the cylinder, it will show up as higher injector duty cycle at the same speed with very little influence on engine vacuum while cruising. There is a fuel efficiency benefit during highway cruise to having a proper turbocharger configuration.
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......
Originally Posted by KingTal0n
Actually engine size has very little to do with economy. It is the vehicle weight, and rolling resistance, which plays the major role. If you install an LSx engine from a 3800lb vehicle (at 25mpg) into a light chassis at 2800lbs, now it will provide 31mpg (the way I tune them). Engine "size" only plays a minor role concerning rotating mass, i.e. heavier internals use more energy just to spin, but often the difference in modern engines (2L vs 6L) is negligible next to the weight of the vehicle's impact, especially when you take into account that the larger (heavier) internals engine often spin more slowly (you will cruise with 1500rpm from an LSx where a 2L would cruise at 3200rpm). In some cases there is an economy boost when going to an engine with a larger displacement and slightly more compression due to this feature alone (being able to idle at a lower rpm for example) when compared to a similar power output, smaller displacement engine.
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.....
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.
Originally Posted by KingTal0n
Sorry, let me consider your question. I discounted it because I thought you were being rhetorical. WRX STI is an all wheel drive vehicle. The rotating mass there is enormous. They are also similar in weight to the vette (I believe both weight around 3300~lbs). As I said already, weight is the largest factor, followed by rotating mass. Simply put, the WRX has more mass to rotate (a more robust drivetrain) more friction coefficients (AWD) and a similar vehicle weight. The engine size (2L vs 6L) as I have mentioned already, plays very little role. The higher compression V8 in your example further reinforces the idea that simply both engines together at idle, the higher compression, lower rotating speed of the V8 would probably give a similar or even better economy.
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.
Originally Posted by KingTal0n
Whatever information I provide that you do not like, you are free not to examine it. Whatever information I provide is never with the intent of distracting or confusing. It only looks wrong sometimes because I use radical examples which could not possible occur to illustrate key points. Even if it makes me look "wrong" the objective was never to be "right" but merely provide a new viewpoint objective lens through which to view automotive finesse.
#85
And efficiency is being discussed.
#86
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.
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.
#87
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.
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.
#88
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.
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.
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.....
#93
I have to disagree, I have yet to see ring butting failure on a stock bottom end due to head and cam configuration but I have seen many on boosted motors, You will always hear to keep the boost level down to X psi due to piston and ring failure, You never hear to keep cam specs at a certain level because catastrophic failure will occur, can you have piston failure on a head and cam setup, of course if the tune is not spot on and it detonates itself to destruction. Yes a turbo will give you better fuel economy but when it comes to Horsepower who gives a hoot about fuel economy, now granted some prefer moderated gains and still maintain fuel mileage, but for myself, I go for the horsepower understanding that "at times" horsepower & fuel economy do not mix...
Last edited by kingtal0n; 02-08-2016 at 11:36 PM.
#94
[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.
The problem is at sea level, like hrcslam was getting at, is that if the turbo/supercharger is not HELPING, it's doing nothing or HURTING your efficiency.
If you drove around right now NA to get a baseline mpg and 1/4 mile pass, then put a turbo on it set at 0.1psi, you would lose mpg and 1/4 mile because you added weight and either are using rotational energy if supercharged to move the belts and turbine, or you restricted your exhaust if turbo.
The wrx vs vette fuel economy comparison isn't fair as you stated, but a car like the new camaro with options for a turbo 4 or a lt1 direct injected v8 is fair, same car/aerodynamics/drag but look at the mpg and HP, the v8 has more HP/mpg. Or look at a 4.8 00-06 Tahoe vs the 5.3 00-06 Tahoe, the 5.3 is better HP/mpg because it doesn't have to work as hard (it's more torque/mpg with the 5.3 vs 4.8).
The ford Ecoboost is a prime example of turbo mpg. Unloaded, they get awesome mpg, I had my buddy's 2014 f150 ecoboost for a weekend and unloaded I drove to get a 7x16 enclosed trailer and got 24mpg highway, because it wasn't under boost unloaded. I got the trailer, and on the way home, same route and speed, it got 12.
The thing you need to consider on this forum with this conversation is that these cars generally are not being used for fuel economy or climbing high mountains. They normally are set up and tuned for their local track, that sees close to the same conditions every time the person runs them.
Heads and cam is the easiest and cheapest way to wipe the floor with 99.99% of the cars on the road, 400-450rwhp in a 3300lb car is damn fast. Most people here do not want to spend more on a turbo kit than they paid for their whole car just to make 400-450rwhp. This crowd is generally looking for a cheap, reliable way to beat the local crowd at their local track.
And 1 big elephant here, is if your car makes that cool WOOSHing sound from boost, it god damn better be able to beat the guy who spent $1600 on an LE2 package, or you look like a poser or rice.
A car that goes potato potato potato potato and runs 11s s way cooler than a car that goes WOOOOOOSH and runs 11s.
A car that goes potato potato potato potato and runs 11s s way cooler than a car that goes WOOOOOOSH and runs 11s.
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.
#95
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......
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).
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.
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.....
#96
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.....
On a side note, I'd like a Gen 5 LT1.....
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.
Last edited by kingtal0n; 02-09-2016 at 01:18 AM.
#97
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.
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.
#99
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.