Vehicle Turbo Efficiency

[Al]

I was curious about the max boost pressure on my vehicle. Found it went to 14+psi. The curious thing is that would be approximately 250 engine HP. And yet at 0 psi turbo the vehicle is maybe 50+ mph on a level road. I think the gauge is likely accurate...bc when windmillint (coasting) the gauge is at -11 psi.

Here is the question... I am speculating that the engine is producing no more than 60 or so HP at that speed (50+ mph). Yet that "should" be half load (125 HP) one would "logically" assume that the engine is not very efficient at 6000 rpm. But that can't be true since at half the theoretical load (0 psi) the engine is actually pulling much less---60HP. The only conclusion is that turbo is very efficient. But its not possible it can be 'that" efficient. I'll think about it later since right now I have stuff to do.

Thoughts?

 

[ammolab]

Why would/should cruise at 50MPH be "half load"? Zero PSI manifold pressure isn't indicative of half load...
Not following the logic here....

 

[dareo]

60hp is enough to maintain speed in a semi truck. For most engines a turbo is most efficient if its not building significant boost. Your negative reading should be due to throttle plate closing causing vacuum.

A small vehicle does not need much power to maintain speed. Especially if it has great aero like Tesla.

If you use all of your power, generally speaking, a gas turbo will be less efficient than a larger non turbo creating that same 250hp. A 250hp V6 is always the size that it is, when a 4 cyl turbo is effectively larger when under boost. This is why you see the Chevy Silverado 2.7 turbo not really beating the chevy 6.2 silverado in fuel economy.

 

[ragtoplvr]

you can not measure efficiency with a vacuum Gage.

 

[barryh]

Nothing like 60 HP needed to cruise at 50 MPH. Could be as low as 10 HP for a small car.

 

[Al]

Originally Posted by ammolab
Why would/should cruise at 50MPH be "half load"? Zero PSI manifold pressure isn't indicative of half load...
Not following the logic here....

Half the pressure half the load. That's not in question (stoichiometric ratio)
I'm screwed up. I see my mistake.

 

[ARCOgraphite]

barrh is correct only need about 15hp to move an "average" car 60 MPH.

Hp is a calculated at the torque output at a specific RPM.

The energy to produce that torque (at rpm) is provided by the fuel and air introduced
for combustion.

I don't understand your reference to 6000 RPM.

I would also think that you would be seeing a vacuum at
Light throttle cruise. and that would be at 1500-2000rpm.
In this scenario you are looking at a near NA motor which would produce approx 1.2 lb-ft of torque
per cu-in displacement at 1ATM and nearly double that at 2ATM

But Al, What is your displacement and vehicle?

My VW Jetta 1.4turbo I am currently leasing is 85 cubic inches.

I cruise at ~1800 rpm at 60 MPH with some engine vacuum (
*1 atmosphere manifold air pressure

This small engine is capable of producing (85 x 1.2) ~ 100 lb-ft at 1atm MAP

100 ft-lb = (100 x 1800 / 5252) = 34 horsepower at 1800 rpm.

the Car at 60 mph is only requiring about 1/2 that HP, so clearly the engine is running well under 1atm MAP

and consequently at a vacuum.

So no "magical turbo efficiency" - just a small displacement engine at low RPM under no boost.

I don't think your boost gauge is accurate if you have a car like in my example

which is a VW that is 3075lbs wet and 2 wheel drive. AWD will add more drivetrain losses.

I average 40 MPG around town and over 50 on the highway with the factory real synthetic oil.

Last oil change I lost about 4 MPG everywhere. I suspect its surely not the same oil.

- HTH

- Ken

 

[ARCOgraphite]

Originally Posted by Al
Originally Posted by ammolab
Why would/should cruise at 50MPH be "half load"? Zero PSI manifold pressure isn't indicative of half load...
Not following the logic here....

Half the pressure half the load. That's not in question (stoichiometric ratio)
I'm screwed up. I see my mistake.

Al, check my post above.
Stoich should only vary toward rich under load.

With the new wideband o2 sensor in most engine exhaust precat

Some are trying lean burn again or a stratified charge with a lean mix in most of the cylinder then a puff of rich fuel near the sparkplug to light it off.

I dont know how they can meet their NOx emission target requirements with the lean burn though.

Turbo fun till she blows!

 

[Cujet]

2000 RPM and 0 boost is not half of 5000 RPM and 14PSI.

As you may have guessed, some engines do make more than double the normal HP on 2 atmospheres of boost. The Ford 2.3L engine I worked on made 88HP normally aspirated and 205HP with 16PSI.

 

[geeman789]

Turbo's are designed to work best at certain engine speeds. A smaller turbo will spool up very quick at lower rpm, giving a nice low rpm boost, but may run out of breath at higher rpm. A larger turbo might make zero boost at low rpm, but once it gets spinning at higher rpm, give a HUGE boost in power. Both are efficient in their optimum operating range, but inefficient outside of it.

 

[KneeGrinder]

I'm sorry, but I could not really make sense of your original post. I did not have the patience to read all through the rest of the post's.

The Chevy in the Sig dyno'd at 505hp, 922TQ, 33ish psi, and the Rollers can't load it down enough to get a real number.
It cruises on the highway @ 55mph, 65mph, 80mph, with no boost. 0 lbs...

The Ford, 302hp, 684TQ, 30 psi, same as above, no boost at highway cruise speeds till throttle is depressed.

Both of these trucks will do over 120 mph which is fast for a diesel or a truck. Turbo's are, can be very efficient if sized correctly, and fueled correctly. The correlation to cruising speed and boost do not exist until very high speeds.

Turbo boost gauges do not read vacuum, which you listed a negative number, which is a vacuum gauge. Boost gauges only read P.S.I. which is a positive pressure number.

Also, Diesels, which I'm familiar with do not make any vacuum what so ever! And do not have a throttle body. I am not familiar with gasoline turbo engines, but they would build vacuum. Maybe gasoline turbo engines have a different type of gauge.

 

[Al]

Originally Posted by KneeGrinder
I'm sorry, but I could not really make sense of your original post. I did not have the patience to read all through the rest of the post's.

Don't feel bad..neither could I after I came to my senses. Of course you can't compare 6000 rpm Turbo with 1200 rpm non turbo

I think someone spiked my coffee!!!

What is interesting though about this engine is that the non-turbo version of this engine makes 205 HP with regular gas (12.6 compression vs 10.5 for the non turbo).

Had this thing been offered on the non turbo..I would have bought it.

 

[KneeGrinder]

I don't feel bad, neither should you! A little Irish cream and coffee sounds good.

One thing to keep in mind, natural aspiration vs. forced induction, horsepower to torque ratios are not equal, they never will be! Forced induction whether turbo, supercharger, ect, torque per horsepower ratio is greatly increased. So, if two motors are both rated at say 200hp and in the same chassis, one turbo, one natural aspirated, the turbo vehicle will walk all over the natural aspirated vehicle all day long.

 

[Al]

Originally Posted by KneeGrinder
I don't feel bad, neither should you! A little Irish cream and coffee sounds good.

One thing to keep in mind, natural aspiration vs. forced induction, horsepower to torque ratios are not equal, they never will be! Forced induction whether turbo, supercharger, ect, torque per horsepower ratio is greatly increased. So, if two motors are both rated at say 200hp and in the same chassis, one turbo, one natural aspirated, the turbo vehicle will walk all over the natural aspirated vehicle all day long.

Lol. Thanks

Since hp=torque x rpm / 5252. Why are the tq/no ratios different. Not doubting you at all! Could you explain this?

 

[KneeGrinder]

https://spicerparts.com/calculators/horsepower-torque-calculator

HP = torque (in lb.ft) x rpm x 5252.

You can't have one without the other.

Forced induction (turbo or super charging) makes for a bigger controlled explosion in the combustion chamber by compressing air, how much depends on a few factors which is felt directly as torque, a force pushing each piston and rod downward on the crankshaft, each piston/rod will contribute to a "PowerStroke" every other stroke making it a four cycle engine. Multiple combustion events from each piston/rod assy continue to produce this torque over a time period, leading to acceleration of the vehicle. Power is force divided by time, foot-pounds per minute.

There are other factors that will throw that calculus out of wack such as rod length, crank stroke, and fuel type will totally throw that calculator out the window. It is surprisingly accurate though!


As said before, RPM does not equate with a turbocharger, so the calculator will only be accurate at full throttle and full boost, or a full dyno pull. With a turbo, you as the driver have almost complete control of the turbo boost with you accelerator peddle. As I mentioned above, at cruise speeds there is no boost, so RPM must be removed from this equation. If it was a constant drive Supercharger, then RPM would always exist in this equation, HP = torque (in lb.ft) x rpm x 5252.

 

[KneeGrinder]

2018, The Corvette Stingray uses a 6.2-liter LT1 V8 making 455 horsepower and 460 pound-feet of torque.

2018, The Z06 uses a supercharged LT4 V8 that's also 6.2 liters, making 650 hp and 650 lb-ft.

My 2002 Chevy 6.6L diesel, 505hp, 922lb-ft, 33ish psi boost. There is a difference.

 

[ZeeOSix]

Originally Posted by KneeGrinder
Turbo boost gauges do not read vacuum, which you listed a negative number, which is a vacuum gauge. Boost gauges only read P.S.I. which is a positive pressure number.


Sounds like he has a gauge that reads vacuum and boost pressure. Seen gauges like that before.

For Al, as others have touched upon, intake manifold pressure (as seen on the vacuum/boost gauge) is basically an indicator of the engine load and corresponding amount of air pressure causing air flow into the cylnders. The more air/oxygen + fuel you can cram into a cylinder and explode, the more torque and HP you can produce. That's as basic as an IC gets.

Forced induction is just an effective way to force more air/oxygen into the engine to make more power, until it goes BOOM if over boosted.

Nitrous oxide (N2O) injection is another way to get more oxygen to burn with more fuel in the cylinders for more combustion pressure and power without forced induction.

 

[DriveHard]

Cruising down the highway my little Fiat is sitting right about 3psi positive boost pressure and between 35 and 45 mpg.

At "full tilt boogie" we are sitting about 25 psi boost...