To be as basic as possible...

"lighting the turbo" is just getting the shaft speed up to the point that the turbine and compressor start operating within its efficiency map (which varies greatly in different turbos)

This is accomplished with both exhaust velocity (rpms) and temperature (load/fueling)

 

heat is a big factor that most people dont always consider, you want to keep thermal energy in, it will help spool your turbo better, but equally as important is the velocity and flow of the exhaust gas through/into the turbine wheel which will spin the shaft in turn build boost pressure off the compressor wheel.

There is no fuel being burned in the exhaust housing of the turbo, all fuel is burned inside the cylinder/combustion chamber in your piston.

 

Thank you guys for making the topic more clear, could some one elaborate on the effect of heat and how keeping that heat in creates faster spooling?

What effect does a turbo blanket, holding in the heat, have on EGT readings?

 

Heat is energy so the more heat you contain(manifold blanket and turbo blanket) the more energy you are turning the turbine wheel with.

 

have you guys seen the newer remote mount turbo kits? they make power and don't get nearly as hot, I wonder how much of a role heat actually plays

 

Those remote turbos usually use a much smaller aspect ratio due to the slower, lower volume, denser exhaust gas passing through them, i cant imagine even an hx35 mounted towards the back of the truck would spool even remotely the same. Its always been my understanding that thermal energy is the major force that drives turbos.

 

It would be interesting to try that, I know heat is important but It would be cool to see how much of a difference it makes in a real life set up

 

The big thing with heat is that hot air has a much larger volume than cool air. So the hotter the air, the more volume of air your moving through the turbo. Blankets help by keeping all of the heat in the exhaust gas rather than letting it seep out through the walls of the manifold.

Hotter air will naturally be forced to move faster because it takes up more room inside the same volume of space. So that's just another reason why heat helps with spool up.

A perfect test is to blow up a balloon outside in 100*f heat or in freezing cold weather then take it into your 70* house and it'll expand or contract quite a bit after an hour or so. Same concept only with a much larger change in volume since we're talking hundreds of degress difference.

The actual spool up or lighting of the turbo is basically an exponential chain reaction. You add more fuel which makes more exhaust that's hotter than at idle which starts the turbo moving more air, which allows for even more fuel which creates more and hotter exhaust which spins the turbo even faster producing even more air in the intake which allows for more fuel again... so on so on. This all happens within a matter of seconds. It starts slow but once its going, the process speeds up exponentially until either the amount of fuel isn't increased anymore or the wastegate opens bleeding off excess drive pressure, or some sort of mechanical limit is reached.

Hope that is what you're asking.

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I don't think so. This explanation has holes.

1. Hot air does not have a larger volume. There is simply less of it per unit volume.

2. Most of us tend to think in terms of atmospheric pressure of, at sea level, 14.7 lbs/sq in. These turbos create a pressurized system. How can this be an "exponential chain reaction" if the starting point is 14.7psi and these motors boosts to what...60 psi or a bit more? In this case, you are 4X the atmospheric ambient. How do you make your exponential calculation here?

3. I am probably wrong here but is seems to me that pressure is the goal with heat being an undesirable by-product. Heat removes density, intercoolers attempt to bring it back. A perfect turbo would make lots of boost and zero heat but since we are using exhaust gasses to spin a blade, heat is a necessity.

 

This exactly the kind of answer I was looking for!

Thank you for your thoughtful replies!

 

Not trying to HJ the thread, but as a follow up question.

Not to sound like an idiot, but.....:confused013:

I've watched drag races and sled pulls, how exactly do they "light the turbo"?
All I could come up with is just shy of doing a brake stand, but that would burn up your clutch and torque converter.

 

thats pretty much how its done, to do a boosted launch hold tight onto the brake add skinny pedal until you reach your desired boost then let her fly, its not like your sitting there for minutes, its only a second or two, and thats why i need a really good torque converter

 

The only reason we cool the air going into the engine is for a better burn, the better burn the more gasses/drive pressures created.

 

Great explanation 12valvedriver :thumbsup:. It's for those exact reasons why keeping the heat in the source is so important for a good spool. Alot of trucks could benefit from a quicker spool using manifold & turbo blankets. The energy is there, but not used to it's fullist potential.
I have a set of blankys for my compounds & manifold waiting to be installed with the rest of my build. Check out Levi's Turbo Blankets. Great guy to do buisniss with & prices are very reasonable.

liteweight

 

Basically what this all boils down to is pressure.

The highest pressurization(via chemical expansion and heat) of the inlet of the turbine in the fastest amount of time will win.

 

BINGO....y'all can throw around what ever terms you want. But PRESSURE (as stated above) is what you need.

 

If thats the case, why does my boost gauge start to move before my drive pressure gauge? As i said before, velocity is what drives the wheel. : thumbsup:

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How do you spool a turbo without flow? Flow is velocity, pressure is pressure. Air doesnt move unless it has velocity. Stick your turbo in a pressure tank and pump it up to whatever number you want....the wheel will never spin without velocity.

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Flow is velocity, pressure is pressure.

I really think we are somewhat talking about the same thing. Velocity is distance traveled in a unit of time (m/s, ft/s, etc.). Flow is Volume per unit time (GPM, LPM, cf/m, etc.) Flow is NOT Velocity. Pressure is in fact pressure though.

Pressure is what causes the velocity, velocity is what drives the wheel.

I really don't feel like breaking out my college literature to get anything to back up this argument BUT, in its simplest form of explanation exhaust gas molecules are colliding with the turbine wheel causing it to spin. When molecules are in a confined space colliding we observe it as pressure. Best way to imagine this is oxygen molecules in a box, they are constantly bouncing off the walls of the box. This force exerted by the molecules divided by the surface area of the box is how we figure the pressure inside the box. This is a dynamic system so just think of it as the same ol' box with two holes in it ( a high pressure and low pressure side). As the molecules move from high to low pressure sides they hit a wheel. So the force of the molecules hitting the wheel divided by the affected surface area of the turbine wheel gives us the pressure exerted on the wheel by the exhaust molecules rushing past it.

I study airflow for a living so maybe a term I use at work will make everyone happy. We measure airflow in systems using a Pitot Tube (commonly found on airplanes), the tube measures the velocity pressure and static pressure inside a system. So just think velocity pressure when thinking about the turbine wheel.

 

Just thought about your example again. Your right, it would not. what i actually should say is it is the flow rate of air through the turbo that forces the exducer wheel to spin. BUT since flow rate is a function of area and velocity, and the area through which the air moves is fixed, VELOCITY is what drives the wheel.

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Look at it this way if u have a boost leak Ur turbo will b laggy and not want to light.this is because the pressure is leaking out.pressure and velocity both play a big part in lighting the turbo

 

So I've been following this thread for some time now. Thinking back on my schooling it seems that wouldn't Bernoulli's Energy equation hold true in this case? It seems like it could be a "simple" pipe-flow problem. z_1+(P_1/gamma)+(V^2/2g) = z_2+(P_2/gamma)+(V_2^2/2g) - FL. The difference between the z variables is the change in elevation which in this case doesn't change much so it's negligible. P_1 would be the intake pressure which we would assume is atmospheric at 14.7 psi, gamma is the unit weight of the material, in this case air. I don't have my thermodynamic tables readily available and I'm not going to look specific numbers up online, but as the air heats up the unit weight (density - they are related) decreases. So gamma_in is higher than gamma_out. As the air heats up, the P_2 increases and gamma_out decreases for an even bigger difference in change between pressure gradients meaning more energy. Okay, now for velocity. The velocity portion of the equation would make more sense if rewritten to view in terms of flow so we can account for the geometry of the piping system. Q = V*A where Q is flow V is velocity and A is the cross-sectional area of the piping. Re-written V=Q/A. So then the velocity portion could be re-written as (Q^2/2gA^2). ugh.... Ok long story short... they work together!

 

Pressure is what causes the velocity, velocity is what drives the wheel. You are getting a little qualitative with Bernoulli's equation because all we are really talking about is, quantitatively, what drives the wheel to spool the turbo. It is a pipe flow problem though.

So again i state my point, pressure cannot drive the wheel. Velocity drives the wheel, but an increase in pressure does force the air to move through the exhaust housing...but there is no possible way that an air pressure can move the wheel

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Let's look at it this way. Wind is the result of atmospheric pressure differences of high pressure to low pressure systems. So pressure differences create wind. Wind has a velocity associated with it that could power a windmill. Now lets say that the windmill is the turbo. The turbo as it begins to spin will create a larger pressure gradient resulting in more "wind" and will result in more velocity to spin the turbo. Follow? Velocity in the system is the result of pressure differences, velocity drives the turbo. Velocity and pressure are directly related and work together in a turbo diesel engine.

 

Heat causes gasses to expand, like stated air in a balloon, so in that aspect, think of it this way, you have a ballon at 300* you take the same ballon and bring it to 800*. The ballon expands more, (a manifold will not because it is ridged, well it does, but very little) so now you have 800* air in the balloon, put one of those little fan windmill deals in front of the balloon filling area and let the air out, the windmill will spin. You need velocity AND pressure to spin the windmill (exhaust turbine). The velocity is the key factor in this,but pressure in the manifold will help spin the turbine because it will push more exhaust out, thus creating more FLOW. pressure alone will not spin the turbine because it has no place to go, like stated put in a pressure chamber, it will not spin unless it the pressure is directed over the turbine and has a pace to escape, like I said creating flow of air. You need both in the manifold and exhaust housing to make a turbine spin

 

with both those statements. This is what i have been saying. Pressure creates the velocity but velocity (air flow, but the only changing factor in this case is velocity) is what drives the wheel

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well ya.....The static pressure in the exhaust system comes from the systems need to flow x amount of exhaust and having too much restriction to do so. Exhaust should never hit the compressor wheel lol. The mov't of the air causing it to spin and the air molecules force on the wheel is literally the exact same thing.

 

Glad folks are finally coming around to the idea that pressure is what you should be watching at the end of the day. I used to get laughed at when I told people to put the magical "flow" word on the back burner for once.

Here's the deal:

Yes velocity matters, but what happens when a mass with velocity(exhaust gas) hits an object? It puts force on it. Force upon and certain area(turbine blade) = pressure!

Theoretically if the exhaust never touched any part of the turbine and just escaped through the open area in the wheel, the turbine would never move.

Remember, the sole purpose of a turbine in to convert pressure ([velocity which = force] / area) to rotational motion.

 

Pressure in your exhaust manifold and pressure caused by air hitting the turbine wheel are completely different. Correlated, yes, but waaaay different

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^^^^^^We have a winner ^^^^^^ :thumbsup:

liteweight

 

I don't know about all the fancy stuff, but my tires brake loose.

 

Lol, well put!