Backpressure

[2turboimports]

I think some ppl are getting mixed up with the 'theory' of N/A exhaust tuning as opposed to turbo charged exhausts.

As far as an exhaust goes for turbo charged cars from the turbo back is, like what was said before, a conical diffuser (think of a cow bell coming out of the back of the turbo).
I always get a little confused when reading about header design for a turbo charged car, it's true you want a higher pressure in the collectors, but there's a sort of limit that comes from the piping itself, effectively causing a sort of 'top-end wall'.
Since you want higher manifold pressures in the higher RPMS's, a larger diameter piping and maybe equal length runners is the solution, but now down low the manifold doesn't have the same pressure so you feel a loss of power that's now placed higher in the power band.
I've always thought it was a tradeoff, one for the other, but I dunno, it's just strange that what's so important is the CFM flow rating of the head/valves, matched with a turbo that can produce more CFM's, then in the exhaust you want slight restriction pre-turbo to keep the temps and pressure high enough to get the turbine spinning to speed. It's just slightly confusing for me to keep in mind flow vs. pressure for head/turbo work, then it's opposite to a point for the pre-turbo exhaust.

And then there's N/A, which is a completely different ball game as far as exhaust design. The whole reason you don't want a 3.5" exhaust on an all motor car is the dimension of the exhaust pulses. IIRC, the diameter of the header/exhaust reflects the peak of the exhaust pulse. If you can imaging an exhaust pulse like a wave. The higher the peak is, the quicker it dies, which means there's a reduction of pressure 'pulling' the next exhaust pulse out of the engine. This is better for higher RPM's where the pulses are closer together and that quick death of one pulse can quickly pull the next pulse. A smaller exhaust is the opposite. However, that's just one general rule, there is still fine tuning, finding the resonance for whatever piping size you choose which means that all the pulses line up in a harmonious hp making effort. That occurs at a specific rpm.


So basically what I'm trying to say is that for n/a an exhaust is 'tuned' for a specific range based on a few factors, and for a turbo back exhaust you really want a cow bell welded to the back of it.

Anyone, feel free to correct if there's something I misunderstood or missed all together.

.02

[UncleBob]

the truth of it is, the exhaust design pre-turbo isn't very important. You obviously don't want the primaries to be so small they restrict the system (ie, combined restriction is higher than turbo restriction) and you don't want it so big that you have a lot of volume. More volume increases the time it takes for pressure to build (velocity takes SOME pressure differential after all)

You can argue about equal length headers, but the sum of it is, what little advantage they offer, is minimal. I've seen plenty of hard-angled log manifold produce very impressive power to believe that its necessarily limiting.

Now of course, a perfectly flowed, smooth transitioned, equal length'd header will keep velocity better, and therefore have more potential, I'm not suggesting otherwise, all I'm saying is its just not crucial. No matter how tortured the manifold is, it will still produce pressure, which will produce decent velocity to drive the turbine.

If you following some of the product reviews, I always find it humorous when some magazine hangs a $600 high foluting manifold on a turbo'd car and then makes really excited "WOW!! UNBELIEVABLE!!" when they gain a whole whopping 10HP with it. I'd rather turn the boost up 1/4 a psi and save the $600 personally

[534BC]

Originally Posted by UncleBob

the truth of it is, the exhaust design pre-turbo isn't very important. You obviously don't want the primaries to be so small they restrict the system (ie, combined restriction is higher than turbo restriction) and you don't want it so big that you have a lot of volume. More volume increases the time it takes for pressure to build (velocity takes SOME pressure differential after all)

You can argue about equal length headers, but the sum of it is, what little advantage they offer, is minimal. I've seen plenty of hard-angled log manifold produce very impressive power to believe that its necessarily limiting.

Now of course, a perfectly flowed, smooth transitioned, equal length'd header will keep velocity better, and therefore have more potential, I'm not suggesting otherwise, all I'm saying is its just not crucial. No matter how tortured the manifold is, it will still produce pressure, which will produce decent velocity to drive the turbine.

If you following some of the product reviews, I always find it humorous when some magazine hangs a $600 high foluting manifold on a turbo'd car and then makes really excited "WOW!! UNBELIEVABLE!!" when they gain a whole whopping 10HP with it. I'd rather turn the boost up 1/4 a psi and save the $600 personally

I would have to agree whole-heartedly

[SaabJohan]

Originally Posted by 534BC

Exhaust driving the turbine is done by flow (cfm) the temp affects slightly, but is really just along for the ride. Whatever it is it is. for temps.

Even though it is not a positive displacement, at the flow rates from engine it is quite similar to sizing the compressor side.

A turbine is a heat engine, it converts heat into kinetic energy. The power output of the turbine is change in enthalpy multiplied by massflow.

P = dh*m

where

dh = (Cp*T1)-(Cp*T2)

but can also be written as (Eulers turbomachine equation)

dh = U1*C1 - U2C2

The power output can also be rewrittem to:

P = nmCpT1[1-(1/er)^((k-1)/k)]

where

n = efficiency
m = massflow
T1 = inlet temperature
er = expansion ratio
k = ratio of specific heats of the gas

The massflow over the turbine is equal to the massflow over the compressor + fuel mass flow. For a gasoline engine we can also relate fuel flow to the air flow.

With turbocharging the volume flow through the engine remain quite constant, but since the density of the air entering the engine is higher mass flow increases.

Originally Posted by 534BC

Terminalvelocity is talking about sizing issue where as a turbine that is too large will not make any boost and one that is too small will be overspeeded. It is size we are talking about, not temperature. I am not saying temp has nothing to do withit, but the temps remain pretty constant for like engines and is determined by mixture and such.

For instance there will be no size difference when sizing turbine for pryro temps of 1100-1400. It is what ever it is.

Sizing of a turbine are done against factors such as corrected gas flow, m*sqrt(T0/P0), blade speed ratio, U/C and pressure ratio (expansion ratio).

Originally Posted by UncleBob

the truth of it is, the exhaust design pre-turbo isn't very important.

The design of the exhaust system is actually quite important. Mostly it will affect the low end power, spool up time and pumping losses during the exhaust phase.

[Alastor187]

Originally Posted by SaabJohan

With turbocharging the volume flow through the engine remain quite constant, but since the density of the air entering the engine is higher mass flow increases.

I am not following the reasoning behind why the volumetric flow rate is constant. If the volumetric flow rate is a function of cross-sectional flow area and flow velocity, the area would obviously be constant but the velocity will change with engine speed.

What am I missing?

[SaabJohan]

Originally Posted by Alastor187

I am not following the reasoning behind why the volumetric flow rate is constant. If the volumetric flow rate is a function of cross-sectional flow area and flow velocity, the area would obviously be constant but the velocity will change with engine speed.

What am I missing?

An engine can only draw a certain volume of air given engine speed, displacement and volumetric efficiency. Under normal conditions 1 m^3 of air has a mass of around 1.2 kg, what turbocharging do is to increase the density of the air. If we have an engine that is consuming .3 m^3/s, that is equal to .36 kg/s under atmospheric conditions, and then a turbocharger is increasing the density to 2.4 kg/m^3 we will get a massflow of .72 kg/s while maintaining the volume flow of .3 m^3/s. Gasoline is added accoring to 14.7:1, hence .0490 kg gasoline/s is burned. Each kg of fuel contains 43,000 kJ, releasing a total of 2100 kJ/s (equals kW) of which about 1/3, 700 kW reaches the crankshaft. Half the density, same volume flow (limited by the engine) and power output is only half at 350 kW.

A turbocharger will not make an engine to flow more air by volume, it can only increase the density of the air that the engine is consuming. It can have a limited effect on volume consumption since it can affect the pressure in the inlet/exhaust. Increasing inlet pressure and/or reducing exhaust pressure will increase volume flow and vice versa.

[zx2guy]

ok sorry to slow everyone down here cuz i follow on everything everyone is saying but one point: heat, how does heat create the kinetic energy if the exhaust gasses themselves do little for boost?

[nissanfanatic]

Originally Posted by UncleBob

the truth of it is, the exhaust design pre-turbo isn't very important. You obviously don't want the primaries to be so small they restrict the system (ie, combined restriction is higher than turbo restriction) and you don't want it so big that you have a lot of volume. More volume increases the time it takes for pressure to build (velocity takes SOME pressure differential after all)

You can argue about equal length headers, but the sum of it is, what little advantage they offer, is minimal. I've seen plenty of hard-angled log manifold produce very impressive power to believe that its necessarily limiting.

Now of course, a perfectly flowed, smooth transitioned, equal length'd header will keep velocity better, and therefore have more potential, I'm not suggesting otherwise, all I'm saying is its just not crucial. No matter how tortured the manifold is, it will still produce pressure, which will produce decent velocity to drive the turbine.

If you following some of the product reviews, I always find it humorous when some magazine hangs a $600 high foluting manifold on a turbo'd car and then makes really excited "WOW!! UNBELIEVABLE!!" when they gain a whole whopping 10HP with it. I'd rather turn the boost up 1/4 a psi and save the $600 personally

I hung a "$600 high foluting manifold" on my car with a smaller turbo and made 50whp more with 1psi less boost than someone with a damn near idential setup. 303whp at 15psi is what he did. I did 353whp at 14psi... Tat II if you want to ask him. $600 for 50whp seems rather good to me. Esp since I didn't have to use any higher octane fuel like you would if you just turned up the boost.

[nissanfanatic]

Originally Posted by SaabJohan

Increasing inlet pressure and/or reducing exhaust pressure will increase volume flow and vice versa.

One thing I do have to ask you...

Wouldn't "critical flow" affect this?

Originally Posted by Hugh Macinnes

Critical flow occurs when teh cylinder pressure is more than twice the exhaust manifold pressure. As long as this condition exists, backpressure will not affect flow.

Would this only apply to port flow? Or could this apply to our discussions on post-turbine backpressure as well? I personally do not see how reducing pressure after the turbine could NOT help. Although maintaining the best exhaust velocity after the turbine should be ideal.... I would think.

Whether or not it works in the math, I know my car is much faster after switching from a 2.5" downpipe to a 3" downpipe on a mediocre 6psi log manifold setup and now switching from a log manifold to a well designed tubular manifold.

[SaabJohan]

Originally Posted by zx2guy

ok sorry to slow everyone down here cuz i follow on everything everyone is saying but one point: heat, how does heat create the kinetic energy if the exhaust gasses themselves do little for boost?

It's heat that turn the turbine. The exhaust gases contains heat and by expanding these gases heat energy can be turned into kinetic energy.
Heat is however not the same thing as temperature, heat is dependant on mass (flow), temperature and gas properties.

The piston engine extracts work by expanding a gas, that is what the piston engine does during the combustion stroke; fuel is burned which increases the temperature of the charge and thereby the pressure and as the piston moves away from top dead center the volume increases and with that the pressure decrease, hence the gas is expanded.

The main difference with a turbine is that it's an open system, there is no trapped charge with change in volume. Instead we have the turbine that separates a high pressure zone from a low pressure zone and by allowing the gases to expand here from the higher to the lower pressure he gases will cool and the heat is turned into kinetic energy on the turbine shaft.

In order for a turbine to work the massflow, temperature and properties of the gas are essential. "Cfm", cubic foot per minute is a unit (one that I recommend against using) of volume flow and that is not the same thing as massflow.

Originally Posted by nissanfanatic

One thing I do have to ask you...

Wouldn't "critical flow" affect this?



Would this only apply to port flow? Or could this apply to our discussions on post-turbine backpressure as well? I personally do not see how reducing pressure after the turbine could NOT help. Although maintaining the best exhaust velocity after the turbine should be ideal.... I would think.

Whether or not it works in the math, I know my car is much faster after switching from a 2.5" downpipe to a 3" downpipe on a mediocre 6psi log manifold setup and now switching from a log manifold to a well designed tubular manifold.

Exhaust pressure always affect the flow through an engine. Partly this pressure is a static pressure caused by flow restriction, partly it's due to exhaust pulses.

The reason why we fit an engine with a tuned exhaust manifold, high flow ports and so on is all to reduce the pressure in the exhaust ports during the exhaust phase. On the intake side we instead fit the engine with tuned intakes, high flow ports, ram air intakes and so on to maximize the pressure during the intake phase. The greater the pressure difference between intake and exhaust is, where the pressure is highest on the intake side the greater the flow will be assuming a specific engine (port sizes, piston velocity and so on).

With a powerful engine using large valve overlaps it's sometimes said that the lack of exhaust pressure is the reason for for the poor low end. Actually the opposite is true, high exhaust pressure due to pulses are partly the reason for the poor low end, this have been stated by for example Yamaha. Because of this reason they have fitted some engines with a throttle in the exhaust system. Many seems to believe that the function of that throttle is to increase the exhaust pressure at low speed, but once again the opposite is true. The throttle can when partially open changed the behavior of the pressure pulses and thereby reduce the pressure in the exhaust port at the time for the exhaust phase (most importantly slightly before the exhaust valve is closing).

So, you always want to have the lowest pressure possible in the exhaust port. This does not always mean the largest exhaust pipe possible though. In the case of the exhaust system efter the turbocharger turbine that will however be the case.

[zx2guy]

sooo saabjohan, in essence its the hot gases that are spinning the turbine?

[SaabJohan]

Originally Posted by zx2guy

sooo saabjohan, in essence its the hot gases that are spinning the turbine?

It's the expansion of hot gases that powers the turbine.

[534BC]

It is the amount of the exhaust that spins the turbine. It is far more significant than the expansion of the gasses across the turbine. There is a pressure and temp drop across the turbine, but is very insignificant compared to the cfm of the exhaust coming thu.

[nissanfanatic]

The point at which pressure changes is where work can or has been done. Pressure is merely resistance to flow, yes... But flow alone can not do anything. There always has to be some total pressure for any work to be done. If there isn't, then no force is being exerted.

Allowing these exhaust gases reach the turbine with as little restriction as possible allows as little energy to be lost as possible. Basically maintaining flow so that a higher pressure upon entry to the turbine can develop. A very low pressure after the turbine allows a very large pressure difference to exist which in turn allows a lot of work to be done.

With pressure drop comes heat loss. All this heat is being converted into mechanical work as it enters/exits the turbine. The turbine wheel acts as a sail. There is typically a 200*f-300*f difference in temperatures from entry into the turbine and exit. This alone shows the importance of heat as an energy source.

Flow alone cannot spin a turbine. You can prove this with a waterhose. Fit the end of it with a twist style spray nozzle.. Get a bicycle and lay it upside down. Spray the tire with the nozzle all the way open...or better yet.. off. Tire doesn't spin for crap.. Tune the nozzle so that you get a steady hard stream.. Now the tire spins right? NO differnce in flow tho...

But if you kinked it(use a restrictive manifold), then no matter what, you get a weaker stream. Right after the nozzle is where work can be done. YOu can spray your sidewalk off, ect. Same with a turbo, Right after these gases enter the turbine housing, work can be done(turn the turbine wheel).. You really don't want to restrict pressure before it gets to the turbine housing.