It's not that simple:
A 4 cycle engine has 4 and not just 1 cycle.
And each cycle adds or substracts torque at the flywheel.

If backpressure is increased, torque will be reduced.

 

Torque is only added during 1 cycle. The power stroke. That is the only one of interest for this discussion.

If backpressure increases the MEP will decrease. Why? Because there is less fuel and O2 in the chamber. If you look at an Otto cycle pressure diagram you will see the pressure from compression alone is virtually insignificant when compared to the pressure created from combustion. So if you have exhaust back pressure, since it is at a higher pressure than the intake charge, it will result in an increased cylinder pressure up until combustion takes place. During and after combustion, the cylinder pressure will be lower due to the exhaust charge contamination using space that would otherwise be occupied by O2 and fuel. In other words, the combustion of fuel will create far more pressure than the existance of exhaust gasses in the chamber. So the MEP will be down as will torque.

 

I'm sorry, but this is the very interest of this discussion:

4 cycles and each cycle adds or substracts torque:
1. Intake subtracts torque
2. Compression subtracts torque
3. Combustion adds torque
4. Exhaust subtracts torque

Backpressure is driving the turbo and not some imp. And the piston or in this case the rotor has to work against this backpressure. If the rotor has to work against this pressure it needs torque and it takes this torque from the flywheel.

This is why 2 engines with the same MEP but different backpressure can have different torque figures.

Besides the Renesis does not have port overlap, which means backpressure cannot displace intake air to the level it could if it had overlap.

 

how many mpg?

 

I know this, but what does it have to do with anything. We are discussing the torque shown on a dyno chart versus cylinder pressure. That torque is only created during the combustion cycle.

How do you know this? When have you had access to equipment capable of measuring the MEP on two identical engines and where able to vary just backpressure?

Not sure what this has to do with our discussion on the relationship between torque and CP. This is a general relationship and not Renesis specific.

I quoted the formula for computing torque from MEP above. Did you see any variable in that formula accounting for backpressure? No, because the relationship between the two is not dependent on it.

 

Obviously you still do not understand. Again there is a reason why it is called a 4 cycle engine and not a 1 cycle engine. Torque shown on your dyno chart is a result of:
Comubstion cycle
minus intake cycle
minus compression cycle
minus exhaust cycle

If you increase backpressure you reduce torque shown on your dyno chart.

This is basics. Are you seriously claiming that you don't believe that backpressure does have an influence on torque and power? If you don't believe it, I suggest to put a Banana in your exhaust and see what happens. The banana will increase backpressure and reduce torque.

This formula is an approximation for a naturally aspirated engines. If it was all that simple we wouldn't have expensive engine simulation software and of course engine test labs wouldn't need pressure sensors measuring the pressure in the combustion chamber. R&D could save millions, if they could just use your formula.

 

smarty pants...

Turbo= GUD!!

 

I honestly don't even think you read my post.

I completely agree that increased backpressure will result in less torque, as I stated here:

I don't think you understand what I'm trying to state. I'm not stating you can calculate the exact torque if you knew the instantaneous cylinder pressure. I'm saying that one is proportional to the other. The expensive simulation software gives actual approximated values for those.

Seriously, I don't even know why I'm having this conversation. How would torque not be proportional to cylinder pressure? What else would push the piston down? If you can describe some other significant force that's involved besides the force from combustion, I would be very interested. If you have more force pushing down the piston, then you will have more torque, if you have less force pushing down on the piston then you will have less torque. I don't know how you're going to argue with this.

Take 5 min and do a google search on MEP vs torque. Find me 1 reference stating you are right.

Here's my list of references:
http://www.google.com/search?hl=en&q=mep+vs+torque

Pick one.

 

Yes but are also claiming that increased backpressure will result in less MEP and that's why it is producing less torque. They do not simply go hand in hand. And even if they did increased backpressure will ALWAYS reduce torque regardless whether MEP is the same or not.

Since we want to increase actual torque shown on the dyno or perceived on the wheels, we have to be concerned about factors that lead to torque reduction as well.

The question was whether this Turbo has more MEP than the Greddy, because torque is higher. And I'm saying that this is not necessarily the case because backpressure of the Greddy set up might easily be double than on this set up.
The Greddy set up at this high boost pressure and efficiency range might easily require 50 HP to generate that boost level at this airflow (Power=p*V). Keep in mind the overall efficiency of a turbo (turbine and compressor) is barely 50% at its optimal operating point.
These 50 HPs are coming from the engine in the form of backpressure and directly lead to a power reduction at the flywheel.

No one claims that torque ist not proportional to cylinder pressure on a given engine, but AGAIN it is not the only factor. There are 4 cycles and NOT just one. And each cycle affects torque measured at the flywheel. And this is not even the whole story. Internal friction, oil pump, water pump, alternator and what not do also reduce torque measured at the flywheel. I don't need references to know that cylinder pressure is not the only parameter affecting torque.
I'm sorry, but this is really far too basic to even have a debate about.

 

If backpressure would not affect torque it would mean that backpressure can be had for free and is therefore not relevant. And in this case turbo efficiency would not matter either.

 

I'm not saying that MEP is the only factor by any means. I'm just saying in general if MEP goes up, torque will go up. Much like the conversion from flywheel hp to wheel hp. There are many thing affecting that conversion (friction, intertia..), but in general the two are proportional. If flywheel HP goes up, wheel HP goes up.

 

Backpressure is a misused word in the context of engine performance. This may or may not be in the context of this discussion but I'll deal with it anyways since the term was mentioned. You never want backpressure. Here's why. You want velocity. A certain velocity of air will help pull a chamber clean, charge an intake, or spin a turbocharger. The term backpressure is commonly used but it is really a misinterpretation of what is going on. For any given load or rpm, there is a certain optimum air velocity. The perfect velocity for 3000 rpm is no good for anything above this point as is true with any other rpm. If we had a pipe sized for max velocity at 3000 rpm, that rpm would be good but we would introduce backpressure above this point which can only hurt performance. Backpressure is bad. So we say we want less backpressure. Want less where? There is an rpm where you don't have any for any given sized pipe. The terms backpressure and velocity are all based on a world of compromise. You don't size something for 8000 rpm but expect it to be optimum at 3000. Sure you'd have "no backpressure" down there but you also have no velocity and that's what counts. You want as little backpressure as possible while still retaining high velocity needed for optimum performance. This applies from everything from intake design, exhaust design, to turbocharger sizing. When most people say they need to add backpressure, they really mean they have too little velocity and need to add it at that spot. Of course above that point will suffer somewhat. It's all a tradeoff.

A turbocharger does not run off of backpressure. It runs off of flow. It doesn't even run off of heat. A turbocharger does introduce backpressure into the exhaust manifold and engine though since it is a restriction. At the flow level it isn't a restriction, it also isn't spooling. Restrictions in the exhaust raise the combustion temperatures of the engine and also raise the likelyhood of detonation. The key to turbo sizing is to get an exhaust housing sized so that it spools up faily quick but at the same time doesn't choke off the motor on the top end any more than neccessary.

To understand the pressures inside a turbo exhaust housing is as easy as just thinking about how air flows through it. The housings takes a certain volume of exhaust area and chokes it down. This speeds up the flow through the turbo to get the wheel moving. Remember the wheel moves based on flow, not pressure. Faster flow is faster spool. What pressure it is at is nearly irrelevant. What happens when you speed air up as is happening inside the exhaust housing? You lower it's pressure. This means you create a high pressure at the entrance to the turbo which in turn affects the flow of gasses out of the engine as well as tries to keep some in it.

I understand you guys are talking about MEP vs torque but I wanted to get the backpressure thing cleared up.

 

But flow is proportional to pressure. (More pressure difference = more flow or no pressure difference = no flow).

pressure = density/2*velocity^2 (according to Bernoulli).

 

That's understood. Flow is proportional to pressure but pressure is not proportional to flow which is why many get them messed up. My air compressor tank in my garage is sitting at 120 psi. There is no flow. It isn't pressure doing the work, it's flow. It is understood that you can't have flow without pressure but that only works one way. You can have pressure without flow. Just because you added pressure does not mean you added flow.

 

In the case of the turbine massflow does work and in case of a piston pressure times Volume does work. (At least that's how we apply these models, since one model works better for one application and vice versa).

But at the end work is work. Work = pressure*Volume = mass/2 *velocity^2.

 

You are correct, the amount of work being done is something that you can figure out that way. What you can't determine based on that is how much you are getting for all the effort. You can show how much work an engine is doing to push air out of a pipe but you can't show how much power it is making based on that or the efficiency of it. Once you hit an average (not peak) airspeed velocity of .6 mach throught the system, efficiency falls off the planet. You can show how much work is being done to a turbo based on exhaust flow through it but you can't show how the turbo is responding to it.

If all you are interested is how much work it takes to do something then that's fine. You should be able to tell how much power it takes to run a turbo or a supercharger this way. That is good information to know. You just can't show how much they are producing based on it alone without knowing alot of other things.

That is a very good formula to know though.

 

Which also means that a turbo run over its operating point might even create more harming backpressure not just because the turbine is past its efficiency range but the whole set up including housing cannot convert that pressure and temperature into mechanical work (and heats housing and header instead).

 

326 hp eh..... more than enough power for drifting.... >_< .... lately its been raining here and i get of work at 3am and just drift every right/left turns at intersections.... so beautiful.... oh yeah anyway, how fast is the 1/4 time for that?

 

Attention people in a 10mile radius of the above poster. Make sure to be in your home and off the road by 2:45am.

Seriously though, you'll be in brothervoodoo's gallery soon enough.

 

I don't like the curve of the power..I have seen alot more straigh lines..I ll put my graph soon from which you will be amazed but i don't have so many hps..I just want you to see what i mean with the "straight" line..

Although very good numbers..Keep it up..

 

Ummm... Your formulae aren't technically correct. You are confusing work and energy. Work done is equal to a change in energy. P*V is a measure of potential energy. 1/2mv^2 is kinetic energy.

 

You've got a point. Actually, I should have said power instead of work or energy.

Power=p*V=m*v^2
and with Volume and mass per time unit. (Meaning a point above V and a point above m).

 

um wat can i contribute to this... u guys r hella smart i am just tuning in and learning all this crazy talk of physics...

 

You cant have your cake and eat it too.If you use a larger turbo that makes more horsepower, your power and torque curves shift to the right. A ball-bearing turbo will help give you the best of both worlds but still expect some compromise when you are dealing with a turbocharger this large.


Tim