SpeedForceRacing-326WHP with dyno chart
#77
Registered
Join Date: Aug 2004
Location: Virginia/Maryland
Posts: 1,294
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
#78
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.
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.
Last edited by globi; 03-27-2006 at 03:39 PM.
#80
Registered
Join Date: Aug 2004
Location: Virginia/Maryland
Posts: 1,294
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by globi
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
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
Originally Posted by globi
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.
This is why 2 engines with the same MEP but different backpressure can have different torque figures.
Originally Posted by globi
Besides the Renesis does not have port overlap, which means backpressure cannot displace intake air to the level it could if it had overlap.
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.
Last edited by rkostolni; 03-27-2006 at 06:13 PM.
#81
Originally Posted by rkostolni
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.
Comubstion cycle
minus intake cycle
minus compression cycle
minus exhaust cycle
If you increase backpressure you reduce torque shown on your dyno chart.
Originally Posted by rkostolni
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?
Originally Posted by rkostolni
I quoted the formula for computing torque from MEP above. Did you see any variable in that formula accounting for backpressure?
#83
Registered
Join Date: Aug 2004
Location: Virginia/Maryland
Posts: 1,294
Likes: 0
Received 0 Likes
on
0 Posts
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.
I completely agree that increased backpressure will result in less torque, as I stated here:
So the MEP will be down as will torque.
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.
Last edited by rkostolni; 03-28-2006 at 07:44 AM.
#84
Originally Posted by rkostolni
I completely agree that increased backpressure will result in less torque
Originally Posted by rkostolni
I still don't know what you're trying to say? I know this, as I've already said. I don't care about any of those other cycles. The torque is created, or the positive addition, or the perceived torque, or however you want me to word it is only added during combustion. Can you explain why you keep mentioning this.
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.
Originally Posted by rkostolni
How would torque not be proportional to cylinder pressure?
I'm sorry, but this is really far too basic to even have a debate about.
#86
Registered
Join Date: Aug 2004
Location: Virginia/Maryland
Posts: 1,294
Likes: 0
Received 0 Likes
on
0 Posts
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.
#87
Registered
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.
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.
#89
Registered
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.
Last edited by rotarygod; 03-28-2006 at 01:06 PM.
#90
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.
But at the end work is work. Work = pressure*Volume = mass/2 *velocity^2.
#91
Registered
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.
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.
#92
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).
#93
Need Rotary Engine!!!
Join Date: Mar 2006
Location: Garden Grove, CA
Posts: 189
Likes: 0
Received 0 Likes
on
0 Posts
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?
#94
Storm Trooper
Join Date: May 2004
Location: Freakmont, CA
Posts: 3,908
Likes: 0
Received 0 Likes
on
0 Posts
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.
Seriously though, you'll be in brothervoodoo's gallery soon enough.
#95
90% Carbon-100% Power
Join Date: Apr 2004
Location: Thessaloniki,GREECE
Posts: 1,706
Likes: 0
Received 0 Likes
on
0 Posts
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..
Although very good numbers..Keep it up..
#96
Consiglieri
Join Date: Feb 2004
Location: yourI'mgirl
Posts: 1,275
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by globi
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.
But at the end work is work. Work = pressure*Volume = mass/2 *velocity^2.
#99
TurboTim
Thread Starter
Join Date: Feb 2005
Location: San Diego
Posts: 251
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by GrRx8MaZdA
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..
Although very good numbers..Keep it up..
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
#100
#1 Legend
iTrader: (1)
Join Date: May 2004
Location: TEXAS
Posts: 3,904
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Moostafa29
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.
Seriously though, you'll be in brothervoodoo's gallery soon enough.