Cylinder Pressure vs. HP/TQ Discussion thread

[Shadow]

All of this talk about DJ's 16v 2.4 and my 2.2 8v being around the same power level got me thinking about the thread where me and JT were swapping views on exhaust resriction and I was trying to explain myself using "conservation of energy" and Nobody was getting it.

Now, since conservation of energy is such a common term, I had to come to the conclusion that I just wasn't explaining it right?

Anyway, it just occured to me that this is a perfect platform to explain what I ment. (long as DJ doesn't mind)

The fact that DJ is running a 2.4 16v and I'm running a 2.2 8v has ZERO to do with anything when it comes to making power. Only 1 thing matters, cylinder preasure. Which after the work is done will = exhaust drive preasure.

The Fact that both DJ and myself are running the exact same turbo AND both running 4" dumps makes this a Great topic to be able to relate to.

DJ made the statement that at 24 psi he would expect his drive preasure ratio to be Higher than mine because we know it's a restrictive turbine wheel and housing. (and I agree with this)

Now here's what I was trying to explain in the other thread; IF we are both making the same power, let's say 500WHP the exhaust energy will be the same! Plain and simple. If DJ had the same exhaust manifold then it would be even more simple, because it would all be apples to apples and I could tell you by drive preasure Alone who's making more power!

Let's say the Charger has 37psi drive preasure @ 34 psi manifold preasure @ 500WHP. Let's also say that DJ IS making the same 500WHP @ 24psi. I would expect his drive preasure to be the Same 37 psi I'm seeing IF his exhaust mani was the same, Get it? The exhaust energy is the Same wether it's coming from a 16v @ 24psi or an 8v at 50psi and you should be able to see this IF the hot sides are the same.

Now, let's take this one step further. DJ is running a nice 4 into 1 header, should be more efficient than my ported stocker, right? Well there's an easy way to see how efficient it actually is. IF we Are at the same power level, than any amount Less drive preasure = more efficient exhaust through the turbine wheel.

So let's say that DJ takes a reading and it's 32 psi drive preasure @ 24 psi boost. IF the two mtrs are making the same power, thus the same exhaust energy, the 5 psi Less drive preasure shows the efficiency of the header.

Make sense?

[turbovanman²]

You talk too much,

But yeah, makes sense,

[Shadow]

You talk too much,

No $hit!

[BadAssPerformance]

The fact that DJ is running a 2.4 16v and I'm running a 2.2 8v has ZERO to do with anything when it comes to making power. Only 1 thing matters, cylinder preasure. Which after the work is done will = exhaust drive preasure.

I think this example may be simplifying the equation a bit as there is no "time" component such as RPM or CFM.

[Shadow]

I think this example may be simplifying the equation a bit as there is no "time" component such as RPM or CFM.

Well anything before the cylinder (intake ect) has no bearing and I'm not so sure rpm would either. ie. mtr #1 makes 500WHP at 5900rpm and mtr#2 makes 500WHP @ 8000rpm, they should still have the same exhaust energy @ 500WHP. Now wether the difference in rpm (and maybe this is what your talking about) has a bearing on how the turbine responds to that exhaust energy I'd have to think about.

Actually, what I was wondering is what is more indicative of the exhaust energy being produced, HP or TQ?

[Reaper1]

It also doesn't take into account that the density of the exhaust won't necessarily be the same, and this also effects the energy delivered to the turbine.

[GLHS60]

I rarely disagree with you but I don't think cylinder pressure = exhaust drive pressure. As in higher boost = equally higher cylinder pressure.


Thanks
Randy

[QUOTE=Shadow;853087]

. Only 1 thing matters, cylinder preasure. Which after the work is done will = exhaust drive preasure.

[BadAssPerformance]

Well anything before the cylinder (intake ect) has no bearing and I'm not so sure rpm would either. ie. mtr #1 makes 500WHP at 5900rpm and mtr#2 makes 500WHP @ 8000rpm, they should still have the same exhaust energy @ 500WHP. Now wether the difference in rpm (and maybe this is what your talking about) has a bearing on how the turbine responds to that exhaust energy I'd have to think about.

Actually, what I was wondering is what is more indicative of the exhaust energy being produced, HP or TQ?

Heat and pressure energy are only part of the equation. Thinking of an engine as a volume pump... naturally aspirated and everything "ideal" without losses:

2.5L @ 5900 RPM moves 261cfm
2.4L @ 7000 RPM moves 300cfm

So there is an additional 39cfm that needs to exit the engine thru the turbine regardless of heat or pressure.

Yeah Chris, density matters too!

[Shadow]

[QUOTE=GLHS60;853190]I rarely disagree with you but I don't think cylinder pressure = exhaust drive pressure. As in higher boost = equally higher cylinder pressure.


Thanks
Randy

Well, maybe I Do think of this stuff in too simple of terms? But Randy, boost preasure surely does not always = cyl preasure, it depends on where the restriction is.

Otherwise that would be like saying the Charger has more cyl preasure than DJ's Shadow simply because I run 34 psi boost and DJ only runs 24. The boost preasure has no relevance or bearing on anything because it is dependant on too many other factors. ie. intake/cam/IC ect.

The only thing that matters is how much cyl preasure you have in the mtr, how it gets there is prob what's throwing a wrench into how ppl are thinking about this.

Try this one; An N/A 300WHP high compression 2.2 vs a 300WHP turbo 2.2. They will both have roughly the same cyl preasure to be able to output 300WHP.

Now the N/A mtr is doing it from high comp pistons and crazy flowing head and cam combo while the turbo is doing it with much less head and cam and raising the cyl preasure to the same level with boost, but in the end, the only reason either mtr is making 300WHP is because of the cyl preasure they are at.

If you can't make/hold more cyl preasure you can't make any more power.

Why did the Charger make 30-40 more WHP with the F4 cam on less boost than the stocker I was running? My cyl preasure increased as a direct result of the extra flow into the cyl.

Anyway, I didn't want to jack DJ's thread with this. I just thought that my explanation had relevance to his thread and what was going on and Might shead some light on what I was trying to say in the other thread.

---------- Post added at 10:14 PM ---------- Previous post was at 10:05 PM ----------

Heat and pressure energy are only part of the equation. Thinking of an engine as a volume pump... naturally aspirated and everything "ideal" without losses:

2.5L @ 5900 RPM moves 261cfm
2.4L @ 7000 RPM moves 300cfm

So there is an additional 39cfm that needs to exit the engine thru the turbine regardless of heat or pressure.

JT, your showing a representation of why the 2.4 will use Less boost with the same turbo and exhaust to make the same HP as the 2.5 on higher boost. The 2.5 will need higher boost in order to reach the amount of cyl preasure to equil the 2.4's output. So the exhaust drive preasure will be roughly the same.......

Crap, should have just kept my big mouth shut! I hate when I do this in other ppls threads! JT, maybe move this somewhere appropriate and call it the truth about cyl preasure, or Does cyl preasure = HP, or something appropriate.

Please move it out of DJ's good project thread!

[Reaper1]

One more thing on the cylinder pressure:

Cylinder pressure is NOT what makes horsepower. That pressure gives you torque. For larger stroke engine you don't necessarily need the same cylinder pressure to achieve the same torque. The shorter stroke engine will need more pressure to achieve the same torque.

Remember that horsepower is a function of torque. Ultimately the engine that can maintain a certain cylinder pressure for more rpm will make more horsepower.

[BadAssPerformance]

Posts moved from DJ's 93 Shadow p-log to this thread. Let me know if I missed any.

JT, your showing a representation of why the 2.4 will use Less boost with the same turbo and exhaust to make the same HP as the 2.5 on higher boost. The 2.5 will need higher boost in order to reach the amount of cyl preasure to equil the 2.4's output. So the exhaust drive preasure will be roughly the same.......

Even if the cylinder pressure and resulting exhaust pressure are the same, if similar displacement engines produce the same power at different RPM, there is more volume of air that needs to get out of the motor on the higher RPM application.

[turbovanman²]

I am not sure I agree with cylinder pressure being equal. I seem to recall that boosting an engine does weird things with the "compression" ratio.

[BadAssPerformance]

That is another age old discussion...

If a theoretical "optimal" compression ratio is 13:1, then theoretically adding boost into the cylinder can artificially raise the cylinder pressure from 8:1 to 13:1 for similar power production.

This is again where "time" must be included in the equation as the peak numbers may be similar, but with different power curves.

[RoadWarrior222]

Hmmm, you can try and generalise things too much. The 8v engine has to "make" more to offset higher pumping loss, to see the same figure at the wheels, the motor with the smaller bore needs higher chamber pressure to put the same torque to the crank given same stroke and rod ratio.... and of course you were able to determine the Brake Specific Fuel Consumption of each motor you're comparing in an engine dyno test cell?

[shelbymonster]

That is another age old discussion...

If a theoretical "optimal" compression ratio is 13:1, then theoretically adding boost into the cylinder can artificially raise the cylinder pressure from 8:1 to 13:1 for similar power production.

This is again where "time" must be included in the equation as the peak numbers may be similar, but with different power curves.

ive heard that mechanical compression got a higher temp that forced induction but in my mind its the same compression ratio

[turbovanman²]

This isn't a bad article on cylinder pressure-

http://www.empirenet.com/pkelley2/DynamicCR.html

What the compression ratio changes to with boost-

http://www.blowerdriveservice.com/techcharts.php

[Shadow]

Let's keep in mind that CR and cyl preasure are two different things....right? JT, you said it yourself, and I totally agree, a mtr is an air pump, the more air you can put through, the more power you make, the More Cyl preasure you have!

I Never said the 2.5 will have the same cyl preasure as the 2.4, but they will be proportional to each other. I said that at the same HP and or tq the exhaust Energy (drive preasure) should be virtually the same.

[turboshad]

I see where you are getting at but I also agree that it isn't that simple as it is a an extremely dynamic system. Like Reaper said HP is a product of torque and we probably make the same power at different RPM points. I have no clue what these numbers actually are so the fluff starts here. If I was at 7000RPM I would need 375 ftlb. If you were at 5500RPM you would need 477 ftlb. Your 2.2 has an 8.3% shorter stroke and therefore needs 9% more pressure to create the same torque. (please take all these numbers loosely). If the mass flow of oxygen could be related linearly to combustion and cylinder pressure you would need 9% more oxygen to create 500hp on the 2.2. If I'm making 500@24psi and you're doing it at 34psi then my turbo has to move air with 30% more oxygen/psi than you so we maybe even call it 30% more thermally efficient air.I doubt this is the case but say my car is running at 1:1 making the drive pressure 24psi giving me 24psi of boost and enough oxygen content for 500hp. We extreeeeemly loosely know I get 30% more oxygen/psi which drops you to 77% of the efficiency I am getting. This extra energy has to come from somewhere so if we say it comes from extra pressure you would have a 1.3:1 ratio of exhaust to intake making your drive pressure 44.2psi with the same thermal losses.

Pretty much everything I wrote above is hogwash and probably no where near accurate but I think it demonstrates a touch of the complexity involved in the situation. I don't think we will have the same drive pressure but if my exhaust is choking up I expect the ratio between drive pressure and output pressure to be greater for me. I think I wouldn't be seeing an increase in power b/c the extra exhaust pressure would be lowering the VE of my engine. If the added exhaust pressure is close to the same as the added intake pressure and the intake air efficiency was dropping then I wouldn't expect to see any substantial power increase. Another interesting comparison in our systems is that we have the same EBC and WGA. I'm curious what your duty cycles are at for 34psi. Mine are around 52-54% for 24psi IIRC. This would show how much energy I am letting escape around the turbine compared to you.

[dgc333]

Well anything before the cylinder (intake ect) has no bearing and I'm not so sure rpm would either. ie. mtr #1 makes 500WHP at 5900rpm and mtr#2 makes 500WHP @ 8000rpm, they should still have the same exhaust energy @ 500WHP. Now wether the difference in rpm (and maybe this is what your talking about) has a bearing on how the turbine responds to that exhaust energy I'd have to think about.

Actually, what I was wondering is what is more indicative of the exhaust energy being produced, HP or TQ?

HP is just a mathematical expression, it can be calculated but can not be measured. You measure torque and rpm.

In your example above the engine that produces 500 HP at 5900 rpm is generating 446 ft-lbs of torque. The engine that is making 500 HP at 8000 rpm is gearing 328 ft-lbs of torque. These two engines are going to have dramatically different operating characteristics, to get similar performance in two
Otherwise identical cars is going to require very different gearing.

Some one mentioned air flow rate through the engine at an rpm. When calculating those numbers the results are for 100% volumetric efficiency and no engine is 100% across it's entire rpm range and most street based designs are less than 90%. It's the restrictions in the intake and exhaust that determine the VE and it is perfectly reasonable to find a smaller engine that moves more air than a bigger one.

Cylinder pressure is determined by the static compression ratio and boost. BUT, the point at which the intake valve closes ABDC ultimately determines how much of the intake charge is captured and what the dynamic compression is.

Also, the design of the engine and specifically the chamber in the head will determine whether ideal timing can be used to cause max cylinder pressure to occur at the ideal point (approx. 20 degrees ATDC).

What this long winded post is getting to is you can't really make any determination about what is happening in two engines based only on the HP the generate.

[RoadWarrior222]

There's so many variables in engine design, that you really can't say much about a general case. If it was easy, we'd have invented the perfect engine 70 years ago under the pressures of looming war and ..... ................. ....................... BRB, just have to go saw a few cylinders off a Merlin and cram it in the Voyager...