Pressure vs. Volume vs power output

[Amelio]

This is just a question I have ratteling around in my head and would like a better understanding of how this works.

Ok so I am talking about cylinder pressure (amount of boost) vs the total amount of volume being packed in the cyclinder.

For example if you have two identicle engines that both make 500hp and 550tq but they are running different turbos.

Engine A: can acomplish this power out put with 26psi

Engine B: can acomplish it at 17psi due to a larger more efficient turbo.

Now here is my question....engine A has to run much more pressure to achieve the power goal so this makes me think that it is a more dangerous tune (meaning everything needs to be dead on and more of a chance of hurting something)

Where engine B is running much less boost so it is somewhat "safer" and maybe a little more room for error right?????

However engine B is running less pressure but the volume of air has to be the same or higher to achieve the same power goal right?

This is where I get confused........

Both engines are making the same power..... hp is hp and tq is tq.....so the strains on both engines are the same or are they?

 

[Jerryl]

Let me try this . . .

All this probably not in the correct order, but it gives some idea . . . .
The variables for engine output are:

• Air Flow: ACFM
• RPM
• Manifold Pressure: Turbo pressure – IC pressure drop
• Compression Ratio: Dynamic pressure
• Temp: Ambient & Charge
• Fuel – Octane/Latent heat
• Drive train efficiency
• Ambient conditions; Barometric pressure/H2O / Ambient temp
• VE
• (And a few others :tongue

Examples:
Fist off, HP is just a calculated number. HP = (TRQ*RPM)/5252
In fact, HP cannot be measured, only calculated.
If we Ass-U-Me that the tune/temp/rpm/fuel/CR/drive train/ambient/weight are all constant, than the engine running lower boost must have higher VE.
I good example is ported vs unported heads where you can run the same number at lower boost. So HP is the same, but the VE/Flow increased.


In one example, if one engine makes 500 HP at 5000 rpm, the other at 8000 rpm, than according to the calculation, the 5000 rpm motor has higher torque value . . . . if the HP=constant.
Next time you see a dyno chart, look at the HP and torque values around 5252 rpm.

So the 5000 rpm motor will most likely win in an 1/8 mile race . . . . . even if the other motor made 600 HP at 10,000 rpm.
It is one of the reasons most S U P R A 's are considered to be "Dyno Queens".

 

[94z286speed]

Subscribing

 

[bison]

There are plenty of posts about engine mass flow and a lot of good calculators and info on the Internet. You would never have the situation described if the turbo sizing was close. Backpressure becomes more of a problem in a high hp wastegated application. The cold side is usually not the limiting factor. Not based on what I read on this site. More mass flow equals more power. You might get the same power at 1 or 2 psi less but it could be the difference in backpressure which will bring up other variables that could be adjusted to offset that power loss and actually make the turbo with higher backpressure out perform the one that had less. For a street car spoolup is a major concern. The quicker it spools the higher the backpressure to an extent.

 

[Bmason]

Jerryl,

The way I'm reading Amelio's question is:

If engine "B" is using less boost to create the same calculated HP rating as engine "A", do they still have the same peak cylinder pressure?

 

[Alky V6]

A turbo that is sized to an engine to run closer to its higher efficiency zones (compressor map) at WOT will heat the compressed intake charge less, resulting in a denser intake charge. A denser intake charge will mean less boost required for a particular hp level when compared to another turbo that is sized for the same engine to not run close to its higher efficiency zones. 75-78% versus 60-65% which is the typical choice for most street applications. The reason most pick a smaller turbo that will run less efficient at WOT is due to the compromise involved with spooling up the turbo. The smallest turbo choice for an application will spool the best, but will run the least efficiently at WOT.
If both engines are identical, and making the same power, then they are both receiving and burning the same amount of air and fuel.
The one engine having to use a higher boost level to achieve the same power is heating the intake charge more and will require a tighter tuneup, since the higher intake charge temperature, before compression and combustion, will put the charge closer to detonation/preignition levels. Extra fuel and/or less timing may be required.
The one engine having to run less boost to obtain the same hp level will be able to do so with a wider safe tuning range than the other engine.
The turbo that is running more efficiently for the application is effectively reducing intercooler requirements, since the charge is leaving the compressor a little cooler to start with than would be with the inefficient turbo.
The mechanical loading on the two engines would be identical assuming both have a safe tuneup. One may be closer to the edge though, than the other.
The thermal loading on the engine running the higher boost will obviously be higher than the other, and that may need to be tightly monitored and controlled through tuning measures. Timing and fueling richness.

 

[Amelio]

Jerryl,

The way I'm reading Amelio's question is:

If engine "B" is using less boost to create the same calculated HP rating as engine "A", do they still have the same peak cylinder pressure?

Yes this is what I am asking......basically to know if running a more efficient turbo to make the same power at less boost is considerd to be more "safe" or is this this just an illusion due to both engines producing the same power.

 

[Amelio]

A turbo that is sized to an engine to run closer to its higher efficiency zones (compressor map) at WOT will heat the compressed intake charge less, resulting in a denser intake charge. A denser intake charge will mean less boost required for a particular hp level when compared to another turbo that is sized for the same engine to not run close to its higher efficiency zones. 75-78% versus 60-65% which is the typical choice for most street applications. The reason most pick a smaller turbo that will run less efficient at WOT is due to the compromise involved with spooling up the turbo. The smallest turbo choice for an application will spool the best, but will run the least efficiently at WOT.
If both engines are identical, and making the same power, then they are both receiving and burning the same amount of air and fuel.
The one engine having to use a higher boost level to achieve the same power is heating the intake charge more and will require a tighter tuneup, since the higher intake charge temperature, before compression and combustion, will put the charge closer to detonation/preignition levels. Extra fuel and/or less timing may be required.
The one engine having to run less boost to obtain the same hp level will be able to do so with a wider safe tuning range than the other engine.
The turbo that is running more efficiently for the application is effectively reducing intercooler requirements, since the charge is leaving the compressor a little cooler to start with than would be with the inefficient turbo.
The mechanical loading on the two engines would be identical assuming both have a safe tuneup. One may be closer to the edge though, than the other.
The thermal loading on the engine running the higher boost will obviously be higher than the other, and that may need to be tightly monitored and controlled through tuning measures. Timing and fueling richness.

Ok Donnie I think you have answerd my question....I had to read your post a few times but basically you are saying that both engines will have the same load given good tunes but the one running higher boost is closer to the edge.....correct?

 

[Alky V6]

Ok Donnie I think you have answerd my question....I had to read your post a few times but basically you are saying that both engines will have the same load given good tunes but the one running higher boost is closer to the edge.....correct?

Yes.

It would be the same as having the choice of one engine that has a compressed intake charge temperature of 300 degrees F compared to one that runs at 75 degrees. This is an extreme case, but the same basic idea.
Who in their right mind would pick the 300 degree temperature over the 75? Isn't that why we run intercoolers in the first place? To get the compressed charge air temperature down as far as possible to get as far away from detonation limits as possible?

Any increase in intake charge temperature puts you closer to the detonation limits of the particular fuel.

 

[Amelio]

Yes.

It would be the same as having the choice of one engine that has a compressed intake charge temperature of 300 degrees F compared to one that runs at 75 degrees. This is an extreme case, but the same basic idea.
Who in their right mind would pick the 300 degree temperature over the 75? Isn't that why we run intercoolers in the first place? To get the compressed charge air temperature down as far as possible to get as far away from detonation limits as possible?

Any increase in intake charge temperature puts you closer to the detonation limits of the particular fuel.

Perfect!

thank you very much for the knowledge!

 

[Street Lethal]

Now here is my question....engine A has to run much more pressure to achieve the power goal so this makes me think that it is a more dangerous tune (meaning everything needs to be dead on and more of a chance of hurting something)...

What makes the tune dangerous is the type of fuel you are using, as pump gas consists of a radical that causes detonation, and detonation kills engines parts. So the quantity of air being ingested is not what kills the engine, the quality of it does (if it is too hot). Certain fuels (like E85) eliminate that radical and keep things cool. When you see the boost pressure gauge in example A embellish 26-psi, that is 26 pounds of forced pressure through a particular intake valve size. The gauge is only a resistance reading, it does not tell you how much air the engine is actually capable of ingesting, it is just measuring the amount of resistance. Look at it this way, 15-psi "doubles" the amount of air the engine will consume at 0" of vacuum, it doesn't double the amount of air that can actually fill an engine bore entirely...

Both engines are making the same power..... hp is hp and tq is tq.....so the strains on both engines are the same or are they?

Ideally, if the boost pressure gauge is reading 20-psi on both engines in your example, then technically they should make the same amount of horsepower. What changes things though in that comparison is the temperature of the air, as well as the RPM being needed to achieve that boost pressure level (because they will vary when comparing a larger turbo to a smaller turbo), and both areas effect horsepower...

 

[Jerryl]

Jerryl,
The way I'm reading Amelio's question is:

If engine "B" is using less boost to create the same calculated HP rating as engine "A", do they still have the same peak cylinder pressure?

I did not read the whole thread but in very basic terms, the pressure would be the same for a constant output.
Pressure = lbs/sq-in
Force = Area (sq-in) * Pressure (lbs/sq-in)
Torque = Leverage (in) X force (lb)

Which brings up an interesting point;
Does a larger bore create more torque than a larger stroke? Area increases by the square of the bore, stroke is a linear measure. I have created a chart on this that is very interesting. I will leave it to the OP if he wants me to post it. Anyway, my apologies to the OP for getting off track.

. . . . Ideally, if the boost pressure gauge is reading 20-psi on both engines in your example, then technically they should make the same amount of horsepower. What changes things though in that comparison is the temperature of the air, as well as the RPM being needed to achieve that boost pressure level (because they will vary when comparing a larger turbo to a smaller turbo), and both areas effect horsepower...

Pretty much sums it up.
Now, a bigger turbo at the same PR should theoretically have the same or higher PEAK HP (Less BP), but that is not the case if you run outside of the efficiency. Example would be if you bolt on a 70mm turbo on a stock motor, and run 15 PSI boost. It is all about the constraints combine with a well matched combo. You are probably already aware that “single point peak performance” is one thing, but true performance is all about area under the curve.

I realize you know already know this, but just wanted to clarify.

 

[Amelio]

No apology needed Jerryl.....

I posted this question for knowledge so all is welcome.

I basically wanted to know this for a better understanding so if and when I make changes to my car I have a better idea of what I am doing. I am currently running a TE-66 and another board member here is running a 67.....now all things are not exactly equal because his tune is much better than mine due to him running an XFI Fast system.

Where I am going is that our combos are almost the same they are very close but he is able to run the same e.t at 14psi as I am at 19psi. Now again his tune is much better than mine and I know if my tune was as good I would be faster on that 19psi pull but I really do not think it would run like his car.

So that is why I posted the original question..............

Any and all thoughts and opinions welcome!

 

[bison]

The pressure before combustion is a lot higher in the higher boost application but the pressure exerted on the crankshaft after combustion is the same as long as there was no detonation.

 

[Jerryl]

I posted this question for knowledge so all is welcome.

I basically wanted to know this for a better understanding so if and when I make changes to my car I have a better idea of what I am doing. I am currently running a TE-66 and another board member here is running a 67.....now all things are not exactly equal because his tune is much better than mine due to him running an XFI Fast system.

Where I am going is that our combos are almost the same they are very close but he is able to run the same e.t at 14psi as I am at 19psi. Now again his tune is much better than mine and I know if my tune was as good I would be faster on that 19psi pull but I really do not think it would run like his car.

So that is why I posted the original question..............

Any and all thoughts and opinions welcome!

Hey man, i am here to learn as well. (BTW, this is a VERY good question!)
Heck, I typically end up reading Bison's and Donnie's posts at least 2X to try and understand what they is trying to say.

So are you running a stroker? (120MPH @ 19 psi is no slouch. )
What set-up is your buddy running and what is his MPH?

 

[bison]

I posted this question for knowledge so all is welcome.

I basically wanted to know this for a better understanding so if and when I make changes to my car I have a better idea of what I am doing. I am currently running a TE-66 and another board member here is running a 67.....now all things are not exactly equal because his tune is much better than mine due to him running an XFI Fast system.

Where I am going is that our combos are almost the same they are very close but he is able to run the same e.t at 14psi as I am at 19psi. Now again his tune is much better than mine and I know if my tune was as good I would be faster on that 19psi pull but I really do not think it would run like his car.

So that is why I posted the original question..............

Any and all thoughts and opinions welcome!

The difference in power there isnt the turbos.

 

[Amelio]

Hey man, i am here to learn as well. (BTW, this is a VERY good question!)
Heck, I typically end up reading Bison's and Donnie's posts at least 2X to try and understand what they is trying to say.

So are you running a stroker? (120MPH @ 19 psi is no slouch. )
What set-up is your buddy running and what is his MPH?

Yeah I wanna learn more as always and right now money is real tight so I can not afford to do anything to the car and I dont wanna try to do any tuning in case there is a mishap because I will not be able to fix it for a while. So for now I am just reading up and thinking of my current combo and if it needs to be improved or left alone.

NO it's not a stroker it is a stoke bore 109, stock ported heads and intake 210/215 roller with a TE66 and all the other supporting mods. The car did not run 120on 19 psi it ran an 11.54 @118 on 19 psi on a fair tune. I say fair because it could have been much better I did that with only a scan master making sure it did not knock..... God only knows what the tune actually looked like. Hell I probably would have run 120mph if I had a locking converter or 28" tires or a good tune:redface:

The 120mph pass was like 6 moths later a 22psi but my et was only 11.8 the reason for that showed on the next pass......the 11.8 pass was lifting the head all the way down the track I heard funny noises but did not know what it was(I have never blown a head gasket or lifted a head) and the next pass it let go.

My buddy's car is pretty much the same, he has stock stroke but .030 over and I believe he has a 212/212 roller...same heads,intake pump he is runninf 72# injectors I only have 55's and the big difference is he is runing XFI so his tune is dead on oh and he is squirting alky.

 

[Amelio]

The difference in power there isnt the turbos.

so I imagine you are thinking it is all in the tune......I agree I do think my car would and will run better at that boost level with a much better tune but I still can not see it running an 11.3@16psi which s what his car ran.......

your thoughts sir.................

 

[hensleyt]

I like this thread I have always wondered the same thing,without a compressor map I don't know the sweet spot on my turbo.So is ALKY just a band to fix the high charge temp at the track to have good street manners? Dose that make TE66 out of the map with a higher charge temp needing more boost to make the HP where the 67 is closer to its sweet spot?But the 67 is squirting ALKY are you on ALKY Amelo ?You read Precision turbo chart you see 61mm compressor wheels making 630-680 HP only difference is turbine wheel.So how dose Amelio (or any body) pick a safe turbo or tune to run his number without a compressor map? I did not spray ALKY at 19 psi but my best MPH was 99.I would like to see a map on my 5857,at 25 psi Im just got in the 11's (11.93)@111mph no where near 118.So if two cars make 500 hp Car A @26psi Car B @17psi what car are you driving and why?

 

[Jerryl]

. . . . NO it's not a stroker it is a stoke bore 109, stock ported heads and intake 210/215 roller with a TE66 and all the other supporting mods. The car did not run 120on 19 psi it ran an 11.54 @118 on 19 psi on a fair tune.
............The 120mph pass was like 6 moths later a 22psi ...... My buddy's car is pretty much the same, he has stock stroke but .030 over and I believe he has a 212/212 roller... on oh and he is squirting alky.

FWIW . . . . I tried to back into the numbers.

If I am even close, on the 19PSI @ 118 MPH run, trap RPM was around 6100 with 9% slip (58 lb/min)
To run that number at 14PSI with alky, it is estimated at 6200 RPM, 0% slip (59 lb/min). With alky I would bet there is more timing in the tune as well.
Now, if I run the same numbers on the OP's car with alky, the estimated MPH is 119. With 5% vs 9% converter slip, it jumps to 121 mph.
Numbers are probably off, but it gives an idea of comparison.

There is definitely a BIG difference and I see why you are asking the question, which I am asking myself as well. I agree 110% with Bison though; It is NOT the turbo since that is not the constraint. Maybe the boost gages are off? (LOL!)

To me, this is pretty interesting and there is definitely more studying to do on my end . . . there is a reason Red runs 123 MPH through a stock IC/Elbow/Air box in TSA. Serious business!