Static and Dynamic Displacement and HP

-
273

273

Well-Known Member
Joined
May 14, 2012
Messages
6,739
Reaction score
4,121
Location
Ontario
Static and Dynamic Displacement


Now these terms are my way of looking at engine displacements and borrowed the static and dynamic idea from CR.


Static displacement is the measured displacement of your engine like 225 340 440 etc…


Dynamic is what your engine can displace “Cfm” at rpm. That carb formula is calculating dynamic displacement at 100% VE. Displacement x rpm / 3456 = CFM, there’s obviously limits of more rpm but it’s pretty variable and most aren’t gonna reach them.


General census on this site is added static displacement adds hp over lesser displacement.


My question is if above true why would static displacement have an advantage over dynamic displacement? What’s stopping the smaller engine from using more rpm to reach similar dynamic displacement of the larger static displacement engine to reach similar hp ?
 
You're way over thinking things. Just go out and make some tire smoke.
 
273 said:
Static and Dynamic Displacement


Now these terms are my way of looking at engine displacements and borrowed the static and dynamic idea from CR.


Static displacement is the measured displacement of your engine like 225 340 440 etc…


Dynamic is what your engine can displace “Cfm” at rpm. That carb formula is calculating dynamic displacement at 100% VE. Displacement x rpm / 3456 = CFM, there’s obviously limits of more rpm but it’s pretty variable and most aren’t gonna reach them.


General census on this site is added static displacement adds hp over lesser displacement.


My question is if above true why would static displacement have an advantage over dynamic displacement? What’s stopping the smaller engine from using more rpm to reach similar dynamic displacement of the larger static displacement engine to reach similar hp ?
Click to expand...
Clearly over thinking this and there is no mention of the cam to be used which has a great effect on dynamic pressures.

This is not a static vs dynamic issue but what can be done with each side of the coin for your engine for your build in what your doing in what vehicle for what purpose under what weight of the vehicle and the concerns of the build.

Your question in this thread is 10 X’s the open ended question of your last thread.
 
Hang on 273, yur confusing me.
As for compression ratios;
I thought Static compression ratio was the relationship of volume of the TOTAL cylinder volume including the chamber to just the chamber, like in a 360SBM;
(737.2 + 108)/108 =7.826
Whereas in the Dynamic , you don't start building pressure until the intake actually closes. So then if the ICA is such that intake does not close until the effective stroke is say 3.0 inches on a 360, then the dynamic is only
(618 +108)/108 =6.72

So how are you
273 said:
borrowed the static and dynamic idea from CR.
Click to expand...
I always enjoy your thinking but I gotta admit I can't always connect the dots, and this is one of those times.
 
19DUSTER73 said:
Have you put any real thought into this? If so let's hear it.
Click to expand...



To me this all combo dependent, for the most part most builds use generic parts.
Take Edelbrock Heads and Air-gap and comp xe 275 or 285 cam it's a general decent combo that can go on many different short blocks it's not tailored for each short block and stands to reason some will work better than others but will that go from largest to smallest? And what with slightly different parts? If we maximize port shape cam timing etc.. For each combo what effect would that have? Or does static displacement will always win out or does it even? What if you factor gearing does that change the results?
 
Last edited:
AJ/FormS said:
I always enjoy your thinking but I gotta admit I can't always connect the dots, and this is one of those times.
Click to expand...

It's not about cr just borrowed the words static and dynamic
And applied them to displacement, don't think most people can see past static displacement what the cylinders physically measure. Not what it displace over time, aka running engine, Cubic Feet per Minute, and how different static displacements can have same dynamic displacements and engines of similar power have similar dynamic displacement "cfm".
 
Last edited:
AJ/FormS said:
So how are you
Click to expand...

Surviving lost my job over this Covid **** figuring out what direction I want to go in life.

I've enjoyed your point of view also, and the info you pass on with your experimenting and fine tuning your combination.

How's life in your part of the universe :?
 
273 said:
To me this all combo dependent, for the most part most builds use generic parts.
Take Edelbrock Heads and Air-gap and comp xe 275 or 285 cam it's a general decent combo that can go on many different short blocks it's not tailored for each short block and stands to reason some will work better than others but will that go from largest to smallest? And what with slightly different parts? If we maximize port shape cam timing etc.. For each combo what effect would that have? Or does static displacement will always win out or does it even? What if you factor gearing does that change the results?
Click to expand...
What exactly are you trolling for? The answers to your questions are all over this forum.
 
Rat Bastid said:
I don’t think he’s trolling. I think he is trying to get people to think in terms other than at least 4 generations of car magazine brain washing.
Click to expand...

I skimmed through the OP and it sort of bit me in the butt I guess. My answer to his questions are money and know how. Those two things are show stoppers in my book for Small cube, high rpm builds for a lot of us.
 
19DUSTER73 said:
I skimmed through the OP and it sort of bit me in the butt I guess. My answer to his questions are money and know how. Those two things are show stoppers in my book for Small cube, high rpm builds for a lot of us.
Click to expand...


You have to define small CID and high RPM.
 
273 said:
It's not about cr just borrowed the words static and dynamic
And applied them to displacement, don't think most people can see past static displacement what the cylinders physically measure. Not what it displace over time, aka running engine, Cubic Feet per Minute, and how different static displacements can have same dynamic displacements and engines of similar power have similar dynamic displacement "cfm".
Click to expand...
Oh yeah now I totally get you.
But, I think you can turn that around by thinking VE, it sorta comes to the same thing.

like if a 318 running at a particular rpm at 100%= a 360 running at 88%... but I think the physical dimensions of bore and stroke, will fudge the efficiency curves such that this can probably happen just over a particular rpm or at least, just a very small window of rpm.

I think the Wallace Calculator does a pretty good job of making this comparison, with their P/V(pressure to volume) index.
Wallace Racing: Dynamic Compression Ratio Calculator
V/P Index Calculation

stuffing the build specs of my engine into the Wallace I get this;
Static compression ratio of 11:1.
Effective stroke is 2.79 inches.
Your dynamic compression ratio is 8.79:1 .
Your dynamic cranking pressure is 184.87 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of PSI is 8.79 :1.
V/P (Volume to Pressure Index) is 159

now sticking a 440 Magnum in there, circa 1968, I get this
Static compression ratio of 10.5:1.
Effective stroke is 2.75 inches.
Your dynamic compression ratio is 7.97:1 .
Your dynamic cranking pressure is 162.75 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of PSI is 7.97 :1.
V/P (Volume to Pressure Index) is 157

Notice the nearly identical P/V numbers
They use this P/V number to compare the bottom end potentials of various engines, and indicate that engines with similar P/V indexs will have similar power... in what they define as; "the lower rpms" , and I assume to be, sub 3500
Here you can also see how the cylinder pressure contributes to performance in this zone; comparing my 367's CCP of 185psi to the 440 at 163psi.
Also notice that the effective strokes are very similar. My 367 is down to 286 effective cubes, compared to the 440 at 322 effective cubes, calculated from the effective strokes.

Now, I have never had nor driven a 440 Magnum, so I cannot say how powerful a bottom-end it has.
But you can come on over and take my car for a ride, and scare the crap out of yourself, lol, testing it's bottom-end.
They say there is no substitution for displacement, which, as for drag-racing which is all top-end power, that would probably be close to correct.
But on the street, IMO, there is no substitution for the P/V index

my previous cam ran this
Static compression ratio of 11.3:1.
Effective stroke is 2.86 inches.
Your dynamic compression ratio is 9.23:1 .
Your dynamic cranking pressure is 196.92 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of PSI is 9.23 :1.
V/P (Volume to Pressure Index) is 173
As you can imagine, with the stronger P/V of 173, I liked it a whole bunch more.
In this configuration, the effective displacement was 293 cubes
 
Last edited:
Rat Bastid said:
No LOL. I was asking for YOUR definition
Click to expand...
Oh lol. I was just making a generalized statement. I guess I don't really have a small cube high rpm definition. My 360 is small in some circles. It's normal to me. I'm pretty happy with the power it made but we had to turn it a little higher rpm than say a 408. You know what I've actually never been around a 408 built with the compression and camshaft used in my 360 so I'd bet the rpm gap would be tighter and the heads would shine more as a limiting factor maybe. I was comfortable pulling it to 7200 which isn't all that high. We didn't see any sense in going higher. Really I guess rpm is probably the killer for the average Joe. We all know rpm generally requires higher quality everything to live and make power. I had a few people during the build asking what rpm it was gonna turn. I was like idk, we will give it what it wants. I knew it would be up there a little bit. It made peak hp at 6700ish and torque at 5400ish I believe.

Judging by your first reply to me the op knows all this and this is more of a thought provoking exercise rather than an "I don't know why so tell me". That makes sense because he appears intelligent enough to figure things out.
 
19DUSTER73 said:
He can tell us.
Click to expand...

I'm not talking any particular displacement or rpm. Could 170 vs 225, 318 vs 360,
340 vs 360, 440 vs 500, 500 vs 543 could even be 289 vs 340, 273 vs 396 etc.. To narrow it down could say 360 vs 408 with same or similar top will the 408 always make more hp from say peak torque to peak hp ? And if so why can't the 360 spin up and extract the same hp out of the top end ?
 
19DUSTER73 said:
He can tell us.
Click to expand...

Plus I'm not trying to say small displacement where's it's at, For most especially street cars where drivablity and rpm is somewhat fixed idle to 5500rpm obviously cid is the main way to make NA power.
 
273 said:
I'm not talking any particular displacement or rpm. Could 170 vs 225, 318 vs 360,
340 vs 360, 440 vs 500, 500 vs 543 could even be 289 vs 340, 273 vs 396 etc.. To narrow it down could say 360 vs 408 with same or similar top will the 408 always make more hp from say peak torque to peak hp ? And if so why can't the 360 spin up and extract the same hp out of the top end ?
Click to expand...
I started off on the wrong foot here. Rat Bastid helped me out a bit. I think usually the 408 will make more power overall but I tend to believe the 360 may make more per cube. My 367 made around 1.6hp/CI I believe I would see more power with that top end on a 408 but I think the heads would be a limiting factor and it may not make the 1.6 or higher mark due to rpm limitations. All just guessing with limited experience here though. Are we starting with heads set up for a 360" or 408"?
 
Last edited:
19DUSTER73 said:
I started off on the wrong foot here. Rat Bastid helped me out a bit. Are we starting with heads set up for a 360" or 408"?
Click to expand...

What I'm trying to get at is the conventional wisdom here is if built same/similar the larger cid gonna always make more peak and under the curve hp and if this true I can't see what stopping the smaller cid spin up to catch up and make similar power since the top end is obviously capable. Everyone says it obvious and been explained to me a million times and I must be really dumb cause I haven't seen it or get it.

For the eg. It shouldn't matter as long the similar level.
 
273 said:
What I'm trying to get at is the conventional wisdom here is if built same/similar the larger cid gonna always make more peak and under the curve hp and if this true I can't see what stopping the smaller cid spin up to catch up and make similar power since the top end is obviously capable. Everyone says it obvious and been explained to me a million times and I must be really dumb cause I haven't seen it or get it.

For the eg. It shouldn't matter as long the similar level.
Click to expand...
Aren't we facing a CSA issue? And maybe what we are seeing when keeping the top ends the same is how hard the bigger cubes are working the induction system? Kinda why I asked what the heads were for. Put a max effort 408 top end on a max effort 360 and watch the power numbers come much closer or eclipse the 408 as the rpm capability is increased vs a smaller head setup for a 360. The guy building the his version of a max effort 360 may not want to see 8000 rpm though. I'm sure I'm a little unclear. I'm Mr. Mom today. I get the kids set up with an activity and here they come halfway through a post completely blowing apart my thought process lol.
 
To put this in very general terms, more cubic inches can burn more air and fuel, generate more combustion and make more power.
 
19DUSTER73 said:
And maybe what we are seeing when keeping the top ends the same is how hard the bigger cubes are working the induction system?
Click to expand...

That could be a reason, I've heard that engines that need more rpm to make similar hp aka smaller engines need larger port cross section then larger displacement lower rpm similar hp engine.

So if port shape and maybe some of the cam timing were tailored for the smaller cid
We would have equal capability ?
 
All things being equal there is no replacement for displacement. The end.
 
You must log in or register to reply here.
-

Back
Top