Figuring out Dynamic compression

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pjc360

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I've got a 300hp crate 360 magnum in a 91 short bed 4x4 truck.
Im curious as to what the dynamic compression ratio is on this engine.
I live at 4000 feet above sea level, and when doing a compression test with the engine at operating temp and the carb wide open I get 150psi across the board on every cylinder.
In told this number would be a little higher at sea level, how much higher I don't know, but doesn't it help to determine what the dynamic compression ratio is by determining cranking compression psi?
 
Short answer - nope.
The dynamic ratio doesn't change and can be figured based on measurements and cam timing events. Actual cylinder pressure will change with rpm so what you have at the starter motor cranking rpm is only valid for that rpm.

Edit - I'll add the MP 300hp 360 crate was a factory 5.9L Magnum with it's stock camshaft, equiped with an intake and carburetor.
 
So do you have an idea of what the factory dynamic compression ratio is on a 5.9 magnum?
 
You need to give us static compression plus the camshaft's advertised IVC event for the DCR.
 
RustyRatRod said:
You need to give us static compression plus the camshaft's advertised IVC event for the DCR.
Click to expand...
Subscribed....I get it,but I don't get it... The intake valve on the closing ramp,against the actual measurements of mechanical compression,are the two major principles..Correct? Sorry for the intrusion,..Maybe if the o.p. learned as well,it would be mutually beneficial...?
 
moper said:
Short answer - nope.
The dynamic ratio doesn't change and can be figured based on measurements and cam timing events. Actual cylinder pressure will change with rpm so what you have at the starter motor cranking rpm is only valid for that rpm.

Edit - I'll add the MP 300hp 360 crate was a factory 5.9L Magnum with it's stock camshaft, equiped with an intake and carburetor.
Click to expand...
Stock 95 Magnum 360 cam specs,from Dakota/Durango late model website....
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HankL
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Like said before in the MP tech line quote, the 5.9/360 V8 Magnum cams seem to have changed a little nearly every model year, perhaps for emissions reasons. In 1998 remember that the HP bumped up - so the above cam specs by MeanGreen98Machine seem about right.

The specs that I have been using for a 1995 5.9 V8 Magnum as a 'best guess' are:

Intake "Advertised" Duration 250
(above is about 197 degrees at 0.050 inch way of rating aftermarket cams)
Intake opens advertised 6 btc
Intake closes advertised 64 abc
Maximum intake valve gross lift 0.385 inches

Exhaust "Advertised" Duration 264
(above is about 206 degrees at 0.050 inch way of rating duration on aftermarket cams)
Exhaust opens advertised 53 bbc
Exhaust closes advertised 31 atc
Maximum exhaust valve gross lift 0.401 inches

The above specs came from the 1996 Ram FSM.
 
It's a 300hp crate 360 magnum, still has the same camshaft that came in the engine.
Advertised compression ratio is 9.0:1
Cam specs are same as what abodybomber shared.
 
Abodybomber said:
Subscribed....I get it,but I don't get it... The intake valve on the closing ramp,against the actual measurements of mechanical compression,are the two major principles..Correct? Sorry for the intrusion,..Maybe if the o.p. learned as well,it would be mutually beneficial...?
Click to expand...

the static ratio is the actual volume the piston squishes, assumign the cylinder is sealed when it's at bottom dead center. Only that's not how an engine works. So dynamic takes into account the time at which the intake valve closes, actually sealing the cylinder. That's why it's always less than the static ratio. However, the OP asked about pressures. Cylinder pressures are dependant on that valve closing, but also the physics involved with filling the cylinder. The higher the rpm, assuming the port and camshaft work well, the more the cylinder will be filled. So the higher the rpm, the higher the pressure, until the system stalls and volumetric efficiency falls off.
 
moper said:
the static ratio is the actual volume the piston squishes, assumign the cylinder is sealed when it's at bottom dead center. Only that's not how an engine works. So dynamic takes into account the time at which the intake valve closes, actually sealing the cylinder. That's why it's always less than the static ratio. However, the OP asked about pressures. Cylinder pressures are dependant on that valve closing, but also the physics involved with filling the cylinder. The higher the rpm, assuming the port and camshaft work well, the more the cylinder will be filled. So the higher the rpm, the higher the pressure, until the system stalls and volumetric efficiency falls off.
Click to expand...
O.k ...a different way, of mathematically calculating volumetric efficiency....Appreciated.....
 
Maybe I didn't say it right if that is your conclusion...

"The intake valve on the closing ramp, against the actual measurements of mechanical compression, are the two major principles..Correct?"

They are two major "principles" yes. But it's not one against the other in some ratio if that is what you mean.
A compression ratio is the difference between measurements of given volumes of a sealed cylinder. If the volume of the sealed cylinder with the piston at BDC is "x", and the volume of that same sealed cylinder with the piston at TDC is "y", the ratio is x divided by y. Static compression ratio is just that. The Dynamic ratio takes into account where the piston is in it's travel up the bore in relation to when the intake valve closes and at what point that same cylinder is actually sealed. Ratios can be calculated and do not change. Call it a variable that is designed in and effects the reading.
A compression reading is the measured output of that design. It is dependent on those physical mechanical relationships, but it is also dependent on the physics of a running engine - or cranking engine as it really would be. Things that cannot be duplicated or predicted with 100% accuracy. As an output that is partially based on the physics, the speed with which it's being turned will change the result.
 
moper said:
Maybe I didn't say it right if that is your conclusion...

"The intake valve on the closing ramp, against the actual measurements of mechanical compression, are the two major principles..Correct?"

They are two major "principles" yes. But it's not one against the other in some ratio if that is what you mean.
A compression ratio is the difference between measurements of given volumes of a sealed cylinder. If the volume of the sealed cylinder with the piston at BDC is "x", and the volume of that same sealed cylinder with the piston at TDC is "y", the ratio is x divided by y. Static compression ratio is just that. The Dynamic ratio takes into account where the piston is in it's travel up the bore in relation to when the intake valve closes and at what point that same cylinder is actually sealed. Ratios can be calculated and do not change. Call it a variable that is designed in and effects the reading.
A compression reading is the measured output of that design. It is dependent on those physical mechanical relationships, but it is also dependent on the physics of a running engine - or cranking engine as it really would be. Things that cannot be duplicated or predicted with 100% accuracy. As an output that is partially based on the physics, the speed with which it's being turned will change the result.
Click to expand...
Thank you, Moper!
If 8.5:1 was the perfect set in stone ratio, we could just have 8.5 static, and close the intake valve at BDC every time, for every engine. Voila! Problem solved, except for that pesky thing called inertia.
 
That gauge pressure at 4000' altitude works out to a DCR at cranking speed of approximately 8.3.

There IS a relationship between cranking pressure and DCR. You can take the DCR and multiply it by local atmospheirc pressure that has been 'modified'. Then you subtract out actual atmospheric pressure to get the actual gauge reading.

This 'modification' accounts for the fact that pressure rises more than the mathematical DCR number. When compressed, air gets hot which in turn raises the pressure even more; this is expressed in what is known as an adiabatic relationship. Anyone who has felt a compressor tank get hot as it fills with compressed air knows this. There are also modifying effects from ring sealing, etc. The 'modified atmospheric pressure' is just the actual atmospheric pressure raised to a numerical power, typically taken as between 1.15 and 1.2 for cold engine computations, and typically ends up being in the range of 20 to 22 psi for common elevations for cold engines.
 
nm9stheham said:
That gauge pressure at 4000' altitude works out to a DCR at cranking speed of approximately 8.3.

There IS a relationship between cranking pressure and DCR. You can take the DCR and multiply it by local atmospheirc pressure that has been 'modified'. Then you subtract out actual atmospheric pressure to get the actual gauge reading.

This 'modification' accounts for the fact that pressure rises more than the mathematical DCR number. When compressed, air gets hot which in turn raises the pressure even more; this is expressed in what is known as an adiabatic relationship. Anyone who has felt a compressor tank get hot as it fills with compressed air knows this. There are also modifying effects from ring sealing, etc. The 'modified atmospheric pressure' is just the actual atmospheric pressure raised to a numerical power, typically taken as between 1.15 and 1.2 for cold engine computations, and typically ends up being in the range of 20 to 22 psi for common elevations for cold engines.
Click to expand...

That makes sense, so with every cylinder reading 150psi at my elevation of 4000 feet above sea level, what would the reading be at sea level?
I've been told the thinner air at 4000 feet above sea level will put out a lower cranking psi on the gauge vs if it were at sea level.
 
This is an old post I put up when I pulled the heads for porting and cam upgrade. All specs were measured carefully. Your static CR has to be the same.

Thought I'd share some specs. on Magnum 360 motor.
93 block OEM crate motor 360/300hp The block is not drilled for head oiling.
Cam 195@050 .433 lift Motor pulls to 4800 and is done.
Upgrading cam and took the heads off today to measure everything.

Heads
True OEM rocker ratio is 1.66
Valve springs and retainers are MP crate motor single with damper.
Retainer hits valve guide seal at .545 -.050 req clearance = .495 max lift as is. I'll cut the guides down as required.
Exhaust valve stem .311 Intake stem .312 outer guide dia .530
Spring installed height is 1.6 so it looks like I'll have to cut the spring seats .050 or get some +.050 retainers.
I'll take the springs to the machine shop tomorrow and see what open and close pressure is.

Compression 8.973
Pistons .053 in the hole. I used 11.23cc for the dish (published spec)
OEM head gasket was .040 compressed 4.02 bore
heads cc'd at 62

Motor only has 10K miles so I plan on a quick bowl clean up, port match intake, and minimum push rod pinch work.

Hope this helps some folks, there's a lot of different info on the web.
 
pjc360 said:
That makes sense, so with every cylinder reading 150psi at my elevation of 4000 feet above sea level, what would the reading be at sea level?
I've been told the thinner air at 4000 feet above sea level will put out a lower cranking psi on the gauge vs if it were at sea level.
Click to expand...
That is right. Your setup would end up around 175-180 psi at sea level.

Just be aware that this is all computed numbers; at least in your case, there is a 4000' number to start from. But there was a recent discussion where these type of computations showed maybe 175 and a poster (an expereinced guy BTW) swore by a consistent 195 psi. So that 'modification' factor to the pressure rise is variable; I forgot to ask in that thread if those readings were hot. I assume your readings were taken cold....Yes? No?
 
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