Pump gas compression

[6pkScamp]

6pk,

I am running a pretty close combo to yours, with 10.6 compression. I run 92/93 octane with about 24 initial, 34 total. Cam is Hughes solid flat #5256. Have about 1500 miles on the engine, so far so good. You can see all my specs in the member photo section. By the way, love the stance and paint of your ride.

Thanks Bro, I appreciate that

Checked out your ride, very sweet

 

[Woods74]

This should get you very close, pick the cam, then enter the intake closing measurement (ABDC) To the rest and you'll get your dynamic ratio.

http://www.wallaceracing.com/dynamic-cr.php

This thread will help too:

http://www.forabodiesonly.com/mopar/showthread.php?t=46600

:burnout:

 

[Mo-Par]

This should get you very close, pick the cam, then enter the intake closing measurement (ABDC) To the rest and you'll get your dynamic ratio.

http://www.wallaceracing.com/dynamic-cr.php

This thread will help too:

http://www.forabodiesonly.com/mopar/showthread.php?t=46600

:burnout:

x2

 

[Lonewolf3165]

No - I think you're just fine. I think you're stopping motion in your head. At peak torque it sure as hell does bleed pressure - but it does it only out the exh port rather than out both ports like it does at idle or low rpm. Plus the "bleeding" action along with the exhaust system evacuation and intake ramming over-fills the available space. So while it bleeds some it's overfilled anyway. So the net change is an inctrease in pressure. But again - the ratio of that volume to be compressed never changes.

cylinder filling % does not = compression ratio which is exactly what I mean. It equals the difference of the cylinder filling as measured by the dyno sensors, and the calculated empty volume - and - that % will rise as the engine approaches peak torque and can remain for rpms after peak torque. It should be well beyond 100% VE at peak torque.

edit - this is all assuming that the engine in question is built properly and has no inherant detonation or preignition inducing features and is being run on the proper fuel.

It's all in the name...lol. as a point of review (and it sounds like you already get this part) - "Static" is the ratio of the volume of area swept by the piston when it's at the bottom of it's bore plus the volume of the chamber and gasket and area on top of the top ring vs. the volume of the piston at the top of it's travel plus the volume of the chamber and gasket and area on top of the top ring. In both positions the cylinder is sealed - both valves closed.
the issue is - the cylinder isn't sealed when the piston is at the bottom of it's travel. Both the exh and intake valves are open. So pressurization doesn't start happening until after the intake valve closes as the piston's moving up the bore on the compression stroke. The "Dynamic" ratio takes that closing event and the rod ratio into account and gives the builder a much better picture of what might be happening in the cylinder. The reason the others are saying the rest of the combo is so crutial is there is a whole bunch of physics that happens in a running engine that is based on what the cam, intake tract, and exh tracts are comprised of. In a running engine - the more efficient the ports and airflow is, the more intake charge is drawn and pushed into the chamber each time. On a racing engine that effectiveness comes at a higher rpm range where the physics works better. So while you can say there's "x" amount of pressure based on the formulas, that amount will rise in a properly designed intake system and the cylinder pressure will go higher than calculated.

Now to answer you - in my opinon - you're saying at peak torque the compression ratios are the same. I don't believe that's the case. The ratios never change. The volume of intake charge that is compressed does. Static and dynamic ratios are a calculated means for a builder to base decisions on. Cylinder pressure and volume of intake charge in the cylinder are (or can be) empirical measurements. That is exactly what volumetric efficiency is calculated from: the measured amount of torque made from a measured amount of air and fuel taken in.

I had this very same discussion(read: argument) :argue: with my brother a few weeks ago & he got all pissed off at meain10: & hung up the phone!:violent1: LOL!

 

[moper]

That's what a forum is all about - discussion. Disagreements will happen but shouldn't be a big deal. I would like to hear others' thoughts on the subject tho - as would Rob I'm sure...

 

[Lonewolf3165]

That's what a forum is all about - discussion. Disagreements will happen but shouldn't be a big deal. I would like to hear others' thoughts on the subject tho - as would Rob I'm sure...

I was trying to make the same point with him as you stated moper. I think he thought I was talking about VE & cylinder pressure because he kept brining up the issue of intake ram effect & density etc. He tends to detonate :angryfir: pretty quickly if I ask him a question & he doesn't have an answer for it. LOL!

 

[blue missile]

First of all listen to MOPER,
then consider this chart from the petroleum institute: This is based on DCR or true compression ratio.
Pat Kelly has a good DCR calculator, google it



Compression Octane Number Brake Thermal Efficiency
Ratio Requirement ( Full Throttle )
5:1 72 -
6:1 81 25 %
7:1 87 28 %
8:1 92 30 %
9:1 96 32 %
10:1 100 33 %

Sorry the table wont copy correctly

Andrew

 

[Dusterguy]

I found this in other forum. This guy have same problem to understant this like i have.

"The problem with dynamic compression ratio is that is doesn't take into account the pressure waves of the intake, exhaust tuning and the velocity of the airflow into the cylinder. The peak pressure (which we use to determine safe octane/timing) depends on how much air gets into the cylinder and compressed to TDC for combustion. Dynamic compression may tell you something at idle and low speed but when the engine is operating in it's power band it tells you very little. If Dynamic compression ratio relies on some of the air slipping back out of the intake valve because the valve is still open when the compression stroke starts. Cams with big duration and overlap would allow lots of air to slip out using this theory. The problem is that the air doesn't slip out. Instead it has developed momentum and is forced in even though the piston is rising. It's forced in through a combination of momentum and pressure waves in the intake and exhaust track. Indeed if a cam were to allow the air to slip out at high RPM the engine would not produce the power you want.

The required information for what you want is volumetric effieciency. Even with a very radical cam a well tuned engine can achieve a VE of 1.0 This means that even though the dynamic CR is low the cylinder pressure due to the factors above gets all the way up to what it would be assuming the static CR.

Yes race cams work better with High CR but you also need race gas if you have a well tuned engine for the cam.

Actually a good indication of VE is torque. If you look at the torque of a 2.2 or 2.4 T, E, or S engine you'll see that despite a lower Dynamic compression ratio the S an E have higher torque than the lower performance versions. This indicates that they have higher cylinder pressures not lower as you would assume from dynamic compression numbers.

Hope this makes sence to you.

-Andy"

 

[moper]

the short short version:
On the first part what he's referring to (pressure not escaping) is only true under two conditions as I understand it: at WOT and at peak torque. I agree 100% and think he's dead right. But - below peak torque, or at less throttle I do not believe that is the case. That's why there are generalities associated with both the calculators and choosing the parts. If you've heard of or seen reversion - that's pressure escaping up the intake tract and it can happen as high as 1000rpm below peak torque on a large cammed race engine. Nothing can completely simulate a running engine - which is why we still use engine dynos.
Torque is the best indicator of VE because it's the only empirical measurement. Horsepower is torque over time and is only calculated. I'll add that VE at peak torque in my opinion should be greater than 1.0. Because a 1.0 (or 100%) VE number represents a complete filling of a volume that includes the total swept area and chamber and a complete burning of that intake charge. However with a good exhaust blow down and a well designed and efficient intake port (meaning all the way to the throttle plates) and "the physics" working it will fill the cylinder beyond 100%. A good natural aspirated performance engine's VE will exceed 1.15 and some go as high as 1.25 plus at and above peak torque.

I think he's talking about a Volvo engine in that last part? I do think some things are comparable but anything that is not naturally aspirated, or varies from the 2-valve, over head valve, single plenum configuration would be a different animal in terms of expected numbers and I wouldn't call it apples to apples. not because the physics is vastly different - but because he said look at the numbers as a reference to back up his comments and I'd expect the numbers to be very different by virtue of the basic design.

 

[1Fast340]

i like the way dusterguy thinks,he is thinking outside the box
from my theoretical point of view one reason that you can get away with that high dynamic presure at max VE or peak torque is probably that there is less time for detonation to actualy take place.