low CR = lag ?

[BRY]

Static compression ratio is calculated using the measurments from your engine and saying that is what it will achieve.
Actually you have a turbo forcing air into your engine under pressure, therefore, you have compressed the air in the cylinder. At atmospheric pressure the volume of this trapped air would be greater than the volume of your cylinder, so what use is the original compression ratio which calculated the volume at atmospheric pressure?
Am I making myself clear, coz I don't think so....
Google it
A team -cheeky fecker
David Vizard more like

[mcandmar]

Have a play here, http://www.not2fast.com/turbo/compre...pression.shtml

When you fill out all the figures i get a 2226cc 5cyl with 9.29CR, the trapped CR is 8.18CR (havent checked the cam timing figures yet, but thats just an example)

http://www.empirenet.com/pkelley2/DynamicCR.html Has a good explanation.

I've been trying to get my head around this for a while now for my engine build. .5mm overbore with the head and block skimmed, so what pistons do i order, how much of a dish etc arrggghhh

[Billman]

Dynamic CR is the CR of an engine when we have the already compressed air thats coming from the turbo!

i.E. a 9:1 CR Turbo engine the Actual CR during full throttle can be ca. 14:1 ! This second CR is called Dynamic CR!

We have Copressed Turbocharger air that is a second time compressed by the Action of the Piston!

[S2central.net]

OK - so this dynamic CR stuff is just a correction for the fact that the engine is inhaling compressed air - as opposed to air at ambient pressure on a non-turbo engine. Small light bulb appears overhead - I'll have to google it later for more info.


Paul

[S2central.net]

Actually - no - its more about valve timing according to http://www.empirenet.com/pkelley2/DynamicCR.html

quoting here "Compression Ratio (SCR) is the ratio most commonly referred to. It is derived from the sweep volume of the cylinder using the full crank stroke (BDC to TDC). Dynamic Compression Ratio, on the other hand, uses the position of the piston at intake valve closing rather than BDC of the crank stroke to determine the sweep volume of the cylinder."

So DCR is a static value as well (always lower than SCR) - calculated from knowledge of cam timing. DCR doesn't change thru different operating conditions of the engine.

Paul

[Greg_S]

I also understood that having a higher static compression ratio would help off boost performance when the engine hasnt spooled the turbo enough where it is acting more like a NA engine.

[Billman]

I also understood that having a higher static compression ratio would help off boost performance when the engine hasnt spooled the turbo enough where it is acting more like a NA engine.

Thats correct!

[S2central.net]

OK so what is our conrod length - Anybody have that data to hand ?

I can throw some Excel together to play with numbers from the calculations described for DCR.

Dangit I need cam timing as well for various options - will have to hit the Audi engine manuals when I get home.

Paul

[BRY]

Actually - no - its more about valve timing according to http://www.empirenet.com/pkelley2/DynamicCR.html

quoting here "Compression Ratio (SCR) is the ratio most commonly referred to. It is derived from the sweep volume of the cylinder using the full crank stroke (BDC to TDC). Dynamic Compression Ratio, on the other hand, uses the position of the piston at intake valve closing rather than BDC of the crank stroke to determine the sweep volume of the cylinder."

So DCR is a static value as well (always lower than SCR) - calculated from knowledge of cam timing. DCR doesn't change thru different operating conditions of the engine.

Paul

Actually Yes - depending on where you look
This is a different view

Connecting rod length and intake valve closing are used to compute trapped compression, i.e., how much of the stroke is really used to compress the intake charge. (Trapped CR is often called "dynamic CR," but the use of "dynamic" is completely incorrect, this measure is static and is a result of engine geometry and not its dynamic behavior; a true dynamic compression ratio would take into account the VE, gas flows and resonances in the system to determine the "swept volume.")

[Greg_S]

Paul, Not 2 fast had a calculator on their site already if that helps.

Have a play here, http://www.not2fast.com/turbo/compre...pression.shtml

[S2central.net]

Yeah but I can grow my own in Excel the way I want it - piece of cake when the calculations are known.

PS - The search function works well on the forum. Here are the important Intake ABDC values needed for comparison purposes.

3B/AAN/ABY = 25 degrees
NM = 32 degrees
ADU = 35 degrees
7A = 38 degrees

Paul

[S2central.net]

And I think stock rod length is 144mm according to some posts I found.

Paul

[Keeper of The Kitten]

The descriptions above sounded a lot like VE - and as was pointed out, you can get better than 100% even in a NA engine.

Dynamic CR is different to static CR, and the amount the piston compresses the intake charge is definitely different.

Static CR would be true by the following formula, approximately:

(swept volume + combustion chamber volume) / (combustion chamber volume)

...this assumes 100% VE.

However as soon as you boost the charge
into the engine, the total volume of charge to be compressed has increased dramatically - but the chamber volume hasn't.

So the dynamic CR would be increased accordingly, something along the lines of:

(swept volume + combustion chamber volume) x (absolute pressure measured in bar) / (combustion chamber volume)

...this means an engine with 1bar boost would effectively be near to twice the CR

Well, that's how I see it anyway - and it explains why we need super fuel and lots of ignition advance.

__MDC__

[S2central.net]

So to summarise results for DST (Dynamic Stroke Length) with stock engine internals we have the following. DST is the operating piston stroke whilst the intake valve is closed.

3B/AAN/ABY = 83.514
NM = 81.667
ADU = 80.735
7A = 79.719

The corresponding DCR values are then as follows:

3B/AAN/ABY = 9.02
NM = 8.84
ADU = 8.75
7A = 8.66

These were calculated with an assumption of 53.65cc chamber volume - which is correct with stock engine on paper gasket to yield static CR = 9.3:1. With a metal head gasket the DCR values would be lower by nature of the extra 1 or 2 cc chamber volume.

Note that the use of the longer duration camshafts reduce the DCR (Dynamic Compression Ratio). The DCR values are all on the high-end of the recommendations from that Pat Kelley website - that would explain the need for REDUCED static CR to sustain aggressive ignition timing and good knock resilience at high boost levels on remapped monster turbo conversions.

I just love it when the real world lines up with the theory...

With 7A cams and 56.65cc chamber volumes (3cc above stock paper gasketted setup)you get a DCR of 8.25... which is about the safe maximum recommended by Pat Kelley. This correlates with a static CR of 8.8 to 1 on our 2226 lumps.

Paul

[S2central.net]

Anybody else doing the calculations - please note the sine and cosine functions are done in RADIANS - not degrees

I can share a spreadsheet on this if anyone wants it - the CR calculations in the Pat Kelley tool are useless without knowing umpteen other variables on engine internals.

My spreadsheet requires knowledge of bore, stroke, intake cam close angle, rod length and combustion chamber volume.

Cheers,

Paul