What is your DCR? Your cam has everything to do with your Dynamic Compression Ratio. Shifting around the lobes will definately alter your DCR and you may end up with a car that may not perform as you vision it.

 

How do you measure or calculate DCR? and how does the cam lobe effect it?

I am shooting to get a CR of 8.3:1, but I have to do a search or something to figure out what DCR is and how it is to be taken into consideration.

Thanks for the info

 

Ok I found out some info here http://members.uia.net/pkelley2/DynamicCR.html about DCR, but after reading it I am still a little confused, what would be a good DCR for a forced induction motor. or how can I calculate DCR + boost to get near what is recommended? they said a good # is around 8.25:1 DCR, I can't download any of these calculators because I am at work and these machines are not able to accept downloads.

thanks for the info and help

 

Depends on the amount of boost you are running. If it's high boost and you are planning pump gas you want as low a static CR as you can reasoanbly get unless you are going to be running a HUGE cam with a very late itnake closing point (ie a cam that will drastically lower your DCR.). The combo you describe sounds about right.

Rich Krause

 

The DCR range is between 8.5 - 7.5. It is best to edge to the lower end of the range for boost applications. The upper range is for NA and/or N2O. At home, download the calculator and it should help you with Head chamber volume, head gasket thickness, quench, etc...

 

I think even 7.5 is quite a bit too high a DCR for a high boost setup. My current combo has a DCR of 6.2 and a static CR of 8:1.

Rich Krause

 

Don't some of the DCR calculators have the ability to include the lbs of boost in the calculation? At least i think i recall coming accross one at some point. If so then you would be going for the same target DCR for the grade gas you want to use.

 

Read the definition of DCR in the link again. Calculation of the DCR is a simple mathematical function determined by the geometry of the engine. It has nothing to do with what happens when the engine is running. IOW, boost, VE, ignition timing, etc. all have no effect on DCR. These things (and more) do effect cylinder pressure though.

Rich Krause

 

I see what your saying about the definition Rich. I didn't visit that link, I was just recalling something from when i was reading up months ago.

For what its worth here is the link i was talking about, i found it again pretty easily.

I guess their calculator takes it a step further and incorperates operating conditions, which isn't consistent with the definition of dynamic compression being used by others. They also recomend using the .05 duration instead of .006 which can have a large effect on the number it spits out.

-brent

 

I have seen that calculator but am not sure what it is purporting to calculate. Is the product an attempt to determine the static CR required to produce the equivalent cylinder pressure? Or the dynamic equivalent? Or what?

Rich Krause

 

Obviously i had nothing to do with developing that calculator, but here are my thoughts on it.

An estimation of cylinder pressure is ultimately what we are interested in to find out if the grade of fuel we desire to run will autoignite under that condition. CR is a ratio of volumes, which does not directly relate to the pressure due to temperature changes, but it is proportional. Raise CR, and your cranking cylinder pressure goes up. Same thing with DCR which is the actual 'Trapped Volume'.
DCR is an attempt to estimate what the cylinder pressures will be by adjusting the volume for the IVC but without accounting for variations in head flow. If you had really crappy heads with a low VE then you could run a higher compression ratio without detonation right? Of course there are other things that contribute like chamber design, but DCR infers the cylinder pressure and thats why we look at it right?

I guess it all boils down to picking a intake vavle closing point close to that of another known configuration that doesn't Knock. The rest of the cam selection is about tuning for the inertial effects of the air to maximize VE.

 

94formulabz,

Thanks for showing me that it was kindof what I was asking about, even though it was set up for a motorcycle turbo calculation it still seems to work.

How do you get the Deg that the intake valve closes?
For example would a 230* advertised duration actualy be 230-180(to get to botom center) = 50* ABDC or is there a calculation to determine it?

Thanks for all the help and suggestions everyone.

 

if that is how I was suposed to get X* ABDC then this was my output from that calculator
That said I should have a CR including boost and altitude conditions added to static = 11.86 :1

So that means I should be fine to run up top 10# of boost and still run pump gas and up the boost at the track and use race gas.

Is this correct???

 

That looks ok to me. You will still need to be careful with tuning. Not run too much timing and be sure the AF ratio is good (not leaner than 12.5:1, maybe a bit richer). Water injection also helps.

Rich Krause

 

Rich,
thanks for the help, I am going to need to follow your lead and run 8:1 static CR to be able to run the pump gas.

Another question for you, since you run a 95 also, what have you found to be the best scan tool to read O2's (or AF ratio) for tuning and such, I have used free scan to read everything and I do my own (NA) tuning with tunercat, but jumping into FI, I don't know how much I can trust free scan since it wasn't even designed for this car.