I'm not so sure about that calculator...it is telling me my DCR is 10.3! Maybe I am entering the data wrong but I have checked again and again. All I know is the valve timing at .050 Tappet Lift. Is that the problem?

 

In the write up it said to use advertised, not 0.05 lift. But in the examples, like the 252 cam one, he used 0.05 lift. I would really like to find a reliable way to determine DCR.

 

Right but I thought advertised duration was at .050 lift.

Further reading shows he wants the .006 durations.

 

he says he wants 0.006 but he used 0.05 when he calculated for the 252 cam

 

I got the numbers at .006 and now I'm coming up with 11.65 SCR, 8.77 DCR with .035 Quench. Ready to go.

 

Good stuff. So it is 0.006 lift then eh, gonna have to make a call to futral I guess

 

DCR is based on the intake and exhaust valve overlap. The more overlap the less DCR.

 

DCR is based on where your piston is when your intake valve closes (effective stroke). Not overlap.

 

My .02 is that SCR , camshaft events and cranking compression are really the only way to really tell how much you can run with pump gas. I have had engines with 11.7 -1 with big cams (245 @ .050" and up) run just fine. These would pump below 195 psi on the cranking compression test. Now other engines that may not be as high SCR but a much smaller, more efficient camshaft thats pumps higher (220 psi and up) will have a harder time utilizing 91 octane fuel. In other words it's going to want much less timing. I have personally seen a small cam gen 1 SBC with 11.5-1 pump 285 psi!!! Torquey as hell made great power but could only take about 27 degrees of total timing. The engine would damn near light off without the ignition turned on!

I guess what I am saying is that using SCR, Cam size, Valve timing events and cranking compression are a better way of seeing how a engine will take pump gas and most of that comes with experience. IMO there are too many variables in "calculating" DCR. Head flow, intake tract flow characteristics, exhaust system (to name a few) will all effect DCR.
According to this link http://members.uia.net/pkelley2/DynamicCR.html given above, DCR is calculated using valve closing events, crank stroke, rod length so on a so forth. This is fine when the engine is on the stand in your garage but when it's running those events are happening SO fast even when cruising speed that there is no way those numbers will apply to the engine in real world conditions. The air flowing through the intake tract is moving so fast that it will not fill the cylinder that same as is does sitting on the stand, therefore rendering the calculations useless.
The only way to actually measure real DCR is with sensors in the combustion chamber to measure actual cylinder pressure. So unless you have Indy or F1 racing technology at your disposal you probrobley would not be able to accurately measure DCR.


So using SCR, Cranking compression and Valve timing events will give you the best guesstimate to how far you can push pump gas.

 

Amen!

 

I've done:
12.25:1
93 octane
254/260 Cam 111 LSA

Combo went low 10s @ 13x.
Phil

 

There are too many other factors involved to give a "this is the max compression for a given octane". Cylinder pressure is one of the bigger factors. For example, if you take two engines, Engine A with a good combo that makes high cylinder pressure, and Engine B with a bad combo that makes lower cylinder pressure than Engine A. The most CR you can run in Eng A is 11:1 before detonation but Engine B could run 12:1 because it doesn't have as much cylinder pressure to deal with.

 

^^Agreed^^ Thanks all for the input....

 

Yea mine was right around in that area. Tuned by the same dude with the same results.

 

Dont forget that the combustion chamber also comes into play, squish, quench.. etc..

Different styles of combustion chambers have different benefits