What info is needed to calculate this? I'll be running 103/104 octane fuel and will have a 12.1 SCR. The cam is a soild roller 249/254 @ 0.050 108LSA with a 106 ICL.

Randy

 

There are a number of online calculators and spreadsheets (including at least one accessible on this message board that will assist you but if you want a down and dirty one that will tell you what information you will need to use on any of them, try the one at:

http://kb-silvolite.com/calc.php?action=comp

I've used several of the various spreadsheets that are available and (as the TV commercials say) "the results may vary." Usually the variations are minimal and you're always looking for a ball park number at the outset anyway.

When you get to the track or the local cruise night you can always do what everyone else does, add a number or two for effect.

Good luck

 

Thanks!

Randy

 

My DCR calculated out to be 9.533, is that to much?

Randy

 

Be sure that the parameters you entered are precise. You can't assume, for example, what the closing point for the intake valve will be without rolling the motor over with a degree wheel and observing it directly. You'll also want to pour your combustion chambers and piston valve reliefs. (I'm not finding fault with your procedure, just pointing out that a small deviation in some of the figures can make a significant difference in terms of the outcome.)

The last NHRA Stocker motor I did came out to 9.28 DCR on the same calculator. It will pull a 3000 pound car to mid-10 second runs. That's enough to require hopped-up gasoline but, then, you mentioned that you were going to run good gas anyway. Note that it's possible to attain a relatively high dynamic number without using exotic parts. Nothing is mentioned about gigantic camshafts (I use stock lift camshafts that fall within NHRA Stock rules), gigantic ports (I'm limited to blue-printed heads), mega-cubic inches (.060 clean-up bore), over-sized throttle bodies or anything else. Common sense and an understanding of what constitutes a workable combination are crucial ingredients to bringing it all together.

Good luck

 

I understand what you are saying and I really appreciate the help. The info that I used to calculate the SCR/DCR were directly from my engine builder and head porter. I'll give you the info and let you look and see if there is something out of the ordinary.
Head vol 53cc
Bore 4.030
Stroke 3.75
Gasket thickness 0.039
Gasket bore 4.155
Deck height 0.021
Piston vol -5cc valve relief
Rod length 6.00
Intake valve close (ABDC) 0.050 50.5 degs

 

you bring up something that I've been wondering about. We've been told on here for years that for a pump gas engine, keep the Dynamic Compression Ratio around 9:1 or less. But what about for an engine that will see higher octane fuel?

 

You need to also use the @.006 duration numbers, not the @.050 ones. I like this calculator better because it also gives you the valve timing events and overlap.

http://www.empirenet.com/pkelley2/DynamicCR.html

 

Yes, DEFINITELY use the .006" numbers! 9.5 sounds WAY too high for a cam that big with only 12:1!

 

In order to be more precise in the calculation it will be necessary to have all the cam timing events from the cam as it's installed in the engine. One of the other calculators I've used includes observed cam timing not only at .050 but also at .006 and .200. Cam cards are not completely reliable, espcially when you've advanced or retarded the cam. When it's done with another calculator, the outcome will be different although not dramatically so.

More octane is required to utilize higher compression. If you're willing to pay for hot gas you can bump the compression substantially. Next time you go to an NHRA national event ask the guys running Competition Eliminator about their compression and the gasoline they run.

 

I got my .006 valve events, and they are IVO 35.5, IVC 67.5, EVO 74, EVC 34.
I believe that puts me at a DCR of 9.4, now with that being said if it's correct what octane fuel should I use?

Randy

 

That's an early IVC for such a large cam. I guess it was made for less compression. I'd like to know why it was designed that way and if it'll work as well.

 

Just to let everybody know this is a race only engine. I can easily run as high a 107 for fuel and I feel this will be sufficent. I believe it will make really good power, but if not, it won't be the first time I've had to change a cam. Thanks for all you guys imput to my ?.

Randy