Compression, Not your average Engine Options

[Mblizzard]

Ok so as many know, I an building my dream engine. With lots of Type 4 Store goodies including Nickies, 78 mm crank, and 102mm P/Cs, LE-200 heads and a decent cam, I am looking at where I want my compression to be.

With a normal engine and quality components it seems that 8.5 to 9.5 is about the limit before heat and detonation become issues.

Don't have any measurements yet but playing around with some calculations and assuming a low deck height because I have a slight dish and or valve pockets in the pistons I could easily get to over 11 to 1.

But I am think that might not be a long lasting engine. Looking at going for 10 to 10.5.

Thoughts? (IMG:style_emoticons/default/blowup.gif)

[PlaysWithCars]

Remember that the cam makes a difference to how much compression the engine can tolerate. More aggressive cam grinds generally allow you to (and require you to!) run more compression, in part because of the overlap where both intake and exhaust valves are open at the same time.

Things like bore size also play a significant role. (I think that has to do with the propagation speed of the combustion flame front, but I'm on very shaky ground with that memory!!!)

Lots of variables, is what I'm trying to say.

--DD

Dave is spot on. With a more aggressive cam you can run higher static compression ratios because the overlap of the cam lower the dynamic compression ratio at low RPMs where you have a higher risk for detonation.

Larger diameter cylinders increase the risk for detonation because it takes longer for the flame front to travel across the whole cylinder. During the longer burn period the piston is still moving up to top dead center and increases the cylinder pressure to a point that it could detonate.

Changes in compression ratio and/or cylinder bore will change the ideal ignition curve as well.