Deck height vs. CR

BenI · 06-19-2003, 11:23 AM

Could someone please explain to me why it is better to keep deck height under 0.060" despite a high CR? I am building an engine and cannot find an explaination for this, just that deck height should be kept between 0.045-0.065" "for performance reasons." On my engine, my heads are cc'd at 47, so running a 0.06" deck height means a CR of around 9.7:1. However, ecerything I have read makes me think I want no more than 9:1. To make matters more confusing for me, I live at 5000ft so the air density is 86% that at sea level. This makes me think I can get away with higher CR, but 9.7:1 seems too high to me. Help?

Ben

PS-for completeness:

2056cc in a 914
71mm stroke, 96mm KBs
42x36mm valves
Weber 163/86b cam (this might be wrong, but it's close)
Dual Weber 44 IDFs
Bursch exhaust, SS heat exchangers

John Rogers · 06-19-2003, 01:06 PM

I run 10.5 to 1 in my engine and so far have had no problems because of it. Of course I use 76 100 octane all the time. At your elevation (Denver) I imagine you could run 92 on the street and use 100 octane for track events.

Dave at Pelican Parts · Dave at Pelican Parts's Garage

If the deck is tighter, it pushes the fuel (and air) more into the hollowed-out part of the combustion chamber. Right next to the spark plug--so the fuel can be ignited nicely. The "squish" also helps swirl the intake charge around and better atomize the fuel. Since fuel vapor burns much better than fuel droplets, this is a good thing. I think I recall that there are a couple of other reasons as well, but I don't remember them off the top of my head.

--DD

CGNJ · 06-19-2003, 02:25 PM

That chamber volume seems tiny. I'd expect hi 58-60cc
Whats the scoop with that?

Carlos

BenI · 06-19-2003, 02:31 PM

They were 2.0L oval-port bus heads. Maybe they've been worked on before. They came on the engine I was rebuilding so I sent them to Headflowmasters for new seats and bigger valves. Didn't know then to be concerned about chamber size...

Zeke · 06-19-2003, 04:04 PM

I couldn't have said it better than DD. I have 53cc heads and .040 deck for 9.5 CR. I use a blend of 106 and 91 at sea level @ 32 degrees BTDC.

BenI · 06-19-2003, 08:34 PM

Thanks for all the help! So I'm planning on learning how to remove 6ccs from each of these. I figure with 53cc combustion chambers I can run a .058" deck and get around 9.1:1 CR. Any advice would be great but I'm going to start with the web and then look to buy a book. Nothing like a novice with tools and a free weekend!