Moper's post: I use 8.25:1 dynamic as a max with open chamber iron heads and no quench. This ends up in most cases around 175psi cylinder pressure. For aluminum, about 8.75:1 with a tight quench is fine, about 185-190psi.
My post: Using moper's aluminum dynamic ratio and the KB-Silvolite calculator. Here's the results:
cyl head volume 63cc Edel LA rpm (not 340) aluminum head
piston volume 2cc Sealed power valve relief volume
head gasket 0.039 Felpro #1008 Quench=0.039 (very good)
gasket bore 4.18 Felpro #1008
cyl bore 4.07 4.04+0.03 (30 over 340 bore)
deck clearance 0 flat top piston flush with top of block deck
stroke 3.31
rod length 6.123
intake closing 59 degrees abdc
RESULTS: 10.57:1 static compression and 8.73:1 dynamic
These are good sounding numbers.
Now apply my test by increasing the combustion chamber volume to a iron head type lower compression. I used 73cc head volume, all else the same. Gives static ratio of 9.4:1 for "regular" grade pump gas at 4000 foot elevation, and 7.8:1 dynamic compression, which is in the 7.7-8:1 that I suggested. So it checks out.
This means you could build an iron head engine with 73cc combustion chamber volume, keeping everything else the same, and you could run regular grade pump gas with a strong running engine. You could reduce the iron head combustion chamber volume and increase the static compression to 10:1 and still run on pump gas.
So the kB hyper pistons for the 340 have too large of valve reliefs to properly set up a strong running engine with aluminum heads and good quench. Sealed Power pistons have smaller valve reliefs.
Raising the piston top 0.004 above the block deck is worth 1cc, and still keeps the quench clearance at 0.035 inch which is a reasonable minimum for street use. Milling the heads for further reduction of the combustion chamber volume by would be required to make up for KB's 5cc valve reliefs.
Topic: 340 build numbers that will make a strong running street engine (Read 1031 times)
