spyder_xlch wrote:If that engine was a 181 and my boss puts the pistons in his 151 that should lower the compression to about 5:1:lol: Should have checked the stroke. I asked if he wanted to keep the crank but he said he had a few good cranks. Oh well.
So, I checked into this. I could be wrong and you guys will tell me.
The Pontiac 151 rotating component dimensions:
3.00" Stroke
6.00" Rod Length
1.60" Piston Compression Height
0.9272" Piston Pin Diameter
2.100": Rod Main Journal Diameter (thanks, Nate!)
9.125": Block Deck Height (Crank-to-Deck Clearance)
So, the piston sits 0.025" below the deck at TDC.
Notice the Rod Length and Stroke of the 151.
The Mercury Marine 181 has a 3.600" Stroke. The crank throw is 0.300" longer than the Pontiac 151. If everything else is equal, the Rod Length of the Mercury Marine 181 would need to be 0.300" shorter at 5.700". So, the 151 pistons would fit in a 181 with no problems assuming that the 181 uses a 5.700" Rod.
These 6.000" and 5.700" Rod Lengths makes sense and don't appear to be arbitrary. These are the two most common small block Chevy size rod lengths.
KB lists KB223 flat-top piston for the 77-78 Pontiac 151 which gives about 9.0:1 static compression ratio.
They have a compression ratio calculator on their web-site.
http://kb-silvolite.com/calc.php?action=comp
Inputting some reasonable numbers and I came up with the 9.0:1 CR as shown below.
The same numbers, but with the stroke increase for the Mercury Marine 181, is a whopping 10.5:1 CR as shown below!
I also added the information for the dynamic CR from the Cam Card on my Schneider Cam - see specs in a previous post.