I'm rebuilding my AV 2.1. This time I decided to measured the actual CC's of the heads, and so have been calculating the barrel shims necessary to get the 8.5:1 ratio for av gas. When I built the engine the first time I had 2 shims on each barrel: a .045 and a .060 (that's what they measure with a micrometer), which is what I calculated using the AeroVee manual's chart. I always seemed to be under powered compared to other AV's and now I'm thinking my compression was too low.
I did the calculations tonight on my #1 cylinder after measuring the deck height and using the actual cc's of that head. I plugged everything in here:
http://www.csgnetwork.com/compcalc.html
I converted all measurements to metric, and used:
Bore = 92mm
Stroke = 82mm
Head Gasket Bore = 92mm
Compressed Head Gasket = 1.0 mm (* see below)
Combustion chamber volume = 59.2 cc
Piston Dome volume = 0 (flat top pistons)
Deck clearance = .89mm (** see below)
* Compressed head gasket: when I measured the thickness of my previous head gasket it was about 33% thinner, at 1.0 mm (a .060" gasket = 1.5mm).
** Deck clearance: without a barrel shim the deck height was -0.61 mm (.61 mm above the top of the cylinder). If I add a .060" (1.5mm) shim, then 1.5mm - 0.61mm = 0.89mm
Using all these numbers, I get a compression ratio of 8.595:1, which is about right.
If I add on another .045" (1.15 mm) shim (like I had originally) I get a deck height of 2.04 mm, and that would make the compression ratio only 7.86:1. My original pistons had a deck height of -0.04 (1.016mm), so adding a 1.5mm and 1.15mm shim would have been a deck height of 1.634, and my compression ratio was 8.1:1, which is still a ways from 8.5:1.
Please let me know if anyone sees any errors in my calculations. It appears that I should only use a single .060" (1.5mm) barrel shim.
Thanks,
Statistics: Posted by mike.smith — Thu Oct 08, 2015 12:03 am