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Shaving cyylinder block head

1K views 7 replies 5 participants last post by  Horn5413 
#1 ·
I need some help getting my head around the reason shaving a cylinder head raises the compression ratio of an engine. As far as I can tell shaving the head does get the head closer to the piston but that should be canceled out by less air being sucked in on the intake stroke. I think you have changed the volume but the compression ratio stays the same. Where am I going wrong on this?
 
#2 ·
The amount of air being pulled into the cylinder doesn't change. That is dictated by the size and stroke of the piston, not the size of the combustion chamber at TDC. Once the head is shaved the air that is pulled in is now being compressed in a smaller space, hence the higher compression ratio.
 
#3 ·
Grossly simplified explanation, with made up numbers to allow the math to be simple:

You have a 9:1 compression ratio. That means the total cylinder volume when the piston is at bottom dead center is 9 times greater than the remaining volume when the piston is at top dead center. At the bottom, let's say you have 900 cc total volume, but at the top you have 100 cc remaining volume (9.00:1). Easy, right?

Now you shave the cylinder head. This shrinks the combustion chamber size by 10 cc, taking you down to 90 cc remaining volume. Now your total volume is 890 cc, and your remaining volume is 90 cc. Doing the math, 890:90 reduces to 9.89:1, a significant bump in your compression ratio.

Make sense?
 
#5 ·
In my made-up example above, you'll see the total volume was only reduced by 1.1%, whereas the combustion chamber volume was reduced by 10%. Shaving the head to this extent therefore won't make a huge difference in the total amount of air that enters the engine (again, 1.1%) but the greatly increased compression can result in a significant increase in the amount of power that will be produced.

It can also put a significantly increased amount of stress on the rods, pistons and crankshaft... and if they aren't strong enough to deal with it... well, let's just say bad things can happen. ;)
 
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