Squish clearance

scooble · Post by **scooble** » Mon Aug 11, 2008 2:11 pm

I have reason to believe that my cylinder head may be warped. My machinist is able to skim the head, but I am not quite sure what a good squish clearance is. Before the trackday, it was about 1.3mm (which I think is a bit wide), how much smaller could I take it and still be reliable?

Also, do I keep the angle the same, or is this something that only needs to be modified when changing the volume?

(I did do a search on squish, but I couldn't find any figures)

the-elf · Post by **the-elf** » Mon Aug 11, 2008 4:09 pm

About 0.8mm squish is ideal. Don't go below 0.65 or the piston can kiss the head. The angle should be a degree or so more than a tanget to the radius of the piston, This is to force unburnt mixture into the centre of the cylinder. IIRC the radius of the standard piston is 120mm. the total width of the squish band should be about 25% of the diameter (about 6-7mm per side)

mj43 · Post by **mj43** » Tue Aug 12, 2008 12:09 am

I did a spreadsheet for head volume squish angle etc calcs - can't remember the link but it is in an old post. Or PM me your email and I will send you a copy.

MJ

scooble · Post by **scooble** » Wed Aug 20, 2008 11:59 am

After my bad track day, I discovered that one of my cylinders wasn't square. The odd one was squared up and the other one was matched to the same height. This brought the squish down from 1.3mm to 0.6mm, so I had the squish band recessed into the head by 0.2mm and angled at 15 degrees.

Please could someone tell me how much difference reducing the squish from 1.3mm to 0.8mm will make to the compression ration? and would it be significant enough to warrant using Avgas or Octane booster?

the-elf · Post by **the-elf** » Wed Aug 20, 2008 12:32 pm

scooble wrote:After my bad track day, I discovered that one of my cylinders wasn't square. The odd one was squared up and the other one was matched to the same height. This brought the squish down from 1.3mm to 0.6mm, so I had the squish band recessed into the head by 0.2mm and angled at 15 degrees.

Please could someone tell me how much difference reducing the squish from 1.3mm to 0.8mm will make to the compression ration? and would it be significant enough to warrant using Avgas or Octane booster?

You'll need to measure the head volumn to be sure.

scooble · Post by **scooble** » Sun Aug 24, 2008 7:52 pm

Ok, I measured the volume of the head and came up with 14cc.

In an effort to work out the Compression Ratio, I made some measurement and came up with the following calculations. I am not sure if this is correct, but here goes;

a. Head Volume measured = 14cc
b. Top of Exhaust port to top of cylinder = 27mm
c. Head gasket thickness = 0.2mm

d. Volume of dome of piston (assumed) = 4.1cc
e. Squish clearance = 0.8mm

f. Distance between the top of the cylinder and the top of the piston edge at TDC = 0.4mm
g. Bore = 56mm

Whole volume of cylinder (after exhaust port is closed) plus volume of 'f' plus the volume of the head gasket plus volume of cylinder head minus the volume of the piston dome. (i.e. swept volume + clearance volume)

Divided by

Volume of head plus volume of head gasket plus volume of 'f' minus the volume of the piston dome (i.e. clearance volume).

I get 77.39/11.38=6.8 to one

does this sound about right?
At what CR should I consider using a higher octane fuel?

the-elf · Post by **the-elf** » Tue Aug 26, 2008 1:50 pm

scooble wrote:Ok, I measured the volume of the head and came up with 14cc.

Is that with or without the spark plug in place? Anyway sounds like it should be fine on unleaded and its not that much different from a standard head. I'm at work ATM but I'll check the figures I've got writtern down when I get back home but, off the top of me head, I think the limit was around 12cc for unleaded (can't remember if that includes the volume taken up by the plug but I don't think so). It will vary between engines and the quality of the fuel you are using, and a hundred other things but 12cc is around the right ball park figure.

scooble · Post by **scooble** » Tue Aug 26, 2008 8:46 pm

I clamped the head to a rubber sheet and squirted water in with a syringe through the plug hole until it just started to go up the plug hole, then drew out a little.....thanks elf for the info.
Also whilst the cylinders were off, I matched the cylinders to the gaskets as per MJ's mini tunning article.