pistons
- Thread starter 851sp2
- Start date
Dellortoman
Hero member
- Location
- Tasmania, Australia
The odds are in favour of Red's answer. Compression reading is a bit too high for flat top pistons. But it's a wee bit low for a healthy Jota.
However, it's not just a simple matter of compression ratio.
Cranking compression is very dependent on cam timing. Specifically the inlet closing angle which determines how much of the stroke is used to compress the gas in the cylinder.
A11 cams close the inlet at 82? ABDC whereas 4C cams close the inlet at 74? ABDC - 8? earlier. So the 4C cams will give a higher compression reading than A11 (all other things being equal and cams properly timed).
Maybe you have a hybrid - H/C pistons with A11 cams. Nothing wrong with that. Good mid-range torque.
On the other hand, you could have flat top pistons with a mis-timed (advanced) inlet cam.
I'd suggest doing a leak-down test (which isn't affected by either CR or cam timing). If that's OK, then identify what cams you have and check the timing.
A reliable way to identify what pistons you have (without removing the head) is to use an adjustable piston stop. Adjust the stop so it just touches the piston at TDC, then remove the stop and measure from the top of the spark plug thread to the business end of the stop. If that measurement is 28mm you have H/C pistons. If it's 30.5 you have flat tops. You can do the same measurement with the depth probe on a vernier caliper, but you'll have to hold it on the centreline of the spark plug hole to get an accurate comparison with the above dimensions.
Cam
However, it's not just a simple matter of compression ratio.
Cranking compression is very dependent on cam timing. Specifically the inlet closing angle which determines how much of the stroke is used to compress the gas in the cylinder.
A11 cams close the inlet at 82? ABDC whereas 4C cams close the inlet at 74? ABDC - 8? earlier. So the 4C cams will give a higher compression reading than A11 (all other things being equal and cams properly timed).
Maybe you have a hybrid - H/C pistons with A11 cams. Nothing wrong with that. Good mid-range torque.
On the other hand, you could have flat top pistons with a mis-timed (advanced) inlet cam.
I'd suggest doing a leak-down test (which isn't affected by either CR or cam timing). If that's OK, then identify what cams you have and check the timing.
A reliable way to identify what pistons you have (without removing the head) is to use an adjustable piston stop. Adjust the stop so it just touches the piston at TDC, then remove the stop and measure from the top of the spark plug thread to the business end of the stop. If that measurement is 28mm you have H/C pistons. If it's 30.5 you have flat tops. You can do the same measurement with the depth probe on a vernier caliper, but you'll have to hold it on the centreline of the spark plug hole to get an accurate comparison with the above dimensions.
Cam
D
dja981
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Is it for sale? 
Dave
Dave