Wildguzzi.com
General Category => General Discussion => Topic started by: 1down5up on April 29, 2019, 02:36:14 PM
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Hi All, a question on the differences (apart from diameter :violent1:) from the stock 850 lemans pistons v the 88mm Gillardoni pistons that are a common replacement for the 850 Lemans to 1000cc.
I am currently tearing down a Mk2 that has the 1000cc Gilardoni barrels & Pistons
I notice that on the Gilardoni 88mm pistons the crown/dome is ~6.5mm and much more pronounced than the standard SP1000/G5 crown/dome for the additional compression and chamber shape that the LeMans uses - make sense
However, the dome/crown is noticeably shallower then the original 850lemans pistons (~9-10mm? from memory).
My question - is the compression ratio maintained by a higher piston deck/top height (distance from gudgon pin to flat of the piston top)? And having never seen a set of OEM 1000 Lemans (mk4/5) pistons, are the Gilardoni’s the same as this or is it an improved design over the OEM 850 & 1000 lemans pistons
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Mk I/II uses mid valve heads, Mk IV/V uses big valve heads. Hence the difference in dome height. You can't use the big valve pistons in mid valve heads due to compression and clearance issues. As for CR with the stockers and Gilardonis? Measure and do the sums. (Shrug.)
Pete
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A bigger bore under the same head chamber will have a higher compression ratio. So to maintain the same, you need less dome, or bigger chambers. not sure if there are other reasons they are different, but that might be the major one.
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CR=SV+CCV
CCV
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CR=SV+CCV
CCV
What Pete said. Given Compression Ratio and Swept Volume (displacement), you can calculate Combustion Chamber Volume. Given CCV and SV, you can calculate CR. If you can measure the real difference in dome volume, you can make a decent estimate of CR with the new pistons.