701 block / Accralites / opinions wanted

[el-saturn]

hi there - hoping you're enjoying your saturday drive to a race circuit ------------ or is it may be shopping
wiz ze wife?? My actual question is: what do you think of the piston (crown, not dome!) which at OT has a deck height (?) of 0,05 + mm (0,002inch) - maybe my memory failed: the piston at OT does not exceed the deck which i thought it should: i measure a gap and i thought (memory??) the piston should go a bit further than (0.1 to 1mm) deck height??? any ideas or even thoughts which relate to the dome which is too high (minus 2mm e.f.?) thanks sandy 36/4982

[rgh0]

The original Lotus design had the piston deck 0.5mm below the top of the block. This ensured that with rod stretch and 6500rpm and normal assembly tolerances that the piston did not exceed the block deck height I believe.

I build my engines using the same 0.5mm piston deck below the block. This is conservative and I know guys who build racing twin cams with the piston deck nominally equal with the top of the block. However you can get a 12.5 compression ratio without this easily enough so I don't see the need to build a higher deck height than Lotus originally used ( ok a higher deck height means lower intruder height and better combustion chamber shape but your now picking nits). At 8000 plus rpm and the smaller cross section H rods, the rod stretch will be greater than originally and I don't want to come to close to the head as the compressed gasket is only 0.7mm thick and not much room for error if your deck height is at the top of the block.

Have you settled on a compression ratio yet?. With a standard 36cc head combustion chamber I believe you will be over 13 with your current 6mm intruders and if you machine to 4mm you will be around 12 to 12.5. I think you need to go down to around 2 mm for your 11 to 11.5 target but you need to measure everything up to determine this.

cheers
Rohan

[el-saturn]

thanks rohan - you?re my man here at elan.net. i have forged (and polished) rods which are the longer type which gets me so close to top deck. i?m always getting a quote of 0,7mm final clearance (head to deck) in torqued down condition - whereas the 0,7 (can be) are the gasket. where i really get variations is the compression from max. 11,3 to 1 to 12,5:1 -- 12,5 would certianly call for 105 octane juice (which you?d
have to order someplace in germany - no circuits here! - the dome is 1/4" and the head?s hemisphere is more or less original - it?s probably night time in melbourne isn?t it?? cheers sandy

[rgh0]

where i really get variations is the compression from max. 11,3 to 1 to 12,5:1 -- 12,5 would certianly call for 105 octane juice (which you?d

Hi Sandy
I don't understand what you mean by variations in compression ratio. can you explain more

cheers
Rohan

[el-saturn]

thanks mate - i just meant the possibilities, because i'd go for at least 12:1 if i wasn't afraid of
the problems - gas e.f.. i have 2 heads at a friends shop so i can't measure the chamber's volume but my
swiss specialist says 11,3 is max. for a road car - another friend says 11,5 --- as i said many variants!
the bottom is safe (brand new accralites, cosworth rods, forged LOTUS crank, not nearly as elaborately designed nor as heavily machined as new ones (farndon), therefore less counterweight mass and i'm pretty sure much lighter. Of course I'll get less rpm - but a road engine won't ever need 8000! i'm basically looking for the best
compromise Rohan! --- the deck height is 0,2mm (i "centered" the piston with 0,1mm wire from the top) so with a 1mm cometic gasket (an minus 2 or 2.4mm doam) i'm getting close ------- at least in theory
.........have a nice sunday - and watch lotus win abu dhabi! sandy

[rgh0]

sitting with a glass of wine waiting for the GP to start!

Personally I really think over 10.5 is going to be to high for a carburettor / mechanical distributor road car on normal 98 EU octane pump fuel. But happy for you to show higher is doable, even if your using a long duration cam that's not really the best for the road - I am always keen to learn new boundaries!!!

cheers
Rohan

[Jeff@Jae]

Rohan sums it up very nicely. That deck height is OK if the valve clearance checks out, the compression ratio will have to measured and adjusted for a road engine and 10.5:1 is iffy on stock cams but fine with higher duration numbers, on a road engine.