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PostPost by: SJ Lambert » Fri May 23, 2014 3:10 am

I've set distributor up for 12.5:1, which I presume is a pretty safe bet.

Just as an aside, I think I may have once read on a BP data sheet that ratio could go as high as 13:1, but I don't know under what circumstances that is possible (and am not totally confident that that was the number as far as my recall is concerned).........
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PostPost by: rgh0 » Fri May 23, 2014 3:46 am

12.5 :1 is what I aim for. This works well for Avgas and leaves a tolerance if it ends up a bit higher in the final assembly. I know others who aim for 13:1 but there is no margin for error and if aiming for reliability rather than ultimate power I think 12.5 :1 as a target is a better bet.

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PostPost by: SJ Lambert » Thu Jul 10, 2014 8:17 pm

Rohan, what effect does taking 2 millimetres, say, off an intruder on compression ratio in your experience?

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PostPost by: rgh0 » Thu Jul 10, 2014 11:59 pm

HI James

What affect depends where you are machining it off given the tapered shape of the intruder and the large valve cutouts in it and some of the other details of the engine build

The Forged JE pistons I buy come with a 10mm intruder as standard with an approx 10cc intrusion volume. This produces a compression ratio about 14:1 if your building a 83.5mm bore standard stroke engine with the pistons coming 0.5mm below the top of the block and with a standard 0.6 mm thick compressed head gasket and a 37cc combustion chamber. I machine the intruder down to about 6mm which gives and intrusion volume of around 5 to 6 cc and my 12 to 12.5:1 target. So roughly a unit of compression ratio for 2mm removal in this case.

You can measure the volume of the intruder at various heights by placing the piston in the bore and sealing it with grease or silicone and progressively filling with a measured volume of suitable fluid ( I use degreasing fluid or kerosene) and measuring the fluid height above the top of the piston deck at intervals . Then do the maths to subtract the volume of fluid added from the cylindrical volume of the bore and your left with the volume of the intruder at the various heights you measured. You can then work out how much to machine off to get your target compression ratio


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PostPost by: SJ Lambert » Fri Jul 11, 2014 4:15 am

Thanks Rohan, another minor point, I've yet to calculate it, but I think I need to factor in the volume occupied from the top of top compression ring to "top" (compression height) of piston as it's too difficult to actually physically measure with the pipette - I imagine it's a very small volume.

It's included in the "piston down the bore" pipette figures, but not in the combustion chamber pipette measurements, for the sake of precision, I think I need to include it in the chamber calculations?
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PostPost by: rgh0 » Fri Jul 11, 2014 4:45 am

Yes I include this volume. The diameter of the piston above the top ring is reduced to give maybe about 0.5mm clearance with the wall. Its easy enough to measure and calculate this volume.

Like you say it does not normally make a big difference but for the sake of precision you want to include it.

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PostPost by: SJ Lambert » Tue Jul 22, 2014 5:22 am

Using a nominal figure of .5 cm top ring to piston deck I get 0.65cc of volume above the ring.

I'll add that to volume below the block deck that the piston deck arrives at at TDC for the sake of precision/completeness.
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PostPost by: SJ Lambert » Wed Jan 14, 2015 5:58 am

image.jpg and
Pistons have had a shave - it's back to the pipette now!
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PostPost by: SJ Lambert » Sun Jan 18, 2015 7:15 am

image.jpg and


With piston deck 8.3 mm below block deck I get 28.7 mils to refill the 81.5 mm bores.
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PostPost by: SJ Lambert » Sun Jan 18, 2015 7:16 am

image.jpg and
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PostPost by: SJ Lambert » Mon Jan 19, 2015 1:16 am

So I make that 43.3 mils volume with no intruder - making intruder volume "14.6cc".
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PostPost by: SJ Lambert » Mon Jan 19, 2015 9:22 am

Ok, with combustion chamber at 40.5, above ring volume 0.63, volume above piston deck @tdc 0.15 a head gasket at 1.5mm compressed by 30% giving 5.68 and intruder volume of 14.6 I get clearance volume (46.96 - 14.6) of 32.36.

So with swept volume 379.52 I get compression ratio of 12.73!

It appears as though I'm 0.6cc away from a 12.5 : 1 ratio.
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PostPost by: rgh0 » Mon Jan 19, 2015 12:06 pm

I would check valve to piston clearances on overlap before taking anything more off just in case you need to deepen the pockets a little.

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PostPost by: SJ Lambert » Mon Jan 19, 2015 12:37 pm

Ta, will do.
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PostPost by: SJ Lambert » Sun Mar 08, 2015 6:01 am

Ok folks, it's "crunch time", er, let me rephrase that, "build it or stop fiddling" time. Closest point is near edge of inlet valve cut out and it's 43 thou on all 4 pistons. From observing the relationship between inlet valve to piston (looking at it through the exhaust port ) whilst turning the engine over by hand it appears as though the closest point occurs some 10 degrees after top dead centre with the valve "catching" the piston up up a bit before the piston gets away.



Comforted by that notion and a head gasket that's hopefully gonna torque down to a in service thickness 20 thou thicker than the one used in the test I reckon it's a goer as is.



Intend to keep the revs to a sensible limit too.



Anyone think otherwise?

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