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8v pistons, 16v pistons.

2K views 37 replies 11 participants last post by  tomstickland 
#1 ·
I took my recently acquired 8v engine apart tonight to look at the pistons. Just like the 16v engine they stick out above the block by about 0.5mm at TDC. The 8v pistons are flat topped but with a bowl where the spark plugs would stick through. So I guess this is all to do with squish and flame front propogation.

The 16v pistons have a very shallow bowl on them.

Ideally I could have the 16v pistons machined so that they are similar to the 8v pistons. This would involve machining a pocket about 4mm deep. Are this pistons thick enough to tolerate this?
 
#29 ·
I've spoken to the engine builder man and he reckons that we can modify 8v pistons to take the 16v rods. Basically just add circlip grooves and bore the hole out a bit. That would be a result.
 
#30 ·
I went round to the engine builder today with an 8v piston and a 16v piston. He recommended machining out the bottom of the 16v piston so it looks a bit like the 8v piston in the picture above. He reckoned the bowl location on the 20SEH/SER pistons is probably emissions stuff and that a piston like the 18SE one would be best. I like the plan.
 
#32 ·
It will involve skimming about 1mm out of the bottom of the small dish that already exists on the 16v pistons. It's only necessary in order to lower the compression ratio. Though I'd better check how far the spark plug sticks out or I could have trouble on my hands!
 
#35 ·
Right I've done some measurements, modelled the bits in 3D CAD and worked out the volume differences.
Essentially it boils down to:
the 16v piston has a 70mm diameter x 1mm deep pocket.
The 8v pistons has a 45mm diameter x 5mm deep pocket.

That makes 3.8cc for the 16v and
7.95cc for the 8v, roughly a 4cc difference.

Fitting 8v head to 16v block would make compression ratio of about 11.something.

The valve pockets on the 16v piston actally remove another 1.5cc, so only 2.5cc is needed to be removed from the 16vv piston.

That's an extra depth of 0.6mm.

So I will be having another 0.5-0.6mm of material removed from the bottom of the pockets on the 16v pistons.

Just going to check the spark plug clearances.
 
#36 ·
nice info tom ;)

was that pockets, or pocket?

70mm pocket 0.6mm deeper to get the right compression ratio?

If you've got it moddelled, could you please check what effect deepening the std pistons valve pockets by 2-3mm would have on compression ratio... pwease.....

:beer:
 
#37 ·
I did analysis today and some measurements with a pipette of water:
16v piston
vol of valve cut outs: 0.2cc from model, 0.2cc from measurement
vol of pocket: 3.7cc from model, about 4.5cc from measurement
total vol: 3.96 from model, 4.7 from measurement.
Call it 4, the measurement was quite inaccurate due to surface tension effects.

8v piston:
vol of bowl: 5.9cc from model, 7cc from measurement.
Call it 6cc.
So there's a 2cc difference, enough to lift the compression ratio by about 0.6.
Therefore I'm going to skim 0.6mm out of the 70mm diameter pockets, adding another 2.3cc to the 16v pistons.

I checked spark plug clearance yesterday and it looks fine. The 8v spark plugs don't protrude below the bottom of the head. So there'll be about 2mm of spark plug clearance at TDC.

Deepening the valve cut outs by 2mm increases the volume at TDC by about 0.7cc. This will lower compression ratio from 10.5 to 10.4. So there's about 0.1 of compression ratio lurking in those valve cut outs.
 
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