dave_424
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| posted on 29/3/13 at 05:57 PM |
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Machining a dish in pistons to lower CR
I have bought a set of 923cc forged pistons for my zx9 engine, they are 12:1 compression and would like to get them a bit lower for my turbo,
somewhere in the 10's or 9's would be great.
The piston crown is approx 9mm in its thickest part, then slightly thinner leading down to the valve reliefs. I have done some reading around and
people say not to go under 6mm thickness.
I need to take out approx 4-5cc from each piston.
I also can do a combination of dishing the pistons and adding a base spacer but would like to keep my squish for detonation reasons.
Who would I talk to about dishing these pistons, I can do the machine work myself but need some guidance about thicknesses I can get away with.
Dave
[Edited on 29/3/13 by dave_424]
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snapper
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| posted on 29/3/13 at 06:58 PM |
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Squish will be reduced even if you bowl the piston as with the valve cutouts you don't have a lot anyway
You can just skim the pistons or use a thicker head gasket
If your not getting Det know you won't get it with less compression and squish
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
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dave_424
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| posted on 29/3/13 at 07:29 PM |
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True, there isn't really much quench area, I think I will probably do a light skim/dish and then go with a little extra cylinder base spacing,
unless anyone has a reason not to?
They only cost me £50 for a set of four with all rings and pins
Dave
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snapper
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| posted on 30/3/13 at 05:50 AM |
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I've researched this a lot but for a Pinto, my problem is I'm using a head designed for 11 to 1 and have cast V6 pistons that sit .382
below the deck.
I'm reducing the compression by using a thicker gasket, my quench was to big from the start, it will be worse now, the only way to get decent
quench was to open the chambers of build a new head
Ill go with what I've got and manage det as necessary
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
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BenB
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| posted on 30/3/13 at 08:27 AM |
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quote:
Originally posted by snapper
If your not getting Det know you won't get it with less compression and squish
Not sure that works- it all depends on how much boost he subsequently runs and the inlet charge temperature. Certainly it would be a shame to have to
run lower boost levels (IE less power) or run on a lower CR (IE less power) or run higher RON fuel (£££££) to make up for removing the squish.
Ferriday engineering copper head gasket would be one option though obviously it will still modify the squish, just in a different way.
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dave_424
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| posted on 30/3/13 at 10:06 AM |
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I will be machining these today, I will skim it flat so that it is level with the part where "XG" is, and then give it as much dish as I
can whilst keeping the thickness at 5.5-6mm
I will CC the head when I send the barrels out to get machined, when I get the barrels back, I will CC the pistons in the cylinders and then determine
the thickness of base gasket that I require.
Something like this pistons which is a custom JE zx9 turbo piston at 9:1, I don't expect to get that much dish though.
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dave_424
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| posted on 30/3/13 at 05:34 PM |
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Machined the pistons today, wasn't able to make any dish into the pistons because the crown would have been too thin, took each piston down so
that the middle was level with the larger bits around the valve pockets.
Ended up with a corn thickness of 6mm which is as thin as I would like to go, the rest of the compression can be taken out with a cylinder base spacer
that I have.
Since I will be losing some of the quench by raising the head, I will look into ceramic coating the piston tops which should lower the crown
temperatures.
I also rubbed down the piston top with very fine wet and dry and then one of those plastic scouring pads, If I don't ceramic coat the piston
tops, this will help to reduce flash heating of certain areas of the piston creating heat spots.
The finished result
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dave_424
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| posted on 31/3/13 at 10:56 AM |
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Anyone know anywhere that can do ceramic coatings on pistons?
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BenB
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| posted on 31/3/13 at 12:14 PM |
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quote:
Originally posted by dave_424
Anyone know anywhere that can do ceramic coatings on pistons?
LOL  that's what I was thinking when you said you were going to get them done.
Nice machining btw, looks quality.
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dave_424
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| posted on 31/3/13 at 01:07 PM |
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Thanks, Ceramic coating pistons is a pretty common practice, found one place that will do it for about £20 per piston, they also offer a teflon like
coating on the piston skirts and a thermal coating on the inside of the piston.
Dave
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