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Piston Selection (need Compression Help)

Careless

By Careless,
in Engines & Forced Induction

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Careless Contributor

Careless

Posted
Posted

Hi everyone.

I have a fellow board member who wants to sell me some custom wiseco forgies, with wrist pins and rings and locks.

87mm

1.260 compression height

.165 dome rise

16cc dome volume

what i've been told is that I can get 11:1 compression by decking the block to get the piston out of the bore by 0.01 inches, get a 1.2 mm (0.47) headgasket, and shave the head to achieve 60cc's or whatever is required to get 11:1. they are for RB25/30 combo, but I'm using an RB26 head.

is this a good idea. by that, i mean, is using the headgasket area an ideal way of building up the quench, or should it ideally be built up using the block and piston at 0 over or under the deck?

does anyone know if the valve clearances will be an issue with 8.7mm cams? Also, what if i want to go higher at one point? will i be able to, if increase the lift (after making the head compatible with the longer/higher cams), and still not hit the pistons?

they are originally to obtain a 9.3:1 CR, so I think they will be pretty close to what I need. But then again, wiseco measured up their RB26 head at 66cc. So It's possible that by putting this into a head with lets say.... 64cc, i'd have closer to 9.8 or something to start with.

I've posted a picture.

The reason I ask is that I'm getting this set (brand new) for half the price of a custom set from a distributor, and it's also free shipping too.

my machinist wants to know if they will be compatible with this setup before i buy it, but I think it is, and i have a feeling he's gonna say no.

(im also posting this in both the n/a and engine forums, because there is no "general engine" forum ,and i'd like both turbo and n/a enthusiasts to be able to view this and help me out. Thanks :P )

post-29116-1184375460_thumb.jpg

Adriano Grand Master

Adriano

Posted
Posted

Sorry, piston height should be close, i would deck it so the pistons sit 10thou(.25mm) proud, and then use a 1mm head gasket, then deck the head for correct comp.(check all the measurements includding ccing the pistons and head before you machine anything)

Careless Contributor

Careless

Posted
  • Author
Posted (edited)
Sorry, piston height should be close, i would deck it so the pistons sit 10thou(.25mm) proud, and then use a 1mm head gasket, then deck the head for correct comp.(check all the measurements includding ccing the pistons and head before you machine anything)

Always coming through adriano. how do i send you a 24 of Toronto's own beer :blink:

1 thou proud is over the deck, correct? with a 1.2mm gasket, that would mean i could get .46 quench area, correct?

I guess I should shoot for .36?

what's everyone's thoughts on quench with the RB head, some engines require different quench is what I hear, depending on combustion chamber design.

is decking the head a smidgen usually a good thing?

the pistons are 16cc dome. however, wiseco states that the RB26 head is 66cc. I don't think that's correct, therefore, I've already got a leg up on the compression, being that usually heads are 64cc and thereabouts.

I think their combustion chamber was polished.

Edited by Careless
Adriano Grand Master

Adriano

Posted
Posted
Always coming through adriano. how do i send you a 24 of Toronto's own beer :blink:

1 thou proud is over the deck, correct? with a 1.2mm gasket, that would mean i could get .46 quench area, correct?

I guess I should shoot for .36?

what's everyone's thoughts on quench with the RB head, some engines require different quench is what I hear, depending on combustion chamber design.

is decking the head a smidgen usually a good thing?

the pistons are 16cc dome, btw.

Expres post of course!LOL, i would try to get 30 thou. The smaller you can run without hitting, the better. If the pistons sit 10 thou proud over the deck, and you run a 40thou(1mm)gasket then you end up with 30 thou. You may have to machine the dome of those pistons down a little, as a rough calc gives it 12.1:1 comp, but you should bore and hone the block, then dummy assemble the bottom end. Then do all the measurements, and figure out what needs to be decked how much, then your home and hosed. Just remember whenever you build an engine like this, treat it as a custom job and dont rely on anything written on the part boxes, as a small difference here can make a huge difference in compression.

Careless Contributor

Careless

Posted
  • Author
Posted
Expres post of course!LOL, i would try to get 30 thou. The smaller you can run without hitting, the better. If the pistons sit 10 thou proud over the deck, and you run a 40thou(1mm)gasket then you end up with 30 thou. You may have to machine the dome of those pistons down a little, as a rough calc gives it 12.1:1 comp, but you should bore and hone the block, then dummy assemble the bottom end. Then do all the measurements, and figure out what needs to be decked how much, then your home and hosed. Just remember whenever you build an engine like this, treat it as a custom job and dont rely on anything written on the part boxes, as a small difference here can make a huge difference in compression.

awesome.

My only beef with the pistons is that the dome height and the dome volume attribute to their weight, sitting at 411 grams per.

I might get them machined out to lighten them to 350 grams if possible.

Adriano Grand Master

Adriano

Posted
Posted
awesome.

My only beef with the pistons is that the dome height and the dome volume attribute to their weight, sitting at 411 grams per.

I might get them machined out to lighten them to 350 grams if possible.

Are you able to check the dome[crown thickness before you buy them?

Careless Contributor

Careless

Posted
  • Author
Posted
Are you able to check the dome[crown thickness before you buy them?

well. the pistons are 1200 dollars new.

i'm getting them for 600, still new, but not useful to the current owner.

therefore, whether they are heavy or not, I bought them because it's a steal of a deal.

sure it's a little heavy for N/A, but whatever.

I'm sure they've got a deep crown, and that they can be machined quite a bit.

Stealth-Z Rookie

Stealth-Z

Posted
Posted
Are you able to check the dome[crown thickness before you buy them?

I have a set of these and emailed Wiseco about shaving the dome. This was the reply.

If there is milling inside of the dome to hollow it out, I would say no.

If it looks to be a solid dome, then yes. Minimum thickness for any part of the

crown would be .150".

Best Regards,

Wiseco Piston Co. Inc.

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