85mm pistons too big for boost?
 

The motor I was building had some scoring in the bore. 84mm Supertechs were the plan. Word came back that it needed a little bit more to clean it up totally. So I'm waiting on 84.5mm Supertechs. Apparently they are 3 months back ordered. I can get 85mm ones though. My understanding is 85mm is the MAX bore for these motors and heard 84mm was the top for boosted. Figured 84.5mm was a stretch, but doable. I'm not concerned with rebuilding this block later, I can get another 1.8 block for cheap. Just don't want to do it now since I already have everything done, but the final boring with this block to my knowledge. More concerned with longevity.

So my understanding is this:
-Wait for 84.5
-Order 85mm and get a bigger head gasket and will this mess with squish/detonation/?????
-Is 85mm too big for boosted and make the bores too thin? Punch it big or go home?

The jump from 84mm to 85mm seems to be the break over point for things going weird. Learn me please.

A word with Supertech gives me the idea that the're pistons and big valves work together. Can't find out if the VVT head will work, but they are claying the block to find out. Basically I'm stuck waiting for pistons. Suggestions? Not interested in super pricey pistons, unless they WILL fit the big valves and VVT. The longer this build takes the more random crap I do to this car while it's apart.

Specs:
'99 Block
'01-05 VVT head
FM big radiator with coolant reroute +oil cooler in case it matters for heat concerns with bigger bore

Sonic test it. I like to stay above. 120 for boosted engines, .150 preferable.

Get a new block and use the 84mm in the plan?

What Emilio said. We actually had a thread a while back where Andrew said the same thing: that he sonic tests all his built blocks or something.

I too would be a bit uncomfortable punching it out that much then boosting it. You're walking on thin ice at that point (literally). Even if you can get another block later, you'll be stuck with pistons that most wouldn't want.

Supertech isn't the only piston company out there either. Better lead times may be available.

I was going to swap to FM's pistons, but aren't big enough. Talking to Supertech they said they do make the valve reliefs for bigger valves. So figured that was my best chance at having them fit +1mm valves. JE is about 8 weeks out for 84.5mm. Might look into CP, but don't know if they would fit bigger valves. I'm not stuck on the big valve thing, just would be nice bonus. Was going with the thought of Supertech pistons and valves being engineered to work together. I admit I may be out of my depth. I haven't kept up with engine internals in years.

CP-nope, just 84mm

Wiseco- yes 84.5mm in 8.5-8.8 compression ratios. Seems a bit low for a street car and off boost performance. Maybe a bigger det threshold?

FM- (Wiseco)nope. only 84mm

JE- yes 84.5 and 9-1, but 9-10 weeks out. Would rather wait for the supertechs at that rate unless someone can say they are amazingly better for some reason.

Other ideas? Go custom, wait, or source a new block. Would rather not source an new block. I would have to ship that out of state, and it's a pain around here to source just a block. All I can find around here is full motors.

What I am afraid of if you go with 85mm pistons is that you will overbore and hone to spec, then sonic test and find out that the walls are to thin to run the pistons. Then you are stuck sourcing a new block anyways and having to go through everything again.

I would source a new block now and just go with 84mm supertechs.

my weiscos are 84.5 9.0:1 and im running +1mm valves in and ex with integral r1/r3 cams (.490 lift) and dont have issues, i clayed the pistons when i assembled i dont remember what i got, but i didnt have to flycut the valve pockets

CP doesn't have them in stock or won't make them? They made me a set of 85mm pistons in 4 weeks or so...

Yes, I called CP and they will make them for me for surprisingly less than I thought. I'm just going to wait for the Supertechs. I still have some piecing together of parts for other stuff. Basically if I have a motor by X-mas I'll probably still need to finish other things. So either way I'll be waiting. Plus moving soon and not having the engine here is one less thing to move. It's the waiting that kills.