Ok, i ordered my stage 5 clutch, new water pump, some coolant, and my new pistons.
I got the pistons for 540$ through integrated engineering.
I chose a 81.5mm Bore, 86.4mm stroke.
With a lower 8.5:1 Compression ratio.
I talked to the guys at the shop doing the work, and they can resurface flywheels for 40$ so i'm gonna take mine to them.
I may also invest in a kevlar timing belt, well see.

 

im building a motor now too. if you have that much piston clearence id think about a larger bore but leave enough to bore again just in case. and from what ive read it would be good to keep the CR near stock especially if youre not doing a stroker but i guess yoiu already bought pistons. should work fine and you can always turn the boost up a bit more and be safe with 8.5:1. personally im going with a kevlar timing belt and also the manual tensioner converter from ie. it seems like a great product and even better insurance against stupid gas charged tensioner hope all goes well for you

 

the pistons were worn, the bore measured out to 81mm the guy at the shop recommended a lower compression with big tubo builds.
Hopefully the lower compression was a good idea? Should i return them, and get a different CR?

And ill definitely go with the Kevlar belt.

 

how much boost lag will i be seeing with 81.50mm and my compression being 8.5:1?
compared to before being 81mm 9.25:1

 

Well I can't answer the above question exactly but I'd venture to guess not as bad as you'd think. There are two reasons that lower CR's are recommended with big turbos - With big boost comes higher pressures all around; the last thing you want to do is have the same cylinder pressure as a built diesel. With that level of pressure the pistons will scream "**** YOU!" and make an attempt to vacate the block, trying to take portions of the block with them. Also, and here's the more likely reason for you: A higher CR along with highish boost makes it all to likely to blow out the spark in the cylinder leaving you with no boom-boom.

Edit: Although if you decide to sleeve then blowing up the block won't really be much of an issue. In the case of Audis, and I assume you're shooting for around 20 psi, blowing out the spark in a bigger issue to watch out for.

 

Ill attempt to explain compression ratios and what your optimally cr is. First off, the higher the CR the better. thats undeniable. the higher the cr the more power. unfortunately the higher the cr in gas engines, the more prone to predetenation you are. this can be counteracted by higher octane gas as it resists predetonation. however, it will only go so far. by the way, cr matters in relation to timing of the spark as well. you want the spark the come on early as possible. it usually happens at or slightly before tdc, but all this combustion takes time. thats how it can spark before tdc. when all timed correctly you have one excellent working enigne. when you drop the cr, your combusted gases are less volatile because there isnt as much of them and at lower pressure. youre throwing a wrench into the works with a turbo though. youre adding a **** ton more air to the mix with that though which throws off the comression ration/air to fuel ration by a lot. as you may know the more air pushed by this turbo the more and more volatile this whole relation of cr/air/fuel is. this is why you cant run 100 psi off your k03 and make 400hp. it would predetonate like a bitch unless 1000 octane fuel existed. you lowering the compression ratio means two things. first, youre making less power off the get go. less air/fuel mixture can fit into 8.5/1 cylinder than 9.25 and less air/fuel equals less power. however, it also means you can crank up the boost a tad more cause the air/fuel mixture is less volatile at 8.5 than at 9.25, making up for that loss of power and then some. bascially it means less power down low when youre spooling and more power at the top when boost is on. by the way predetonation is a very, very bad thing. you want to avoid this. not only is it bad preformance wise, its terrible for a motor. its why you have a knock sensor.

but wait?! i got a buddy with a duramax and he runs 50psi at the drag strip and his compression is 16:1? how does that work? (i dont really got a buddy just a possible scenario) according to the above statements he would predetonate like a ************! but he wouldnt. its how diesel work. they have such high cr's because thats how diesel ignites. the fuel gets injected dirrectly at the top of the cylinder or slightly before. because they have cr's that are so high, the compressed air is very hot. the fuel gets injected and reacts with the heated air to combust. this is why diesels need glow plugs to start when its cold out. the cylinders are just too cold to heat the air up enough to combust. But how can they run 50psi you ask? cause predetonation (as long as the injectors are timed correctly) cannot possibly predetonate. the pistons are only compressing air until the fuel is injected directly into the cylinder. this is why diesels will never have a carb on them and why their fuel injection systems run in the 1000's of psi. if it wasn that high, the compressed air in the cylinder would blow right out the injectors. this is why most diesels run mechanical injectors. it is/was capable of much higher fuel pressure.

i understand this is a lot, and maybe you already knew all this or more, but im saying lower cr has its advantages just as higher cr has. without a doubt, especially in non-forced induction cars, the highest cr is always best. its more powerful and more economical, but predetonation limits this. im sure youre fine running 8.5, but you would have more power while spooling with 9.25. how much? probably only a few, but noticeable. youre trading off a few hp's at the 2-4k rpm range for a few more (assuming youd crank the boost up a bit) from 5-7k range

 

Thanks for all the info it helped a lot, the only part of that i was really educated on was the diesels, My best friend is a diesel mechanic.

I think i'm going to end up going to the stock compression, i like spooling as early as possible.

Thank you for all the help guys.

 

So, last but not least.
What am i gonna want, for injectors?
630cc,750cc,870cc, or 1000cc?
I'm going to be going with, Eurodyne tuning. They told me they could set me up with a very accurate base file for my setup.
Which would be better than running a tune that wasn't for my turbo...
So i will probably ask them what they suggest also.

 

ask what injectors to run with the specific software. they will tell you. on a side note im actually going with 8.5 to 1 cr on my engine build. i got a smokin deal on mahle powerpak pistons and couldnt pass them up. i only paid 370 for 81.25mm bore with 8.5 cr (i know i gotta pay to bore it but well worth considering these are 700 dollar pistons)

 

Oh right on!
i went back to the old compresion, You will have to let me know how you like it, are you thinking of doing Eurodyne when you finish your build? and what turbo are you gonna be running?