Hi guys not sure if this has been discussed before but im just wondering what pistons and compression ratio people are running and what power there making?? Cheers guys

value for money acl hypertectics (spelling) 7.8:1 are a good bet.

Quote: Originally Posted by WOGPSI [View Post] value for money acl hypertectics (spelling) 7.8:1 are a good bet. high silicon content stock cast piston yeah? what would you recommend for a 25/30? only after 230kw to begin with

for that power level, standard pistons, NA block, not turbo block, has too low of a compression ratio, the NA pistons have perfect ratio for 25 heads. Higher than say 450hp, you can get good forged piston and rod packages from spool for good prices, and they cater for 26/30 and 25/30 stuff too. Because really you want to change the rods if anything at that power level

I still have heard way too many stories of those Hypertecs shattering from detonation - although detonation is the cause, they tend to do more damange then they are worth in the end. Alot of people will simply reccomend that you are better off freshening up the original Niassan pistons with a new set of bearings, rings etc etc - which seems to be the go. But if your going to pop the top, do it properly and spend the money. Alot of the time the crap you have to replace is that part you skimped out on a few hundred bucks for. And it ends up costing you thousands to learn that lesson. 7.5:1 --- Ross Racing Pistons - Made 330rwkw on 27psi without breaking a sweat.

Where did u source the 7.5 :1 Ross racing pistons from petros?

Quote: Originally Posted by xAsh- [View Post] for that power level, standard pistons, NA block, not turbo block, has too low of a compression ratio, the NA pistons have perfect ratio for 25 heads. Higher than say 450hp, you can get good forged piston and rod packages from spool for good prices, and they cater for 26/30 and 25/30 stuff too. Because really you want to change the rods if anything at that power level Thats my plan actually. I'll be looking at the spool rod /cp piston combo when i feel the time is right. For now what i have is a fully rebuilt A6 rb30 that was from a engine builders sons vl that got rear ended. that motor was thrown in with the purchase of my old wagon. I didnt know the builder so i went down to his workshop and had a chat and he said it had maybe done a year of k's since the rebuild and has new cast pistons and rings etc, bit off the head which i wont be using(sohc), 20 or 30 thou overbore, welsh plugs, gaskets, and maybe decked the block.. he cant remember. so im using this bottom end with a standard 25det head modified to keep the VCT. What i want to know is would the 20-30thou overbore and possible decked block make a dramatic difference to the compression ratio with the 25 head? ie. would this raise the compression? the bore i should be able the measure but the deck i have no idea. the motor has also been sitting for a couple of years in a shed without an exhaust manifold so i hope it wont need a rebuild Some original photos have not been recovered yet.

Im wondering why it has the overbore when he used standard cast pistons to be honest :S Never heard of anything like that being done If i was you i'd measure the block cc and the head cc (using a syringe) and then work out the compression ratio, if its too high with a standard head gasket, get a cometic made in whatever thickness you need

Quote: Originally Posted by xAsh- [View Post] Im wondering why it has the overbore when he used standard cast pistons to be honest :S Never heard of anything like that being done If i was you i'd measure the block cc and the head cc (using a syringe) and then work out the compression ratio, if its too high with a standard head gasket, get a cometic made in whatever thickness you need He would have done it just because he has the shop and could and also because the engine needed a rebuild and his son was on his P's and wanted more power. he did take the car to willowbank. the pistons are cast but would be 20-30 thou oversized or whatever to match the bore. So to cc block/head you put perspex with a small hole over the block and head and inject oil while counting the amount injected. I dont understand how you stop the oil escaping to gauge a correct reading??? especially the head which will have some valves open.. ---------- Post added 17-02-12 at 10:01 ---------- I'll get an inside micrometre to measure the bore diameter.

As far as ive seen people doing it you use a combustion chamber with the valves closed, and do as you said with the oil, stopping once you get to the rim of the chamber (this means it has to be perfectly level.) The perspex idea could be good though

Quote: Originally Posted by xAsh- [View Post] As far as ive seen people doing it you use a combustion chamber with the valves closed derrrr *slaps head*. Yep, sorry i wasnt even thinking. theres always going to be a couple of cylinders closed as theres different power strokes. as long as the head is straight and flat i should be fine otherwise i could maybe do 3 chambers to find the average.. I'll just do one. i think cc'ing the block may prove harder. maybe if the block was blocked at the deck and flipped upside down on a bench... however my motor is complete and im not pulling it down otherwise i would be going straight to the spool/cp option on my a8 block.

cp pistons best i seen - wit carrillos rods 8.6 rb26/30 should be around 550rwkw

mine are going to be 7.6:1, and im slightly worried its too low, will lag and scull fuel (has a huge cometic gasket though), more boost i guess to make up for it

Quote: Originally Posted by xAsh- [View Post] mine are going to be 7.6:1, and im slightly worried its too low, will lag and scull fuel (has a huge cometic gasket though), more boost i guess to make up for it Yep, low comp & more psi = win!!