Bores/Comp ratio questions

[DaviE-K 0]

Alrite guys, I am a relatively new person here after being swayed towards the "dark side" by VR6pete, so do forgive me for jumping straight in with the questions! ive had a flick through the results after a search, but not really found answers to questions.

To start off, iam hopefully going to be building a VRT lump from the ground up for my mk2, but need to answer a few questions before i start looking into it any further and possibly wasting money on stuff i dont need to buy?

First one, Bores - Ive read that the blocks can be taken as far as 3.1? Would this benefit a turbo setup at all? At this stage does heat transfer between the cylinders start to become a problem due to the wall thickness being reduced?

Secondly - compression ratios. I understand the reason for lowering the compression ratios with forced induction applications, but iam unsure how the optimum compression ratio is decided? what effects to different compression ratios have?

That probably covers everything iam looking at for the moment, but no doubt ill be back with another barrage of stupid questions later lol

thanks for the help guys :]

[VWVR6T 0]

First one: I will use 3.0lt piston but no bigger than that because heat. and with a turbo a 2.9lt that is good for 500whp with the proper internals. Pass 450hp is ridiculous HP cus you get in another problem TRACTION. Just keep it between 400 to 450HP and get the car light to compensate.

second: stock internals with C2 head spacer 9.0:1 you can run 14 to 15 lbs boost. With a head spacer of 8.5:1 you can run 20lbs boost. Lower the compresion more mix you can push in the cylinder. But if you use a forge pistons the lowest CR is 9.0:1 that is OK to push 30 or 40lbs of boost cus is forge, and this is with the proper internals like forge rods.

In a MK2 a VR engine is very powerfull in any matter, because the power vs weight ratio. Get a VR6 refresh the engine get ARP rod bolts with a head spacer of 8.5:1 and stock internals with proper tuning and turbo set up you can keep it @ 400HP. You will have a hell of a fun. no PORSHE will catch you in the highway. Good luck, have fun and be safe.

[FishWick 21]

But if you use a forge pistons the lowest CR is 9.0:1 that is OK to push 30 or 40lbs of boost cus is forge' date=' and this is with the proper internals like forge rods.[/quote']

You can get any CR you like with forged pistons. I've seen some turbo engines running as low as 7:1 CR!

30 to 40lbs is fine so long as you don't run into det. Forged pistons are chosen because they are stronger and don't shatter easily under det like cast ones do. plust they are easier to customise in terms of bowl shape & capacity and skirt designs etc etc.

With 30 to 40lbs, you also run into the risk of bend rods if the manifold / wastegate doesn't flow enough, spinning rod bearings, smashing gearboxes etc etc. So I agree with you entirely, keep it to 400hp. Any more than that and it's unusable and just a pointless and very expensive numbers game.

[VWVR6T 0]

But if you use a forge pistons the lowest CR is 9.0:1 that is OK to push 30 or 40lbs of boost cus is forge' date=' and this is with the proper internals like forge rods.[/quote']

You can get any CR you like with forged pistons. I've seen some turbo engines running as low as 7:1 CR!

I woul love to have brand and part # for a set of 7.0:1cr pistons, I'm very interested.

[Lizard Racing 0]

I woul love to have brand and part # for a set of 7.0:1cr pistons' date=' I'm very interested.

[/quote']

Why? It would drive like a dog off boost!

[rigs 0]

And given the comments about sticking to 400hp... there's not a lot of point overboring your cylinders. A 2.8 can make the same power as a 3.1 if you turn the boost up by 1psi.

[VWVR6T 0]

I woul love to have brand and part # for a set of 7.0:1cr pistons' date=' I'm very interested.

[/quote']

Why? It would drive like a dog off boost!

That was my point mate, I never seen pistons that low for a VR. lol.

[FishWick 21]

I didn't say I saw a VW turbo engine @ 7:1 ;-)

It was some kind of Jap thing. I was just making a point that you can get forged pistons for any engine in any compression ratio.

8.5:1 is a nice number for a boosted VR6.

And whilst on the subject, Mahle are the best bar none. But they are $220 per piston for custom work.

Wossner it turns out are cheap chinese pistons, but branded as German. Cheeky gits.

JE are the real deal.

Wiseco, not sure about them.

Any others?

[LadVR 0]

Wossner it turns out are cheap chinese pistons' date=' but branded as German. Cheeky gits.

[/quote']

That fills me with confidence then!

[FishWick 21]

Shouldn't worry about it. I've been using mine for 30K and only just found out! Apparently they're OK for low to mid boost, but high boost (30+ psi) they probably won't handle.

[Phat VR6 3]

oh Wossner and Wiseco where the ones to get? But now what? JE but theirs ment to be alot of piston slap

ah ffs tbf as long as there uprated forged items it probably not worth worrying about

[FishWick 21]

I know mate. The trouble with the motor parts industry is you don't know what to believe half the time. Whose to say the Wossner being chinese information wasn't fabricated by a rival piston maker?

I wish I looked on the back of my Wossner box for "Made in xxxxxxx" when I had the chance!!

Even if they are chinese made, they're still designed by a german company so will have had the proper R&D put into them.

Yes JEs do run looser in the bores, but they are made of a better quality material and have better weight balancing across a set of 6 than Wossner.....apparently.

As end users of these parts, we don't have the luxury of being able try every part on the market to verify the claims!