I know when fitting a turbo to a gt engine it needs a decompression plate or decompression piston but if the engine was to be bored out to increase piston size and cc would this do the job and what effect would it have and would I still need a decompression plate or decompression pistons?

Ull still have high compreasion just bigger pistons?

Depends what the new pistons you're using are!

Compression Ratio = Swept Volume + Clearance Volume over Clearance Volume. (Its normal to measure the clearance volume of an irregular shape by pouring oil from a measuring cylinder into it and noting how much oil it has taken.)

As you can see, the difference in the diameter of the bore after a re-bore isn't going to make a lot of difference to CR, but will slightly increase it.

So with reading them replies am I right in thinking that if I have the engine re-bored to a larger size it won't decompress the engine just increase the capacity so to get a bigger capacity and decompress I would need to buy decompression pistons which with research is the more reliable and best way to go for decompression

I do like the sound of where this is going, but for the money involved, would it be easier to take another engine from VWs back catalogue such as an AAV or ABD that's got a lower compression ratio 'out of the box' and combine it with a GT cam to make the most of the turbo?

Just a thought and some would say I'm a killjoy for mentioning it.

I do like the sound of where this is going, but for the money involved, would it be easier to take another engine from VWs back catalogue such as an AAV or ABD that's got a lower compression ratio 'out of thebox' and combine it with a GT cam to make the most of the turbo?

Just a thought and some would say I'm a killjoy for mentioning it.

I've currently got the a AV in my car and I'm picking my GT engine up at vw action at the end of the month most of the work I'm going to do myself but machine work on the block and head I'm going to need have someone else do it for me if there is anyone that could give,me any recommendations that would be helpful or if there is someone that could do it for me I would be happy to buy beers and pay :-)

You know the 3F block, crank and rods are no stronger than an AAV? So why not use a cheaper bottom end as a starting point like Karl suggests? Both AAV and ABD have been boosted successfully in the past - the former on standard internals!

Yeah, bang the GT lump in yours to run around on and prep the AAV for boost :)

Yeah, bang the GT lump in yours to run around on and prep the AAV for boost :)

could i not get more power out of the gt lump as I'm starting with 75 bhp with the gt and 55bhp on the aav

The power you are starting with is almost irrelevant. You will be changing most of the things that dictate how much power one of these engines has over the other anyway. if you dropped the cr of the 3f lump to that of the AAV and put a GT cam in both you would have pretty much the same engine generally speaking, its only really the shit bolted to them that would separate them and most of that is going in the bin.

The AAV has already been proven to take boost well, so why waste money making a 3f lump take it when the engine you already have could already do it?

I think you take a step back little and do some research before committing to buying any more stuff. get a proper idea of where you are heading and what route is best to take. Researching costs nothing but can save you a fortune in the long run. It's best to get a good idea of what will and won't work before you put your hand in your pocket, what you save on not buying the wrong parts or having to modify stuff that you really didn't need to, can be spent on much more worthwhile parts of the project