Compression Ratios - what does it all mean?

[Lee172]

My block decided to fall apart on my 1.8t so im building another. I come across a 2.0 TFSI lump for a good price so my plan is to use the TFSI crank in the 1.8T, i already have IE rods so it leaves me with pistons.. is it possible to use the TFSI pistons?

I believe the standard size piston in the 1.8T is 81mm and the compression is around 9.3:1 (?) whereas the TFSI is 82.5mm and the compression is around 10:1. I understand that on older engines, high compression and high boost is a no no. But is the 1.8T capable of running both?

 

[aragorn]

you cant use the TFSI pistons, they're designed for a 4valve engine and would probably also stick out the top of the block.

You need specific aftermarket pistons that are designed for 2.0 conversions on 1.8T engines.

Something like this:
JE 1.8T 20V Stroker Piston Set: 83MM Bore, 9.5:1 CR, 92.8MM Stroke - Pistons


What exactly happened to your old engine? If you've blown a hole in it, what makes you think your existing rods are reusable?

 

[Lee172]

Thanks Aragorn, is it still possible to run higher compression alongside higher boost on a 1.8T though?

I didnt blow a hole in the original engine, i found a lump of casting in the sump.. couldnt see where it came from so im guessing it was towards the top of the engine.. could of hydro-locked when we last put the head on - may of missed some water or something like that. I seem to have slightly low oil pressure as well so thought it could be part of an oil gallery but if it were there wouldnt be any pressure at all, and there is.

 

[<tuffty/>]

TFSI pistons are a different design to the 1.8t's...

Std CR for 150/180 motors is 9.5:1 while 210/225 motors are 9:1

Bigger turbo/large boost applications generally run lower compression to allow to run high boost... you need to factor the turbo capabilities too.. your current turbo for example won't produce massive amounts of boost so you can get away with higher comp pistons (I run 9.5:1's as does welly) but on the TT we built recently the GT3076r on one run held 35psi to the redline... to sustain that 'reliably' you would want to use lower comp pistons

The pistons in the link aragorn posted is the sort of thing you want...

<tuffty/>

 

[Andrew@A.L.D]

Badger is running high comp with mega high boost

 

[aragorn]

"high comp" or just the standard 9-9.5?

10:1+ and high boost arent going to mix IMO.

 

[<tuffty/>]

9.5:1 JE's in Bill's car IICR... in race trim it doesn't run all the boost though....

<tuffty/>

 

[badger5]

new engine, which is as yet unmapped, just run in, is an unknown.. its 9.25:1 CR where my previous engine was 8.5:1

its higher CR than before.. and potentially less timing for the same boost max.. but in practical terms, max boost is pointless in it being only fwd. more spritely pickup is the aim from its new spec, and active vvt usage..

 

[Lee172]

So what sort of CR should I go for? once I have built this bottom end, I dont intend to do it again. Me being me I will probably change to a bigger / better turbo in the future. Would I be better off going for around 9:1 CR?

 

[<tuffty/>]

If going for a stroker then 9:1 is fine and tbh no different to what you have now...

<tuffty/>

 

[BuuBBa]

Hmm.. Wössner catalog gives pistons for AEB, AGN, ALT (92,8mm stroke, stroke 82.50mm, dome -10,3cm3, CR 9.2:1).
I asked about the pistons and got answer that on my S3 I would get CR around 8.5:1.

So what are the differences between BAM or AGU and with those I listed above?

 

[badger5]

9:1 vs 9.5:1

 

[BuuBBa]

Are the pistons different in those engines which would make lower/higher CR?
Or is there some differences between cylinder heads combustion chamber volumes or in other components which would affect CR?

 

[superkarl]

a3 are slightly different to s3.
as bill said 9.5:1 compared to 9:1

 

[aragorn]

Its all in the piston, the heads are identical.

So fitting 9.2:1 Wossners would give you 9.2:1, regardless of wether you started with an S3 or A3.

 

[BuuBBa]

Its all in the piston, the heads are identical.

So fitting 9.2:1 Wossners would give you 9.2:1, regardless of wether you started with an S3 or A3.

Thanks Aragorn.. Thats exactly what I wanted to hear!

I think that I have to find another company who dont try to fool me to get me to buy other pistons with twice price..

 

[Gops]

Why don't you just fit the entire tfsi lump? Be a good project that