Cheap Low Comp Gti6

[allanallen 528 2 Cars]

Right, a friend of mine is building a turbo gti6 and wants to lower the compression a bit, he's got a pulsar t28 and external wastegate fitted up and is wanting to run around 12psi.

I suggested fitting some skimmed s16 pistons and rods as I know from pocketing them in the past they have a very meaty crown, not to mention the cars on the continent with de-comp plates running silly power.

 

Has anyone done this? AFAIK everything will bolt straight in but what sort of CR will we get with say 1-1.5mm removed from the pistons?

 

Cheers, Al

[Doof 11]

I'll be interested to hear how he gets on with an otherwise standard engine. What power is he expecting?

[welshpug 1,657]

I'm sure a few have skimmed meat off the S16 piston to run a longer stroke crank, pip470 and Paul Higgs from memory.

 

 

Having measured how much clearance there is in an MI16 piston I bet you could run monster cams even with a longer stroke.

[allanallen 528 2 Cars]

I would think he'll be looking for around 280bhp

 

Welshpug, I'm sure in the past you've posted up the CR of a gti6 with s16 rods and pistons? Have you a rough idea what sort of CR it'd be with a mm or so old the piston?

[welshpug 1,657]

yeah, not too difficult to figure out, I'll stick all the numbers I remember up.

 

 

RFS engine:

 

capacity=1998cc

 

total clearance volume given a known compression ratio of 10.8-1, [499.5/9.8]=50.96cc

 

combustion chamber volume is known to be 37cc.

 

gasket IIRC is 87mm bore x 1.2mm thickness [(8.7x8.7xPi/4=59.44cm²)x0.12]= 7.13cc

 

 

so dish + cutout volume [50.96-(37+7.13)]=6.87cc.

 

 

 

RFY engine:

 

Same block and crank, therefore same 1998cc,

 

total clearance volume given a known compression ratio of 10.4, [499.5/9.4]=53.13

 

combustion chamber vol - 40cc

 

gasket 87mm x 1.3mm [(8.7x8.7xPi/4=59.44cm²)x0.13]=7.72cc

 

dish+cutout volume = [53.13-(40+7.72)]=5.41cc.

 

 

 

 

Therefore, RFS head over RFY piston, using 1.2mm MLS o.e RFS gasket = [499.5+(37+7.13+5.41=49.54)] /49.54= 11.08-1

 

 

 

Take 1mm off S16 piston which is flat topped adds (8.6x8.6x3.14159)/4=58.08cm² x 0.10=5.8cc to total clearance volume.

 

499.5+(49.54+5.8=55.34)/55.34=10.02-1.

 

 

there is a standard 1.45mm repair gasket, which would add 1.488cc, that would take it down to 9.78-1

 

 

[allanallen 528 2 Cars]

Well the standard 6 engine expired a few weeks ago so we're going ahead with this now I believe. Watch this space.....

Edited January 8, 2015 by allanallen

[Tom Fenton 1,541]

Al, would the 2.0 Mi16 head not be a better place to start, larger chamber to start with plus bigger stem valves?

 

What went wrong with the old one in the end, has it been post mortem'd?

[allanallen 528 2 Cars]

He hasn't got an s16 head, he's got a j4r but he'd rather use the rs for some unknown reason!!

 

Big end went, then really went and let the pistons bosh the valves.

[petert 586]

I'd just get a custom head gasket made, say 2.5mm thick. There was a member on here from Argentinia who made them. Probably a sore point for UK members however.

Edited January 8, 2015 by petert

[allanallen 528 2 Cars]

I can skim the pistons for free peter and he already has a new headgasket.

[wicked 103 3 Cars]

@ Welshpug: in your calculation you assume that the pistons are sitting flush at deck for both engines; I don't know if they do.

 

Secondly; RFS has 158mm rods, RFY has 152mm rods, so probably you need to take of another 6mm from the piston??

edit: no issue, since the rods are taken from RFY as well... ​

Edited January 8, 2015 by wicked