Gti6 2.1 compression

[blandy 29]

Can anyone help with my compression calculations I’m plugging measurements into a online calculator 

Basically I have 

87mm bore 
88mm crank 
Piston dome 2.8cc
Head 37cc (yet to accurately measure) 
Deck height -1.1mm 

 

cat cams 110 (hydraulic lifters) 

I was advised by a well know builder who has built a similar engine with same components to run a 2.2mm cometic headgasket which I also have and that along with a “light skim” of the pistons to bring comp ratio down to a sensible level.

 

Now playing on a calculator before touching anything I get 13.27:1 does this sound correct. I’m yet to have the block/head skimmed yet until all the measurements make 100% sense. Also still have my standard 86mm crank as a worst case scenario 

 

i have all the above parts from either new or 2nd hand (and yes there not the most expensive or sought after makes being wossner/pec and I am aware of the various views but unfortunately that’s what I have so need to work with them best I can) and also have my standard 86mm crank if the 88 just wont work. 

[welshpug 1,657]

whats the crown height and rod length?

[blandy 29]

Rod 158mm 

Comp height 34mm 

piston specs below k9181D050 (1st owner never got round to using pistons or rods so are brand new in box hence finding the part number)

Edited December 10, 2017 by blandy

[blandy 29]

[welshpug 1,657]

so with that rod and piston you will have a zero clearance with an 86mm crank, so I'm not sure how you were planning to go 88mm?!

 

you need a 157mm rod or a 33mm crown height if you go 88mm stroke and retain one of the two parts you have, using a thicker gasket isn't ideal, taking material off the Dome is best practice.

 

 

Guessing you also have new valve springs given the 11.50mm peak lift also.

Edited December 10, 2017 by welshpug

[blandy 29]

Exactly I was following advice that as they go approx 1mm over deck that a 2.2mm gasket would compensate for that which all it seems to do is screw up the compression and my concern is How much needs to be removed off the piston to make it work. In a ideal money no object world I’d get custom rods but Need to work with what I have.

 

Using the calculators ive been using would seem I would be better off sticking with the 86mm crank as looking online the pistons and rods I have seem to be the common “package” sold by everyone as a gti6 high comp piston and rod combo. Although even with the 86mm crank and a 1.2mm pug gasket there level with the deck and gives a 13.27:1 compression ratio

 

 

Not got the springs yet but there next thing on my list as know I need them for the cams need a few valves as well so will get all done and installed when head goes to machine shop 

 

Really appreciate your help by the way 

Edited December 10, 2017 by blandy

[welshpug 1,657]

yes it does look like it would be much simpler to stick at 86mm, from the spec sheet the piston doesn't come right up to deck, but not by very much!

 

 

one detail overlooked is the valve cutout cc, it doesnt state if these are included in the total volume or not, which may explain the discrepancy in the compression volume.

 

 

to get 12.2-1 on 2044 cc you need a 45.625cc total compression volume,

 

59.44cm2 is the surface area of an 87mm bore.    (gasket bore is 87mm from memory, so the cc at 1.2mm is 7.13)

 

 

37.2cc (std head) plus 7.13 is 44.33, so if they've done a brainfart with the total compression volume and forgotten about the valve cutouts in the list then they'll be about 3.095cc

 

[blandy 29]

Yeah I dropped the 86mm crank in with a piston and it sits around 0.05mm below deck doing the best I could measure with feeler gauges. 

 

Thats the bit im struggling with as you say the specs state a 12.2:1 on a standard engine so would assume that’s with a standard gasket at 1.2mm thick.. 

 

only downside is ending up at around 2050 cc rather than a 2.1... what in reality terms would that do to performance? 

[dcc 855]

Start with a new block and take 1mm off the piston, go 88mm crank

[blandy 29]

As far as I’m aware the block hasn’t been decked before so should be standard height - the pistons on the 86mm crank confirm they come ever so slightly below deck. 

 

With the piston maching being a domed piston would they take say 1mm of uniformly so 1mm off dome part and also 1mm off lowest part? I.e if I could get 1 mm off uniformly would that keep the piston cc the same and bring piston back level with the deck 

 

whats the highest comp ratio I could get away with - only use super unleaded 

Edited December 10, 2017 by blandy

[welshpug 1,657]

if you go longer stroke you will need to modify the cutouts as well as the dome.

[blandy 29]

That’s the bit I’m really unsure of as also had a quick dummy measure with a inlet and exhaust valve and have stacks of room at tdc of around 5mm so plenty considering cam spec of 2.95mm (again if I’m reading right that is). 

 

Im guessing if they were to be machined it’s the whole lot that needs machining unimformly i.e not just a skim off the top of the dome as it’s the deck height figure were trying to amend I,e from -1.1 to say -0.5? 

 

That said im guessing if you were to just machine the top of the dome it would affect the piston dome cc which again would affect compression? 

 

 

[welshpug 1,657]

its the radial clearance to the valve that needs checking as well as the depth of the cutout.

[petert 586]

Those high dome Wossner pistons are a bit of a false economy. I have a set here that I also intended to use with an 88mm crank. Even with skimming 3mm off the dome and using a 2mm head gasket, I still end up with approx. 12.8:1. Straight out of the box, they're approx 14:1 as you said. Luckily the valve pockets are very deep and wide. However, the whole design doesn't promote good flame travel. I can't take any more than 3mm off the dome, as the crown has been hollowed out.  A flat top piston would be better.

 

Additionally, are your hydraulic cams big enough to run that much compression? I couldn't find on the Catcams site what a '110' was.

 

[blandy 29]

In a way  glad it’s not just me then with dodgy measurements as this is what has been most confusing as like you say even with the 86mm crank it’s looking like I’d be better off with the 2.2 gasket To get around 11.7:1 which when the spec suggests 12.2:1 with a normal pug 1.2 gasket. 

 

definitely looks like the 88mm crank is going to be difficult so 86 is prob going to be the safe bet with the items I have 

 

the cam cam spec I have is below I have no idea what a ideal comp ratio would be though? 

Edited December 11, 2017 by blandy

[petert 586]

Have a look under the crown and see if they've been hollowed out. I'd still be cutting the dome away in order to lower the CR. Those cams are quite tame and suit 11:1-11.5:1. If you can skim the piston, I'd still consider the 88mm crank and the thicker head gasket. What you need to avoid is a large squish distance.

 

Those cams don't have a lot of lift at TDC. Thus with the 88mm crank and the 2.2mm head gasket, you might be able to avoid machining the pockets deeper, which is a reasonably expensive exercise. Skimming the piston is cheap.

Edited December 11, 2017 by petert

[blandy 29]

thanks for your help ill grab the pistons out tonight and have a look. the cams are the wildest they do whilst retaining the hydraulic lifters which is why i went for them well that and the fact there is another supposedly identical set up running albeit it with the pistons skimmed so in theory there should be a work around but thought wise to measure as much as pos