Hi all,

just wondering what the compression ratio should be and what head gaskets I would need to achieve this?

have had a change of plan with my build so no longer need the v1 sti heads, power fc and 750 cc injectors.

i can only presume this has been done before?

 

You really need to volume the combustion chambers on both heads/ measure piston deck height/ volume at tdc etc etc yourself to get accurate measurements of what you are actually building.................. .but a Guesstimate of 46cc chambers on the STI V3 heads and using a 0.78mm head gasket with the pistons sitting 1.48mm below deck at TDC would give around 8.5 :1 static CR

Mick

 

Right ok,

I sort of understand what you are saying, I don’t suppose you have a walkthrough for beginners on the prices of doing this so I can make sure I get it right first time by any chance?

 

sorry predictive text I mean process lol.

so basically if I choose the 0.78mm cosworth gasket this would be ok, what about the Rodger Clark 0.8 mm would this also work?

what compression ratio would it be running with these or should it be running ??

 

Hi Mick,
Would you please show your calculations?
I'm doing a similar build but with 48cc combustion chambers & the STI piston has a 6cc dish.
I've spent a fair bit of time measuring things accurately, my calculations come out with 7.24:1CR as shown below. I'd be more than happy to be wrong because its causing me considerable problems.
The interesting thing is that this is oem data, so with a compressed 0.8 gasket = 0.7mm the piston to head clearance would be 2.2mm and that's about twice as much as i'd really want.
Please discuss : )

Regards Kev

92mm bore & 75mm stroke

Area of circle = Pi x radius 2 (3.145 x R x R = Area)

3.145 x 4.6 x 4.6 = 66.44

Cylinder volume = Area x Height (66.44 x 7.5 = 498.31)

Swept volume = 498.31cc



Combustion chamber 48cc

+ 93.5mm gasket @ 0.7mm = 4.8cc

+ piston/deck clearance @ -1.5mm = 9.96cc

+ piston crown 6cc

Total squish volume @ -1.5mm piston deck = 68cc

Swept volume 498.31cc / 68.76cc = 7.24:1cr

 

[QUOTE=sparks_c/s;12072291]Hi Mick,
Would you please show your calculations?
I'm doing a similar build but with 48cc combustion chambers & the STI piston has a 6cc dish.
I've spent a fair bit of time measuring things accurately, my calculations come out with 7.24:1CR as shown below. I'd be more than happy to be wrong because its causing me considerable problems.
The interesting thing is that this is oem data, so with a compressed 0.8 gasket = 0.7mm the piston to head clearance would be 2.2mm and that's about twice as much as i'd really want.
Please discuss : )

Regards Kev

92mm bore & 75mm stroke

Area of circle = Pi x radius 2 (3.145 x R x R = Area)

3.145 x 4.6 x 4.6 = 66.44

Cylinder volume = Area x Height (66.44 x 7.5 = 498.31)

Swept volume = 498.31cc



Combustion chamber 48cc

+ 93.5mm gasket @ 0.7mm = 4.8cc

+ piston/deck clearance @ -1.5mm = 9.96cc

+ piston crown 6cc

Total squish volume @ -1.5mm piston deck = 68cc

Total squish + Swept volume = total cylinder displacement

68.76 + 498.31 = 567.07cc

567.07 / 68.76 = 8.24:1cr

Looks like I've missed a crucial step in the process.

I used a number of online calculators from the US, all of them came up with 8.24:1, what I'd been expecting in the first place but a full compression point below what I'd had.
It became clear that I'd forgotten to add the full displacement of the cylinder together before dividing it by the combined combustion area at TDC. Hence the missing full compression point. My bad..
I've edited the calculations above.
I am still querying the OEM combined piston to head clearance of 2.2mm but at least for now things are making sense.
I hope this helps a few people.

Kev