4AGE Smallport Compression Increase

[*E9*]

I'm looking to increase the CR of my 4AGE Smallport by running a thinner headgasket, or decking the head/block.

From memory a Smallport 4AGE has 10.3:1 compression. How high can I go with compression until the motor becomes an interference motor? And what would be the highest compression ratio, that is safe to run on 98 RON, all year around?

[oldeskewltoy]

if you carefully deshroud the chambers...



you can run far more compression without detonation

[xero]

running a thinner headgasket is going to net you a MAXIMUM 0.5 of a compression increase.
decking or skimming the block/head 1mm about 0.5-0.7 of a compression increase. without replacing pistons and headwork your looking at maybe 11:1 at best, using a thinner gasket and head skim.

you will need to replace pistions and head work (such as OST has suggested) to really get an increase in compression.

becoming an interference motor will depend on the LIFT of the cams you run, and again the compression pistons you are using. for example, i have 12:1 pistons 304/296 cams with 11mm and 10.5mm lift. it is now an interference motor due to the lift.
i can run 8-9mm lift with the same duration cams and it wont be an interference motor (its fucking very very close though), but really any bigger than 9mm lift and it start to become a problem.

i have 12.5:1 compression ratio and run it on 98ron on the street (its not an everyday car) and run this all year round. this is really the highest that could be run SAFELY on the street. it will depend on your the tuner and the ECU you choose, given the large cams and high compression because i chose a tuner well versed in angry N/A engines mine starts first go, and runs like clockwork.