Increasing compression ratio by skimming

A1ex · Post by **A1ex** » Mon Apr 16, 2012 5:16 pm

So was talking to LLL on the weekend and we got onto the topic of increasing compression ratios. Then it reminded me that the CR is increased on the type S by having domed pistons to reduce the cylinder volume.

So did some googling to find the piston dome size for the a5 (-2cc) and the S (+3.9cc). You can then use this to create a relationship between cylinder volume and CR (providing its linear) and work out the CR increase through skimming the head

Not sure how accurate it is as there maybe other factors involved but I worked it out as for each 1mm skimmed off the head it gives a 0.04 increase in the compression ratio. Only trouble is I can't remember how much was skimmed off my head

but I'm guessing it won't be much more than a couple of mm.

Good news is my a5 could be close to 10.1:1 now, get in!

The calcs would be strictly for a h22 motor, anyway enough geeking for one day!

Post by **Gayno** » Mon Apr 16, 2012 5:27 pm

...and you don't even share the calculation with us!!

I imagine it will be along the lines of "cylinder radius (in cms) squared" x "pi" x 0.1 (1mm in cm) x 4 (number of cylinders) = the number of CC lost due to 1mm of skim.

A1ex · Post by **A1ex** » Mon Apr 16, 2012 5:32 pm

Calculation? I did it all in my head, I'm like rain man! Not really, I'm just lazy

I did it on a single cylinder, it'll be the same for each one but other than that your calc is the same as part 1. You then need to link the CR to the change in CC through the use of different pistons before you can use the change in cylinder volume to get the change in CR.

Post by **Donald** » Mon Apr 16, 2012 5:45 pm

Very interesting but a bit like

if you're not too gifted with maths.

Post by **Gayno** » Mon Apr 16, 2012 5:50 pm

Gorgon wrote:Very interesting but a bit like

if you're not too gifted with maths.

Really? I failed A Level maths!!

judderod · Post by **judderod** » Mon Apr 16, 2012 6:13 pm

http://www.zealautowerks.com/hfseries.html

Enjoy

Post by **Donald** » Mon Apr 16, 2012 6:27 pm

Gayno wrote:
Gorgon wrote:Very interesting but a bit like

if you're not too gifted with maths.
Really? I failed A Level maths!!

Me too, for a reason I guess

It's not too bad but when you want to get really accurate and get down to the small numbers and other stuff, it's easy to get tangled up with what goes where

'92 Si VTEC · Post by **jjmartin349571** » Mon Apr 16, 2012 6:40 pm

Gayno wrote:
Gorgon wrote:Very interesting but a bit like

if you're not too gifted with maths.
Really? I failed A Level maths!!

I failed A level maths too, now I work in accountancy... Shhh don't tell anyone

As for the increased compression ratio, I considered getting my Accord's block skimmed in order to increase the compression ratio, but is there anything else you'd have to uprate or could the stock internals take it?

A1ex · Post by **A1ex** » Mon Apr 16, 2012 6:51 pm

It's a comparatively small increase so stock internals would be fine, mine are.

Skimming is pretty common practice when swapping heads etc

'92 Si VTEC · Post by **jjmartin349571** » Mon Apr 16, 2012 7:13 pm

Yeah had my Golf one done for the head gasket, I thought in that scenario it was fractions of a millimetre removed though, I'm imagining that for a decent increase in compression you'd have to lose a lot of metal? (comparatively speaking)