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Model specific boards => Golf mk2 => Topic started by: Winso on 24 November 2008, 13:01

Title: Increasing compression
Post by: Winso on 24 November 2008, 13:01

I had the head skimmed on my KR valver a few months ago, quite a bit was taken off as it was fairly warped so I'm guessing the compression is a fair bit higher now. I've read a few times that higher compression gives more power, is this right? If so, why? and what negative effect does it have on the engine? (I'm assuming theres something bad, otherwise they would run higher compression out of the factory, right?)

Also, the car was set up by Stealth back in 2005, since then its done a few miles, had various bits taken off and put back on again and obviously had the headwork done, I'm guessing it could probably do with a tune up again by now?

Cheers for replies!

Title: Re: Increasing compression
Post by: Organisys on 24 November 2008, 13:12

Here's some reading (yes I know it's Wikipedia)

Concept
http://en.wikipedia.org/wiki/Compression_ratio

Higher ratio downside
http://en.wikipedia.org/wiki/Engine_knocking

Title: Re: Increasing compression
Post by: shakeyjake on 24 November 2008, 14:40

increasing the compression too much can f**k up the compression ratio and actually reduce power

Title: Re: Increasing compression
Post by: DarnPB on 24 November 2008, 18:16

The compression ratio is swept volume + clearance volume divided by clearance volume.
The swept volume is the amount of volume take due to one stroke of the piston.
The clearance volume is the gap that is left on the top of the piston at top dead centre. This gap comprises of the head volume and the rest of the cylinder and the head gasket.
So if you reduce the gap on the top of the piston by skimming the head, the swept volume will be forced into a smaller space, thus increasing total pressure.
If the compression ratio tops 10.5/1, then you may need to start using fuel additives to prevent detonation.
Increased pressure means higher temperatures before ignition. So the total temperature during and after ignition will be higher than normal. The temperature increase may go high enough to cause spontanious combustion, (explosion) rather than a regulated burn. This can cause significant damage to the pistons, valves and if too severe, the head itself. Look on youtube at drag racing and you will find some footage of cylinder heads being blown off or apart due to extreme pressures within the combustion chamber.

For my next lecture, we will be going into the pro's and cons of nitrous charging! :lipsrsealed:

Title: Re: Increasing compression
Post by: Jay on 24 November 2008, 19:04

Quote from: Darnpistonbroke on 24 November 2008, 18:16

For my next lecture, we will be going into the pro's and cons of nitrous charging! :lipsrsealed:

Oh go on! Enlighten us :smiley:

Title: Re: Increasing compression
Post by: danny_p on 24 November 2008, 19:34

meh  fit a 2nd row of injectors and use various blends of nitromethane and methanol   aka model engine fuel