4k Comression

[tioklou]

hello! i have a 4k -u with a weber dgv carburator and i want rise the comression! now the engine has 175 comression! its a good value and if not,how much do you suggest to plane my head in order to rise it! the enfine has a stock camshaft! thank you very much!

[Felix]

Couldn't tell you offhand exactly how much to plane your head.

 

I'd be fitting a reground cam and skimming the head 40thou or so. You'll be making a lot more power from a cam change than from the increase in compression.

[kiahn]

from what ive read 175 sounds like pretty good compression

40 thou on the head would be as far as id go so that you have space to skim it again if you blow head gasket

I'm not 100% sure on this but everywhere ive read so far says that the higher compression you have the more aggressive cam you need

[Taz_Rx]

You could probably re-phrase that to 'The more aggressive cam you have, the more you need to increase compression.' :dance:

 

Pretty sure 4ku's have higher comp than a 4kc already. Can somebody confirm this?

[corolla_bitten]

i have a 4ku, actually 3 of them, compression is almost the same as a normal 4kc, BUT, as they have dished pistons, the heads on these motors are already alot shallower, (as in they are already shaved compared to a 4kc head). good luck shaving these heads as the valve seats are almost already level with the head surface. mine are any ways, don't know if mine have been shaved, I'm guessing not. If you want a high comp 4k get a 4ku head and put it on a 4kc with a thickish copper head gasket, standard head gasket gave me little valve marks in my flat top pistons haha. 4ku also have an extra 20hp from the factory. mine all came out of ke70 panel vans.

[philbey]

For starters, I'm guessing your value of 175(psi?) is your cylinder compression value?

 

Just keep in mind that this value IS NOT the compression ratio. If you want to calculate your compression ratio, and then work out how to increase it with a head skimming, then you need to take a number of measurements of your bore, stroke, dish and head volumes, piston crown to deck height, Crown to Ring land etc, HG Thickness and then calculate from there.

 

Compression ratio: a Volumetric function

Cylinder Pressure: a Pressure function

 

175 is sounds like a good healthy value. You should talk to your engine builder about how much planing you will need to increase comp ratio.