Xu10j4r / Rfv Cr

[sub205 5]

Hello,

 

what CR does the RFV have?

The pistons got hollows, but some sources say the engine has 10.4:1, is that correct?

 

Whats the reason for that hollow?

Economy? Performance?

Edited February 14, 2010 by sub205

[welshpug 1,661]

yes they are 10.4, why the circular hollow? no idea! gti6 are ever so slightly dished rather than having the pronounced hollow, and come up to the same deck height but are 10.8.

[sub205 5]

yes they are 10.4, why the circular hollow? no idea! gti6 are ever so slightly dished rather than having the pronounced hollow, and come up to the same deck height but are 10.8.

ok, thanks for the info. i was curious to know because i brought an rfv-block to the engine rebuilder to hone and give it new piston rings. when this block would not be that good or have lower cr, i would try to find a J4/J4RS-one.

But 10.4:1 is ok, i'll shim the head a bit and then i got my 10.8:1, thanks to the variable campulleys :-)

[welshpug 1,661]

I don't understand what you mean there?

[jackherer 543]

I don't understand what you mean there?

 

It makes more sense if you substitute 'skim' for 'shim'!

[sub205 5]

It makes more sense if you substitute 'skim' for 'shim'!

yeah, thats what i wanted to say :-)

morning hours are bad for me, especially when there's not enough coffee !

[welshpug 1,661]

I wouldn't skim these heads for the purpose of raising the compression, they are very close with regards to valve-piston clearance as standard.

 

 

Best route is to leave it as it is, you wont gain a massive amount at all from trying to get a 0.4 rise in the ratio, if you do want a higher ratio fit RS pistons or aftermarket items.

 

Still don't know what the adjustable camshaft pulleys have top do with this, they have no effect on the static compression ratio!

[sub205 5]

Still don't know what the adjustable camshaft pulleys have top do with this, they have no effect on the static compression ratio!

they can be used to correct the timing after skimming the head.

[welshpug 1,661]

Ok that I understand yes, but please bear in mind the lack of clearance these engines have.

 

You can machine the standard RS piston to give more valve clearance.

[sub205 5]

Ok that I understand yes, but please bear in mind the lack of clearance these engines have.

 

You can machine the standard RS piston to give more valve clearance.

in what way is the clearance small?

 

valve - piston-clearance? what about a high lift cam?

 

i thought about putting an xu9 crank in the engine, that way i have to skim the pistons for sure. but that was only a thought.

[welshpug 1,661]

yes, valve to piston, even with the stock RS cam its very close.

 

XU9 crank would make it even worse, the pistons come right up to deck height, so I cannot say if there's enough material in the piston to remove that much from it.