Raising The Compression

[taffycrook 2]

I am building a xu9 with sri pistons.

Static CR will be above 11:1

However the camshaft I am using is such that it should work well in this set up.

Too much CR is as much of a problem as too little.

As always the results will be posted.

Edited April 12, 2008 by taffycrook

[zander205 0]

i have used with my 1.9 engine the 1.6 pistons with the head skimmed down to it's tolerance rite now not to good with working out the cr but when using a compression gauge it was reading at 270 psi on all 4 cylinders. rite now i had to pull back down due to a crack liner but if anyone can help with telling me what my compression reading is i would be great full

[ifcho 0]

Decking the block and liners achieves two things. Because the piston will be pushed above the deck and into the gasket, the CR is increased and the squish distance is decreased. Correct me if I'm wrong, but doesn't the XU5JA piston have a 37.50mm compression height? ie the top of the piston is already flush with the deck?

 

As Sandy said, decking the liners is not for the faint hearted. The risk of failure is very high if you're not tooled up for the job. I have a friend who decks them for me, but he has special mandrel to hold the liners, then faces them to length in a lathe. I suggest finding someone who has done the job before if you take that route.

 

An easier method to achieve both of the above objectives is to fit a thinner gasket. I've successfully used copper gaskets on alloy/liner engines, using 0.5mm and 0.7mm thicknesses. They work very well providing liner protrusion is good to begin with. So long as you're aware of their limitations and have the skills to fit them, they're a cheap and effective upgrade. You can buy direct from www.ridgrecrest.com.au.

 

 

Thanks Peter, I have already gone this route and now I'm waiting for the workshop to finish with my engine....

A thinner gasket is a good advice, I may try to find someone to cut one for me, as delivery from Australia to Europe won't be very cheap I guess..

[brianthemagical 1]

pardon my ignorance.. what benefit does higher CR have over lower CR? Been meaning to ask this question for a while

 

Basically, it makes combustion more efficient, not that noticable on its own tho. Dynamic CR needs to corrected when using big cams or altering the crank and rod stuff. Then theres squish to add to the equation.