Compression Test And Increasing Cr

[pete1986 2]

So the next thing on my imaginary list of goodies to buy is a Stage 2 inlet cam from Peter T, but he advises having at least 10.4:1 compression ratio. I have a cat version mi (xu9j4z) so have 9.7:1 as standard. But i think mine may have had some head work done in the past, so i got the compression tested and the results were as follows (from left to right):

 

1. 195psi

2. 185psi

3. 175psi

4. 175psi

 

My thoughts are that the results are pretty good, there is a small leak from the head gasket which could explain why 3 & 4 are slightly lower i suppose. But i dont understand how i work out the CR from these? I found the formula in a search, but i dont see how these results fit into it?!

 

The second part of my post is about changing piston/liners, if CR isnt up to 10.4:1. I understand that getting the liner/block decked will achieve the correct CR but i dont really want the expense. So i suppose the cheaper option would be to get some xu9j4 pistons (a mate has some spare). Is this an 'engine out' job? Also i have read about mi's suffering from liner wear, however mine has only covered about 65k, so would it actually be nessesary to replace and if so at what cost?

 

Any comments/advise apreciated

[pete1986 2]

Anyone??

[Tom Fenton 1,542]

You cannot determine the CR from a compression test. CR as referred to by Peter as required 10.4:1 is STATIC compression ratio and is a figure that is derived from the swept and unswept volume of the engine.

 

A compression tester measures dynamic compression, basically how well the piston rings and valves are sealing.

 

The only way to determine the static compression is to pull the head and do some measuring of volumes.

[pete1986 2]

ahhhh, i see, thanks tom. Ignore the first part then :ph34: Im no mechanic, but im learning bits and pieces. Praise the forum! Alelouya! lol

 

As for the second part, is changing pistons an engine out job? Would liners be ok after 65k? And if they do need replacing are they expensive?

 

Sorry for the quickfire questions, its just that i dont wanna get my mate to start the work, only for me to run out of cash half way through.

[Mikey S 29 1 Cars]

ahhhh, i see, thanks tom. Ignore the first part then :ph34: Im no mechanic, but im learning bits and pieces. Praise the forum! Alelouya! lol

 

As for the second part, is changing pistons an engine out job? Would liners be ok after 65k? And if they do need replacing are they expensive?

 

Sorry for the quickfire questions, its just that i dont wanna get my mate to start the work, only for me to run out of cash half way through.

 

 

pistons out is a head off job. not too difficult a job too do though. my own mi16 engine has a reputed 70k on the clock and theres only minor bore wear, nothing that im worried about.

 

mike

[projectpug 0]

I did mine and got 200psi to 205psi across the range just becuase i have firing problems thought it may have dropped a valve or something silly. Does 205psi sound ok? My next task is to replace all the HT's and see hot it runs then.

[pug309twin40s 1]

I did mine and got 200psi to 205psi across the range just becuase i have firing problems thought it may have dropped a valve or something silly. Does 205psi sound ok? My next task is to replace all the HT's and see hot it runs then.

 

 

that is a very good result. a good standard engine should have about 200psi.

[projectpug 0]

Cheers , any particular HT leads or will some genuine ones be ok ?

[Super Josh 4]

Cheers , any particular HT leads or will some genuine ones be ok ?

 

The GSF Bougicord ones are cheap enough and OE quality.

 

 

Josh

[pete1986 2]

pistons out is a head off job. not too difficult a job too do though. my own mi16 engine has a reputed 70k on the clock and theres only minor bore wear, nothing that im worried about.

 

mike

 

Sweet, just the answer i was hoping for. Cheers pugger

 

that is a very good result. a good standard engine should have about 200psi.

 

Is that standard for mi16? If mine are ranging between 175-195psi, what could be causing it? There is a slight head gasket leak, or maybe HT leads?

[pug309twin40s 1]

Sweet, just the answer i was hoping for. Cheers pugger

Is that standard for mi16? If mine are ranging between 175-195psi, what could be causing it? There is a slight head gasket leak, or maybe HT leads?

 

 

ht leads wont have any effect on compression!! if there faulty your have a missfire.

 

low compression on 2 cylinders could be headgasket. are you using water or oil??

[projectpug 0]

Valves, Piston rings and H/G are the main causes for loss of compression