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Compression Ratio - Working It Out


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#1 happydude2012

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Posted 04 August 2018 - 10:35 PM

I need to calculate my compression ratio - i understand i can do this with the head off or with a manometer with the head on via a spark plug hole.

 

Anyone know the proper tool to do this? id like to work it out without having to remove the head just for the moment if this is viable.

 

thanks



#2 Dusky

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Posted 05 August 2018 - 05:06 AM

You cant, need to measure the size of the combustion chambers.

#3 happydude2012

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Posted 05 August 2018 - 05:19 AM

You cant, need to measure the size of the combustion chambers.

 

are you sure?

https://www.yourmech...mpression-ratio



#4 Bat

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Posted 05 August 2018 - 06:38 AM

Hi 

It's a compression tester. 

It's a badly worded article in my opinion.

In the calculation ignore the bit with the 15 in it and use the guage psi and divide by 15.

If your CR is 9:1 your guage should read around 135.

Not sure how accurate it will be but should give you an idea.

Also there should be no more than 10% difference between cylinders.

Cheers  :proud:


Edited by Bat, 05 August 2018 - 06:41 AM.


#5 unburntfuelinthemorning

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Posted 05 August 2018 - 01:52 PM

The results are going to vary depending on the cam profile aren't they?



#6 Dusky

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Posted 05 August 2018 - 05:03 PM


You cant, need to measure the size of the combustion chambers.

are you sure?

https://www.yourmech...mpression-ratio

Thats a compression test wich will vaguely give you an idea about your dynamic compression and how good your battery is charged.

#7 Moke Spider

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Posted 05 August 2018 - 07:58 PM

 

I like the way the guy is measuring the bore diameter down to 0.1 of a thousandth with a Bore gauge, then using a ruler to measure the stroke!

 

That's well enough for me to have a good laugh while closing that page.

 

As Dusky has mentioned, the only way is to lift the head. Anything else is a guess that will be within 1 or 2 points at best.



#8 Retroman

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Posted 05 August 2018 - 11:52 PM

I need to calculate my compression ratio - i understand i can do this with the head off or with a manometer with the head on via a spark plug hole.

 

Anyone know the proper tool to do this? id like to work it out without having to remove the head just for the moment if this is viable.

 

thanks

 

Got ask why do you need to calculate it when the head is on ?

 

It can only be done accurately with the head off

 

You can find out approximately what it is, but its of litte or no use, a workshop manual will tell you what it should be.

 

The only way to 'calculate' it is to measure everything

 

https://jscalc.io/ca...RNEglVkKLQir11T

 

A useful link



#9 Pigeonto

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Posted 06 August 2018 - 07:16 AM

From my findings, if you use an on-line calculator,( way better than trying to do it yourself I find) use 2 or 3 different ones just to make sure.



#10 happydude2012

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Posted 06 August 2018 - 07:55 AM

 

I need to calculate my compression ratio - i understand i can do this with the head off or with a manometer with the head on via a spark plug hole.

 

Anyone know the proper tool to do this? id like to work it out without having to remove the head just for the moment if this is viable.

 

thanks

 

Got ask why do you need to calculate it when the head is on ?

 

It can only be done accurately with the head off

 

You can find out approximately what it is, but its of litte or no use, a workshop manual will tell you what it should be.

 

The only way to 'calculate' it is to measure everything

 

https://jscalc.io/ca...RNEglVkKLQir11T

 

A useful link

 

well in honesty - i cant afford for the car to be off the road for even one day at the moment, and so im just seeing what all the options are before i strip anything off and spend money on gasket sets and all that - ill learn in time what i can and cant do but its more a general learning question - and lucky for me asking questions here is ok ha ha.

Its a pretty easy job so im told to remove the head but i dont know as ive never done it  :lol:

If this tapping turns out to be more likely a follower than cam lobe (pleaaaasseee goooddd!!!) then ill have to remove the head anyway to replace the followers rather than a full engine out for the whole cam - and i can only determine cam wear by using the DTI on the rockers and seeing if a lobe has gone whilst maintinaing an operational engine

 

My method is to whittle everything down by elimination until ive got nothing left but to start dismantling - im in medicine and quite thorough about ruling out until i have to get stuck in.

 

Anyway, compression test commences in 30 mins - ill report back on that if there are any anomalies.

 

thanks guys.



#11 GraemeC

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Posted 06 August 2018 - 08:16 AM

I thought you had a 1275?  If so (unless it is a genuine Cooper S block) you'll have to take the engine out, and split it from the gearbox, to do the followers.



#12 happydude2012

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Posted 06 August 2018 - 09:05 AM

I thought you had a 1275?  If so (unless it is a genuine Cooper S block) you'll have to take the engine out, and split it from the gearbox, to do the followers.

 

 

:ohno: ............... :mmkay: ............ :xxx:  



#13 Pigeonto

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Posted 06 August 2018 - 09:17 AM

I thought you had a 1275?  If so (unless it is a genuine Cooper S block) you'll have to take the engine out, and split it from the gearbox, to do the followers.

Interesting thing............

Couple of days ago I wanted the cam out of one of my spare 1275 non S engines to see what it is.

Stood engine upright, clutch end down

Removed camshaft, good condition 544 fitted !!

Thought I'd have a look at the followers  and  managed to remove them all with one of those larger  telescopic magnets.

The one I did drop simply landed on the oil pump and was easy to get hold of.

I wouldn't have cared if any did fall in and dissappear  which is probably why none did !

But it struck me that if I wanted to fit new ones I reckon I could.

Just one of my 'It's a forum and all knowledge is good' type posts  :highfive:



#14 Retroman

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Posted 06 August 2018 - 11:42 AM

Happydude have you got yourself a DTi yet ?

 

Your next move is to measure the lift on each pushrod / rocker

 

Its the best diagnosis



#15 happydude2012

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Posted 06 August 2018 - 12:06 PM

still waiting for delivery - wont be long

am i setting the valve clearance to zero for this? literally no gap






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