[Healeys] 4 Cyl Compression Measurement

[Chris Dimmock]

I totally agree with Kees

"Most important is that all cilinders show the same pressure within  
ca. 5% of each other and that there is no significant oil burning or  
crankcase pressure."

Forget the number - look at the variance between the numbers, and look  
for the visible signals of wear - exactly what Kees said.

A Healey engine is made of 4 or 6 cylinders, which should all be  
operating the same. The critical issue is making each cylinder to be  
operating identically . Identical height pistons, identical combustion  
chamber volumes per cylinder, identical cam lift etc and so on. Why do  
you think balancing is so important in any performance reciprocating  
internal combustion engine?

Once you start playing with high compression engines, with huge valve  
overlaps, you'll realise how little the actual compression guage  
numbers mean on their own at starter motor/ idle rpm. The goal is as  
close to identical as humanly possible. Not a specific number.

Chris
www.myaustinhealey.com


Sent from my iPhone

On 03/12/2009, at 9:28 AM, Oudesluys <coudesluijs at chello.nl> wrote:

> Allthough there are ways to compute the final pressure, it is very  
> complicated and probably not very accurate.
> You have to take into account pressure loss through leakage, heat  
> exchange air/cilinder/piston, engine and ambient temperature,  
> cranking speed, valve timing and overlap etc.
> Most important is that all cilinders show the same pressure within  
> ca. 5% of each other and that there is no significant oil burning or  
> crankcase pressure.
> Kees Oudesluijs
> NL
>
> Bob Spidell schreef:
>> Found a link:
>> http://en.wikipedia.org/wiki/Compression_ratio
>> Look at: " Fault finding and diagnosis"
>>
>> bs -------------------------------- Bob Spidell - San Jose, CA
>>
>> Compressing a gas causes it to heat and (try to) expand, raising  
>> pressure beyond nominal.
>> There are formulas for computing the total compression, but I'm to  
>> lazy to google it right now.
>>
>> bs
>> -------------------------------- Bob Spidell - San Jose, CA
>>
>>
>> Sorry, but I'm not following the math here. Atmospheric pressure is  
>> 14.7 psi (at sea level) so a compression ratio of 8.5 should yeild  
>> appx. 125 psi. Assuming I'm missing something, can anyone splain  
>> what it is?