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Compression ratios 

 

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 Post subject: Compression ratios
Posted: Mon Jul 17, 2006 7:23 pm 
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This is a dumb question for an egine reconditioner to ask and I don't know if it is in the right forum but anyway *shrugs*. How do I work out the compression ratio of an engine? I have the swept volume of the cylinders and the volume of the combustion chamber (including the head gasket). The reason I ask is I have been asked to build a 250 x-flow that I was told (never actually saw) was breathing hard. Have pulled motor down and cant find anything wrong with it (still got hone marks in the bores, no broken rings etc). But apparently was running 215 psi which sounds excessive. The head doesn't look as if it has had a lot machined off it (judging by the steps on the sides). It does have high compression pistons in it but I think the machinist must have machined too much off the block :?

Can anyone help?

Embarressed and confused

 

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Posted: Mon Jul 17, 2006 7:52 pm 
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compression ratio=swept volume+combustion chamber volume divided by combustion chamber volume, though you have to factor in piston dome/dish into the combustion chamber volume

 

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Posted: Mon Jul 17, 2006 7:57 pm 
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intercepter, whats your proffession? You sure know a lot of technical know how, i'm curious and in awe.

 

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Posted: Mon Jul 17, 2006 7:59 pm 
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Thanks interceptor. Once you said it like that it made sense. Numbers I am getting don't though. Works out to 30:1! Think my measurements might be wrong! :lol:

 

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Posted: Mon Jul 17, 2006 8:03 pm 
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smiley235 wrote:
intercepter, whats your proffession? You sure know a lot of technical know how, i'm curious and in awe.

according to a couple of people on here, i'm a magazine mechanic.....

no profession, but worked for engine rebuilders, mechanics, diesel fitters, etc

i should add that my version above is kinda simplified

quick search on google revealed this

http://www.muller.net/sonny/crx/compression/

 

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Posted: Mon Jul 17, 2006 10:42 pm 
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YEP...
I use this one...Has been upgraded...
If you know specs and you should? It's easy...
http://www.rosspistons.com/calculator.php

 

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Posted: Tue Jul 18, 2006 9:53 pm 
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tell them to fit a bigger cam! that should reduce the cranking pressure.
bigger cams can get away with larger compression ratios for a given fuel.

 

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Posted: Tue Jul 18, 2006 10:02 pm 
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The cam that was in it is the second biggest cam that crow could supply! Don't remember duration but had approx 515thou lift. Duration may have been 320 but don't quote me. Think compression problem is actually coming from the head being a late injected head. It has a smaller chamber than the early heads

 

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 Post subject: Re: Compression ratios
Posted: Wed Jul 19, 2006 12:54 am 
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xhute0 wrote:
This is a dumb question for an egine reconditioner to ask and I don't know if it is in the right forum but anyway *shrugs*. How do I work out the compression ratio of an engine? I have the swept volume of the cylinders and the volume of the combustion chamber (including the head gasket). The reason I ask is I have been asked to build a 250 x-flow that I was told (never actually saw) was breathing hard. Have pulled motor down and cant find anything wrong with it (still got hone marks in the bores, no broken rings etc). But apparently was running 215 psi which sounds excessive. The head doesn't look as if it has had a lot machined off it (judging by the steps on the sides). It does have high compression pistons in it but I think the machinist must have machined too much off the block :?

Can anyone help?

Embarressed and confused



also suggest check the length of the rods. if they used 200 rods previously with the wrong pistons it does all kinds of nasty things.

215psi compression?? must have one hell of a starter motor to crank it over

 

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Posted: Wed Jul 19, 2006 3:17 pm 
Getting Side Ways
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Ride: XH B&S ute, Navara 4x4

Power: 202 rwkw

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Its definately all 250. This thing used to hammer.

Those cortinas of yours are pretty wild. When we've finished this one will have to exchange pics.

 

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Bulldog the Diamantina pin its ears down flat

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Posted: Thu Jul 20, 2006 2:50 am 
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xhute0 wrote:
Its definately all 250. This thing used to hammer.

Those cortinas of yours are pretty wild. When we've finished this one will have to exchange pics.


yeah cool

as soon as i get my s**t together i'll update the page as it's been about 18 months. there's been heaps of changes and money spent in the meantime. there's also a 302W TC to put on the pile

 

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