Compresion ratio

[jonbills]

Why does a powerful engine need a high CR?

To get a high Compression and a better explosion.

A diesel wouldn't work at a low CR because it needs the Compression pressure to generate heat (like the bike pump does in your hand) to ignite the fuel.

I don't think that's quite right Rob - in terms of "A better explosion" - 'better' is a bit short on specific meaning and actually in a petrol engine there shouldn't be an explosion.There's combustion (and it's not the same.)
The reason higher compression is better is not about the burn being better, it's about the gas expansion extracting more heat - that wikipedia article explained this well I think.

 

[Farmer42]

Coming back to compression and testing it, is there a way it can be tested without the engine being in the car and not running?

Having just stripped and re-assembled an engine with refurbished head and other new parts, I wanted to make sure that the compression was ok and that I hadn't messed up anywhere before I transplanted it into the engine bay of my car.

I haven't got any oil in it yet apart from a splash to keep everything moving so I don't want to be turning it over too much.

Thanks

 

[jonbills]

you can test the compression on an engine outside of the car and without it running. But you do need to crank it over with the starter motor, so you will want all in it.

 

[andrew muir]

Thanks for the replies.
Sorry I meant compression.
I am hoping not to have to do a rebore, just replace main bearings and get some head work done.
Its goimg to run on 2inch su's hopefully.
I know my L26 measured about 165 across the board just wanted to know if i should expect more or less compression.

 

[racer]

Andrew to answer your question.
From memory, an early 280 engine used to give about a 140psi reading. A late engine gives about a 150 reading.

 

[andrew muir]

Thanks for that, it actually measured a little bit more than than so must be late flat top pistons in the motor, read about the same as my L26 engine.
Although cyl 2 did not have any compression initially but afer testing others came on song , whould this be caused by a sticking valve not seating properly?

Andrew to answer your question.
From memory, an early 280 engine used to give about a 140psi reading. A late engine gives about a 150 reading.