What 10.1 or 8.5 compression mean?

[cavemanno1]

Hi guys!

Sorry for the stupid question but have seen these numbers and have no idea what they mean!Can someone explain to me why the compression is lower the more pumpgas friendly it is?How do you choose or why the compression when building an engine?

Thank you,

Peter

[2Luke2]

Hmm, should probably read up a bit.

http://en.wikipedia.org/wiki/Compression_ratio
http://en.wikipedia.org/wiki/Engine_knocking

Let me know if you have questions about it after those two short reads.

[b5blue]

Higher the number the higher the octane needed. 8.5 will run 87 Oct. 10 will want 93+ Oct. (One atmosphere is compressed X 10 for 10 to 1.) 

[Ghoste]

They simply represent a ratio.  Its comparing the volume of an engine cylinder when the piston is at the bottom of the hole and when its at the top.  The tighter you can squeeze the fuel /air charge, the more power it will make but there are a lot of physics laws conspiring against this basic explanation.

[71 SE3834V]

Higher compression creates more heat in the air/fuel charge.
The higher the octane rating the more the fuel will resist igniting before the spark plug fires.

Diesel engines have a much higher compression ratio than gasoline engines. So much heat is built up when the piston compresses the air that when the fuel is injected into the cylinder it ignites. No spark plug needed!

[1974dodgecharger]

Higher the number the higher the octane needed. 8.5 will run 87 Oct. 10 will want 93+ Oct. (One atmosphere is compressed X 10 for 10 to 1.) 

That's a general rule right not concrete? I don't have oct 93 in az as a regular but do have 91 and then the race fuels i found out 110 with lead and 100 with no lead.

I have 10.5 to 1 that's why I ask.

[cavemanno1]

Thanks guys!

2Luke2,sorry if i bothered you man!If don't wanna help just be sarcastic you shouldn't write just ignore my stupid question!
However you are right,i should have read up before i asked the question!I was being lazy 

Thanks anyway

[b5blue]

Higher the number the higher the octane needed. 8.5 will run 87 Oct. 10 will want 93+ Oct. (One atmosphere is compressed X 10 for 10 to 1.) 

That's a general rule right not concrete? I don't have oct 93 in az as a regular but do have 91 and then the race fuels i found out 110 with lead and 100 with no lead.

I have 10.5 to 1 that's why I ask.

"Generally" To each engine's build, are the heads aluminum? Aluminum transfers heat faster so a 10 to 1 aluminum head can run less octane. Octane is the ability of a fuel to not pre-ignite under heat and pressure. The entire build and even what altitude effect what octane these old motors will need. Getting "quench" just right, a very specific tolerance for the combustion chamber can also allow higher compression. You have to back off timing advance to eliminate ping or increase octane. Reducing advance timing reduces power.
  Pete the questions you ask are never dumb! It's actually smart to find what you don't know but want to understand.
I have a friend who got a call from his wife, she said honey you told me to always check the oil and water so I did and I'm worried. The Cherokee took over 2 gallons of water to top up the system this morning so I thought I would call you. Good call dear don't drive it, I'll come take you to work. 
When he got back he told us his wife had filled the engine to the top of the valve cover with water. 

[cudaken]

 

[Ghoste]

And as for generalizations, your camshaft plays a role too inasmuch as it affects cylinder pressure.  Valve overlap etc.  There are a few factors at work.

[six-tee-nine]

Yeah, and when you start with forced induction physics change again

Low Comp. Means less power but it also runs cooler, my '72 unimog only has 7,5 to 1 comp ratio. But it wont overheat even after 30 minutes at WOT @ 0 mph to get out of a mudhole.
Another reason they had such low comp engines was to be able to keep running the engines even when fuel quality was poor. These things were designed for the army back in the Day's...

[1974dodgecharger]

good info blue...goes to show ya how complicated engine building is.  Thats why I like it when I can build a copy of someones engine give me the parts and just let me build away kinda like legos with instructions everything thought out for me.

[2Luke2]

Thanks guys!

2Luke2,sorry if i bothered you man!If don't wanna help just be sarcastic you shouldn't write just ignore my stupid question!
However you are right,i should have read up before i asked the question!I was being lazy 

Thanks anyway

Oh heck no you didn't bother me at all! Sorry if my post came off as rude or short. I just wanted to give you some quick links to most likely a better explanation than I could provide. I'm glad a few other chimed in here and helped you out. I personally like the idea of someone being able to ask a basic question on the forums and get good feedback. Again my apologies if you felt like I was being rude as it was not my intention at all.  Hope you found the answer you were looking for!

 

[cavemanno1]

2Luke2,

No need to apologise you have done nothing wrong!I was too lazy and stupid to look it up that simple question!Let's just leave it like that!
Thanks for the link by the way!

Have a good day!