best compression ratio

[CHARGIN68]

What is compression ratio? The compression created per stroke? Why is this number important? Is the higher the compression ratio the more horse power? How do you measure compression ratio? Thanks for your help. I know this is important to the combo of the engine, but I don't know why. 

I am running;
383 .030 over
727 auto
750 4bbl
tti headers
eddy aluminum heads
.455 purple shaft cam

[cudaken]

 Oh God, this could take 3 days for me to miss spell. There are so many thing that come in to play with this question, no wonder no one has answer yet.

Ok first the simplified verson. Let say 1 cylinder holds 10 cubic inches of air. Then the head holds 1 cubic inch air. Piston's push 10 CI in to 1 CI that is 10 to 1 compression. That is what is called mechincal compression and what most of us talk about.

Next is what I would concered the real compression. Static compression! It is based on what is called Volumetric Efficieny. (There is a big word for old Cuda Ken) this is where things get a little more diffacult for me to spell.

OK, lets stick with the 10 CI cylinder and 1 cubic in head. Volumetric Efficieny is how much air your engine can get in to the cylinder. Most stock HP engines Volumetric Efficieny is 80%, well around 1977 that was ture . So, only 8 cubic inches of air and gas is getting into the 10 cubic inch cylinder, that is being compressed into the 1 cubic inch head means the Static compression is 8 to 1.

Basicly that is the answer, but there is more if you really need to know.

Other thing's can come in to play, lift and duration of cam (at higher RPM's have a better Volumetric Efficieny) over lap of the intake and exhaust valves (lowers Volumetric Efficieny at lower RPM's and less clyinder pressuer), over bore of the clyinder, head gasket thickness, head CC's.

My Runner has a good sizes cam with a lot of over lap and 10 to 1 pistons, but yet I can run cheap gas 89 octane unless I want to beat on her over 4500 RPM's. After that the Volumetric Efficieny picks up and I have spark knock.

My head hurts, does your?

                           Cuda Ken

[charger_72_318]

now, that's a really good kindergarden explanation, even i understood

[firefighter3931]

Difficult question to answer....the best compression ratio is the one that is matched to the other components for the car.  

Basicly the static compression ratio is as Ken describes.....cylinder volume compressed into the chamber crates a mechanical compression ratio. The height of the pistons along with the chamber volume and the head gasket volume all contribute to the mechanical or static compression ratio.

Dynamic compression is a combination of the static compression and valve timing events. Those events are controlled by the camshaft which lets air in and lets air out of the cylinder. Cylinder pressure is what makes power. The cam determines to a large extent when that power will be made and how much. Cylinder head efficiency also comes into play and needs to be matched up with the cam....specificly the lift. If the heads stall at .500 lift, there's no point opening the valve to .600 or more. The cam's duration needs to be matched to the rest of the combo...especially the gearing and stall speed. As Ken mentioned, volumetric efficiency increases with engine speed. The engine's peak volumetric efficiency is usually where it makes peak torque as measured on a dyno.

As a rule, higher compression equals more power...all things being equal. It's really more complicated than that, but that's the general idea. Of course everything needs to be matched to take advantage of that increased compression. There are other considerations that come in to play, such as fuel....race gas or pump gas and how streetable the car needs to be. Streetable means different things to different people and everyone has their own definition of what they consider streetable.

Ron

[mbandit]

With the aluminium heads it should be possible to get up to around 10:1 on 93 octane.  There's plenty of good calculators out there, try Not2Fast.com are the kb-silvolite piston website.  Don't get too hung up on the CR.  What you are looking for is a good quench area with about .040' clearance between the piston and head and a cylinder pressure around 170 lbs.  This way you won't have detonation problems.

Your cam choice will also affect the dynamic CR - cam duration helps bleed off pressure.

Mel

[CHARGIN68]

Thanks for the answers. Very helpful. So if I stroke the 383 I would not only gain by extending the stroke but also through a higher compression ratio. Right. Can the compression ratio be too high?

Thanks

[golden73]

Can the compression ratio be too high?

Thanks

Yes. This is called detonation. It creates a pinging/knock in the engine. The solution to that is running a higher octane fuel.

Higher the octane, higher compression you can run.

[cudaken]

 With your heads should be on the boder on detonation at 10.0 to 1.

Stoking the engine, boy that is a whole new can of worms. That brings wirst pin height rod length, piston height over block deck into play. Sorry to say I will pass on this part. 

Way I am feeling right now I hate to say it but if it runs good leave it a lone. Just pulled a 85 mile $7000.00 440 and there is problems. There is a shock. :icon_smile_angry:m Only BB I did not do my self. OK, Time to shut up, lick my wounds and have at it again.


                               Cuda Ken

[Chryco Psycho]

with alum heads a lot of the combustion heat is absorbed by the alum so you really want to be approx 1 point higher
so if you would run 9.8:1 on the edge of detonation with premium pump fuel you will want 10.8:1 with the alum heads to get the same amount of combustion heat /  power