A laymans guide to an engine rebuild part one

Kev · Post by **Kev** » Tue Nov 20, 07 6:43 pm

10.5 static? Work out your dynamic cr as this is real world....

Blue · Post by **Blue** » Tue Nov 20, 07 7:25 pm

Pete, need to know how far the piston is down the bore at TDC and how many cc the heads are to work out the static compression. Factoring in the cam profile will give the dynamic compression which is the important bit. Ideally you would want the piston at zero deck or very close to it (flush with the cylinder head face) to get maximum quench.

MrNorm · Post by **MrNorm** » Tue Nov 20, 07 7:50 pm

You didn't say what heads you have either? (Doesn't change what you need to do but will change what an acceptable static, therefore dynamic CR is).

What makes you say it is 10.5:1? If it is just the piston spec it doesn't really mean much without the other stuff.

Anonymous · Post by **Anonymous** » Thu Dec 13, 07 5:07 pm

Hi Techies,

Whats the formula for dynamic compression?

I assume its knowing when the inlet valve closes on compression stroke, then refiguring the swept volume?

Am I right with that or is it much more complex?

Baccy.

Ivor · Post by **Ivor** » Thu Dec 13, 07 5:22 pm

I'm a big fan of cylinder pressure, but that can change with all sorts of variables, not least the cam!

The way I understand it Baccy is that a static 12:1 static compression ratio means that the swept volume of the cylinder is 12 times the volume of the combustion area.

Dynamic compression ratio is the swept area of the cylinder from when the intake valve closes so that depends on the cam timing and is therefore lower than static compression ratio.

Does that make sense?

It does to me!

Oops, forghot to mention, I reckon you are right!

dustymopar · Post by **dustymopar** » Thu Dec 13, 07 5:24 pm

Baccy, Have mailed you a thang that sort of calculates it

Ivor · Post by **Ivor** » Thu Dec 13, 07 5:28 pm

Can you stick it on the board Dusty? I reckon we could all use that!

dustymopar · Post by **dustymopar** » Thu Dec 13, 07 5:34 pm

This may help....

Ivor · Post by **Ivor** » Thu Dec 13, 07 5:57 pm

Fantastic, thanks mate, I'm going to use that!

Blue · Post by **Blue** » Thu Dec 13, 07 6:16 pm

This is a good article,
http://www.popularhotrodding.com/engine ... index.html

Based on what I've read I'm planning to start off with a static compression of 11.5, and choose a cam to end up with a dynamic ratio of about 9.5 with around 200psi of cylinder pressure. This hopefully will run OK on pump gas, unless I'm somehow misunderstanding the tech......

Anonymous · Post by **Anonymous** » Thu Dec 13, 07 10:27 pm

From what i understand , anything over 10.5 static compression will yield detonation on pump gas , not worth the risk , my intended stroker will be zero decked , 84cc combustion chambers , twelve cc dished flat top pistons with 40 thou compressed head gasket , anything more or less than a 40 thou quench can lead to detonation.

Those yanks like throwing ideas around.

mopar_mark · Post by **mopar_mark** » Thu Dec 13, 07 10:47 pm

Adam68 wrote:From what i understand , anything over 10.5 static compression will yield detonation on pump gas , not worth the risk , my intended stroker will be zero decked , 84cc combustion chambers , twelve cc dished flat top pistons with 40 thou compressed head gasket , anything more or less than a 40 thou quench can lead to detonation.

Those yanks like throwing ideas around.

Adam,
Please refrain from from giving to much information away, this is the UK Mopar Muscle board, not the USA version

Anonymous · Post by **Anonymous** » Thu Dec 13, 07 10:54 pm

mopar_mark wrote:
Adam68 wrote:From what i understand , anything over 10.5 static compression will yield detonation on pump gas , not worth the risk , my intended stroker will be zero decked , 84cc combustion chambers , twelve cc dished flat top pistons with 40 thou compressed head gasket , anything more or less than a 40 thou quench can lead to detonation.

Those yanks like throwing ideas around.

Adam,
Please refrain from from giving to much information away, this is the UK Mopar Muscle board, not the USA version

, too right Mark , am i gonna have to start lookin over me shoulder , or will i be doin that @ the track.