Compression vs overbore

**Silverhatch** · 03-02-2008, 03:25 PM

How much does overbore affect overall compression ratio? I realize it will be lower, but is there any equation to determine exactly how much. If an engine is 11.0-1 on a standard 4.00 bore, what would the compression drop to on a .030, .040, or .060 overbore? Its obviously a larger diameter/ larger combustion chamber.

Brian

**coralcoupe1993** · 03-02-2008, 03:28 PM

Originally Posted by luxurysportscar

How much does overbore affect overall compression ratio? I realize it will be lower, but is there any equation to determine exactly how much. If an engine is 11.0-1 on a standard 4.00 bore, what would the compression drop to on a .030, .040, or .060 overbore? Its obviously a larger diameter/ larger combustion chamber.

Brian

Why would it effect compression? When I order pistions .030 or what ever, I still pick the compression ratio. The head CC wont change.

**cam303** · 03-02-2008, 03:31 PM

Actually a larger bore would result in a higher compression ratio when used with the same size stroke and combustion chamber. But if you are increasing the bore you are going to need different pistons anyway so you can make your CR anything you want with the dish of the piston.

**SonofaBish** · 03-02-2008, 03:35 PM

Cam303 is right... if keeping the same heads, and same cc pistons, an overbore will increase the compression ratio...

Check out any of the MANY compression ratio calculators on the net.... they'll help you out

**Pure Stock** · 03-02-2008, 03:42 PM

And do not forget to factor in this variable. A four-eyebrow piston contributes roughly 4 to 6 cc’s worth of volume. This is the same as adding that amount to the combustion chamber size. So the 65 cc heads you have would become larger in that sense.

**Pure Stock** · 03-02-2008, 04:08 PM

Another variable is not only head gasket thickness, *but* also if the head gasket is round (sometimes they are not) and the head gasket bore is the *same* as the bore of the block. Instead of trying to calculate the C.R. from the thickness exclusively, I look to the manufacturer of the head gasket in question and record the gasket volume in cc's.

Lastly crevice volume is another factor. Although it is a negligible amount it must be included to have an accurate C.R. Crevice volume can be defined as the tiny volume between the compression ring and the top of the piston. Typically this will not increase total volume by more than 1 cc.

These small factors may seem miniscule, however, they will ultimately affect the final C.R. numbers.

**Silverhatch** · 03-02-2008, 05:43 PM

Well the only number I can't measure is the distance from the top of the flat edge of the piston at TDC to the deck of the block. Only because the kit isn't installed in the block. Using a calculator on the net, I have some wiggle room using different thickness head gaskets to try to keep it around 10.5-1. All the calculators are setup to have the shortblock already put together. Any ideas on what a typical distance from top of piston to top of the block??? I know, dumb question. I really don't want to have to mill the heads later on to up the compression a full point or two. Are all pistons pretty much the same distance to the top of the block? whether its a stocker or stroker?

**mlowry1260** · 03-02-2008, 06:39 PM

Originally Posted by luxurysportscar

Well the only number I can't measure is the distance from the top of the flat edge of the piston at TDC to the deck of the block. Only because the kit isn't installed in the block. Using a calculator on the net, I have some wiggle room using different thickness head gaskets to try to keep it around 10.5-1. All the calculators are setup to have the shortblock already put together. Any ideas on what a typical distance from top of piston to top of the block??? I know, dumb question. I really don't want to have to mill the heads later on to up the compression a full point or two. Are all pistons pretty much the same distance to the top of the block? whether its a stocker or stroker?

They aren't all the same. On a stock motor the pistons are usually in the hole about .030-.040". What are you building? Stroke kits vary due to different compression heights from different manufacturers.

The compression height of the pistons plus the rod length plus 1/2 of the total stroke (1.5" for a 302) = the installed height. It's best to mock-up the setup & measure amount it's in the hole.

Most builders recommend you keep the quench distance under .060" to minimize chances of detonation. This means if the piston is in the hole is in the hole .030-.040", you'd need a very thin head-gasket to maintain .060" clearance between the piston & flat surface of the head. On a n/a car zero-decking the block is usually needed or recommended. Not only for quench height but, most stock block surfaces are off end to end as well.

**mlowry1260** · 03-02-2008, 07:41 PM

Give me what you know about the parts & I'll plug it into my c/r spreadsheet. What cc on the head's combustion chamber? What cc are the pockets on the pistons? No need for a headache.

**Silverhatch** · 03-02-2008, 10:15 PM

65cc on the heads
stroke 4.0
bore 4.06
either a 11.5 cc dished piston or a 22cc dished not sure which to use.
just don't know what thickness head gasket or what the deck height would be. A guy gets f'd if he has a stock rebuilt block, a set of heads, then tries to purchase a stroker kit later on. Theres no way to get an accurate measurement prior to purchase. Want to get a cr of around 10.5-1.
I plugged in a deck height of .04 and a headgasket of .021 and came up with a cr of 10.48

**mlowry1260** · 03-02-2008, 10:29 PM

If you had the pistons .020" down in the hole, use a .041" thick h/g and the 11.5 cc pistons it will have 10.79 to 1 CR. The 22cc pistons would put it at 9.73 to 1 CR. If it will be aluminum heads, go with the 11.5 cc pistons.

It will be a 414 ci motor btw.