Finding Compression

[Dougthe37th]

hey guys just a quick question I'm about to get my VT Dyno Tuned and befor I get it tuned I want to take the heads off and get them shaved to raise the compression a little bit. When I built the engine I was just learning and didnt ask what kind of pistons he supplied me with I know I asked for flat top ones and they where Hypertec pistons. is there any way of finding the compression ratio? and how much would I shave of the heads? the shop I'm going to said no more then 20tho because I would have issues with the intake manifold.

Thanks Guys

 

[Immortality]

Do you know what cam has been fitted? do a compression test, it will tell you what the dynamic compression is (when cranking) and will give a good indication if indeed you need more comp or not. Ideally on a fresh engine you would want to see 180psi.

There is no actual way to calculate the static CR without making a few measurements which will require the heads to come off.

If flat top pistons have been fitted then it will have in excess of 10:1

 

[Dougthe37th]

Hey thanks for the reply its a comp cam from mace stage 2 no idea what the product ID is for the cam because I wanted to look at the specs on the compcams web site. Now that I think off it they wernt complety flat at all I'm getting so pissed off now thinking about how he ripped me off when I told him what I wanted! I'm taking the heads off any way so what measurements would I need to take to find out the static CR?

Thanks heaps for the help

 

[TI3VOM]

Hey thanks for the reply its a comp cam from mace stage 2 no idea what the product ID is for the cam because I wanted to look at the specs on the compcams web site. Now that I think off it they wernt complety flat at all I'm getting so pissed off now thinking about how he ripped me off when I told him what I wanted! I'm taking the heads off any way so what measurements would I need to take to find out the static CR?

Thanks heaps for the help

Who ripped you off? And how do you know he ripped you off? If you built the engine wouldn't you know what went into it?

 

[Dougthe37th]

I thought I new what was going into it back then but now that I have a lot more experience and knowlage I know that what he was saying one thing and giving me something else.

 

[GTP 85]

you can go way more than 20 thou. Find a machine shop who knows what they are doing and they will machine the intake manifold to suit.

 

[TI3VOM]

I thought I new what was going into it back then but now that I have a lot more experience and knowlage I know that what he was saying one thing and giving me something else.

Fair enough mate, you should go back to him and question him.

 

[Cheap6]

Hey thanks for the reply its a comp cam from mace stage 2 no idea what the product ID is for the cam because I wanted to look at the specs on the compcams web site. Now that I think off it they wernt complety flat at all I'm getting so pissed off now thinking about how he ripped me off when I told him what I wanted! I'm taking the heads off any way so what measurements would I need to take to find out the static CR?

Thanks heaps for the help

You will need the swept volume and the clearance volume. Any units are OK as long as they are consistent.

The swept volume is bore area times stroke; (Pi x Bore Diameter squared / 4) x Stroke.

The clearance volume aka combustion chamber volume is the volume of any piston dish plus (or minus) the volume (if any) due to the difference in height between the block deck and flat part of the piston plus the volume due to the compressed head gasket thickness plus the chamber volume in the head. The gasket and deck height volumes are bore area (or gasket piston hole area) times the height or thickness.

Typically the chamber volume and the piston dish volume would be measured by measuring the amount of fluid - usually kerosene - it takes to fill them. A piece of plexiglass with holes to allow the fluid in and air out is sealed to the head sealing surface or piston top with a film of grease and a burette (lab equipment) used to measure the amount of fluid it takes to fill the volume completely.

CR is then swept volume plus clearance volume divided by clearance volume.

From the VS workshop manual, 0.15mm off the cylinder head thickness is ~ 1.00cc.

On the front and rear of the heads are little pads that indicate the amount of material that has been machined off the head sealing surface. 1.37-1.67mm is the new dimension, 1.12mm is the limit.

Apart from the inlet manifold alignment, machining the head will also bring the (open) valves closer to the piston and a potential issue of interference. I don't think it will be an issue on an Ecotec (it depends on cam timing and valve angle) but worth at least considering. It will also have some (minor) effect on rocker geometry.

As a check/indication, a standard 96.52mm bore, 86.36mm stroke and 9.4:1 CR gives a swept volume of 631.883cc and a clearance volume of 75.224cc.

 

[Immortality]

Google is your friend, plenty of CR calculators online

RSR Static Compression Ratio Calculator

 

[Atkins4]

There are machines around, particularly in the speedway world that can tell you your compression ratio.. They are called a "whistler"..