SuperTeg95

May be a stupid ?, but I'm building a motor and I'd like to know that I will reach the CR that I am looking for. I know that you can't just buy certain pistons and expect them to be the exact CR that you requested. How can you be certain that you know what your CR is.

b18bturbo

Checking the compression ratio
how to check the theoretical compression ratio of an engine.
Detonation, due to high compression and low fuel octane, is
becoming an unwelcome factor with our old engines.
The tools we need are as follows. 100cc burette (a graduated
glass tube with a ground glass tap – for measuring the volume
of liquid run out from it) or large syringe, small piece of Perspex,
DTI gauge and a micrometer. First we must get familiar with
some abbreviations which are in our formulae.
SV = SWEPT VOLUME
CV = CLEARANCE VOLUME
CR = COMPRESSION RATIO
D = Diameter of bore in mm
S = Stroke or height in mm
The formulae we need are as follows
Formula 1 SV = p x D2 x S or H Formula 2 CR = SV + CV
4000 CV
Your engine may have been rebored, so the new SV
needs to be worked out. Measure the diameter of the
bore, or alternatively you may be able to see the
oversize markings on the top of the piston. The stroke
on both the 3.4 and 3.8 engines is 106mm. Put the
dimensions into the formula and work out the SV. To
find the cubic capacity of the engine, the SV needs to
be multiplied by the number of cylinders. The area
above the piston, when the piston is at TDC, is called
the clearance volume {CV}.
SV = p x D2 x S
4000
Example 3.142 x 87.5 x 87.5 x 106 = 637.48 cc { SV = 637.48 }
4000
Measuring the size of your combustion chamber
With a spark plug fitted, and assuming the valve seats are in
good order, place a piece of Perspex over the combustion
chamber and seal to the head with grease. Through a small hole
in the Perspex carefully inject paraffin into the chamber and
read the volume from the burette. We will call this Volume 1.
The figure will be approximately 98cc.
Calculating the size of the piston dome
Turn the engine over so that the piston is half way down the
bore. Put some grease around the piston land to seal the piston
to the bore. Turn the engine over so that the piston comes up
to the point that the piston dome is flush with the block face.
Wipe any excess grease from the top of the piston. Place a
10
piece of Perspex on top of the block, covering the piston. The
Perspex can also be sealed with grease. With the burette inject
paraffin into the space created above the piston. The figure will
be approximately 50cc. We will call this Volume 2
Remove the Perspex and mop up the paraffin. We need to
measure how far the piston moves up the bore to TDC. Put a
DTI gauge above the piston and allow the needle of the gauge
to rest on the piston dome. Zero the gauge and turn the engine
over to TDC. The piston will have moved up approximately
0.500in.
Using Formula 1 we must work out a theoretical volume Volume
3. The diameter is our cylinder bore size and our stroke or height
is the 0. 500in. that we have just measured - 0. 500in. = 12.5 mm
Formula 1 V = p x D2 x H 3.142 x 87.5 x 87.5 x 12.5 =75.17
4000 4000
Volume 2 = 50 Volume 3 = 75.17
If we subtract Volume 2 from Volume 3 we will have the volume
of the piston dome in cubic centimetres – 75.17-50=25.17cc
Piston dome = 25.17
Calculating the volume of the head gasket
The head gasket volume should be worked out using Formula
1. Measure the thickness of the gasket with a micrometer and
put the dimension into our formula. It is vital that the diameter
of the gasket is larger than the cylinder bore, as the thin metal
gasket overlapping the bore will create a hot spot and cause
severe pre-ignition. I often use a 3.8 gasket on a 3.4 engine. We
will call this Volume 4. The figure will be approximately 2.5cc
We now have all the information we need.
Clearance volume = Volume1 plus Volume 4 minus piston dome
98 + 2. 5 – 25.17 = 75. 33
Swept volume = 637.47
Clearance volume = 75.33
We can now work out the compression ratio using Formula 2
CR = SV + CV CR = 637.47 + 75.33 = 9.46 : 1
CV 75.33
With a standard engine running standard camshafts we must
be sure that the compression ratio is under 8.6:1. If the
compression is higher than 8.6 we have three options - remove
metal carefully from the combustion chamber, fit a thicker head
gasket or fit high performance camshafts. With the first two
options we can simply carry on measuring and checking until
we are happy with the results. The third option is not as reliable
and we are now getting into areas that are beyond the scope of
the article. A high performance cam will normally have a longer
duration and more overlap. This will reduce the true
compression ratio and help us with our problem.

earl

or for us dummies
http://www.c-speedracing.com/howto/c...c/compcalc.php

b18bturbo

yeah that will work too..

SuperTeg95

Well alright. THank you very much.