I am about to calculate the compression ratio for my motor, and I thought I would ask a few questions:


1. What do you use for a fluid in the burette?

2. I have seen examples where people talk about using 1/4" plexiglass, etc and seal it onto the surface of the head, and then add fluid with the burette.

I dont want to go through the effort of finding plexiglass and cutting it up, etc. Could I just use a CD? Its the right size, and it already has a hole in it.

3. any really good writeups on doing this lurking around here?


Thanx,

Kirk

 

The case for a cd might be a better choice .
Its hard to see what going on , on the other side of a cd .

"What do you use for a fluid in the burette?"
Non flammable , non toxic , doesn't eat plastic , cheap ? Water .

"any really good writeups"
What info are you looking for ?

 

check out http://www.shadetreetuner.com/conten...RatioCalc.html

 

1) I use alochol with food coloring
2) Yes, you need something that can seal to the head when measuring it and plexiglass with vasoline works well. I also use vasoline on the valve to valve seat mating surfaces. You need the plexiglass so you can see as you are filling it up
3) You also need to get the net dome volume of the piston to calculate the compression ratio.

 

Does this look correct for my compression ratio calculation:

COMPRESSION RATIO = (V1 + V2 + V3 - V4) / (V2+V3 - V4)
WHERE:
V1 = SWEPT VOLUME
V2 = DECK HEIGHT VOLUME
V3 = CYLINDER HEAD VOLUME
V4 = DOME VOLUME

3.6 L
BORE = 100mm
STROKE = 76.4 mm

CYLINDER VOLUME = PI/4 * BORE² * STROKE
CYLINDER VOLUME = 0.7853982 * BORE² * STROKE

V1 = SWEPT VOLUME
V1 = BORE² * STROKE * 0.7854
= 10cm² * 7.64cm * 0.7854
= 600.05 cc

V2 = DECK HEIGHT VOLUME
V2 = BORE² * STROKE * 0.7854
= 10cm² * .095cm * 0.7854
= 7.46 cc
(.95 mm DECK HEIGHT MEASURED DURING REBUILD LAST YEAR USING CRUSHED SOLDER METHOD)

V3 = CYLINDER HEAD VOLUME
V3 = 89.57 CC (MEASURED WITH BURETTE)

V4 = DOME VOLUME
WITH PISTON 1cm BELOW DECK, THEORETICAL VOLUME (WITHOUT DOME) WOULD BE:
= BORE² * STROKE * 0.7854
= 10cm² * 1cm * 0.7854
= 78.54 cc
MEASURED VOLUME USING BURETTE = 44.4 CC
DOME VOLUME = THEORETICAL VOLUME - MEASURED VOLUME
= 78.54CC - 44.4CC
= 34.14 CC

COMPRESSION RATIO = (V1 + V2 + V3 - V4) / (V2+V3 - V4)
= (600.05 + 7.46 + 89.57 - 34.14) / (7.46 + 89.57 - 34.14)
= 662.94 / 62.89
= 10.54:1

The deck height was measured when I rebuilt the engine. Although the pistons were machined to lower compression, I dont see how the deck height measurement would change, as only the dome of the piston was machined.

The compression ratio I wanted was about 9:1. How can I still be at 10.5:1 after having the pistons machined down?!?!?! Where did I go wrong on this?!?!

Kirk

 

kirk, you can try this also http://www.imperialclub.com/Repair/E...ompression.htm

it is late and i'm tired. all the numbers just seem to run together.

 

Can someone tell me how many CCs a factory 964 piston dome is?
What is a typical factory cylinder head volume of a 964 head?

Thanx
Kirk

 

I went and got out my auto math book and read it , yet again . Some day before I pass away , I might remember ...

Anyway I think I'm ready to kinda help .

"Can someone tell me how many CCs a factory 964 piston dome is?"
I have unused pistons that I could check , if no one comes up with a number .

 

Hi Kirk,

I have run your numbers and unless I missed something they look good. I have looked for stock volumes with no success so far.

How much material did they remove from the dome? Could they have "missed" one of the pistons they removed material in "steps" and you are measuring that particular piston? Have you cross checked the measured the piston you used against another one to determine they are indeed the dome height with a depth gauge?

Since I can't offer anything else at the moment and the compression ratio dropped only a point I thought either they didn't cut enough or they mis-cut the piston you are using. I don't know but I would think they would need to cut a lot to drop almost 2.5 points.

I'll keep looking. Good luck.

 

Hey! Wait a minute, the V2 deck height does change because the machine shop removed material from the top of the piston. TDC remains the same, but the volume is increased becasue of the material removal. I think you have to re-measure the deck height:

Deck Height=
dome height+Depth #1 - Jo-block height

Yes???

 

Well since the deck height measurement i did, measured crushed solder along the edge of the piston only, I dont see how it could be any different now.
Admittedly I didnt really understand what they were talking about int he anderson book about measuring the dome, in regards to the deck height.

Kirk

 

I am going to do the piston dome volume measurement again today.
And I think I will post a new thread asking about the standard dome volume and head volume. That would answer alot of questions right away about whether I screwed up up the measurement.

Kirk

 

Here is what I am thinking. The dome height was shaved reducing the space it occupies, while increasing the total sum volume of the combustion chamber. I think that will impact your equation and your result. Unless I have completely missed something that should reduce your compression ratio some more.

Its the only thing I can find that could skew the results. I don't pretend to know this blind, I'm just a fellow Rennlister trying to help.

I cannot find stock numbers anywhere. There aren't too many folks that go this deep or even care to. Once you verify the deck height, I do have a racing source that can crunch the numbers if you like.

 

In the for what its worth dept, I just found this statement while searching for stock numbers.

http://www.356registry.org/Articles/LaDow/

"It is commonly and mistakenly measured by putting clay or solder on the piston top, rotating the assembled engine, and measuring the compressed thickness. The resulting dimension tells you something about the relationship between the piston dome and the combustion chamber, but has only an accidental connection with the deck height.

Accurate measurements here start with finding the piston deck. It really doesn’t matter where it is, so long as what you measure to for the deck height is the same surface you measure to for the dome volume. With the tail of a caliper against that established surface, cylinders and pistons in place on a mock-build engine, you can determine the height of the cylinder above the deck; deck height dimension.

Commonly, the values will wander a bit on the first measurements. Like using the tools to find the chamber and dome volumes, it takes a bit of practice to produce repeatable numbers. And you may find some inconsistencies as a result of those five variables mentioned above. You’ll have to find where the variance is and correct it."

 

I think your math went wrong with the net dome volume of the piston. It is nowhere near 44cc. A 3.8RSR piston is 35-36cc net dome volume and I'd have to look at some notes to see what the last 964 one was. There is also the arear between the piston and cylinder from the first compression ring to the piston deck that should be included in the measurements.

Your cylinder head volume looks in the range a standard head should be at.