Deck Height, Dome/Combustion volume chart

[rodoredas]

Hi folks,

I'm new to the engine rebuild forum with plans to start such a project maybe next year, meanwhile doing lots of theory.

I've built a Compression ratio chart to get an understanding of the tolerances of Deck height, dome volume and combustion volume in an attempt to reverse engineer the dome and combustion volume of a stock standard 2.7L engine #6370345 911/81 CR 8.5 121KW/165HP.

The absence of an engine in pieces to measure has led me to create this chart which shows for a given Bore/Stroke/Deck height various combination of Dome and Combustion volumes that give a specific Compression ratio.

Here's a sample showing various volumes give CR 8.5 for 90/70.4/1
I've cross checked the calculations against other known data eg from the JE Pistons tables for various bore/stroke/deck combinations.

2.7L engine #6370345 911/81 CR 8.5 121KW/165HP.


Here's an example of a 3.0L engine showing in 2 pictures the change in compression when deck height changes from 1mm (.040) to 0.75mm (.030) for each of the various combinations of dome and combustion volumes - Engine 6411313 930/16 CR 9.3

CR 9.3 on 930/16 engine


CR 9.8 on 930/10 engine



So what's the point of all this - my question is - what is the factory dome/combustion volumes for this engine (2.7L engine #6370345 911/81 CR 8.5 121KW/165HP.) given the data above in the relevant chart.

cheers
Shane

[boyt911sc]

Shane,

I could not follow or understand your tables. What equation or formula did you use to calculate compression ratio. Give as an example how you arrived to such number. Thanks.

Tony

[dannobee]

Here. Compare these with your numbers.

PORSCHE - PORSCHE 911 - JE Pistons

[rodoredas]

Hi Tony and dannobee,

I'm using the standard CR=Max Volume / Min Volume using the V1,V2,V3,V4 variable definitions to work out Max and Min volumes.

V1 Swept volume
V2 Deck Height volume
V3 Cylinder Head volume
V4 Piston Dome volume

CR=V1+V2+v3-V4 / V2+V3-V4

The chart use inputs for Bore/Stroke and Deck Height shown at the top.
in the chart Dome volume runs left to right, Combustion Volume top to bottom and they can run in any increment I choose 1 or .1 or .2, below I've changed the Dome Volume increment to .1

Once you enter the Bore/Stroke and Deck at the top, the cells in the matrix simply display the compression ratio for the Dome and Combustion grid cell.

I've run many test against the JE Piston chart, here's a couple of random examples showing same CR.








You can see in the first chart in my first post that there are many variations of Dome/Combustion cc that give a compression ration of 8.5 - I'm just curious as to what the real factory values are so made a calculator to see all possibilities and it works for any bore/stroke/deck height combinations.


Cheers

[Bigtoe32067]

Man, some people have a lot of free time on their hands. My brain hurts just looking at that.
When I have free time I watch Ridiculous with Rob Dyrdek. Much easier on my brain.

[Trackrash]

When I built my last motor, I did something similar. I created a spread sheet with the CR formula and spaces for all my measurements. That way after I measured my deck height, piston dome, and head volume all I had to do was input the numbers. I also validated all the published numbers for the pistons and heads.

Then I could adjust my deck height and see what affect it had. Then the spread sheet would automatically recalculate after each change I entered.
Edit to fix typo in equation

[boyt911sc]

Gordon,

Could you explain why your equation for calculating compression ratio is different from that of the OP? I have used several times in my rebuilds the equation similar to OP’s equation and got very close values to known CR’s. Thanks.

Tony

[Walt Fricke]

Gordon - I assume your spread sheet used "-V4" and not +V4?

[Trackrash]

I was wondering if anyone would notice that I had incorrectly displayed the equation.

In fact I confirmed I used the correct equation - V4 in the actual calcs and will edit my post accordingly.

[rodoredas]

Good on you Gordon a like mind!

In mine I'm coming from the point of view of not knowing Dome and Chamber volume so made it to see the possible variations of both (across and down) for given Bore/Stoke/Deck.

I assume the factory Dome/Combustion cc for a 2.7 engine 90/70.4/1 is something like 15cc/68cc or 16cc/69cc or something close to those based on the matrix in my first chart.

cheers

[rodoredas]

Hey Gordon just for fun plugged your numbers into mine with same answers, and can see how small changes in Dome cc and Combustion cc affect CR

Cheers

[targa72e]

Quick thought. If you can hit desired compression with less piston dome volume that's generally better. In general if you need less dome volume to achieve the desired compression ratio with a hemispherical combustion chamber, with two valves you will make more HP and require less ignition advance. This is particularly true with a single plug. The reason is a smaller piston dome doesn't implead flame propagation as much as a larger dome. Less important with twin plug as the flame front starts from two locations. CIS pistons are designed to concentrate the combustion around the single plug, That is why they have a offset dome.

john

[rodoredas]

Thanks John, good information to know.
cheers