working out compression ratios

[Colin Morgan]

Looking at the spreadsheets - the original version is missing x Pi in the Clearance Volume calculation (B11), and as Mark says the ratio at the end needs to be total volume (not just the swept volume) divided by the compressed volume.  With these corrections the two spreadsheets agree - compression ratio of 5.9:1.

Colin

PS - Notice from playing with the numbers in the spreadsheet that a change of only 0.1mm (4 thou) in the assumed head gasket thickness makes a 0.1 difference in the calculated compression ratio (e.g. a decrease in thickness of 0.1mm increases the calculated compression ratio from 5.9 to 6.0).

[jansens]

Thanks Colin and Mark for checking it for me, I had a feeling something was off. I've fixed the mistakes and re-uploaded the correct file. Mine might need lowering a bit if I blow the engine but I have the spacer plate that can go under the block when I do that.

Simon

[JonE]

I hadn't fully appreciated the effect of the boring process and oversize pistons. Having worked out the LC head (an early one with priming brass bolts) I get about 5.2, (its like filling up a swimming pool by comparison!) which I suppose is the effect of the larger +040 cylinder being compressed into the same size head.

Btw, how are LC water outlet repair doobries affixed to the head, assuming they create a whole new mating surface? If I have a little cavity in the original mating face on the iron head, can I just fix it with JBweld or will it not last?

[JonE]

The standard Compression Ratios seem to be

1923-1933 4.8 (or 4.9) to 1

1933-1936 5.2 ? to 1

Having observed the depths of the cavities of the early LC head for the priming arrangements, does the increase in compression ratio essentially come down to the effects of their removal? I didn't think they went on as long as 1933...

[GK5268]

Btw, how are LC water outlet repair doobries affixed to the head, assuming they create a whole new mating surface? If I have a little cavity in the original mating face on the iron head, can I just fix it with JBweld or will it not last?

Do you mean the stainless steel mending plates - these have holes for drilling and tapping into the cast iron and they look as if they should fit flush against the original mating surface - as you say, to add the JB Weld to the mating surfaces should fill any imperfection and give it a permanent seal, I would not be inclined to trust to JB Weld on its own to keep it all secured, a mechanical fixing should give certainty.

On my LC head, I had the bridge properly repaired, that was quite expensive I seem to recall, now that the Ruby has an HC head, the LC head was spare and has been "wedged" and is destined for the Swallow!

[Bob Culver]

Manual for 1600cc ohv Rootes engine gives tolerance of 44 +/- 1 cc but may not be within same head.
Any old engine with c.i valve seats is sure to have at lest 1mm variation in valve height, say.5cc. it is a pity to deliberately sink valves.
On th old site I was referred to some meaurements of Seven heads.  Sample of 4 varied .3 to .7 cc within each..
Seems  .1 of a cr could be reasonable.

[Tony Press]

The standard Compression Ratios seem to be

1923-1933 4.8 (or 4.9) to 1

1933-1936 5.2 ? to 1

Having observed the depths of the cavities of the early LC head for the priming arrangements, does the increase in compression ratio essentially come down to the effects of their removal? I didn't think they went on as long as 1933...

Under the post Head Identification I wrote-

I think this is the progression after reading the cards and parts books -

The earlier low compression head with tap primer plug holes was 1A 38 (Part No BC 28)

The primer holes were deleted, but other than the parts listings deleting the plugs there wasn't a change in the part number or casting number shown on the cards for this later style of 'low compression' head up to 1933.

(no mention of a higher compression ratio at the removal of the priming plugs)

Casting number 1A 670 was introduced under Part Number BC 82 in August 1933 with a slightly raised compression (5.2?) .

(the cards specifically mention this higher compression ratio)

Casting number 1A 684 for BC 82 appeared sometime before 1936 (not sure of the change here but it is listed for the three bearing engine ?). 

In 1936 head number is listed as 1A911 - the new so called 'high compression' with two bolt water outlet.

Out of interest the later head gasket is shown as 1A 912 Klingerite or 1A 915 C/A.

[Bob Culver]

Hi Jon E

I am a bit confused about which heads you are concerned, but as per another post recently the pre and post 1933  standard heads seem to differ in the depth from the face to the plateau area

[JonE]

Bob - is that description essentially the top and the bottom of the bathtub if you turn the head upside down?

Tony - I suppose I was just seeing if anyone had measured or compared a non tap LC head vs a tapped one vs a pre1933 already before I try and look! I see its ambiguous, but it seems plain that a big "mineshaft" in the corner of the bathtub must be responsible for something of a reduction, so unless they did something else to counteract - like make the head taller - the CR must have risen at exactly that point of their removal...?

[Ian Williams]

Jon, they changed the shape and or depth of the Chamber at least three times not including the removal of the compression taps. I will dig out examples if you are interested and can wait.