variations in compression ratio in VB

[Danny.Lee]

Hi Brenton

you must be up late!

I think the worst of the 5mm gap will be the complete lack of squish and consequent slow uneven burn, but as I say, my days of wanting to be the fastest on the street are now over.

Danny

[pete.collings]

As an aside to this thread, there is in the knowledgebase an article by Hartley on tuning side valves, that may be of interest in deciding what to do with a VB motor.

[Danny.Lee]

Pete

As I have two heads, (the bike not me) and two barrels and pistons, I may well use the spare to try out some side valve tuning, just out of interest.

In fact the main reason I started this thread was simply interest to see if anyone was aware that there were diferences in VB heads and pistons. I have never seen anything printed that would have spelt this out, though I have seen diff CR figures quoted in the Ariel literature but no explanations.

Perhaps a bit of carefully recorded tuning would be worth doing just for the fun of it

[will.wallace]

Thanks Danny, very interesting stuff. My VB's head seems to have met the same grinder

[Danny.Lee]

Will

like I said at the top of this thread, I have recently seen another head just like the one I assumed had been molested, so there is either a lunatic with a grinder wandering the earth ruining Ariel VB heads, or, they are supposed to be like that.

Danny

[will.wallace]

I'll knock out some photos and see what you think...

[Danny.Lee]

Hi Will

I am attempting for the first time to add a photo. If this works you should see two VB heads, the dirty looking one looks like it might have been skimmed, notice no upstand above the level of the lowest fin. The lighter coloured one has an upstand about 1mm, but more than that it is actually about 5mm deeper in the valve pockets than the dirty one. With the same piston filling the cylinder the clean gives around 5:1 CR the dirty one around 7:1CR This difference could not done by skimming, this is two very different heads by Ariel.

I have reason to believe the dirty one is 1952 it has an 18mm spark plug hole, the other one is presumably later with a 14mm plug hole

Danny

[david.anderson]

Well I don’t know where I got 7 t0 1 from for the side valve. Like Danny I have just measured my head and found it to have a 140cc capacity plus 8cc cylinder volume at tdc which gives a total allowing for my cylinder skim of 150cc which comes out at 5 to 1.
I have just checked the specifications for several of the side valves and found the early ones were 5 to 1 with the last few years being 6 to 1, but my 1952VB should be 5 to 1. The maximum cylinder head skim possible for my head would be 1.5mm which will reduce the volume by 8cc which will raise the compression ratio to just over 5.2 to 1.
David

[brenton.roy]

Hi David and Danny, I checked a cast iron head - admittedly with a 20mm syringe (so error margin = lots) and get 120 ish - maybe 125 cc to the top of the meniscus.
Brenton

[Danny.Lee]

This is a puzzle, the dirty looking head came with the engine numbered TC which is '52 but it is obviously older cos of the size of the spark plug hole. With the heplex piston giving just 1-1.6mm to the top of the cylinder this would come out at 7:1 unless my sums are wrong. The cleaner head came from autojumble so no idea of age but it is bigger, 137cc as far as I can measure, this would equate with the heplex to 5:1, but it is almost certainly younger cos of the 14mm plug hole.

My problem is I cant use the heplex piston so will have to use the hepolite which is much lower CR with either head. Best I can do is 5:1 with the hepolite and the older head....

Brenton. I got to say the accuracy of my measuring is not good. I filled the heads with water and then weighed it 1cc=1grm but I cant pretend the results are dead accurate, but it seems David got very similar results 137cc by whatever method he used so I think all our figures are probably useable if not dead right.