Another CCing question

Nourish

Joined
Dec 2, 2009
Messages
93
Country flag
My 850 head volume measures that of previousley mentioned figures at 51.6 cc's. With no base gasket fitted the pistons sit pretty much at the deck height - so without a head gasket am I right in thinking that 51.6 + 414 (stroke x Bore) divided by 51.6 = 9.02 :1 CR? - does this seem low considering there isn't a head or base gasket involved?
 
There is a flaw in your math.

Stroke x bore = length x length which is not a volume!
You cannot divide CC volume (length pwr3 by length pwr 2 and obtain CR, a dimensionless number.

CR = (cylinder volume + CC. Volume) ÷ CC Volume = (850/2 + 51.6)/51.6 = 9.23

Since an 850 is actually an 830, the above formula becomes (830/2 + 51.6)/ 51.6 = 9.04

Slick
 
Leaking valves have more effect on compression than combustion chamber volume. When all things are ship- shape and upright, compression ratio probably does not matter much. A slight amount of wear in the needle jets can probably slow a motor much more than a slightly lower compression ratio. The two sizes 0.106 and 0.107 in needle jets, when used with quick taper needles with petrol represent a huge difference -the available adjustment is not fine enough.
Have a look at the needle jets and needles in VM Mikuni carbs - do you believe the Japanese did not need to do that ?
I like Amal carbs, but I would never use their needles, or needle jets.
I suggest a standard 850 motor is a quick motor - what holds it back is carburation and it needs a better gearbox to work with. Mk2 amals use the same jets and needles as larger Mikuni carbs, and the available ranges are much better. No British manufacturer would ever tell you that. What they sell does a job, but often not very well.
 
Last edited:
acotrel said:
Leaking valves have more effect on compression than combustion chamber volume. When all things are ship- shape and upright, compression ratio probably does not matter much. A slight amount of wear in the needle jets can probably slow a motor much more than a slightly lower compression ratio. The two sizes 0.106 and 0.107 in needle jets, when used with quick taper needles with petrol represent a huge difference -the available adjustment is not fine enough.
Have a look at the needle jets and needles in VM Mikuni carbs - do you believe the Japanese did not need to do that ?
I like Amal carbs, but I would never use their needles, or needle jets.
I suggest a standard 850 motor is a quick motor - what holds it back is carburation and it needs a better gearbox to work with. Mk2 amals use the same jets and needles as larger Mikuni carbs, and the available ranges are much better. No British manufacturer would ever tell you that. What they sell does a job, but often not very well.
Click to expand...
"When all things are ship- shape and upright, compression ratio probably does not matter much. "


Tell us WHY compression ratio probably doesn't matter much.
Substantiate your claim with facts, so we can follow along.
 
acotrel said:
Leaking valves have more effect on compression than combustion chamber volume. When all things are ship- shape and upright, compression ratio probably does not matter much. A slight amount of wear in the needle jets can probably slow a motor much more than a slightly lower compression ratio. The two sizes 0.106 and 0.107 in needle jets, when used with quick taper needles with petrol represent a huge difference -the available adjustment is not fine enough.
Have a look at the needle jets and needles in VM Mikuni carbs - do you believe the Japanese did not need to do that ?
I like Amal carbs, but I would never use their needles, or needle jets.
I suggest a standard 850 motor is a quick motor - what holds it back is carburation and it needs a better gearbox to work with. Mk2 amals use the same jets and needles as larger Mikuni carbs, and the available ranges are much better. No British manufacturer would ever tell you that. What they sell does a job, but often not very well.
Click to expand...
I'm wondering what this has to do with the question?
And everyone knows that amal designed the mikuni VM so likelyhood of parts fitting from a MK2 amal I'd say were pretty high
 
acotrel said:
Leaking valves have more effect on compression than combustion chamber volume. When all things are ship- shape and upright, compression ratio probably does not matter much. A slight amount of wear in the needle jets can probably slow a motor much more than a slightly lower compression ratio. The two sizes 0.106 and 0.107 in needle jets, when used with quick taper needles with petrol represent a huge difference -the available adjustment is not fine enough.
Have a look at the needle jets and needles in VM Mikuni carbs - do you believe the Japanese did not need to do that ?
I like Amal carbs, but I would never use their needles, or needle jets.
I suggest a standard 850 motor is a quick motor - what holds it back is carburation and it needs a better gearbox to work with. Mk2 amals use the same jets and needles as larger Mikuni carbs, and the available ranges are much better. No British manufacturer would ever tell you that. What they sell does a job, but often not very well.
Click to expand...
Thanks for your thoughts there but I was only asking about the maths!
 
concours said:
"When all things are ship- shape and upright, compression ratio probably does not matter much. "


Tell us WHY compression ratio probably doesn't matter much.
Substantiate your claim with facts, so we can follow along.
Click to expand...
Go on - start your own blooming post on why Compression matters!!! - just ignore him😢
 
Last edited:
Standard 77mm bore?

(Swept volume + combustion chamber volume)/combustion chamber volume.
​
I get 9.03.
 
texasSlick said:
There is a flaw in your math.

Stroke x bore = length x length which is not a volume!
You cannot divide CC volume (length pwr3 by length pwr 2 and obtain CR, a dimensionless number.

CR = (cylinder volume + CC. Volume) ÷ CC Volume = (850/2 + 51.6)/51.6 = 9.23

Since an 850 is actually an 830, the above formula becomes (830/2 + 51.6)/ 51.6 = 9.04

Slick
Click to expand...
True number is 828..
He did the take the actual (CV 414 + CC 51.6) divided by CC 51.6 = 9.0233
 
MichaelB said:
True number is 828..
He did the take the actual (CV 414 + CC 51.6) divided by CC 51.6 = 9.0233
Click to expand...

Yes, after posting my reply, I surmised the OP was taking 414 as the cylinder volume.
His principal error was stating "414 (spoke x bore)" , which I took as 414 = stroke x bore, which obviously does not compute.
The cylinder volume is stroke x bore x bore x Pi/4

Slick
 
Does the C.R. seem low at 9 to 1 as measured , No. With the head gasket installed, your actual C.R. will be in the region of 8 to 1 Pocketing the pistons will reduce the C.R. further (because of the 2S cam) It says in the stage 1 tuning notes to shave 75 thou off the head to get the C.R. up to 10 to 1 and in the stage two notes to take 90 to 95 thou off to get 10.5 to 1 ( there is an error in the metric measurements stated on the stage two notes) I suspect the piston cutouts seen on the 750 pistons are machined over size so there is no chance of valve/piston hitting with whatever cam/valves are used.
 
There is a resource (page 7) that allows calculation of CR ratio change when adding a shim under the barrels. The formula works for skimming the head if a negative shim thickness, h, is used to represent a head skim.

Slick
 
dobba99 said:
Does the C.R. seem low at 9 to 1 as measured , No. With the head gasket installed, your actual C.R. will be in the region of 8 to 1 Pocketing the pistons will reduce the C.R. further (because of the 2S cam) It says in the stage 1 tuning notes to shave 75 thou off the head to get the C.R. up to 10 to 1 and in the stage two notes to take 90 to 95 thou off to get 10.5 to 1 ( there is an error in the metric measurements stated on the stage two notes) I suspect the piston cutouts seen on the 750 pistons are machined over size so there is no chance of valve/piston hitting with whatever cam/valves are used.
Click to expand...
Not fitting a head gasket and base gasket must equate to something like a .060"skim? The 75mm skim quoted in the tuning notes would still include the gaskets?
 
Nourish said:
Not fitting a head gasket and base gasket must equate to something like a .060"skim? The 75mm skim quoted in the tuning notes would still include the gaskets?
Click to expand...
AFAIK 850s did not have a base gasket as standard.

And the original flame ring gasket is closer .025” crushed thickness IIRC.

You’ll have to crush a new flame ring gasket to find out for sure what it’s crushed thickness is and then ensure you only use that type in future.

If you want real accuracy and dependability of thickness, use copper, and check out the large range JS offers.
 
Last edited:
Nourish said:
Not fitting a head gasket and base gasket must equate to something like a .060"skim? The 75mm skim quoted in the tuning notes would still include the gaskets?
Click to expand...
Include the head gasket? Probably. An engine builder that knows what he is doing would use the tuning notes as a guide and not follow them to the letter. They will have decided on what state of tune they want the engine to be and build it to that requirement. Measuring everything along the way and altering things accordingly to get to that. You won't get the best out of the engine unless you treat it as a whole, from the air entry (carbs) to the exit (exhaust)
And as for a 75mm skim, thats chopping the head in half....
Finally, when you get the C.R. up to around 10 to 1 or 10.5 to 1 to suit the 2S cam you will have the problem of starting the engine. An engine in standard tune is bad enough on the leg muscles, a high compression 850? good luck with that.
 
Last edited:
You must log in or register to reply here.
Back
Top