It's a long way to 920 type(rary)

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Fast Eddie said:
Surely that’s a job to look forward to for next winter Yves?

I’m pretty sure your valve to piston clearnece with the new JS2 will be fine, but, of course, you should check.
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Fast Eddie, you still don't know Yves very well...I betcha his squish band will be down to 0.30 before the Easter Bunny cometh! :p:cool:
 
Hi Nigel and Bruno,
I was in the workshop this moning to tcheck the head gasket I have left, I have a 0.8mm in stock, this will replace the 1.00mm I have on the engine, I hope to find the time to do it before the Dyno, but this week I have to work on Bruno's bike
Yves
 
Hi there,
I take the decision to day about my squish, my squish clearence is 1.5mm now with a gasket from 1mm
I will ask a specialized co. to make me a head gasket from 0.6mm, that will give me a squish from 1.1mm
Your advise please
Thanks
Yves
 
Yves, Every few thousanths you reduce the head gasket also reduces your valve to piston clearance. What method do you use to determin the valve to piston clearance? thanks... awesome bike btw!
 
o0norton0o said:
Yves, Every few thousanths you reduce the head gasket also reduces your valve to piston clearance. What method do you use to determin the valve to piston clearance? thanks... awesome bike btw!
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I already tchek the valve to piston clearance with the 1mm head gasket.
I did it with plasticine and there is enough clearance to go down to 0.6 head gasket
Yves
 
Hi there,
Here wath I did to day:
I take the compression from the engine; first warm up the engine and put the compression meter on it, here are the results:
8.5 Bar or 138 PSI, to me, this is not very high, so I ask a water and lazer cut Co. to make a gasket from 0.6mm to replace the 1mm I have at the engine now.
With the 0.6mm my squish will go down from 1.5mm to 1.1mm and the compression will raise (in theory) with 0.4 bar, my compression will be close to 10 Bar. My squish will be more effective also. your opinions please...
Keep you posted
Yves
 
Yves, I cannot compute PSI or bar. I understand the figures, but not relatively. I do it the old way, get the piston to TDC on the compression stroke (so valves are closed), lean the bike over so the spark plug thread is vertical, then carefully measure how much oil it takes to filll the combustion chamber, using a syringe.

I was taught to fill to the second thread to compensate for the hollow around the plug electrode (a minor detail really).

Then you simply calculate your static CR thus (combustion chamber volume + swept volume) / combustion chamber volume.

It’s a bit of a faff, but it’s the only way I know of to get a dependable figure.

For reference, my 850 runs 10.5:1 on pump fuel without issue. I put this down to the good combustion chamber shape and effective squish band effect.

Personally, I wouldn’t go to so much effort just to make such a small change! I’d put it on the list of things to do for next winter. But I guess I’m just showing how much lazier I am than you!!
 
Fast Eddie said:
Yves, I cannot compute PSI or bar. I understand the figures, but not relatively. I do it the old way, get the piston to TDC on the compression stroke (so valves are closed), lean the bike over so the spark plug thread is vertical, then carefully measure how much oil it takes to filll the combustion chamber, using a syringe.

I was taught to fill to the second thread to compensate for the hollow around the plug electrode (a minor detail really).

Then you simply calculate your static CR thus (combustion chamber volume + swept volume) / combustion chamber volume.

It’s a bit of a faff, but it’s the only way I know of to get a dependable figure.

For reference, my 850 runs 10.5:1 on pump fuel without issue. I put this down to the good combustion chamber shape and effective squish band effect.

Personally, I wouldn’t go to so much effort just to make such a small change! I’d put it on the list of things to do for next winter. But I guess I’m just showing how much lazier I am than you!!
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Hi Nigel,
I know how to calculate the CR, if I find the time I will do it, I first need to finish Bruno's bike.
But the PSI or Bars give me already an idee witch way I need to go, more compression and a smaller Squish
Yves
 
another issue of gasket thickness changes is effect on rocker on stem/cap geometry. 138 psi implies lowish 8'ish effective CR or something leaky which is not likely. Racers on high octane have found squish is not much a factor in Norton power and competitive power made w/o any squish, but may help on pump gas operation. A more radial cam distinctly tolerates low octane throttle up better but to get the max out of any engine requires it tolerates mild detonation or ain't tuned up or operated to optimal engine glee. If really want the best detonation tolerance in lower rpm zone, study up on Singh's Grooves. Did it on 18 hp and no longer bogs when blades jammed just gets deeper power tone as it smokes belts, cool. Not a power adder per say, just allows more ign advance, low down grunt before detonation so totally useless if not operating/loading to otherwise detonating conditions. Of course can go w/o a gasket if nothing fouls or skews rockers off centering.
https://www.google.com/search?clien...y-ab..3.8.1020...0i67k1j0i131k1.0.V60j-pNwLtc
 
yves norton seeley said:
Hi Nigel,
I know how to calculate the CR, if I find the time I will do it, I first need to finish Bruno's bike.
But the PSI or Bars give me already an idee witch way I need to go, more compression and a smaller Squish
Yves
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Can anyone on here give a comparison of a bikes CR and matching PSI?

IE “my 920 with PW3 can has a CR of x and a PSI of y”?

Maybe it’s just me, but 138 PSI just doesn’t compute. What do you think it should be Yves?
 
"Can anyone on here give a comparison of a bikes CR and matching PSI?"

My understanding of such things is shallow, but it seems impossible.

You cd have an engine set up for CR of 10:1 (just a ratio of volumes, a pure number with no "unit of measurement" attached) with a set of blown rings and yr compression (how much pressure you can generate in the chamber, called out in units of measurement that must be specified, such as PSI or bars) wd suck, be under 100 maybe, or less.
You cd have an engine with CR 8.5 to 1, perhaps a stock 850 with a fresh top end, and find over 170 PSI on both sides with a compression gauge.

The word "incommensurate" comes to mind, but cd be wrong...
 
Ok thanks, so Yves 138psi is actually quite low then.

I’d still want to measur it with oil Yves.

For reference, if I recall correctly, taking approx .5mm out of the equation will add approx .5 point to your CR.
 
Fast Eddie said:
Ok thanks, so Yves 138psi is actually quite low then.

I’d still want to measur it with oil Yves.

For reference, if I recall correctly, taking approx .5mm out of the equation will add approx .5 point to your CR.
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Hi Nigel and Rick
I will do the CR on saturday
The compression gauge is only to have a reference to compare after a few thousend Kms and of course the compression will change when the engine is ruining at higher RPM, for sure with overlap cams
Thanks
Yves
 
I don’t want to throw this thread off track, but I’m confused about this discussion about CR vs compression. Using the KISS approach:
My understanding is Compression Ratio is the total volume of the cylinder and head area at BDC compared to the total volume of the cylinder and head area at TDC, expressed as a ratio. Where actual compression is the maximum amount of air pressure the cylinder can hold before it begins to leak past the rings, valves, head gasket or what ever place it can escape. Two seperate but related measurements. CR is a hard number determined by math, compression is more or less determined by build tolerances. An engine with no rings or valves will have a CR but no compression.
Am I close to understanding this correctly?
 
I think you’re correct Pete.

I may have accidentally caused confusion by using abbreviation CR and the word compression ratio.

My understanding is that ‘static compression ratio’ is arrived at as I described above with my oil based calculation.

‘Effective compression ratio’ is the real life compression ratio that the engine will have when it is running. The main issue is not leaking rings etc. The main issue is cam duration, big sporty cams with long duration will allow some of the charge to escape before it is compressed, thus creating a lower ‘effective compression ratio’ when compared to the same engines ‘static compression ratio’.

Yves has a JS2 cam, which is very similar to a PW3, which I think we’d say is quite a hot cam. Thus, it’s gonna lose some effective compression as described above. Hence compression ratio is an important topic.

I don’t know how to calculate effective compression ratio, some info has been posted on this previously (by Hobot I think), but it was above my pay grade!
 
I don’t know how to calculate effective compression ratio, some info has been posted on this previously (by Hobot I think), but it was above my pay grade![/QUOTE]

Sorry to say above my pay grade too Nigel, as are quite a few other things. That’s why I let someone else engineer it and I’ll bolt it together, by the third time I might have it right, or not.

Sorry Yves didn’t mean to high jack
Pete
 
Regardless of how compression ratio computes at TDC, piston does not start compressing (at slow tick over rpm) until the intake valve closes. Hot cams close valve with piston further up bore so lowers compression pressure testing results. Hotter cams are easier to kick over though may make idle lumpy or require higher rpm to keep running. Real race only engines must often be blipped a lot at idle or may stall out d/t such low compression pressure d/t over lapped cam. Mz Peel has 10.5:1 "constructed" compression ratio but Norris D cam late closing makes it only 7.5 effective compression ratio, until mixture inertial or blower kicks in to raise ECR over 13: 1. Btw rebore to next size up raises effective CR w/o changing the stroke,( constructed CR) plus increasing displacement some. There are charts to quessitmate constructed CR by just compression PSI but only apply to standard non over lap cams. 140 PSI in Yves engine implies PW3 hot enough cam to lower effective CR almost a couple points. Cool.
 
Hi There,
To day I find the time to calculate the CR:
At TDC I fill 50cc of oil in the head, with the spark plug hole vertical
After calculation I find a CR from 10.172/ 1, this with ahead gasket from 1mm and a squish from 1.5MM.
So if I go down to a 0.6mm head gasket I will have a CR from 10.55/1 and a squish from 1.1mm
I hope to receive the 0.6mm head gasket on monday and have it fitted before friday, the day of the Dyno run
Keep you posted
Yves
 
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