pf1
Hero member
- Location
- Portsmouth UK
Yes the Ceriani loops always snap if they aren't rubber mounted.
Engine compression results
- Thread starter 78jotadave
- Start date
piranha-bro2
Hero member
- Location
- Melbourne
My SF1 mudguard 'braces' never even looked like breaking in 160,000+km of riding on every type of road surface thrown at them.
Can anyone explain to me how there can be a connection between blown fork seals and a fork brace???
Just doesn't make any sense to me.
Can anyone explain to me how there can be a connection between blown fork seals and a fork brace???
Just doesn't make any sense to me.
Laverdalothar
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- Location
- Germany
Can anyone explain to me how there can be a connection between blown fork seals and a fork brace???
In theorie, a fork brace could "bent" the fork which would cause the chromed tube to "scrap" a bit more on one side of the O-Ring. I would assume though, that if this happens, the fork will get very stiff and sticky and it should be recognized something is wrong. Who knows - never had that. Ride my fork with the same seals now since more than 20 years and more than 40.000km
Dellortoman
Hero member
- Location
- Tasmania, Australia
It's interesting that such a simple question doesn't have a simple answer.
My advice is that if the bike is running OK and not using heaps of oil or blowing clouds of exhaust smoke, or have pistons rattling around in their bores, then don't worry about the numbers. Just enjoy riding it. That way a happy new year is pretty much guaranteed.
Your question only has a truly meaningful answer if you previously measured the compression on that engine under exactly the same environmental conditions, with the same gauge, when the engine was new and in tip-top condition. Then you can tell if it has changed over time.
120psi sounds reasonable to me, but someone with more experience at compression testing than me might be able to tell you if it's in the "normal" ballpark for your engine.
We can do some arithmetic if you like. Anyone who's bored shitless with maths can stop reading at this point with my apologies. Maths is how I make sense of the world.
Compression ratio is a bit misleading because for a good part of the stroke the inlet valve is still open, during which time the piston does no compression at all. With A11 cams, the inlet valve closes at 82° ABDC, or 98° BTDC, at which point the piston has already covered 36.7mm of its stroke. The remaining effective stroke as far as actual compression is concerned is only 37.3mm (note that these numbers will vary a bit depending on valve clearance).
It's interesting to note at this point that the 4C camshaft closes the inlet valve earlier, at 74° ABDC, giving a compression stroke of 42.6mm. 14% more than the A11.
The maths for working out piston travel vs crank degrees is nothing more complicated than high school trigonometry and Pythagoras's squaw on the hippopotamus. I can assemble the general calculation on a spreadsheet for anyone who's interested in applying it to other engines.
If your CR is nominally 8:1, then the effective CR from when the valve closes so the piston can actually start to compress the gas mixture is only 4.7:1. Compressing atmospheric air at 14.7psi by a factor of 4.7, you end up with 68.6psi (absolute). Gauge pressure would be . So why does your gauge read 120psi?
Well, all of the above numbers are simple enough to calculate, but it only applies to isothermal compression (no change in temperature from start to finish of the compression stroke). What happens in reality is not isothermal. The air heats up as it's compressed so there is a thermal component that's not so easy to quantify. In theory, if there's no heat gain or loss through the walls of the cylinder as the gas is compressed, it's called Adiabatic compression. The maths for adiabatic compression is way too complicated to go into here (if you're interested, Wikipedia covers it quite well, but be warned, there's pages of mind-boggling mathematics https://en.wikipedia.org/wiki/Adiabatic_process)
Suffice to say that the gas gets hot enough during compression to significantly increase its temperature. The interesting bit is that we can work out how hot the compressed gas got to produce the indicated temperature. It might surprise you.
Let's take a snapshot of the compressed volume. As calculated above, if it stays at room temperature, it'll be sitting there at 68.6psi. The universal gas law says that PV/T is constant, so forgetting the volume change for now (that's already happened) it reduces to P/T = constant.
or T(final) = P(final)/P(initial) x T(initial) (temperatures must be in degrees Kelvin).
Taking the 120psi gauge reading as the final pressure and adding atmospheric pressure to get an absolute pressure of 134.7 psi, we can see that the pressure increase due to temperature is a factor of 134.7/68.6 =1.97. Assuming a starting temperature of 20C (293K), the final temperature must be 293 x 1.75 = 576K, or 303C. That's pretty hot. You only need about 190C to roast a chicken. (Hmm ... I think I just invented the compression BBQ).
When you think about diesel engines that use the compression to ignite the mixture. The fuel mixture needs to get to around 600C to ignite. That's why diesels typically have a CR of around 20:1. I guess 300C gas compression temp in a petrol engine is not so surprising after all.
Happy new year.
My advice is that if the bike is running OK and not using heaps of oil or blowing clouds of exhaust smoke, or have pistons rattling around in their bores, then don't worry about the numbers. Just enjoy riding it. That way a happy new year is pretty much guaranteed.
Your question only has a truly meaningful answer if you previously measured the compression on that engine under exactly the same environmental conditions, with the same gauge, when the engine was new and in tip-top condition. Then you can tell if it has changed over time.
120psi sounds reasonable to me, but someone with more experience at compression testing than me might be able to tell you if it's in the "normal" ballpark for your engine.
We can do some arithmetic if you like. Anyone who's bored shitless with maths can stop reading at this point with my apologies. Maths is how I make sense of the world.
Compression ratio is a bit misleading because for a good part of the stroke the inlet valve is still open, during which time the piston does no compression at all. With A11 cams, the inlet valve closes at 82° ABDC, or 98° BTDC, at which point the piston has already covered 36.7mm of its stroke. The remaining effective stroke as far as actual compression is concerned is only 37.3mm (note that these numbers will vary a bit depending on valve clearance).
It's interesting to note at this point that the 4C camshaft closes the inlet valve earlier, at 74° ABDC, giving a compression stroke of 42.6mm. 14% more than the A11.
The maths for working out piston travel vs crank degrees is nothing more complicated than high school trigonometry and Pythagoras's squaw on the hippopotamus. I can assemble the general calculation on a spreadsheet for anyone who's interested in applying it to other engines.
If your CR is nominally 8:1, then the effective CR from when the valve closes so the piston can actually start to compress the gas mixture is only 4.7:1. Compressing atmospheric air at 14.7psi by a factor of 4.7, you end up with 68.6psi (absolute). Gauge pressure would be . So why does your gauge read 120psi?
Well, all of the above numbers are simple enough to calculate, but it only applies to isothermal compression (no change in temperature from start to finish of the compression stroke). What happens in reality is not isothermal. The air heats up as it's compressed so there is a thermal component that's not so easy to quantify. In theory, if there's no heat gain or loss through the walls of the cylinder as the gas is compressed, it's called Adiabatic compression. The maths for adiabatic compression is way too complicated to go into here (if you're interested, Wikipedia covers it quite well, but be warned, there's pages of mind-boggling mathematics https://en.wikipedia.org/wiki/Adiabatic_process)
Suffice to say that the gas gets hot enough during compression to significantly increase its temperature. The interesting bit is that we can work out how hot the compressed gas got to produce the indicated temperature. It might surprise you.
Let's take a snapshot of the compressed volume. As calculated above, if it stays at room temperature, it'll be sitting there at 68.6psi. The universal gas law says that PV/T is constant, so forgetting the volume change for now (that's already happened) it reduces to P/T = constant.
or T(final) = P(final)/P(initial) x T(initial) (temperatures must be in degrees Kelvin).
Taking the 120psi gauge reading as the final pressure and adding atmospheric pressure to get an absolute pressure of 134.7 psi, we can see that the pressure increase due to temperature is a factor of 134.7/68.6 =1.97. Assuming a starting temperature of 20C (293K), the final temperature must be 293 x 1.75 = 576K, or 303C. That's pretty hot. You only need about 190C to roast a chicken. (Hmm ... I think I just invented the compression BBQ).
When you think about diesel engines that use the compression to ignite the mixture. The fuel mixture needs to get to around 600C to ignite. That's why diesels typically have a CR of around 20:1. I guess 300C gas compression temp in a petrol engine is not so surprising after all.
Happy new year.
That tops the interesting scale to the max Cam, so what temperate did my 3c reach using its F1 Keith Cam, the same spec as used in RGS but obviously configured to my 180 at a measured Compression of GLUP 205psi, yes it blew my mind at the time as well, when it melted the ring lands off 2 cylinders, answers on the back of an envelope. No pinging was heard but I wear earplugs. Yes, it's a fast bike. Built to Corsa Spec plus a bit more.
Dellortoman
Hero member
- Location
- Tasmania, Australia
What's the nominal compression ratio Vince?
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martymoose
Hero member
- Location
- Sydney, Australia
2015 triumph Street triple, road use, 30,000km.
L 250 psi
C 100 psi
R 230 psi
Regular servicing, no outward signs of abuse, valve clearances ok. Hard to start.
L 250 psi
C 100 psi
R 230 psi
Regular servicing, no outward signs of abuse, valve clearances ok. Hard to start.
It will be interesting to see if it reached alloy melting temps, well it did melt so obviously it did. Was the damage to the piston skirts from shock loads or thermal loads? It hasn't happened since, I try to rev it harder but even then it was always over 3k so who knows? But it would be good to know what temps could be involved, ambient that day from memory was maybe 30c but it happened right at the start of a big climb, the first 35k bend just as I was starting to wind it on and I looked in the mirror to see solid thick blue smoke and could hardly believe my eyes.
Dellortoman
Hero member
- Location
- Tasmania, Australia
Yeah Vince, I would have guessed it's around 10:1. I plugged your numbers into the spreadsheet I made, and it tells me you're getting cylinder compression temperature of around 480C before the plug fires. I don't know whether that's high enough to ignite the mixture from compression only. Probably not, and it's not in the alloy melting range. I think pre-ignition is normally caused by an overly hot centre electrode in the spark plug acting like a glow plug.
Also bear in mind that the engine components are cooled by air and oil. That peak temp of 480C is just the little pocket of gas in the combustion chamber. It gets a whole lot hotter after the spark plug fires. I'd guess the peak combustion temperature would be somewhere around 1500C, but that's only for a very short time, and most of that heat goes out the exhaust pipe. It doesn't necessarily heat up the engine.
The F1 camshaft closes the inlet valve at 50 ABDC, so you're getting a lot more effective compression stroke (58mm) with that camshaft than the A11 or 4C. You also have a longer power stroke on the other side of TDC because the exhaust valve opens later.
I'd guess that the F1 is designed for good low to mid-range power/torque, but I could be entirely wrong. My interpretation of valve timing is pretty simplistic. There's gotta be a whole lot of dynamic effects with gas flow that I've never made any attempt to get my head around. Someone who builds race engines (like Julian from Belgium) would have a better idea of what variations in valve timing actually achieves.
Thinking of Julian, we haven't had an update on his highly modified SF for a while. I wonder how it's progressing.
Also bear in mind that the engine components are cooled by air and oil. That peak temp of 480C is just the little pocket of gas in the combustion chamber. It gets a whole lot hotter after the spark plug fires. I'd guess the peak combustion temperature would be somewhere around 1500C, but that's only for a very short time, and most of that heat goes out the exhaust pipe. It doesn't necessarily heat up the engine.
The F1 camshaft closes the inlet valve at 50 ABDC, so you're getting a lot more effective compression stroke (58mm) with that camshaft than the A11 or 4C. You also have a longer power stroke on the other side of TDC because the exhaust valve opens later.
I'd guess that the F1 is designed for good low to mid-range power/torque, but I could be entirely wrong. My interpretation of valve timing is pretty simplistic. There's gotta be a whole lot of dynamic effects with gas flow that I've never made any attempt to get my head around. Someone who builds race engines (like Julian from Belgium) would have a better idea of what variations in valve timing actually achieves.
Thinking of Julian, we haven't had an update on his highly modified SF for a while. I wonder how it's progressing.
He popped up just before Christmas with a comment but nothing more. I was looking forward to his yearly update on his out there build. It's a shame Keith's F1 180 cam turned into a giant nightmare, I think it was the last factory cam design and seems to work great right through the range in my 180. But eating cam buckets wasn't in the design brief. That was right out of left field. Good to see you back crunching numbers, I hate to see anyone not feel comfortable doing their Laverda thing. It's a dying breed here and none of us is getting any more reputable or younger. Want a giggle, I am trying to resurrect my Atlas horn, I got one tiny squeak and then silence. It's hidden behind the oil cooler and an absolute arsehole to get out. My spare horn works on the wiring so it's probably rusty inside and typically it's riveted together. It's soaking in rust dissolver, where have a seen this before?
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F1 cams never ate one cam block ever, change your post to buckets
Standard comp test here pre motor disassembly, reference figures in assessing motor issues. WOT, dry test, all plugs removed, ignition system rendered safe or disabled, 120 to 165 across three cylinders, as Piet indicates it is the close related values that is indicator of worth, not so much the actual reading in each cylinder. Squirt new oil into one cylinder plug hole ( not too much!! ) retest , 8-10 crank cycles, note the new much higher readings can reach high 100's up to 250 ( later is rare, about 220 has been noted here ), ditto other 2 cylinders. As may be of worth information. j
Dellortoman
Hero member
- Location
- Tasmania, Australia
Just to indicate how much affect the cam timing can have on engine compression tests, I did compression and dyno tests before and after changing the cam timing on my Jota.
My Jota started life as a 1976 3CL. After a minor crash in about 1980 it got re-built to Jota specs. The previous owner got Stanco to fit all the factory Jota bits. 4C cams, HC pistons, etc.
The bike came into my possession in 2007. I thought it was a bit down on power, and eventually discovered that the Stanco mechanic had installed the 4C camshafts using the sprockets off the A11 shafts, so the valve timing was out by a fair amount (don't remember the actual numbers). Compression test in that condition was around 120psi on all cylinders.
After dialling in the cam timing to the correct 4C specs, the compression test came up 180psi and the bike picked up about 10 HP on the dyno.
No other changes were made to the engine at that time, so the difference was entirely down to valve timing.
I also found about another 10HP by replacing the awful 3-1 exhaust and putting sensible sized main jets in it. But that's another story.
The thing I can't figure out is how the previous owner rode it for 27 years without noticing that the engine was making less power than it did in its original 3CL tune. Maybe he just tootled around without ever giving it full throttle.
My Jota started life as a 1976 3CL. After a minor crash in about 1980 it got re-built to Jota specs. The previous owner got Stanco to fit all the factory Jota bits. 4C cams, HC pistons, etc.
The bike came into my possession in 2007. I thought it was a bit down on power, and eventually discovered that the Stanco mechanic had installed the 4C camshafts using the sprockets off the A11 shafts, so the valve timing was out by a fair amount (don't remember the actual numbers). Compression test in that condition was around 120psi on all cylinders.
After dialling in the cam timing to the correct 4C specs, the compression test came up 180psi and the bike picked up about 10 HP on the dyno.
No other changes were made to the engine at that time, so the difference was entirely down to valve timing.
I also found about another 10HP by replacing the awful 3-1 exhaust and putting sensible sized main jets in it. But that's another story.
The thing I can't figure out is how the previous owner rode it for 27 years without noticing that the engine was making less power than it did in its original 3CL tune. Maybe he just tootled around without ever giving it full throttle.
As long as it makes enough racket, everything's fine...
Most riders associate excessive noise with power.
piet
Dellortoman
Hero member
- Location
- Tasmania, Australia
As long as it makes enough racket, everything's fine...
Most riders associate excessive noise with power.
piet
It certainly did that. The 3-1 exhaust was so loud it would make your ears bleed. It was also very good at killing horsepower.
hooksey
Hero member
- Location
- Vic , Australia
Maybe at the top end Cam...... factory 3 into 1 on my 76 proved better (6 ponies) on the dyno from 4 to 7k rpm and that's in the sweet spot for road riding
I do enjoy wakeing up the neighbours in the wee hours on the w/ends.......
Dellortoman
Hero member
- Location
- Tasmania, Australia
The 3-1 that came on my bike wasn't a factory item. It looked like it was cobbled together by someone who had a pipe bender and some basic welding skills. For example, the flanges where it bolted to the cylinder head were cut out of steel plate and were permanently attached to the pipes. The whole thing was one fully welded assembly from beginning to end. I don't think any consideration of gas flow went into its design. If the only design criterion was to get the exhaust gas from the front of the engine to the back, then it was a success. But as a performance upgrade or a noise reduction device, it was a dismal failure.