Dyno run (2017)

worntorn said:

I'll go against the grain again in that
I think the factory CR number is right. I just syringe measured two 850s, both with untouched heads and I get a CR of 8.53 to one with a thin 20 thou flame ring gasket or 8.23 to one with a 40 thou copper gasket.
One engine is assembled ( the MK3) the other is disassembled and on the bench.................... Glen

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Not at all against the grain.

Did you conduct a direct measure of the combustion chamber and account for the volumes of the valve pockets in the piston crown as well as the actual piston deck height and between the cylinder and piston crown above the rings? They all need to be accounted for to get an accurate value. A typical way to do this is to get the engine at TDC with the valves closed and then use graduated syringe, graduated pipette or burette with pinch clamp (best) and direct fill the combustion chamber with an oil such as ATF. With the Commando, the plug hole needs to be facing up to avoid trapped air.

850 Commandos came from the factory with a flame ring head gasket, correct?

As stated above, I was only going with anecdotal evidence from this forum - maybe a risky thing, eh?

So bumping the compression above 8.5:1 really would be a modification from stock. A good valve seat job could also be construed as modification from stock but both "improvements" alone, and certainly together, will render improvements in power. Whether they make the difference between 108 mph and 118 mph I do not know for fact.

Yes to all of the measurement questions.
Piston top at TDC is .030" below cylinder deck. I calculated the volume of that space then double checked it by filling the space with oil from the syringe. The numbers agreed +- a small amount.
My question then was did the factory use a thin flame ring gasket as I have on the MK 3?
If so their listed CR is spot on for these two 850s. As it should be!
Glen

Good stuff. Something that may be relevant is the question as to whether all Commandos came from the factory with a cylinder base gasket.

FWIW, I cc'd a stock '74 850 that I took apart back in the '70s. With no base gasket and stock flame ring head gasket, the CR was 8.47.

Ken

Best performance is achieved by a balance of compression ratio, ignition advance and jetting. If the compression ratio is low the other two components end up being adjusted so that the end result is pretty much the same. High compression only gives more power because combustion temperatures are higher, so more fuel is used to maintain the balance. In effect, if you are using methanol fuel, at 12 to 1 comp., the jets are much larger, but if you run at 9 to 1 comp. and jet to suit, you get almost the same power output. With petrol as fuel, the effects of jetting differently are much more critical , because the jets are half the size they are with methanol.
For anyone setting a commando engine up to the manufacturer's specification, I would say that the specification is necessarily 'one size fits all'. If you are seeking performance, you need to move away from it.

acotrel said:

Best performance is achieved by a balance of compression ratio, ignition advance and jetting. If the compression ratio is low the other two components end up being adjusted so that the end result is pretty much the same. High compression only gives more power because combustion temperatures are higher, so more fuel is used to maintain the balance. In effect, if you are using methanol fuel, at 12 to 1 comp., the jets are much larger, but if you run at 9 to 1 comp. and jet to suit, you get almost the same power output. With petrol as fuel, the effects of jetting differently are much more critical , because the jets are half the size they are with methanol.
For anyone setting a commando engine up to the manufacturer's specification, I would say that the specification is necessarily 'one size fits all'. If you are seeking performance, you need to move away from it.

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Alan that’s wrong.

An engine set up perfectly at 12:1 has to perform better than an equal engine set up perfectly at 9:1. It’s basic physics surely (I confess I’m on thin ice here as I’m no physicist).

Even as a non physicist, I argue that if you were right, all modern manufacturers would be building engines with low CR.

But they’re not. Even your average Joe petrol engined family hatch back is over 12:1 these days, and that’s for a reason.

Fast Eddie said:

Alan that’s wrong.

An engine set up perfectly at 12:1 has to perform better than an equal engine set up perfectly at 9:1. It’s basic physics surely (I confess I’m on thin ice here as I’m no physicist).

Even as a non physicist, I argue that if you were right, all modern manufacturers would be building engines with low CR.

But they’re not. Even your average Joe petrol engined family hatch back is over 12:1 these days, and that’s for a reason.

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Agreed. What passes for credible information on this site is sometimes downright laughable, not to mention, in some case dangerous. I am constantly amazed by the total disregard of physics by some when it comes to hydraulic brakes. Seems some are under the impression it is the brand of components, rather than correct sizing of components that is the determining factor for brake performance.

The power increases with compression ratio, but maybe not as much as one might expect.
With a 50 HP motor, Raising CR from 8.5 /1 to 10.5 /1 will bump power 1.3 HP to 51.3 according to the Wallace Racing Calculator.

Glen

Fast Eddie said:

It would be very interesting to get that Beeza on a dyno Glen and we could then clearly see the difference between crank and rear wheel hp.
!

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On a dynojet that distinction is pretty blurry.
Some of the stock Thruxton R s are showing 92-93 rwhp on a Dynojet. The owners are thrilled of course, but how is this possible with 97 at crank?

Sometimes I think the test hill is the best dyno. It's always the same.

Glen

Fast Eddie said:

Alan that’s wrong.

An engine set up perfectly at 12:1 has to perform better than an equal engine set up perfectly at 9:1. It’s basic physics surely (I confess I’m on thin ice here as I’m no physicist).

Even as a non physicist, I argue that if you were right, all modern manufacturers would be building engines with low CR.

But they’re not. Even your average Joe petrol engined family hatch back is over 12:1 these days, and that’s for a reason.

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Quite right, Nigel. And yes, I am a physicist (and an engineer), or at least I was before I got so old. But you don't have to be one to understand this issue. All other things being equal (correct timing, carburetion, etc.), volumetric efficiency and power increase with increasing compression ratio. It's not linear, and at the higher compression ratios the percentage gain from increasing another point is less than at the lower ratios, but it's still there up to the point where you're limited by the anti-knock level of the fuel. Somewhere around 17:1 in a 4-stroke gasoline engine you reach the point where the power curve flattens off and starts to drop with increasing CR. In the real world of street Commandos, the most bang for the buck comes from raising the stock CR a point or so, or more if you run higher octane fuel. A longer overlap cam lets you go a little higher, but the practical limit for a typical Commando with the kind of pump gas we get where I live seems to be around 9.5, maybe as high as 10.0 with a cam and careful choice of gas.

Ken

All good stuff (other than some misguidance from someone).

As Glen points out, compression ratio alone is not the end all and be all but that estimated 1.3 HP increase that Glen cites is nearly a 3% bump in peak power; not to mention what is does for mid range torque, basically across the torque curve. My point about bumping/blue printing compression and dressing up the valve seats was in the context of max mph reported, where 1up3down only saw 108 mph and some literature stated 114 mph to 124 mph. So does a %3 bump in HP due to compression and whatever dressed up valve seats contributes get you from 108 mph to 114 mph - 124 mph? I don't know but it is certainly moving in the correct direction. I can speculate here and say that to get that peak MPH rating one would also look at overall top gear gearing; in other words, where is peak HP with respect to peak max speed. I would certainly look at leaning out and tuning (jetting and ignition timing) for peak power. I am reasonably sure that main jetting on a stock Commando is where it needs to be to prevent toasting the engine, not peak power. A max speed test for reporting purposes could easily be a one off short duration exercise where a leaner condition can be tolerated.

As for compression ratio increasing power, I don't remember what text it was (Fayette Taylor, G. P. Blair or other(s)) where they state that increasing compression ratio increases efficiency. Intuition tells me that more efficient translates to more usable power but I don't recall exactly why there is more efficiency. Higher temperatures was mentioned here. I also speculate that with a higher compression ratio you get a more compact combustion chamber which should translate to less ignition advance; that is something I can get my head around.

As Ken states, the incremental increase in power drops off dramatically as you go from say 5:1 up to 13:1. There are graphs out there to illustrate this. The top graph in the link below illustrates this in terms of efficiency (not power). It's interesting to see that an increase in compression ratio from 8.5:1 to 10:1 on this graph indicates around %3 increase in Otto Cycle efficiency which is in line with Glen's estimate. I am reasonably sure there's more to it than the simple Otto Cycle Efficiency.

http://www.motoiq.com/MagazineArticles/ID/1950/PageID/3097/Compression-Ratio--The-Squish.aspx

Aspects of increased compression which I believe are above and beyond the Otto Cycle Efficiency include:

• Higher compression ratio usually results in a more compact combustion chamber which usually requires less ignition advance = more usable power.
• Higher compression ratio usually results in a more compact combustion chamber which usually has less combustion chamber surface exposed to peak combustion temperatures = less thermal loss = more usable power
• Higher compression ratio usually results in a more compact combustion chamber which may enhance turbulence = better A/F mixing = more efficient & complete early combustion = more usable power

Dances with Shrapnel said:

As for compression ratio increasing power, I don't remember what text it was (Fayette Taylor, G. P. Blair or other(s)) where they state that increasing compression ratio increases efficiency.

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It's in both Taylor and Blair, as well as Ricardo, Heywood, Stone, and any other good text on internal combustion engine design. I love the availability of info on the web, but it's still not the same for me as sitting down with a really good technical book by a knowledgeable author.

Ken

If increasing comp. ratio involves poking a piston crown up into the combustion chamber, as it does when you raise the comp. ratio to 12 to 1 in a 650cc Triumph twin, perhaps the flame front is obstructed and the fuel is not completely burnt ? If you have a look at the pistons out of some bikes, the coking on one side of the piston is often very thick, but almost non-existent on the other side near the plug. When you raise the comp. ratio in a Commando engine, the squish effect probably increases, but not as much as in a modern 4-valve motor. Something which has an effect on performance is using the three cutters to cut the valve seats. Raising the comp. ratio comes at a cost and the benefit might not be as much as expected. Getting the tuning right might be as important.

how i have rationlised the compression ratio benefits - layman here so feel free to shoot me down.
- the internal combustion engine aims to convert the heat of combustion into forward motion.
-the more heat that is converted the more efficient the engine will be.
-those who have used CO2 canisters to inflate tyres will have noticed how they quickly go from ambient to very cold when the gas is released in rush.
-something similar happens in the engine, gases cool due to the sudden expansion on the power stroke and this amount of this cooling reflects the conversion of energy to forward motion.
-the greater the expansion (higher comp ratio = more expansion) the more heat that has been converted to forward motion.
A gross simplification but sounded good to me in my armchair - fire away.

acotrel said:

If increasing comp. ratio involves poking a piston crown up into the combustion chamber, as it does when you raise the comp. ratio to 12 to 1 in a 650cc Triumph twin, perhaps the flame front is obstructed and the fuel is not completely burnt ? If you have a look at the pistons out of some bikes, the coking on one side of the piston is often very thick, but almost non-existent on the other side near the plug. When you raise the comp. ratio in a Commando engine, the squish effect probably increases, but not as much as in a modern 4-valve motor. Something which has an effect on performance is using the three cutters to cut the valve seats. Raising the comp. ratio comes at a cost and the benefit might not be as much as expected. Getting the tuning right might be as important.

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So you agree about better valve seat preparation and tuning (ignition and jetting) that I suggested above. If you are disputing the basic physics behind increased efficiency (see my link above regarding the Otto Cycle) then please carry on and explain why.

Contrary to your assertion, you can get more squish effect out of a Commando (two valves) when compared to comparable bore diameter four valve engine if you really worked at it BUT there is an ideal amount of squish which is expressed as a percent of cross sectional area of the cylinder bore so too much is not good. Again, one of the better known text referenced above spell out the ideal % squish.

The 650cc Triumph twins are at a bit of a disadvantage due to the angle between the valves necessitating (big word for requiring) more of a dome for compression, but this is ACCESSNORTON under the subtitle Norton Commando Classic Motorcycles. Have you not seen bathtub shaped combustion chambers on a Norton; not only do they work but from first hand experience, they work very well. Furthermore, the original Norton factory short strokes used full hemisphere heads with domed pistons to match and they made very good power and torque. How about the Ducati singles, they do a good job.How about the Norton Manx or the original or modern G50's.

Nice summary ntst8. In this case, there is no free lunch and the one thing that should be added is that more energy is required to compress to a higher compression yet the higher compression ratio nets and overall increase in power.

Take a look at the last two Pressure-Volume graphs in the link I provided. The net is the area under the curve; that is the net with a lower compression ratio as compared to the net with a higher compression ratio.

From the link "So a higher compression ratio means getting more work out of the same fuel."

Nuff said?

Nice link thanks Dances, i was guilty of only skimming the last few posts before posting myself.

On the subject of interposing a piston head/dome into a combustion chamber and how this might affect ignition timing and the ensuing combustion event, as a worst case example consider for a moment an ironhead sportster, e.g., an XLR. These engines employed very high domed pistons, and the factory ignition timing spec was 48 deg BTDC, which is the highest “best power spark advance” setting I’ve ever heard of. This range of timing speaks to a serious issue, namely, having this very high piston head positioned deep into the combustion chamber requires 15-20 degrees more lead than would be required for a very good combustion chamber.



Just thought this example was germane to the earlier inquiry by Alan regarding the consequence of sticking a big ugly piston head far into a combustion chamber.

Back to the subject of Otto cycle efficiency as a function of compression ratio, thought the following may have merit. Below is a table that allows one to determine changes in Otto cycle efficiency that occur as a function of changes in compression ratio. The equation employed to create the table is shown and explained at the following link.

http://victorylibrary.com/mopar/otto-c.htm

Briefly, the equation is as follows.

E = 1-(1/R^K-1)

where E = Otto cycle efficiency, R = compression ratio and K = 1.4 (the ratio of variable heats for air). To determine an efficiency divide the efficiency value of the new compression ratio by the efficiency value of original compression ratio.

To use the table find the original compression ratio in the left hand column, then read across the top to find the new compression ratio and at the intersection of the original row and the new column is the change in efficiency that results.

That XLR piston looks horrible, hardly a performance piece and largely suffering from a large included angle between the valves. One really needs to see the whole picture including the cylinder head combustion chamber shape. So would one expect to have to increase or decrease ignition advance if the compression ratio on that XLR motor were increased? In other words, would the combustion chamber become more compact or less compact (and efficient)? I guess it has a lot to do with how one goes about this. As I mentioned earlier, the original Commando short stroke had full hemisphere domes and the domes were substantial and they went like stink; unfortunately I don't know what their best power ignition timing was. From my experience with the various short strokes I have worked with, best power was less than factory 28 degrees.

Interesting that your table indicates 4.6% increase in thermal efficiency alone when raising the compression ratio from 8.5:1 to 10:1. For a 50 hp motor, that would indicate 2.3 hp gain from thermal efficiency alone.

Dances with Shrapnel said:

That XLR piston looks horrible, hardly a performance piece and largely suffering from a large included angle between the valves. One really needs to see the whole picture including the cylinder head combustion chamber shape. So would one expect to have to increase or decrease ignition advance if the compression ratio on that XLR motor were increased? In other words, would the combustion chamber become more compact or less compact (and efficient)? I guess it has a lot to do with how one goes about this. As I mentioned earlier, the original Commando short stroke had full hemisphere domes and the domes were substantial and they went like stink; unfortunately I don't know what their best power ignition timing was. From my experience with the various short strokes I have worked with, best power was less than factory 28 degrees.

Interesting that your table indicates 4.6% increase in thermal efficiency alone when raising the compression ratio from 8.5:1 to 10:1. For a 50 hp motor, that would indicate 2.3 hp gain from thermal efficiency alone.

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Between '76 and '80 I had one of those 'original Commando short stroke' heads with 'full hemisphere domes' pistons, but at 89mm stroke not 80.4mm (pistons machined from blank with standard pin holes, as per things like Barcelona works race bikes. Ignition set at 28 degrees with points, and later with electronic ignition, the same. Works were doing the same. I was guided by an experimental department engineer who supplied me the engine in parts, and Tony Smith then a very successful Commando racer who rode alongside Tony Holland and who worked with Thruxton Motorcycles. Tony supplied me an ex works modified timing cover with ignition pickup and a Lucas Rita ignition. On investigation in the Rita the advance circuit was jumpered out and it ran at full advance.

Since I now have a short stroke, Standard Fullauto head with JSM flat tops at around 10.25:1, I would be interested in your suggested advance figures. Currently running 29.