Compression ratio (2021)

Fast Eddie said:

Fuel economy is arrived at (in this context) by improving combustion efficiency. Thats the same thing that produces power.

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Hence increased CR is a "win/win" if A/F ratio and ignition timing can be managed effectively.

Dan1950 said:

Hence increased CR is a "win/win" if A/F ratio and ignition timing can be managed effectively.

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Agreed !

Fast Eddie said:

What is your thinking re the link between high compression pistons and increased crank case pressure ?

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Good question. I guess it makes no sense. It sounded good in my head at 6:30 this morning. Probably just too much RPM. It was a hair brained theory based on never having a cylinder base gasket weep with the 2S cam flat top Commando pistons stock rods and same breather and the same head riding the same way. Longer thinner rods and thinner wall pistons pushing more air volume down in the cylinder and into the crank case than the breather in the timing case can handle? Heck, I don't really know, but something is different. I'm not saying I did not screw up with the sealant. That is top of the list.

Dan1950 said:

Hence increased CR is a "win/win" if A/F ratio and ignition timing can be managed effectively.

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Not so much on a shakey old air cooled engine. If going for durability then a moderate Cr is so much easier on things.
If you accept that a rebuild interval might shrink to 25,000 miles from 50,000, then a high CR can add to the fun.
Same with the big cam. Not ideal for longevity. If you only put a couple of thousand miles on per year and you only have ten years of riding left to do ( most of us here) , longevity of the engine might not matter.

Schwany said:

Good question. I guess it makes no sense. It sounded good in my head at 6:30 this morning. Probably just too much RPM. It was a hair brained theory based on never having a cylinder base gasket weep with the 2S cam flat top Commando pistons stock rods and same breather and the same head riding the same way. Longer thinner rods and thinner wall pistons pushing more air volume down in the cylinder and into the crank case than the breather in the timing case can handle? Heck, I don't really know, but something is different. I'm not saying I did not screw up with the sealant. That is top of the list.

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Actually, the stock paper base gaskets were famous for weeping, and even blowing completely out in some instances. I can't say for sure, but I've always assumed that's why Norton eventually recommended leaving the gasket out (late 750s and pre-MK3 850s) and sealing the base with gasket sealer (Loctite Plastic Gasket per Service Releases N3/23) and 3/67). Norton went back to a base gasket with the MK3, supposedly to stop owners from blocking the oil drain back hole with goop (I have no idea if that is actually the reason, but I recall hearing it somewhere).

Ken

lcrken said:

Actually, the stock paper base gaskets were famous for weeping, and even blowing completely out in some instances. I can't say for sure, but I've always assumed that's why Norton eventually recommended leaving the gasket out (late 750s and pre-MK3 850s) and sealing the base with gasket sealer (octite Plastic Gasket per Service Releases N3/23) and 3/67). Norton went back to a base gasket with the MK3, supposedly to stop owners from blocking the oil drain back hole with goop (I have no idea if that is actually the reason, but I recall hearing it somewhere).

Ken

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Thanks for that Ken.

I used gasgacinch on the old thin paper gasket when I stuffed the 2S in the Atlas (P11) cases. It lasted a long time. Pure luck I suppose.

This time I used a .021 copper base gasket to drop compression some and prevent the valves from banging on the tops of the pistons. I had to pull the cylinders due to a brain fart and forgetting the saftey wire for the lifter tabs. I didn't seal both sides of the base gasket with a modern sealer when I put the cylinders back on. Same with one side of the head gasket. Only what I did has held up on the head so far. It was an experiment. I went old old school and painted the tops of the copper gaskets with brush on aluminum paint I've had 50 years. JS will laugh at the stupidity of that if he reads this, but I felt like doing it, so did. I am paying for it. It's all good though. It only weeps when I thrash it. I'm going to plumb the timed breather in and see if that helps or makes it worse.

Triton Thrasher said:

Two of those about four factors look very similar.

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I always thought the factors were fear, surprise, ruthless efficiency, an almost fanatical devotion to the Pope, and nice red uniforms....

They say roughly 4% increase per point in the ratio . i:e 9:1 to 10:1 , but there would obviously be other mods .

Schwany said:

Thanks for that Ken.

I used gasgacinch on the old thin paper gasket when I stuffed the 2S in the Atlas (P11) cases. It lasted a long time. Pure luck I suppose.

This time I used a .021 copper base gasket to drop compression some and prevent the valves from banging on the tops of the pistons. I had to pull the cylinders due to a brain fart and forgetting the saftey wire for the lifter tabs. I didn't seal both sides of the base gasket with a modern sealer when I put the cylinders back on. Same with one side of the head gasket. Only what I did has held up on the head so far. It was an experiment. I went old old school and painted the tops of the copper gaskets with brush on aluminum paint I've had 50 years. JS will laugh at the stupidity of that if he reads this, but I felt like doing it, so did. I am paying for it. It's all good though. It only weeps when I thrash it. I'm going to plumb the timed breather in and see if that helps or makes it worse.

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My experiences with Commando head gaskets started with the stock flame-ring 750 gaskets in the '70s. I really liked those. In several years of seriously racing a 750 Commando PR, rebuilding engines regularly, I never never used any sealer with them, and never had a problem, even with 11+ CR. I got most of my experience with blown head gaskets when I started building them up to 920 cc. Copper gaskets were all that was available, except for some aftermarket flame ring style ones that were useless. Over the years I tried aluminum paint, copper based spray sealants, Hylomar, Permatex, and Pliobond, with and without the use of silk thread (old school aircraft practice) or fine copper wire (Jim Schmidt recommendation) in sealant around the pushrod tunnels. This was with 920 cc, 10.0 and up CR, and 7,000 (or maybe 7200 now and then) rpm red line. They mostly worked, at least for a while, but eventually started weeping oil. It wasn't a real issue on the race bike, just needed cleaning now and then. The silk thread/thin copper wire technique definitely helped. But I did have blown head gaskets a time or two. Machining the cylinders for copper o-rings around the bores solved that problem, and that's what I used for the 920s from then on. In the end, I think the best combination was an .040" copper head gasket with Pliobond sealer and the fine copper wire technique around the pushrod tunnels, along with the copper o-rings in grooves cut around the bore.

What we really need is for someone to offer Commando head gaskets using modern technologies, but I don't see that happening any time soon.

Ken

worntorn said:

Not so much on a shakey old air cooled engine. If going for durability then a moderate Cr is so much easier on things.
If you accept that a rebuild interval might shrink to 25,000 miles from 50,000, then a high CR can add to the fun.
Same with the big cam. Not ideal for longevity. If you only put a couple of thousand miles on per year and you only have ten years of riding left to do ( most of us here) , longevity of the engine might not matter.

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Your point about potential longevity is valid. As with most tuning really. Building a hot rod for long distance touring with the aim of a high mileages with minimum maintenance / intervention would never be a wise idea!

But it’s not an argument against Dans point as, fundamentally, a higher CR WILL improve combustion efficiency (how desirable or possible it is, is another matter).

IF fuel economy is the main aim, this helps to allow an engine to be set to run leaner.

If power is the main aim, altering fuelling and timing will help to increase power and torque.

Or a bit of both can be achieved.

The limiting factors are just what’s possible with the fuelling and management of heat etc.

Which is why none of us run 14:1 on an air cooled, carb fuelled, street Commando!

I like the "durability" vs "riding time remaining" consideration. That was a major part of my thinking when I JB-welded long velocity stacks/ no filters to my new set of Premiers a couple of years ago!

Actually, the main reason I JB welded them is due to stupidity; I should have "glued' them with silicone seal! For those who may wonder, Vstacks attached with JB WELD are NOT removable!!

I think a higher compression ratio is 'the direction of goodness' for the most part, as long as you only approach the Reliability Cliff and don't step off. The cliff is in a different place for any given engine. It appears for our Commandos that 10:1 is a practical limit without serious upgrade work.

I've always thought a great target for CCP was about 180 psi, maybe a bit higher. I've had occasion to question this recently. After installing a performance cam in my 114 cubic inch Harley (new series with four valve heads fwiw) I tested CCP and got an unbelievable 250 psi. I borrowed a buddy's gauge and proceeded to test our Kia Soul with both gauges. It's at the other extreme with my gauge yielding 100 psi and his showing 110 psi. A man with two watches doesn't know what time it is, so naturally I've ordered a new digital gauge in hopes of yielding another result on the Harley, perhaps more believable since I know it's not a diesel.
I watched a Youtube compression test on the very similar but stock 107 inch motor and they got a comforting 180 psi. The 107's are advertised as having a 10:1 compression ratio. Then I read a discussion on the T-Man web page (high performance HD parts people) and they claim that the factory 117 inch motor in the top of the line CVO models comes with a factory cam that yields compression test results between 225 and 230 psi. The compression ratio is 10.5:1 as measured at the brochure, same as my motor. Maybe their gauge is no more believable than mine....
I'm coming around to the idea that the CCP figures are not all that meaningful when looked at in isolation. Get a figure high enough to feel comfortable that valves and rings are sound, similar numbers for each cylinder, go riding.
It might be a couple of months before I find time to test my Harley with my new digital gauge (it's a royal pain in the butt, long boring story) but I will stick my findings in this thread as and when, just for academic interest.

Maybe your cam is too advanced or has advance ground into it early intake closing , and that is bumping comp on the high side .

Yes, that would definitely explain my high CCP. After riding the bike about 1500 miles (runs great by the way) it annoyed me enough that I tore it down and checked my work. I fully expected to see a miss-indexed cam but in fact the timing was spot-on. Was the cam ground incorrectly? Maybe, but I inquired about that and was assured that every single cam goes through a detailed quality inspection before shipping. It's a very small company and allegedly the inspection is done personally by the owner/CEO/designer. Certainly no guarantee that the cam is correct. On the other hand, the bike is running really well, so I'm pretty okay with it 'as-is'. No signs of pinging or detonation either, but this engine has knock detection and will pull out timing as needed, so I might not even know.
I will absolutely do another test but I'm riding to North Carolina in the morning, then after the trip life's hectic activities may push it to late September.

If

Atlas Commando said:

Yes, that would definitely explain my high CCP. After riding the bike about 1500 miles (runs great by the way) it annoyed me enough that I tore it down and checked my work. I fully expected to see a miss-indexed cam but in fact the timing was spot-on. Was the cam ground incorrectly? Maybe, but I inquired about that and was assured that every single cam goes through a detailed quality inspection before shipping. It's a very small company and allegedly the inspection is done personally by the owner/CEO/designer. Certainly no guarantee that the cam is correct. On the other hand, the bike is running really well, so I'm pretty okay with it 'as-is'. No signs of pinging or detonation either, but this engine has knock detection and will pull out timing as needed, so I might not even know.
I will absolutely do another test but I'm riding to North Carolina in the morning, then after the trip life's hectic activities may push it to late September.

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If it doesn't rev enough or you have pinking type issues down low maybe retarding the cam 2-4 degrees could help .you would need to recheck your piston to valve clearance as well . Cheers.

As indicated earlier, engine setup is a game of compromises. For example, raising compression alone may REDUCE max power IF you have to excessively retard the timing to prevent detonation.

MexicoMike said:

As indicated earlier, engine setup is a game of compromises. For example, raising compression alone may REDUCE max power IF you have to excessively retard the timing tof prevent detonation.

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Yep I've been down that road before

MexicoMike said:

As indicated earlier, engine setup is a game of compromises. For example, raising compression alone may REDUCE max power IF you have to excessively retard the timing to prevent detonation.

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Quite right.

And in some engines (like Triumphs) fitting excessively pointy pistons to increase CR can negatively impact other aspects of combustion efficiency, requiring ign retard etc to compensate, and subsequently giving a net loss.

What I like about the Norton is that decent CR is achievable with a flat top piston (and flat combustion chamber). And in doing so, the squish band gap is simultaneously tightened up to the point whereby it actually works. Then the squish band effect actually allows a higher CR than would otherwise be ideal, or even possible. In fact, I believe that doing this to tighten the squish, and accepting a higher CR, allows the engine to run better than it would with a lower CR and no squish effect. This is what’s been demonstrated a lot in the car world in the USA (and referenced / linked on this forum in the past).

This is what I did in my 920, I wanted 10.25-10.5:1 but miscalculated my piston dish and got 11:1. I had to decide between adding thicker gaskets to lower the CR to the desired level but also lose the squish, or leave it at the higher CR in order to keep the squish. I decided on the latter and run it at 11.1 and it runs great ! Yes I use Super Unleaded where possible, but I’ve put regular in it before too (not from fully empty, so it would still have been a slightly higher octane mix).

Disclaimer: I’m not recommending to anyone else, especially those who live in areas that only have access to low octane petrol!

MexicoMike said:

As indicated earlier, engine setup is a game of compromises. For example, raising compression alone may REDUCE max power IF you have to excessively retard the timing to prevent detonation.

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Hi mike , I did mean retard camshaft timing , which means making the valve event happen 2-4 later , so that the intake is fully closed later and reduces comp a little, which could reduce that high 250 psi compression reading and lesson low end pink and detonation or longevity issues ,shifting the rpm range a little higher..... Retard not go be confused with ignition timing , I just mentioned this as some cam manufacturers grind in advance , even if your cam is installed "straight up" this maybe for the timing chain that is going to wear and slowly retard your cam .you don't really know unless it is all dyno tested , or a knock sensor installed to the cylinder head and data logged would tell the real story. Cheers

Fast Eddie said:

Quite right.

And in some engines (like Triumphs) fitting excessively pointy pistons to increase CR can negatively impact other aspects of combustion efficiency, requiring ign retard etc to compensate, and subsequently giving a net loss.

What I like about the Norton is that decent CR is achievable with a flat top piston (and flat combustion chamber). And in doing so, the squish band gap is simultaneously tightened up to the point whereby it actually works. Then the squish band effect actually allows a higher CR than would otherwise be ideal, or even possible. In fact, I believe that doing this to tighten the squish, and accepting a higher CR, allows the engine to run better than it would with a lower CR and no squish effect. This is what’s been demonstrated a lot in the car world in the USA (and referenced / linked on this forum in the past).

This is what I did in my 920, I wanted 10.25-10.5:1 but miscalculated my piston dish and got 11:1. I had to decide between adding thicker gaskets to lower the CR to the desired level but also lose the squish, or leave it at the higher CR in order to keep the squish. I decided on the latter and run it at 11.1 and it runs great ! Yes I use Super Unleaded where possible, but I’ve put regular in it before too (not from fully empty, so it would still have been a slightly higher octane mix).

Disclaimer: I’m not recommending to anyone else, especially those who live in areas that only have access to low octane petrol!

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The squish is good for tumble and swirl and a more homogeneous mixture , it is the way to go .