Compression ratio (2021)

Madnorton said:

Lots of claim and counter claim of what works and what doesn't, but where are the dyno print outs. If you are confident to put these engines together then no doubt you will be happy to run it on a dyno, it is not expensive and most can be compensated for which method is used.

So come lets see the curves, we all like curves in the right places!!

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Yeah... good luck with that...

It’s only relevant if you can show ‘before vs after’ taken from the same Dyno / operator etc.

But comparing individual tuning aspects like this is futile as things are intended to function as a package.

Comparing before and after packages of work will still leave the debate open regarding the relative benefit of individual tuning aspects.

So, you’re damned if you do and damned if you don’t, and whatever you do or don’t do, the debate will go on !

YEP, The only way a dyno (or track) run is really valid to check changes is, as Fast Ed said, if it's done on the same dyno/track on the same day. Further, only one change at a time should be made.

The concept that a dyno can "compensate" for changes in atmospheric pressure/temp changes/humidity is just that...a CONCEPT. It's not real, at least not in the typical dynos at most speed shops. The F1 guys might have such dynos!

The ‘one change at a time’ is not always correct as you‘ll often want to ascertain how things work together.

Big carbs, big valves, big cams, big CR, big bores... could all be rejected if tested individually. In fact there have been many threads on here doing just that.

But put them together and it’s another story entirely.

Well, let's see some initial runs and then the updates when modification takes place. Before and after, even better.
Agreed, a dyno run only tells one half of the story, data gathering during actual laps helps, then dovetail the two together. A race team member complained his bike was down on power at full throttle, the track data revealed he only used full throttle 3% of a lap, nothing new mapping a bike to a track, bit more tricky on a Norton though.

I agree that often you will make multiple changes - but that is usually in accordance with some "known" set of mods for a specific engine. IOW, very little is done "in the blind." The starting point for major engine mods normally starts with a "package" that is proven/sold to work with the engine/application. From that point, tweaking is done to get the optimum performance for the specific engine with each adjustment/change checked on the dyno/track. As an example, I have seen identical engine models require notably different ignition timing for maximum power. Also, I have seen identical big-block OEM factory engines produce dramatically different max HP on the same dyno. My favorite example - a low of 420 and a high of 510HP on two different same model factory engine with no changes and all settings to stock.

MexicoMike said:

NEVER believe the butt dyno when dealing with engine mods. It's the same as the "louder is faster" phenomena!

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Often the "louder" phenomenon is the result of mistuning that is sending more energy (noise) out of the exhaust rather than sending the energy onto the crank. (torque)

I have often experienced the exhaust note getting quieter as an engine is incrementally tuned to make more power.

Most of the increase of noise from a motor comes from advancing the timing of the exhaust valve opening point. But with a 2 into 1 exhaust system, you need to do that, because it runs with more back-pressure and needs a stronger pulse to be effective. Also the tail pipe must offer no restriction or you lose too much off the top of the rev range.
Jim Schmidt sells light 12 to 1 comp. pistons and long rods. I would not hesitate to use them. But I use methanol fuel which runs cooler.
When you raise the comp. ratio, you probably fool yourself. Because the comp. is higher, the combustion temperatures increase, so you either jet larger or retard the ignition timing. Theoretically, if you use more fuel efficiently at the same revs, you should get more power. But it might not work like that if there is a quench effect. I think that overall a motor on 12 to 1 compression with high octane fuel, with jetting and ignition advance optimised.,has to be faster, than similar on low comp. But there might not be much in it between them.
Even when you go big bore, you effectively raise the comp, ratio. That has the same effect leaning-off the jetting. So where does the increase in power come from , if you are already slightly too rich. ?

With methanol fuel, I cannot tell the difference in performance between low comp. and high comp. It is just that most people do not run low comp. with methanol or lean it right off
Phil Irving's book says 'if you run methanol rich, you still get good performance'. That is true, but it does not mean what most people think it means.

When I was young, I was a real idiot and believed all the bullshit. I'd had Triumph 650d for years and had used high comp. and race cams. Then I bought my mate's 500cc short stroked Triton after the motor had blown up. I fitted a 650 cc Triumph motor into it and too it to a drag meeting at Calder Raceway. It was a real blur. The motor was nothing. all it had was E3134 race cams. The pistons were 7 to1 comp., but it was on methanol. It had been rebalanced . I sold it to a sidecar guy for 35 pounds who immediately had his first win with it, in an Allpowers sidecar race. Then he blew it up. I spent the money fixing the 500cc short stroke motor, which had everything done to it. It was never as fast. If you look at the crowns of Triumph 12 to 1 comp. pistons which have been raced, there is always coke on the side which is away from the spark plug.
In the 1950s in Australia, most Manx Nortons ran on methanol. When our guys raced in the UK, they found the Brits' Manx Nortons were just as fast using pool petrol. Tuning for petrol is like cricket - designed to drive you insane. The main reason our guys did OK over there, was they were used to the speeds and they were a long way from home.

The convergence of opinion (probably based on experience) is that about 10:1 is a reasonable maximum target for a 750, which should give a cold cranking pressure of about 180 psig, depending on when the intake valve closes. With a given motor running somewhere below the reasonable maximum compression ratio/ cold cranking pressure for it, I've always thought that more compression is the direction of goodness for performance and efficiency, especially for a key variable not mentioned in this thread. What's the impact on torque when compression is boosted by shaving heads, fitting thinner head gaskets, or changing cams for an earlier intake valve closing? No doubt, 'it depends', but I'd rather have a boost in torque down low than a boost in horsepower up top for a bike that's more fun and easier to ride fast. My experience is very limited compared to some of you, having modified just 2 engines over the years, so would greatly value some further discussion on boosting torque.

I've been on several backroad USA tours where the only available fuel at a small town gas stop is reg 87 octane.
The 8 to 1 Rapide runs fine on it.
My riding friends with 9.4 to 1 and 10 to 1 chose to try to make it to the next town rather than fill up with 87 and melt some pistons. One made it, the other ran out of fuel, but at least they did not destroy their engines.
High compression and the need for high octane fuel does limit your touring capabilities.
That is one disadvantage to the modern bikes. At 12 to 1 compression they need fairly high octane fuel. I have to think about this when on the Thruxton R or Daytona.
Likewise the two specials I have here are limited to high octane.
The 8 to 1 Rapide and 8.5 to one 850 can run on just about any fuel I find along the way.

Glen

I’d recommend to your hot rod Vincent mates that they carry a small bottle of octane booster!

The fellow with the 10.4 to 1 bike did have some of that but when it came down to it he didn't trust it would work.
Most places have premium but there are always a few that don't.
Then there was the little town that showed as having one fuel station according to the TomTom.
It turned out that the pumps had been removed a few months earlier. I managed to buy one gallon of lawn mower gas from the lady who operated the General Store.
That and a lot of motor off coasting got me to the next town.



Glen

The mention of octane booster reminded me of this older post from Jim Comstock

"Well I can now say what regular fuel does with a 10.2 to 1 compression ratio. I got a tank full of low octane fuel at a premium pump in Northern Texas. Around 4000 ft elevation.

It was hot and I was riding with my visor up and my earplugs in so I didn't notice the pinging until too late. As I topped the second hill from the gas station one piston seized.

It freed up after a short cool-down and I got some octane booster to stop the severe pinging but the damage is done. I have ridden 600 miles since and compression is down on the left side and it rattles pretty seriously. Definitely going to shorten the trip.
I guess I will be doing that new motor for sure now -at least it happened on the old motor.

I am still doing better than my brother who was accompanying me on the trip. His weld re-enforced Combat cases split about one inch below the cylinder base about 400 miles into the trip. His bike went home on a truck. Jim"

Without doubt if we're going to take longer trips on a bike, the ability to burn lower octane regular fuel is very valuable. A set of lower compression pistons would be a great modification to get a bike ready for some 3rd world countries (or 2nd world like some of the states in the US). However, high compression can be compatible with 'regular' gas. My 2004 Suzuki SV1000S had a compression ratio of 11.3 to 1, and the owner's manual specifically advised me not to run any higher octane than 87, so that's what I did and it ran great. It was clearly designed to run on the lower octane. I would be curious to know what the cold cranking pressure on that motor was, but I never had occasion to test it. Maybe the CCP is the more important variable?
A much more famous example is the Hayabusa. The earlier years, at least to 2009, were also set up to run on 87 octane. It did not seem to hurt performance. At some point they began requiring higher octane fuel. The back story would be pretty interesting, at least to me.

It might not have occurred to you guys, but a Commando engine is probably as good as they come. To my mind, you cannot use one at high revs unless you balance the crank. Mine has a streel plug screwed into the hole in the bob weight with Loctite. If it comes out, I am dead. Road bikes usually have separate exhaust pipes with mufflers. Whatever exhaust system you have, the timing of the exhaust valve opening has to suit it. Commando cams are almost as hot as Triumph E3134 race cams, but inlet and exhaust cannot be moved independently. My cam sprocket has two more keyways broached into it. If you advance the cam timing you get more torque, but the bike becomes louder. Compression ratio, fuel octane rating and mixture and ignition timing are all in balance. Your comp. ratio does not natter as long as the other 3 factors are in balance with it. I use standard compression. I advance the ignition timing to suit the fuel, and jet to suit.
Another way is to put the bike on a dyno, jet to an oxygen probe, then advance the timing to get maximum torque.
I suggest when you are jetting with petrol , you cannot get the jetting adjusted close enough to get best power, using the usual Commando carbs. I use methanol fuel which is much more forgiving of tuning errors.
Within reason, it probably does not matter what comp. ratio or ignition advance you use, you can jet to suit it and end up with a quick motor, as long as the cam and exhaust system are a good combination.
I use a 2 into 1 exhaust system with a near-standard cam which I advance 12 degrees,. In my Amal carbs I use the leanest Mikuni needles.
Judging from my experience with methanol, when you jet for petrol, a quarter of a thou of an inch oversize in needle jet ID, would be the difference between fast and slow. With methanol, it is half a thou, but the jets flow twice as much. You use 0,108 - I use 0,1165 inch. I make my own needle jets - you could not do that when you use petrol.

If I had a road going Commando, I would rebalance the crank and fix the gearbox, then tune the carbs and ignition timing. Other than that - nothing.
My Seeley 850 is an idiot bike. The motor is nothing, but it is fast enough to beat methanol-fuelled 1100cc CB750 Hondas.
When I built it, I never believed in it. A motor that ugly should not be quick.

The opposition :

Temporarily...
Even if the engine is tuned/designed for higher octane fuel, you can get by with lower octane fuel on a trip if you are a bit careful - no loading the engine. Use a lower gear than you would normally when climbing a hill, no full throttle in a high gear to pass someone, etc. Of course you would want to fill with appropriate octane as soon as possible. If you were traveling through a country or large area that does NOT have suitable octane fuel available, you could retard the timing a few degrees.

Obviously, as mentioned, you could also carry a bottle (or three) of octane booster...

MexicoMike said:

Temporarily...
Even if the engine is tuned/designed for higher octane fuel, you can get by with lower octane fuel on a trip if you are a bit careful - no loading the engine. Use a lower gear than you would normally when climbing a hill, no full throttle in a high gear to pass someone, etc. Of course you would want to fill with appropriate octane as soon as possible. If you were traveling through a country or large area that does NOT have suitable octane fuel available, you could retard the timing a few degrees.

Obviously, as mentioned, you could also carry a bottle (or three) of octane booster...

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These tablets look promising

Here's the gain.
On a 50 HP engine 10 to 1 will give you .5 bhp more than 9 to 1 at peak and something less at hill pulling rpm, maybe .3 bhp?
It's less than I would have guessed.
11 to 1 vs 10 nets an even smaller gain.

Glen, as I’ve said before, I don’t think that calculator works for our small engines.
I dyno’d my 850 at 64rwhp.
According to the calculator it’ll give 51 even with 5:1 CR. I somehow doubt that…

Your old Compression Ratio of 10.5:1 and HP of 64 is now calculated
as a Compression Ratio of 5:1 and 51.01 Horsepower.​