Maximum Streetable Compression Ratio?

Something doesn't make sense here. I don't understand the reason for increasing comp. ratio on a street bike. Why do you need to do that? Are your roads full of people who really should be circuit racing ? Surely a standard commando has enough go, and if you need more you could always buy a modern twin cylinder bike like a Ducati or Guzzi instead of destroying a classic ? It would probably be cheaper in the long run.
I raced a short stroked Triumph engine in a featherbed frame for about 12 years , continually developing it. It was 500cc however used 12 to 1 650 pistons because of the 63mm stroke. The side of the crown away from the plug was always black while under the plug it was the correct colour. The major advantage of the commando engine, is surely in the cylinder head with the squish band ? My friend won the Australian Historic Championships Period 3 Unlimited class for several years with a 750cc Triton and found the 750 Nortons extremely difficult to beat. He only won because of his larger input of funds, and greater skill as a tuner and rider. With my own bike I am using a near standard 850 commando engine on methanol with a two into one pipe, advanced cam timing to compensate, and tapered inlet ports to maintain the torque characteristic. From experience, I know the bike is good enough to win races against over-capacity four cylinder bikes and two strokes. I don't understand when guys hemisphere the head on a commando and increase the inlet port size, then fit cams and pipes which shift the usable rev range up to where the cranks and bearings start breaking. Surely increasing the comp. ratio while doing that process makes it all so much nastier ?
I played with 650 Triumph engines for many years . The best result I achieved was when I fitted flat top 72mm 350cc BSA Gold Star pistons and machined the crowns at the edges to fit the head as a squish band. The pistons were lighter than the 12 to 1 comp. items usually used for racing, and the comp. ratio I achieved would have been considerably lower. Norton engines are much better.

Now now acotrel don't go throwing snesable logic into a motorcycle power discussion as we all know its illogical to desire more power than required to reach legal speeds in reasonable time to flow with normal traffic all the while risking life and limb. There is only a handful of power percentage points to be gained going from 9 to 10 CR and less above that. But does sound better the higher the CR.

To use this chart locate your original CR horizontally. Next, locate the new CR down the first column. Where the two intersect is the gains that can be expected. For instance if the CR is raised from 9:1 to 12:1 we find the two values intersect at the box with 7.7 in it. This is the percentage increase that can theoretically be had by raising the compression from a lower to a higher level.
Read more: http://www.popularhotrodding.com/tech/0 ... z2gb9KoNva

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Original CR across top new down the side.
New
CR 8:1 9:1 10:1 11:1 12:1 13:1 14:1 15:1
9:1 3.5
10:1 6.5 2.9
11:1 9.2 5.5 2.5
12:1 11.5 7.7 4.7 2.1
13:1 13.6 9.7 6.6 4.0 1.9
14:1 15.4 11.5 8.3 5.7 3.5 1.6
15:1 17.0 13.0 9.8 7.1 4.9 3.0 1.4
16:1 18.6 14.5 11.3 8.6 6.4 4.4 2.8 1.4

Read more: http://www.popularhotrodding.com/tech/0 ... z2gbAHkO9r

hobot said:

Now now acotrel don't go throwing snesable logic into a motorcycle power discussion as we all know its illogical to desire more power than required to reach legal speeds in reasonable time to flow with normal traffic all the while risking life and limb. There is only a handful of power percentage points to be gained going from 9 to 10 CR and less above that. But does sound better the higher the CR.

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Yes, the lure is strong but I think acotrel simply states that you just cannot dial one up, ordering up this head, that cam, these pipes and those carbs, etc, etc.

I think I have attained what bitchin is looking for but it has taken years to get there. Cruzin at 80, throttling up to 90 in a few second with rock solid handling at all speeds. I think I could easily race it by just changing the sprocket from 20 to a 19.

So if we figure a good Commando might have 50 rwhp at 9 to one then it will have 51.5 at ten to one, according to the chart.
Less change than I would have guessed.

Glen

For the low compression 850 models, where the factory really wanted to do a sports model but were prevented from doing so because of noise legislation, owners have some justification for at least looking at compression ratios - and cams. Especially when the factory put out Tuning Leaflets #1 and #2 to tell 850 owners precisely how to do this.
And some owners have reported doing so here, with good results.
AFTER the somewhat leisurely braking has been attended to, we should perhaps note...

For Aco, with a purpose built race bike on methanol no less, not bumping the compression from a lowly 8:1 up to something like 12:1 and getting an approx 10% gain in power, (and probably more, who/how was assembled that CR/power gain table) with no loss of anything, for free in effect !!!, this seems tough to understand not doing it ?? Obviously the will to win just isn't/wasn't there ?

worntorn said:

So if we figure a good Commando might have 50 rwhp at 9 to one then it will have 51.5 at ten to one, according to the chart.
Less change than I would have guessed.

Glen

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Raising the static CR alone is only part of the equation. Higher CR coupled W/a more aggressive cam grind can yields bigger gains.

A more aggressive grind can decrease TQ @ low to mid RPM ranges. Adding a higher static CR to the mix will gain back some of that TQ @ the lower RPMs & maintain that TQ as the RPM rises. That can result in significant HP gains.

On the flip side, a more aggressive cam grind will lower "dynamic" CR thus reducing detonation tendencies @ normal operating RPM.

My 10.5:1 static CR 86" Panhead would run on 87 octane unleaded if needed & the only time it spark knocked W/87 octane gas was in hot weather, in traffic just as the clutch was engaged. The cam was a regrind W/a lot of overlap that did not produce high cylinder pressure below 3000 RPM. It was "docile" around town until the wick was twisted & RPM hit the 3000 RPM range. Then, it came on like a freight train.

It's all a give & take situation & the "sweet spot" is the combo that gives the maximum power W/the best street manners & reliability W/the riders normal habits & environment taken into consideration..

For what it is worth, 3000rpm IS the putter around town range on these Commando's. For the most part, but not absolutely, this is where the charging system becomes effective.


So, 3000rpm kick off is a fair target. As stated, mine is around 3750 yet below that it is still as strong a stump puller as I have ridden.

It's ok to brag isn't it?

I lost my desire to make big changes to the 850 after looking at a dyno chart posted by one of the racers here. It was one of Herb Becker's motors and he is one of the top Norton race engine builders. As I recall it had about an extra ten hp up on top vs the stock chart, but had a big hp hole right where a road bike spends most of its life- between 3,000 and 4500 rpm. The stock engine had a hump in that area and made about 5-8 hp more than the hotrodded engine.
A stock or nearly stock Commando makes really good power for the road just as they came. They were quite a hotrod back in the day and still can really get up and leave town if needed.
I'm always surprised when I get off my 160hp Triumph and onto the Commando. The Commando still feels plenty strong.

Glen

Yes, there are lots of numbers and charts but the seat of the pants don't lie.
Bitchin, I hope you have had the opportunity to ride a good example.

pete.v said:

For what it is worth, 3000rpm IS the putter around town range on these Commando's. For the most part, but not absolutely, this is where the charging system becomes effective.


So, 3000rpm kick off is a fair target. As stated, mine is around 3750 yet below that it is still as strong a stump puller as I have ridden.

It's ok to brag isn't it?

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There was a time, when I had the 8mm pitch belt drive, the speed per 1000 RPM was 25.25 MPH. It could hit 5200 in 4th easily & I even did it once W/a leggy blonde on the back. (The road was straight the sun was shining & she was into it!)

3200 was the sweet spot, where all you could here was a faint purr in the backgraound & the wind in your ears. All you could feel was a slight pulsing rhythm. That was just a bit more than 80 MPH.

Want a "stump puller"? W/70# flathead wheels it was the reigning champion of the local shovel races. For those not familiar W/a shovel race, you get some skinny guy to sit in a large scoop shovel hitched to the motorcycle via a stout rope. The races are usually done in horse/cow pasture. It's from a dead stop & usually about 100 yds or less. It was a drag race. I could let it lurch out of the hole in 1st, ease on into 2nd & roll the throttle in as it dug for traction. I ran an Avon "Safety Mileage " 5.00 X 16 on the back. It was a good all terrain tire but you had to be careful on wet tarmac if you got down onto the shoulder of the tread.

I had a hard time finding anyone W/the guts to hang onto the shovel when I would get it up on the power band in 2nd gear. The Pres. of the local MC was a wiry guy about 150#. He jumped on & said to "hit it". I got up on the HP in 2nd gear & dragged him 1/4 mile up the pasture @ over 60 MPH. When I got near the end, I backed off the throttle & made a big sweeping turn. I could see Mike bouncing along on the shovel in my mirror. Just like a water skier, he swung out wide on the arc of the turn. Once I got it straightened out, I rolled the throttle back in. When I came to a stop back where we started, Mike jumped up & pulled his now very hot pants down around his knees. :shock:

Great tales of wild abandon BB! I'm in Arkansas - had 2 fellas tell me they spent a lot on their engines both put in choppers and got their kix taking on all comers in the 70's by WOT clutch drop standing starts in 4th and just hang on till the end of the torque rise. They would of had to break into mid 11 sec 1/4's to be king of the hill in those triple 2 toker days as well as others. They had 16x5 rear wheels too. Of course then hi test super ethyl lead was cheap as water.

Draging Nortons
http://www.youtube.com/watch?v=8grKW1iC_mQ
http://www.youtube.com/watch?v=hw6HQOKZG5w

I think this is our xbackslider on his 850.
http://www.youtube.com/watch?v=7CZTGqk0Gno

and couple moderns cutting up on it
http://www.youtube.com/watch?v=0AXewjhzdP4

hobot said:

Of course then hi test super ethyl lead was cheap as water.

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Back in my late teens/early 20s I ran Sunoco 260 in my BSA. As I was in my "learning curve" days as far as wrenching, I once had to pull the top end off the Beezer when it had relatively few miles on it after a re-build. I was amazed at the amount of carbon that had built up on the valves/pistons in such a short time.

AMOCO had “unleaded” premium as far back as the late 60s as I remember. After the revelation of carbon build up from super high octane leaded gasoline, I started using AMOCO “unleaded premium” almost exclusively in my motorcycles. The only problem I encountered in those many years since the early ‘70s was a problem the old leaded engine Harleys had due to the valves sticking in the valve guides when unleaded fuel became the only choice. One time, as my engine came to a halt after turning off the ignition, I heard a slight, but distinct “squeak”.

I monitored the noise for a few days & yes, there was something that was not quite getting proper lubrication. Besides the squeak at shutdown, my engine was performing flawlessly, but some of the Shovelhead riders were having their exhaust valve stick. The solution was to add 1 ½ oz of Marvel Mystery Oil to 3 gallons of fuel. It wasn’t necessary to do this each & every fill-up as long as it was done on a semi regular basis. I kept a shot glass & an 8oz can of MMO handy to keep the valve stems/guides lubricated.

Unleaded fuel, especially E-10 has never been an octane problem for me. It has however been a top end lubricant problem. Valve guide & valve seat material can alleviate the problems. I never had valve seat problems W/the bronze seats in the Panhead engine but old OEM steel valve guides were an issue. It appears that the Norton head has bronze seats & guides.

Yep them were the days of engine coal making fuels. What I do now and then is put a dash of caster bean oil in the gas for a hint of old fashioned race smell. I find it in big cycle shops for 2 tokes. Tried it in my chain saws but becomes stifling odor when wind not blowing it away deep in dense brush huffing and puffing. Don't add much oil to fuel as lowers its detonation resistance.

Of all the features [besides H2O] in over blown 10.5 CR Ms Peel, alloy barrel, tight squish, small 43 ml chambers, late valve closing cam, insulating coatings, the Powerarc tri-spark programmable ignition system is her main protection to push the envelope. Had a long pow wow with Gary that designed the units - told me of very hopped up Harley that could not tame detonation on best pump fuel and all the hi end after market ignitions till the Powerarc, then ran great on hi test, so Gary said go try the cheapest crap 87 gas he could find, and owner discovered it made even more power on the faster burn low octane w/o going ping pang Pow. There was report of the fit of the trigger on cam taper was unstable in Nortons so a fella here made a better adapter for us. Also mentioned by a couple of Cdo users that the tri spark function didn't work out for them so set it to single spark. Give Gary a ping and bring these and your own concerns up for a good sales pitch that sold me. It is also one of the few kits that has a rev limiter.
http://www.youtube.com/results?search_q ... SFKYK9PHYM

hobot said:

Yep them were the days of engine coal making fuels. What I do now and then is put a dash of caster bean oil in the gas for a hint of old fashioned race smell. I find it in big cycle shops for 2 tokes. Tried it in my chain saws but becomes stifling odor when wind not blowing it away deep in dense brush huffing and puffing. Don't add much oil to fuel as lowers its detonation resistance.

Of all the features [besides H2O] in over blown 10.5 CR Ms Peel, alloy barrel, tight squish, small 43 ml chambers, late valve closing cam, insulating coatings, the Powerarc tri-spark programmable ignition system is her main protection to push the envelope. Had a long pow wow with Gary that designed the units - told me of very hopped up Harley that could not tame detonation on best pump fuel and all the hi end after market ignitions till the Powerarc, then ran great on hi test, so Gary said go try the cheapest crap 87 gas he could find, and owner discovered it made even more power on the faster burn low octane w/o going ping pang Pow. There was report of the fit of the trigger on cam taper was unstable in Nortons so a fella here made a better adapter for us. Also mentioned by a couple of Cdo users that the tri spark function didn't work out for them so set it to single spark. Give Gary a ping and bring these and your own concerns up for a good sales pitch that sold me. It is also one of the few kits that has a rev limiter.

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I put together a 10.5:1 CR 355 SBC for my ’83 C20 P/U by using a pre '71 rebuild kit (flat top pistons) W/041 casting# small chamber (68cc) heads. It ran OK but was weak off idle W/the factory 0° BTDC ignition timing setting. I tried a little more advance & that helped the throttle response but as soon as the advance weights threw in more timing, it would rattle like a can full of rocks even on 93 octane unleaded.

I bought a re-curve kit for the stock HEI distributor W/several different springs & lighter flyweights. Best HP for the $$$$ I ever spent. I was able to dial in 12° initial advance for 36° total when it was all in.

Much better midrange TQ, throttle response & over all power. No more rocks in a can W/93 octane, but it did rattle on 89. I could have detuned it for lower octane, but (even W/4.10 axle ratio)the MPG went from a dismal 12 MPG stock to over 16 MPG making more than double the original 145 HP.

Programmable spark curve you say? Now that sounds interesting!

I suggest that with a lot of these things, much depends on the end use of the bike. There are about 8 variables in tuning a bike for a certain situation. In my own case, these days I only usually ride at one circuit which has a twisting tight part, three long straights and a long sweeping bend which tightens and has a twist at the end. You can choose how you go - either top end motor, point and squirt for the straights, and struggle with the tight stuff - or strong bottom end motor with lots of close gears for the tight stuff, and you still have to get down the straights quickly. The programmable advance curve has much to recommend it on a race bike, and a friend of mine manufactures a well-known engine management system. The problem is that there is not usually a 'one size fits all' solution, and a lot depends on your level of riding skill. Years ago I raced at Phillip Island several times with a Triton which had a severe top end motor which had been pretty useless on short tight circuits. I found myself passing guys at very high speeds at the ends of the straights. I still cannot figure out what ignition advance curve should be used for general purpose - I always use fixed timing and comp.ratio and cam - then tune carburation and gearing to suit. When I change my setup, I usually only change one variable at a time, then see what I have got by riding the bike.
If I ever decide to increase comp. ratio, it would be by fitting Jim Scmidt's rods and pistons, not by machining the head on a near standard motor. Incidentally, methanol gives more power than petrol, even at comp.ratios as low a 7 to one, in the commando motor it is excellent - try it !
At tne moment I still have the 6 speed TTI box to try out, and even practise sessions don't help much. The real test is using it in anger. I expect to have to increase the overall gearing to get the max out of it. I'm using a single row primary chain with floating Jawa engine sprockets, so it looks like I will be broaching splines into blanks.

Whoa aren't we motorcyler's such an excitable opinionated bunch with billboard size expressions : )
Ho hum on regular promgramables, the one I got for Ms Peel has 4 manual selectable or sensor switched curves down to when each of 3 sparks hits in order to fine tune the lower rpm to say heat the plug and chambers some for extra cold starts or restart stalled or blown out flame kernel and follow up on the squished out jets past TDC follow though. Open the youtube URL I sent above and then ping here for wheeling and dealing as this level unit is not in the Brit Iron vender catalogs who could order it but not up to snuff on supporting it. I dealt direct with Gary. http://www.powerarc.com/

Thankgoodness some of us got to live through the best times ever in human transportation history in the Northern hemisphere and nowadays in a More Golden Age of Norton Commando support in full swing. One big reason for so many Commando to have survived to today is the parts and payment and communications sending methods were such poor and expensive error prone situation.

Realistcally if ya get your engine about right and bike mass reduced some you should get a reliable below 12 sec 1/4 mile sprinter that can top 130 mph w/o waiting much and curise all day at 5000 rpm/90's mph in good area of max torque curve. But then you will run into THE Hinge on fast but wavy or windy sweepers closing in on the ton leaning on the power curve. Its a proven no brainer to configure to Combat spec's kind of between stage 1 & 2 that's pleasant to idle around camp ground rallys and city stop go then wake T-F up with a down shift or pass most traffic easy w/o bothering to down shift from top.

Here's pre-Peel no name factory Combat getting the jump on an over cammed for public use wheelied bared 1200 Sportster till the tire fold up leaped a vertical wheelie just past the x-mass tree. Prior runs I lead the Sportster to 11+ sec/85 mph 1/10 mile then got out horse powered in 3rd. Must remember to add more air to rear next time... btw on the 8x10 glossy hard copy photo you can see that ain't a shadow seen behind rear tire but tire and asphalt jet fan just easier to see the part against the lighter tract side.

Hobot, have you ever concentrated on only one given situation with a bit of variety involved, then tuned and geared your bike to suit it ? The biggest mistake I make is not keeping enough notes about what works.

Ah crap Alan I was such a newbie with fates of lost head etc that forced my 1st ever rebuild into just a slapped together expedient combo for initial break in and then was so pensive timid for 1st 1000 miles I had no idea what i had created going by the prior performance till about a month later being so so so depressed I'd wasted $1000's and years on a stupid antique clunker that could not dream of keeping up real sports bikes, so actually tried to punish it and me trying to break it and be done with it... So like you and your surprising 850 the shock was on me so strong I'm the one that died that day and bike became alive with a name to call out. I know what I did but it goes against the grain of universal power wisdoms but the final key factor was opening up the megaphone just enough with 12 ga slug and the rest is history that still returns with the sport riders returning and hunting up The Norton Nut for some more fun and games on their 1198's, GRSX;s R1's and BMW/s etc.

If Next Peel detonates too much will just have to go down to 10 CR pistons and fudge plates and gaskets and push rod geometry till she don't on 87 octane. If works out blown Peel will have more torque over mass than all but GP or drag bikes so should be able to take em up to some hp power or tire limited top speed. Realize Peel don't have to slow down in the sense all others do for sharp turns but can enter them with front lifted out of traction where all others are hardest on brake then add more power as she increases traction by shifting all mass on rear tire while levering its extra thrust against the centrifugal force throwing front back down but unable too :shock: > so just wheelie even faster out of there. Sorry dudes but Ms Peel don't get thrilling till front is out of the picture and spoiled me for anything else as I'm over 60 now and shun the athletics and risks all other cycles require just to cruise let alone hot dog. Tri-links excite me more than hi hp power plant.

Hobot, about the exhaust system. The theories are great but in practice the results can be far different from expected, When I first raced my short stroke 500cc Triton, it was fitted with 4 inch megaphones and it was simply nasty and scary. I didn't get decent lap times until I used a two into one system, and even that took modification after about 3 race meetings to become respectable. It is never as simple as fitting something you believe might work. A restrictive muffler can easily stop a race cam from doing its thing

oh yeah it takes tedious experiments with this and that to get optimalized and even though you'd done something right - that will eventually really help kick ass - it may fall on its face until another vital detail dealt with then surprise surprise. Each exhaust side of a 750 Cdo with brackets weighs like 10 lb. Headers ~3, silencers ~ 6. 360' firing pulses are made for 2-1 extractor system. Hollow Dunstall mega with just end plate weights under 4 lb. W/o end plate it both bogs down low and injures ears of people and animals to point I'd feared farmers shooting me from running the weigh off their cattle and horses over fences. Wish there was a simple way to measure octane ahead of time like we can our moonshine by flame proof test and size of shaken bubbles suspended.