Advice on common sense engine modifications

And, we are officially derailed... :shock:

SquareHead said:

Dances with Shrapnel; sorry about not having more specific parameters, but I don't know what parameters to set other than price, reliability and the fact that I don't want to lose any of the low to mid range torque, based on my riding. But I do appreciate what you posted, it was very helpful, except I am going to have to pass on the supercharger idea! :wink:

Click to expand...

Back on rail. The supercharger was clearly tongue in cheek but accurate in terms of power for the dollar.

I'll revert back to my original comment, assuming you keep the same engine configuration, bump up the compression ratio and flat slide carbs. You could also afford to dabble a little bit in a more aggressive cam.

Another route which is effective is going with a 920 kit. Assuming and 850 (828) this bumps your displacement by 11% (920/828). I will take a SWAG at pistons $400, over bore $150, throw in a little more cam $400, and flat slide Kiehen ~$800; your into around $1,800 - $2,000 and you should have a nice ride with reliability, improved torque, power and throttle response. Not sure if Jim Schmidt offers light weight 920 pistons but with his rods and pistons ~$1,000 you are now looking at around $2,400 to $2,600 and have reduced vibration; always a good thing.

'I'll revert back to my original comment, assuming you keep the same engine configuration, bump up the compression ratio and flat slide carbs. You could also afford to dabble a little bit in a more aggressive cam.'

I agree - cam plus exhaust system can improve power right across the rev range, however enlarging the inlet ports might not be good if you lose the bottom end. Bigger carbs and a tapered port might be better.

And what say ye about 72/73 750 vs. 73/74 850 as a build platform with the original parameters?

My first thought is that the 850 would be a better platform. More cc's right off the bat, for instance. However, I have had a number of experienced, knowledgeable people tell me they would seriously consider a 750 over the 850 because of the 750 tends to be "snappier".

SquareHead said:

And what say ye about 72/73 750 vs. 73/74 850 as a build platform with the original parameters?

My first thought is that the 850 would be a better platform. More cc's right off the bat, for instance. However, I have had a number of experienced, knowledgeable people tell me they would seriously consider a 750 over the 850 because of the 750 tends to be "snappier".

Click to expand...

My understanding is thus:
Early 750s were 9:1 with 28.5mm ports.
Combats were 10:1 with a 2S cam.
These went well. Giving the 750 a good reputation.
850s were heavier, ESP with elec start. It seems most 850s were lower than the claimed 8.5:1. My own 850 was only 7.67:1! This low CR will have a big negative effect on performance.
Late 850s also had 32mm ports, as Comnoz has proven, these also lower performance.
And lastly, the 850s torque can make it feel slower than it really is, as there is less 'kick'.
All of this has led to the 850s reputation being 'knocked' a tad.
But as a platform to build a bike, an 850 has to be better, 100ccs cannot hurt performance! Plus the cases etc are stronger.
But, it will need a RH10 head (with 30mm ports) and raised compression to put on equal terms with a 750. It will then be better.
Only IMHO of course.

ignore 99% of these post just as most are by gray beards toting huge beer guts - just add a good EI, eat more salads & exercise regularly - you'll live longer, your bike will be much more enjoyable without lugging about any extra poundage , and you wallet (and your wife!) will thank you!

Salad and lobster for dinner , shave every day and cycle (bicycle ) every day in winter (balloon tires). , - 20 right now, Off to Tanzania- Zanzibar- Istanbul, Turkey now, :shock: so see you all in the spring !

mikegray660 said:

ignore 99% of these post just as most are by gray beards toting huge beer guts - just add a good EI, eat more salads & exercise regularly - you'll live longer, your bike will be much more enjoyable without lugging about any extra poundage , and you wallet (and your wife!) will thank you!

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And a GT750, no less!

Do you want common sense modifications that increase the power or common sense modifications that improve the bike ?

Power ? How much is enough ? How much can you use ? What is the point of a 160 HP bike ? Are you going to use that power in 1st gear or in 6th ? On the street ? Really. Do not get me started on 4WD pickup trucks.

Put superblends in a 9:1 750. Clean out the sludge trap. Electronic ignition. Oil filter. Use 30 mm carbs. Spend enough time to get the jetting right and balance the carbs. Adjust the throttle cables. Make sure the timing chain is adjusted. Lighter lifters. Lap the end plate on your oil pump. Belt drive primary. Higher output alternator and rectifier. Carefully rebuild the transmission. Make sure to grab the second handful of throttle to get the slides all the way up.

Greg

Fast Eddie said:

SquareHead said:

And what say ye about 72/73 750 vs. 73/74 850 as a build platform with the original parameters?

My first thought is that the 850 would be a better platform. More cc's right off the bat, for instance. However, I have had a number of experienced, knowledgeable people tell me they would seriously consider a 750 over the 850 because of the 750 tends to be "snappier".

Click to expand...

My understanding is thus:
Early 750s were 9:1 with 28.5mm ports.
Combats were 10:1 with a 2S cam.
These went well. Giving the 750 a good reputation.
850s were heavier, ESP with elec start. It seems most 850s were lower than the claimed 8.5:1. My own 850 was only 7.67:1! This low CR will have a big negative effect on performance.
Late 850s also had 32mm ports, as Comnoz has proven, these also lower performance.
And lastly, the 850s torque can make it feel slower than it really is, as there is less 'kick'.
All of this has led to the 850s reputation being 'knocked' a tad.
But as a platform to build a bike, an 850 has to be better, 100ccs cannot hurt performance! Plus the cases etc are stronger.
But, it will need a RH10 head (with 30mm ports) and raised compression to put on equal terms with a 750. It will then be beter.
Only IMHO of course.

Click to expand...

As far as weight goes, I cant think of much of anything on an 850 non estart that isnt on a same model 750, Maybe the bigger holes of the 850 weigh more
If you are using an estart mk3 as starting point, one can always remove the starter and related gear to reduce weight. That way you get the stronger MK3 cases and crank, rear disc, vernier isolastics and about 100 other improvements, but still have a bike that weighs roughly the same as earlier models. When you get old and dottery or just sprain your ankle, the estart can always go back in.

A friend bought a new 750 Roadster in 1970. In 1975 he moved from Canada to the Uk for one year. He sold the 750 and bought an new 850, rode it in the UK and Europe for the year then sold it to move home. I asked the obvious question, which bike did he prefer?
He liked both but found the later bike with the 850 to be more powerful and more refined, so it was his favourite.
56 ft pounds torque beats 48.

Glen

'56 ft pounds torque beats 48. '

At what RPM ?

Its takes about a year to cover all the content in Capt. Norton site and NOC-UK archives or short cut the learning curve with this.

http://victorylibrary.com/NOR.htm
Want better performance from your Norton twin?
A must for every Norton racer - Novice or expert. This manual contains priceless info gleaned from some of the greatest tuners of our time including notes from Peter Williams, Ron Wood, Leo Goff and many others. Twelve 8” × 11” pages of text. Eleven pages of illustrations and text. Three pages of photos. Subject material includes crankshaft lightening, reshaping and balancing (how to balance your own crank). Strengthening of crank PTO shaft (very important). Detailed porting specs for three stages of tune. Exhaust pipe and megaphone specs for various states of tune. Making and fitting your own big valves (1mm oversize). Racing cam specs for Norton factory cams, C.R.Axtell cams’s and other high performance cams. Piston and pin lightening. Handling/suspension improvements for Norton forks. Photos of rear monoshock layouts (racing frame) and much more for only $25.00 (plus shipping). Order now and make your bike a winner. See other Norton racing parts on my site here: Norton racing parts.

And for the rest of the story

Two more: Balance the pistons and rods. CC the combustion chambers.

worntorn said:

Fast Eddie said:

SquareHead said:

And what say ye about 72/73 750 vs. 73/74 850 as a build platform with the original parameters?

My first thought is that the 850 would be a better platform. More cc's right off the bat, for instance. However, I have had a number of experienced, knowledgeable people tell me they would seriously consider a 750 over the 850 because of the 750 tends to be "snappier".

Click to expand...

My understanding is thus:
Early 750s were 9:1 with 28.5mm ports.
Combats were 10:1 with a 2S cam.
These went well. Giving the 750 a good reputation.
850s were heavier, ESP with elec start. It seems most 850s were lower than the claimed 8.5:1. My own 850 was only 7.67:1! This low CR will have a big negative effect on performance.
Late 850s also had 32mm ports, as Comnoz has proven, these also lower performance.
And lastly, the 850s torque can make it feel slower than it really is, as there is less 'kick'.
All of this has led to the 850s reputation being 'knocked' a tad.
But as a platform to build a bike, an 850 has to be better, 100ccs cannot hurt performance! Plus the cases etc are stronger.
But, it will need a RH10 head (with 30mm ports) and raised compression to put on equal terms with a 750. It will then be beter.
Only IMHO of course.

Click to expand...

As far as weight goes, I cant think of much of anything on an 850 non estart that isnt on a same model 750, Maybe the bigger holes of the 850 weigh more
If you are using an estart mk3 as starting point, one can always remove the starter and related gear to reduce weight. That way you get the stronger MK3 cases and crank, rear disc, vernier isolastics and about 100 other improvements, but still have a bike that weighs roughly the same as earlier models. When you get old and dottery or just sprain your ankle, the estart can always go back in.

A friend bought a new 750 Roadster in 1970. In 1975 he moved from Canada to the Uk for one year. He sold the 750 and bought an new 850, rode it in the UK and Europe for the year then sold it to move home. I asked the obvious question, which bike did he prefer?
He liked both but found the later bike with the 850 to be more powerful and more refined, so it was his favourite.
56 ft pounds torque beats 48.

Glen

Click to expand...

I was meaning to compare the early bikes (glass tanks, no indicators, no black caps, etc) with later 850s. I thought they were lighter, if not, it just goes to prove how wrong the folklore can be!
My primary point was that the 850 is a better base, which if I was wrong about the weight difference, simply reinforces the point I feel.

Most of the mods on this forum are about getting more performance for competition purposes, not about improving bikes for road use. There is such a thing as 'blue printing' a motor. My feeling is that the commando motors were a slight compromise however a pretty good design as old British road bikes go. I doubt there would be big differences in the end to end weights of conrods, and piston weights. The standard flywheel should have enough mass to cope with the differences. As with all bikes, you have to choose how you intend to use the commando, the compromise situation is not good - often there is no satisfactory answer and you really need two bikes if you have two different purposes in the way you intend to use it.

Eddie, I think the 850 s rep for extra weight might come from the mk 3 which is heavier mainly by the weight of the starter and related gear. There are also all of the small strengthening mods that were added over the years, but those might total less than ten pounds, all in critical areas, frame, crankcases(850, then more strength added for mk3) crankshaft (mk3) swingarm (850)
Glen

acotrel said:

'56 ft pounds torque beats 48. '

At what RPM ?

Click to expand...

Not mentioned in the specs below is that the Combat max torque of 49 ft lbs comes in higher, from memory at 5800 rpm.

Since torque to horsepower is a simple calculation, the horsepower output of the two engines at 5,000 rpm can be compared from this torque spec. Horsepower of the 850 @ 5,000 is 53 , 750 is 45. We dont know the torque of the Combat at 5,000, but I suspect it is less than the standard 750, so horsepower would also be lower, probably, 42 or so.
So you can see where the Combat would really start to pull after this, since it quickly goes from a level of lower horsepower than a standard 750 to one of higher horsepower than standard.



http://jerrydoe.com/nortonCommandoTechnical.html

norton73 said:

$3000-$5000 will get you a low mileage Monster with all the good mods (airbox, pipes, carb/injection) done by the PO.

Click to expand...

+1 On this...lost of options to have something else...like a cruiser or a sportbike...hmmm...I have a bunch of those for some reason.

I go with the comment earlier that money spent on suspension, brakes and other "rideability" related stuff is a good option...hmmm...that's what I am doing.

I understand racer type Common Sense, which is always staying in hard hearted engine damaging tire wasting traction limits rpm's, so no common sense racer would let rpms drop below 6000 on shifts or his 750. Before 7000ish rpm comnoz has reported the 850+ bore/ring friction goes up faster than hp gains, but not the 750 size. Public street Common Sense makes an 850 more pleasant to leave lights and pass normal speed traffic but its limited on how much extra speed the extra pull lasts in each gear. So 850's need more gears than say a Combat which just gets stronger through red zone so can stay in higher hp-more fuel burning power rpms longer/further/faster. If more hp being developed then don't matter what gear ya in, its putting down more power its gonna be quicker for same weight to accelerate. Every shift cost a few tenths a second delay that can add up over a few laps of a track too. To over come the big bore rpm friction limits one needs to add race only special fuels or boost like TC did using his Common Sense essentially all factory competent 850 engines and two speed tranny. The fastest land speeders in Norton history books are not 850's, unless in streamliners ya got miles of run way to work up to speed. Hp to weight is all the really matters for acceleration then the total hp determines what speed it can punch air aside. Racing around in pulbic especially the canyon and bluff face twisties usually means-requires staying in lower gears for miles slowing up and speeding up in wave after wave of lean tossing and exiting spurts, while regulated road racing generally means getting pass lower gears fast as can for top end speed in opens. To stay ahead of best moderns up to the ton on my Peel Combat could sure of used two gears after the higher 850 2nd gear ratio she had. I hated to have to wait on 3rd if I let rpm get as low as 5800, so tended to stay in 2nd till 7500ish/90 mph and short shift 3rd about 6800 so 4th better pull started about 6200. Rather strange and confusing sensation on Peel vs factory Trixie geared lower 19T vs ~21T, 3rd helps Trixie get along but not Peel which really only needed 2nd and 4th, if I was hard hearted or gear confused enough to launch in 2nd and not just spin out or wheelie on clutch drop in upper 5000ish rpm along with throttle snap WOT. Then again my Ms Peel engine was against all Common Sense combos, too small ports and valves, mis matched step down lip of manifold to head with intruding rough gasket edges and single 34 Miki with 2-1-mega lower CR, 2S cam straight up not degree'd optimal and lightened flywheel. The better the power to weight the less gear peddling needed.

If you continually run at 7,000 rpm then you can enjoy the extra 5 hp of the Combat, at least for twenty minutes or perhaps two hours so until it blows up if held above that rpm.
If you run at highway speeds, 4,000 rpm for seventy up to 5,000 rpm for a blast at ninety mph, then the 850 has more available hp on tap than the Combat 750. According to the official numbers given, quite a bit more actually, probably in the region of ten hp at 5,000