H.P mods ?

Status
Not open for further replies.
worntorn said:
Going back to the original question here, the question of how much extra power can be made with the 750 via hot-rodding. Assuming that durability needs to be at least couple of hundred operating hours, not just minutes, the answer is not much.
The factory hotrodded the 750 from a rated crank bhp of 58 HP up to ,65 with the Combat.
It was only 7 hp extra, only on top with some lost in the middle, but it was the proverbial straw that broke the Camels back. Those engines experienced a lot of catastrophic failure and Norton nearly went under from this.
So the stock 58 HP pre-Combat Commando must be very close to max safe output for the original 750 cases and crank.

Glen
Click to expand...
Like ever other engine that is subjected to a higher engine tune, it will require more attention and money spent on it to maintain this performance.
 
In this case the extra maintenance was often done by Norton on warranty, as in a new engine to replace the destroyed engine.

Glen
 
A single Mikuni carb will probably give more power than twin Amals because it is easier to get one carb jetted really right than it is to match the jetting of twin carbs and get them both right - especially when you use petrol. A single carb with the correct taper needle, using the highest octane petrol with optimised advance curve and exhaust system, would be pretty good
Fitting a race cam usually increases power both across the usable rev range and below where the cam comes on. . But it is probably better to slightly increase the lift than extend the cam timings. If you change the timings, you usually move the power band up the rev range. Cut-off for a Commando motor is 7000 RPM, unless you go silly.
Joe Craig claimed 50 BHP for his 500cc Manx Nortons using petrol as fuel. So 75 BHP would probably be the ultimate for a 750 cc Commando, using petrol. - Methanol is better - unlimited antiknock and a supercharging effect from latent heat of vaporisation..
 
Fast Eddie said:
Very true...

If you want more power, buy a modern bike. If you want more comfort, buy a modern bike. If you want a better tourer, buy a modern bike.

Makes you wonder what kind of fools ride these old nails !
Click to expand...
Most modern bikes would have a hard time catching me around the back of Winton Raceway. Motorcycles are about having fun. Fun is when you improve your bike and achieve. A good Commando will out-handle many faster bikes. And a Seeley is even better. A lot depends on mindset - 'if you have a victim's mindset, you will be a victim'. The way my bike handles the last time I raced, it was so goo that it worried me.
Around the back of Winton, an RS125 Honda is faster that a 600cc sports bike. - Horses for couses ?
 
Last edited:
A single Mikuni carb will probably give more power than twin Amals because it is easier to get one carb jetted really right than it is to match the jetting of twin carbs and get them both right - especially when you use petrol. A single carb with the correct taper needle, using the highest octane petrol with optimised advance curve and exhaust system, would be pretty good
Fitting a race cam usually increases power both across the usable rev range and below where the cam comes on. . But it is probably better to slightly increase the lift than extend the cam timings. If you change the timings, you usually move the power band up the rev range. Cut-off for a Commando motor is 7000 RPM, unless you go silly.
Joe Craig claimed 50 BHP for his 500cc Manx Nortons using petrol as fuel. So 75 BHP would probably be the ultimate for a 750 cc Commando, using petrol. - Methanol is better - unlimited antiknock and a supercharging effect from latent heat of vaporisation..

Bullsh-t I purchased my '69 fastback with a single 34 Mk2 Amal, It was lazy up top, mucked with it a bit but finally put it back to twin carbs- a different bike!
 
I agree! If the argument is that a perfectly tuned single Mikuni will outperform a badly tuned/poorly synchronized/improperly jetted pair of Amals on a stock Commando, I might go along with that. But a single Mik will not perform as well as a pair of properly set up Amals. There are too many real life examples - head to head - that prove the opposite. To be fair, either bike might win a race if you limit the max RPM to 4500. But beyond that the Amals will easily pull away. I've been involved personally in several of those specific "races."
 
I don't think your question as phrased is unanswerable! But that is probably my history as a systems engineer kicking in.

First you need to state your requirements in terms of what you want to achieve! Write them down in clear language we can all understand.

List them and prioritise them. Classify as essential, desirable, nice to have. Scratch out anything that in analysis is nice to have, and a few of those desirable ones, and concentrate on essential. Rideability is ESSENTIAL for any road bike, and great to have on any successful bike, reliability should be part of the requirements for any build you don't want to keep repeating!

Determine how you are going to test and evaluate results, dyno, drag strip, test track or the test hill, or 200 mile Sunday ride. Honest answers here should drive a review of the requirements.

Modifications to inlet/port flow, exhaust flow etc. should not affect reliability, but excesses will limit performance more than enhance it. Good carb choices can help but standard fit works with most mods with little need to spend weeks optimising things and will avoid bad choices. (carbs, exhaust and ignition timing are also the easiest things to change when the motor is built, installed and running)

Camshaft and valve gear can add or subtract reliability and or performance dependent on choices, but avoidance of excessive lift, wild timing and big valves, coupled with modest reductions in valve train weight will most likely improve reliability.

Modest compression ratio increases have positive impact, high compressions increase risk of internal impacts and pose fuel problems for road use. I have 98 octane road fuel working well enough at around 10.5:1 but you would be better placed considering 9.5:1 a limit on a ride anywhere bike.

The biggest benefits are going to come from intelligent and diligent assembly of best quality and sound parts, including several dry builds to test and measure everything! Make sure nothing is modified to reveal weakness and that nothing is at risk of undesired contact. This will address reliability head on.
 
Last edited:
acotrel said:
Joe Craig claimed 50 BHP for his 500cc Manx Nortons using petrol as fuel. So 75 BHP would probably be the ultimate for a 750 cc Commando, using petrol. - Methanol is better - unlimited antiknock and a supercharging effect from latent heat of vaporisation..
Click to expand...
There is no logic in your suggestion that an extra 50% capacity can readily translate in to 50% more power when you have doubled the potential for friction and added more valve train components in a more compromised cylinder head design using push rods rather than OHC mechanisms.

I think you will find a 750 long or short stroke that makes 60RWHP a lively and rewarding bike to ride! Norton managed around 76hp at the crank from 750s on their Thruxton Dyno. These figures are not that far apart after transmission and measurement differences and other losses. However, a modern Manx runs nearer 60RWHP and is lighter than a period Manx and pretty much any Commando based bike!
 
Free engine boost can be obtained via forced air induction, all I need is a small plastic funnel, 1/2 inch bore pipe and a free extra kick from 60 mph! It works every time!
 
Cam timing is one of the main criteria on ANY four stroke, and getting one that is 100% accurate is harder than you think.
The Dunstall Street cam that I had was not worth a piss as only one lobe out of the for was anywhere near what was claimed.
When I put back in the Norton cam I had took out, I did a mini wheelie from standstill !
 
Knuckles,

In some editions of the Clymer Workshop manual (I have the 1977 edition) there is a chapter on go-faster mods. I believe it answers your questions.

-Knut
 
Bernhard said:
Free engine boost can be obtained via forced air induction, all I need is a small plastic funnel, 1/2 inch bore pipe and a free extra kick from 60 mph! It works every time!
Click to expand...
Won't work. As the air hits the neck-down it will choke and reverse as it jams up. You'd be better off to maintain the funnel big diameter all the way to the carb, catching clean air. I forget the formulas I had in college, but the pressure available at 60mph is almost negligible, like .05 psi or so, goes up with the square of velocity.
Jaydee
 
There's lots of ways to hop things up, but none of it is of any practical use if the cases and crank can't take it, as with the 750.
Norton unintentionally found the limit of the 750. That is right around 56-58 bhp at crank. It could be that with Superblends in place, 60 would be ok.
The 850 cases were strengthened then strengthened again for the 850MK3. The MK3 also got a stronger crank.
Kenny Dreer found the limit on those components with his 880s. I've read that those were capable of about 65 rwhp for a short time.
That might correspond to about 75 bhp. But those engines did not hold together, so the safe limit on the 850 isn't all that much higher than the 750.
Maybe 65-70 at crank?

Glen
 
jaydee75 said:
Won't work. As the air hits the neck-down it will choke and reverse as it jams up. You'd be better off to maintain the funnel big diameter all the way to the carb, catching clean air. I forget the formulas I had in college, but the pressure available at 60mph is almost negligible, like .05 psi or so, goes up with the square of velocity.
Jaydee
Click to expand...
When you say the air 'jams up', you are probably saying that the air pressure is locally higher. The amount of air transferred through the funnel probably does not change. One thing which can help a race bike go faster is to carry a supply of cold air to the carburetor. I think it was Bob McIntyre who was into that. There was one top rider who used to do it one every bike he rode in races.. It is the main thing about methanol as a fuel - the carbs are much colder, so the incoming charge is denser and you use more fuel to compensate for the extra air..
I think a pressurised air box around the carbs on a Commendo, might be good. But the most power is achieved by getting the jetting right, regardless of anything else. An air box makes adjusting the jetting more difficult.
When you use petrol for racing, the motor usually gets too hot.
 
I use 34mm carbs on my Seeley 850 with 30mm ports. The ports taper in 2mm per side over the first half inch of the port. If the air was going to 'jam up', it would do it there. The air in the ports is sonic - behaves like a sound wave. If you duct air to the carbs, the whole system probably resonates. Air temperature is important - many guys find there bikes are much faster on very cold mornings. You get that if you are jetted slightly too rich. You can also get it if the race circuit is within a pine forest. Mount Gambier race circuit in South Australia is like that.
 
Last edited:
jaydee75 said:
Won't work. As the air hits the neck-down it will choke and reverse as it jams up. You'd be better off to maintain the funnel big diameter all the way to the carb, catching clean air. I forget the formulas I had in college, but the pressure available at 60mph is almost negligible, like .05 psi or so, goes up with the square of velocity.
Jaydee
Click to expand...
Sorry this is b.s.
I've done this on petrol and diesel engines and all give a better on/ off/ on throttle response at exactly 60mph.
It is dependent on the size of the engine, some car manufacturers put their air intake trumpets into the front or side of the front mounted radiator. I suggest you try it first before you slag off a successful method.
What you learn in theory and practice can result in two different things. . . . . .
 
@Knucles, I'm not going to read all the replies, the first few seemed to immediately stray from your likely direction.

What I WILL say is:

1. Get the shop manual, open to the Service / Maintenance / Tune-Up sections near the front, and go through the ENTIRE process. Be very thorough and methodical, EXACTLY in the order that the sections are presented in the book (there is a good reason for the order, every step is primarily dependent on the previous step being correct, so as not to affect the next step's correct completion).

2. Use high quality consumables. I'm NOT going to start an oil war, and (some) modern lubricants do indeed have something to offer. PERSONALLY, I use dino-based Castrol, per the book. Same for spark plugs, Champion N7Y(C) (Iridium plugs are now highly touted).

3. Go thru all the adjustments on the bike, checking for loose, worn, and broken/missing hardware, and any cable fraying (both ends of each). Nip everything up accordingly. Especially critical is looseness in the swingarm spindle, easily checked by setting the bike on the center stand and pushing side-to-side on the back of the rear wheel.

4. You want your bike to stop, as good as it goes. Go thru your brakes to ensure best operation.

THIS IS YOUR STARTING POINT!

Attempting to perform performance modifications on a less-than-optimum bike is an exercise in wasted time and money. Maybe not a total waste, and maybe you get away with a slight performance boost, but you will NOT arrive at your upgrade's optimum performance because it is dependent on the basic bike being as good as IT can be.
 
Status
Not open for further replies.
Back
Top