H.P mods ?

what can be done to increase h.p. In the 750 cc engines ?
what is the most h.p. You can get to with street fuel ?
i just have an interest in what can be done .
not that I want to do it on mine . I want to stay with reliability .
I can remember about 40 years ago a friend had a bunch of work done on his . He seemed to have a lot of problems and the bike was being fixed very often .
I don’t remember what he had done .
what mods can be done on a street bike and what hp increases do they give ?

Better fuel , higher octane, pure gas, no ethanol , Methanol too. Easiest route for the reliability factor.

I have run both a 750 & 828 motors in a supercharged methanol burning speedway sidecar, I blew my first 750 motor as it was a combat- breather which acted as a better scavenge than the oil pump, I tried to catch the Castor oil out of the breather and blew the motor as a result [late 1970's young and stupid, now probably old and still stupid] run a second 750 the first season then fitted a 828 motor for the second season, run it to 7500rpm through the gears in top, end of the season rebuilds showed egg shaped conrods [still alloy].

Knucles said:

what can be done to increase h.p. In the 750 cc engines ?

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You could blueprint a stock engine and run a AMR300 at around 6 psi and 60 rwhp @ 6800 rpm maximum.

I think people used to add all manner of things, cams, carb, compression without making sure all the bits were happy going round and round, up and down.

Knucles said:

what is an AMR ?
a supercharger ?

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Yep

Knucles said:

what can be done to increase h.p. In the 750 cc engines ?
what is the most h.p. You can get to with street fuel ?
i just have an interest in what can be done .
not that I want to do it on mine . I want to stay with reliability .
I can remember about 40 years ago a friend had a bunch of work done on his . He seemed to have a lot of problems and the bike was being fixed very often .
I don’t remember what he had done .
what mods can be done on a street bike and what hp increases do they give ?

Click to expand...

find out what Ron Wood did ( if you can)and copy it... based on cam, gas flowing and compression, I believe.

No matter what you do as far as engine mods, generally if you increase max HP, you reduce low/mid power, which is more useful in street riding. The only mod to a conventional engine that will increase power across the RPM range is an increase in compression. However, it doesn't increase power as much as people generally think it will. As an example that would be applicable for a Norton engine, raising from 9:1 to 10:1 would produce 1.5-2 HP.

SO...it depends on what you want. You could certainly get more max HP with a cam change, exhaust change and appropriate fueling. Even more if you get into head porting, etc but you would end up with what most people would consider a very annoying machine to routinely ride on the street.

Of course, often you can get some additional HP just by ensuring that everything is set up perfectly for the specific engine using a Dyno. Also, blueprinting an engine can produce dramatic results but that will vary from engine to engine. For example, I have seen identical factory v8 engines produce from a low of 430 HP to a high of 510. After blueprinting, the 430 HP motor made 525, the 510 motor made 530. So how they came out of production has much to do with it. I suspect the same was true of Nortons!

I photo copied from an old Haynes or Clymer manual , tuning process of the fastest guys before us , not sure if I posted it here or sent the pages to those that had asked ....

The idea that a tuned engine loses mid range torque is a relative concept. Meaning, when I tuned my 850 for the first time it produced more power everywhere, period. Now, I could have altered the tune to lower peak power and give it more bottom end of course, but relative to the stock engine, the tuned engine produced more low end torque.

My 920 engine takes off like a scalded cat at 4500rpm and it’s temptimg to consider it weak below that, but it’s not, not compared to a stock engine! It’s just that the increase in power above 4500 is exponential.

Also, ‘mid range’ means very different things to different people. I once had a 650 Triton rider telling me what a rocket ship his Triton was... that he never revved above 4000! A lot of people swear by single Mikuni’s on Commandos as they ‘increase the mid range at the expense of losing a little top end’. In reality a single carb strangles the engine above 5000rpm... to me, 5000rpm IS the mid range.

Different horses for different courses... and riders !

Fast Eddie said:

The idea that a tuned engine loses mid range torque is a relative concept. Meaning, when I tuned my 850 for the first time it produced more power everywhere, period. Now, I could have altered the tune to lower peak power and give it more bottom end of course, but relative to the stock engine, the tuned engine produced more low end torque.

My 920 engine takes off like a scalded cat at 4500rpm and it’s temptimg to consider it weak below that, but it’s not, not compared to a stock engine! It’s just that the increase in power above 4500 is exponential.

Also, ‘mid range’ means very different things to different people. I once had a 650 Triton rider telling me what a rocket ship his Triton was... that he never revved above 4000! A lot of people swear by single Mikuni’s on Commandos as they ‘increase the mid range at the expense of losing a little top end’. In reality a single carb strangles the engine above 5000rpm... to me, 5000rpm IS the mid range.

Different horses for different courses... and riders !

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+1

Same on my bike. 9.5:1, hot street cam, head porting, balanced, Mike's Maney pipe. I get past 4500 FAST!. And i bet there is at least as much torque as a stock cam.

All other things being the same, you will get more power by increasing capacity, increasing compression ratio, or supercharging.

Then there’s remedying design faults, such as over-restrictive exhausts or bad timing specs.

Anything else you do is likely to be a trade-off (of something) and is likely to require skill, patience and resources to produce an actual gain.

Jim Comstock informs that the greatest midrange power comes from the stock cam profile.
Unless you have a high rpm capable aftermarket cases, crank and rods, like Nigel has, it's best to stick to the recommended 6500 rpm redline.
For a stock-ish bike the useful rpm range is 3000 to 6500. 3000 is good as the bottom as that where the ISOs start to really give smooth running.
So midrange for me is about 3500-5000. 5000 to 6500 is top end.
I'm sure there are cams that will bump up the top end a little, but if they will steal a bit from the midrange.
In any case the stock trans is already highly stressed with a standard output Commando.
As Jim Bush , owner of multiple Commandos, a 961 and a Thruxton R says, " The best power upgrade for a Commando is a Thruxton R"

Glen

Glen, mentioning the Thruxton R reminds me of something you pointed out some time ago...

The more highly tuned ‘high power’ Thruxton R actually makes significantly more torque, everywhere, than the lower tuned ‘high torque’ version of the same engine.

The high torque motor feels like it has more torque low down because the torque levels off. Whereas the high power motors’ torque continues to climb as the revs rise. Could the Thruxton be tuned for more torque at the expense of some peak power? Of course it could. But, it still makes more torque relative to the lower tuned motor.

That’s probably the best example there is demonstrating the ‘relative’ nature of the argument IMHO.

For more H P you need to understand where your priorities lie. Tuning for top end power will leave a big gap between bottom end power and coming "on the cam"
Blue printing an engine can get you some surprising results and reveal some not so obvious engine faults, like finding your camshaft is inaccurate, in the old days of yore you could try 10 camshafts before you got a good one.
Also opening the inlet port too much is a power killer as it will be over ported.
However, done in a specific way and polished, it can, on this alone, knock half a second off your SS 1/4 mile time.

Bernhard said:

Also opening the inlet port too much is a power killer as it will be over ported.
However, done in a specific way...

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And as a home mechanic/tuner, I might destroy more cylinder heads than I can afford, yet never find that specific way.

Guys , thanks for the responses. It is interesting to see what others have done.
I want to leave mine stock , for reliability and just want it tuned right , run good .
any compression over 9.5 is not good in this state , the high test gas is crap here .
it took the cam out of my Harley , with detonation .
had to get a new cam and lower the compression pistons to last .
it was a great , modified , running bike till I moved here . on thing I had done on my Harley was get the heads ported , polished and springs changed by a company in Ontario Canada called Head Quarters . The guy does a nice job .
not sure if he does anything besides harleys or if he is still around ?

Triton Thrasher said:

And as a home mechanic/tuner, I might destroy more cylinder heads than I can afford, yet never find that specific way.

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You are saying that never studied how other people worked on a cylinder head to improve the gas flow?
There's plenty of tuning information out there.

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

I never thought trying to increase the hp on these engines was worth any effort , at least for street bikes.
the slight increase makes little difference in performance .
I think that correct tuning is the right and best way to set up these bikes