750cc Norton Commando Salt Flats Build

worntorn said:

A couple of years back , I ran my stock ( except blackcaps swapped for 74 type 850 silencers) 850 MK3 slug against a hopped up 750 with D exhaust ports, reshaped intake ports, raised compression and a PW3 cam. There is a video of it somewhere on this site.
It was pretty much a dead heat to 80 mph, at which speed we had agreed to shut off.
My friend kept his 750 pinned past 80 so I did too. As my stock cam 850 went past its power peak and his 750 PW3 went into its powerband, his bike pulled away.
When I finally put the MK3 in top gear at about 100 mph, his bike stopped pulling away.
If we ever have a rematch I will know the stop at 80 thing isn't happening, so I'll shift into top gear at the right rpm. With that, the bikes will be very close to equal for acceleration from 0 to 110 mph.

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Maybe he let off the throttle when he stopped pulling away? kidding

I remember the tail of 850 power on the hill.

Are you saying maybe the D-ports were a power bottle neck on the 750, or is this about the exhaust? Or just a good example of engine displacement making a difference?

Im saying that the DPorts didnt seem to make a huge difference in this particular case.

I have read many times that 750s are faster than 850s and that MK3 850s are the slowest of the slow. Even some experts from back in the day have made that claim.
A friend had a 750 and then added an 850 to the herd. He informed me that his new to him stock 850 was very slow compared to his stock 750.
I have always wondered about this topic so we compared his stock 750 to my 850 MK3. It wasn't really close but I don't think he gave his 750 a fair chance, short shifting it etc.
Another friend who owns the hotted up 750 saw that video of the MK3 vs stock 750 and said " I will run against your MK3 850 and I will totally smite it!" I thought about backing down as a fair fight would be stock to stock, but then thought what the hell, if nothing else it will interesting. I suppose that if his machine had romped away as expected by both of us, my 850 would be heavily modded like his 750 by now. Always good to keep an open mind on these things and be prepared to learn.
He had told me all about this modified 750. He said that it was the quickest Commando he had ever owned ( he's owned quite a few over the years)
I assumed I would get my ass kicked badly, but it didn't go that way.
Perhaps the stock setup is actually pretty good, ( other than those awful black caps!)
The exhaust system on both bikes was the same , stock with open peashooters

worntorn said:

Im saying that the DPorts didnt seem to make a huge difference in this particular case.

I have read many times that 750s are faster than 850s and that MK3 850s are the slowest of the slow. Even some experts from back in the day have made that claim.
A friend had a 750 and then added an 850 to the herd. He informed me that his new to him stock 850 was very slow compared to his stock 750.
I have always wondered about this topic so we compared his stock 750 to my 850 MK3. It wasn't really close but I don't think he gave his 750 a fair chance, short shifting it etc.
Another friend who owns the hotted up 750 saw that video of the MK3 vs stock 750 and said " I will run against your MK3 850 and I will totally smite it!" I thought about backing down as a fair fight would be stock to stock, but then thought what the hell, if nothing else it will interesting. I suppose that if his machine had romped away as expected by both of us, my 850 would be heavily modded like his 750 by now. Always good to keep an open mind on these things and be prepared to learn.
He had told me all about this modified 750. He said that it was the quickest Commando he had ever owned ( he's owned quite a few over the years)
I assumed I would get my ass kicked badly, but it didn't go that way.
Perhaps the stock setup is actually pretty good, ( other than those awful black caps!)
The exhaust system on both bikes was the same , stock with open peashooters

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But was it a one kick start every time 750 Commando. They are the fastest ever. lol

I would think the 750 and 850 are kind of even. I have never touched an 850, so don't know what they are like performance wise.

I've got a story about using D-port exhaust inserts in a 750 Norton engine, but I am not posting it in a forum here. The experience is why I brought up the possible D-port and cam not getting along in the first place. However, if Zacks breather was blocked that would explain a lot. That and one cylinder running leaner than the other.

I know my P11 accelerates a heck of a lot faster than it did when it was stock. However I have no idea how it compares to a stock Commando 750 running at sustained WOT. Probably never will know.

Yes, the 1/4 miles tests from back in the day had the 750s and 850s very close, at least until the black cap silencers went on the 850s.
The D ports must work in some situations as many modern high performance auto engines use them.

I have D exhaust Ports on the Vincent 1360 but have no way of knowing if they are helping or hurting performance there.

Glen

When tuning the 750 we must consider that it is just a 500SS with a larger bore and stroke. The combustion chamber shape is the same. The exhaust port and valve are identical. The inlet ports up to 1972 are identical with the minor difference of slightly larger areas at the valve throat due to a slightly larger valve on the 750. The standard cam, followers and rockers are identical. The 500, I think, had a larger exhaust pipe diameter (which explains the step up of the pipe on early commandos at the head and silencer). Doug Hele assured me that domiracer pipe internal diameter was 1 5/8”, chosen because he found that a low mean exhaust back pressure was better than having a smaller pipe diameter with stronger pressure pulses. For the same engine speed the 750 will therefore be trying to move air through the ports 50% faster than the 500. That is what gives these early commandos “there’s a lotta torque about Norton”, but they run out of breath at high revs and the 500 doesn’t. It runs out of valve control instead, which is why Doug Hele put so much effort into reducing the valve train weight. I’m starting to drift of topic so will end this paragraph by saying that the commandos are limited by having a cylinder head designed for a smaller engine and somehow the porting / valves in that head must be altered to suit the increased capacity.

The fullauto head improves the inlet ports but the D shape stepped exhaust is there to reduce gas reversion, mainly at engine speeds outside optimal for the exhaust system which is not a problem if tuning for maximum speed where a broad spread of power is not required. All that matters is getting maximum power.

A 6 speed gearbox with close ratios for the top 3 gears is a good choice as it keeps the engine within it’s power band. A good selection of gearbox sprockets is a must so maximum speed can be reached in top. I am not familiar with your gearbox but the type with a direct top gear is most efficient in top gear, the only losses being due to stirring the oil (the gearbox doesn’t need a long life so a thin oil will loose the least power).

To help with selecting the most likely sprockets there is software available ( even I wrote one some 35 years ago) but you need accurate values for CdA, back wheel slip % and back wheel power curve. If you have one good run giving a recorded top speed, an accurate tachometer(to enable you to calculate the slip) and the back wheel power curve you should be able to use the software to find the CdA. If the software can’t do that then it is rubbish. I’ve droned on for too long and it’s teatime. I’m just envious of your bike.

maaseyracer said:

Also advance your JS2 a degree or two and compare it the SS cam, I would be curious to hear your thoughts on the comparison.

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I went for a short ride on my Norton yesterday and turned the wick up a little in 2nd. Hit 70 mph still accelerating hard getting into 3rd with a 5 speed before the next traffic light. Pissed off the old folk in the apartments along the route I took. My 2 into 1 open exhaust is unfortunately nasty loud when on the throttle. With the 2 degrees of advance (relevant or not) set on the SS cam with the stock valve train and my other components the SS is better than the JS2 for me. Assuming my memory is still somewhat accurate the JS2 cam is a mellower linear ride with a nice top end.

The JS2 would probably be a great choice for the membership here that isn't concerned about using a mild race profile. I on the other hand like the violent hit from the SS cam. It really is more fun to me. If I wear the SS cam out and have to take the engine apart again, I would probably get a PW3 and some new tappets. The PW3 would mellow things out again.

By the way, other people's opinions of a SS cam with Amal carburetion on a 750 Commando valid as they may be are irrelevant. I have too many one off modifications to my power train for the comparison to be close.

Thanks for trying to learn me something. Futile as it is.

To assure I am tracking along with you, I've always heard the stock CDO cam referred to as the SS cam (same as 650SS Dominator), and the Combat cam referred to as the 2S. Is the cam in your bike the stock SS CDO cam, or something else?

WZ507 said:

To assure I am tracking along with you, I've always heard the stock CDO cam referred to as the SS cam (same as 650SS Dominator), and the Combat cam referred to as the 2S. Is the cam in your bike the stock SS CDO cam, or something else?

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It is something else. It is a Commando cam that supports ignition in the timing cover with SS stamped on the side of one exhaust lobe and it does not support the timed breather. I asked Bob Raber for a hot cam and this SS is the cam I got from him new in the late 80's early 90's. It wouldn't be for a Dominator if the Dominator cam has the tach drive at the end of the camshaft and has timed breather machine work done to it.

Edit: In case somebody wants to say that cam is not new. Well dah..
Below is the SS cam after less than 10K miles on it. I has been dressed since these pics were taken. I took the pics when I replaced the SS with the JS2 cam. After dressing the SS up I removed the JS2 and put the SS back in. I did some other things as well, but that is the cam story, like it or not.





Sorry for the side tracking BS Zackery, but they want to know things, or they are looking to discredit me, but I can do that on my own without help.

Schwany said:

It is something else. It is a Commando cam that supports ignition in the timing cover with SS stamped on the side of one exhaust lobe and it does not support the timed breather. I asked Bob Raber for a hot cam and this SS is the cam I got from him new in the late 80's early 90's. It wouldn't be for a Dominator if the Dominator cam has the tach drive at the end of the camshaft and has timed breather machine work done to it.

Edit: In case somebody wants to say that cam is not new. Well dah..
Below is the SS cam after less than 10K miles on it. I has been dressed since these pics were taken. I took the pics when I replaced the SS with the JS2 cam. After dressing the SS up I removed the JS2 and put the SS back in. I did some other things as well, but that is the cam story, like it or not.

View attachment 124962

View attachment 124963

Sorry for the side tracking BS Zackery, but they want to know things, or they are looking to discredit me, but I can do that on my own without help.

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That is indeed a combat 2S cam.