500 domi exhaust tunning

johnm said:

I'll be interested to hear how that mag holds up. I may need to get a JS condenser for my Morris mag. It's been excellent but condenser is 20 years old now

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I’ve done a full season on it John with the brightspark easycap condensor and it’s going great here’s a picture of it, it sits in the place of the old condensor with some plastic washers under the screw head it’s small and compact

Brian Riley said:

I’ve done a full season on it John with the brightspark easycap condensor and it’s going great here’s a picture of it, it sits in the place of the old condensor with some plastic washers under the screw head it’s small and compact

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We had problems with those Jo Hunt mags but 25 years ago now. They were poorly made at time. I would strongly advise you to check the two lobes on the cam are exactly opposite and your timing is identical on both cylinders. We bought two new Hunt mags from Hunt in California and among the many issues we found the timing mismatched by 3 degree or so side to side.

johnm said:

We had problems with those Jo Hunt mags but 25 years ago now. They were poorly made at time. I would strongly advise you to check the two lobes on the cam are exactly opposite and your timing is identical on both cylinders. We bought two new Hunt mags from Hunt in California and among the many issues we found the timing mismatched by 3 degree or so side to side.

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Funny enough mine are 2 degrees out 28 and 30 just like having a Lucas mag, would a morris rota cam fit in the joe hunt?

Hi Brian

Changing the cam over might not help because it can be the mounting process that causes the difference. I don't know how Hunt mount it these days so cannot really advise.

Two degrees is not too bad and on a road bike would be fine. You would just split it between cylinders.

Have you run any dyno tests to see how ignition timing changes engine performance.? My experience is that these motors don't change much over about 3 degrees but this could be engine specific I guess. Mine liked 28 degrees full advance but was fine from 28 to 30.

If your motor doesn't care too much on precise timing then I would keep it as it is. Check the plugs for any sign of distress including the black pepper spots described by Jennings

One thing. Is the points gap the same on both lobes of the cam. Sometimes the shaft is not perfectly parallel to the case and that can give you asymmetrical timing. In this case it will take a good machine shop to fix it and I'd probability leave it alone for only 2 degrees.

If the gaps the same and you really wanted to fix it I would gently stone the cam front face. Very gently.

Agreed, I’d split the difference and not worry about it. It’s highly unlikely to make much / any measurable difference IMO and is definitely not the cause of your missing 10 ponies.

I’ve not noticed any difference even when swapped the timming around, so I’ll leave them be.
I do often check the plugs after every race and they are always nice with no signs of distress.

It sounds like the only way to get those ten horses from the 500cc Dommie is to sacrifice power below 5000 rpm. Brian says the bike currently pulls well from 3500 on.
Is it worth losing that big piece of the powerband?
On a roadbike the answer is a definite no however on a race bike... maybe?

Glen

worntorn said:

It sounds like the only way to get those ten horses from the 500cc Dommie is to sacrifice power below 5000 rpm. Brian says the bike currently pulls well from 3500 on.
Is it worth losing that big piece of the powerband?
On a roadbike the answer is a definite no however on a race bike... maybe?

Glen

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I think his power band is primarily defined by his cam choice.

Over porting the head would probably do as you suggest ie lose some bottom / mid range for some increase up top.

Hence my belief that getting the ‘right’ head and getting it ported ‘right’ is the only real way to move forward in his missing pony search.

Comnoz did my head specifically to be ‘streetable’ but it still gave a 9rwhp increase. I therefore reckon he got it ‘right’.

However, I’d suggest that a 500cc racer should not really be that strong from 3,500 as there’s simply no need for it and sacrificing some 3,500-4,500 to give max spread of power from 4,500-7,500 would still be a plenty wide enough power band when coupled to a 5 speed box.

Actually I’m surprised Al hasn’t piped up yet, after all, having less power than the opposition should make Brian lap faster according to Al !

There is a difference between horsepower and torque, and what you need depends on the type of bike. A Norton which can win a drag race down a straight is probably pretty rare, so the bike has to be better everywhere else. With a Dominator, the rev limit is probably 8000 RPM, and most race cams start to work at 4000 RPM. If you have 5 gears close ratio, then start raising the overall gearing, you should experience the bike's capability. But with a British twin, you need to be able to accelerate through the corners where the other guys can not. Bikes with a lot of top end power often cannot use the power until much later in the corner. So where the transition point occurs is important. If you can advance it to very early in corners, you can come out of the corners much faster, and the other guys must make up the difference.
When you can flick the bike into a corner after braking, then almost immediately accelerate flat out, it can feel as though you should not be doing it. We are all programmed to ride in corners at full lean and dependent on our tyres. - That is only one way, and most guys do it. Always work up to speed slowly and progressively.

I think the point where a race cam has advantage over a stock cam in a Norton twin is way up there, much higher than 4 k rpm.. That's been my limited experience anyway. The bike with the race cam pulled away from the stock bike once above 6000 k rpm. This makes sense as the stock cam peaks at 5800 (850) and then starts to fall off. That bike with the big cam had other things to make it go fast, Comstock ported D head and raised compression. Usually bikes with a hot cam do have other mods to make it all work.

My take is that up to 5 k the stock cam does best, 5 to 6 about equal then over 6 its the race cam all the way doing its thing, stock cam all done, don't even bother going to that rpm in search of power.

That's for 750 and 850 but a 500 is probably a different kettle of fish entirely.

Glen

worntorn said:

I think the point where a race cam has advantage over a stock cam in a Norton twin is way up there, much higher than 4 k rpm.. That's been my limited experience anyway. The bike with the race cam pulled away from the stock bike once above 6000 k rpm. This makes sense as the stock cam peaks at 5800 (850) and then starts to fall off. That bike with the big cam had other things to make it go fast, Comstock ported D head and raised compression. Usually bikes with a hot cam do have other mods to make it all work.

My take is that up to 5 k the stock cam does best, 5 to 6 about equal then over 6 its the race cam all the way doing its thing, stock cam all done, don't even bother going to that rpm in search of power.

That's for 750 and 850 but a 500 is probably a different kettle of fish entirely.

Glen

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I personally think the tip over point is closer to 4.5 or maybe 5k IF the rest of the engine has been built accordingly.

But whatever the number is, the point here is that if Brian wants to find some of those missing ponies he’s gonna have to sacrifice some of that ability to pull strong from 3.5k. There’s just no need for this on the track anyway.

Here’s the dyno sheet the red line is from last years run with GP carbs which put a huge hole in the midrange the top blue line is this years with Monobloc carbs and 12.5” total lengh inlets, you can see where the power builds from and ends,

worntorn said:

I think the point where a race cam has advantage over a stock cam in a Norton twin is way up there, much higher than 4 k rpm.. That's been my limited experience anyway. The bike with the race cam pulled away from the stock bike once above 6000 k rpm. This makes sense as the stock cam peaks at 5800 (850) and then starts to fall off. That bike with the big cam had other things to make it go fast, Comstock ported D head and raised compression. Usually bikes with a hot cam do have other mods to make it all work.

My take is that up to 5 k the stock cam does best, 5 to 6 about equal then over 6 its the race cam all the way doing its thing, stock cam all done, don't even bother going to that rpm in search of power.

That's for 750 and 850 but a 500 is probably a different kettle of fish entirely.

Glen

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I am fairly certain the cam in my 850 is 650ss - which is stock Commando. From the timings it is close to an E3134 Triumph 650 race cam. That is enough for any bike. I run it 12 degrees advanced and with a 2 into 1 exhaust to get more midrange. Getting the bike to pull hard while the revs rise is more important than having more power above 6,500 RPM.
My usable rev range is from 5,500 RPM to 7300 RPM. For racing, the way the Commando engine needs to be used is different. With a 650cc Triumph engine, the revs can go as high as 8000 RPM. However my mates Triumph 650 never gets revved above 6300 RPM, and is still extremely fast. With our bikes, making them pull hard is more important than revving the tits off the motor to get more power. Many race circuits are about 50% corners. Two-strokes and Kawasaki 900s are fast, do not play their game. They are stuffed in corners. When I get passed in races, it is usually near the ends of the longest straights. Gearing and handling are extremely important. On a straight, any of Rex Wolfenden's 1100cc CB750s can blow my Seeley 850 to the weeds. It does not matter.

I'm going by the listed maximum torque rpm for the stock cam in an 850, 5300. Sorry, I wish I knew more about the smaller engines but am lacking in that.
I don't think that a race type cam will offer more torque than the stock cam but it will move the max torque rpm up a few hundred rpm so that more maximum hp is possible. At the point where the stock cam is producing max torque, it's going to be hard to beat, is my guess. Above that it will lose out at some point. That point for me was somewhere around 6000 rpm, hard to say exactly.

Note that two owners of said to be healthy original spec Combats on this site found the bikes incapable of pulling past 85 mph in top gear, with just the stock 19 tooth gearbox sprocket changed to a 21.
This seemed to indicate a pretty big hole in the top of the midrange with a Combat cam. 85 mph in top with a 21 tooth is just under 5000 rpm.
The Combat cam might have a greater hole in the middle than some of the other choices, but I thought that was an interesting happening. All sorts of things were tried to resolve the lack of grunt, but iirc nothing was successful.
Shifting back down into 3rd ought to work!

Glen

With close ratio gears, it is easier to get more go. but nobody would ride a road bike using the gearbox that way. Personally, I only ever ride motorcycles at high speed, so I don't usually ride on public roads. I have ridden a VFR400 Honda on country roads. It belonged to one of my wife's girl friends. While I had the Honda maxed out at 180 KPH on a bend, her boyfriend rode around the outside of me on a GSXR1000 Suzuki, regardless of slippery roads, trees and wet leaves. With the Honda, riding it was a mindless exercise - it did everything right.

From experience, if I race, I usually know how to get the most out of my bike. When I had wide ratio gears, I knew immediately why my bike was too slow. A lot of guys would probably not identify that problem. Also, with close gears, much higher overall gearing can be used. The only way to discover that is by actually running higher overall gearing. With lower gearing the bike can appear to be going as fast as it can.

johnm said:

Hi Brian

Changing the cam over might not help because it can be the mounting process that causes the difference. I don't know how Hunt mount it these days so cannot really advise.

Two degrees is not too bad and on a road bike would be fine. You would just split it between cylinders.

Have you run any dyno tests to see how ignition timing changes engine performance.? My experience is that these motors don't change much over about 3 degrees but this could be engine specific I guess. Mine liked 28 degrees full advance but was fine from 28 to 30.

If your motor doesn't care too much on precise timing then I would keep it as it is. Check the plugs for any sign of distress including the black pepper spots described by Jennings

One thing. Is the points gap the same on both lobes of the cam. Sometimes the shaft is not perfectly parallel to the case and that can give you asymmetrical timing. In this case it will take a good machine shop to fix it and I'd probability leave it alone for only 2 degrees.

If the gaps the same and you really wanted to fix it I would gently stone the cam front face. Very gently.

Click to expand...

I’ve found my notes on the different settings on the cam timing with the JS2/PW3 here’s my findings at present it’s on at 105 lob centre with 6 degrees on the RGM sprocket, and also piston clearence with a 0.020” copper head gasket