RH4 to RH10 JS Intake Port Sleeves

seattle##gs said:

I will mention this again for you Mikuni people...there is a company called Thunder Products which has done a lot of work on the simple round slide VM carbs and his inventions really works. The best item is very simple to install and requires NO fiddling around with jets. The main improvement is in low speed to mid range. It is called The Torque Wing (this NOT the swarup and is NOT a gimmick). It really works, I've used it for several years. Look it up on their website for more info.

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What do you have yours fitted to?
And what benefits did you notice exactly?

I have a 34mm Mikuni on a 750 motor. Everything is stock. It boosts the power from just off idle to midrange or more. By full throttle it is no longer in play. The increase in power is really noticeable when the throttle is opened quickly. The guy who designed it works mainly with racing snowmobiles. I sent him an amal carb body hoping he could make a torque wing for them but no luck. Any waay, where can you spend $100 and actually get results you can feel? You can install one in about an hour.

jan nelder said:

this seems aimed at two strokes. How is it on Commandos?

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It works great on commandos

The first race bike I had was a 500cc Triton. The motor was made from 650 Triumph parts with a 63mm billet crank. The bike had been built with all the theories by one of my mates. It had almost killed him at Bathurst in about 1958. When I first got it, it haseparate pipes with 4 inch megaphones. It was really exciting to ride - gave a great adrenalin rush - however it was impossible to get a decent lap time around Calder Raceway with it. From the beginning, I had a never-ending search for torque. Manx Nortons had really big inlet ports, so that was obviously the way go - NOT !
The Commando 850 engine is really lovely with 30mm inlet ports. I am just amazed that it is quick enough. These days in Australia, we race in 'historic' classes - not Allpowers C grade against ZI and H2 Kawasakis. The fastest bikes are 1100cc methanol fuelled CB750 Hondas. When I came out of corners faster than them , they could just catch me at the ends of the longer straights.

Fast Eddie said:

I think that’s a fairly safe bet !

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Three things work in conjunction - the torque characteristic ,the gearing and the shape of the needles in the carburettor. If you have high overall gearing, you tend to use more throttle, so you are more onto the taper of the needles. A slightly over-rich mixture gives less power. I use 6D Mikuni needles which are the leanest for my 34mm Amals. With close ratio gears and very high overall gearing - that gives the fastest acceleration. Commando motors are different because of the heavy crank. When you open the throttle, you compete with the crank inertia. If you open the throttle quickly, you often use more of the needle taper than you need, so even with more throttle, the bike does not accelerate as fast as it can. It is very deceptive. The only way to findout what works is to try all options, but the only way to find out you have improved torque is to raise the overall gearing. If you don't do that, the bike gives the impression that it is accelerating as fast as it can. If you lower the gearing with the heavy crank, the bike does not usually accelerate faster.

The port leading up to the valve guide should be the venturi (smaller diameter) and then open up a bit to get around the valve guide. Norton did just the opposite. They opened up the port before the guide and left the choked down area at the guide - hurting the midrange and doing nothing for the top end. See illustration of RH4 port. If you run your finger down the port you can feel the smaller diameter ridge near the guide. The answer is to reduce the port diameter leading up to the guide and restore the high velocity venturi effect.

Fast Eddie said:

I recall him saying the difference was much greater Glen. The velocity in the low and mid range was much higher with the RH10 but the increased flow of the RH4 was more than the engine needed, hence they performed (more or less) the same at the top end.

Different engine sizes, and tunes, will of course behave differently though.

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What cfm an engine of a given size needs depends on the displacement, the rpm and the volumetric efficiency of the engine. The last factor being a product of engine tune, cylinder head flow and inlet speed. Power is directly proportional to burning charge. Burning more equals more hp. And the thing to know is an engine can be overfilled to the point of around 127% if all the ducks are lined up. 127% VE. Anything near 100% is really good, but GP engines need to be higher and even the Manx and 7R made very high VE making them super hard to beat. Modern GP engines are no better, but they rev harder and so burn more charge. It isn't the size of the engine that determines cfm requirement, a 900 at 73% VE burns a charge of 657cc that's what it needs to do that. A 750 at 127% VE burns a charge of 952cc, and is what it needs in cfm, not only that but if the compression ratio is 11-1, that overfilling means that bigger charge is compressed in the same small combustion chamber and the real compression ratio goes to almost 14-1, so that little motor punches very hard and is very alive to throttle.

So, the actual balance you are looking for is enough cfm to feed high VE, and enough speed in the port to push it in. So, the size runner you run needs to be the smallest possible before it starts lowering the cfm. I have a big engine with a head flowing 200cfm through 38mm ports, it's together and runs well but I'll guarantee I would be able to retain that flow and close down the size of those runners. What that would do is speed the intake and force more into the cylinders, at 6,000 it is already 111%VE but it would increase that and whatever that increase is it means more hp at 6,000. I think it's around 85hp so at 124% it would be around 94hp. And that 85hp would have happened at lower rpm, and all lower rpm points would have more hp. At the moment that engine is peaking close to 8,000 so VE can move the power down from that rpm because it will run out of available cfm earlier.

Running bigger ports needs them to flow more for their size, for example a BSA 30mm port (109cfm ) increased by 13%, is 34mm and only 123cfm, both are super inefficient, the 34mm loses bottom end for a little more top end. The 30mm could be around 145cfm if it's the right shape. And the 34 around 180cfm with a slightly O/s valve. More air through the same port size and speed goes up, an engine with the 34mm will kill a stock engine in the midrange and the difference will kick in harder when it revs.

It's interesting how they choke the port near the valve stem, BSA did the same. It acts as a blockage. If there is metal and head studs are not in the way widening the port around the guide alleviates the blockage. And air goes through easier. There is a limit to height. That blockage does not speed up the air going through the carb and port it has to slow it down because it cannot get through. The engine tune is designed to pull that air trough and that blockage impedes it in volume and speed. The reason for 32mm carb on a 30mm port is it flows more. A 30mm Concentric does not flow like a smooth bore and restricts what a port can otherwise do. The faster the port the worse it is, fast air that is disturbed block the flow. I measured a few carbs once, VMs are about the same as a concentric, TMs are better and Lectrons much better. I'd like to test a GP2 but $$$. Std BSA ports on the twins are not fast enough to lose much flow with a concentric but as speed goes up it limits it and a bigger version on the same port flows more. I test with a bell and with a carb and pwk are cheap and pretty good.

Why the bell flows well is because the air enters smoothly and isn't broken up by an edge, and blocking, the faster the port the more important it becomes. Without the bell the port flows less and the air down the centre increases speed, that isn't a good thing because the volume is reduced.

With a two-stroke engine a lot of noise comes from the intake. If you can hear it, it is sonic. What happens at high velocities is different from what happens at low velocities. With aircraft they use transonic wind tunnels to get better flow. With motorcycles, it is what works which counts. Three things work together - rev range, port size and the shape of the needle in the carb. If you have less vacuum (big ports) when you open the throttle, you need more fuel - so more taper. If you have a quick taper needle but wind the throttle on slowly, the mixture can be too rich and that dramatically reduces power, But when you are geared relatively high you do not notice the loss. If you have big inlet ports, the whole power output can be too high in the rev range. With 30mm ports and the leanest Mikuni needles, the throttle needs to be wound on in a more controlled manner, but the bike will accelerate much faster than it will if you have the same size port with quick taper needles, and open the throttle more rapidly.
It might sound silly, but to go faster,you need to wind the throttle on slower. A lot of guys get around corners somehow, get the bike pointed, then whack the throttle open. With a Commando engine, the next straight needs to begin at the start of the preceding corner. So handling and smooth power delivery is more important.
You need to watch out for the guy you pass as you leave the corner.

When you race you need to think in terms of combinations of variables. When you change only one variable you get an indication of it's effect, but sometimes optimisation to suit a circuit and the weather can involve changing several variables. When I stopped racing a few years ago, I was only racing on two similar circuits.

acotrel said:

When you race you need to think in terms of combinations of variables. When you change only one variable you get an indication of it's effect, but sometimes optimisation to suit a circuit and the weather can involve changing several variables. When I stopped racing a few years ago, I was only racing on two similar circuits.

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I have Air/fuel gages on 3 bikes, one is mine, for setting up the carbs. They have a wide band sensor in one header, I just jet the carbs the same once I find the settings. It's easy to do the main and power jet though the pilot jet affects that as it still flows when wide open. I try for 12.6-1 wide open and power is very good, richer or leaner gets progressively worse though it's still good around 13 or 14-1. So, I set light throttle pretty lean up to 15-1 cruising but as you open it, it goes richer into the 13s and flat out or near there around 12.5 12.7 area. That's what I aim for. This is my son's 734 it needs a GoPro set up to watch the A/f gauge and see where it goes as it is no place to be looking on full noise. But when it's all set up in those zones it feels very strong. I need to check this again because the mutes are now out of the mufflers. It is still silenced but not quite so much.