Big valves in a Fullauto head

acotrel · Jun 7, 2021

In the old days there was a theory that the strongest motor in the best frame made the best bike. So guys built Norvins and Tritons. Anyone who has ever ridden a genuine Manx knows differently.

mark parker · Jun 7, 2021

acotrel said:
With anything such as this, you never know what the benefits are until you get the modified part onto the bike. If you succeed in getting more torque, you often do not know it until you change the gearing. If the power band moves up, you can reach the rev limit before you experience the benefits. Dynos and flow benches probably only give an indication of what might happen in service. The rest is supposition until you try the changes in the situations which represent normal use. When you do that, the motor needs to be set up with everything optimised. It is not as simple as an additive process. You can buy all the good parts and think you will build the perfect motor, but it is usually the way in which the parts are assembled which is more important. If you change the flow in the combustion chamber, you might do better if you also incorporate a different cam or exhaust system. One of the main strengths of the Commando engine is it's smooth strong power delivery. In road racing and on the dirt, a bike which is peaky is usually slower.

That's why I'm trying the 34mm entry and carb and port on the big valve head. The little 34mm port with 42mm valve works really well. The one on the 745cc sidecar has won 8 out of 10 races it's been in. Last years P3 Queensland champions using a Thunderbolt head and revving to only 6800 max. They got a second because it jumped out of gear and a dnf because a carb rubber split. It has std bore and A10 crank. Max power should be around 7500 or 8,000 at least, not 6800, but it uses a hard to get fragile A10 crank. So with an 80x74 engine and 90degree crank, that can rev, we will try this. Though it's got to be Thunderbolt based. They race against stuff on methanol, 840 Triumph 920 Norton and 1200cc Vincents. In the Queensland championship races it broke the lap record by 4.4seconds, it's pretty dominant, but the national titles this year promise more competition. They need a potent spare motor for the rig. Until a head is done and tested on a bike we will not know if we will lose and of the midrange grunt, which is so nice. Peaky is not what we want. And I think it will be ok.

I actually thought the 34mm with 42mm valves was probably crap till they messaged and said they won the Qld titles. And then I got my Firebird registered. The vacs which I had doubted were actually on the money, even though the ports were small.

acotrel · Jun 7, 2021

From what Jim Schmidt said in a similar topic on this forum, I suspect the search for ways of making the Commando engine faster now has a lot to do with racing against Harley XR750s on flat track. On dirt, the balance between slide and drive is important. A V-twin gets better drive than a vertical twin. There are ways of getting better drive, but they apply equally to both types of bike. The perceived difference on horsepower might be due to the way the power is put on the ground. It also happens in road racing, but to a lesser extent. A single cylinder motor usually gives better drive than a vertical twin of the same capacity.

acotrel · Jun 7, 2021

mark parker said:
That's why I'm trying the 34mm entry and carb and port on the big valve head. The little 34mm port with 42mm valve works really well. The one on the 745cc sidecar has won 8 out of 10 races it's been in. Last years P3 Queensland champions using a Thunderbolt head and revving to only 6800 max. They got a second because it jumped out of gear and a dnf because a carb rubber split. It has std bore and A10 crank. Max power should be around 7500 or 8,000 at least, not 6800, but it uses a hard to get fragile A10 crank. So with an 80x74 engine and 90degree crank, that can rev, we will try this. Though it's got to be Thunderbolt based. They race against stuff on methanol, 840 Triumph 920 Norton and 1200cc Vincents. In the Queensland championship races it broke the lap record by 4.4seconds, it's pretty dominant, but the national titles this year promise more competition. They need a potent spare motor for the rig. Until a head is done and tested on a bike we will not know if we will lose and of the midrange grunt, which is so nice. Peaky is not what we want. And I think it will be ok.

I actually thought the 34mm with 42mm valves was probably crap till they messaged and said they won the Qld titles. And then I got my Firebird registered. The vacs which I had doubted were actually on the money, even though the ports were small.

I use 34mm Amal carbs with 30mm ports. The first 25mm of the port is tapered. I don't use oversize valves, mainly because of the difference in valve mass. Norton valve gear should be able to cop 9,000 RPM reliably, if the cams are not too radical. But springs fatigue, and I am too lousy to keep replacing them.

mark parker · Jun 8, 2021

acotrel said:
I use 34mm Amal carbs with 30mm ports. The first 25mm of the port is tapered. I don't use oversize valves, mainly because of the difference in valve mass. Norton valve gear should be able to cop 9,000 RPM reliably, if the cams are not too radical. But springs fatigue, and I am too lousy to keep replacing them.

SRM have pretty cheap springs they call 'race springs' they are very good and not very heavy. The cams I use are std Lightning/Spitfire profile the guys racing are using an SRM race cam, they say isn't particularly radical but is really good. They swapped it in after 4 races and the outfit seems the same except it now idles as the one before was very radical. I was interested to know what the power spread was like, where it started to stop pulling, Matt the rider said it pulls like a train to redline... 6,800. And the milder cam may be better, but they get a lot of wheelspin which a 90degree would be less inclined to do as they drive well. I'll have to wait for a LSR bike in the states to be dynoed with one of the heads and maybe see some higher rpm with the A10 crank. They strengthened the A70 cranks but they are not around. We have had the big valves to 9,000 in a big bore 750. You need control at least for another 1000rpm if you miss shifts.

I have no idea what this head will do at least till I set up and measure air speed in the port, it's an odd shape expanding a bit going in. But filling it hasn't lost much, possibly 2cfm, I'll be chasing that tomorrow if I get time. The air is ripping through the carb and manifold, how much it may slow down through the big valve I don't know, but its getting about 20cfm more so may not I hope.

This is a std stroke 750 90degree. It has a good 38mm carb head but this 34 carbed head may be better as flow is close but faster which should give it more grunt. And the compression could be raised a point as well. Which will make either headwork better.