Rebuilt RH4 head with Black Diamonds and flow tested. What is a reasonable price to pay?

Status
Not open for further replies.
Flow testing does not tell you if the ports are oversize. If they are too big, you might find you don't get the performance you might expect. A lot depends on maintaining vacuum at full throttle. With big ports, that becomes more difficult. A motor with that usually develops it's power at much higher revs than a Commando bottom end will cop.
Some guys create sucker-bait.
 
acotrel said:
Flow testing does not tell you if the ports are oversize. If they are too big, you might find you don't get the performance you might expect. A lot depends on maintaining vacuum at full throttle. With big ports, that becomes more difficult. A motor with that usually develops it's power at much higher revs than a Commando bottom end will cop.
Some guys create sucker-bait.
Click to expand...
Why assume the worst?
 
acotrel said:
Flow testing does not tell you if the ports are oversize. If they are too big, you might find you don't get the performance you might expect. A lot depends on maintaining vacuum at full throttle. With big ports, that becomes more difficult. A motor with that usually develops it's power at much higher revs than a Commando bottom end will cop.
Some guys create sucker-bait.
Click to expand...
Which is why a good head guy would always look at flow AND velocity, and aim for the best balanced increase of both, with the precise ‘best balance’ being somewhat dependant on the desired application.

Which is what Sir Comnoz does.
 
Fast Eddie said:
Which is why a good head guy would always look at flow AND velocity, and aim for the best balanced increase of both, with the precise ‘best balance’ being somewhat dependant on the desired application.

Which is what Sir Comnoz does.
Click to expand...
Flow testing is not usually done under sonic conditions. When the motor is running, there are standing waves in inlet ports and exhausts - you can hear them, so they are sonic. When you fly a jet aircraft at the speed of sound, normal aerodynamics do not apply. If you only look at ports in terms of flow, you might be getting the wrong answers. A slight bump in a sound wave can change things a lot. In the end it comes down to what works - trial and error. I have seen ports you could fit your fist down, it does not necessarily mean the bike will be fast. I suggest the most common mistake most guys make when racing a British twin, is over-porting.
When the guys first started playing with piston-port two strokes, it was common to have the inlet tract too long. You might think you would get more ram, but that often stopped the motor from revving high The answer was to get the carb hard up against the cylinder. Even Peter Inchley found his motor only revved high after the bell-mouth fell off the carb on the Greeves..
 
Last edited:
If you want to develop better ports for your motor, you need to work progressively, testing as you go. But if you go too far, you have stuffed the head. So you need two heads and port the second one that little bit less. When the motor is running, the internal shape of the ports might be more important than the results on the flow bench
 
Fast Eddie said:
So what ?
Click to expand...
You are assuming cause and effect. - More flow means more power. But what happens when a motor is running is under different conditions. When there is a sound wave, the rules are different The shape of the port becomes important. I use 34mm Mk2 Amals, and my ports are gently tapered back to the original 30 mm over the first 25mm of the port. It seems to work well. - ARSE BEATS CLASS !
.
 
Many years ago, there was a guy in Victoria supplying services with a flow bench. He raced a 350cc Ducati which seemed to be fast. But how would you know if he really got results from his customers' race bikes ?
 
acotrel said:
You are assuming cause and effect. - More flow means more power. But what happens when a motor is running is under different conditions. When there is a sound wave, the rules are different The shape of the port becomes important. I use 34mm Mk2 Amals, and my ports are gently tapered back to the original 30 mm over the first 25mm of the port. It seems to work well. - ARSE BEATS CLASS !
.
Click to expand...
So let me understand your point here... someone like Comnoz who does flow and velocity testing is wasting his time, but your engine that’s never been near a flow or velocity test... or a dyno... is good because “it seems to work well” ?!

I remain convinced that you’re are actually a 20 something art student Al, and that your posts are part of some kind of art project...
 
Last edited:
So, regarding my “assumptions of cause and effect” Al...

Below are two charts.

The first is showing the CFM (flow) of my RH10 head, before and after Comnoz’ work.

The second is the velocity of my RH10 head, before and after Comnoz’ work.

You can see the clear differences. What makes these particularly relative is that this bike was dyno’d before and after as well. The only other difference to the engine apart from Comnoz’ head work was to reduce valve duration by putting a slightly smaller radius on the cam flowers. This would reduce peak HP a little and increase mid range power.

Despite this ‘hit’ to the peak power, the recorded difference was 9rwhp... NINE. On my engine that was approximately a 16% increase.

So, a clear link between a defined increase in flow, a defined increase in velocity, and a resultant increase in output.

As a scientist Al, I would assume you would approve of the methodology and findings.



ECCA3AC5-CAAE-4E45-B93F-516DF4C73E66.jpeg
6195AF13-E2E6-4897-8F9F-697FFC0A7F60.jpeg
 
Last edited:
Fast Eddie said:
So let me understand your point here... someone like Comnoz who does flow and velocity testing is wasting his time, but you engine that’s never been near a flow or velocity test... or a dyno... is good because “it seems to work well” ?!

I remain convinced that you’re are actually a 20 something art student Al, and that your posts are part of some kind of art project...
Click to expand...

You forgot to add track ;)
 
Dkt26 said:
is fullauto still around? Last I spoke to him he had just got his bike back on the road after being t boned.
I was saddened to hear of John's (Landsdown) passing in my absence. I spoke to John regularly and he was a really lovely guy. As I said, I have been away for some time. Just before my absence, Steve had stopped producing his exhaust systems which I always wanted and couldn't convince him to make just one more. He had a bad fall somewhere and had cut back his service quite a lot.
There was a fella who was always a laugh, I'm sure he would agree with me that he was a bit of a nutter when it came to his Norton's! I think Hobbit was his name. There's been quite the change since coming back but its great to see SteveA and fast eddie still kicking around.
Click to expand...
@Brooking 850 is making Maney repro exhausts in both street and race configurations here in NZ. They are lovely and work very well. Black or silver.

We know which is faster...

Rebuilt RH4 head with Black Diamonds and flow tested. What is a reasonable price to pay?
 
Dkt26 said:
The last FA I saw was for sale in Britain foe £2900 from memory. It was New in Box.
Wasn't Doug and Steve running FA heads along with fast eddie and a few other racers? Hardly an oddity more like a piece of unobtainium racing kit.
Anyway off topic. What should one pay for a bolt on ride away rh4?
Click to expand...
Andover Norton has a FA head listed as in stock for 2498 GBP
 
Fast Eddie said:
So, regarding my “assumptions of cause and effect” Al...

Below are two charts.

The first is showing the CFM (flow) of my RH10 head, before and after Comnoz’ work.

The second is the velocity of my RH10 head, before and after Comnoz’ work.

You can see the clear differences. What makes these particularly relative is that this bike was dyno’d before and after as well. The only other difference to the engine apart from Comnoz’ head work was to reduce valve duration by putting a slightly smaller radius on the cam flowers. This would reduce peak HP a little and increase mid range power.

Despite this ‘hit’ to the peak power, the recorded difference was 9rwhp... NINE. On my engine that was approximately a 16% increase.

So, a clear link between a defined increase in flow, a defined increase in velocity, and a resultant increase in output.

As a scientist Al, I would assume you would approve of the methodology and findings.



View attachment 21233View attachment 21234
Click to expand...
 
That seems to be conclusive evidence that flow testing give an improvement in horsepower, but what about torque. ? It is possible to get a gain in horsepower but still have a motor which does not pull so well in the mid-range. Most dyno runs are done at full throttle - does that tell you how well the bike accelerates when coming up through the gears. A commando engine is very strange. It is different from motors with light cranks and short strokes, but probably just as good if you get it geared right.
 
In 1973 there were short stroke 750cc engine Commandos which were sold at the same time as the normal long stroke 750cc models. I have never seen anybody on this forum making claims about the short stroke Commandos. Perhaps there was no discernable advantage in the shorter stroke - or were they slower ? I know where there is one of those short stroke motors - a few years ago I tried to buy it. But I have always wondered whether it would be better ? The torque thing is very deceptive. You don't know whether you have an improvement until you raise the overall gearing
 
Last edited:
acotrel said:
That seems to be conclusive evidence that flow testing give an improvement in horsepower, but what about torque. ? It is possible to get a gain in horsepower but still have a motor which does not pull so well in the mid-range. Most dyno runs are done at full throttle - does that tell you how well the bike accelerates when coming up through the gears. A commando engine is very strange. It is different from motors with light cranks and short strokes, but probably just as good if you get it geared right.
Click to expand...

Below 3,000rpm it was difficult to judge either way, so my assumption is, backed up by riding it, that below that there really wasn’t much change. Hard to tell in reality of course, as when the throttle is opened, even from a standstill, 3,000 is past so quickly.

Above that there were substantial gains in torque and power everywhere. Biggest gains in torque were in the 4,000-5,000 region and the biggest gains in power were above 4,500. But there was more everywhere. Peak power was still same figure of around 6,250rpm which I found as a pleasant surprise as I had expected the point of peak power to increase up the rpm scale.

What was most interesting was to ride it back to back against my mates near stock mk111 (ham can, peashooters and EI being the only mods). There was just no comparison.
 
To me, an un-modified cyllinder head is worth more than one which has been ported. I would say one thing- if there is a bike at race meetings which is very fast and winning everything, and you want to go that fast - buy it. But buy it at meeting, just after it has won a race. Don't let the guys take it home, then buy it from them later.
 
Status
Not open for further replies.
Back
Top