Airflow -port taper

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comnoz said:
And Yes, the setup in the last picture was the winner on the dyno on a 750 cc race engine. Jim
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OK, in an attempt to get the thread back on track from pig lips and quench.......................

I have to ask, did you every try the equivalent 34mm carb, 34mm to 30mm to 35mm and if so, how did it stack up?
 
Between the straight 30 mm manifold with a radius inlet and the 34 to 30 tapered manifold.
The peak horsepower was about the same.
The powerband started a lot earlier with the straight manifold and peaked at the same RPM. Megaphonitis was much improved.
 
comnoz said:
tricatcent said:
BSAs experiments showed that the step should be pretty sharp to provide the horsepower increase that they noticed on the dyno.

What I am planning to do to reduce reversion is shape the exhaust and inlet valves a bit differently. There is reversion at slow speeds on this engine. You can see it puffing out of the carburettor at idle. I am also switching to a cam with a bit less duration for the intake as well. The overlap will be about the same though so that may not make much difference. I just want the engine to run a bit better at the slower speeds. I don't think reversion is happening at higher engine speeds. Jim if you have any other hints about what I might do please tell me about it.
Click to expand...

The sharp edge on the port would effectively make the port smaller which would increase the velocity in the active area of the port. They might find a bit more power by making the port smaller yet and matching the carb with a radius.

I would not think air "puffing out of the carb" at idle would be from reversion. Real reversion comes from the returning exhaust wave during overlap. The exhaust system would have to be really "off" to return the wave at idle. I guess you could change the exhaust length and see if the puff goes away.

It sounds more like very late intake valve closing is allowing the air to push back out the intake after BDC. I suspect any valve or seat damming would hurt the intake flow but I can't say for sure.

I would make sure the intake valve is closing at the right time and make sure the valve is centered on the seat. Beyond that you may have to live with it or install a softer cam. Jim
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I've always believed that the appearance of mixture blowing back through the carbs was due to the fact that the tuned lengths of inlet and exhaust only suit a certain rev range, as do the cam timings. If the motor is running below the cam spot, the standing wave tries to establish itself beyond the bell mouth ? Normally you don't ride the bike revving below the cam spot, so unless your bike is too peaky, I wouldn't change to a milder cam to get better performance. If you slowly increase the revs while watching the vapour coming out of the bellmouth, you should see it disappear as the motor revs past the cam spot, and the tuned system starts to work properly.
 
I think that Comnoz and Acotrel are both right about the cause of the puffing of the carb on my BSA at idle. It is not caused by reversion of the exhaust gasses. It is because it has an inlet cam with 320 degrees duration and the inlet valve closes somewhere around half way up the stroke (about 75 degrees after BDC). The piston pushes the fresh mixture back out. I am installing an inlet cam with 20 degrees less duration to reduce this (the BSA scrambler cams) Apparently this cam will give about 3 less horsepower at 7000 RPMs and more power everywhere else. I am hoping it might improve the fuel economy a bit as well. This cam has the same profile as the exhaust cam instead of the extra high lift one that was used on the Clubmans GS.

I still plan on making some changes to the shape of the intake valve to reduce this effect. Apparently sharp edges will help.

One thing I am not going to alter is the sharp lip where the 1 1/2" carb bolts onto the 1 7/16" port. This is a feature that is proven to work on this engine. I am not really to sure what it does, but I had better not change it.
 
A loss of 3 BHP at 7000 RPM is not as important as having decent power below 6,500 RPM. Gearing changes the importance of top end and my feeling is that regularly revving a commando engine above about 6,800 RPM can become very expensive. If you don't buy the billet crank, long rods, and thick crankcases before the blow-up, you will pay the bill after it - so why go there ? At first I did not believe in the design of the commando engine, I now believe it is quite adequate if it is always operated with certain parameters.
 
Fuel stand off is the term for the fog seen out in front of intakes which is mostly the exhaust gas pressure reversion pushing through valve over lap that far. The section in front of the carb likes being exponential funnel down but on the other side if funneled slightly back out, similar to comnoz higher expanding flow tube it acts like a megaphone resisting reversion by both the mass wave of back flow pressure and the massless sonic energy waves. Below is another almost completely unrelated principle in handling standing intake waves. Looks like a non super sonic shock loss improver to me Dances. Says only works in carb seeing air flow 90' past throat but that's pretty much the wind direction that hits Norton throats after it slaps back together around barrel.

http://www.baggersmag.com/novi-twinstac ... w?image=06
Airflow -port taper
 
After looking at the findings, if a bike had 32mm carbs, manifolds and ports, could you expect a gain by replacing the manifold with one that tapered 30mm at the carb to 32mm at the inlet port, this would give the step at the carb/manifold joint, or would it work better with a radius as opposed to a straight step or would it even work at all?

Yours curiously
Paul.
 
That is exactly what I plan to do on my factory Combat, hehe and likely for same reasons the idea popped up in your mind against the grain fluid resonant wave front turbulence vortex mass flow to stifle the standing excess fuel cloud in carb throats. Insides will not be smoothly contoured either so likely not able to CNC one out.
 
brxpb said:
After looking at the findings, if a bike had 32mm carbs, manifolds and ports, could you expect a gain by replacing the manifold with one that tapered 30mm at the carb to 32mm at the inlet port, this would give the step at the carb/manifold joint, or would it work better with a radius as opposed to a straight step or would it even work at all?

Yours curiously
Paul.
Click to expand...

With a 32mm carb I would use a 32mm manifold and then radius the 30mm inlet on the head to match. That will help keep the air velocity in the manifold low so it can make the curve with less turbulence.
That combo flows the best on the flowbench but the difference is pretty small. Jim
 
As I understand the results of comoz reverse cone flow bench report and intermediate intake pulses and reflected sonic waves and how some turbulence can energize boundary layer-eddies I think we might be surprised in practice if anyone ever does it to see fro sure. Then again might make engine run backwards.
 
I "hogged out" the ports on a 650ss, not a clue what i was doing....just making them BIG! It's easy to get carried away with a carbide bur and a view that larger is better.
Anyway arnt 650 ports are way to small to start with? YES get im BIGGER¬! I guess the finnished holes where 20% larger,and looked considerably increased.
All the alloy dust on the bench later,made me feel like Pops yoss, Great i thought....
The engine was stock every thing..cam, comp...but for a two/one megga . Length guessed...LOL
Test day ONE.......It started , but liked one cylinder better than the other.
I took it out on the road....1st gear it lept to 7000 plus...the rev counter was jumping around a bit...and my vision had gone blurd!
2nd gear the same.
3rd ...wow this is storming along! 7000 plus and rapide responce..had i really put the engine into super tune?
4th....O dear...the rev counter refused to please me...5000 rpm max....changing down give me my double vision back...but back into top it died.
Mayby i had over done the port enlarging? gas speed down? over geared? it had one extra on the gear box.
But if the ports where to large why did it go like the clappers in 3rd? changing the mains did not inprove matters..from 220 -280 in steps, not a great lot differance.
Thinking now, had Norton really got it right?
 
The shape of those stock ports was created by none other than Doug Hele , the man behind the record setting 500 Domiracer. When the 650SS first appeared, the roadtesters of the day said they were the fastest of the fast bikes. So, yeah, Norton got it right.
Having said that, my 650 SS received Port surgery from Herb Becker before I bought the bike. He had the bike in his shop for some valve work and just for fun decided to port the thing the same was as he does the racing Commandos. The port is still standard sized where the carb stub bolts up, however he changed the bowl shape further in. He did get a result which made the clutch slip.
I struggled with the clutch for some time and finally installed a New by Belt drive yesterday. Haven't tried full out in fourth, but a quick blast thru the first three gears showed 105 MPH in third.

Glen
 
comnoz said:
brxpb said:
After looking at the findings, if a bike had 32mm carbs, manifolds and ports, could you expect a gain by replacing the manifold with one that tapered 30mm at the carb to 32mm at the inlet port, this would give the step at the carb/manifold joint, or would it work better with a radius as opposed to a straight step or would it even work at all?

Yours curiously
Paul.
Click to expand...

With a 32mm carb I would use a 32mm manifold and then radius the 30mm inlet on the head to match. That will help keep the air velocity in the manifold low so it can make the curve with less turbulence.
That combo flows the best on the flowbench but the difference is pretty small. Jim
Click to expand...

Jim, unfortunately my ports are already 32mm, along with the carbs and manifolds also, so given this scenario what would the preferred plan of action be. I realize that It could be just left alone but where's the fun in that??

Paul.
 
brxpb said:
Jim, unfortunately my ports are already 32mm, along with the carbs and manifolds also, so given this scenario what would the preferred plan of action be. I realize that It could be just left alone but where's the fun in that??

Paul.
Click to expand...


I used to make an aluminum shoe that filled the bottom .2 inch of the port and went all the way to the valve seat. Then I would widen the port near the valve guide. That gave pretty good results but it was a lot of work. On race only engines I have been known to fill the bottom of the port with JB weld but it only lasts for a while before pieces start flaking off and going through the motor. Jim
 
When I built my Seeley 850 I never really believed in it so it stayed unraced for twenty years while I did other things. In about 2003 I got silly and decided to race it. I now know my lack of faith in the Norton motor was unjustified, - I believe the motor as standard is very right in many ways. Mine has the minimum done to it, however I've fiddled the cam and slightly reshaped the inlet ports, and I use a two into one exhaust. I've set the handling of the bike up to suit the motor and it is excellent on the short circuit. When I was a kid, I had a lot of racing years. These days I cannot really afford to race, however when I get onto the Seeley it is all so easy to do without any anxiety. To me the motor looks ridiculous, however it really does it's job well.
 
john robert bould said:
I "hogged out" the ports on a 650ss, not a clue what i was doing....just making them BIG! It's easy to get carried away with a carbide bur and a view that larger is better.
Anyway arnt 650 ports are way to small to start with? YES get im BIGGER¬! I guess the finnished holes where 20% larger,and looked considerably increased.
All the alloy dust on the bench later,made me feel like Pops yoss, Great i thought....
The engine was stock every thing..cam, comp...but for a two/one megga . Length guessed...LOL
Test day ONE.......It started , but liked one cylinder better than the other.I took it out on the road....1st gear it lept to 7000 plus...the rev counter was jumping around a bit...and my vision had gone blurd!
2nd gear the same. 3rd ...wow this is storming along! 7000 plus and rapide responce..had i really put the engine into super tune?
4th....O dear...the rev counter refused to please me...5000 rpm max....changing down give me my double vision back...but back into top it died.
Mayby i had over done the port enlarging? gas speed down? over geared? it had one extra on the gear box.
But if the ports where to large why did it go like the clappers in 3rd? changing the mains did not inprove matters..from 220 -280 in steps, not a great lot differance.
Thinking now, had Norton really got it right?
Click to expand...

When I purchased my 750 Norton it came with a single carb manifold with split reducing sleeves in the inlet ports.
I too, under the impression that bigger is better, obtained a pair of 32mm Concentrics and manifolds, I even took the head off and stripped the rockers ground & polished them. The inlet ports were bored out with a 32mm drill to the valve guides and blended in between the guides and the seat.
Did it make the bike faster :?:
No.
Did it increase the gas speed :?:
No.
With the benefit of hindsight, and knowing what I know now, I would have been better off making a pair of split sleeves to the orginal port diameter to fit into the inlet ports of the head to INCREASE /SPEED UP the gas flow.
Enlarging the cylinder head port might appear to be a good way of improving air-flow but it has a major effect on mean gas velocity, which we actually need for better power and performance. A small port, relative to the cylinder, will have a high mean gas velocity at low RPM but it will struggle to fill the cylinder at high RPM. Thus Volumetric Efficiency will tail off at high RPM and power will fall off quickly. Conversely, a relatively large port will have a low mean gas velocity at low RPM, and when we have a low mean gas velocity the fuel in the air/fuel mixture could drop out of suspension and dribble into the combustion chamber as droplets. This leads to poor fuel economy and poor performance as fuel droplets do not burn efficiently and as quickly enough as it would while in suspension. It also removes lubricant from the cylinder walls, which increases emissions and reduces engine life. To maintain fuel atomization, i.e., to keep the fuel droplets suspended in the air flow, a high mean gas velocity is required. Therefore, enlarging the cylinder head ports might not be as beneficial as it might appear.
 
brxpb said:
comnoz said:
brxpb said:
After looking at the findings, if a bike had 32mm carbs, manifolds and ports, could you expect a gain by replacing the manifold with one that tapered 30mm at the carb to 32mm at the inlet port, this would give the step at the carb/manifold joint, or would it work better with a radius as opposed to a straight step or would it even work at all?

Yours curiously
Paul.
Click to expand...

With a 32mm carb I would use a 32mm manifold and then radius the 30mm inlet on the head to match. That will help keep the air velocity in the manifold low so it can make the curve with less turbulence.
That combo flows the best on the flowbench but the difference is pretty small. Jim
Click to expand...

Jim, unfortunately my ports are already 32mm, along with the carbs and manifolds also, so given this scenario what would the preferred plan of action be. I realize that It could be just left alone but where's the fun in that??
Paul.
Click to expand...
I refer to my answer above.
 
I suspect that looking at port size in terms of gas flow might not really be the answer. I believe that when the motor is running fast there is a standing wave in the inlet port, and that a smaller diameter tube resonates more easily at lower revs than a large one. (7,000 RPM limit is not a lot of revs.) I think the same applies to exhausts - skinny pipes are better than large at lower revs. With my two into one exhaust, I use skinny header pipes , a non-restrictive collector, and a tail pipe with a cross sectional area as big as the sum of the areas of the header pipes. I believe the tail pipe resonates at twice the frequency of one header pipe.
With the inlet port, it is operating over a range of revs and getting a stable resonating column of gas with a large port could be a problem. Obviously the cam used is highly relevant to the situation, when the 'cam spot' is high.
I always find it difficult to think of standing waves and mass transfer of material through the wave at the same time, however I don't believe it is as simple as gas flow. (How much water from a tsunami actually comes from the epicenter where it was created ? )
 
Ugh, there is no such a thing as optimal everything but at one narrow rpm zone, OR Worse several bands of pressure waves adding up then subtracting as ya run to redline, which is why all modern race engines have electronics to artificially dampen out the crashing tendiency unpredicable throttle response induces. There are many ways moderns do this sliding tuning scale from sliding thrombone exhausts and mainifolds with valves to pass gas through easier or restrict it. STudy on stepped headers and length clculations is very fun area which in a 2>1 its cleavage point can be part of the reflective features, each coming on in turn or at least mellowing out the bigger power variations with a narrowly tuned system. In the olden dazed GM had the QuadraJet 4 bbl, 2 small ones for the low down that stayed open as the 2 flaps opened up with accelerator pumps to boot for the fun old school response w/o electronics. I've a pair of 32mm flat sldie Lectrons that I"m thinking to put on 920 Peel with a X type manifold so both carbs can feed each side in turn and maybe not give up the low down rush of mixture. I've had 2 cycles and two cages that ya could test traction up over hi way speed by tire spin on punching it straight ahead while trying to stay straight ahead then ease off a bit then back on for the best press backwards. If ya can swirl the intakes right it tends to condense the cooler molecules to the center of flow which lessens the loss of the boundary and over come sound wave effects.
 
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