Airflow -port taper

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comnoz

comnoz

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For you DIY porters out there.
I see this problem all the time when I am porting heads. I made a little demo of what happens with a common mistake.

Here is a flow tube. It is 35 mm on one end and 30 mm on the other end. It has the same 3/4 inch radius entrance on both ends and a steady taper of 2 degrees included angle.

Airflow -port taper


Airflow -port taper


Airflow -port taper


Here it is flowing with the air entering the 35mm end. See the airflow at 112.5 CFM

Airflow -port taper


Here it is flowing with the air entering the 30 mm end . See the airflow at 139.9 CFM

Airflow -port taper


It's a good demonstration of what happens when the port is larger at the flange than it is at the entrance to the bowl. Air does not like to speed up as it is traveling down a tube. Jim
 
The Factory-publishing Tuning Sheets on how to build a Combat-spec 850 call for 34 mm Amals to be fitted.
And all the mods to get it to a 2 way average speed of 143 mph - Elvington 1973 by the Rawlins/Baker bike.

Tuning sheets show the required port cross sections and mods.
So there is more than 1 way to skin a cat ?
(apologies to any cats reading this.)

Anyone done it by the book ?
 
Rohan said:
The Factory-publishing Tuning Sheets on how to build a Combat-spec 850 call for 34 mm Amals to be fitted.
And all the mods to get it to a 2 way average speed of 143 mph - Elvington 1973 by the Rawlins/Baker bike.

Tuning sheets show the required port cross sections and mods.
So there is more than 1 way to skin a cat ?
(apologies to any cats reading this.)

Anyone done it by the book ?
Click to expand...

Yes, I have done it by the book many times. Then I learned how to make it better. Jim
 
Intriguing Jim. If there is enough pressure differential the flow will speed up by funneling down, like rocket nozzles before the exit flair. Might try test again with a 30 mm dia or other dia. restrictors on the 35 mm intake end. What does a straight throat tube flow? Try a restrictor with saw tooth inner rim roughness.
If bored glue in a pair of spiral raised rifle strips tightened up the spiral a little bit towards the exist. Maybe try some spin vanes inside too.

Airflow -port taper

Airflow -port taper
 
"And all the mods to get it to a 2 way average speed of 143 mph - Elvington 1973 by the Rawlins/Baker bike."

143 MPH? WOW, must have been some really freaky wind conditions that managed to produce a 20MPH tailwind in both directions! :)




Sorry, couldn't resist...
 
hobot said:
Intriguing Jim. If there is enough pressure differential the flow will speed up by funneling down, like rocket nozzles before the exit flair. Might try test again with a 30 mm dia or other dia. restrictors on the 35 mm intake end. What does a straight throat tube flow? Try a restrictor with saw tooth inner rim roughness.
If bored glue in a pair of spiral raised rifle strips tightened up the spiral a little bit towards the exist. Maybe try some spin vanes inside too.

Airflow -port taper

Airflow -port taper
Click to expand...

A straight 30 mm tube flows slightly less than the taperd tube does when flowing into the 30 mm end. It is only when you are trying to compress the flow that turbulence in the tube kills the flow.

If you really want to see a problem, stick a valve guide into the 30 mm end and flow the air into the 35mm end. Jim
 
I will do many things to a motorcycle including carving space into a set of Molnar cases for a 102 mm stroke Bobweight race crank (dont screw up!) but will never mess with the ports, that I will leave to experienced pros.
I am pretty sure I would get it wrong and reduce performance.
Glen
 
Interesting demonstration comnoz. I am pretty sure the difference in the flows is due to the difference in shock losses which is a function of velocity. The 35mm has 36% greater cross sectional area so velocity will be 73% of that of the 30mm side. A higher velocity discharge will incur a greater shock loss. The greater the air velocity at the discharge the greater it wants to contract (vena contracta).

If interested, do a Google search on the words <mine fan evase>. Basically evases reduce the shock loss of discharged mine ventilation air.

Same thing as comnoz demonstration and how it applies to porting.
 
I have been told the reason for this is -the air goes to a high velocity as it enters the small opening. Then the walls open up so the air will expand to fill the space. This means it has to slow down but since the air has momentum it creates a depression behind it which increases the pressure differential across the inlet.

When the ports cross section get's smaller, the converging walls create turbulence. I can watch the turbulence with my velocity probe.

If you look at the ram air ports on late model motorcycles you can see they take advantage of this -they have a very small opening at the front of the fairing which expands as it goes back to the airbox.

Car air cleaners often take advantage also by having a small inlet that tapers up in size. Jim
 
If I remember from college (40 yrs ago), a diverging tube with a seven degree included angle is the most efficient for flow thru a tube. That's why exhaust megaphones are made the way they are. I know Jim is on the right track, I'm just trying to remember the optimum angle.
Jaydee
 
jaydee75 said:
If I remember from college (40 yrs ago), a diverging tube with a seven degree included angle is the most efficient for flow thru a tube. That's why exhaust megaphones are made the way they are. I know Jim is on the right track, I'm just trying to remember the optimum angle.
Jaydee
Click to expand...

I believe the optimum angle depends on the speed of the air. Jim
 
All kinds of influences on air flow, < area rule > is one that concerns the guide taking up space narrowing the port cross section area, maybe remove some bulges on either side. Wasp waist jet fighters used in to get super sonic before strong enough engines they can just power through it.
http://www.youtube.com/watch?v=xZWBVgL8I54

Some engine factoids on mostly straight vs mostly bent tubeage flows
http://oldjohnno.id.au/page150.html
 
Just wondering what differences there are between a regular engine with pistons drawing fuel/air in verses superchargers or turbos and ram air effect. One is sucking air in the other is forcing air in.
 
Are we talking about fluid flow, or standing waves and mass transfer of combustion mixture ? Do flow benches pulse the gas as it flows through the ports ? Pressure reductions in pitot tubes are caused by laminar flow conditions up to certain speeds. If they go supersonic the conditions and results are different. Why is the noise from inlet tracts a significant factor when considering noise laws ?

I suggest it is the results in practice which are important, not the theory. 'Big is better' does not always work.
About megaphones, why wouldn't you fit expansion chambers to a four stroke motor ?
 
RennieK said:
Just wondering what differences there are between a regular engine with pistons drawing fuel/air in verses superchargers or turbos and ram air effect. One is sucking air in the other is forcing air in.
Click to expand...

The air is being pushed in whether it's supercharged or not. Just that one has the air pressure increased by a supercharger.
So there is no difference other than if your not getting enough air and you have a supercharger then you just turn up the boost. Jim
 
Alan its all of what you said at once plus the highly significant sliding scale of the temperatures of fuel amount burnt than the rpm timed tuning alone might lead ya to do. Is comnoz testing for intake or exhaust tube? Turbo's are weird exception to most exhaust principles, as can search up for extended periods. Driven boosters' exhaust concepts/mods are the most fascinating of all as the rest just suck in comparison to the benefits.

Never hurts to review basic phenomena of horns and shot guns.

http://www.epi-eng.com/piston_engine_te ... nology.htm
 
I think the problem here is that the flow test was done with a radius on BOTH ends of the tube.That won't be the case in reality.For many years,everyone else has been finding that air flows better into a reducing taper.Some are using 1.5 degrees reducing taper,and others will say that even 0.002" diameter change per foot flows better when the flow is heading to the smaller diameter http://www.bmw-m.net/techdata/cylinder.htm.There's a lot of reading in that link,but you will find it mentioned.

I think the difference in flow in this case is caused by the exit conditions,and not the taper in the tube.You only need the radius on the entry edge.Connect the tapered tube (radius on entry end only) to a real port with a real valve,and see what happens.
 
hobot said:
Alan its all of what you said at once plus the highly significant sliding scale of the temperatures of fuel amount burnt than the rpm timed tuning alone might lead ya to do. Is comnoz testing for intake or exhaust tube? Turbo's are weird exception to most exhaust principles, as can search up for extended periods. Driven boosters' exhaust concepts/mods are the most fascinating of all as the rest just suck in comparison to the benefits.

Never hurts to review basic phenomena of horns and shot guns.

http://www.epi-eng.com/piston_engine_te ... nology.htm
Click to expand...


This wasn't something I had just discovered. I just got a refresher course. I made the tapered flow tube years ago.

I just happened to be doing some repairs on a rh1 racebike head yesterday that had the intakes opened up to match big carbs. It had a lot of taper all the way to the bowl.
It flowed a lot less than a stock rh1 head and the bike was a dog. Once that has been done it's hard to fix. Jim
 
X-file said:
I think the problem here is that the flow test was done with a radius on BOTH ends of the tube.That won't be the case in reality.For many years,everyone else has been finding that air flows better into a reducing taper.Some are using 1.5 degrees reducing taper,and others will say that even 0.002" diameter change per foot flows better when the flow is heading to the smaller diameter http://www.bmw-m.net/techdata/cylinder.htm.There's a lot of reading in that link,but you will find it mentioned.

I think the difference in flow in this case is caused by the exit conditions,and not the taper in the tube.You only need the radius on the entry edge.Connect the tapered tube (radius on entry end only) to a real port with a real valve,and see what happens.
Click to expand...

I have applied it to a real head -the results are the same. Jim
 
x-file has interesting point about the radius exits back wash or detached boundary layer. How much, only one way to find out.

Jim your failed funneling experiment is a real head cocker surprise to me [bfd] so must be evidence of trading funnel down compression for velocity at lower pressure. I've been thinking [bfd] that if the whole port surrounded in JBW ya could crave it to shape and it'd stay in place by its shear brute strength perfect jamming w/o depending much on adhesion to the port walls. Intake side don't get hot enough to discompose standard JBW, just soften some so knife point can nick its surface. Might try hobot hop up now, place thick fiberous rubber rough cut gasket at head so intrudes ~1/16" but more so on the floor side.
 
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