Airflow -port taper

worntorn said:

This suggests that the 32 to 30 mm intake stubs used with a stock rh10 head might not be a great thing?

Glen

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I use 34mm stubs and taper down to 30mm over the first 25mm of the port. Not that it means anything - all my tuning is 'seat-of-the-pants' and I use methanol fuel which the 850 motor seems to love. It all seems to work well, however I don't have any comparisons. I suggest the only way to find out the truth is to test the bikes side by side on various circuits, although anecdotal evidence also has it's place. My feeling is, after my own experience and looking at the MotoGP Workshop videos on Youtube, that what must be avoided is the tendency to run wide in corners, and what should be promoted is to get the bike pulling hard out of corners on tight circuits - the opposite happens on many bikes which have been set up for big circuits.
The effects of gearing changes for various circuits are very deceptive. One thing I noticed in the MotoGP videos was about angle of lean in corners. On most of the smaller circuits on which I have raced, I've never 'rolled' much in the middle of corners, and I wonder how much time can actually be made up in the middle of corners by high speed with extreme lean angles.
What Jim hasn't told us is where he has gained advantage on various circuits from his mods. I suggest it is impossible to predict outcomes, even with the help of dynos and flow benches.

acotrel said:

worntorn said:

This suggests that the 32 to 30 mm intake stubs used with a stock rh10 head might not be a great thing?

Glen

Click to expand...

I use 34mm stubs and taper down to 30mm over the first 25mm of the port. Not that it means anything - all my tuning is 'seat-of-the-pants' and I use methanol fuel which the 850 motor seems to love. It all seems to work well, however I don't have any comparisons. I suggest the only way to find out the truth is to test the bikes side by side on various circuits, although anecdotal evidence also has it's place. My feeling is, after my own experience and looking at the MotoGP Workshop videos on Youtube, that what must be avoided is the tendency to run wide in corners, and what should be promoted is to get the bike pulling hard out of corners on tight circuits - the opposite happens on many bikes which have been set up for big circuits.
The effects of gearing changes for various circuits are very deceptive. One thing I noticed in the MotoGP videos was about angle of lean in corners. On most of the smaller circuits on which I have raced, I've never 'rolled' much in the middle of corners, and I wonder how much time can actually be made up in the middle of corners by high speed with extreme lean angles.
What Jim hasn't told us is where he has gained advantage on various circuits from his mods. I suggest it is impossible to predict outcomes, even with the help of dynos and flow benches.

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I know I'm not very clever... but I'm struggling with the link between understeer / oversteer and porting...

I wonder how much time can actually be made up in the middle of corners by high speed with extreme lean angles.

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Al that's my whole point of Ms Peel and can tell ya it makes others seem like parking lot cones, but since no one seems to have ever ridden a strange combo like Peel, no one can relate to the fantastic advantage of entering sharpies on WOT where everyone else is hardest on the brake, so mid turn is the funnest part highest acceleration by cutting power to let frame unwind and tire re-grip to leap forward better than mere fuel burn on fast ports can do. But once ya got that kind of power handling capacity so comfortably solved road racing hi sides into chicances is too boring for much adrenalin hits so only off road unpredictable conditions do it for me anymore. Also part the reason I ain't following the beaten path to power bands nor gear spacing.

Following on from SteveA, I'll recite my port story, not as an argument either way, just as an observation:
I had a Triumph with a Norton crank, 77.5 x 89 = approx. 840cc. It was quite highly tuned and class rules limited carb choices, I had mk1 concentrics bored out to 33.5mm.
It produced 62 ish BHP on the brake at first.
Myself and 'Student Boy' (a student working at Dresda at the time) decided to 'give the cylinder head a going over' it was already ported etc, but we focused on (our ideas of) fine tuning. This meant mainly working around the valve / valve seat area. We also removed the inlet guides, had straight cylindrical guides made, then filed them into a propeller shape, and inserted them inline with gas flow.
When I put it back on the brake, I also fitted new 32mm concentrics... the power output was 62bhp... exactly the same as before!
I was gutted! Until I realised the carb difference... and fitted the 33.5mm carbs and... 67bhp!
The moral of this story...? Before my 'cylinder head going over' but fitted with 33.5 carbs, the cylinder head was the bottleneck. After the 'cylinder head going over' but fitted with 32mm carbs, the carbs were the bottleneck! With the 'cylinder head going over' work AND the 33.5mm carbs, the power had increased by circa 5bhp!
The ports on this head were approx. 30mm... with the manifolds tapering down to suit as gradually and smoothly as I could get them.
Later cam work, and the addition of 34mm smoothbores yielded approx. 75bhp.
Then I went 8 valves, and that's another story altogether...

If you change the inlet port diameter and lose bottom end and increase the top end, (usable rev range moves up) you change the way the bike behaves in corners. A savage top end motor dictates a wide sweeping line coming out of corners. A more gentle but extremely torquey motor gives you the option of getting the bike to tighten its line in the corners and get on the power much earlier. It also permits the use of higher overall gearing which can also make the bike easier to ride - less jerky. The worst situation is the ultra short stroke four stroke engined racer fitted with extreme cams, big ports, high comp, and separate pipes with open megaphones. If it comes on with a bang, it will often step out - thus the hi-side, or the simple slide drop. - Very difficult to ride well on tight circuits.
A lot depends on the circuit. I don't race at Phillip Island, I hate the thought of my Seeley 850 going around turn one on full scream - get-off there is a big one. There is another thing - it has been said that bikes are set up to suit riders' riding styles - I suggest that riders adjust to the handling of the bike and it's power characteristics. Point and squirt in corners is not necessarily the fastest way around them.

acotrel said:

If you change the inlet port diameter and lose bottom end and increase the top end, (usable rev range moves up) you change the way the bike behaves in corners. A savage top end motor dictates a wide sweeping line coming out of corners. A more gentle but extremely torquey motor gives you the option of getting the bike to tighten its line in the corners and get on the power much earlier. It also permits the use of higher overall gearing which can also make the bike easier to ride - less jerky. The worst situation is the ultra short stroke four stroke engined racer fitted with extreme cams, big ports, high comp, and separate pipes with open megaphones. If it comes on with a bang, it will often step out - thus the hi-side, or the simple slide drop. - Very difficult to ride well on tight circuits.
A lot depends on the circuit. I don't race at Phillip Island, I hate the thought of my Seeley 850 going around turn one on full scream - get-off there is a big one. There is another thing - it has been said that bikes are set up to suit riders' riding styles - I suggest that riders adjust to the handling of the bike and it's power characteristics. Point and squirt in corners is not necessarily the fastest way around them.

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I understand, and agree with you, at the extremes, ie TZ vs Manx... but do not personally believe that simply changing port sizes by a mill or two, on an otherwise unchanged bike, will have that degree of effect.

If we [not me] could jump fences and tracks to re-design the whole head...
How cool would be bell mounts poking up out gas tank into a ram air plenum ya could lay on w/o choking the mouths.

At the Mega Mile (1 1/4 mile) AMA Pro flat track race last summer the Kaw Versis powered bikes were way faster than everything else. Looking at them in the pits, they had intake ports that looked parallel with valve stems. Dan W on Micheil Moore's mc-engine list

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I think when I get Trixie pure factory Combat KS going to start I'll rev up to 6800 and spray in white paint till comes out the peashooters then kill with throttle open, then take apart [ just to reseal everything] and take some photo's. If I see some places the paint ain't sticking I'll punch some notches in just ahead of that to see what happens again when next ring change needed. If paint gets into places it shouldn't then I'll follow up on the diagnostic clues.

You can obviously increase the inlet port size on a commando and still get around corners OK. However if you are looking for advantage, that increase in port size can make you lose a bit of bottom end and make a bike which tightens it's line less desirable. Where are you if somebody turns under you and gets on the gas quicker ? With my Seeley, I can get on the gas almost back at the apex of a hairpin bend, and give it a good strong squirt without it going sideways, and stay centre track coming out. It never runs wide. The main thing is that even if you cannot afford all the hot bits, you can still be competitive, depending on how you set the bike up.

Looks like no takers on my "acceleration takes energy" idea of slowing down air flow. I once had a high pressure hose set up for washing mud off of trucks up on a ramp. We had a nozzle that tapered quickly from 3/4 to 3/16 of an inch (over about 1 1/4 inches) and then went straight for about 1/4 inch. We got good pressure out of it but I thought that giving the water time to accelerate to full velocity would do even better so I machined a nozzle that tapered from 3/4 to 3/16 over about 4 1/2 inches. The flow was so violent that we had to get rid of it for fear that some kid would let go and kill themselves.

Hehe Motorson, I jetted mud off of cranes and almost invented the water jet toy. Some references state a certain range of slope down in ports for some help rather than hurt power packing. Any change of speed faster or slower and any direction change and any shape or size change costs flow energy in the trades.
Here's extensive interesting read to me along these lines, especially hemi's.

just for Research , i placed a small 1/8" buildup of Epoxy on the Short Turn and gained about 3 PerCent more Flow on a FlowBench...also did that to help make the Port legal in CC volume.....
Results=> Lost Mid to Top End HP/TQ in NHRA SS Race-Hemi
the FlowBench CFM numbers can lie to you in one respect and you must use a Pitot Probe to verify what you did does not create **too much** velocity at the wrong spot in a Port.

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http://speedtalk.com/forum/viewtopic.php?t=840
and for the main thread
http://speedtalk.com/forum/viewtopic.php?t=860

I think it might be important to blend in carefully, and variations in port diameters. I always try to keep the differences in angles smooth and as straight as is sensible without increasing the middle section ID of the inlet port. When I matched the 34mm stubs to the 30mm port, it was done over about 25mm with a gentle curve. My exhaust system gives me a bit of concern, it is made up of bends welded together, and there is always welding flash on the inside at the joins. The ID of the header pipes exactly matches the ID of the stub at the head - no step. However the rest experiences the same sound waves

Several different versions of the BSA Goldstar left the factory with carburetors 1/16" larger bore than the intake ports. There was always a step just after the carburetor. It was found on the dyno that these bikes gave more power when set up this way. Often owners would blend the step so it was smooth. By doing that they reduced the power of their engines by two or three horsepower. BSA found out about this tuning secret by mistake when an apprentice made a mistake and put the wrong size carburettor on an engine that was being tested on the Dyno. All BSA Goldstar engines were run on the Dyno and came with the Dyno report.

Most models of the Amal GP carburettor had this step built in as well. I have never heard anyone give a really accurate reason why these steps give more power. I doubt they give more flow. I think it may be that the Amal GP carburettor is a bit to smooth inside.

Several different versions of the BSA Goldstar left the factory with carburetors 1/16" larger bore than the intake ports. There was always a step just after the carburetor. It was found on the dyno that these bikes gave more power when set up this way. Often owners would blend the step so it was smooth. By doing that they reduced the power of their engines by two or three horsepower. BSA found out about this tuning secret by mistake when an apprentice made a mistake and put the wrong size carburettor on an engine that was being tested on the Dyno. All BSA Goldstar engines were run on the Dyno and came with the Dyno report.

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Well how about that - hobot did same thing on purpose against all wisdom's expecting low performance just to break in 1000 miles before seeing if would break on WOT red line+ tests to be driven out of my mind by blood slosh forever more. Jim has not yet tried the similar concept testing entry lips or intruding gasket causing similar flow vortexes that stream line the flow around bends into narrows. BSA also ran into fuel flow limits of Amals so created a slot into bowel to get around that.

hobot said:

Several different versions of the BSA Goldstar left the factory with carburetors 1/16" larger bore than the intake ports. There was always a step just after the carburetor. It was found on the dyno that these bikes gave more power when set up this way. Often owners would blend the step so it was smooth. By doing that they reduced the power of their engines by two or three horsepower. BSA found out about this tuning secret by mistake when an apprentice made a mistake and put the wrong size carburettor on an engine that was being tested on the Dyno. All BSA Goldstar engines were run on the Dyno and came with the Dyno report.

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Well how about that - hobot did same thing on purpose against all wisdom's expecting low performance just to break in 1000 miles before seeing if would break on WOT red line+ tests to be driven out of my mind by blood slosh forever more. Jim has not yet tried the similar concept testing entry lips or intruding gasket causing similar flow vortexes that stream line the flow around bends into narrows. BSA also ran into fuel flow limits of Amals so created a slot into bowel to get around that.

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What's your scource there, Captain Smuggo? We must concider it. Anyone can type between quotes.
I am not saying you are wrong, though.

I've had two engines that surprised the snot out of me and strangers, one a 20hp twin outboard I ran w/o a engine cover with single carb facing wind, which i tried conical funnel for nil results but woke up to over rev and run half prop out of water when I placed hand about 45' angle and few inches in front & below throat to cause a standing pressure wave so tapped up a single playing card and went out to take on more powerful EXPENSIVE boats, 2nd engine was Peel's with 34 Miki, 32 manifold into 28.5 standard head, with step down lips at each transition, because Combat head was lost over a year by MAP Cycle's shop quiting heads w/o telling anyone, so just set up for break in power only, but OH LA LA when I finally tired to break it in anger. I swear Peel must of made like 70 hp, as no way could my SV650 Intruder or others on their faired SV's keep up with the early 2000 era inline 600's but Peel could and then some until speedo showing 130 then was neck and neck till 135 indicated and they could ease past me if I slowed for a crest or blind I'd have bad surprises in prior. I became a safer rider with such power on hand, so would slow to 90 over crests on commutes then nail it back to 125+ in couple seconds in 4th, so not much delay to work.

By far the worse Peel combo was Combat hogged out head, dual Amals and 2-1 open pipe, could hardly idle across grass w/o stalling and barely able to pass the ton after a long wait. Just before I was going to put back Peel's accidental power combo - throttle stuck open on a freeezing day start to work as I barely stepped on KS she fired to drop KS lever out from under which casued me to stumble backwards and let go bars still watching tach as it hit mid red zone and freaking out panic strenght slapped my hand back on throttle to be horriiffically shocked as it got second breath and rev'd up so much harder the engine flipped up on iso rubbers so hard it slapped my adrenalized grip right back at me to cause me to stumble again, as tach needle completely dissapeared from bouncing off peg stop for couple more seconds it took to regain stance and slam shut, and get sick at stomach over the few more seconds before needle became visable again as a blur then a fast twiching bounce off peg then about a second more as the needle hovered in red then to zero. Every engine seam opeen up like waves breaking on beach and expelled so much oil smoke I could not see engine top for a few more seconds even fanning smoke away til finally could see through fog that head was still on. Few minutes later started up and rode off but couldn't hit but 110 with the huge winter screen on instead of 120+ prior. Rode 2000 more mile on reduced power then 1098 cam out and decided to go big block boosted to stay in the game with the super duper that play out here. I believe the hogged out 'CHO' ports really came into flow play after about 9 grand. ugh.

tricatcent said:

Several different versions of the BSA Goldstar left the factory with carburetors 1/16" larger bore than the intake ports. There was always a step just after the carburetor. It was found on the dyno that these bikes gave more power when set up this way. Often owners would blend the step so it was smooth. By doing that they reduced the power of their engines by two or three horsepower. BSA found out about this tuning secret by mistake when an apprentice made a mistake and put the wrong size carburettor on an engine that was being tested on the Dyno. All BSA Goldstar engines were run on the Dyno and came with the Dyno report.

Most models of the Amal GP carburettor had this step built in as well. I have never heard anyone give a really accurate reason why these steps give more power. I doubt they give more flow. I think it may be that the Amal GP carburettor is a bit to smooth inside.

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Hmmm....seems to me the step would trip a laminar boundary layer into a turbulent one, and trip the entire flow from laminar to turbulent.
Refer to Hobot's velocity profile diagrams on page 5. The turbulent profile has a greater skin friction drag which would tend to decrease the flow, on the other hand, the turbulent flow hugs turns and gets around obstructions (read valve guides) better, which increases mass flow. It would seem the latter factor is more important.

I wonder if a gasket, cut with a 1/16" smaller bore than the carb, and placed between the carb and manifold, would give a similar boost?
Such a gasket would have even more turbulent flow inducement than a step.

So I dug out my box of flowtubes that I had experimented with. I have a bunch of them.

I gathered a few for some more show and tell.

The tube on the left is a 2 inch long tube tapered from 34mm to 30 mm over its full length. About 5 degrees included angle.
The next tube is 30 mm straight through with a partial 3/4 radius on the top. The od of the radius cut is 34mm.
Tube 3 is 30mm straight through with a 3/16 radius on the top.
On the flange is a straight through 30 mm tube with a 3/4 inch radius on the bottom.
The last one is a 30mm to 35 mm tube tapered over it's full length with a 3/4 inch radius on the bottom.

To imitate a carb I will be using a 34mm Amal.



The first test is the 30mm straight through "manifold" with the 3/16 radius on the carb end on the 30mm straight "port" 96.9 CFM



The second test is the 30mm straight through manifold with the 3/4 inch partial radius on the carb end. 30mm straight 'Port" 97.4 CFM



The third test is the full tapered manifold on the 30mm straight port. 95.9 CFM



So basically it does not make much difference how you reduce the size -when you go from a large carb to the smaller port.
Any time you go from a larger opening to a smaller opening you loose some airflow.
So how do you get it back -plus some. You leave room in the port for a slow increase in area. It slows the air down so it can better make the turn into the cylinder and get around the guide. And it pulls additional air behind it.

Here are the same three manifolds setting on a 30 to 35mm tapered port.

Peel's prime, had both infamous single carb poor performance installed plus a lip and an intruding 1/16" rough cut rubber/fiberous gasket as who in their right mind going by the 'most widely accepted and followed engine flow logic' in the world, would bother to trim gasket back with an obvious flow obstructing lip, so would be like putting lip stick on a pig, but instead turned into a voluptuous sexy flowing Ms Piggy. Stranger sportriders sneer'd and made snide remarks rather more intense than the polite bunch here, till showed em a cute turn of an ankle... and multiple road orgasms.

John was much more diplomatic than I. Jim

?