Carb-intake manifold-intake port matching

The facts of my matter is I made same accidental discovery as BSA and face a lot of flak exclaiming how good is was, plus with above intriguing references i wonder why others do not try this fairly simple mod to test and report back. Peel was from needing any head i fit of depression when M.A.P. Florida sent Combat head to their shop that decided no more Norton head w/o telling anyone for over a year and thought a runner would sell better as I was still in same camp as you that no matter what Cdos are not up to modern fun and games. I had just put on Peels first set of real road tires to break in before stuck throttle threw me and her down and out for a decade now. Interested followers might search up >dyno guided port mis match>.

Rohan said:

hobot said:

Seems you do not understand there is not a sudden sharp choke down effect nearing super sonic just less and less flow increase even with more and more pressure.

Click to expand...

We are struggling to see that you have any solid knowledge of this subject.

And I made no such statements anywhere Steve.
If you are going to fabricate what others are (not) saying, your usefulness here is about zero.

How can there be 'pressure' in the inlet manifold when it is a (partial) vacuum that is pulling it in.
Lets get even the simple facts somewhat straight here, to begin with.
Science = measured stuff.
hobot = ??

Click to expand...

How can there not be pressure in the manifold?

Guess that depends on whether you consider atmo pressure to be a pressure.
Or background, to what else is happening.
Most pressure gauges register zero at atmospheric...

Ever driven a car or truck with vacuum wipers - or brakes. ?
When the engine is working, HARD, there is no wipers.
What does that tell us is happening in the manifold.
And what is happening other times...

So far, we have seen nothing/nada of the speed of gas flow in the manifold.
You'd think this sort of basic stuff would be all over the net.
The slides in the carbies go up and down, for a reason.
CV Constant Velocity carbs work like that for a reason.

Unless you have a supercharger, or turbocharger, when there WILL be (positive) pressure in the manifold.
Thats the gauge that says Inches of Hg (Mercury).

And all the turbo boy racers have their blowoff valves set really low,
so they go pheeew pheeew pheeew through the gears in suburbia ...

hobot is Useless if not believing how i happened to be so pleased on Peel. Flow issues divide between sub & super sonic so applying the concepts of flow bench pressure to sonic choke will confuse discussion. Neutral non charged gases only move by higher pressure pushing into lower pressure. Vacuum does not pull. Only know of one Combat head install that might choke on instants of 750 f/s flow bursts. So going by Rohan logic if hobot claims he was also very pleased by ugly mis match then that nullifies real race builder reports. There is real potential lurking for those that take it to heart and maybe find out how to optimize the benefit. You will know if works or not before dyno to put numbers on it.

If hobot can spot a couple of hp by the seat-of-the-pants method, then good luck to him.
Most folks probably wouldn't even spot if the carbs weren't synchronised, according to magazine reports.

(One of the brit mags did a survey of bike dealerships, some years back.)
Apparently they reported that about 70% of bikes through the workshop
had carbs that were considered to be significantly out of synch.
And this was when bikes typically had 2 cylinders, not the ubiquitous 4s that dominate todays cycles...

Back on engine manifolds.

This is the vacuum gauge in my baby V8, of some years back.
With the engine running, the mm of hg vacuum reading its pulling is right there in front of you.
The only time is up to zero vacuum (ie atmospheric pressure) is when its stopped (like in the pic),
or when its at full throttle, and its sucking juice like its going out of style.



Driven with the needle always in the red, it did about 6 mpg, as a street car.
You got there quick though...

Just out of curiousity, one day I disconnected the accelerator pump (squirt of fuel when you pushed the pedal down).
It did more than 30 mpg. But the acceleration was woeful, and hills were a worry...

Not terribly Commando related though.

The Commando connection is that if you connect a pair of vacuum gauges into the balance pipe connections on the carb manifolds,
you can perfectly synch the carbs, visually, on the vacuum readings.
My pair of gauges disappeared in a move somewhere, must look for another pair.
It is fiddly work though, and the needles jump wildly unless the connection is choked right down to an 'average' reading.

Hi Cheesy.
Re your: "How can there not be pressure in the manifold?"
Of course there is pressure in the manifold.
It just happens to be negative (compared to ambient air pressure) when the valve opens as the number of molecules available to exert their force is less due to the rapid change in cylinder volume for only one of the four piston strokes. This is our 'Gas Laws' at work.
Ta.

needing said:

Hi Cheesy.
Re your: "How can there not be pressure in the manifold?"
Of course there is pressure in the manifold.
It just happens to be negative (compared to ambient air pressure) when the valve opens as the number of molecules available to exert their force is less due to the rapid change in cylinder volume for only one of the four piston strokes. This is our 'Gas Laws' at work.
Ta.

Click to expand...

Well yes, although I would have said lower instead of negative though, not that it really makes any difference. Then just for Rohan it is entirely possible to have a higher than atmospheric pressure in the cylinder at valve closing

Cheesy said:

Well yes, although I would have said lower instead of negative though, not that it really makes any difference.

Click to expand...

Indeed - a pressure lower than atmospheric IS a negative pressure, is it not, by definition ?
On the overrun, the gas pressure in the manifold is near a total vacuum - the engine is
pulling max vacuum, and with the throttle closed v little air is getting in.

Cheesy said:

Then just for Rohan it is entirely possible to have a higher than atmospheric pressure in the cylinder at valve closing

Click to expand...

That then would mean more than 100% cylinder filling, would it not ?
The NIRVANA of engine designers/tuners...

It was said that manx nortons achieved 100+% cylinder filling at some rpms.
Although in the light of more recent engines achieving higher and higher outputs,
either they are getting 100+++% filling, or the manxs claims were exaggerated.

Its not that simple to measure such claims either.

Of course, supercharged and turbocharged engines can achieve way more than 100% cylinder filling,
depending on how much boost is applied. WW2 Spits with compound supercharging were (eventually)
boosted to +2x atmospheric pressure (30 psi) (?), to make up for the fact that they were generally
much smaller capacity than the opposition. This is considerably more than they ever would have been
permitted in peacetime aviation.

We still have seen nothing of what sort of gas speeds are achieved under the restriction of the carby slide there.
Anyone with any feeling for what these may be ??
Someone with a flow bench maybe, that has let the cigarette smoke waft into the intake,
and checked how fast it gets whipped into the works.
The exhaust gas comes out at a fair old velocity.
But then there is a lot more of it by then...

Note: when the mis match effect works right it works better than just fuel suspending turbulence, which is a good thing dyno and E.T.s can detect but *sharp* lip steps can streamline flow enough just seat of the pants is all ya need to know ya done good. Eons of pilot interviews vs dyno and E.T.s imply it takes about 10% power changes for average pilot to detect by feel. Btw using slang terms and concepts on exams is bad juju but fine in non technical discussions, if ya know its slang & pub level concepts, negative pressure does not exist in our 3D+time cosmos only pressure differences. Vacuum measure is sliding scale of just lower pressures in metered life but can only reach as low as no or zero pressure, never negative pressure. So who will be first to ignore Rohan unsupported counter points and try to follow hobot Peels old school joys? Wonder what a biggish Miki carb with UFO and cavitation plate venturi behind slide would do with its mis matched flow screw up?

Hi hobot.
If lower or higher than a quantified ambient is the conventional language I shall attempt to adopt this.
Negative or positive to a datum of zero at ambient is my current way of thinking on such matters.
This is due to using pump positive and manifold negative style gauging during my formative years.
Ultimately, the semantics are irrelevant - the gases do what they do regardless of how we think of them.
Your ignore question is easy to answer with the ignore function available to forum members - it then requires 'opt-in' to not ignore such posts (i.e. vitriolic diatribe and puerile one liners are no more). Please apply this option to my original threads (and perceived reactive, belligerent posts) if they ever irritate to such an extent.
Ta.

Ugh thot we-me beat this old horse to dog food by now. Excuse me nic pics on lingo poking back at Rohan and maybe un-confusing others - as we know w/o back peddling what we mean by negative Relative pressure in our engines. Engine level pressures/temps type Labs use Standard Pressure and Temperature as zero points *generally* close to sea level ~14.5 PSI and room temp commonly 60F but also water at sea level pressure freezing.

Peel had a vac/boost guage on while waiting for Bruce Megregor to send upgraded Drouin back, but was so good with mis match single carb into sharp step small ports + 2-1>open mega that Peel was able race ahead of famous Duci Monster squadron to pull out their only brave track trained rider doing knee draging turns while I was still in daily commute posture and loads as tires under inflated 18 rear/ 14 front to cross quick sand mud getting to hwy to arrive so far ahead of the squad I had time to park and go into clinic to pee to come out unable to see my Peel for the crowd of asses and elbows squatting around looking and arguring where the hidden turbo or super charger was. When I got there a few of them fell backward off heels with the rest stepped back recoiled when I said its was not yet installed... That is day Peel took out 3rd gear teeth - when waiting behind car in a straight to let pack hot shot catch up to see what I was on, which they did but only for an instant in passing lane to freak out and down shift race away so I forgot I was still in 4th as Peel still so responsive I only snick to 3rd not 2nd to catch up and heard a tinkle with instant of acceleration hesitation I thot was just normal catching rubber snicking 2nd in Peel with too much throttle. I had a few things that to go quickest one could not willy nilly just open throttle, Peel was the slowest responding of them in bee lines but made up for it with more Gs in turns. Pure random fate I lucked out with P!! and Peel unbelievable power plants. Only much later did I stumble on BSA type reports of mis matching tricks. To keep up with 100+ hp cycles limited by wheelies take about 1 hp- 4 lb and to out leap em like Peel did takes even better power to weight and resistance to wheelie. Until i hear folks having to deal with roll on power band front floating or wheelies may not yet know what they are missing out on nor the reversal in controls it can thrust ya into. I get sense yoose guys relate to quick response like letting go of sling shot > zooommMMM, I feared P!! and Peel throttle like pulling trigger on full auto shot gun the instant recoil can hurt. BANG GONE!

hobot said:

negative pressure does not exist in our 3D+time cosmos only pressure differences. Vacuum measure is sliding scale of just lower pressures in metered life but can only reach as low as no or zero pressure, never negative pressure.

Click to expand...

Steve, the pressure gauge on my air compressor says zero when it gets down to ambient,
so your preaching here is not going to change that.
And a manifold vacuum gauge measures VACUUM , unsurprisingly.
Depending on a common datum point...

Its important that we lay the ground rules when discussing these disparate things, or folks ARE going to be confused. ?
Otto cycle engines restrict or throttle the air inlet, and its important that this be understood.
As distinct from diesel engines, which have unrestricted air access into the cylinders.
And supercharged or turbocharged engines, which DO have positive manifold boost pressures.

Its all pretty simple, when you categorise them all ??

And doesn't diminish that pulses of air or fuel/air are going down the inlet throats of all of them.

Re: Carb-intake manifold-intake port matchi

Come on guys, it is just words, you are basically saying the same thing, simply not putting your ''0" (origin) at the same place. Hobot is right, there can't be negative pressure, yet of course Rohan is right to use atmospheric pressure as "0" (origin) for practical reasons, since we don't live in a vaccum...this discussion being proof of that. :mrgreen:

Now, can we get back on comments on the issue at hand, ie. the positive (or not) influence of a venturi in the admission manifold, stepped or not, etc.?

It is possible to have negative temperatures in the Centigrade or Fahrenheit scales, however not in the Kelvin scale ? What pressure has a perfect vacuum ? I suggest it is minus 32 feet of water or minus 30 inches of mercury.

There are a couple of problems with calling atmospheric pressure 0, firstly it is not constant and second, other things related to engine function are related to absolute pressure such as air mass.

Try finding a manifold vacuum gauge calibrated in absolute pressure.
Or a boost gauge calibrated in absolute pressure either, WITH corrections for atmospheric on the day..

Better to stick to plan A, and go to plan B only as required.
hobot isn't generally noted for his maths or calcs, so is almost odd him being there... ?

Rohan said:

Try finding a manifold vacuum gauge calibrated in absolute pressure.
Or a boost gauge calibrated in absolute pressure either, WITH corrections for atmospheric on the day..

Click to expand...

A simple manometer self adjusts to atmospheric pressure, then it's up to the user to calibrate it to "0".