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2 hours ago, FastNCurious said:

borrowed Zstore trumpets and a head torch! They pushed the AFRs one whole number down! I can only assume that they are significantly improving air velocity across the bridge.

Ali, The explanation for the richening is complicated. It involves understanding there are two pressure drops between the filter and butterlfy. One is at theh mouth of the carb and the other is set by the "constant depression " operation of the piston mass plus spring. Fittting a trumpet removes the small pressure drop at the mouth but has only a small effect on the constant depression principle. The trumpet will slightyl raise the pressure under the area of the piston that enters the choke tube and  carries the needle. Normally this would see the reduced pressure  set at the mouth. When a trumpet is fitted that "suction" on the exposed piston is lost and the piston will lift, along with the needle, richening the mixture. The constant depression is a bit less but the bigger fuel area in the jet wins out.

With trumpets the air particle velocity over the bridge drops a bit due the piston riding higher and that tends to non-linearly impair atomisation of fuel and worsen combustion and power. The mass air flow thru the carb will increase marginally as the trumpets remove a fraction of the constant depression. That is calculable from the mouth pressure  drop change and the exposed area of the piston. Even so, compared with the prssure drop across the butterly it will be small.

SUs prefer turbulent flow to avoid vreating a venturi effect thaty would pull the piston down. SU Company as far as I know never advised fitting trumpets, perhaps because they would promote laminar flows and introduce unwanted venturi effects.

Peter

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5 hours ago, JohnC said:

I can't tell you how much I laughed. Go Norm! :D

Excellent! Glad to be of service ;)

2 hours ago, Peter Cobbold said:

Ali, The explanation for the richening is complicated. It involves understanding there are two pressure drops between the filter and butterfly. One is at the mouth of the carb and the other is set by the "constant depression " operation of the piston mass plus spring. Fitting a trumpet removes the small pressure drop at the mouth but has only a small effect on the constant depression principle. The trumpet will slightly raise the pressure under the area of the piston that enters the choke tube and  carries the needle. Normally this would see the reduced pressure  set at the mouth. When a trumpet is fitted that "suction" on the exposed piston is lost and the piston will lift, along with the needle, richening the mixture. The constant depression is a bit less but the bigger fuel area in the jet wins out.

With trumpets the air particle velocity over the bridge drops a bit due the piston riding higher and that tends to non-linearly impair atomisation of fuel and worsen combustion and power. The mass air flow thru the carb will increase marginally as the trumpets remove a fraction of the constant depression. That is calculable from the mouth pressure  drop change and the exposed area of the piston. Even so, compared with the pressure drop across the butterfly it will be small.

SUs prefer turbulent flow to avoid creating a venturi effect that would pull the piston down. SU Company as far as I know never advised fitting trumpets, perhaps because they would promote laminar flows and introduce unwanted venture effects.

Peter

Thanks Peter! You really do understand these bits of kit so well. I'm going to read that again carefully later and try to understand it!! Before I met you, I used to think I was quite clever!! Now I find my head is hurting. 

 

Edited by FastNCurious
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12 hours ago, Peter Cobbold said:

Ali, The explanation for the richening is complicated. It involves understanding there are two pressure drops between the filter and butterlfy. One is at theh mouth of the carb and the other is set by the "constant depression " operation of the piston mass plus spring. Fittting a trumpet removes the small pressure drop at the mouth but has only a small effect on the constant depression principle. The trumpet will slightyl raise the pressure under the area of the piston that enters the choke tube and  carries the needle. Normally this would see the reduced pressure  set at the mouth. When a trumpet is fitted that "suction" on the exposed piston is lost and the piston will lift, along with the needle, richening the mixture. The constant depression is a bit less but the bigger fuel area in the jet wins out.

With trumpets the air particle velocity over the bridge drops a bit due the piston riding higher and that tends to non-linearly impair atomisation of fuel and worsen combustion and power. The mass air flow thru the carb will increase marginally as the trumpets remove a fraction of the constant depression. That is calculable from the mouth pressure  drop change and the exposed area of theo pistn. Even so, compared with the prssure drop across the butterly it will be small.

SUs prefer turbulent flow to avoid vreating a venturi effect thaty would pull the piston down. SU Company as far as I know never advised fitting trumpets, perhaps because they would promote laminar flows and introduce unwanted venturi effects.

Peter

Bold text: I got that wrong, Its easier to see whhats happening if we imagine the mouth pressure drop increasing. The piston then rises to keep the constant depression constant. The mixure will richen due to the lifitng the needle. Conversely when a trumpet is fitted the small pressure drop at the mouth is ablished and the pisotn sinks a bit to keep the constnat depression nad the mixture weakens. Mass air flow follows the pressure drop which is....consntant, so trumpets dont add more air. However the "air particle velocity" over the jet will be a bit faster and this non-linearly improves fuel atomisation and hence distributionn between cylinders and combusiton, which is where a power gain might arise.

Peter

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10 hours ago, FastNCurious said:

 

Tks Ali, pls see correction above, I got it wrong. With a trumpet fitted the mixture should weaken a bit, move to a bigger AFR number. Your richening may be related to the stand-off vapour being captured by the trumpet and not dispersed away from the carb mouth.

I had to delve into SU fludi mechanics when seeking to tune a single HS8 feeding a blower. It took months.

Peter

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3 hours ago, Peter Cobbold said:

Tks Ali, pls see correction above, I got it wrong. With a trumpet fitted the mixture should weaken a bit, move to a bigger AFR number. Your richening may be related to the stand-off vapour being captured by the trumpet and not dispersed away from the carb mouth.

I had to delve into SU fludi mechanics when seeking to tune a single HS8 feeding a blower. It took months.

Peter

Thanks Peter, appreciate the clarification. That’s kind of what was baking my noodle - I was also expecting a weaker mix, but now with your latest explanation it makes a lot of sense as to what is happening. 

To support your logic: I always wondered why putting on the factory air-box would bring about the same richening effect. I had put it down to a restrictive box or my K&N filter being a little clogged. But then the factory box has built-in 40mm tall trumpets on its back-plate which, I NOW realise, were doing exactly what you have described! Big thumbs up to you sir.

 

Someone (back in the day) undoubtedly did some maths on their slide rule and came up with 40mm for good reasons!! ;)  Probably why 40-50mm trumpets work so well on those L6 engines. I’m now thinking that with my (longer than stock duration) cam and tighter LSA, causing more stand-off than stock, it probably would benefit from even longer trumpets. But that’s an experiment for another day with a webcam and some homemade trumpet spacers!

 

Speaking of intake air velocity, pulses and tuning, here’s a very interesting diagram I saw back in the 90s explaining how the Porsche M64/21 engine with “Varioram“ took care of it. They had it for years before but in ‘95 they added the third chamber. Still, fascinating stuff is this fluid dynamics business!

A - up to 5k rpm

B - between 5k-5.8k

C - above 5.8k

80765975-C856-4A7B-BDC8-FDE4ADCBC789.jpeg.d102e6adc0658d9414018da806b7e21b.jpeg


219C8336-E8F4-4C16-BE15-A765A1B86B5D.jpeg.cc116464530c7f5aed43bd3d37614b7b.jpeg

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Ali,

That stand-off bothers me. As the manifold runners are asymmetrical to the SU shock waves causung the stand-off may  be greater on some cylinders than others. The stood-off fuel might enrich the subsequent cylinder not the one causing the bigggest shock wave.   The SU is designed to deliver, for the same needle position, a richer mixture at wider butterly openings than 30deg. That allows pulsating air flows towards the inlet valves to transiently inrease depression at the jet and pull more fuel, richening the mixture. SU factory tuned the effect by the size of the balance pipe. I dont see retrograde pulses causing the stand-off improving the mixure spread between cylinders so experimenting with  a bigger balance pipe might cure the stand-off.

I think the Porsche inlet ram tuning worked with fuel injection, not sure it will work with SUs.

Peter

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Now you raise an interesting point there Peter. As you can see the intake manifold I have in the picture, I absolutely abhor the asymmetry and the paths to cyl 2 and 5!

 

There is a balance tube on these but changing the size is frankly a fabrication nightmare! 
 

I think I read somewhere that Mr Vizzard didn’t like balance tubes - so I was considering blocking mine off with a home made blanking gasket to observe the effect it has - am I about to commit a sin?

 

B965EEE8-5645-4CB6-8C01-5BC978C0D686.jpeg.2218577d58f1fbe410a5c8320898fe5b.jpeg

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11 hours ago, FastNCurious said:

Now you raise an interesting point there Peter. As you can see the intake manifold I have in the picture, I absolutely abhor the asymmetry and the paths to cyl 2 and 5!

 

There is a balance tube on these but changing the size is frankly a fabrication nightmare! 
 

I think I read somewhere that Mr Vizzard didn’t like balance tubes - so I was considering blocking mine off with a home made blanking gasket to observe the effect it has - am I about to commit a sin?

 

B965EEE8-5645-4CB6-8C01-5BC978C0D686.jpeg.2218577d58f1fbe410a5c8320898fe5b.jpeg

Ali, Indeed worse asymmetry than TR 6-pot. But is your firing order different, TR is 153624

Nothing like an experiment to blank it off.

Am thinking that trumpets by containing the ejected fuel droplets may cool the inlet charge as they evaporate. By max 20C if evaporation is complete.

Peter

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