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Something that intrigues me guys, supersonic port theory. With some strategic porting using SMALLER ports in some specific locations, you can see noticeable gains in performance. Interesting...Thoughts? I've been following it off and on for a couple years, but it has yet to catch on.

I know our Opels tend to suffer from the whole intake system being huge and sloppy (if I understand correctly) and I'm curious some of you pro-tuners thoughts on this, especially RallyBob and others like him. Sorry for the eye-burner website link, but I can't find a more visually-pleasant website on supersonic theory when I'm as tired as I am right now, haha!

Super Sonic Nozzle
 

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To add to this engines are using 4 different valves sizes (progressively smaller) on these 4 valve heads. Things are advancing from the standard valve setup.
John
 

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I won't claim to know anything about supersonic port theory, but I do know that in the case of an engine restrictor (such as an SIR used in SCCA GT-lite) or a turbo restrictpr as used in open class rally cars, once the airflow goes supersonic, hp gains come to a stop. Torque can increase somewhat if tuning is altered to suit the restrictor.

I remember doing some experiments on Opel heads in the early 1990's on my flow bench, and on some racing-prepped big valve heads I was able to fill the intake port floor with 1/2" of clay (tapering to the short side valve radius) with ZERO loss of airflow on my flow bench. Velocity went up accordingly. If I filled it 5/8" the high pitched whistle the port made at max flow was deafening.

I did mess around with set of 327 Chevy heads on a 400 small block for a friend of mine. After porting, I tried the same clay technique with similar results. So we epoxied the ports and intake runners with the same shape as a cast iron 2.8 Chevy intake port (sort of a rounded shark fin protruding from the port floor) and gained massive flow. That 5400 lb pickup could also roast the tires at will. Later on he did replace the valve guides and when he pulled the heads some of the epoxy was missing. He chipped all of the epoxy away to avoid ingesting more into the engine, and when he put it all back together the engine was a dog comparatively. I saw an increase from 132 cfm to 148 cfm on those Chevy heads with 'smaller' ports.

Regarding the Opel (1.9) head having too big intake ports....yes they are too big relative to the STOCK valve sizes. With bigger valves not as much. With really big valves you do need to enlarge the ports, but it's more about the port shape. Some parts of the port remain untouched and other parts are enlarged massively.

I have yet to run any Opel heads with epoxied ports because I've never found an epoxy that lasts. In SCCA I really wanted to try it but it was (and still is) illegal to add material to any ports. For street use I have a simple solution which us to make an aluminum insert that protrudes into the intake ports but bolts to the intake manifold.
 
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Discussion Starter · #4 · (Edited)
I would be very interested in hearing more about your street solution. Would it just be a plate cut and bent to change the intake shape and then bolted in place? I'm still really new to the tuning scene. I have some experience with motors, but so far very little is with the Opel. The reason I ask is twofold. First I am genuinely intrigued by the science behind supersonic intake theory, and all forms of tuning, conventional or not. Secondly I am looking for ways I can increase the performance of my 1.9 CIH on a very tight budget. I've typically gone more the bolt on route in the past, but I have more time than money and would really like to learn about tuning rather than just bolting parts up.

I've heard or some people using JB weld to good effect for intake epoxy. Also good catch guyopel. I didn't notice thatdifthat, though I'm not sure it would completely change the concept, it would definitely have some effect on performance overall.
 

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You might get close to supersonic speeds on the exhaust ports.
Take a close look at 1 & 4 then match 2 & 3 to them.

On the intake side I would be very impressed with ~150 g/s.
 

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I would be very interested in hearing more about your street solution. Would it just be a plate cut and bent to change the intake shape and then bolted in place? I'm still really new to the tuning scene. I have some experience with motors, but so far very little is with the Opel. The reason I ask is twofold. First I am genuinely intrigued by the science behind supersonic intake theory, and all forms of tuning, conventional or not. Secondly I am looking for ways I can increase the performance of my 1.9 CIH on a very tight budget. I've typically gone more the bolt on route in the past, but I have more time than money and would really like to learn about tuning rather than just bolting parts up.

I've heard or some people using JB weld to good effect for intake epoxy. Also good catch guyopel. I didn't notice thatdifthat, though I'm not sure it would completely change the concept, it would definitely have some effect on performance overall.
More than bending up a plate I'm afraid. More like machine an insert or cast one (well, 4 really). Trouble is the heads are finicky. I figure everything on a running flow bench. If you're off a little bit it goes to crap in a hurry.

Opels run pretty high temps, so even JB will let go eventually. I've tried Marine-tex, JB Weld, Moroso (POR-15) epoxy, Devcon, etc......nothing lasts in the long haul.
 

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Something that intrigues me guys, supersonic port theory. With some strategic porting using SMALLER ports in some specific locations, you can see noticeable gains in performance. Interesting...Thoughts? I've been following it off and on for a couple years, but it has yet to catch on.
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What a crock.

This 'theory' hasn't caught on simply because it isn't true. The author of your linked website is confusing most of his nozzle theory, or at least selectively choosing what science he likes to quote.

Nozzle theory goes back a long time (A few hundred years!). And ever since WWII, there has been a HUGE amount of published experimental data essentially proving all of the super-sonic nozzle theory out there. Since Wernher von Braun sorted out much of the practical applications in one of the first long-range rockets, the Nazi A4/V2. And he was later instrumental in the development of the most powerful nozzle ever made, the Rocketdyne F1 (We used 5 of them on the Saturn V's to get our boys to the moon).

In rockets, you want to get the exhaust gasses moving as fast as freaking possible without removing too much energy in the process of accelerating them. There can be a bit of a balancing act to moving them faster and removing some energy to get them going faster. The reason you want faster gas in a rocket nozzle is because of the whole Energy = 0.5*m*v^2 concept. The faster you can make some mass go, the more kinetic energy that mass will have, and thanks to the equal-opposite rule, the more kick in the keister our astronauts will get on their trip to space.

The velocity part is really important because mass is such a difficult thing to get into space. It takes so much more mass of fuel to carry the additional fuel necessary to carry additional cargo. Its a squared scalar.

Back on topic. Supersonic nozzles DO EXIST! Every rocket has them! BUT the supersonic part is happening in the diffuser bell after the narrow part of the rocket. While in the throat of the rocket, the gasses are moving at Mach 1 plain and simple. They may be really hot and so the speed of Mach 1 could be in the 1000s of MPH, but its still Mach 1. In the diffuser they will speed up to Mach 4+.

Application in car land. The reason the guy in your provided link mentions the FSAE kids in hi anecdote is because there is an intake air restrictor in FSAE to limit the power output of the engines. The current rule says you must have an engine smaller than 610cc, 4-stroke, with a 20mm circular restrictor. There is no limit on the entry and exit geometry around the restrictor.

With typical BSFCs, the theoretical power limit of FSAE engines is about 120HP sustained, with most 600cc teams making about 90 to 95 HP.

You simply cannot get more air mass through the restrictor than that.

However! That doesn't not mean nozzles do not have a place in engine tuning. Ever hear of Helmholtz? The idea of wave resonance adding momentary pressures at tuned RPMs to increase HP has been around for a long time and makes a lot of sense in IC piston engines. Helmholtz resonances is the reason people say longer intake runners are good for low-end torque and shorter runners are good for high-end HP. The long runners momentarily compress the air at the valve in lower RPMs, which makes the pressure differential when the valve opens larger, forcing more air into the cylinder. At higher RPMs, the waves are shorter so the runner needs to be shorter.

The idea of using runner and port geometry to increase pressure at the valve is not a misnomer and could be used in your car. If you made a nozzle in a runner which did not add restriction to your engine, it is in theory possible to get the sped-up air to crash into the closed valve at the right time, increasing the pressure momentarily, which would increase HP at the RPM.

If you look closely at a lot of cylinder head port geometries, especially modern engines they are using this theory in application. The intake ports on the Honda engine I use daily in my research start at ~30mm, neck down to ~24mm and then broaden for the valve at ~35mm. Like Bob says, if you don't know what you are doing you can very quickly make your ports work horribly with the wrong kinds of modifications. You might end up 'tuning' them to work between 200 and 500 RPM, a place you will never use when running your engine.

TLDR: Supersonic nozzle theory is a very real and normal thing. Usually it is simply referred to a nozzle theory and there are 1000s of resources on the subject. You can use it in your engine and it has been used for many years already. Author of the website is selling snake oil and is imagining that this is an 'undiscovered science because of the closed-minds of the students'

Paper from 1950

Paper from 1961

Website from 2006
 

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Back when I was tuning modified late model Fords.
Block off plate for the long intake runners. Short runners only for HP.
Had to use IMRC deletes in the software to keep the check money light off.
 
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