The perfect Intake/Exhaust concepts

[Dessertrunner]

Okay I plane to throw some ideas out for discussion and would like to hear a bit of feed back,
As everyone is aware I have been working on roller throttles and Matt has also installed a set of IDTB ( he said the car goes like a stung cat), also I got a bunch flow bench work done. All this has confused me which mean its all running around in my brain till I sort it out.

- Harvey has always said that on the exhaust side you need a pipe coming off, a head with out a exhaust won't work as well. In the past I was convinced he was wrong but good manours stopped me saying so. Now I am 100% convinced he is right.
- Over the years you read about how people redo the intake on the head to get maxium flow and expect it to make a big difference. I am convinced that is a wrong assumption UNLESS.
- People talked about how long the intake ram pipe has to be eg, longer for low revs, shorter for high rev, its suppose to do with harmonics.
- Mark suggested on my roller throttles to go back to a 40mm inlet size instead of the current 45mm the SVX have standard now. (common sense would tell you that is a really stupid idea) BUT HE IS RIGHT.

Lets look at the intake, I fully understand the harmonic (shock waves) have a part to play but I would like to start on the simple steps first.
Here goes, when the intake valve opens the pistion is roughly on the down stroke so air and fuel runs in due to the vacum. In simple terms the amount of air that fills the chamber is directly related to the sucking pressure less the drag (restriction) on the intake parth. So reduce the restriction (port the head for example) and you will get more power. As the piston reaches the bottom of the stroke suction decreases so airflow reduces. If a engine has a long intake part then it needs to be as large as possiable to get the maxium flow into the head.

So why does IDTB get such a bang for there buck if it all about in airflow restriction. Why did Mark think cutting the intake size was the right thing to do. I think the answer lays with the movment of air. With a narrower dia intake pipe but short we may have the same sort of restriction when the intake valve first opens but when the piston is no longer suction the air in the intake pipe is at such a high volicity that in continues to pump into the cyclinder under PRESSURE not vacum. Simple rule is that air under pressure contains more fuel and Oxygen then when its under vacum. The flip side to this is that you can delay the closing of the intake valve for a longer time when its on the up stoke due to the air raming against it.
Back to harmonics/sound waves, I would suggest that the issue is closing the valve before the air/pressure wave gets back out the intake. In other words lower revs mean slower air velocity in the intake for a given size, the soluation is to either lengthen the parth (Ram Pipe) and or reduce the intake diameter.

So team jump in, feel free to disagree with me I won't ake it personally.
Have a great day.
Tony

[K_Dub]

What about variable length intake runners?

Link

Here's a pic of Farrari's F1 system from 2005, I think.



You did say "perfect Intake..." didn't you? Just throwing out an idea.

There's also this thingy which may help tune the harmonics.

[Jay Wrix]

variable length would be awesome!

I love how the fuel rails blow down into the intake, such a gorgeous piece of high horsepower machinery



yes, smaller intakes can yield higher velocity, allowing
air into the chambers allowing for more power, but longer tubes can make more useable torque for summer applications, Ford Duratecs
have split ports (theres many more motors) where butterflys cover the short
runners under 2.4k and opens after to allow more air to flow in but keep
the large vacuum from having a longer narrower port. my cousin has a
monster 2002 sport cougar with a 3.0 duratec, you can feel when they open.

when I was building some sick demented flathead motors, (jr drag minibike motors... haha... 25hp briggs) I learned out the hard way that biggerintakes wasn't always the way to go, and in the case of exhausts on the little briggs you added a reducer to bring the ID down just a little, or you couldn't gain velocity out of the exhaust, you also added material to the inside of the intake to make it smaller, yet it was bigger around the throat and valve head to make more power, It was also important not to smooth the intake side mirror finished as this didn't allow for atomization of the gas via turbulence.

its a key blend of velocity and good flow, smooth edges, and ive always assumed my ford truck would run better high with short runners, but i like my low end tourqe of the long monster,

and I completely agree that it could be a much better route. just dont go too small! and dessertrunner, awesome work!

[Dessertrunner]

Hi Guys,
Would love the variable intake design if I could make it work.

A question how do we define what to small or to large for the intake, I fully understand its a matter of trial and error but I wonder if I can fit a pressure transducer in the intake against the valve to see what is happening. Have to check if the PLC can collect the data. Would need to do 4,000 reads per minute at a minium which should be no big deal also would need to have a timing point such as valve closing.
Thanks for the thought food.

One other crazy question has anyone heard of heating the fuel before its injected. The reason I as is that it would mix better, I don't think it will increase pre ignation much but the amount of heat it would be carrying is low in terms of the total cyclinder fill.
Tony

[oab_au]

Tony.” All this has confused me which mean its all running around in my brain till I sort it out.”

Don’t let it worry you Tony.

I have been studying it since I got my first two-stroke bike, in ’53. Intake design has evolved over that time.‘60s early ‘70s the engines were designed to use the gas inertia, that occurred in the inlet and the exhaust. This way the gas running into the cylinder continues to ram in, even though the piston is coming up on the compression stroke, trying to push it out.

The same inertia process is use in the exhaust pipe. When the exhaust valve opens the 65psi of gas pressure charges down the exhaust pipe, the inertia of the gas flow causes a low pressure to develop in the cylinder to evacuate the remaining gas and reduce the pumping the piston would have to do when it is rising on the exhaust stroke. If the engine was a low speed engine, the low pressure caused by the exhaust inertia, lasted till the inlet valve opened, to start the inlet flow.

To this end the engines had long exhaust pipes, and inlet tracts of a smaller diameter to enhance the gas velocity and inertia pressure, but these type of engines developed their maximum torque at a low speed around 3000 rpm, so the maximum power was developed around 5500. To get more power, the engine has to run at a higher speed, and the speed of the gas inertia is too slow to continue to do the job.
Instead of the exhaust low pressure lasting till the inlet valve opens, it only lasts long enough to evacuate the gas and reduce some of the piston’s pumping loses.

So we have lost the effect of the exhaust pulling the inlet gas in on overlap. It is the length and weight of gas that takes too long to start moving, for the faster rpm. This is why the SVX has the inertia system working till 4000 rpm, then changing to the resonate system to extend the rpm that the maximum torque is delivered.

The work that the inertia pressure was doing to push the gas in from outside the cylinder, the sound pressure waves will do from inside the cylinder, using the resonate system.
The Inlet tract has to be shorter to reduce the time, the gas takes to start moving and a larger diameter to reduce friction. The SVX has the inlet resonate length, from the inlet valve to the plenum, of about 15” to 17“ to produce the torque at the desired speed of 4800. To raise the rpm of maximum torque to 6500, we need to reduce the length of the inlet tract to about 13” to 14”, and reduce the length of the exhaust pipe to about 44”. All these lengths depend on the actual valve timing used.

With this system the exhaust is the start of the induction sequence. When the exhaust valve opens the 65psi of gas pressure still charges down the pipe to produce the low pressure to evacuate the cylinder and reduce the pumping loses. At the same time a sound pressure wave starts down the exhaust pipe till it reaches a box, expansion chamber, where it expands to be replaced with a negative pressure wave, that travels back up the pipe to reach the cylinder in time for the inlet valve to open. The low pressure created, starts the inlet gas flowing into the cylinder even before the piston starts down.

The negative wave from the exhaust continues to travel up the inlet pipe as a negative pressure, reaching the open end of the ram tube to expand and be replaced by a positive pressure wave that travels back down the tract, meeting the piston about half way down the bore. Because it can’t expand, it is reflected back as the same positive pressure wave to the top of the ram tube, where it does expand and is replaced with a negative pressure wave to travel down to the piston around the bottom of the stroke and is reflected back as the same negative pressure.
So we have a negitive pressure in the cylinder right through the last 60* of the inlet stroke. It is this negative pressure that is created in the cylinder to increase the flow into the cylinder, that the older inertia wave did in the old engines. Instead of a positive pressure outside the cylinder pushing the gas in, we have a negative pressure inside the cylinder pulling it in. .

“So why does IDTB get such a bang for there buck if it all about in airflow restriction.”

In Matt’s case it is just the short inlet tract that has moved the maximum torque up the rpm range, so that the power is developed at the higher rpm to make more power. He already had the cam duration to suit the higher rpm that he wanted it do, but because of the long inlet tract, it would not run at the higher speed. Fitting the shorter tract provided the missing link to let the engines modifications, all work at the same higher rpm.

”A question how do we define what to small or to large for the intake, I fully understand its a matter of trial and error but I wonder if I can fit a pressure transducer in the intake against the valve to see what is happening.”

The sort of engine that is modified for higher output, is not too interested in engine output below 4000, so the inlet tract is for higher speed, a short large diameter tract will have the less effect on restriction, but we still need the engine to run clean at low rpms. So we have to pay attention to the velocity of the gas as it enters the cylinder through the valve. The gas direction and velocity still have to be able to give good turbulence in the combustion chamber to have clean running, so the valve throat has to be large enough to give good filling and small enough to give good low speed running. Using an engine simulation program will show the velocity at any rpm, to find the most effective size.

Harvey.

[Dessertrunner]

I have a couple of thing that need clearing up.
You and other mention about long intake lengths I accept that if they are to long you can't get the veloicty of the air up at higher speed due to friction.

If the lenght of the inlet is fixed then its the diameter of the inlet that counts. I dispute the general statment of all the so call experts (I am not picking on you) that at lower revs you need longer inlet pipes. What it does not seem to be taken on board is that the diameter is more critical. You post talked about air velocity which I think is the most important point.

Next burning question for me is, It is my belief that the standard SVX engine will give a major lift in power just by change to ITB even at revs of 4k. My logic is simple the standard manifold on the SVX has 45mm inlets, at that size there is no way in the world that you can get the benfits of pressures feed from high velocity air. The designers of the SVX engine are all about compromise. According to the manual the current SVX engine is getting plenty of air at that rev.

Have you run the current SVX intake on a simulator to see what the intake velocity was?

Tony