General article in Exhaust science

[SKiddell]

Great referance exhaust science and tech from the legend David Vizard
Exhaust System Science -by David Vizard |

 

[datsun dave]

Great read.

 

[morbias]

Thanks a lot for the link, I thought I would reply in this thread rather than take the other one further off track.

So, if for instance your cam provides peak torque at 6000 rpm and you have selected velocity stacks on your carbs to make peak torque at 6000 rpm, and you have attached an air box that will provide a resonant frequency also at 6000 rpm, then by using an appropriate length secondary on the exhaust that provides maximum scavenging around 6k rpm and primaries which have a max scavenging range between 3-7k rpm you should theoretically end up with the most torque you can get out of a particular engine at 6k rpm?

Does the size of pipe after the secondary matter that much, ie. is bigger actually better after the secondary to help pull exhaust gas away from the engine?

 

[SKiddell]

Does the size of pipe after the secondary matter that much

I think its fair to say that for the entire system, size matters, for example as the gas travels along the length, it cools and thus slows down which is undesirable, the ideal is to minimise resistance and maximise speed.

We also need to factor in that resonant pulses can travel at huge velocity, sometimes close if not exeeding the speed of sound

Most people either dont understand or dont want to understand scavenging on overlap and yet its one of the most important factors in taking an engine and getting the maximum out of it.
The ovelap is a post combustion cam event and is when both valves are open, the inlet is opening, the exhaust is closing, a good cam/exhaust combo will utilise this "5th cycle" to its maximum, completely evacuating the burnt waste gas, creating aditional inertia (and thus velocity) in the new inbound charge helping to ram it in above and beyond atmospheric pressure, then if the cam is designed correctly the inlet will shut holding the excessive charge in the cylinder ready for compression...... used correctly it can have a massive effect on overall power and torque as you can cram in more air/fuel than you normally would, its a little like a free low pressure turbo.

And to do this well, you need an aggressive cam profile with lift on overlap around 5mm (mind the valve to piston clearance) a free flowing inlet tract of the right length and a well tuned exhaust, too big and it wont create the velocity, too small and the back pressure will be excessive.

So if you were building a performance engine from scratch you would flow bench the standard head, perform gradual porting techniques and re measure each time until you get to an optimum point that will give you the flow rates of both inlet and exhaust ports at various valve lift, this data is then used to design the inlet tract, and the exhaust header, then re measure to get new flow data then design the cam to match the results (no point in opening a valve further than you need, its a waste of time and effort, however most people stick a cam in without paying any attention to the clearly important science behind it.......no point in sticking in an agressive stage 4 cam into an unmodified head, at best youll get a few hp gain but lose out on drivability and fuel economy at worse you can lose power and it will drive like a donkey.

 

[morbias]

Thanks for the explanation

If you were to time the air charge from the resonant frequency in the airbox to coincide with the resonant frequency of the exhaust, would that result in fastest flow in and out of the head at cam overlap and therefore maximum pressure in the cylinder for the next combustion cycle?

Do the resonant pulses in the exhaust bounce between the back of the exhaust valve and the collector or all the way to end of the exhaust pipe?

Just for discussion's sake, if it were possible to fit some sort of large check valve on the neck of the airbox would that cause the reflected pulses in the plenum to build higher?

 

[SKiddell]

If you were to time the air charge from the resonant frequency in the airbox to coincide with the resonant frequency of the exhaust, would that result in fastest flow in and out of the head at cam overlap and therefore maximum pressure in the cylinder for the next combustion cycle?

I believe the best performance comes from a cascade effect ie a series of episodes in rapid succession rather than everything at once

Do the resonant pulses in the exhaust bounce between the back of the exhaust valve and the collector or all the way to end of the exhaust pipe?

BTH The pressure wave (which is traveling about 5 times faster than the expanding gases) will create a resonant pulse at each major pressure change so transition from primary to secondary, secondary to end of collector, collector to muffler and muffler to open end

Just for discussion's sake, if it were possible to fit some sort of large check valve on the neck of the airbox would that cause the reflected pulses in the plenum to build higher?

I'm not quiet sure exactly what you are describing but from the limited experiance I have, the strength of the reflected pulse is largely dependant on the general design of the air box, inner wing, battery tray or anything else around the open inlet tract, what weakens it is an out of phase reflected pulse partially cancelling it out

Try this article, http://www.formula1-dictionary.net/exhaust_road_perf.html

 

[morbias]

So I guess having one airbox per cylinder would reduce crosstalk (for want of a better word) between pulses and make it easier to tune each airbox to a specific frequency and also give the strongest reflection.

Also, I was wondering what the effect of having the airbox inlet pipe protruding into the plenum would be, whether that would help bounce the wave around or simply mess up airflow:

In my mind that would help amplify the reflected wave but I'm sure there is a reason it's not done.