Exhaust system back pressure

[FreeBug]

My limited "understanding" : backpressure was this thing they needed in the 50's to keep the engine "on cam", without too much lost out the exhaust. Then it was discovered that what was really important about keeping charge in the cylinder was more to do with valve timing. Then they figured out how acoustics can get "magic" done, closing intake on a high pressure wave, close exhaust on a low pressure wave, giving intake a "headstart", and it's a balance between wanting to lower resistance to flow, and having enough tubing there to get the wanted acoustic resonance at the rpm you want them at. I think.

No exhaust was the method of choice in ww2 non-turbocharged air engines. Ideal, or missing out on benefits of resonant pressure waves? I dunno.

[oprn]

Resonant pressure waves are really only effective at a narrow RPM range or to a lesser degree, multiples thereof. Open exhaust may not gain you as much but it works at any RPM. Also tuning a pipe at the RPMs aircraft engines run would end up with a very long exhaust system. That adds weight and airplanes are highly allergic to weight gain!

They tend to break out in gravel rash at the end of the runway on take off!
_________________
We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[oprn]

It would be very interesting to get exhaust pressure numbers from factory stock cars and see how close to zero back pressure they are. It sounds like it's just a matter of poling the info on a scan tool... no experiance here with using those... Maybe one of the auto techs here knows?
_________________
We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[PhillipM]

[raygreenwood]

[oprn]

I just talked to a Buddy that's been a GM and Mazda tech forever. He tells my that the scan tools he uses diagnose Cat problems by comparing upstream and downstream O2 sensor readings not pressure. He has never heard of using pressure on an exhaust system but will talk to his co-workers and see if they ever have.
_________________
We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[FreeBug]

[oprn]

The vast majority of light aircraft, factory built as well as home built have very little for exhaust systems. It's mostly a weight saving thing. The power gain at 2100 to 2600 rpm where they run is just not worth the weight gain of a tuned system. That is why you can hear them long before you can see them and long after they are out of sight.

The exception to that is the ones powered by two stroke engines. Two strokes respond much better to tuned systems and run at 6000 rpm with gear reduction drives to get the prop speed down to 3000 rpm or less range. Also being tuned at that rpm rang the exhaust length is so much shorter as to make a tuned system more practical.
_________________
We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[oprn]

I finally got around to buying a 2” muffler (they don’t make them smaller) with an internal construction like the Flow Master. I put it on temporarily and took it for a drive last night. It’s a bit louder but tolerable. The biggest issue is that the engine runs TERRIBLE now! Bogs, sputters and misses up to about 1/3 throttle. Light throttle cruise is full of missing too. Feels lean, pumping the gas peddle helps. Idles fine. What the heck?

I need to recheck timing and see if that has changed. Then put my wide band back on and see what has happened.
_________________
We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[modok]

HA!
So much for "back pressure"
That's how tuning is.
If trying something works worse, try the opposite.
Clearly it doesn't like that,
tho we dont know if it's the Length, volume, or size that it doesn't like.

[oprn]

Yes at that throttle setting there should be virtually no back pressure with either muffler. Due to the bulk of the new muffler I had to move it which involved adding some length after the last “Y” in the system which will change the tune no doubt. So we do have an increase of length and volume.

Now there are other possibilities too. The car sat out through a bit of a shower a couple days ago so - moisture in the ignition system needs to be ruled out. This is a points distributor too but I did grease the rubbing block but a check on that is in order too.
_________________
We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[anthax]

From what I understand its the low pressure pulse previous cylinder produced when pushing exhaust fumes out that needs to be timed to when next cylinder opens the exhaust valve to help scavenge exhaust gases out and if cam has some overlap it helps suck fresh mixture in.

[Ohio Tom]

A few things to add:

Short tubes on Top fuel and other applications are required to prevent oxygen from sucking in the port and setting the exh valve on fire.
They need to be long enough to prevent oxygen back flow to the exh valve.

The other use for "zoomies" are they actually provide down-force on a TF car.
And if angled back, provide thrust.
They are always tweaking the angle to balance down force with added thrust.


Low restriction exhaust typically need a fatter main jet as some intake charge is sucked right out the exhaust.
Valve timing overlap works with the reflected wave pulse to pull more A/F mixture into the chamber.
As stated before, this only works best at one rpm.

Turbo pressure ratios are a big consideration when selecting the cam you wish to run.
If you have a higher pressure ratio (greater than 1:1), you can benefit from a cam with wider lobe separation angles. Early exh opening and less overlap help to pump the exh gasses thru the turbine.

However, Some high dollar turbos with ball bearings and larger exh turbines are designed to work based on flow velocity. Not so much on the pressure. They are designed to operate at much lower pressure ratios. (they spool really fast too).
Those benefit from a Narrow LSA cam. The exh pulse is held later, then let go with a sudden rush of flow that quickly exits and keeps the exh pressure lower.

[oprn]

I was expecting this muffler change to effect the full throttle higher rpm mixture but not the off idle transition. Maybe it does but there are a couple other things to eliminate first… as soon as I get a bit of time I will dig into it.

Stock cam, stock SP heads, no turbo Tom. I’m just tinkering with a few exhaust ideas here. Nothing wild and woolly.
_________________
We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[raygreenwood]

[oprn]

The exhaust on all aircraft point back to direct the exhaust away from the prop wash. Is the force measurable? Possibly. Is it of any practical value? Not likely.

Just put me down as a skeptic on the practicality of that one. Sorry!
_________________
We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[oprn]

Ok, I found the article in question and you are right, they did do the study and the facts were that it added 25 mph to the top speed of a 338 mph Spitfire. The conclusion was that the redesign of the exhaust added 70 pounds of thrust. Now I would have to ask, is that actually physical thrust from the exhaust gasses leaving the aircraft? Or was it that the redesigned exhaust stacks presented less aerodynamic drag? Or was there a Venturi scavenging effect that helped evacuate the combustion camber and the engines made more power?

I suspect it was a bit of all three.
_________________
We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)

[VWporscheGT3]

[Ohio Tom]

There was a TF run a few years ago where a Funny Car hit the wall and broke the header clean off the head.
when the driver (inadvertently) slapped the throttle, the car lifted off the ground and started to roll over due to the thrust.
Slo-mo video made it obvious that's what was happening.

[oprn]

Ok sufficient for that, let’s get back on track here.

Just for a quick test just now I took the muffler off and did a little blast up the road with open header. Yup! Running like a bunch of junk! I have a problem somewhere. Idled nice and goes good flat out but in between… nope, not good! And getting worse.
_________________
We had the stone age, the bronze age, the industrial age and now we are in the age of mass deception and mind control for corporate profit. (The mass media age)