4.0L cubic feet per minute (just some numbers)

[XJ Dreamin']

We've all read that a motor is essentially an air pump. The piston moves down - air flows in equal to the volume vacated by the piston. After the power stroke, the combustion gases are vented. But, how much air can a 4.0L engine move.

In a hypothetically perfect air flow model (i.e. atmospheric intake manifold pressure, no blow-by, no exhaust restrictions) the 4.0L would displace 216 cfm at 3000 rpm, and 360 cfm at 5000 rpm. Of course, unless you've added a blower or tubo the intake pressure is below atmospheric and, of course, some exhaust back pressure is necessary. However, back pressure is really a matter of designing a system appropriate to the existing flow rate and is not a direct result of absolute air flow. That is, you can and should have some back pressure even if you've augmented the induction system.

Where did the numbers come from? One linear inch is 2.54 cm. One linear foot is 30.48 cm. One cubic foot is 28,316.85 cc's. One liter is 1,000 cc's. Therefore, one liter is 0.035315 cubic foot, and 4 litres is 0.141259 cubic foot. Since intake to exhaust takes 2 revolutions of the crank, then the six cylinders would displace 3000rpm times 0.141259 cubic foot divided by 2 revolutions per 4-stroke cycle, or 216 cubic feet per minute. The same formula yields 360 cfm at 5000 rpm. Of course, the actual flow rate is something less in the real world.

I got into this little exersise by reading the promotional copy at an air filter retailer's web site. They quoted 240 cfm for the oem factory air filter. Solving for rpm at 240 cfm means (even in the perfect model) you would be in trouble above 3398 rpm. It seems like any filter that will flow 400 cfm or better would be fine, but given that a filter's performance is impacted by dirt, you really need to know what the flow rate is just before it's time to clean/replace it. I would think that anything in the 600-800 cfm range would be fine, but, of course, bigger is always better :laugh3:

XJ Dreamin'
stock 2WD XJ, 4.0L HO, AW4, d35c, LT 235/75R15 A/T's

 

[dmillion]

Back pressure is not something you "should" have. Back pressure does not improve torque or anything else. That's a myth. The physical limitations of exhaust system design, however, dictate that there will always be some back pressure at some engine speeds, in exchange for getting the exhaust pulses to go through the system at the right speed. It's all about the speed of the exhaust pulses, not back pressure. The question is, what engine speed do you want to optimize the exhaust system for, and where are you going to accept back pressure as a consequence?

Exhaust systems operate best within fairly narrow RPM ranges. At the very best RPM for the exhaust system there will not only be no back pressure, the exhaust pulses already in the system will actually help to SUCK the burnt gasses out of each cylinder. Sort of negative back pressure. This is referred to as "scavenging" and two-stroke builders refer to that particular engine speed as being "on the pipe."

The thing is that if you design an engine and exhaust system to provide maximum torque at low engine speeds, you will end up with back pressure at higher engine speeds. This is where the myth that back pressure helps torque came from. But is IS a myth. The back pressure does not help the torque at all. In fact, as the engine RPMs go up, and the back pressure increases, the torque begins to drop. This is simply an unavoidable consequence of a system designed NOT to have back pressure at lower engine speeds!

If you want to understand back pressure and exhaust system design I suggest reading up on two-stroke motorcycle exhaust systems. In a two-stroke, the exhaust system design is an EXTREMELY influential element of overall engine operation, since scavenging is the primary way that exhaust gasses are removed from the cylinder.

 

[BLSXJ]

to add to dmillion post
some people wouldthen conclude that best exhaust would be no exhaust at all (except for the scavenging effect). most people will tell you never to run a motor with out headers. this is not for backpressure this is to stop the valves from burning. valves burn when exhaust is sucked back in to the cylender right before the valve completely closes, and the exhaust contians O2 (sort of like a cutting torch)

I'm sure the good doctor Dyno will chip in soon but the bottom line there is a differnt exhaust and intake design for every RPM.

 

[XJ Dreamin']

Back pressure is not something you "should" have. Back pressure does not improve torque or anything else. That's a myth. The physical limitations of exhaust system design, however, dictate that there will always...

Thank you, dmillion. I should not have exposed my ignorance by getting off track in my own message. Just to illuminate what I had in mind when I said that some back pressure was needed... When designing a tuned exhaust one can set up a standing wave in the pipe at a particular rpm, or a particular narrow rpm range. What I have been told is to be careful of holding a trough of that standing wave form on the exhaust valve. It promotes scavaging but it is difficult to dissapate heat from the exhaust valve in that low pressure condition. It does not matter as much on short runs, such as dragging, where maximum scavaging might be most desired, but could be a problem if you expect an engine to operate within a particular rpm range for extended periods of time. So, it is as you say. Back pressure is not needed - I mispoke (again) but it may be a necessary consequence to prevent burning a valve.

In any case, I should not have ventured off topic. Do the cfm numbers seem right to you? It seems to me that the flow rate cannot be anything but the 1/2 the rpm times displacement - minus restrictions, blow-by, and other inefficiencies.

XJ Dreamin'

 

[XJ Dreamin']

to add to dmillion post
some people wouldthen conclude that best exhaust would be no exhaust at all (except for the scavenging effect). most people will tell you never to run a motor with out headers. this is not for backpressure this is to stop the valves from burning. valves burn when exhaust is sucked back in to the cylender right before the valve completely closes, and the exhaust contians O2 (sort of like a cutting torch)

I'm sure the good doctor Dyno will chip in soon but the bottom line there is a differnt exhaust and intake design for every RPM.

I was always told not to run an engine without some kind of exhaust pipe. Since it was my Dad tellin' me I never questioned it There were consiquences to damaging an engine in one of Dad's cars :twak: .

XJ Dreamin'

 

[Dr. Dyno]

dmillion has hit the nail right on the head, so it should come as no surprise that the ideal exhaust system would have a valve that stays virtually closed at low engine speeds and opens gradually as you increase the rpm, thus minimizing back pressure and maximizing exhaust gas velocity right across the rpm range. Manufacturers are already using variable intake technology so why not variable exhaust as well?
As far as cfm goes, the 4.0L engine is actually 3960cc or 241.5ci. At 5250rpm, the airflow through the engine would be 367cfm @ 1.5"Hg if the volumetric efficiency (VE) was 100%. In reality it's only about 72% at 5250rpm on a stock 4.0 so the airflow is closer to 265cfm. The stock XJ air filter flows 370cfm so it doesn't pose much restriction when clean but does become restrictive when dirty.
Ideally you'll want an air filter that flows at least double what the engine can handle. If you improve the VE to 80% at 5250rpm with bolt-on performance mods, the engine's demand for air will be 294cfm so you'll want an air filter that flows at least 600cfm.

 

[XJ Dreamin']

I'm sure the good doctor Dyno will chip in soon but the bottom line there is a differnt exhaust and intake design for every RPM.

I hope so. I will be modifying the intake, along with other upgrades over the next year. I had an idea of stickin' a big ol' 1000 cfm round air cleaner on top of the TB but Dr. Dyno advised caution. The thought occured to me, then, how much air can the engine move, anyway? That's where the numbers came from.

XJ Dreamin'

Dr. Dyno beat me to it! Thanks for the numbers. I knew it wasn't exactly 4.0L but I was lazy enough to figure that it wouldn't be too far off. I was hoping that the VE would be better than 50%: 72% is not bad but that's the number i didn't have (although I'm sure it's somewhere in your web pages). I'm thinking your recomendation for some pipe between the TB and the filter is to generate some velocity to feed the TB. I can't break the temptation to treat it like a big V-8 and stick a filter on top. I know I should just listen and shut up, but is velocity the issue? That would make some sense to me.

Thanks again,
XJ Dreamin'