highvoltagefeathers

So there is the myth that backpressure is necessary for the engine to run properly.

It makes sense in a carbureted engine, but the inline six in our cherokees actively adjust for mixture and pressure issues.

Backpressure would keep lightly pressurized, hot, oxygen-depleted gas in the combustion chamber when the exhaust valve closes at TDC, which would then purge and expand into the intake runner, before the piston begins to fall again, sucking in fresh air on top of the hot, burned gas.

Seems to me that the hot, pressurized, o2 depleted gas left in the combustion chamber effectively lowers the amount of mixture each combustion cycle gets to burn and would hurt power.

By removing any restriction, the pressure in the combustion chamber comes a lot closer to atmospheric pressure at TDC, so any burned gasses are less prone to expand upon opening the intake valve, and more charge air can be inducted, improving power.

Now a carbureted engine that can't actively detect and fix mixture issues might have trouble and need re-jetting to accomodate the higher volume of air the engine wants to suck at a given RPM, but the FMS in our cherokees would be fixing that as soon as lean gas reaches the o2 sensors.

Am I thinking correctly? Please advise!

Thanks,

Feathers

5-90

Backpressure is "necessary" on no four-stroke engine, regardless of induction.

Backpressure has been used to eliminate the EGR valve (in conjunction with exhaust valve closure timing) to promote exhaust gas reversion in an attempt to reduce NOx emissions (the theory and principle are sound, but the method could be done better. EGR is stupid, reversion is only slightly less so. There are better ways to solve the problem - and they've been around longer.)

Backpressure interferes with scavenging of exhaust gas and with cylinder emptying & filling - and both of those are limits on engine output.

I'd just love to know how that myth got started. I heard that myself for more years than I'd care to admit to. However, definitive literature on the subject all agree that any backpressure is detrimental to engine output and performance.

There are titles of some of the books that can be considered "Definitive" on the subject of internal combustion engine operation & performance listed on my site under "Books & Recommended Reading." I don't know if the typical City library will have them - a university with an Engineering school should have them, but even that can be hit-or-miss. Start with Taylor and Morrison.

hankthetank

Then why do Hondas have such serious valve overlap? Why do h-pipes work so well on 5.0 'stangs. I've seen dyno charts that say your thinking is a little skewed.

cherokee90

Its BS!!! You do not need back pressure, a motor is nothing more then a air compressor. The more air that can come and go the more power you will make. People are scared of straight exhaust because the the exhaust cools faster, meaning the cool air can go back in and make the hot valves warp. but as long as you have at least headers this is not going to be a prob.

hidperf

I wish I could upvote both of you.

hankthetank

Ok, disregard my previous statement...I guess I'm another victim of a baseless myth. After some googling, it seems that velocity and pipe diameter are the most important factors in an efficient exhaust system. Bigger is not always better...

redjeep1983

I agree about pipe size. Look up bernolli's law on convergent and divergent passages of gasses. Convergent is narrowing which increases velocity but reduces pressure. Divergent is opposite. Including the cat and muffler and pipe, these passages counteract each other, to keep check on gas velocity and pressure.

ct67_72

This debate always kills me. I will tell you where it comes from, and why it is wrong.
First of all people are confused when they use the term backpressure in reference to exhaust size.
People blame performance loss from an oversized exhaust on a "loss of backpressure" because they cant understand anything else.
This is how it works, and why exhaust systems are sized the way they are.
This is the easiest way to explain it.
No matter what you do, the engine has to move a certain amount of air in, and out to run.
SO. The smaller the exhaust is, the faster the gasses have to move to be expelled from it.
The larger the pipe is, the slower it is expelled from it.
SO, (now remember this is with typical things within reason, were not talking about crazy valve overlap or anything like that, and without getting into thermal expansion) when sized correctly, there is actually no, or very little backpressure in the system. In race car applications, you can actually get the gasses flowing fast enough, to scavenge, or pull. This can be beneficial with the right cam configuration, because with enough scavenging, and valve overlap, you can actually get more exhaust out, for less new charge contamination, and actually start to pull the new charge in before BDC.
Now of course if its too small, there is just no way to move the volume of gas out of the pipe. This instance would actaully be a backpressure issue, but not that common, (in typical street car stuff anyway)
So when you go too big, you reduce the speed of exhause gas exiting the pipe, so that the engine has to work harder to expel it. (and can actually cause higher measured backpressure than smaller piping)
This means that with no scavenging from the cylinder, you are not pulling any of the left over exhaust out of the cylinder and contaminating your new charge with it.

So it really comes down to the fact that it has nothing to do with backpressure at all, (typically) but actual exhaust velocity.

Remember that this is true with NA engines, and supercharged engines, turbo engines are a different story.

CobraMarty

This engine's cam is part of the problem also. In it's effort to not use a EGR valve, It has much more I/E valve overlap than usual and and to keep all the intake charge from going out the exhaust, they intentionally used the exhaust 'backpressure' to control this. Good for 'smog' readings, poor for user. Better to put in a reasonable less overlap cam and a free flowing exhaust. The 'stock' cam makes it worse with forced induction.

ct67_72

Are you sure that " backpressure" is what's being used for this.
Typically you can use more overlap like this is intended to hold some of the unburnt gasses in the chamber to allow them to reburn.
I'm not sure that backpressure has anything to do with it.

Outlaw Star

The EGR valve also helps to increase the life of an engine by reducing cylinder temps. The average cylinder is about 2500 degrees, the average temperature of exhaust is around 1500 degrees. Now think for a minute, does your engine run better when its cooler or hotter? The introduction of exhaust gases into the combustion chamber not only burn NOX but cools the fuel charge, thus keeping engine temps down for more power and reduced cylinder wear.

xj_maniac_newb

I just learned alot.

5-90

valve overlap has nothing to do with backpressure - and it assists scavenging by allowing more intake charge to enter and push the exhaust gas out of the way.

H-pipes don't generate backpressure - they help relieve it, by allowing the pressure wave pulses on one side to assist with "draw" on the other side. An X-pipe is better than an H-pipe, because it improves acoustic performance (and when you're talking about pressure wave dynamics in fluid flow, it comes back to acoustic theory.

You've (probably inadvertently) made my point for me - thank you.

5-90

More overlap allow for greater throughflow of charge gasses - which tends to improve scavenging, but typically at the expense of fuel economy and an increase in HC and CO emissions. Overlap can be used to improve efficiency for forced induction (often done with superchargers) and you can tie it in with a supercharger to achieve a different method of engine operation (cf: "Miller Cycle," as used in the Mazda Millenia.)

Reducing overlap can allow exhaust gasses to be retained (simulating the effect of EGR,) and you can also achieve reversion through removing exhaust port size, baffling the exhaust primaries, or other methods (these two are most common.)

EGR achieves NOx reduction through "dampening" the fire - although using hot exhaust gas to dampen the fire doesn't make so much sense to me.

How would I do it? The way we did it in WWII - water/MeOH injection/fogging. It's cleaner, it has better results, it doesn't run carbon-bearing exhaust gasses through the system, and it actually reduces carbon deposits in the chamber relative to what would be present in a non-EGR engine!

Size the water/MeOH tank to last a bit more than a tankful of fuel, have water/MeOH fill pumps available at filling stations, you just fill both tanks at the same time (a 20-gallon or so fuel tank is probably just fine with two to three quarts of water/MeOH mix, mix at about 3:1 or so.)

I'd take water/MeOH injection over EGR in a heartbeat, I don't even have to think twice about it. I've run the system on high-CR engines before with excellent results - being able to run a 10.5:1DCR engine on 87AKI pump fuel is all of the "result" I need - dyno tests showed that I got 98% of projected output, running timing at a useful level.

I tore the engine down for inspection after a year - it hardly needed cleaning. The oil drainbacks were dirtier than the combustion chambers!

Instead, we get saddled with a device that routes dirty exhaust gas back into the intake stream, craps everything up with carbon, blows HC and CO up in exchange for reducing NOx - and we get saddled with the thing on engines that don't even need it!

(My 1987 had a failed EGR when I bought it - HC and CO were non-existant, NOx was nearly so. I failed - because they manually tested the EGR.

(I replaced the EGR valve - HC and CO spiked, no real change in NOx. I replaced the cat six months later out of hand, but it consistently tested better without the EGR functioning than with...)

This is the sort of thing that happens when you get people who pass rules to control things they don't understand...

Outlaw Star

For Turbo cars they sell those same injection units. But I've never installed one. But your rundown of it makes sense. If you think about it, keeping the combustion chamber free of carbon and deposits would increase the efficiency of the engine.