help! 1/04/08

[sahara0207]

hmmm... i see well you prolly know more the me lol but i do know having it to open can cuase issues cuz my engine runs alot better now that i have my lil glass pack comparred to open. it feels more controlled? can you elaberate on how it works?

[Kodiak Spirit]

PeteyG, Rat, You guys are such know-it-all's! :shout:

I love it! That's why this MJR thing is so sweet!

[DEADMEAT]

first off i have to agree with kirkandsylvia, i would change the fuel filter and run and injector cleaner. start with the cheap easy stuff first.
with the back pressure, i wont get too into it, but the only thing you need and exhaust is to sound cool and emissions. very few cases actually require back pressure (turbocharges). just look at your fundamental of engines books.
with that, your air and fuel always have to be proportional.

[brokenujoint]

you do not want any back pressure, an engine is a vaccum, the faster it gets the air in and out the faster it goes, if it needed back pressure why would people take the cat and muffler off to get more power, wouldnt those add power if it needed back pressure?

WAY OFF my friend.

I am a certified Aircraft mechanic. i work on 6 cyl, 9 cyl, turbine jet, radial and inline aircraft engines that put out hp ratings in the thousands of hp and I can agree that an engine DOES need some back pressure. the amounts that most of you guys are thinking about is more than what is actually needed. a piston engine is not a vacuum, it is a breathing machine just like you are. inhale and exhale.

when an intake valve opens, the exhaust valve is closing. when an exhaust valve is opening an intake valve is closing.(valve overlap). This design is to take avantage of the inertia of the outflowing exh gases to provide more complete scavengin and to allow the entering mixture to flow into the combustion chamber at the earliest possible moment, invproving volumetric efficiency. with that understood, you need a little back pressure in the exhaust to circulate and use fuel/air in the combustion chamber to the best efficiency possible. to much back pressure and you will get pre-ignition and pre-detination due to hot exh gases pushing there way thru the intake and starving the engine of O2 and cooler undetonated air. to little and you will get crappy performance on the other end of the scale due to unused gases on the exh end.
for the most part, the backpressure sales-pitch is a bit much but the theory and reality is that yes you need a little bit of back pressure.
Unless you are versed in piston engine theory, history, and tech, don't make such a bold statement.
cut off your cat, run a 70mm tb and run 4" exh tubing and see how well your engine runs.
For the poster who mentioned that backpressure was only needed during the carburator days, i understand your reasoning but regardless of how well vaporized and atomized the fuel is when it is ignited in the combustion chamber it is not completely used and properly circulated. you WANT some hot and cold air mixing together during the release of the exh during the good ol 4 stroke five event cycle principle "Otto cycle"'s exhaust stroke.

for the backfiring comment. a backfire occurs if an intake valve opens to early, pre-igniting incoming fuel-air with hot exh gases.

This debate can go on for ever, ive done over 2200 hrs of training and if you want an engine to run at full throttle and care little about the mpg or efficiency of the engine then back pressure is of little importance to you, hence the race car talk. if you want a long lasting, highly efficient motor then understand that they are indeed designed to burn the most gas and to keep the engine as cool as possible.

[Schmo]

And in the left corner, weighing in at one hundred eighty six and one half pounds . . .

[sarah]

And in the left corner, weighing in at one hundred eighty six and one half pounds . . .

... GUNS 'N' ROSES!! *cheer * cheer * Get in the ring!

Sorry for the ginormous hijack, but I couldn't help myself!

[RAT]

Like Jesse says, that's why MJR Rocks. So many differing opinions. The advice all us "smart" people should have given right off the bat is to take your heep to a certified mechanic. Obviously you are not going to get anyone to agree on one answer here and real time data is what is needed to determine your problem. So read this thread for entertainment, then go see a tech that can do the "laying on of hands".

[RAT]

... GUNS 'N' ROSES!! *cheer * cheer * Get in the ring!

Sorry for the ginormous hijack, but I couldn't help myself!

And you, missy.... :hijack:

[peteyg]

Like Jesse says, that's why MJR Rocks. So many differing opinions. The advice all us "smart" people should have given right off the bat is to take your heep to a certified mechanic. Obviously you are not going to get anyone to agree on one answer here and real time data is what is needed to determine your problem. So read this thread for entertainment, then go see a tech that can do the "laying on of hands".

Very sound advice.

However, it's not a matter of opinions, it's just plain physics. It's amusing to see people stubbornly cling to the myth because they have been believing in it for so long. This isn't metaphysics, where you go on faith.

But whatever. I have weighed in, and I'm so far the only one who backed myself up with additional references and reading materials. Take it for what it's worth (which is about 2.04 cents, accounting for inflation).

[Mr. Orange]

Whoa...hey now.....

:hijack:
Cheezus....is this still going on???


No more hostile posting.....:gun:
Face it....some engines need back pressure, and some don't. Ask any 'real' mechanic. There is no 'right' answer, it all depends on the 'physics' of your particular setup.

Let's stop this post before the 'pressure':fire: builds up any more than it has.

It does need 'real time' evaluation, so, yeah, take it to a mechanic:wrench: that charges the 85/hour, and see what he says. That is why they charge so much, most of them actually know how to fix certain problems, if not narrow it down....
All this speculation could end up with the wrong result.

[Kodiak Spirit]

What this thread needs now is a BIG GROUP HUG! :haha:

[RAT]

Well,
Here you guys go. None of this is going help fix Justin's junk, and it has nothing to do with his problem anyway, but since there is so much debate on the subject I offer just a few of the tens of thousands of publications out there that deal with engine exhaust backpressure. It's all application specific, but the bottom line is that, overall, exhaust backpressure is not desireable. Even aircraft powerplant manufacturers list it in their nomeclature. Racers do what ever they can get away with to reduce it, even old deisel submarines tried to reduce it to 0. I could fill an entire thread on this subject alone but ain't gonna. Deal with it...

http://www.magnaflow.com/05news/magazine/05sportc.asp

http://www.warnertechnology.com/Cars/backpressure.shtml

http://www.fordcobraengines.com/exhaust_hedder_size.htm

http://www.powersportaviation.com/Ho...es/engines.htm

Remember, it's all in good fun, so don't get all riled up. :thumbs_up

[Paul B]

The correct amount of backpressure is an absolute must!!!! :hide: Of course, I'm talking about two stroke engines

[flatpoint]

Have to put my 2 cents in. I was a builder in an engine shop for 14 yrs. Had a passion for it. Bore vs. stroke, rod liengh, port size, and liengh, and cam timming all decide where the engine will make its max power. Picture your self in a helocopter above downtown LA, watching traffic. This is how air flows through an engine.

[flatpoint]

Backpressure should be thought of as a "pulse".Reversion. An exhaust valve held open too long,(big cam) will over scavinge the cylinder. Header dia. and tube liengh will effect this. Tape up a piece of conduit at one end,and slap the open end. This is how a header works.