Bags

iTrader: (3)

Is vaccum controlled or related ot exhaust?

I have a HUGE exhaust leak at both manifolds.

My vaccum is 10 HG.

I was trying to figure out if they are related.

Thanks

ejj

iTrader: (5)

I can't see how it could be.

Vaccum is before the engine, exhaust is after. In my mind they don't seem releated (concerning leaks).

20th maxstyle

iTrader: (4)

Well kinda. When the Exhaust valve is open, there is a minnor piont in time when the intake valve is open as well. So when the Exhaust leaves, there is a slight type of vacuum which pulls air into the combustion chamber. That is another reason why your exhaust system can not be completely open. You need that kind of back preasure, like a vacuum line, to pull some of that new air in. You can't put a Huge vacuum line on a little port. The the vacuum energy is lost to excessive volume. Kinda harder to explain so ask me if you need me to go futher into it.

20th maxstyle

iTrader: (4)

hmm did I spell vacuum wrong. LOL

nick

iTrader: (10)

how do you know its exhaust leak ???
do you have a valve clatter type of noise when the engine is cold and driven at low RPM's , and it goes away once engine is at operating temp ?

I might have the same problem ,
I pin-pointed the source of noise to either EGR vlave or EGR Backpreesure transducer valve.

Nick

Quicksilver

iTrader: (1)

Ummm, what the hell are you talking about? There is no air pulled back into the combustion chamber (from the exhaust side) when the exhaust valve opens in this motor...

If there was, you'd have pistons/rings melting left and right from the massive amount of preignition that your little theory would cause...

Care to try again?

bonzelite

iTrader: (2)

yes. he is referring to exhaust scavenging. it is normal in combustion engines. there is a brief moment in the 4-stroke cycle when both intake and exhaust valves overlap, creating a vacuum effect. when building up an engine, you can play this up to maximize combustion: cam choice and valve timing can either retard or promote this overlap. there is no such meltdown as you have described.

Quicksilver

iTrader: (1)

So you're saying that air is pulled back into the combustion chamber thru the exhaust valves? That's what I'm calling BS on. If you have overlap with forced induction, the compressor blows some of the mixture right through the intake past the exhaust valves without ever burning it...

It does not somehow suck it back through the exhaust valves into the combustion chamber again...

Jeff92se

iTrader: (82)

Well vacuum is related to exhaust I guess.

But if you are on boost, there is no vacuum. So what we are talking about is in general terms with na or idle or < postive boost conditions on a boosted car.

You have to remember that the combustion process is not a smooth line like _________ it's more like a bunch of pulses like **********. Even then these pulses don't always move at a certain speed even if the engine is at a certain rpm. Sometimes the pulses can even move backwards. This is true with the intake charges.

With the exhaust, the way the pulses move is a derivative of rpm/load/pipe size/design etc.... blah, blah. If you have a very large exhaust in a na car, the individual combustion pulse from ie.. #1 cylinder will move down the exhaust manifold/pipe and then lose VELOCITY. Like ***** * * * * *. If it slows down too much before another cylinder pulse can help it "move" along by either pushing or pulling, you get crappy lowend power.

If these pulses are allowed to slow down too much or even move backwards via a large exhaust leak, it could affect the vacuum reading at idle/part throttle.

For example, imagine your hand as an intake/exhaust valve. As you know these valves move up/down to open/close. Imagine your hand move though water. Move it one way, then move it the other direction. See the waves it creates? It will create waves in both directions. Without a good scavenging exhaust system(ie.. no leaks), the waves made by the valves could actually make the exhaust go the other direction(like a sucking action)

hakk97se

The action of the exhaust leaving the chamber momentarily jerks air in through the intake valve as the pressure in the chamber is equalized. I think that the overlap with our cams is extremely small tho, as scavenging is not good for emissions. I would imagine that the vacuum you are usually measuring is more due to the effect of the piston rising in the camber (moslty, while the exhaust valve is closed).

bonzelite

iTrader: (2)

what we are saying is the open positions of both intake and exhaust valves coninciding as an event, ie, overlappping, allows the outgoing exhaust gases to literally "suck," enable, even more air into the intake, thus creating denser charge air. this promotes more bang for your buck. so, yes, the exhaust valves, when *open*, facilitate movement of air *into* the intake valves *open*. like a condensed milk can punctured on 2 sides >>> the cream flows out better.

it sounds like, too, you are referring to blow-by of fuel, thus setting the stage for a backfire/detonation. it can puddle in the intake itself or it can blow-by and detonate in the exhaust pipe. this can be catastrophic with wet N2O setups incorrectly installed. is this what you mean? or do you mean excessive overlap in general? if that is the case, then too much overlap will make the car run like sh*t. or you may be referring to turbo fuel/air mixtures being incorrectly managed. like a too-rich condition.

and jeff, i enjoy and fully understand your _______ v ****** * * * * explanation. very well put and illustrated; idiot-proof tech talk. thanks, man.

Quicksilver

iTrader: (1)

OK, you guys are clueless...

Read the post I was referring to, then read my post. I was asking him (20th maxstyle) if he was trying to say if the air was being drawn into the combustion chamber via the intake valve or the exhaust valve. That was my question. You guys start talking BASIC engine dynamics, which I am already well aware of. I know how scavenging works, and I know how overlap works, but I was confused by his response since he also mentioned the word "backpressure"...which doesn't exist in a N/A 4 stroke engine application. Since he mentioned backpressure, I assumed he was either confused about his engine applications, or didn't know what he was talking about.

You'll have to forgive my frustration, but when you folks don't actually and directly answer a very simple question, it's kind of annoying to me...but that's my bad for being short on patience. I was also specifically hoping for an answer from 20th maxstyle, since he is the originator of the post in question...

bonzelite

iTrader: (2)

backpressure is exhaust pipe related. not engine related. if that is what the issue was/is for 20th maxstyle. sorry i lauched into overlap 101. at least some readers may have learned from that. and jeff's explanation was fun.

Quicksilver

iTrader: (1)

Jeff's explanations are usually pretty darn good...

Engine Tech for Dummies

And backpressure is not the correct term still...it's velocity and temp/thermal energy potential...

Jeff92se

iTrader: (82)

Well Quicksilver. Who said we were talking to you?

bonzelite

iTrader: (2)

i praise your knowledge, but lighten up.

Quicksilver

iTrader: (1)

Well I did, of course

Jeff92se

iTrader: (82)

hehe.

Yes, you are 100% correct. Exhaust is more about gas velocities and not "backpressure". If you think about exhaust as a bunch of little pulses moving along one after another and think what affect a big/small pipes will do to the flow, everyone would be less confused. ie.. blowing though a small straw vs blowing though a toliet paper roll etc.... I know you know all this anyway.

nismology

iTrader: (8)

Fluid dynamics/mechanics here I come!!!!!

Edit: Hey!! Post # 500!!!!

bonzelite

iTrader: (2)

when people throw out the term 'backpressure,' they really are meaning the above information. but, now, how or why did that term originate? if it's meaning is not literally correct, then why is it still used? in essence, the muffler and pipes are like a cork. maybe a good layperson term is "bottleneck."

"headers with straight-through pipes will reduce bottleneck."

20th maxstyle

iTrader: (4)

oh ****, I didn't mean to get this goin so big, Word of advice. UTI sucks now, as opposed to what it used to be.

Quicksilver

iTrader: (1)

Well, one simple explanation for where the term backpressure came from has to do with 2 stroke engines. Some older two strokes used a specially designed exhaust manifold/tube that during it's running process would actually help to reverse the flow of the exhaust (and unburnt fuel/oil mixture) back into the combustion chamber in an attempt to be more fuel efficient and also to help control the combustion chamber temps. Old chainsaws, maybe some handheld rotary saws...stuff like that. It was never really used in powered vehicles except for some older two stroke dirt bikes and stuff...

bonzelite

iTrader: (2)

so it is a vestige of the past, a term now used to mean something else, but not thought about literally anymore. so what am i supposed to say, then?

Quicksilver

iTrader: (1)

I guess the key words/terms to remember now are exhaust pulses, exhaust velocity, and exhaust gas temps/thermal energy. It's actually a lot more complicated, but those are the general terms...

bonzelite

iTrader: (2)

right. but nobody is going to ever refer to all of those terms, as it is too esoteric, engineering-minded and too long. they are just going to say "backpressure" even if it is technically incorrect. but i hear what you are saying. i have learned something from all of this. i just don't know what i will say from now on when referring to outgoing exhaust pressure. i suppose i can say that: "outgoing exhaust pressure." at least that is literally what it is. as "backpressure," per what i have learned from you, has become this inappropriately directed catchy garage jargon, pseudo tech-word.

Quicksilver

iTrader: (1)

Perhaps we should call it exhaust energy...which would be a combination of all those things.

bonzelite

iTrader: (2)

agreed, then. i will refer to backpressure as exhaust energy. whenever it comes up again on other threads, i will just deal with the people who do not understand.

thanks.

Bags

iTrader: (3)

It is golden. I changed the gasket while the motor was out. I put my hand back there while it was running and no exhaust leaks.



ejj- There are the 2 diapharm looking things on the rear of the motor. 1 of them attaches to the rear manifold. When I get home I'll point you to the page in the FSM. I just found it and I am still tyring to understand it all.



20th maxstyle - you do have a point

nick- the fumes coming from the rear manifold and the hiss say it's a leak

bonzelite & justin - english muthafu*ka, do you speak it? ( watch pulp fiction )

Jeff- AWESOME explination. And you do speak english And yes idle only, I have not even gone over 3500 rpm and no where NEAR boost. Makes lots of sense. I will be putting on a stock Y pipe to see if that solves the problem


bonzelite & justin- do they speak english in what? ( pulp fiction again )

I am glad this was discussed. I see a few different views and thougths on this.

Justin- Who's your daddy

And I am glad yall agreed that backpressure is needed for a NA mtor

bonzelite

iTrader: (2)

yo yo yo muthaf*&*Ka, i speek 'a inglissh.

Quicksilver

iTrader: (1)

What???


Say what again!!!!!!!!!!!!!