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Discussion Starter · #1 ·
Well since I enjoy working on cars so much, I decided that this summer I would see if I could get paid doing something I enjoy doing anyways. So I got me a job as an assistant to a mechaninc in the service department of a Licoln Mercury and Chrysler dealership. Basically, I just do Oil and Transmission fluid changes, Alignments, Tire Rotations, Fuel injector cleaning etc. etc. etc. But today, I got to change a headgasket on a Grand Marquis(gotta start somewhere) and I have two questions that the mechanic that I work with didnt even know.

1. Why is it that whenever you are putting something back on that requires a torque wrench, they tell you to set the torque to something low like 30 ft-lbs. and then tell you to turn it another, say, 85 to 95 degrees? Why dont they just put in the torque needed in the first place and have you put it in at whatever 30 ft-lbs. plus 85 degrees would be?


2. Why do you have to tighten something, like heads, to torque specs, loosen it, and then tighten it again to torque specs? This is very time consuming and I cant figure out why they make you do something twice.

I am one of those people that likes to know why I am doing something and I just cant figure these two things out. Thanks in advance for the help.
 

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i don't know but for #2 i would venture out and guess it's so that things such as gaskets etc. gets to compress and align themselves (get seated) properly before the final assembly is done?

just a guess but that's what i would think.
 

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weeztones said:
Well since I enjoy working on cars so much, I decided that this summer I would see if I could get paid doing something I enjoy doing anyways. So I got me a job as an assistant to a mechaninc in the service department of a Licoln Mercury and Chrysler dealership. Basically, I just do Oil and Transmission fluid changes, Alignments, Tire Rotations, Fuel injector cleaning etc. etc. etc. But today, I got to change a headgasket on a Grand Marquis(gotta start somewhere) and I have two questions that the mechanic that I work with didnt even know.

1. Why is it that whenever you are putting something back on that requires a torque wrench, they tell you to set the torque to something low like 30 ft-lbs. and then tell you to turn it another, say, 85 to 95 degrees? Why dont they just put in the torque needed in the first place and have you put it in at whatever 30 ft-lbs. plus 85 degrees would be?


2. Why do you have to tighten something, like heads, to torque specs, loosen it, and then tighten it again to torque specs? This is very time consuming and I cant figure out why they make you do something twice.

I am one of those people that likes to know why I am doing something and I just cant figure these two things out. Thanks in advance for the help.
1. when you put something back together and torque it to 30 ft lbs the gasket compresses it is no longer at 30 ft lbs turing the bolt another 1/8 might only add a few inch pounds to the torque but it will be close to the the 30 ft pounds.

2. your compressing the gasket then taking the torque away so the gasket is now compressed to the head. Now retorqing the head the gasket will be at a true torque. It use to be torque the heads get the car hot and then retorque the heads.

Next lesson, head bolts VS studs or torque sequence?
 

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Don't worry after about 2-3 years of wrenching it will be simple...just grab your air wrench...crank it up and ram it home. If it doesn't come back in 6 months to a year...your doing OK! (you think I am kidding) /images/graemlins/supergrin.gif
 

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The answers given are correct, but an additional comment on the extra degrees
of turn added to a torque reading. This is usually on a bolt where the
head of the bolt or a washer has a friction force that interferes with a
true torque reading. I have always had a feeling the extra degrees of turn
was a way of getting a truer torque value - because it takes the friction drag
out of the equation, where friction is highest.

Another words, it's the last 1/2 to 3/4th turn that are the most inaccurate on
the torque sequence. So they take you up to an accurate stopping point,
and by calculation they guide you to a more precise end point.
 

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They are torque to yield bolts.

When your torque a bolt you are stretching it, that is just another way to insure proper stretch.

Some bolts will be specified like that with no lube(oil) and some will say to lube when you torque them. Follow recommended procedure. If in doubt, do it dry.

FYI, I always do rod bolts etc. with a bolt stretch gauge. And I Never use rod, head, main bolts/studs more then once.



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Discussion Starter · #9 ·
Cool. Thanks for the responese. Now, who wants to tell me why dumping exahaust gas back into the combustion chamber LOWERS the temperature inside the combustion chamber? I am talking about EGR valves here. Anybody wanna splain this one?
 

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Different emissions control methods have been used over the past years
to help control pollution. One was pumping outside air into the heads in the
exhaust port - this helped burn the remaining hydrocarbons, and resulted in
an increase in temperature. Whenever you add oxygen - you get a temperature
increase. In the case of pumping a burnt gas(exhaust)into the combustion mix, with little oxygen(less than the air it replaces), you
get a temperature decrease. But you re-burn the exhaust (again)- so you
expel less hydrocarbons.
 

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Discussion Starter · #12 ·
HP said:
Different emissions control methods have been used over the past years
to help control pollution. One was pumping outside air into the heads in the
exhaust port - this helped burn the remaining hydrocarbons, and resulted in
an increase in temperature. Whenever you add oxygen - you get a temperature
increase. In the case of pumping a burnt gas(exhaust)into the combustion mix, with little oxygen(less than the air it replaces), you
get a temperature decrease. But you re-burn the exhaust (again)- so you
expel less hydrocarbons.
Excellent explanation, thanks.
 

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HP said:
Different emissions control methods have been used over the past years
to help control pollution. One was pumping outside air into the heads in the
exhaust port - this helped burn the remaining hydrocarbons, and resulted in
an increase in temperature. Whenever you add oxygen - you get a temperature
increase. In the case of pumping a burnt gas(exhaust)into the combustion mix, with little oxygen(less than the air it replaces), you
get a temperature decrease. But you re-burn the exhaust (again)- so you
expel less hydrocarbons.
OK lets mess him up now. And tell him about the unburnt fuel in the cylinder.
 

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Weez I enjoy answering your questions, But it makes me wonder what kind of jerk you work with, he either doesn't know or isn't doing his job.
 

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HP said:
Different emissions control methods have been used over the past years
to help control pollution. One was pumping outside air into the heads in the
exhaust port - this helped burn the remaining hydrocarbons, and resulted in
an increase in temperature. Whenever you add oxygen - you get a temperature
increase. In the case of pumping a burnt gas(exhaust)into the combustion mix, with little oxygen(less than the air it replaces), you
get a temperature decrease. But you re-burn the exhaust (again)- so you
expel less hydrocarbons.
So, a question for you. How do air injection and EGR affect horsepower?

I would assume that the air injection would not affect an egine's horsepower output since it's only burning the expelled HCs, but that the EGR would potentially reduce HP because it's taking the place of intake oxygen, true?
 

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Discussion Starter · #16 ·
brianflynn said:
Weez I enjoy answering your questions, But it makes me wonder what kind of jerk you work with, he either doesn't know or isn't doing his job.
He's one of those guys that knows what something does, but doesnt always know why. You see what I am saying. He knew what the EGR valve did, but not why.
 

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Matt said:
HP said:
Different emissions control methods have been used over the past years
to help control pollution. One was pumping outside air into the heads in the
exhaust port - this helped burn the remaining hydrocarbons, and resulted in
an increase in temperature. Whenever you add oxygen - you get a temperature
increase. In the case of pumping a burnt gas(exhaust)into the combustion mix, with little oxygen(less than the air it replaces), you
get a temperature decrease. But you re-burn the exhaust (again)- so you
expel less hydrocarbons.
So, a question for you. How do air injection and EGR affect horsepower?

I would assume that the air injection would not affect an egine's horsepower output since it's only burning the expelled HCs, but that the EGR would potentially reduce HP because it's taking the place of intake oxygen, true?
All emission add-ons(except charcoal cannister) - decrease HP and efficiency - at least, all that I can possibly think of. In the case of air injection, with the old air pump,
the main disadvantage was increased heat, but the air inlets in the exhaust
ports interrupted air flow, and in the old days(before good aftermarket replacements) these air inlets were sealed off to make the heads flow better.
EGR decrease performance because they deprive the intake of oxygen. Luckily newer performance cars can pass emission standards with better computers to manage fuel flow, and improved cats, so there is no need for EGR valves and air pumps.
 

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weeztones said:
brianflynn said:
Weez I enjoy answering your questions, But it makes me wonder what kind of jerk you work with, he either doesn't know or isn't doing his job.
He's one of those guys that knows what something does, but doesnt always know why. You see what I am saying. He knew what the EGR valve did, but not why.
Yeah I know what your saying.
 
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