• 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Did you know an exhaust leak can make an engine run lean.
#1
Sort of interesting experience on my end.

a long time ago i installed a Kit exhaust on my car in an effort to save money going to a real muffler shop.

From day one i knew the kit was garbage the fit of the pipes was awful nothing really fit and nothing sealed right. i said screw it and just went ahead with it.

Basically for the next 4 years i was content to poison myself with carbon monoxide every time i took the car out, but i didn't care, for the amount of time i spend inside the car i felt it wasn't worth a proper repair as i was so fed up with the entire project.

So i knew i had pretty much an open down pipe setup. i was constantly fighting a mid throttle lean issue and it forced me to richen up the throttle curve a lot. I would ask people about it and nobody could explain why.

so last year i got more fed up then usual and i bought Band Camps for the exhaust to better seal the pipes. my poison level went down 90% and down the road i went.

about a month ago i noticed my poison level had gone up again and i knew i had to get underneath the car and deal with the exhaust issue. so last week i jacked it up and discovered the reason my poison level was up was because 2 of the band clamps had failed on the exhaust.

screw it i welded up the exhaust once and for all in a fit of anger.

3 days later i went for a spin, OH joy no more exhaust leak. i have to say night and day difference on human health. (:rolls eyes, I'm an idiot). the idle was much better. I also noticed a change in the Unloaded verse Loaded Idle Rpms. before out of gear i would have the engine idling at 1000rpms because dropped into gear it would load the motor and drop it to 650 rpms, now out of gear with the increase back pressure the engine was holding in the 800 rpm range.

then i got on the highway and punched it.... no power.... the car was very hesitant and held back all the power i'm used to. i came home and just did a quick timing check and vacuum check and there was no change.

hummmm,,,,, I googled exhaust leak and making an engine run rich. but nothing would come up except an engine running lean due to the incorrect signal from the Oxygen sensor on a modern car. I suspected the difference in exhaust back pressure was causing the change but i just could not find anything on the subject.

then i came upon a very old article taking about the myth of burnt Valves and exhaust leaks. it was talking about modern cars where an exhaust leak can not cause a burnt valve due to the computer compensating for the lean condition caused by an incorrect oxygen sensor reading in the exhaust on a modern car which in turn riches the intake mixture which is why on a modern car they talk about an exhaust leak effecting your MPG.

then the article went back and said this was the problem on carbureted cars it went on to explain a loss of back pressure cannot be compensated for with a carburetor on the fly so the engine will experience a lean condition because the back pressure causes a reverse flow of air through the carb, and this causes a double dip of fuel going back down into the carb. so what happens is back pressure causes a shock of reverse air signal back through the intake causing the carburetor to release more fuel. it acts like 2 shots of gas at the same time going into the intake. Basically the back pressure causes the motor to run richer and it was a bad design that was taken advantage of by engineers.

It went on to say this was why when people changed to open headers or increased the size of the exhaust pipe over stock suddenly people were burning valves inside the motor and it would cause a lean condition that people would compensate for by using a larger carb or jetting up.

Oh very interesting,,,, i ran out to my car and changed out my accelerator shooter size from .37 to .31 went for a spin and the power was back.

so here as an extreme case where a change or repair to an exhaust completely changed the behavior of a motor because of something nobody ever heard of.

i can say i have a much better understanding of back pressure now and how it effects, Idle, engine vacuum, air/fuel mixture.

I called up a buddy of mine that runs 3" pipes and headers and has had years of mid throttle problems like i did. like me was constantly tuning the motor all the time. I explained what i had learned and he decided to remove his 3" lines and go back to 2.25". a few days ago he called me back he had a reducer made up for his header and bolted it on and then he threw an old 2.25" setup he had and welded it to the new reducer and went for a spin, his mid throttle issues were gone and he had to jet down the carb and adjust the accelerator pump circuit just like i had to.

he told me it was night and day and the car ran much better.

So just something to think about bigger is not always better, and seriously fix any exhaust leaks you may have.

maybe professional mechanics know this stuff but novices are rediscovering things all the time.

one other thing from the article i found interesting. It spoke about how when people increased the exhaust pipe diameter, and decreased the back pressure how there was a surge in power that people felt. the main reason for this was due to the sudden leaning of the intake mixture, if the car was overly rich from the factory then this sudden lean out would of made the car have more power the problem was if nobody doubled checked how lean or you didn't retune things then it would lead to burnt valves.

lastly the article spoke about reduced back pressure also flowing too much air through the carb contributing to the leaning condition. its a very interesting fine balance of air verse fuel.
  Reply
#2
Man, that's good info! My dad had a larger exhaust put on the Mustang so I'll have to take a close look at that.

Steve



[Image: 25yvyp3.jpg]
  Reply
#3
I have found my headers leaking at # 3&4 cylinders. Am going to change the gaskets out and see if I have an improvement in power. Thanks for the info.
Jim

drive it like you stole it
  Reply
#4
Very interesting, thanks for the information.
I am presently considering which way to go on the exhaust system. The questions being should I spend the add'l $$$ and have a real muffler shop bend a system (around $900) or try to order a system (around $500)and install it myself and have to fab a piece or two myself because of the exhaust manifolds I am using on my 460. Any suggections ?
Any help would be appreciated.
Thanks, Jay
  Reply
#5
Personally if I had to do it again, I would just spend the extra and have a muffler shop fabricate a system.

The kit exhausts are terrible, you will need a pipe expander because they take no care when they bend the pipes so you may get a pipe squished on the end. The bends are not consistent the left side might fit fine while the right side won't clear everything properly. I went through 2 kit exhausts the first one was so bad as far as bends and matching clearance under the car I returned it the second one was slightly better junk.

After having to go back and constantly tring to repair it and make it better what sense did it make I could of spent more upfront and been done with it.
  Reply
#6
That is some great info. Told by an older engine builder in the late 70's never go over 2.25 unless it was a BB looking for the 6-7k range. And then was told to never go over 2.75. My BB is running 2.5 with a 780 duel feed and never misses a beat.

[Image: 2rr7aiv.png]

Just cruising along minding our own business when BAM!!! The LAWS show up.
  Reply
#7
Yes I did. Smile

'Mike'
73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

Pics of modifications included in:
  Reply
#8
Years ago I had a header gasket leak on one cylinder and went to change the plugs, all old plugs were a dark tan color except the one with the header leak.
I was extremely white in color, and was running very lean on that plug.

[Image: mustangnight010.jpg]
1972 Mustang Convertible 351C 4V
1966 Ford Galaxie 7 litre-4speed

Jorge
  Reply
#9
clevelandcoupe;84308 Wrote:Very interesting, thanks for the information.
I am presently considering which way to go on the exhaust system. The questions being should I spend the add'l $$$ and have a real muffler shop bend a system (around $900) or try to order a system (around $500)and install it myself and have to fab a piece or two myself because of the exhaust manifolds I am using on my 460. Any suggections ?
Any help would be appreciated.
Thanks, Jay
Jay as other have said..Have an exhaust shop do the fab install....I had a guy who specializes in exhaust systems do mine on the Camaro x pipe & all 1100.00 Best money I spent car runs a thousand times better..I wouldn't do a system without an x pipe...
http://www.youtube.com/watch?v=WerMlnKnHgU

LOVE OF BEAUTY IS TASTE..THE CREATION OF BEAUTY IS ART
  Reply
#10
I need to convert to an x pipe or at a minimum a crossover pipe.

Stage 8 header bolts work very well at keeping the bolts tight to help assure a good seal. Not super cheap, kind of a pain to install/remove but worth the effort.

'Mike'
73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

Pics of modifications included in:
  Reply
Share Thread:  


Possibly Related Threads...
Thread Author Replies Views Last Post
  2 1/2 "Exhaust Cainsable 8 190 Yesterday, 05:10 PM
Last Post: Mikes73
  351C M-Code engine Timing issue! Meyer 25 451 06-17-2018, 08:41 AM
Last Post: Stanglover
  Engine buffs: why do our engines need to warm up? Vinnie 20 503 06-03-2018, 03:58 PM
Last Post: Don C
  Any good engine builders in my area? 73' mach 1 7 315 05-21-2018, 11:51 AM
Last Post: 73' mach 1
  Performance Part 302 engine Cresee 7 331 05-12-2018, 06:12 PM
Last Post: Ed Raver
  Can FMX be removed from car without dismounting exhaust? Spechti 4 288 05-11-2018, 11:12 AM
Last Post: Spechti
  Head and Exhaust Confusion digithead 14 455 04-20-2018, 07:47 AM
Last Post: Hemikiller
  Engine mount orientation CalgaryChris 34 1,127 04-06-2018, 05:05 AM
Last Post: EBSTANG



Users browsing this thread: 1 Guest(s)