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gutted cat , timing?
Topic Started: Feb 23 2010, 01:09 AM (1,099 Views)
dnantha
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[ *  * ]
Did I read somewhere , maybe Johnny M, saying that the timing could be advanced a little to compensate for the less back pressure. This has got to be the lamest post Ive made so far, but its sticking in my brains and I need the help. It ran well and hasnt thrown a code yet (we'll see). Seemed like it didnt have quite as much torque on the hills though..Ill play around with it some more tomorrow....Dan out.
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Bad Bent
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Facetious Educated Donkey

I don't recall any compensating for lack of back pressure. Less back pressure seems to run a little easier, for moi. The advance timing is usually suggested to get a little cleaner burn to reduce carbon build up. May add some pep to the throttle and some "power." B-)
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mwebb
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FOG

with a stock 10G engine
my test results show there is no benefit to advancing base timing to more that
8 degrees BTDC

in fact advancing the base ignition timing past that reduces fuel economy
on my special car
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dnantha
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Yes, I agree I played with the timing today driving up and down the same steep hills. My best performance was at 8% timing. I had a completle shot muffler on it when I ran my first tank of fuel on my just rebiult motor, and I got 53mpg. I had a new muffler put on it and gutted the cat yesterday and I now have to shift down on hills I could do in 5th yesterday morning. I sort of miss the exaust note too. Maybe I should swap the muffler for a glass pack....Any thoughts?
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Bad Bent
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Facetious Educated Donkey

Well, since you asked... BadBent's Loud Geo Under Load B-)
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mwebb
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FOG

someone , started a thread regarding header design for economy NOT performance
at first i thought there was little to be gained there ...

i was wrong
i have been studying theory on that topic .

it turns out that :

the OEM exhaust has pipes about 42mm OD and the front exhaust pipe in front of the converter is double walled , to insulate against heat loss between the cat converter and manifold .

there is a resonator after the cat which is tuned to the exhaust to maximize economy and power out put .

larger diameter exhaust pipes are detrimental to fuel economy AND power output .

anywhere off of the race track or except where the engine is used at MAX output more often than any other load .

the exact location of the resonator in the exhaust matters and removing the resonator is detrimental to both economy and power output .

there is a thread running in ecomodder about this as well , i would go there and study
FIRST
before fraking with your exhaust .

there are two sets of pulses in the exhaust ,
one set of pulses flows at the speed of sound which the resonator is used to maximize the flow of ,
and
the other is the actual slug of exhaust gas , equal to the volume of the engine displacement divided by the # of cylinders , which in a perfect world should be traveling at 240 to 300 fps INSIDE the exhaust pipes AT the most efficient load and RPM that the exhaust is designed for
the velocity of the gases in the pipe is determined by exhaust pipe diameter AND engine speed and load .

the volume of one primary tube on a header system should be equal to the volume of the exhaust gas slug described above ,
the diameter of the primary pipe should provide a gas slug velocity of between 240 and 300 fps ,
AT the load and RPM the system is designed to be the most efficient at .
i have the formulas as posted in ecomoder
the cat converter would be the collector

======================

so bottom line
to maximize fuel economy OR performance , there is / are exhaust design rules that should be followed - if they are NOT followed
you will make the engine sound different , maybe good maybe bad , depending on your own tastes .
but
you will frak up
max power out put AND fuel economy - a loose loose situation

Edited by mwebb, Feb 23 2010, 11:41 PM.
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metroschultz
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Please just call me; "Schultz"

:gp
:+1
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dnantha
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Mwebb, I appreciate your technical response. I dont understand it all, but it is proven by my experience. When I gutted my Cat yesterday and with the bad muffler. I drove the 15 miles to town to get one installed. It seemed to me on that drive that I had less power on the many hills between here and town. Ater installing a non OEM muffler I have even less power on the hills. I think I will play with the exaust for a bit as I just like playing with things. But I will probably put it all back on straight again soon. Thanks for the help....Dan out.
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Potter
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Col. Potter

well when my exaust got riped off and it was dumping out from behind the cat i did notice a signifacant diffrance, i was able to chirp in 3rd gear while doing a burnout (wasnt able to do it before) and i gutted the cat while i was bracket raceing and it made the car .05 faster (granted that cat was probably pluged afer dad driveing it 40 miles with a droped valve.... that was the first engine) also Schultz moved my ingition timeing from stock to...10 btdc i belive it was? and that took me from 48 mpg to 51.8? wasn't it Schultz on the Geopalooza run? all in all i belive it works... granted im not getting those numbers now since i have a bad valve and a stickin lifter.
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