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Gas Mileage DECREASE!?!?

80Vette

Well-known member
Joined
Jul 3, 2006
Messages
63
Location
Portland OR
Corvette
1980 pearl white vette
Hi there, I have a 1980 with a remanufactured 350 AT.
It has less than 5k miles on it.
When it had the original factory exhaust, carb & intake I was getting around 17 MPG (avg hwy/ city). The car however failed emissions.
But, I've since put the new Edelbrock Performer Intake, Edelbrock performer 650 carb & brand new 3" exhaust all the way to the flowmaster 40's. The car passed emissions spectacularily.
The downside...I am now only getting 10-12 MPG.

What in the world could be causing this? Or, is this more enviromentally friendly engine getting worse gas mileage?
Are the new gadgets supposed to DECREASE MPG so greatly?
 
Joined
Apr 29, 2001
Messages
2,141
Location
Rio Rancho, NM
Corvette
1981 HD Suspension; ZN1 Option
80Vette,

Ignition timing is an important part in balancing the emissions readings. Generally speaking, more timing will yield higher emission by-products, but it will also improve performance which in turn could mean better gas mileage.

But the first step is for you to make sure that everything is working as it should. Check the spark plugs, ignition wires, fuel/air, idle settings, timing and the like. If everything is good :ugh , then perhaps you can go on to the next step. Just remember that a mal-adjusted brake could effect your mileage.

Next, since you passed your test already, record the emission-friendly conditions now present in the engine (i.e. vacuum signal, initial timing, vacuum canister setting, spark plug gap, and the like). Then, change one setting at-a-time, and see what kind of improvements (or losses) you gain.

Keep a log of every change made, and change only one parameter at-a-time.

Surely, this is the old fashion way and it will take you a while to see what gains you could attain, but it is only one way it could be approached.

The fast track way is with the use of a dyno and a wide-band sensor. The same applies here, though. One-change-at-a-time.

There may be other ways to go about it, so perhaps some other members will chime-in.

GerryLP:cool
 

80Vette

Well-known member
Joined
Jul 3, 2006
Messages
63
Location
Portland OR
Corvette
1980 pearl white vette
MPG

Thanks Gerry, funny thing...I had the work done by the corvette doctor here in Portland. Once the engine work was done, he directed me to then get the new 3" exhaust put on THEN, once that was completed, I returned back to his shop to then "tune" then engine (carb-timing etc). Before I returned the car back to get "tuned" it was still getting decent MPG.

Only after the new-exhaust tuning did the MPG get worse. I recall him saying something about making it more rich and that my HC's (hydro carbons) would be too high to pass emissions again but, it would be good for performance.


I'm wondering now what exactly did he do during the after new exhaust "tuning?"
It only took about 5 minutes, we had to let the catalytics cool to get an accurate reading. (whatever in the h#ll that means :) )
 

AKRAY4PLAY

Well-known member
Joined
Oct 14, 2001
Messages
584
Location
Wasilla, Alaska, U.S.A.
Corvette
'77 L-82, black on black. Full mod 406 small block
it sounds like your mechanic is using an oxygen sensor to set the idle air/fuel mixture. which is a very good way to tune and most likely the reason for letting the cats cool. cats mess with 02 sensor readings, usually giving a faulse lean reading. the colder the cat, the less they effect the readings. unfortunately, the colder the cat, the colder the engine. if your engine is not up to temp, adjusting the idle mixture will yield a rich setting for when the engine is warm. this clould be the very reason your mileage is way down. also, swapping from the stock Q-jet to an Edelbrock will hurt mileage at cruise speeds. the Edelbrock is a square bore carb, all four ports are the same size. the Q-jet is a spread bore, primary ports are small and secondaries are large. the square bores give a better linear fuel curve and more even distribution to all cylinders whick is better in the high performance world. but most street applications do not really benefit from this since they spend most of thier time near idle rpm. the advantage on the street goes to spread bore. the smaller primaries give better throttle responce and atomization at low rpm which improove mileage and streetability. bottom line, try leaning out your idle mixture and checking mileage again. first, as Gerry has mentioned, record your settings before adjusting. with the Edelbrock, the 2 idle mixture screws are the big ones on the front (headlight side) of the carb near the baseplate. it takes a regular screwdriver to adjust them. turn them in counting the turns until lightly set, then back out an extra 1/2 turn for starters. also have your engine up to full operating temp before playing with the adjustments! happy tinkering! Brian.
 

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