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Plug Gap, RPM's and fuel consumption questions...

widewhites

Active member
Joined
Jan 16, 2002
Messages
34
Location
Austin, Texas
Corvette
1979
I have a some of questions on spark plug gaps, RPM’s and fuel consumption. I gapped my plugs (R45TS) to the correct gap-.45 (running HEI). After researching, and realizing that the gap was previously at .35, I regapped. Timed the engine everything is good. BUT, my RPM’s are now way off, before I would hit 3000 RPM’s when I got to 60 mph and fuel consumption was okay for your average 350 Chevy engine gulp. Now I hit 50 mph and am at 3000 RPM’s and forget mileage. Gas is going through the roof. You can actually see the gauge move to empty when you are driving 60 – 65 mph (35-3700 RPM’s) . The response if good, better than before but is this normal for the RPM’s to increase due to the increased plug gap. I have the older Torker package with a 650 carb, an automatic trans. The reason I regapped was because I was having to go through plugs every 5 months. Any suggestions, comments??? I have run .35 gap for about 4 years.

I was also always asked why I would hit 3000 RPM’s at 60 MPH. I always thought that was normal……….

Thanks guys!!!!!!!!!
 

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
your plug gap determines the length and heat of the spark. the bigger the gap the better burn you get in the combustion chamber. i have seen hei ignitions run with .090 plug gap. the spark was weak but it ran. you have a trade off between plug gap and coil life. the bigger the gap the shorter the coil lives. the smaller the gap the hotter you spark. .045 is a good compromise for all around applications. your timing may be what is causing the poor mileage. did you set the timing with a hot engine and the vacuum advance disconnected and plugged?? timing effects vacuum! if the carb was tuned for a certain vacuum and a timing adjustment was made, then you may have to retune the carb. your plug gap can also effect vacuum, but nothing you will notice unless your engine is running on the ragged edge of not being tuned. example, if you are running rich the motor will load up and not want to pull as much fuel/air mixture. then you open your plug gap and get a better burn, and the engine clears out easier resulting in better vacuum. a two inch change in vacuum will effect your jetting. i hope i did not confuse you. these are some of the things you need to look at when tuning. a vacuum guage is your friend!!!! best of luck, brian
 
Joined
Jan 1, 2002
Messages
7,246
Location
Washington, Michigan
Corvette
'67 Marina Blue Convertible
3700 @ 65mph? That's a 4.56 axle. Spark plug gap will have no effect whatsoever on rpm at a given speed. The only variable your car has that could change rpm vs. a given speed is your torque converter.
 
F

fc3

Guest
?

John, How do you know the ratio? My '72 is between 2700-2800 at 70 mph. I do not know what my rear end ratio is...FC.
 
F

fc3

Guest
Car is in for AC/Heater redo...BIG $$$- I think 15 inch 70 series. 400 Turbo trans.
 

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