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Discussion Starter · #1 · (Edited)
how tight is the correlation between fuel economy and "reading" plugs?

hey guys...been tuning as best as i know how to do...

have the plugs to where i am thinking they are still a tad rich but i would rather be a tad rich than lean...

original prosystems hp1000 setup:
primaries are 84
secondaries are 92
idle air are 76
idle feed are 35

it was bucking alot below 1800 and i changed the IABs to 59 (per patrick) which made a huge difference
plugs were always black before...they seem better now with the 59 IABs...(would like to understand that..i really thought smaller IABS would make it richer and the plugs would be more sooty)
ran it a good bit tonight aroun15-1600 and the plugs look better



timing is via msd digital e curve
set to 38* total electronic mechanical all in by 2500 as per below
Initial 18*
Vacuum Advance 15* @ 10"
Mechanical Advance 20* all in by 2500 rpms
actual timing 43* at idle (w/ fulll 15* of vacuum advance) 47* at 2500+ rpms limited to 9* vacuum advance here)
but timing will not exceed 47* at cruise

it is very nice and smooth here

so here is the question
i am getting about 6.5 MPG with easy around town regular driving w/ a/c on....

if the plugs are looking like i am in the correct mixture range, will any further adjustments/ different carbs yield better economy??

if so why, how?...
what is the correlation to okay looking plugs and maximun fuel efficiency??

looking to learn so open to all comments & enlightenment

TIA
 

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Re: how tight is the correlation between fuel economy and "reading" pl

Insulator looks rich to me up top... It should be whiter up top and down in the bottom of the valley it should be more that color... For gas mileage, Concentrate on the idle circuit and mains... You're on the idle ciruit alot, so, if you can lean that out, it would greatly help your gas mileage... If you are happy with WOT, then, reduce the main jets and you can up the secondary jets to compensate since all circuits are all in at WOT... Just do not go too far of a spread... By doing the above, I increased my mpg by 30%. I have a 750 dp on my Chevelle...
 

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Re: how tight is the correlation between fuel economy and "reading" pl

Oh yeah, also timing... My cam needs lots of initial timing, comp cams 292h. When I was running 19 deg. of timing, I did not like the off idle response which made me gas it more.... So, I reset my pro-billet MSD to 14 deg. advance, special bushing, and set my base timing at 22 degrees.... this made everything more "crisp", reduced smell, burned the gas better at idle.... which allows you to reduce the idle mixture and get better throttle response... So, check what your cam wants in terms of timing...
 

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Re: how tight is the correlation between fuel economy and "reading" pl

Oh yeah again, I also went to an adjustable vacuum advance... the accel one... At cruise, you can add more timing depending on your compression... I am running 52 total deg. at cruise and the motor likes it... Of course, the biggest one contributer to my gas mileage is the TKO 600 5 speed... I drive alot of fwy and at 80 mph, I'm at around 2100 rpm...
 

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Re: how tight is the correlation between fuel economy and "reading" pl

6!!!! Hey Doc, I gotta get you setup wth a tripower so you can drive that thing! Ouch!
 

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Re: how tight is the correlation between fuel economy and "reading" pl

Oh yeah again, I also went to an adjustable vacuum advance... the accel one... At cruise, you can add more timing depending on your compression... I am running 52 total deg. at cruise and the motor likes it... Of course, the biggest one contributer to my gas mileage is the TKO 600 5 speed... I drive alot of fwy and at 80 mph, I'm at around 2100 rpm...
The Accel fits in the pro billet dizzy?

Also, where did ya get the custom bushings for it?

Thanks

Chris
 

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Re: how tight is the correlation between fuel economy and "reading" pl

The Accel fits in the pro billet dizzy?

Also, where did ya get the custom bushings for it?

Thanks

Chris
Yes, the accel adjustable vacuum advance fits the pro billet distributor... You have to get the HEI adjustable vacuum advance... The standard vacuum advance for a non-HEI distributor do not fit... They look the same, but, they are "short"... I am running about 6 turns out on the accel which gives me about 14 degrees of advance at cruise... Note: You also can install the adjustable vaccuum advance without pulling the distributor, if you grind the "hook" on the vacuum advance 1/16", you can wedge it in there.... Note2: With my comp cam 292h, the accel will not give me advance at idle, only at cruise, ported or not ported vacuum... Anyway, I did not buy it for that because I am running 22 deg. of base timing, but, I thought I should mention that....
I ordered the bushing kit from 4secondsflat.com... You get 2 bushings a 14 deg. and a 10 deg... You have to pull the distributor to get the bushing in... I thought I might be able to do it without pulling the distributor, but, honestly, there is no way, it is just too hard to get in there... I am currently running the 14 deg. bushing, but, I am thinking to go to the 10 degree and run 26 deg. of base timing... TBC... You have to call 4secondsflat, the guy is cool..., ask him to send you his tuning guide also, it is really good info on tuning a carb... :beers:
 

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Re: how tight is the correlation between fuel economy and "reading" pl

Yes, the accel adjustable vacuum advance fits the pro billet distributor... You have to get the HEI adjustable vacuum advance... The standard vacuum advance for a non-HEI distributor do not fit... They look the same, but, they are "short"... I am running about 6 turns out on the accel which gives me about 14 degrees of advance at cruise... Note: You also can install the adjustable vaccuum advance without pulling the distributor, if you grind the "hook" on the vacuum advance 1/16", you can wedge it in there.... Note2: With my comp cam 292h, the accel will not give me advance at idle, only at cruise, ported or not ported vacuum... Anyway, I did not buy it for that because I am running 22 deg. of base timing, but, I thought I should mention that....
I ordered the bushing kit from 4secondsflat.com... You get 2 bushings a 14 deg. and a 10 deg... You have to pull the distributor to get the bushing in... I thought I might be able to do it without pulling the distributor, but, honestly, there is no way, it is just too hard to get in there... I am currently running the 14 deg. bushing, but, I am thinking to go to the 10 degree and run 26 deg. of base timing... TBC... You have to call 4secondsflat, the guy is cool..., ask him to send you his tuning guide also, it is really good info on tuning a carb... :beers:
Also, if you change the bushing, make sure to increase your springs... since it is advancing less, the advance will be "all in" way too soon with your existing "18 deg bushing" springs... With the 14 deg. bushing, I am running the "silver heavy" and "silver light" springs... They seem about right...
 

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Discussion Starter · #10 ·
Re: how tight is the correlation between fuel economy and "reading" pl

496 10.5:1
afr 290 heads
carb specs in first post
cam is:
.391/.355 lobe lift
.6647/.6035 Valve Lift
301/306 adv.
244/248 @ .050
164/163 @ .200
106/118 Centers ICL/ECL
112 LSA

dist is digital e curve so i can set it to anything i want easily

what do you guys recommend that i try??
 

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Re: how tight is the correlation between fuel economy and "reading" pl

496 10.5:1
afr 290 heads
carb specs in first post
cam is:
.391/.355 lobe lift
.6647/.6035 Valve Lift
301/306 adv.
244/248 @ .050
164/163 @ .200
106/118 Centers ICL/ECL
112 LSA

dist is digital e curve so i can set it to anything i want easily

what do you guys recommend that i try??

You could check over/call at AFR's site to see what those heads like.

I know on small blocks they usually like 18/20 int to 36/38 total.

Track testing told me 37deg total on my sb.
 

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Re: how tight is the correlation between fuel economy and "reading" pl

hey guys...been tuning as best as i know how to do...

have the plugs to where i am thinking they are still a tad rich but i would rather be a tad rich than lean...
Rich, I have had good luck dialing back the pri jet and bumping up the PV opening rate. It may also require a small tweak of the pump. Everything you need to do will be on the primary side, other than to ensure there is no fuel pullover on the sec side.

At less than 3000 rpms, the MSD could mask a rich condition?



The timing seems spot on, if not maybe too much. Have you checked different settings? 35 or 41??
 

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Discussion Starter · #13 ·
Re: how tight is the correlation between fuel economy and "reading" pl

gene
when i first got it i set timing at 35* to be sure that i was not to far advanced.
then went to 37*.
then to 38* which is where mike told me it should be and where he did the dyno run he sent me.
ran fine and nice and cool at all these total timings so how do i differentiate and pick the best?

i cant say that i really feel/can tell a difference.

but generally i dont really drive it hard anyway.

i tend to cruise and get on it occasionally up to the speed limit.
i prefer not to smoke the tires or let then squeal for mercy because it is a highly obvious car in this area that gets alot of attention.

couple years ago i was only keeping up with traffic and i got pulled out of a long line of cars and got the speeding ticket for the bunch of them.
 

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Re: how tight is the correlation between fuel economy and "reading" pl

"Reading" plugs is pretty subjective, and only a backward looking static snapshot of one plug at a time, as you pull & replace them.


Whereas, a WB Air:Fuel Ratio gauge is actual, objective, dynamic and real-time, giving an average of 4 plugs-per-WB. :thumbsup:
 
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