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Discussion Starter · #1 · (Edited)
I have been toying with the idea of using an O2 sensor to help me get my jetting, vacuum advance, and distributor curve dialed in. I installed an O2 sensor into the side of my gutted cast iron heat riser valve, connected a digital voltmeter, and took it for a spin. My first run indicated my cruising air/fuel ratio was around 13 to 1 using #72 primary jets so I came back home and changed my primary jets to #70's. I took it for another spin and it indicated my cruising air/fuel ratio was now 14 to 14.2 to 1. I also found my full power air/fuel ratio was around 12.5 to 1 so I drilled my secondary metering block .003" larger. Then to increase my vacuum advance's total I lengthened the slot another 1/4" and now it'll produce 16 degrees of advance; 4 more than the 12 degrees it had.

So yes, you CAN use an O2 sensor to help get your fuel mixtures where you want them to be. Now that I've found my fuel mixture changed dramatically with a simple jet change I'm now in the process of adjusting my vacuum advance and distributor curves to obtain a more aggressive advance rate. With some further tuning I'm hoping to achieve a 14.7 to 1 air/fuel cruising ratio.











 

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Just think of the free horsepower you gained not to mention the increase in MPG. One thing I found odd though, you increased the primary jet size from 0.070" to 0.072" and the A/F mixture got leaner. I would have expected the A/F to get richer. :doh:
 

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Discussion Starter · #3 · (Edited)
Just think of the free horsepower you gained not to mention the increase in MPG. One thing I found odd though, you increased the primary jet size from 0.070" to 0.072" and the A/F mixture got leaner. I would have expected the A/F to get richer. :doh:

Read my post one more time. My air/fuel ratio at cruising was around 13 to 1 with the #72 jets then changed to a 14 to 14.2 to 1 with the #70 jets. So the smaller #70's leaned it a full point. Now I'm playing with my advance curves with the hope I can achieve a perfect 14.7 to 1 at my usual cruising speeds. I found my gutted heat riser valve was a perfect place to put the sensor into as it's so close to the heat source and well protected...............





The air/fuel ratio changes to reflect every throttle movement and grade change and it changes almost instantly like the reading on a vacuum gauge. When I did my initial testing yesterday I was on a short stretch of level road but I did have the chance to observe the output voltage when I was climbing a few hills when I was coming back home. The sensor cost me $17 and the 18mm tap cost $12 so my total expense was around $30 and installation time was about an hour.

 

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Discussion Starter · #6 ·
maybe i'm confused or this is a stupid question but the sensor in the exhaust gives an accurate reading of the air/fuel mix prior to ignition?

No, the O2 sensor tells you how complete the combustion was. High voltages like .9 indicates a rich mixture and a lower voltage of around .45 indicates a perfect 14.7 to 1 mixture. I found my vacuum advance wasn't needed at all so now I'm just running the initial 10 degrees and my HEI's 25 degrees for a total of 35 degrees above 2800 rpm. So as I'm not running my vacuum advance any more I cut it off flush with the side of my HEI's housing (but leaving the inside part of it intact to keep the breaker plate anchored).



 

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Discussion Starter · #7 ·
Getting the air/fuel mixture lit off is only half the battle as it's equally important to get it lit off at the right time to get the most heat energy out of the fuel.

An air/fuel ratio burns completely when it's at a ratio of 14.7 parts air to 1 part fuel by weight. And the more complete the burn is the hotter the exhaust temperature will be for that amount of fuel.

So an O2 sensor merely reads the temperature of the exhaust to determine what the air/fuel ratio was after the burn is completed. A 12 to 1 mixture needed for maximum power will give a higher voltage reading like a .9 to 1.0. And a 14.7 to 1 mixture needed for the best fuel mileage will give a lower voltage reading like a 4.5 to 5. Seems backwards but that's how they work.

Originally I was running #72 primary jets and about 50 degrees of total advance at my usual 60 mph cruising speed and getting a voltage reading of around 9.5 volts. By putting #70 jets in my cruising voltage dropped into the high 7's to low 8's and by pulling my advance back about 25 degrees my voltage fell to around 4.5 to 5.25 volts.

So the amount of timing plays a very critical role in how complete the burn is and an O2 sensor will quickly show you what's happening inside your cylinders. For the miniscule $30 I paid for the O2 sensor and the 18mm tap this has been a real learning experience for me and I got a hefty increase in my fuel mileage too!
 

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No, the O2 sensor tells you how complete the combustion was. High voltages like .9 indicates a rich mixture and a lower voltage of around .45 indicates a perfect 14.7 to 1 mixture. I found my vacuum advance wasn't needed at all so now I'm just running the initial 10 degrees and my HEI's 25 degrees for a total of 35 degrees above 2800 rpm. So as I'm not running my vacuum advance any more I cut it off flush with the side of my HEI's housing (but leaving the inside part of it intact to keep the breaker plate anchored).



You of all people arnt running a vaccum advance??????? Lol. What are you running for the air fuel ratio at wot. I found that 12.3 was the best when i had the l48 built up some.
 

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Discussion Starter · #9 · (Edited)
So far I have only given it one brief full throttle and my O2 sensor indicated a 12.3 to 12.4 air/fuel ratio. I drilled the two holes in my secondary metering plate .003" larger and I'll be running another test today if it doesn't rain (any time the roads are wet you do NOT give big blocks full power).

This has been a real eye opening experience for me as I had always assumed fuel mileage would go out the window if the vacuum advance wasn't working because the total advance wouldn't be anywhere near the 50-52 degrees recommended by the "experts". After thinking about it more I couldn't see how it was possible to run 50-52 degrees of advance in an engine only spinning 2000-2200 rpm and get a consistent burn. At an advance of 52 degrees the piston would be less than 3/4 the way up when compression pressures were still very low.

The moment I disconnected my vacuum advance (and lost 16 degrees) I felt an immediate increase in power as my air/fuel mixture was getting lit off when compression pressures were much higher. I can certainly see how too much advance can be detrimental to consistent ignition as there's a certain point in which there's simply not enough cylinder pressure present to support combustion. The 50-52 degrees may have been needed back in the 50's when engines were spinning 3000 rpm at 60 mph but for those of us running overdrives and spinning our engines at 1500-2000 rpm the 50-52 degrees is way too much (as my O2 sensor indicated).

I'll keep you guys posted on my progress. My goal is to achieve 18 mpg at 65-70 mph cruising speeds and so far I think my goal is obtainable with some further tinkering.
 

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Discussion Starter · #10 ·
50-52 Degrees Of Total Advance?

I have always questioned the 50-52 degrees of total advance recommended by the "experts". For an engine spinning REAL fast I can certainly understand the need to light the mixture off real early to ensure complete combustion. But for today's engines (using high gears and overdrive transmissions) and spinning only 1500-1800 rpm at cruising speeds that 50-52 degrees seemed excessive.

I have been running the recommended 50-52 degrees ever since I got my 454" running several years ago and assumed all was well until I installed my O2 sensor a few days ago. Right away I found my cruising mixture was way too rich as my new O2 sensor was indicating a 13 to 1 air/fuel mixture. By installing primary jets two sizes smaller my O2 sensor indicated more complete combustion which prompted me to pursue the air/fuel ratio and timing issues even further.

The next thing I did was re-curve my HEI to produce a quicker advance and my O2 sensor indicated that was the wrong thing to do as my exhaust temperature increased (indicating some combustion taking place in the hedder pipe). So on my way to Fresno yesterday I disconnected my vacuum advance (losing 16 degrees) and instantly felt a power increase and my O2 sensor indicated a substantial decrease in my exhaust temperature.

The moment I got back home I re-curved my HEI back to where it had been (full advance by 2800 rpm) and removed my vacuum advance as I could see it wasn't needed. Running the initial 10 degree advance and the HEI's 25 degrees of mechanical advance I headed back out and found I had achieved the 14.7 ratio I had been wanting. I haven't checked it yet but I would guess my total advance at cruising speeds is now around 28-30 degrees; far less than the 50-52 degrees I have been running.

With my O2 sensor now indicating near-perfect combustion at cruising speeds I'll check my fuel mileage again and I expect it'll be the best it has ever been; hopefully 18 mpg at 65-70 mph.
 

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I have always questioned the 50-52 degrees of total advance recommended by the "experts". For an engine spinning REAL fast I can certainly understand the need to light the mixture off real early to ensure complete combustion. But for today's engines (using high gears and overdrive transmissions) and spinning only 1500-1800 rpm at cruising speeds that 50-52 degrees seemed excessive.

I have been running the recommended 50-52 degrees ever since I got my 454" running several years ago and assumed all was well until I installed my O2 sensor a few days ago. Right away I found my cruising mixture was way too rich as my new O2 sensor was indicating a 13 to 1 air/fuel mixture. By installing primary jets two sizes smaller my O2 sensor indicated more complete combustion which prompted me to pursue the air/fuel ratio and timing issues even further.

The next thing I did was re-curve my HEI to produce a quicker advance and my O2 sensor indicated that was the wrong thing to do as my exhaust temperature increased (indicating some combustion taking place in the hedder pipe). So on my way to Fresno yesterday I disconnected my vacuum advance (losing 16 degrees) and instantly felt a power increase and my O2 sensor indicated a substantial decrease in my exhaust temperature.

The moment I got back home I re-curved my HEI back to where it had been (full advance by 2800 rpm) and removed my vacuum advance as I could see it wasn't needed. Running the initial 10 degree advance and the HEI's 25 degrees of mechanical advance I headed back out and found I had achieved the 14.7 ratio I had been wanting. I haven't checked it yet but I would guess my total advance at cruising speeds is now around 28-30 degrees; far less than the 50-52 degrees I have been running.

With my O2 sensor now indicating near-perfect combustion at cruising speeds I'll check my fuel mileage again and I expect it'll be the best it has ever been; hopefully 18 mpg at 65-70 mph.
With my timing ive found both light springs gives me the best performance.
 

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Discussion Starter · #12 · (Edited)
With my timing ive found both light springs gives me the best performance.

I ordered a spring/weight kit for my Proform HEI then found it already had light springs and heavy weights in it. Then I found it produced 25 degrees of advance at 2800 rpm which is just about perfect. By putting in the two lightest springs (from the kit) I found it started advancing at only 500 rpm and when using only one light spring it started advancing around 1100 rpm. So in the end I put it's original springs back into it as they gave me the desired curve for my engine/transmission/rear gear combination.

Your full mechanical advance should come in somewhere above your usual cruising rpm. With my 3.70's my 60 mph cruising rpm is about 2050 rpm so the 2800 rpm of my HEI is perfect.

As you're also running a TH700R4 I suggest disconnecting your vacuum advance like I did and just run it's initial and mechanical advance. You'll quickly notice a big difference as you can feel the individual power impulses from your cylinders lighting off when they should be lighting off.

Had I not installed my O2 sensor I never would have known I had an ignition problem. But in my pursuit of better fuel mileage I decided to install an O2 sensor to help me get my jetting and timing fine tuned. Mighty glad I did as my fuel mileage has already taken a healthy spike. I'm planning on taking a trip to Pismo Beach (about 175 miles away) by the end of this month and I'm hoping to see 18 mpg.
 

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I ordered a spring/weight kit for my Proform HEI then found it already had light springs and heavy weights in it. Then I found it produced 25 degrees of advance at 2800 rpm which is just about perfect. By putting in the two lightest springs (from the kit) I found it started advancing at only 500 rpm and when using only one light spring it started advancing around 1100 rpm. So in the end I put it's original springs back into it as they gave me the desired curve for my engine/transmission/rear gear combination.

Your full mechanical advance should come in somewhere above your usual cruising rpm. With my 3.70's my 60 mph cruising rpm is about 2050 rpm so the 2800 rpm of my HEI is perfect.

As you're also running a TH700R4 I suggest disconnecting your vacuum advance like I did and just run it's initial and mechanical advance. You'll quickly notice a big difference as you can feel the individual power impulses from your cylinders lighting off when they should be lighting off.

Had I not installed my O2 sensor I never would have known I had an ignition problem. But in my pursuit of better fuel mileage I decided to install an O2 sensor to help me get my jetting and timing fine tuned. Mighty glad I did as my fuel mileage has already taken a healthy spike. I'm planning on taking a trip to Pismo Beach (about 175 miles away) by the end of this month and I'm hoping to see 18 mpg.
I quit running the advance can when i dynoed the l48, it didnt make any more power and i wasnt concerned about cruise advance. Now then once i put a bigger cam in the 400 it idles better with the advance, but like you i found it didn t work with cruise rpm and the 700r4 as well. Once you get up there in duration the ignition advance burns idle air fuel better with more advance. .but cruise with the 3.08 and 50 degrees advance tends not to wanna work so well.
 

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I run 14* initial and 22* mechanical adv. , for a total of 36*adv ,all in by 2500rpm..I have an adjustable vac adv, set for plus 10* max at light throttle cruise..but at 2000 rpm, I only have 30* plus 10* vac, = 40* total...... that also gives me 44-46* at 2500rpm, which is 80 PMH ...works beautifully in my stroker , BUT, I do have 3.70 gears .. those 3.08 s are probably not the best match with your big cam and Overdrive 4th gear
 
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