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[Frnknsteen]

Thanks Mattax, that was interesting information.

If I am understanding it correctly, the motor liked 18Initial/36 degrees the best and dropping to 17/35 not only hurt power, but narrowed (and lowered) the torque curve. I did notice that advance timing (36 or 37) didn't seem as different at upper RPMs, and that 37 was maybe even better. Judging from what I am seeing, It looks like it liked 18 degrees at initial and liked a little extra total advance at the upper RPM ranges.

I see what you are saying about the A/F changing with changes in timing. What I found interesting was that as timing was reduced, the A/F appeared to get leaner. Not what I expected for some reason. I guess I always expected more advance to show up leaner, not more rich

So am I reading the last graph correctly that 4000 RPM (WOT start) is designated by the vertical line going up just to the right of "11.5" on the bottom access? I'm guessing that from the way the A/F changed and low vacuum leveled out.

So tell me this,... Am I correct in assuming the reason the A/F went rich at that vertical line is because the secondaries kicked in? I am assuming that because of how the A/F stayed more stable running primaries only.

You mentioned above that mixture should go leaner as throttle is opened, up to 70-90% throttle, then go richer. Can you explain why that is? I would expect just the opposite, or is it because opening the throttle opens the butterflies and lets more air flow through the carb?

 

[Mattax]

If I am understanding it correctly, the motor liked 18Initial/36 degrees the best and dropping to 17/35 not only hurt power, but narrowed (and lowered) the torque curve. I did notice that advance timing (36 or 37) didn't seem as different at upper RPMs, and that 37 was maybe even better. Judging from what I am seeing, It looks like it liked 18 degrees at initial and liked a little extra total advance at the upper RPM ranges.

Yes. That is how I interpreted it. To know the curve better, I'd have to go back to see which distributor was on the engine during those runs, and whether I checked the full curve later that week. Since I was paying for time, I did the minimum to have reference points.

I see what you are saying about the A/F changing with changes in timing. What I found interesting was that as timing was reduced, the A/F appeared to get leaner. Not what I expected for some reason. I guess I always expected more advance to show up leaner, not more rich

I'm not well enough steeped in the combustion or the sensor operations to explain it myself! I remember reading debates and discussions about how oxygen sensors work and if CO effects them directly. Certainly when some of combustion results in a different proportion of CO to O2 than is normal, the O2 sensor alone will not know that. The post by Shrinker (earlier link) was after those discussions and much more investigation. (Me, I just watched from the sidelines). Another anamoly (sp?) in the chemical balance that widebands can't know is from a misfire. I think they (WBO2) are great tools, but I thought it worth showing some situations where they should not be believed.

So am I reading the last graph correctly that 4000 RPM (WOT start) is designated by the vertical line going up just to the right of "11.5" on the bottom access? I'm guessing that from the way the A/F changed and low vacuum leveled out.

Yes. Mostly from the vacuum going to near zero. With only two barrels open, it then when up slowly with the higher rpms.
(Sorry about not identifying the bottom axis as time in seconds. 11.5 seconds from beginning of the logging session)

So tell me this,... Am I correct in assuming the reason the A/F went rich at that vertical line is because the secondaries kicked in? I am assuming that because of how the A/F stayed more stable running primaries only.

No. It went rich because the power valve opened when manifold vacuum dropped below 8.5 Hg.
These particular runs were with a '750 cfm' vacuum secondary carb with plain spring IIRC. So based on some other runs, I'd say the secondaries were not fully open at 4000 rpm. With mechanical secondaries of course they will be fully open when the primaries are fully open. The purpose of the secondaries is less restriction at high rpm. As long as fuel distribution isn't hurt, this is more efficient.

You mentioned above that mixture should go leaner as throttle is opened, up to 70-90% throttle, then go richer. Can you explain why that is? I would expect just the opposite, or is it because opening the throttle opens the butterflies and lets more air flow through the carb?

Probably better, I'll requote from Racingfuelsystems and other forums. I would have just posted the link, but Tapatalk took over the hosting and screwed up all the preexisting images and attachments. It makes it hard to hard read.

 

[Mattax]

"The clue [relationship] here is rich idle, lean cruise, leaner (leanest, actually) part-throttle acceleration and rich WOT. The leanest is at mid-load, half-throttle or so.

The thing to understand is most engines respond to being leaner than stoichiometric at part-throttle because the lean exhaust gas has hot unburned oxygen, and hot oxygen improves combustion"

In summary "at moderate to mid load, engines will run lean and like it, and burn much less gas while doing so. They must be rich at idle and very low load, lean in the middle, and rich at WOT." The load where richer is needed varies with engine, gearing and vehicle. It may be 60-70% as shown here, or as high as 90%. Load relates to manifold vacuum, and therefore is used to signal the opening of the power valve, letting more fuel into the main circuits.

"This ... graph is from Walter B. Larew, Carburetors and Carburetion. At the time he wrote his book on carburetors he was a retired Brigadier General who taught Military Science at Cornell, among his other accomplishments. He published this carb book in 1967.

He didn't specify an engine type for this graph but his information is in the context of engines in general. His sources were most likely military aviation research. The math in his book is from NACA TR-49 and similar publications.

This graph is representative of a richer part-throttle that may be necessary to tolerate with an engine that has radical valve timing and perhaps not so good A/F distribution at part-throttle."
quote and graph from Tuner on Innovate Motorsports Forum and reposted at RFS.

F is Fuel/Air ratio for gasoline. Invert the numbers on the vertical axis to convert to Air/Fuel ratio.
0.08 = 12.5
0.667 = 14.7
0.06 = 16.6

Constant power is any steady throttle condition.
Steady 15% might be something like driving 40 mph on a flat.
Steady 25% might be sameat 60 mph or 40 mph up a long a grade.
Steady 100 % might be something like towing or dragging max load up up a really steep long grade, foot to the floor, without losing or gaining speed.

The acceleration loop shows that maximum acceleration (full throttle) has about the the same fuel-air needs as constant 100 % power.

 

[Frnknsteen]

Thanks Mattax. That's good information. I now see why you questioned the A/F curves on my Dyno graph looking so different and inconsistent. It would lead me to believe he wasn't applying the throttle consistently. The missing portions of data in the middle of runs doesn't help much either considering we lost most of the dark blue run and chunks of the green run.

I just bought a bunch of parts from someone who just left where I work. He will be moving and is trying to downsize some of his parts inventory. Picked up a complete set of 915 heads, HP Exhaust manifolds, a spare Demon Speed Demon carb, and this weekend I am picking up the complete 400 (906 heads and Edelbrock dual plane intake) that wouldn't quite fit in my back seat . I can use a lot of those parts for my Charger rebuild.

Reason I am mentioning is he also has a WBO2 sensor, complete with all the wiring, connectors, and led display (red & green lights) that he said he'd sell me for $25. He was running it in his '72 pro-charged 440 Charger, but says he is building a new 383 that will have ported fuel injection and says he will have no need for it.

I'm thinking I will buy it from him and wire it up. I don't know how accurate they are, but I figure at least I will be able to see if I am having any major A/F ratio issues. I don't know how else to check it on an older car like ours. I tried to look up A/F sensors, but they all seemed to be designed to plug into the OBDII port (I think that's what that connection under the dash is called).

Thanks again man! I really do appreciate your patience and the learning opportunity you provided!

Mark

 

[Mattax]

A wideband can show the impact of changes. Its a handy tool, but not a neccessary tool. Tune to the performance. Check the spark plugs for the best view of what was going on inside.

That said, I would't bother with buying a sensor like that. A oxygen sensor needs a controller and interpretor to give AFR. AFR gages and loggers work with specific types of wide bands. This is true even within a brand. My older LM1 uses a different WBO2 than the newer LM2.

Its nice to have extra parts, second engine, etc. I hope you continue tuning with your current set up as well.

As far as the missing data, it may not be entirely gone. If they still have the *.drf file, those datapoints may be there as mph vs Hp. It just may not show on Torque and RPM. On a dynojet, RPM is based on the inductive pickup and the torque is caluculated based on assumed or entered gear ratio. OTH maybe its not important - water under the bridge.

 

[Mattax]

↑
So tell me this,... Am I correct in assuming the reason the A/F went rich at that vertical line is because the secondaries kicked in? I am assuming that because of how the A/F stayed more stable running primaries only.

No. It went rich because the power valve opened when manifold vacuum dropped below 8.5 Hg..

But also YES. Your observation about the primary AFR staying flat and the AFR with the secondary working does indicate the secondary side needs serious tuning.
The slope from rich to lean at the end suggests too large of a main air bleed.
The rich dip as the secondary starts to open could be too large of a main air bleed, too many or too large "emulsion holes" or secondary "idle circuit". So the first change I should havemade was to reduce the size of the secondary's main air bleed. I've done that, but haven't had a chance to retest.
I put "idle circuit" in quotes because these circuits also supply the fuel off-idle. In this sense the name is misleading, and most of us spent too many years not getting that fact. (This is why early in this thread I suggested if you were brave to reduce the primary jets and try driving steady at interstate speeds. Cruising at 35 or 40 mph, most of the fuel is coming from the "idle circuit". I personally think that its best to do that test after establishing best jetting for WOT. Less risky and in many cases best for interstate cruising will be the same as best for WOT. )

 

[Frnknsteen]

That said, I would't bother with buying a sensor like that. A oxygen sensor needs a controller and interpretor to give AFR. AFR gages and loggers work with specific types of wide bands. This is true even within a brand. My older LM1 uses a different WBO2 than the newer LM2.

Ok. Makes sense. This just seemed like a handy simple system. He showed it to me and it seemed pretty simple. It had the sensor and controller and the readout was just a row of red and green LED's. Center group was green and outer edges were red. He said it was handy because he knew if it started tripping into the reds, it was time to look to see what was going on. Only thing it was missing was the bung to weld into the exhaust pipe.

Its nice to have extra parts, second engine, etc. I hope you continue tuning with your current set up as well.

Oh, I have no intention on giving up on my current set-up. It runs good and I will play around with it. Prices were just so good on the extras that if I didn't end up using them on my Charger, I could always resell them.

1976 Complete 400 (oil pan to intake) with the Edelbrock intake and 906 heads - $300 Figured if I ever did want to build another 400 (or 451) it would be a decent starting point.
Mildly used Demon Speed Demon 750 with a lot of new in package parts - $100
Pair of 915 heads that have been reworked with hardened seats. Have valves and springs but no rockers - $150
HP exhaust manifolds complete with heat shields - $150
Plus he threw in some other misc items like new in package 440 intake gasket sets, water pump housings, a mini starter, etc.

 

[Mattax]

That sounds great.
Perhaps with a blower, having a warning setup is good idea. I think in your situation its going to be too coarse and a distraction. If you hear pinging at WOT, that's the one situation where you need to get off the gas because the engine can be damaged. (The spark plugs will also show even light detonation in the form of tiny speckles of metal on the porclain).

Glad to help. Its always good to help someone who is as interested in learning as you are.
I meant to mention my favorite books on carb basics are the ones written by Mike Urich. Either the little one on 4150 Holleys or Holley Carburetors and Manifolds. Used copies are pretty cheap.
On-line, Chrysler's 1970 Carburetor Fundementals and Facts is pretty good. (Just note that Holleys vent to the air cleaner whereas Carters are often vented to increase pressure in the bowls.)
Another Chrysler booklet from that year gets into the basics burning of the fuel.
I can't think of any great overviews of the ignition timing concepts, but this page showing the piston position and these on purpose of centrifical vs vacuum advance
basicly cover it.

 

[Kendog 170]

When I set up my holley 850 on my 500 motor I jetted and change the power valve (Holley) and insstalled wideband sensor . My #'s are very close to what you posted oi post 53.

F is Fuel/Air ratio for gasoline. Invert the numbers on the vertical axis to convert to Air/Fuel ratio.
0.08 = 12.5
0.667 = 14.7
0.06 = 16.6

 

[Kendog 170]

Lot's of great information Matax Thanks!

 

[plymouth67]

I was just doing some reading about Demon Carbs and read that of the two ports I was mentioning, the rear port is constant vacuum and the front only has vacuum when the engine is revved. They said that the vacuum advance should go on the front port. I do not recall if there was vacuum on my front port when I rev it. I just know there was no vacuum at idle.

I will put a vacuum gauge on that front port and rev the engine to see if it develops vacuum when on the throttle. Could it be that I am hooked to constant vaccum so advance is coming in too early?

Just a thought

If your hooked to a constant vacuum it will max out the dist at idle, when this happens your running it so retarted (timing wise..lol) that it will be very lazy. The engine literally cant keep up with the distributor. Ask me how I know...lol

 

[AJ/FormS]

your running it so retarted (timing wise..lol)

I don't get what your meaning is.
Unless you floor it from idle and 20*of V-can timing drops out leaving you with zero, lol. Which is exactly why I speak against running full-time vacuum to the can.... but nobody I know nails first gear with a 4-speed from idle,lol. It's just about impossible.
But usually when running full-time Vcan, your base timing will still be set to around 14/16, with maybe 10 in the can

And yes I agree the Vcan should be hooked up to the sparkport, and not considered in any power timing measurements.
Furthermore, the higher the cylinder pressure is, the more powerful the individual power pulses will be, and that leads to a bucking bronco situation at very low MPH with the timing jumping around as the rpm pulses back and forth on the curve. For this reason, with a manual trans, I fix the idle-timing waaay lower than with an automatic (fluid-coupling), and delay the onset of advance to at least 1000 rpm. Now you don't have to toe the clutch so bad under ~8 mph to avert the bucking.With a stick and a 2.66low and 3.55s, you gotta be sharp down there.
Whereas those automatic guys,in comparison, can get away with a pretty sloppy low-rpm timing tune.