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My first time to ever have actual data to tell me if I have a rich or lean condition. Now, more questions than answers.
I think I am running a bit rich. This is a Quick Fuel 750 vac secondary (Holley clone) on my 427 stroker with 272 degree cam.
Steady state 2,000 rpm in the driveway 14.5
Steady 60mph/1,800 rpm 14 to 14.3 on level ground
Steady 60mph 13.5 uphill
Steady 60mph downhill 14 to 14.5
Idle about 800rpm. 11-13.5
Mild acceleration at 60mph 11-12
Am thinking of dropping one jet size. Currently 68’s in it.
What should I be looking for at idle? Trying to get the magic 14.7 at cruise
Has a 6.5 power valve.
What would the tuning experts suggest? I have never had this much data to look at before.
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I'm with Alessandro on that, Steve. For a pot it looks pretty darn good, I think. It's a touch fat at idle but you might get that closer with the idle bleeders and the cam can be doing that also. Power demand goes a bit rich and i think that's what you want. Steady state is very close to Stoich so it should get decent highway econ. You can try one jet size but I won't be surprised if you come back.
Bob
Last edited by Bullet Bob (5/18/2019 7:37 AM)
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So why do you think you need help tuning it Steve? Those numbers look like you've done a swell job. Those 68's look pretty spot on to me. I'm with BB on changing them. If you've got some extra time and enjoy bending over fenders, have at it. Me, I'd drive it as is.
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For a carb that calibration is quite good. You may be able to lean out the cruise a bit, but it will probably come at the expense of also being lean elsewhere in the curve, where it would be detrimental. The big thing is the acceleration enrichment looks good. That's where people have fits trying to tune a carb, usually because its too lean and they think its too rich and just make it worse trying to fix it.
Whether or not the idle can be improved in terms of A/f ratio really depends on what the cam profile will allow it to tolerate. You could try changing air bleeds as BB suggests. I'm assuming you already adjusted the screws to max vacuum reading.
If it were EFI where you didn't have to make any compromises it would be a different story, but a carb is never going to be perfect everywhere. This setup looks to be running great.
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Like the others said it's pretty darn good. Maybe try to lean the idle out a tad, I'd be interested in a 3rd gear WOT pull on level ground.
When tuning I always went for what the vehicle "wanted" rather than shoot for a specific number. Some would buck at part throttle when at a certain AFR and others were totally happy. Same thing with idle, some like it a little richer some like it leaner. So again just tune until you get the desired behavior. But again now that you have the wideband I'd look at WOT A/Fs.
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In the interest of learning, where should the probe be installed with dual exhaust and and H pipe? I thinking right in the middle of the H to get the best reading that would be either too rich or too lean compared to the rest of the cylinders.???
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HudginJ3 wrote:
In the interest of learning, where should the probe be installed with dual exhaust and and H pipe? I thinking right in the middle of the H to get the best reading that would be either too rich or too lean compared to the rest of the cylinders.???
Hi , no the h pipe is not the correct spot . It must be welded after the manifold exhaust , or with long tubes , just after the 4 into 1 . The sensor must work at high temp and crosspipe generally are away from hot .
top notch should be one on each side and two readings on the gauge . Have seen only one doin like that
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So do you have a switch on it to read each side individually or hook them together to get an average reading?
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HudginJ3 wrote:
So do you have a switch on it to read each side individually or hook them together to get an average reading?
Generally reading a single bank is fine and is what most people do. If it varies widely between both banks there's something wrong.
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The sensor needs to be as close to the merge point in the header as possible. A single sensor in one bank is fine, especially for a carb application. EFI applications use two if they have the ability to trim injector pulsewidth in each cylinder, though early batch fire systems used a single unheated sensor because they lacked this ability. Even with the ability to trim individual cylinders having only two sensors still ends in guesswork, because what you really need is a probe in each cylinder. High energy ignitions render plug reading impossible, so without that there's really no way to know what's going on in each cylinder. Typically the factory uses a probe per cylinder when dyno testing an engine and sets the base calibration based off those results. BUT, modern EFI systems are on engines that are typically so well designed that individual cylinder variances due to cylinder head, intake manifold, and exhaust manifold differences cylinder to cylinder are essentially insignificant. So the tunability is there, but usually there's little to no trim in the base calibration from one cylinder to the next, though the system may use the trim function if it "senses" idle instability caused by a misfire, etc.
With a carb there's not much you could do cylinder to cylinder anyway, except maybe change plug heat range around a bit to try and cool or heat a cylinder that is lean or rich respectively.
And, as Hakan said, a 3rd gear pull on level ground at WOT is sort of the standard for power tuning. BUT, without a time component the readings are somewhat academic, because it is all about what the engine wants. Some of that you can do by feel, this will help you sort out a lean condition, but on the other side its hard to tell how rich to go without track testing for best ET. Normally at the egnine's peak torque point I wold start at 12.5:1 and go from there, but that's also tricky because without dyno pulls how do you know exactly where that is? Around here chassis dyno time is actually reasonable and there's a place that does a ton of it that's 20 minutes from my house. Though, I do live in the most populous of the three counties in our nation's second smallest state But, chassis dynos have popped up all over in the past decade, so with a little Googling you should be able to find one near you.
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Ed Winfield told us that back in the 20's he had a known distance up a fair grade near his home in Glendale Calif. that he'd run against a stopwatch to tune his Model T race cars. Things have changed a bit since then but the physics are still the same.
BB
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There is not a level stretch of road within 20 miles of my house! So, I guess my “tune it by ear” method must have worked ok. It would be nice to lean out the idle, but it just doesn’t want to run if I lean it any.
It will be fun to see what mileage it gets on the bash trip.
Thanks for the input, guys.
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May I ask how much base timing you're running?
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If I recall, I am at about 12 BTDC
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My guess would be that on a high performance engine like that you are going to limit total timing to no more than 32 degrees. Compression and chamber efficiency shouldn't require more spark lead than that. So if you have 20 degrees of mechanical advance and 12 initial that's right where I would set it, at least to start tuning.
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Maybe I missed this, but what A/F gauge are you running, is it permanent/temporay, and are you using a wide band O2 sensor?
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Permanent gauge. Digital with an O2 sensor mounted in the pipe just aft of the header collector.
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MS wrote:
If I recall, I am at about 12 BTDC
Reason I ask is because I noticed when I installed an A/F gauge I tended to focus on fuel circuits at first, and your #'s are looking so good across the board I thought it may be worth playing with base timing just to watch the gauge and observe its effect at idle.
I run up to 17* base even on stock motors and limit mechanical advance to 20*. It's tougher on the starter but always seems happier at idle for me.
It seems feasible to me anyhow that advancing the base timing, being such a simple check, might do the trick.
And I ofcourse agree with TKO and am not suggesting that is the fix, if it did clean up at idle you'd then have to do it right by adjusting the timing curve so as not to have too much total timing.
Last edited by Jon Richard (5/29/2019 10:52 AM)
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Thanks for the timing suggestion. I am actually going to retard a bit, as I am getting some pinging on hard acceleration. I have a custom Duraspark II curved for my engine by Davis.
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MS wrote:
Thanks for the timing suggestion. I am actually going to retard a bit, as I am getting some pinging on hard acceleration. I have a custom Duraspark II curved for my engine by Davis.
Absolutely. I just think it's cool having an O2 sensor rather than reading plugs. Kinda makes me want to get other sensors and start data logging, just not sure I'm ready for all that.
Sounds like a nice ignition set up. You know I bought an old Allen syncrograph intending to restore it and do my own curving, but I've been bitten by the crank fired ignition bug.
I've been a member since the blue forum, I hope I can someday make it to a bash and put a face to these screen names!
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The AEM gauge I use has an interface for data logging. Very helpful, especially when tuning EFI engines and I can log essentially every sensor stream in real time.
The problem with reading plugs is that once ignition voltage gets north of 40,000 it becomes unreliable at best. The spark is so hot that it burns the plugs clean making it essentially impossible to use it as a guide anymore. If using any kind of multi spark discharge system its even worse. On newer vehicles with coil on plug and EFI voltage is often in excess of 100,000, and whether its pig rich or super lean the plugs always look the same. Its truly amazing ow far we've come since the '60s.
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I only use plug cuts when I doubt the O2 sensor which happens now and again because I have to run leaded fuel. I replace sensor often. Plug cuts are a pain. I start with a brand new set of plugs run a couple of laps and cut power towards the end of the longest straight. Pop it in neutral and coast as far as I can hopefully to pit lane. The thing I have found is that richer is sometimes better than leaner since that whole combustion thing is so complicated. Based on looking at the torque curve, sometimes there seems to be a hole in it then it clears off and goes good. If that hole is in an important spot in the curve, I usually find that richer is better for lap times.
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Hey MS, I'm interested in an update. I'm thinking of installing a wide band O2 sensor and gauge myself. Been reading up on it and it seems that the consensus is that at WOT you should be in the 12.5 to 13.5 range. The thinking being that you're measuring an average of all the cylinders (at least on the bank you O2 sensor is on) so it's likely one or more are leaner and if you're reading is at stochiometric (14.7) then those leaner cylinders may be too lean. But I'm only in the research stage now.
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