F/A Ratio for blowthru

[oldhead]

I am happy with full power setting at 12.5 and idle at 14.2-14.5. But cruise is down at 12.2 -12.4...should cruise be in 13.s....what about 20 mph or 80 mph. Oldhead

[panic]

If you don't have knock or mixture distribution problem, closer to 15:1.

[PackardV8]

At cruise, a blow-through shouldn't be making any boost. Main question is what is the vacuum reading? Are you using any vacuum advance? Either way, what is your total ignition advance at cruise RPM?

And agree, at highway cruise, the A/F wants to in the 15:1 range.

Bottom line - since you know your A/F readings, maybe you've installed a wideband 02 sensor and a direct reading gauge or hand-held unit. After fifty years of farting around with carbs, I only learned how much there still was to learn when I started using a real time 02 readout on the track and highway. Learning to use the data logging feature is my next goal.

[oldhead]

The cruise vacuum is up at 10" or more......There is boost above the carb but not below Oldhead

[oldhead]

timing locked at 32....no vacuum advance Oldhead

[ClassicComp]

As long as the engine feels ok you can lean it out up to 14.5:1 approx

[ap72]

timing locked at 32....no vacuum advance Oldhead

In my limited experience you'll see your cruise A/F reading change if you hook up vac advance, it'll show it leaning up a bit since you'll be getting a more complete burn.

[PackardV8]

As long as the engine feels ok you can lean it out up to 14.5:1 approx

Not my experience. What do you feel would be the negatives to 15:1 or even 16:1 at cruise with 32 degrees locked?

Personally, if many highway miles were contemplated and fuel economy were a consideration, I'd want some means of achieving much more total advance, either electronic or mechanical vacuum.

jack vines

[ClassicComp]

if I recall he runs a tight torque converter. when you run the timing up higher than 14's the car tends to "trailer' or you feel it buck sorta when at cruise.

[Caprimaniac]

As long as the engine feels ok you can lean it out up to 14.5:1 approx

Not my experience. What do you feel would be the negatives to 15:1 or even 16:1 at cruise with 32 degrees locked?

Personally, if many highway miles were contemplated and fuel economy were a consideration, I'd want some means of achieving much more total advance, either electronic or mechanical vacuum.

jack vines

No load, 15- 16 should be OK? Only "downside" might be engine running warm, cooling needed.

But then, depending on alot of Things, on pump gas going to he 11's on boost might be healthy

[dave brode]

oldhead,

Fwiw, imo, 13s-1 cruise is fat, and will still suck gas. You might be surprise just how lean it'll tolerate at light throttle, but it'll want add'l timing running lean. Leaner than stoich = less heat, safe at light throttle. It'll surge/buck if not happy, you'll know. 4500s will need t-slot restrictions afaik. My carb is a 4781, it did not need t-slot feeds restricted. I would start by pulling say 5 jets sizes out and cruise around easy to see how it acts. Then again, I'm Assuming that you run a pv, at least in the front.

See june 5th tuner viewtopic.php?f=1&t=36018&start=45

Fwiw, I have a mild street blowthrough. I run it a little fat at wot to be safe on pump gas, but it is happy at 15 to 16-1 at light throttle. At 16.5-1 or leaner it is not happy. I have my jetting/pv opening set up so it richens up to 13-1s at around 5" manifold vac. It works well imo. It is easy on gas for a 512".

It was a fight getting the timing, jet/pvrc/air bleed / ifr combo and the PV opening just right, but the results are worth it imo.
Dave

[oldhead]

Are you guys saying cruise can be leaner then idle(14.5) ? and thats cruise at 20 to 80???Oldhead

[dave brode]

I have found that lumpy cam stuff will not tolerate a very lean idle mix, but at higher rpm they seem to do fine VERY lean at light throttle. I do not believe that it would be happy much leaner at idle.

repeat: Leaner than stoich will not cause it to run hotter. Leaner than stoich = not as much heat as is made at stoich. Running lean will do much better with add'l ignition advance at cruise. The lean mix needs add'l advance to get going.

Ditto Mr. Vines;

"Personally, if many highway miles were contemplated and fuel economy were a consideration, I'd want some means of achieving much more total advance, either electronic or mechanical vacuum.

jack vines"

That said, if you don't care all that much about mileage, imo, if you can get it into the 13-1s at cruise, you'll be much better off than you are now. More repeat: assuming a power valve in front, reduce primary jetting, see if it does better at light throttle, then enlarge the PVRCs to put the missing fuel back at heavy throttle. If you don't at least have a PV in the front, I can't say much more. I've found that running no PV with proper jetting makes for a MUCH richer cruise afr, vs a PV and proper jet to pvrc balance. My PV is externally referenced. I can cause it to stay open by disconnecting the "manifold signal" line. My AFR goes full rich with not much throttle with the PV opened. Even right off of idle is much richer with PV open.

I'll quote Mr. Vines again:

" After fifty years of farting around with carbs, I only learned how much there still was to learn when I started using a real time 02 readout on the track and highway."

I'll say. It's satisfying when you get it close to right though.

Dave



Dave

[PackardV8]

Are you guys saying cruise can be leaner then idle(14.5) ? and thats cruise at 20 to 80???Oldhead

Yes, in the 15-16:1 range works. As dave brode says:

I have found that lumpy cam stuff will not tolerate a very lean idle mix, but at higher rpm they seem to do fine VERY lean at light throttle. I do not believe that it would be happy much leaner at idle.

[af2]

Why the 32 locked??? That seems to be the when it is not to be?