Maximum afrs in individual cylinders
 

Heres a question for those in the "know", what is the real world, safe maximum lean afr # a bbc motor can live with at sustained high speed? Yes, I KNOW what tailpipe target readings to use. I normally target mid to low 12's at wot depending on rpm, etc. I target sub 12 #s on blown applications, thats NOT the question here.
I recently finished upgrading the software on.my dyno and inputing my daytona 8 pack o2 system into it, i am now reliably and consistently able to see, read, review the individual cyl afr's. I was able to before but constantly had connectivity problems with the aem interfsce boxes and laptop.
so, dynoing a zz502, vic jr intake, customer chose to put 502 mpi cam.in (HE assembled and planned this motor, NOT me). Typical "hot" cylinders are hot, 1, 7 and 8. And when i say hot, i mean BLOW up hot.

So, begin dynoing this, 17-1 on #7/# 8, 12.5 ish on coldest, drops down to mid to high 15's by end of pull, coldest cylinders in sub 12 range. NOT a carb problem, stagger jetted , high speed bleeds, etc, etc, spacers tried. Numbers look like a disaster. Once you fatten back up enough to finally get 7/8 into mid 14's to high 13's at wot, youve got #6 so fat (sub 11's) that you almost get below point o2 sensors can even read (10.5)

I previous saw a 3 to 3.5 spread in afrs on.my 548 when dynoing carbed with various open plenum intakes, was able to get wot numbers in high 12'w enough stagger jetting, adjusting on carb etc and close gap up on afrs to around 1.8 #s from.lean to rich. Not the case on this 502 though

So, guy who owns/built this motor created a mis matched combination with this vic jr intake, totally in appropraite cam etc, yes, these intakes suck unless you are rpming the **** out of something or have some cubic inches, but just like dominator carbs, everyone THINKS they need one!
So we know a firing swap cam or even a a cam with corrected lobes in lean corners/rich corners would help but what about the "typical"guy whos trying to run without getting that fancy?

So i wanted to see where numbers goe w intake swap to dual plane. We swapped on a old school performer 2.0, sum things up, just shifted lean afrs to differemt cylinders, made 20 to 30 more ft lbs tq (545 ish) and just shy of 500 hp (490 ish) about 6 to 10 less than the vic jr (from 5000 up).
all the kings horses and all the kings men cant get cylinder 3 to something that DOESNT put visions of exhaust valve out ex port and molten aluminum! Put enough fuel into this to get #3 to high 13's at wot, its "paired" cylinder on intake, number 2 , is below limits of 02 sensor, sub 10.5.

So, ive been on alot of dynos in my life before owning one, most had egts, none had individual 02 sensors ,wasnt uncommon to see same lean cylinders in the high 1300's, even low 1400's and rich/cold cylinders in the low 1200'even high 1100s. We always tried to get those 1400's into the 1300's . I have a feelung those high "1300's" when translated to a 02# prob end up in the high 13's to 14's.

Average tail pipe readings, these are numbers were used to seeing, tune for 12.5 lets say, motors usually "safe" , ive known for long time that AVERAGE number can get a guy in trouble in a endurance application. Lets say for sake of discussion, number 1 is 13.5, 3 is 11.5, 5 is 10.5 and 7 is 14.5, guess what, tailpipe average will show you 12.5, number 7 is in burn down territory. So, point of this thread, what IS that maximum cyl afr on a typical bbc, forged pistons, decent parts where it melts down vs chugs along a happy life?

Ill say this smitty. Todays technology, should be changing the way we all think. That information you have gathered, is pretty darn critical in the engine build/tuning process.

i really have never understood the infatuation with a single carb single plane intake setup. Especially on a higher end build. If i was targeting over 700hp with an NA deal, I likely would be looking at a tunnel ram setup with twin carbs.

the bbc is such an imbalanced engine. Head runners are uneven, most intake manifolds have uneven runners, it be dam near impossible to get a 800hp bbc to make 100hp per cylinder.

back in the day, used to hear guys say "number 5 is the lean cyl on a bbc" or whatever number they felt applied to all bbc engine builds. Its just not the case. Every build is different.

I wish i had an answer to your question. Seems to me, that typically 12.5 afr makes best power on pure gasoline, and 12.2 with a 10% ethanol blend. Id prob shoot for that, and adjust timing from there?

The problem with that Joe is to get those hot cylinders into the mid 12s you've got other cylinders below 10 5

I do know on mefi 3 stuff i tuned w 502 mpi i closed up the afr gap even though you cant tweak individual cyl fiel trims by double firing the injectors vs once per 4 cycle revolution

I was surprised to see that a dual plane manifold was just as imbalanced. I was going to suggest you try one until I read that you did. Those are some absolutely horrible AFR imbalances! Is Norm scratching his head too?

Has anyone tried a old school dual quad low-rise manifold on the dyno to see if that evens out the AFR? Would be interesting to compare that to tunnel ram results.

I was thinking mine my be running a little fat (seeing black soot build up on tips), but after reading this I may be in the "safer" place

https://cimg7.ibsrv.net/gimg/www.off...094928bed1.jpg

Given your knowledge Im sure you have thought of this....but I will say it anyway just in case.

Are you SURE the lean readings are from a lack of fuel to that cylinder.....are you SURE it isnt a misfire causing a false lean condition?

EDIT: looking at the graphs all the transitions appear smooth....usually false leans (from misfires) show up as erratic jumps...however I do not know what type of data conditioning that DAQ is using.

Do the plug readings per cylinder correlate with your wideband readings per cylinder?
Have you switched the widebands around to different cylinders?

Im not saying you dont have an imbalance issue...im simply encouraging you to double check so you dont chase your tail if you dont.....

Ron, we put so.much fuel into it w performer to try to get #3's afrs below 14 at wot that we drove #2s afrs below 10.5 to point where bsfc was in the .62 range and #2 was breaking up. We thought the vic jr was bad, the performer was worse, we gained 20-30 ft lbs tq sub 4600 but none of that matters if you have a cylinder thats 15-1 to get rich cylinders to run.

Well this would explain why Mother Merc's setups are so pig rich! Trying to keep it together through the warranty!

What was the AFR spread when you dynoed the MPI intake with even length runners?

Ss496, yes, i swapped 02 sensors, free air calibrated them etc. Plug in #8 when we started out looked mightly lean and lean cylinders moved when we swapped to dual plane. These uneven afrs are nothing new, just most dynos dont read individual cylinders. The real question is, how lean can these hot cylinders really be because you cant add fuel to point the rich cylinders wash rings and foul plugs. In past, before the 8 02 setup,we always used tailpipe readings. Like shown with yhe averaged numbers, you could easily see 12.5 and have cylinders in 10s, low 11s and cylinders in 14's but when you add all 4 up, divide by 4 you have a number we all feel is "safe". How many people have torched # 7 or 8 w open plenum intakes and thought it was "bad gas"?
i would love to dyno a bone stock, good running carbed hp 500 and see what THOSE numbers look like, im confident its real similar!

Ron, i remember seeing afr spreads of +2 leanest to rich, certain ones started out lean, others rich, matched up somewhat in 4000 to 4800 range THEN flip flopped, rich went lean, lean went rich from there up. Blamed ecus, harnesses, swapped a million things around. When i configured the mefi ecu to double fire the injectors (once per revilutuin vs once per 4 cycle revolution) it got much better. I had to increase the pw accordingly because of having 2 sets of dead bands vs 1, ie, pw is 10 ms, 1 ms of dead band/latency, you have 9 ms of actual flow, fire injector twice f0r 5 ms, now you have 2 ms of dead band w 8 ms of flow, you change it to a total of 11ms in table to compensate to over simplify it

This is exactly why I love the megasquirt 3 pro systems with CANegt.....individual cylinder fueling control at different RPM/loads.

Im not knowledgeable in the MEFI systems. However, if they are anything like OEM SFI automotive systems the "factory" calibration corrects for these imbalances per cylinder and the "aftermarket" tuning software do not bother to unlock those features.

The cylinders I would be most concerned about would be in the ~13.7-14.7 areas @ WOT....However, there are some factory calibrations (gasoline trucks) that stay in closed loop/stoich all the time versus entering power enrichment.....but their specific power output is far less.

As with many things...there isnt one answer....it is application specific.....like the undergarmets...it depends....haha

Ss496, your giving mercruiser mefi too.much credit, lol, its blind speed density w batch fire, it basically looks at rpms, map and a knock sensor
Its tuned pig rich from mercruiser, for good reason!

So basically an open loop GM 80's TPI applied to a BBC

2 fuel channels, 1 spark channel

Yeah....merc gets by with it by running rich and running the timing far before MBT......but I would think that would lead to high EGT and be murder on exhaust valves.

Exactly, tuned fat AF so between low flowing imbalanced injectors over time and lean cylinders it dont blow up. Funny thing is a 500efi tune double fires injectirs, a 502 mpi tune doesn't, i havent dynoed a mpi 454 or 502 yet w stock cam so not sure if it would be better or worse to double fire them with stock cam, it definately helped on the 560 hp 502 i dynoed w 500efi cam and modded intake/throttle body

Not a big boat...but one of my toys is a low buck twin turbo 460 jet boat....I have a microsquirt ECM that has 2 fuel channels and 4 spark channels (no individual cylinder control, waste firing GM LS coils).

The intake is an old tunnel ram that I converted to EFI with injectors in spacers between the upper and lower parts of the tunnel ram. I used the carbs as throtte bodies because I am too cheap to buy aftermarket ones, i put a TPS on the rear carb.

I run it at about 15.5:1 AFR up to about 80 KPA.....then slope to 12.5:1 at 100 KPA....then at 150 KPA I target 11.5:1

I too noticed some driveablity improvements (especially low speed) by firing the injectors multiple times/cycle......I do not have an individual cylinder EGT on it....but the plugs look dead even...im trying to post a pic....but my work computer is fighting me.

https://cimg9.ibsrv.net/gimg/www.off...723ed3ed36.jpg
https://cimg0.ibsrv.net/gimg/www.off...696b2080bb.jpg

I think some of the old school tricks for distribution on carbs intakes is the modify the bottom of the manifold to "direct or block rivers" until you get even plug readings.

However, this would only work for one RPM/load case region in the operating range.

These issues makes it easy to understand why OEM carbureted auto engines typically had an exhaust heat cross over for manifold heat


My intake set up is far from perfect......but it does a great job for the application and resources put into it.

Smitty,
Excuse my ignorance, but are all O2's equidistant? Could some cylinders be burning more or less in the pipe? Or, is 1 cylinder just scavenging more from the other, or fuel distribution from the carb? How tall carb spacers are you using? I have some 3" and 4" spacers Jim designed I could send you. You obviously you have the technology to monitor, what would a colder plug do in those high cylinders more or less burn?
Just throwing out thoughts. Probably some dumb ones. lol
(Now we are all worried about our builds after your findings) :)

you look at the plugs?do they show anything,color,piston.......

Id think if the o2 readings, correlate to the egt readings, that would make sense to me. I know I have seen some engines with big differences in egt temps across the cylinders. Like 200, 300+ degrees in some NA builds with a single 4 barrel single plane.

Heres a video talking about this issue

https://cimg8.ibsrv.net/gimg/www.off...38df30d46f.jpg
https://cimg1.ibsrv.net/gimg/www.off...7f493346b5.jpg

#7, 8 after a couple lean initial pulls were very lean, slightly sand blasted looking porcelain, strap heat discolored to ring, definately lean. As mentioned top, swapped to dual plane, lean cylinder moved to #3, number 7 and 8 went to middle of range.

yes, all 02s are as close to equally distanced from head as possible. I dont feel what im seeing is anything new, just no one measured this much till recent past. We'v sll seen Egts vary from high 1100s to 1400s on open plenum single 4bbl.motors, so we we would stagger jet motor a little, egt would maybe drop a titch snd we'd call it done. So, when we saw egts at 1380/1400 on 7/8 in past , what afr were THOSE cylinders at, i bet it wasnt 12.5! So, if they were at 14-1 at wot and lasted years, thats the question.

I realize it is a bit ham fisted and archaic....but have you considered putting extremely small zip ties through the boosters (or notching boosters)? Non symmetrical boosters are semi common in OEM carbs to help with distribution.

http://speedtalk.com/forum/viewtopic.php?t=39608