AFR numbers on dual fuel setup/300A tuning tip?

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Billhilly
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AFR numbers on dual fuel setup/300A tuning tip?

Post by Billhilly » Mon Jun 05, 2017 3:40 am

I've recently got my 72GMC K2500 with a 400 small block running on LPG. I'm going to run dual fuel with a 300A setup. All Impco components rebuilt. I have an Innovate wideband O2 setup installed.
On first start up we got it running/idling well showing 12 on the O2 gauge. Over the next couple of runs I got it idling above 15, cruising above 14. I did around 400 miles over a couple of days last week and the problem I have is man does this truck like LPG! Like 40% more than petrol! I know LPG has less BTU's, I know on some/most setups you do use more LPG than petrol, but I was expecting a couple of litres per 100k, not 40%! There is limited information out there on AFR's on LPG, and not a lot on tuning 300A's either. Besides the excessive consumption, I'm lead to believe that with acceleration there should be a richening of AFR's by 2 point's or more. I'm going in the opposite direction, leaning up by two points!
Here is what I've got on AFR's on LPG -
Idle 16.5 - 17
Low cruise same
60 -70 mph 15.5 - 16.2
Power 12 -13
WOT Rich
Stoichiometric 15.5
Have no idea where I copied that down from. The reason I was running 15/14, instead of 16.5/15.5 was to try and get lower numbers on acceleration. Didn't work......

So several questions. Are these AFR numbers right? Can you tune by them, or like the Quadrajet, you tune by other traditional methods, then look at the AFR gauge! If that's the case, what are the basic steps to tuning a 300A on the road, without a dyno?
Again, not sure where I got this graph.... and a picture of my current setup...
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Steptoe
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Re: AFR numbers on dual fuel setup/300A tuning tip?

Post by Steptoe » Mon Jun 05, 2017 4:39 pm

Thats what I run at.. or very close.
There is far more than just getting the O2 right, you need to combine that with the timing curve.
The timing curve will vary due to load on the engine.. ie final gear ratios, weight of the vehicle and if running EGR or nor
Also Vacuum advance at idle (enables low initial for low loads on starter motor , easy start..) and at low loads , eg cruising and small slopes.
Theres a general rule, do not exceed 42 deg all up (initial+cent+ VA) unless dial in with a knock sensor
I with the innovate you can put in more sensors, tach, knock sensor and a couple more .. IMAP, TPS sensors.
O2 and knock will end up getting very close if not bang on timing/ mixture

Also timing for petrol is very different for LPG any compromise between the 2 will hurt economy anty and power significantly.
I run around a 8 to 10 initial, VA comes in at idle just enough giving a stable idle ..initial+VA 14 to 16 (depends on cylinder pressure and cam ) Then a total (intial + cent) between 28 to 32
higher your cylinder pressure at a given load/ rpm the less advance you need (this will be only a couple degrees at most)
I get the timing curves into ball park (springs, weights) then get the O2 pretty close, re do timings, re do O2, generally don't need a further cycle.
I did around 400 miles over a couple of days last week and the problem I have is man does this truck like LPG! Like 40% more than petrol! I know LPG has less BTU's, I know on some/most setups you do use more LPG than petrol, but I was expecting a couple of litres per 100k, not 40%!
you mean less BTUs per volume.. but far more per weight.. If we measured by weight it would be the other way around...
Even so BTU per dollar is still far less.
Converting LPG to $ per mile, the old school camaro is about as economic as a modern 2 or 2.5L car..
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Billhilly
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Re: AFR numbers on dual fuel setup/300A tuning tip?

Post by Billhilly » Mon Jun 05, 2017 9:00 pm

So the 400 in the truck is probabley stock, has the second worse heads Chevy ever made on it, and unknown miles. It does have even compression, steady vacuum, full MSD ignition/leads, and starts great petrol/LPG.
I've got the timing set up more for LPG. All the advance in before 2500rpm. I am running alot of advance. 11 initial + 25 mechanical + 16 vacuum. It does pink slightly on full WOT on petrol, no sign at all on LPG as yet.
4:10 diff, TH350, I'm in third a couple of hundred meters down the road, after that it's just rev's! Runs around 2800 rpm at 100k. Rarely (if ever?) would see much over 3500rpm. 2500kg brick shaped 4X4 truck, is what it is in that respect.
I guess the crux of the questions from my end is how I'm managing to run lean on acceleration, not richer? If I'm not getting audible pinking more advance is good right? Take it as far as you can then back it off a touch is what I've been told with petrol.... I can't see that affecting a fundermental like more rich on acceleration? What do you reakon?
Steptoe wrote: you mean less BTUs per volume.. but far more per weight.. If we measured by weight it would be the other way around...
Even so BTU per dollar is still far less.
Converting LPG to $ per mile, the old school camaro is about as economic as a modern 2 or 2.5L car..
Yes, I meant by volume, as in I need to squeeze more through to get same output. Relating to what's a 'normal' percentage increase in consumption in a dual fuel setup. As stated, large, heavy, brick-shaped object looovvves fuel of all types!

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Re: AFR numbers on dual fuel setup/300A tuning tip?

Post by storm » Tue Jun 06, 2017 12:11 am

Billhilly I think you have the whole tuning thing a bit screwed up.
1st we need to clarify that Petrol and LPG are very different and require very different tunes to get optimum performance.
2nd pinging can be inaudible yet still destroy an engine quick smart. The old adage of take it till it pings and then back off 2 degrees needs to be forgotten. What you are looking for is MBT (Minimum timing for Best Torque). Old Chev v8s are extremely knock limited on pump gas, they will never reach MBT on regular unleaded (that is unless they are low compression engines) and will ping their heads off damaging the engine if you try. The only way to tune is either on a dyno, or on the street (highly dangerous), with knock sensors which can "hear" pinging up to 5 degrees before it is audible.
3rd going beyond MBT does nothing for performance and uses more fuel than required.
4th any engine that has as much timing as you have told us your has needs to be retuned. The figures of 11 initial + 25 mechanical + 16 vacuum which you gave us are extreme for a traditional US based iron block and head V8. All in all you have 51 degrees of timing all in by 2500 rpm at steady cruise. This indicates a highly inefficient engine, or tune, and is a good indicator of your poor fuel usage stats.

Here's what I'd do if I were you.
1st set your base timing (i.e. no advance at all so disconnect the vacuum line) at oem specs which will be anywhere between 4 and 12 degrees depending on the engines configuration which Steptoe has already mentioned. What we are trying to do here is get the base timing low so the starter motor isn't fighting against cylinder pressures. The engine needs to be able to start cleanly and relatively quickly without loading the starter motor more than the manufacturer designed it to be.
2nd once the engine is running slowly increase the revs of the engine and see what the timing is at each 500 rpm increment, write these down. This is your Centrifugal Advance
3rd reconnect the vacuum line to the distributor and and again slowly increase the revs like you did in step 2. You need to let the engine stabilise at each 500 rpm increment. This will give you your Vacuum Advance.
4th make sure it is all in by 3000 rpm, I'd say not 2500 but 3000.
5th sit down and work out a timing map where you have no more than about 35 degrees (40 maximum) all up because Chev V8s aren't that inefficient that they require 40+ let alone 50+ degrees. I have read on this site 14 Initial + 14 Centrifugal + 14 Vacuum by 3000 rpm is about right for starting to tune on LPG engine and you then work your way from there adjusting each one accordingly. 14+14+14 is 42 degrees so I'd be taking down the initial to somewhere between 4-8 degrees and then seeing how it goes.
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Re: AFR numbers on dual fuel setup/300A tuning tip?

Post by Billhilly » Tue Jun 06, 2017 5:55 am

Hey Storm. I am obviously going to have to make some changes. In my defence. I set the timing after reading alot of stuff, and alot of stuff by Lars Grimsrud. I've put nearly 12000 kms on the truck in the last 13 months. Completely trouble free. Fires up cold after a couple of pumps on the Quadrajet, rest of the day a flick of the starter with no pedal and it fires right up. On LPG it takes around four seconds to fire up in the morning (coldest about 4C) and then the rest of the day it's just a flick. This paragraph summarises some of Lar's stuff...

Most GM V8 engines (not including “fast-burn” style heads), and specifically Chevys, will produce peak torque and power at wide open throttle with a total timing advance of 36 degrees (some will take 38). Also, a GM V8 engine, under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” at cruise due to the over-advanced timing condition. Anything less than 52 degrees produces less than optimum fuel economy at cruise speed.

More light reading here -
http://www.digitalcorvettes.com/forums/ ... hp?t=75830
http://www.crankshaftcoalition.com/wiki ... _Specs.pdf

Dyno's down here are few and far between and expensive! Might have to save my pennies and get the truck on it and see how much different my two curves need to be... So obviously on LPG I need to take the top out of what I am currently running. So do we agree that the 14 14 14 deal is something to aim for on LPG then?

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Re: AFR numbers on dual fuel setup/300A tuning tip?

Post by Steptoe » Tue Jun 06, 2017 4:56 pm

Little to ad to storms
I take a slightly different approach in setting up timing.. both end up achieving same results...method in older posts
Few things you need to understand.
1/ Never dial in timing increasing advance...as advance increases the tone changes, sounds better..this is sort of like a optical illusion that results in the feel , of the vehicle having more power... put on a dyno and it will not. Start high go down till you feel power drop off.. will be sudden within 2 degs.. and thats where peak will be.
2/ over advanced was a common thing back in the day.. knock the initial up, that knocks the whole curve up, that sort of makes pistons still coming up and firing at light cruise.. one of the inaudible knock Storm mentions and s;ow death to the engine.
3/ ppl swing on a dizzy...ideal timing at a given load and rpms, octane and cylinder pressure is within a small 6 deg window 12 to 18 degs after TDC.. The ideal is only +/- 1 to 1.5 deg either side of that..
Timing is a very much a pressision thing measured in micro seconds
4/ forget old school stuff, fast burn.. we have different fuels now, different chemicals that burn at different speeds across chambers. Sped of the burn is primary dependant on the pressure at ignition and the octane. Due to duration and rate of lift of a cam, this determines how long both valves are open.. then increase rpms, that decreases that time... both valves open reduces the cylinder pressure... Compression rato means squat. eg a small chamber head with say a 10:1 compression may give say 140/ 160 lbs at crank.. throw a bottom end torque , low duration slow ramp cam in and you will have 220 lbs.

Therefore there is no overall total should be,, in saying that...
compared to petrol LPG has a higher octane, therefore for best operation needs higher cylinder pressures..chemically has a faster burn (higher the octane of petrol the slower the burn).
So LPG has a fast cent.. generally coming off a 100/ 150 rpms above idle (often the lightest spring combo possible without the weights bouncing at idle speed... also LPG will idle stable far lower than petrol.. (camaro idles 450 rpms.. in drive)
Althu comes off fast, the cent is all in at lower rpms and degs.
A old school petrol on modern fuels ball park will strat around 1200 rpms, have around 32/ 34 degs all in around 3200 / 3600 rpms depending on final gear ratio and cylinder pressure... with 10 to 20 deg VA.
LPG will start around 850/ 900 rpms, all in around 2600/ 2800 and between 28 and 32 deg with around 8 to 10 deg VA
Now re read point 3/ above.
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Re: AFR numbers on dual fuel setup/300A tuning tip?

Post by storm » Tue Jun 06, 2017 8:21 pm

Billhilly I hope I'm not coming across as harsh but there is a lot on the internet and much of it is generic or wrong. When I post, and I think I can say the same for all regulars here as well, we hope the reader will be a wise consumer and take the information provided and see if it passes the BS test.

Ok lets think about timing this way. When setting up timing what you are trying to do is to create peak cylinder pressure approximately 16 degrees ATDC so your timing needs to be set so the flame peak occurs at that point. This give maximum downward force on the piston on the power stroke and creates maximum torque. It is about leverage. If your timing starts the ignition cycle to early you loose the leverage effect but, and this is the important part, you are making the engine work harder because you have more ignition occurring as the piston is coming up the bore. In effect the engine is fighting against the pressure build up more than it needs to. Timing is a great indicator of overall engine efficiency, with all else being equal and in good condition the lower the required timing the more efficient the engine is. While no 2 engines are the exact same 2 engines built using the same parts with the same tolerances the timing specs should be practically the same and nor more than 2 degrees of each other. If 1 engine has max timing of 34 degrees and the other is 36 degrees yet they make the same hp and torque across the rpm range the 34 degree engine is more efficient because the ignition process take less time to reach peak pressure. Changing the 34 degree engine to 36 degrees will do nothing for hp and torque but will use more fuel because the engine is pushing against a flame front and pressure increase btdc for longer.

The original purpose of vacuum advance (v/a) was to add timing at cruise speeds before the centrifugal advance (c/a) is fully in. This helped with fuel economy and also as a positive side effect emissions. An engine running at 2000 rpm at 100km/h could have full v/a but no where near full c/a. The higher the engines rpm the more throttle is applied the lower the vacuum will be therefore you will have less v/a. You have set your engine up to have it all at the same point. In all honesty I have only seen 1 street chev require anywhere near 40 degrees all up. I have not been to the US, I've never been out of Australia actually, so I have never actually seen the engines others talk about in the file and forums you linked to (thanks it was very interesting reading). Please note the forum thread you linked to does state
• 36 degrees total timing (vacuum advance hose disconnected), all “in” by 2500 rpm
• 18 degrees initial timing at idle (vacuum advance hose disconnected). Note that it may not be possible to achieve the 18-degree initial spec with the 36-degree total without modifying the distributor advance stop system. It is more important to achieve the 36 total than to hit an exact 18 initial. However, if your initial timing is very low (below 12 degrees) with the 36 total, it is important that you repair or modify your distributor in order to achieve correct engine performance
• 16 degree vacuum advance control unit with a pull-in spec that allows the full range of vacuum advance to be pulled in at the engine’s idle manifold vacuum level. Connect to manifold vacuum for most applications (this will allow the engine to idle with actual timing at idle of 34 degrees).
I highlighted the important section. The post says the Vacuum advance, and this is with petrol not LPG, needs to be set so that the engine idles with 34 degrees. The thing is as you drive, put load on the engine, your v/a should decrees otherwise your engine will buck as you try to drive off. Also note there aren't many starter motors that will start an engine with timing set at 18 degrees initial, I think you will find this isn't the base timing but rather the timing that happens once then engine is running meaning the c/a has been forced to kick in as soon as the engine is idling.

I also disagree with the idea that these timing hints suit "most GM V8s" as the file you linked to suggests. If I tried that with my old Pontiac 400 I'd be looking like a rodeo rider with the car bucking and kicking and I'd have a dead engine quick smart. Oldsmobile's were the same, I don't know about Buick V8s I've never worked on them. I also wouldn't be pushing a Holden V8 to 50+ degrees either, I have worked on them and know they don't like it.

This has now gone way past your original intention of getting help with your 300A and has concentrated on timing rather than fuel. I apologise for this but I believe your timing curve may be alot of your fuel economy issues.
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Re: AFR numbers on dual fuel setup/300A tuning tip?

Post by storm » Tue Jun 06, 2017 8:36 pm

Just to add to my previous post to reinforce the idea something on the net are written to be generic there was a post made on that forum thread later on asking about timing under 2000 rpm. The reply was
Timing below 2000 rpm is irrelevant in a performance application provided the engine idles smoothly and has good throttle response off idle. You want the timing curve to start coming in just above idle speed and pull a straight line to 36 degrees at 2500-3000 rpm; you can draw it on a piece of graph paper. Cam specs are also fairly irrelevant - you want to hit 36 degrees total as quickly as the engine can take it without detonatrion regardless of cam, but the 2500-3000 spec works pretty good for every application I have done. A larger cam will want more initial timing than a small cam, but that's about the only setuyp difference. What's a PCM...?
This type of thing scares me. The point being made in this post is you get your timing from base to 36 degrees as fast as possible by 2500-3000 rpm. There is no mention of MBT and it basically says get the timing to 36 degrees as quickly as possible as long as you don't have knock. So lets think this engine isn't knock limited but it is highly efficient (he does say performance application and it relates to EFI) and the correct timing curve would show this to be true. The suggestion here is regardless of how efficient the engine is and regardless of how much timing it does need take it to 36 degrees and be done with it.
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Re: AFR numbers on dual fuel setup/300A tuning tip?

Post by Steptoe » Tue Jun 06, 2017 8:51 pm

Billhilly I hope I'm not coming across as harsh but there is a lot on the internet and much of it is generic or wrong.
I will also add.. most old school pre dizzy mechanics didnt/dont even understand timing and the background behind it..
Damn near ALL modern mechanics.. or 'technicians' who only know ECU havnt a bloody clue..
"Harsh".. Kiwis/Aussies tend to call a spade a spade and dont worry about PC diplomacy.. we just get on with the job :wink:
Add to that most dyno operators who are meant to be able dial in (set tuning specs) havnt a clue.. the top end guys where costs a fortune usually do, and keep it close to the chest
If one goes thru the American GM electrical spec manual and updated bulletins, you will find just between 1954 and 1986 there are many thousands of dizzy , and dizzy specs.. then throw in the VA specs....and hundreds for each engine type or size .
About the only thing in common with GM timing is If one sets the points (pre ECU days) to 30 deg dwell, be it German , Aussie, British US you will be in the tuning spec.. maybe bottom or top, but will be in spec.

Storms explanation above about fighting the pistons. here is a practical illustration of it
If you push a piston ( the fuel exploding) down on a short block at TDC.. you will not move it.. wasting power that should be at the flywheel... as you turn the crank, drops in the cylinder, it becomes easier, till it reaches a point that moves down very easy... but as the piston gets very low in the pot, you loose all significant cylinder pressure and the explosion is very weak no power
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Re: AFR numbers on dual fuel setup/300A tuning tip?

Post by storm » Wed Jun 07, 2017 1:15 am

Steptoe wrote: "Harsh".. Kiwis/Aussies tend to call a spade a spade and dont worry about PC diplomacy.. we just get on with the job :wink:
I know but in my old age I've come to realise some people are a bit touchy about these things and react in very negative, and sometimes even socially unacceptable, ways.
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Re: AFR numbers on dual fuel setup/300A tuning tip?

Post by Billhilly » Wed Jun 07, 2017 6:09 am

I'm starting to get the feeling you's think I should change my timing!? :mrgreen:
Storm, I'm all ears. Not harsh at all. The graphs below show the curve I currently have, and the advance stop options. I may have to make/buy an adjustable vacuum unit too.... I can't find an OEM initial setting yet. Because the motor is not out of my vehicle, and the repair manual I have that covers the right era says check under the bonnet... One thing I read tonight regarding 400's is 20 idle degrees (initial + vacuum) and 32 degrees with initial + mechanical. This would also be pretty close to your 42 degrees Storm.
no more than about 35 degrees (40 maximum) all up because Chev V8s aren't that inefficient that they require 40+ let alone 50+ degrees. I have read on this site 14 Initial + 14 Centrifugal + 14 Vacuum by 3000 rpm is about right for starting to tune on LPG engine and you then work your way from there adjusting each one accordingly. 14+14+14 is 42 degrees
I go adjust things!

Thanks for the input so far.
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Re: AFR numbers on dual fuel setup/300A tuning tip?

Post by Steptoe » Wed Jun 07, 2017 4:47 pm

Forget trying to relate petrol tuning (any specs) specs to and other fuel... doesnt work.
I mention dial in.. this is what you have to do...Dial in means establishing the best tuning specs (as one gets the tuning specs in the manual)
idle rpms... that is best stable idle at good afr..."best " means you may prefer a idle little loopy, or a smooth one, so even on the same engine you may prefer a different end of the "tuning spec" than the nest guy.
Same thing goes for everything else when dialing in.

I find the best way to establish idle tuning specs is
Mark off the harmonic balancer CIRCUMFERENCE in 5 and 10 deg increments out to 45 degs
tie back the cent weights, disconnect the VA. fire the engine.. get idle in ball park... say 600 rpms, then set the timing (will be 12 to 16 degs) and afrs (17 to 18.5) so get the stable idle you want and note the engine vacuum.. say 20"
Note the advance say 14 degs ....this will be what you need idle +VA
therefore you need say an initial around the 6/8 degs and enough in the VA (total.. has to be all the way otherwise the engine will speed up slow down) which will be about 7 deg VA.
Now you can do the same with the cent...set the intial at your say 7 degs, disconnect the VA...with the data logger going record your AFRs, and at full load at around 2800/3000 rpms and around 36 deg initial+ cent.. do a run.. then drop the timing back couple deg till you get a drop off in power...you should have a rich AFR 13/14 .
Note the drop off point, add 2 deg to that and that will be real close to your best (its a truck , not a race engine where the tiniest edge is required to win a race) full load initial+cent total timing. say its 34 deg (its not a hi compression and/or very low duration cam engine which couple put the total timing down around the 28 degs)

So now you have your idle tuning timing spec, and your total tuning timing spec , an initial spec , and a VA total degs spec and the vacuum at which the VA has to be all in by.
Its just a matter of playing with weights stops to achieve this.
petrol dizzies often have far too much cent advance for LPG....
The top plate of a chevy dizzy.. be it HEI or points, has a couple holes.. tap that hole, get a grub screw, file the outside off into flats at different distances from the center. You now have an adjustable cam stop to reduce the amount of cent in the dizzy. set to your required cent amount of degs required.
Now play with weight springs.. which will be very light.. even combinations of springs to get the rate at which the cent comes in... sometimes one requires a ford slotted spring to get the very fast initial cent to come off fast enough... important thu, springs must be able to hold the weights at idle rpms
Now the VA.. the amount of degs in a VA is the amount of distance the lever in the back moves.
The vaccuum that it operates at is in the spring in the inside of the cannister.
'adjustable VAs only adjust the vacuum bu chopping off the top end.. so if screw is wound out maybe 5 to 18" and have 22 deg... screw the screw in and the top end of the VA drops...have say 5" to 9" but the degs will also reduce. It is very rare to have both meet your requirements.
With a couple bts scrap sheet metal a couple tags can be made that fit under the VA mounting screws...each end of the stoppers can have little slots made at different distance (3 or 4) to make choices easier
Now you have TRUE fully adjustable VA.
Your idle vacuum was (from above) say 20".. you need the VA to be all in at around 18" (1 to 2" under what you need)
you know how many degs need for idle (initial say 7 and VA say 7 degs) adjust the stoppers to give this.

Sounds complex, sound a lot...once you start into it, everything falls into place...you ge a engine thats makes good power, and if has the cylinder pressures to take the higher LPG octanes well, far more power than petrol....also huge economy numbers...bottom line economy is not mpg .. it is $ per mile.
I dont know what one would expect out of a truck on economy , but an old school original 350 69 camaro running a custom bottom end high torque, low duration, higher lift cam runs 15/16 mpg of LPG around town in traffic and open road little less as not sitting at lights and intersections.
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Re: AFR numbers on dual fuel setup/300A tuning tip?

Post by storm » Wed Jun 07, 2017 6:14 pm

Steptoe wrote:Forget trying to relate petrol tuning (any specs) specs to and other fuel... doesnt work.
I agree with Steptoe, this cannot be stressed enough. This is because Petrol (or Gas as our North American friends call it) has very different properties to LPG. Different flame speeds, different pressures, different ignition points, etc etc etc. The only similarity between the 2 is they are both ignitable fuels. So far everything you have mentioned has been based on someone else's generic thoughts on tuning for Petrol, and even then I think the other people are way over optimistic :lol:

The 14+14+14 is a start point to get things running, for example you have just built an engine with all new parts and you need to run it in before you do anything else 14+14+14 will do that for you. After the engine has been run in, and this should only take a couple of hours at the most on an engine using modern parts and good machine work and building practices, you setup the timing to suit the engine. You can either use the method I posted or the method Steptoe posted they both get the same result and I dare say they are both equally conservative in the long run. The difference between the 2 methods is simply because we have both developed a preference and it works for us and we are comfortable with it.

I personally don't start with 14+14+14, I start with oem base timing and then follow the method I outlined simply because base timing is a very good start and it can always be increased easily just by turning the distributor a little. All other adjustments require the distributor to be adjusted internally and that takes more effort.
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Re: AFR numbers on dual fuel setup/300A tuning tip?

Post by BigBlockMopar » Sun Jun 11, 2017 7:46 am

This is where a programmable digital ignition system can be of great help to visualise what's happening at what rpm and vacuumrange, and also find out what your engine likes best.
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Re: AFR numbers on dual fuel setup/300A tuning tip?

Post by BigBlockMopar » Sun Jun 11, 2017 5:57 pm

Just for fun, I tried out the 14+14+14 timing map today to compare it to my current ignition map.
During regular steady state driving there was no obvious difference to be noticed.
Only during accelerations I noticed the 3x14 timing map felt a tad slower then my regular timing table.
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