Best A/F ratio for HP & reliability with 87 Octane
 

'Whipple Charged' said that stock NA motors, cast iron heads, with 91 octane fuel should run around 12.5:1 for peak HP and reliability.

What would be the recommended A/F ratio for peak HP and reliability with 87 octane for the same stock NA motor, cast iron heads ~9.1:1 compression ratio and 28 degrees max advance?

Re: Best A/F ratio for HP & reliability with 87 Octane
 

RAGE
I had this conversation with a friend last night that runs the fastest v6 buick turbo dragster in the country Barry was telling me they are running 11:5.1 and they found it makes more power then the leaner ratios now on the 556 I just finished that I also posted on the af ratio was real close to that and that was also the best power the plug is not going to be the nicest color you've ever seen but it will be the best power Good luck Laz Mesa

Re: Best A/F ratio for HP & reliability with 87 Octane
 

MESABALANCING,

Thanks for the information.

The dragster is running on 87 octane?

Re: Best A/F ratio for HP & reliability with 87 Octane
 

no not 87 but we run the minimum oct we can for the power were making like the 556 I just finished with 89 oct the buick runs 112 oct with 8.1 comp with 45 lbs of boost .

Re: Best A/F ratio for HP & reliability with 87 Octane
 

Very interesting to see the fatter AF ratios making more peak HP. Does the motor rate as fast with the fatter ratio? It seems alot of drag race guys are running super lean to get the best ET. Is it possible the leaner mixture is making less peak HP but the motor rates faster thus better ET's in these cases?

Re: Best A/F ratio for HP & reliability with 87 Octane
 

Acceleration(WOT) Enrichment is needed to compensate for increase in air velocity in the intake manifold. Fuel is heavier than air and does not accelerate as quickly. The mixture will become lean under normal fuel control (14.7:1 A/F).

At Full Load/Full Throttle extra fuel is needed for two reasons,To develop maximum power. Normally reached at a value of λ=0.9 (12:1 A/F) and to reduce combustion and exhaust gas temperature

Re: Best A/F ratio for HP & reliability with 87 Octane
 

Does the difference between 91 octane and 87 octane gas have anything to do with the optimum A/F ratio for an engine?

Re: Best A/F ratio for HP & reliability with 87 Octane
 

There is not a one size fits all Optimal A/F ratio for every engine. Each engine design is going to be different, and require a different amount. Also the fuel you run will require a different A/F ratio even if they are both 87 octane, they may have different additives in the fuel to get there, which will require different tuning.

Re: Best A/F ratio for HP & reliability with 87 Octane
 

Thanks Mike&Paula,

Then λ=0.9 (12:1 A/F) @WOT would be a good starting point for me after improving the air flow in my stock 496HO and adjust from there based upon on the water results?

Re: Best A/F ratio for HP & reliability with 87 Octane
 

If your looking to Increase power you need to be doing this on a dyno. In the boat I think this will be harder to do. I guess if you watch for RPM increase I guess that would mean a gain in your power output. I would also monitor you’re plugs for aluminum on the outer ring where the threads end and on the center electrode area, this would be a telltail sigh of detonation..

Re: Best A/F ratio for HP & reliability with 87 Octane
 

Mike&Paula,

A dyno is not in the cards for me, everything will be on the water. I have a 496HO which runs on 87 octane, has 28 degrees max advance and knock sensors feedinf the stock Merc ECU. This works at a particular stock A/F ratio which seems quite rich based on the heavy soot build up on the transum after a WOT run. I will be increasing the cfm with new intake manifold, heads and exhaust mods. I want to insure that the new setup will still have sufficiently rich A/F ratio to work with 87 octane and the stock Merc ECU and timing and get what additional hp I can by adjusting fuel pressure.

I assume that there is a rule of thumb A/F ratio that would be a good target for my set up of 9.1:1 c/r, 87 octane, stock Merc ECU/timingof 28 degrees max and stock fuel map. I plan to make WOT runs with A/F sensor and data logger to dial in the proper fuel pressure to to achieve this target A/F ratio. I wish to error on the side of reliability rather that trying to get the last hp on the ragged edge of disaster. The back up will be to monitor the spark plugs as you suggest though should not the Merc ECU knock sensor/redardation control circuit loop prevent detonation issues?

Any quidance you can give as to flaws in my approach and or suggestions on a better approach (excluding the dyno) will be greatly appreciated.

Best Regards,

Bill

Re: Best A/F ratio for HP & reliability with 87 Octane
 

Isn't the A/F ratio programmed into the ECU from Merc? If so turning the fuel pressure up and down is only going to confuse the ECU. If it's possible you should have the ECU reprogrammed to match your performance upgrades.

Re: Best A/F ratio for HP & reliability with 87 Octane
 

that 28 degrees advance sounds a little scary to me. overheat potential. wouldnt you be far better off at 34.

Re: Best A/F ratio for HP & reliability with 87 Octane
 

The fuel curve is pre-programmed but it doesm't get feedback from the exhaust (no factory o2 sensors) so the a/f ratio won't "correct" itself. You could proceed and see what your add-on wideband 02 sensor shows then try adjusting the fuel press to band-aid it. Tyler crockett had me add a 02 sensor on my boat then we drove it at ALL throttle settings and he made corrections with his laptop while on the fly :eek: . It got pretty hairy trying to cruise down st claire river at 75-90 mph in my 272 baja long enough for him to get steady readings but he did it (he's a frickinn baddazz programmer). On the dyno he set fuel curve/fuel tables to yield 13-1 down to 12.5-1 under wot at lower rpm bands (2800-3500) then between 11.8-11.2 at wot under moderate to upper boost and rpm ranges. When we tested it as installed in the boat on the water under real world conditions we saw as low as 9.8-1 and he ended up tweaking fuel tables leaner to get back to mid 11-1 fuel ratio.My motor is supercharged though and as Tyler says "you need to fire hose the fuel to it".
Rage in your normally aspirated application I would look for 13.5-1-13-1 at light throttle settings and very easy/light cruising and then closer to 12.5-1 at wot settings. I am no expert however,these are just my opinion from my limited experiences. You can adjust your fuel pressure to change what your seeing on your 02 sensor,the problem with that is lets say for example your base pressure is 43.5 psi and you have a spot at 4500 that is 13.5-1 and you want to lower it to 12.5-1 so you turn fuel pressure up 10 percent (4.5psi) you might end up too rich at a different rpm that was already good or actually too rich to start with. You could proceed and if that happens buy one of these programming guys a airline ticket and pay them to come to your location and program your boat on the water, Tyler will do this for you if he's not super busy but expect to compensate him for it but hey,a good running boat is priceless,Smitty
My boat runs PERFECT after Tyler re-programmed it on the water and believe it or not actually gets over 2 mpg cruising (when not under boost) even with a 950 hp motor.

Re: Best A/F ratio for HP & reliability with 87 Octane
 

Smitty, I didn't think it had any 02 correction, does it use TPS and MAP?

Re: Best A/F ratio for HP & reliability with 87 Octane
 

articfriends,

Good suggestions. Thanks!
I guess I am hoping that the increased cfm will need ~the same fuel pressure increase at all rpms and therefore still run acceptably at all rpms when dialed in for the correct WOT A/F ratio. It is my understanding that the Merc ECU can not be reprogramed and I would also like to not have to start from scratch with a new GM ECU. I really want to retain the stock ECU and all its features , such as Gardian and the like.

You may be right and then I will have to change ECUs and proceed as you suggest.

Re: Best A/F ratio for HP & reliability with 87 Octane
 

I forgot,you must have the pcm555 or whatever they call it,I was thinking of mefi3. I hear that new mefi3's are non-existent but a new mefi4 and harness would be readily available plus I hear you can wire in 02 sensors to give the computer feedback although if you have anything resembling water reversion at all they would never last,Smitty

Re: Best A/F ratio for HP & reliability with 87 Octane
 

You'd know more than I would Gary,isn't it called blind speed density with a iat,tps,iac,coolant temp,and a preprogrammed fuel/timing curve? I didn't realize he had the new 555 deal,not a mefi3,Smitty

Re: Best A/F ratio for HP & reliability with 87 Octane
 

Point was, if it runs speed density, the bigger cam and heads would probably lower manifold vacuuum. The ECU would think the motor was under load and add fuel. The motor would probably run to rich,to a certain rpm. Then he turns up the fuel pressure thinking he needs more fuel accross the board, which probably makes the rich condition worse. If he can stumble to his new top end he may be fine. The bottom and middle may stink, I could be wrong, it was just a thought. I think your way would be better, reprogram it if possible, or replace it.

Re: Best A/F ratio for HP & reliability with 87 Octane
 

GPM,

Sounds like I should first run it with stock fuel pressure and record an A/F ratio sweep through the full rpm range and then adjust the fuel pressure up as required to get the best balance of A/F ratio if a suitable balance is possible.

Will cam design be capable of midigating some of the mid range vacuum drop should it occur and still contribute to increase in peak horse power for better top end?

Re: Best A/F ratio for HP & reliability with 87 Octane
 

Depending on the cam you may see a vacuum drop at idle. Turning up the fuel pressure will richen up the motor across the board. The only way your motor will see increased air flow at idle is if you turn up the idle rpm. You could run stock and keep a close eye on it, play with fuel pressure if you want. The safest way would be to reprogram if possible. I don't know what kind of HP gain you're looking for. Increased HP, 87 octane and 28 degrees of timing sounds like trouble. This is only an opinion.

Re: Best A/F ratio for HP & reliability with 87 Octane
 

GPM,

Is 28 degrees considered a lot of advance for this engine? I believe that is the stock setting for the 425hp engine. If too aggressive what would be a more tollerable max advance for 87 octane and increased hp?

Re: Best A/F ratio for HP & reliability with 87 Octane
 

On the other side of the rpm range, what I am reading about the Merc PCM555 ECU stock on my 496HO, thay say that the unit individually monitours each cylinder for knock and will automatically adjust the timing and the fuel delivery to a cylinder on a cylinder by cylinder as needed basis to control knock in each individual cylinder.

Does this make the issue of A/F ratio as it pertains to engine knock go away?

Does the GM Gen 4 ECU have the same capability?

Re: Best A/F ratio for HP & reliability with 87 Octane
 

No, its too low. You should be around 34 on a NA engine. too late timing is as bad as too early timing.

Although if you have a 496 im not sure on that setup with a computor running things.

Re: Best A/F ratio for HP & reliability with 87 Octane
 

I called Mercruiser to confirm my understanding. The 496HO maximum timing advance (stock engine set up) is 27 degrees BTDC for the rated 87 octane gas. David Moore's book also says that the big block GM heads in general have an inherent tendency for detonation / engine knock events for timing above 28 degrees BTDC though he did not cover MPFI only carburators.

The Merc PCM555 ECU on the 2005 496HO also uses the knock sensors to adjust both the fuel delivery and the timing on an individual cylinder by cylinder basis to control knock / detonation events cylinder by cylinder. With this sort of failsafe one would think that they could be a little less consercative with the timing.

Finally with all this designed in boiler plate it also seems to me that my basic stock set up is very much on the rich side when looking at the plugs and the collection of soot on the transum,the latter after a hard run.

Can you tell me more about the other supporting setup and engine that works with the 35 degrees max BTDC timing advance. Does this work for you with 87 octane gas?

Re: Best A/F ratio for HP & reliability with 87 Octane
 

Doug, very well explanned . There is no given A/F Ratio that will make optimal power.There is lots of conseption of 12.5:1 being perfect. As you well know "lean is mean" and this qoute is derived from what it's results are POWER! We have been doing this stuff since 1956 and ever engine combination and application will very for maximum horsepower.

Dean Gellner

Re: Best A/F ratio for HP & reliability with 87 Octane
 

Dean,

I very much appreciate what you, Doug and others have shared with me based on your long history in this field. Doug's comments were very clear to me and totally under stood. I probably did not phrase my question apporiately for what I am actually looking for. Given that I am very much a rookie in this area I am looking for a ball park A/F ratio 'range', not a precise target, to shoot for. My primary goal for this A/F ratio range is not to wring out the absolute last HP from my engine but to instead shoot for a more conservative set up that will protect the engine from detonation issues since its modifications from stock.

Excluding dyno work how would you suggest that I proceed?

Thanks for your help!

Bill

Re: Best A/F ratio for HP & reliability with 87 Octane
 

Are you sure that was TOTAL timing? For total you have to add the initial idle advance.

[/QUOTE]David Moore's book also says that the big block GM heads in general have an inherent tendency for detonation / engine knock events for timing above 28 degrees BTDC though he did not cover MPFI only carburators.[/QUOTE]

Be careful what you read.

[/QUOTE]Can you tell me more about the other supporting setup and engine that works with the 35 degrees max BTDC timing advance. Does this work for you with 87 octane gas?
[/QUOTE]

The Merc v-8 engine service manual (carb) reads a minimum of 31 and max of 36 degrees of total timing for all the v-8 engines. The cyclones were 33-36. The TB-4's are 31 -34.

Everyone of my BBC books and years of research with engine builders indicates total timing anywhere from 32-42 depending on the engine. Id never run one over 38. My combination seems to like 34 although I have run everywhere from 28-38. Anything below 34 I loose some power and temps start to rise. This is all stock GM head stuff with reasonable CR's. There are guys running later timing (below 30) with big CR's or blowers. I had a friend with overheating problems this summer and after we exhausted all the cooling issues, I put my digital timing light on, wound it up to 4000 and advance was only 26 even though his initial was right on, put a new module in and rechecked, 34 degrees, over heating gone.

I use 89 but my engines can run on 87. I am not up on the latest head designs and maybe they will allow for such late timing. But for the old world bbc's, I think 34 is a good safe advance. If anyone has a different idea, Im all ears.

Re: Best A/F ratio for HP & reliability with 87 Octane
 

forgive my marine ignorance.... i come from the race car industry... not boats... but in that arena, we use the exhaust gas temps to define the fuel curve as nec within the ecu... the motors go on the dyno and are programmed for max power and those exhaust gas temps are recorded at a high sample rate and a curve defined relative to rpm.... this is necessary because the motors see different conditions in the car ( and boat) then they do in the dyno room.... so then the car is fitted with egt sensors , the output of which is recorded and the map adjusted to make them look like the dyno room.

in the alternative an egt gage or two on the dash like aircraft use will do the same thing. now i can see on these non header type exhaust where that would be a problem.... but that's just execution... a lambda sensor ( af ratio gage) is pretty crude and is not really what you wuld want to use in situation where you are actually trying to tune for power... af ratio is rpm and load specific ... no one number is a be all answer... way to many variables for that to be the case.

Re: Best A/F ratio for HP & reliability with 87 Octane
 

The Merc v-8 engine service manual (carb) reads a minimum of 31 and max of 36 degrees of total timing for all the v-8 engines. The cyclones were 33-36. The TB-4's are 31 -34.

Everyone of my BBC books and years of research with engine builders indicates total timing anywhere from 32-42 depending on the engine. Id never run one over 38. My combination seems to like 34 although I have run everywhere from 28-38. Anything below 34 I loose some power and temps start to rise. This is all stock GM head stuff with reasonable CR's. There are guys running later timing (below 30) with big CR's or blowers. I had a friend with overheating problems this summer and after we exhausted all the cooling issues, I put my digital timing light on, wound it up to 4000 and advance was only 26 even though his initial was right on, put a new module in and rechecked, 34 degrees, over heating gone.

I use 89 but my engines can run on 87. I am not up on the latest head designs and maybe they will allow for such late timing. But for the old world bbc's, I think 34 is a good safe advance. If anyone has a different idea, Im all ears.[/QUOTE]

Formula31,

This is amazing stuff. Thanks for the information. I obviously have to do more digging.

Your friend with the overheating problem because of the 26 degrees total advance defective ECU instead of the required 34 degrees, what engine was this? Multipoint fuel injection or carb?

Re: Best A/F ratio for HP & reliability with 87 Octane
 

Carb, old 454 mag, 24 degree tb-4 module that was really only advancing 14.

Even my 2, 24 degree modules are different, one only advances 20. I always set timing at full in now and ignore initial.

I know squat about fule injection but I was led to believe it advances the timing until it picks up knock and then retards it a little. I think the Northstar in my caddy must do that when I run crap fuel, It doesnt ping but looses power.

Re: Best A/F ratio for HP & reliability with 87 Octane
 

fyi... essentially correct about knock sensor function altho each manufacturer has a dif soultion as to what happens when knock detected... always timing retarded instantly but often defaults to a very conservative fuel/ign map and goes into " learning mode" where it keeps edging the parameters back up to optimum until knock detected again... this allows best case setup for whatever fuel you ended up with that caused the prob in the first place.

Re: Best A/F ratio for HP & reliability with 87 Octane
 

stevesxm,

Where/how can I get more detailed information on how my Merc PCM555 actually operates as regards its response parameters to a knock event?

Re: Best A/F ratio for HP & reliability with 87 Octane
 

formula31,

Can you clarify why the above timing numbers seem to be in the 20's and your preceeding post they were in the 30's

Re: Best A/F ratio for HP & reliability with 87 Octane
 

well... good question. can't imagine that merc is going to tell you however the factory workshop manual for my motors has a very comprehensive and detailed explanation of the parameters they use. also i see advertisements for guys that are re programing the factory ecu's so they obviiously know what THEY are doing with the knock sensor signal ... bottom line is that getting the fuel and ignition curve correct for the fuel and application is absolutely critical and the single most important benefit that efi offers.... it can make your mixture absolutely perfect for every condition... as long as you are consitsent with the fuel you put in and don't make any big changes in hardware. as to your question...i would buy the factory workshop manual for you motor and read it... and i mean FACTORY not after market. you answer may be right there

Re: Best A/F ratio for HP & reliability with 87 Octane
 

stevesxm,

I will pursue getting the Merc Factory Workshop Manual today.

Did you discover about the Mercury PCM555 ECU '...goes into " learning mode" where it keeps edging the parameters back up to optimum until knock detected again...' from the Mercury Factory Workshop Manual? If not there from where?
That is what I am really interested in learning more about.

Thanks!

Bill

Re: Best A/F ratio for HP & reliability with 87 Octane
 

The TB-4 modules are rated by the amount of advance they have. The 24 advances the initial timing by 24 degrees. If you are at 10 initial, thats 34 degrees. They have 20 and 16 modules too so you can run more initial timing. Some setups dont idle well unless they are around 14-16 degrees initial timing.

Re: Best A/F ratio for HP & reliability with 87 Octane
 

formula31

I thought that might be the case but wanted to make sure.

Thanks!

Re: Best A/F ratio for HP & reliability with 87 Octane
 

How do you plan to measure the HP and TQ?
With an adjustable fuel pressure regulator you can raise the FP + or - 5-20% and maybe see a 1.0-1.5 difference in A/F numbers. Without a measuring device/dyno, it is just a crap shot as to what A/F ratio will give the best/max HP/TQ. I am sure that Merc. has done their homework.
What are you after here? With all the fiddling maybe you will find 10-15HP but you might blow the motor along the way. Leave your motors alone. Go boating with your motors and build a 540.

Re: Best A/F ratio for HP & reliability with 87 Octane
 

i am not saying i have ANY specific knowledge of the merc efi programming. when i refer to " learning mode" i am refering to specific knowledge of OTHER manufacturers ecu's for efi specifically automotive . but most manufactures use the same approach. i have no direct experience w/ marine efi. having said that , however, the theory , concept and execution are precisely the same. all that changes are the fuel and ignition curves which may or may not be marine specific but are far more likely to like aircraft where the engines run at steady state rpm more than they do varying throttle position... but regardless.... what is critical is getting your hardware defined once and for all and the fuel and ignition curves defined for what you plan to do with it... the factory stuff i am certain has a very wide power band is is made for the average joe who is spending most of his time at 60 -80 % throttle and wants good acceleration and good cruise... it is my experience that the factory curves are a great place for most people to be. smart people working there.... the aftermarket guys dump the rev limiter, pump the timing and richen it up a bit to cool the combustion chambers and claim they are wizards.... more often than not the area under the curve ( hp vs rpm ) is LESS than what the factory gave you... a few more hp at the extreme top end ? sure ... but who cares.... you don't spend much time there and the first time you get average fuel and burn a hole in a piston or rattle the top ring out you are really gonna hate yourself.