GPM

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.

Rage

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?

Rage

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?

formula31

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.

Rage

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?

mrhorsepower1

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

Rage

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

formula31

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.

stevesxm

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.

Rage

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?