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.