greggreen

Guy I v'e been talking to on another forum says he checked his 496 HO with the Merc scan tool and noticed the computer was pulling back timing and fuel at WOT while running recommended 87. Switched to 93 and gained 400 RPM @ WOT. Anyone else seen this happen? Sorry if this has been discussed before.

hoozeyurdaddy

It could happen, if the 87 is not that good of fuel and the engine is detecting a knock, it will retard the timing and fuel. with better fuel, there is no knock and runs the timing up higher.

jeffswav

I saw a post somewhere and they dyno'd a 496 HO with 93 then 87. No difference in HP. I have heard the Raylar kits advance the timing in that case I think you must use 93. It sounds like you friend may have had some bad gas. If he uses E10 blended gas, do not put more in the tank than you plan on using in a couple of weeks.

stevesxm

the second post is correct. the electronics are very clever and spend a lot of time running the timing and the fuel as close to " perfect" as it can. if, as mentioned, the 87 was getting it into detonation, the box would have added fuel and retarded the timing as noted. the ideal fuel and ignition map for each fuel is unique to that fuel. i have seen more than one occassion where higher octane fuels made less power on the dyno until the maps were changed and in some cases never made better power in a specific case than the lower octane did. the old addage still holds true:

" what's right is right and everything else is wrong to some degree"

Raylar

This sounds like one of those HE Said, A Guy Said commments? Notice only 7 posts?

Just a note for bad info here on this post. Raylar kits do not require 91 or 93 octane as posted here.

If you have had any detonation or knock events they should be listed in ecm history file.

If you have no knock events listed then the ecm should not be altering spark timing at all.
The stock Merc PCM555 program for 496HO's has only 27 degrees of timing max and that should not cause any knock on 87 octane unless you've got lousey fuel.
Stay away from fuels laced with lots of ethanol such as BP or Arco, they will not make the same power as some others and they collect water from atmosphere like crazy!

Raylar has never seen a 496 run 400 rpms lower from retarted timing unless engine went into a 90% power mode which should show on the scan tool when running?

Just the Facts MAM, Just the Facts!

Best Regards,
Ray @ Raylar

jeffswav

Good info Ray, Thanks for clearing that up. I am not familiar with your products. I read an article somewere and it stated that one of your kits advanced the timing and required 93 octain fuel.

greggreen

[QUOTE=Raylar;2924577]That's exactly why I asked the question here. I knew I 'd get the straight answers. I lurk frequently but rarely post. I don't consider myself a performance boater although my 23 ft. Chaparral with the 496 does surprise other boaters sometimes.

Might be fun to put a Raylar kit on it but I don't think it would fit. My boat didn't come with the Merc engine cover because there's no room for it. Anyway, thanks for the info.