Rage

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

GPM

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.

formula31

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

articfriends

iTrader: (1)

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 . 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.

GPM

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

Rage

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.

articfriends

iTrader: (1)

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

articfriends

iTrader: (1)

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

GPM

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.

Rage

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?