HP500EFI with premature detonation
 

I have an HP500EFI with a premature detonation problem. Merc. Racing has provided me with another series inline fuel pump, replaced my fuel regulator valve and upgraded my ECM. This seems to have solved the problem when the motor was under the most load (getting on plane at about 2000 - 2200 rpm). However during our lake test the motor did it again at about 5000 to 5200 rpm (ran well at 2000 - 2200 rpm). My current prop is too small and I hit the rev limiter before reaching WOT. My mechanic feels that we hit the rev limiter and lack of fuel caused the detonation. Is it typical for an HP500EFI to prematurely detonate during periods when the motor hits the rev limiter? Is the fuel injection sequential? If it is it seems like it would have had a better chance of recovering. Is it possible that the symptom was moved to a higher RPM range due to being too lean at WOT?

I've been told efi's cut the fuel at the rev limit ,,bad deal ,,,,real bad if its blown ,raise the limit , 02 worth;) ;)

I have been told the same thing (cut fuel) ... its not blown ... I think that I will start by getting the correct prop for my elevation.

Thanks!

Wow that would be a lean condition for sure, what is your elevation F1? you might get Dustin to raise the RPM limiter then install an MSD softtouch{if legal in class} they at least cut spark instead of fuel

I just moved from 5000 ft to sea level, the boat was proped for 5000 ft, and that is why I am in the process of re-proping and currently over reving. H2Xmark can you go into more detail on MSD softtouch. I do not want to get the boat properly proped and have a lean condition at WOT.

The stock limiter is at 5450 at which time it cuts all fuel until it see's 5350. Depending on the way the boat is propped, conditions, etc, you can constatnly touch the rev limiter and will raise your average air fuel ratio from 12:1 to 13-14:1 range which it will detonate. In most cases, you can barely feel the limiter and constant abuse will kill the engine. Also, just for info, it's just detonation, not premature detonation, I think your thinking about pre-ignition, but what your seeing is detonation.

The rev limiter is the most common cure, but you will also see the same damage from low water pressure and low fuel pressure.

Thanks,
Dustin

Dustin,
What A/F ratio do you set your blown engines at?

Yes, what Dustin said: Preignition and detonation are two different beasts.

As far as the MSD Soft Touch, they cut spark in different cylinders. They use a replaceable chip to select the limit rpm. I have two brand new in boxes that I would part with.

Check it out!
 

Check your compression, it should be 155 (aprox.)
Check your fuel rail pressure (40 - 45 psi ? I don't recall) while running the engine at high rpm.
Check timing (it should be about 34 degrees, check it while running at high rpm on a sterndrive dyno).
Check engine temps (overheating?)
The engine could be full of sand creating hot spots in the block or heads, backflush the engine.
Check the heat range of the spark plugs.
Is it using oil? Oil in the combustion chamber will make it detonate.

Gotta go now, I am writing a magazine article about the LA Boat Show for the "Wet Toys" section of Truckin' magazine.

Sincerely
Dennis Moore

Thank you everyone for your responses. I would like to keep the RPM's in a safe range by controling it with prop size. However I have not chosen a prop yet, therefore if I choose a prop that is closer to the rev limiter than I would be interested in the MDS soft Touch (mcollinstn how much $$?) just to be on the safe side. Dustin and Dennis thanks for the technical info. I will review all with my mechanic.