Re: ZZ502 Cam in a Merc 502MagMPI
 

My mechanic told me that even the stock HP500s have had problems with reversion and that Merc' put out a service bulletin reccomending not running for extended periods of time under 1100 rpm. Is he right or do I need to buy a vowel?
Rich

Re: ZZ502 Cam in a Merc 502MagMPI
 

I've read the same thing on this site and the other one. The ZZ502 cam is similar to the hp500 cam which is why I keep above 800 R's during extended idle, and the throttle cracks at shut down. But I did the same thing when the 7.4 was in the boat.

Re: ZZ502 Cam in a Merc 502MagMPI
 

I am responding to this much after the fact but the Merc MHP-301 Cam is the later cam used in the EFI HP500 enignes. The specs in the cam at .050" are I=230 E=236 114 LSA and .600 Lift. I've used this in a ZZ502 with all valve train replaced with Crane's stuff. On the Dyno it made 501 HP at 5400 through Revolution Marine Exhausts. We used the crapiest 87 octane gas and the timing was at 36 degrees!! Aluminum heads must be resposible for this resistanceto detonation. In the boat, it will idle indefinitely, Idles at 500 Revs and No Reversion. No stalls on shifting. This is the 114 doing it's thing. This is the hot setup for the ZZ502.

Re: ZZ502 Cam in a Merc 502MagMPI
 

Thank you for the responce.
Last year when I did my rebuild the stock 502Mag cam went back in so i would not have EFI calibration problems.
I have aquired a cam from a 500EFI and most of the rest of the parts to convert my engine to a true HP500EFI.
That will be next years project though...

Re: ZZ502 Cam in a Merc 502MagMPI
 

I just went thru this with my '97 502 MPI. I did much research before diving in and talked to as many "experts" as I could first. Here are some important things to consider with this engine.
Because this is a speed-density type fuel injection, it is very important to maintain a stable strong vacume signal at idle to communicate with the MAP sensor. Cams with aggresive overlapp or too much duration will not work well. I would avoid the 110 LCA on the ZZ cam...not only for this reason but to avoid water reversion. I had Cam Motion grind mine at 228/233-.600/.600-114. The higher lift did require a new valvetrain. Limit yours to about .550 if your not going to upgrade the valvetrain. You should at least use new spings though!
To take advantage of this (mild) cam and better exhaust, I did have to remap the ecu and bump my fuel pressure up to about 45-48 psi. AZM did the remap. I know all of the negatives about having someone remapp over the phone but they do so many similar combinations, I think they have the specs nailed down pretty well. Mine worked great but I will say that AZM's customer service is terrible and if I had it to do again I would use Mark at Precision. Hope that helps a little.