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ZZ502 Cam in a Merc 502MagMPI

ZZ502 Cam in a Merc 502MagMPI

Old 11-21-2004, 12:52 PM
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Default ZZ502 Cam in a Merc 502MagMPI

I just bought a 1997 Merc 502MagMPI. She is still running but has 480+ hours and questionable oil pressure. The plan is to rebuild it this winter.
My question is whether I should go with a ZZ502 cam or leave it stock???
I am not going to spend the $$ to convert the whole valve train to an after market set-up ie. Crane 731...
Has anyone had success with recalibrating the Merc fuel-injection using a ZZ502 cam?
I built up my 1999 7.4MPI last year and after 3 attempts, AZ Speed could not calibrate the ECU properly...
I have heard that a lot of guys are running the ZZ502 cam; Any in a 502MPI ???
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Old 11-21-2004, 01:30 PM
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Default Re: ZZ502 Cam in a Merc 502MagMPI

I was always told that cam had to much overlap to run in a wet exhaust system and it would ingest water, try to find you a cam with just a little more lift than the stock merc cam and change your stock 36psi fuel regulators to the higher pressure 43 psi merc regulators to make up for the lean condition, if you have the VST system the part number for the 43psi fuel regulator is 806808A2. if you have the cool fuel system the 43psi regulator part number is 861126A1. I used the 43 psi regulator on my 454 mag efi[VST system ] to make up for a lean condition whem I installed new EMI exhaust on my boat. price is around 75buck,s each.
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Old 11-21-2004, 06:59 PM
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Default Re: ZZ502 Cam in a Merc 502MagMPI

The stock cam is 224°/224°, .483/.483, 114°LSA.
The ZZ502 cam is 224°/234°, .527/.544, 110°LSA so it is not all that much more radical. The HP500 cam is real close to a ZZ502 cam with 222°/230°, .576/.598, 110°LSA.
Desk top dyno shows a 30-35HP increase with either the ZZ502 or HP500 cams...
With 110°LSA and 234° exhaust duration there is quite a bit more overlap than stock.
Stock exhaust would not likely work, but I plan to use a set of Stainless Marines that are currently on my 7.4MPI.
Upping the fuel pressure is a good idea. That should cover the added cam lift and better flowing exhaust ?
One problem I had with my 7.4 mods. was the idle speed. The engine speed would wonder all over.
Maybe if the ECU was set up to have a 800-850 idle speed ?
I am just not sure what else needs to be done to the ECU calibration ???
I am not impressed with AZ Speed... Any others I should talk to ?
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Old 11-21-2004, 08:27 PM
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Default Re: ZZ502 Cam in a Merc 502MagMPI

I believe that your idle would vary due to the extra reversion caused by a larger than stock cam. This would be picked up by the MAP sensor and cause this problem. I have heard of several dissatisfied customers with AZ Speed and Marine regarding their ECU mods. I have no faith in anyone who claims they can modify your ecu over the phone. I feel it can only be done properly on a dyno where the BSFC curve can be monitored and adjusted accordingly. Ofcourse this can be expensive, but it is better than burning a hole in a piston.
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Old 11-21-2004, 09:48 PM
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Default Re: ZZ502 Cam in a Merc 502MagMPI

whipple can do the ecu.
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Old 11-21-2004, 10:00 PM
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Default Re: ZZ502 Cam in a Merc 502MagMPI

I ran my ZZ502/502 for +,-40 hours with stock Merc exhaust and Imco Sound Advantage from the 7.4 that was pulled out. I pulled the exhaust off and replaced it with Revolution Marine's this spring. There was a little rust on the bottom of the Merc manifolds in the two center holes on one side. And a little at the bottom on the other side. Very little on both sides. So yes, there is reversion with the ZZ502 cam but very minor. Never any signs of moisture/water in the oil.

Since realizing the potential for sucking water, regardless of how minor it seems to be, I keep the idle up around 800 rpms or so in no wake zones. And before shutting down I goose the throttle a few times. When I change the oil I feel around the bottom of the valve cover after removing the cap, noth'n there but oil.

Hope this helps.

Last edited by Kidnova; 11-21-2004 at 10:04 PM.
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Old 11-21-2004, 11:02 PM
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Default Re: ZZ502 Cam in a Merc 502MagMPI

I have a zz motor that I modified a bit and with the cam reground to a 112 lsa and instaled at 110 atdc and it still sucks water back at low idle.My exhaust is stainless marine with the short risers that I modified to be dry to the transom(still a low riser).With the stainless marine exhaust its easy to pull a riser and check for reversion.I use a high idle selenoid to keep the idle up when warming up and just try to avoid letting the motor go below 800 rpm.(especialy after a hard run).As mentioned above,give it a shot of throttle when shutting down to clear the manifolds.So many boats suffer from reversion without the owner knowing.
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Old 11-22-2004, 08:03 AM
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Default Re: ZZ502 Cam in a Merc 502MagMPI

Pat,
I just thought id drop you this info...
i decided not to run the zz cam in my build, i also am going to upgrade the lifters.
my zz cam and lifters are out of my brand new gen6 zz502 shortblock. i made my decision based on all the feed back here.

BTW,
if you want to tray the cam/lifters, ill make you a heck of a deal, cuz otherwise theyll be sitting on my shelf!!!

kevin
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Old 11-22-2004, 09:43 AM
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Default Re: ZZ502 Cam in a Merc 502MagMPI

Thanks for all the feedback guys.
I was hoping to here that someone has had success with a cam and ecu change on an engine with the Merc MPI...
It is probably best to leave her stock unless I'm going to ditch the MPI for a Holley.
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Old 11-22-2004, 12:17 PM
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Default Re: ZZ502 Cam in a Merc 502MagMPI

Originally Posted by formulaz583
kidnova,

have you checked your exhaust for water since you changed it? with stock Merc crappy exhaust, I would ever bee surprised to see rust in them.

No, never checked since changing to the Revolution's. But like I mentioned, rust at the bottom of the Merc manifolds was very minor and no signs of reversion when changing the oil.

The Rev Marine exhaust is higher giving greater angle down to the tips. Plus they have a small plate in the riser before the dump. I think Todd at Revolution refers to it as a "reversion dam".

Regardless, I haven't had a problem with reversion that I'm aware of. But as I mentioned, I do take a few minor precautions i.e. higher idle through no wake zones and a couple of throttle cracks before shutting down. I'm not the only one that does that. Most go fast guys do the same thing after any idle time more than a couple of minutes. Even with stock power.
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