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BBC mercruiser marine engine tune reflash

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HI everyone.

I am mostly interested in mefi controllers that are used in mercruiser marine engines 7.4l and 8.2l engines (Mefi 1,2,3 and 4)

My new engine is currently not playing nice.

Mercruiser 500 efi clone bbc engine. its basically a GM 502 ramjet engine. 1100cfm mono blade throttle body 50# injectors 41# fuel pressure slightly larger 330cc alum. heads. with wet exhaust.

(I have yet to be able to install the o2 sensors as most marine exhaust is wet. So I don't have that info as yet.)

Any way it is running well other that the fact that everytime it is put into gear it stalls the engine out and it can not be started in gear.

I think that this could be a lean condition at idle to the point that it chokes the motor down.

The base tune I am using is a proven tune for this motor combination but I upped than cam for this engine. and yes I realize the cam is probably the single biggest change for a tune. I just didn't think 25hp would make this much issue.

(500EFI tune for base)

I attached the cam specs below. The later being the newer larger cam currently in use.

500 EFI

230/236 .598/.610


525 EFI

236/244 .610/.632

114 LSA

Any input at all that could be helpful. please do. boating season is almost over

That's a pretty decent size cam - I'm not too surprised you're having some difficulty. That's quite similar in spec to one of the cams we used a lot in LS road cars. In particular it requires a higher idle speed which in itself may be problematic in a marine application. You're also going to see a reduction in VE at low rpm and a subsequent increase in VE at high rpm. How is this tuned - MAF or SD? I found I got better results using an SD OS with the LS engines I tuned with cams around this size and you may find the MAF is affected by reversion at idle.

Thanks Andre

there is a Maf sensor in the plenum. and yes it is a decently large cam. It is the same cam that mercruiser uses in there 525efi motors. it has to idle at 750 rpm for slow movements around docking, since it doesn't have a tranny. but u r correct once its up around 950 rpm it runs like a champ. however shifting in and out of gear at that rpm will destroy gears and maneuvering around boats and docks or trailing is next to impossible

ok Andre

if the VE is doing as u say it is how much of a change should I make at the bottom end

You really should get this engine on a dyno to get it set properly. I build Marine engines for a living and re-tune them. I just took it customers 500 EFI s bored and stroked them to 540s put a larger cam in than what you are using with the Arizona Speed monoblade when I was done tuning them they made the 675 horsepower. Play idle between 7:50 and 800 RPM with very good dock manners. If you have a stand-alone O2 you can take one of your tail pipes out and have someone weld a bung into it. Just keep in mind that you want to turn your O2 sensor off while getting on plane. If you get any reversion it will trash the O2.