Purchased a boat with 500efi's in it and one of the motors has always sounded like it was doing a lot more hissing through the air intake than the other motor and it always acted lazy. Yesterday I had some time to look into this and discovered that the motor was in base timing mode. I took it out of base timing mode and put it into normal mode with my Mercruiser Scan Tool and now it sounds normal.
While I was in there I checked the timing on both motors and the base is set at 8 deg. When putting the system back into normal mode and reving the motor the total timing only goes to 27 deg.
In all of my enging tuning days we always set BBC's at 34-36deg. I do realize that Mercruiser has these motors set up to run on 87 octane and they basically live their whole life on a dyno being in a boat, but does anyone know if it is possible to pull a little more HP out of these things by bumping the base timing to 10 or 12 deg.
It appears to me that the module is putting an additional 19 deg of timing to the base setting. The only thing that I am not shure of is if the total advance changes under a load.
Any solid feedback on this issue would be greatly appreciated.

Is the 27* you mention what the timing light says
or what the scan tool reads ?

I rebuilt a set of 500EFI's this spring and when they were on the dyno, we checked the timing ( under load, with light) and it came up 35*

How high did you rev it???? Max advance does not happen until the 5200rpm range. Not sure if load affects it.

You can't set timing on the efi with no load ,thats why you put it into base on the dyno you can check timing on scan tool and then put timing light on it to see its the same,i have found it is always the same on the 500efi

Sorry it has taken so long for me to get back to conversation, been out boating for the last 4 days.
It reads 27 deg on the scanner as well as on the timing light.
I had a chance to hook up my scanner on another buddies tall deck EFI's that have reprogramed ECM's and his base was at 10 deg and the total when reving motor went to 40 deg. What that tells me is that they put 30 deg of adv in his ECM and mine has 19 deg. I really don't see how big blocks in a 46' boat would not detonate running 40 deg total adv on pump gas.
What really puzzles me is how my 42' did not gain 1 or 2 MPH by taking the one motor out of base timing mode. You would think it would have had to be down a few HP being locked into base timing mode.
Any thoughts on this?

Your buddys boat may have 40 degrees of timing in his timing tables at 50 or 60 map at 5000 rpm's but when actually under a load it is going to be at 95-100 map in his timing tables and probably less timing, the timing table on a mefi controller goes by map (manifold vacuum) and rpm primarily, boat would never see that low of a map reading/40 degrees of timing at that high of a rpm because it will be under a much greater load unless free revving. I don't know how to post a timing map or I would show you, Smitty
this is what a 502 ram jet factory timing table looks like, if your buddies motor had a few degrees more timing acrossed the table you see how he would have a high timing reading free revving it but not under a load
30--- 40--- 50-- 60-- 70-- 80-- 90-- 100 map
40.1 39.0 38.0 36.9 36.2 34.1 32.0 32.0 3600 (rpm)
40.1 39.0 38.0 36.9 36.2 34.1 32.0 32.0 3800
40.1 39.0 38.0 36.9 36.2 34.1 32.0 32.0 4000
40.1 39.0 38.0 36.9 36.2 34.1 32.0 32.0 4200
40.1 39.0 38.0 36.9 36.2 34.1 32.0 32.0 4400
40.1 39.0 38.0 36.9 36.2 34.1 32.0 32.0 4600
40.1 39.0 38.0 36.9 36.2 34.1 32.0 32.0 4800
40.1 39.0 38.0 36.9 36.2 34.1 32.0 32.0 5000
40.1 39.0 38.0 36.9 36.2 34.1 32.0 32.0 5200
40.1 39.0 38.0 36.9 36.2 34.1 32.0 32.0 5400
40.1 39.0 38.0 36.9 36.2 34.1 32.0 32.0 5600
40.1 39.0 38.0 36.9 36.2 34.1 32.0 32.0 5800
40.1 39.0 38.0 36.9 36.2 34.1 32.0 32.0 6000

OK, that clarifies it a little more for me. Would it be safe for me to put 2 more deg in my stock 500 efi's or won't it gain anything?

You could try it, if you were going 2 degrees acrossed the board obviously you could do it with a timing gun. Make sure you have the octane to do it and see if you gain anything, if boat goes no faster and runs no different then turn it back, Smitty