https://documents.holley.com/frm2889...OTY1OS4wLjAuMA..

Good morning guys,

The end of last season I finally got my boat up and running for a month to get the fresh Mercruiser based 509 BBC broken in. Details are:

Brand new GM Performance 502 Block bored .030
New JE forged pistons
9.2:1 compression
Stock Mag crank & rods
All ARP hardware and studded
High volume melling oil pump
Stock heads cleaned up
Full Roller with new roller rockers
Holley 850 Carb
Holley single plane EFI intake as I have a complete Holley EFI system "not sniper its HP" I will switch over too i the fall (I just want to enjoy the boat this season)
Isky cam:

• Racing Cam
• Hyd. Roller
• Valve Lift Int/Ext. 0.553/0.578
• Valve Lash Hot Int/Ext. 0.000
• ADV Duration Int/Ext. 275/284
• 0.050 Duration Int/Ext. 228/238
• Lobe Center 112

In the link above is the Holley distributer I am running for now while it is carb'd but I am not sure what springs and timing stop to run. Right now it has the factory setup which is the 21 degree stop with the heaviest springs so it doesn't go full advance until 4K I believe. The engine was not dyno'd but my builder has done enough of these to estimate in the 530-550hp range.

I have the base timing set at 10 degrees so I should see all in at 31 degrees but what is a safe amount of timing for an engine like this?

I'm not an engine builder(standard disclaimer). But typically, you want around 14-16 degrees of base and 34-36 total. Also, I would put a lighter spring in so you get full advance alot sooner than 4K RPM. More like 2500 or so.

John

I was thinking the same on the springs I just didn't know if I brought it in too soon I could hurt something. Am I better off adding base timing and leaving the 21 degree stop or leave base at 10-12 and go 1 up on the stop. The next size busing is a 23 degree so base at 10-12 would be 33-35 degrees of total timing.

I would install lighter spring, add base and leave the 20-21 of advance. I would check base and total with the light. Your only concern is detonation. Which obviously isn't good. If you hear that your obviously to far advanced. Very unlikely you will get detonation at 34-36 total. Once everything is set run it, shut it down and pull a plug. Good article linked below.

https://www.enginebuildermag.com/202...ur-spark-plug/

I would set the base timing by where it likes to idle and start well. That may be 10 deg or 20 deg or anywhere in between. Give it 35deg total and I would use the springs that are all in at about 4000rpm. Sometimes a little less timing through the midrange makes more power.

Wouldn’t 35 degrees upstairs be rather aggressive for iron heads with no way to detect knock? I’d think that 32 would be a safer bet.

I read the article about reading the plugs, so I guess go WOT for a little blast and just kill it don't idle it down? Isn't that bad to go from 5K rpm pull to 0 instantly by just shutting it down? If I back it down then kill it will it toss off the reading?

I have personally never seen a BBC combo that wouldn't tolerate 35 deg timing, but I guess it could happen. I would ask the engine builders opinion if there is any type of warranty involved.

You could do it this way, just be mindful of water surge at the transom. Working flaps are a must.

i know the builder I just asked his advice and he said bring it all in by 3k and 35° + or - a degree so I’m going to set at 34° all in and ramped in by around 3k. See how it likes it.