mjb

I recently rebuilt the MEFI 1 502 mpi's in my top gun. Prior to the rebuild I could turn my bravo 1-24's at 4800-4900 rpm. The new engines have bigger cams(228/233-.600-114), 9:1 comp., good flowing dart heads with larger valves, stainless marine manifolds, ASM modified intakes and other improvements....all with a goal of an axtra 100 HP per engine peaking at around 5300-5400 rpm. Most shops in the know that I have talked to(ASM, Precision, Crocket) says that this combination should accomplish that goal. ASM remapped my ecu's and reset my rev limit to 5500 so I could keep the same props. I was told to set my fuel psi at 48 and my base timing at 10 BTC, which I have done. ASM told me the new timing curve will result in 32* total advance at 5000 RPM.
I can only seem to get about 5100 rpm's with the new combination. I have fuel psi gauges mounted on the rail and I am maintaining about 45 psi at 5000, so I don't think the problem is fuel?? I am wondering if the spark timing/curve is too conservative for my setup?
- Shouldn't total timing come in earlier than 5000 rpm?
- isn't 32* a little low..maybe 34-36?
-should I simply bump my base timing to 12* or should I have the curve remapped?
That's a lot of money for 3 mph. Hoping for some good advice. THANKS-

800XCR

iTrader: (2)

Bumping your initial timing will affect the whole range. The ecm is a series of tables. For conversation sake, and most likely true. You may have 34-36* of timing in the low 2000 to low 3000 rpm range and only 32* total. Timing is changeable per 100 rpm.

Long story short, the table isn't linear and you may cause detonation at an early rpm trying to achieve a particular wot target. If you want more timing, the ecm should be reprogrammed or someone that can monitor detonation or timing retard while the boat is underway to make sure you don't have issues by bumping it.

This is why changes like yours should be mapped on a dyno.

FYI..the cam you have is only slightly larger than a 500 efi's, but with better idle characteristics. Your 3 mph gain is in my opinion very close. The 500 efi only makes 75 more hp than a 502 and figure 20 hp per mph gain. A little top end timing probably wouldn't hurt it, but another 20 hp is again, probably 1 mph. Best of luck.

Pat McPherson

800XCR is probably right.
I played the same game with Jim at AZ Speed. I sent my ECU with spark plugs back to AZ Speed 3 times only to get more frustrated...
Mark at Precision has the Merc program for the HP500EFI. It might be the closest thing to your set up.
Good Luck!