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Demon 850 Baseline Jetting

Old 07-23-2021, 02:46 PM
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Default Demon 850 Baseline Jetting

Guys, I'll have my 509s in the boat soon and will be ready to test. I have not dyno'd the engines, but one has already been test fired and the other will be done this weekend. We couldn't get dyno time worked out and I'd rather get the boat back together than wait. The starboard test fire was good, strong oil pressure, not leaks, etc.

Engines are 509s with as-cast CNC chamber AFR 325s, 112cc chambers, .646/.646 235/241 113 hydraulic roller (Rookie's old cam), Merlin 4150 intakes, HVH spacer, and Mighty Demon 850s (down-leg boosters not annular).

I've baselined the carbs per Demon's instructions. Based on what I've read regarding the Mighty Demon 850 I went ahead and increased jets to 88 primary and 96 secondary with a 4.5 PV on the primary side (originals are 80/88 and 6.5 PV).

Good start? I will be tuning with a A/F meter in the boat but curious with regard to how close (or far) I may be.

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Old 07-23-2021, 02:57 PM
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Why the 4.5 Power Valve? Me think you going the wrong direction on that.
Tom, what afr system did you go with?
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Old 07-23-2021, 03:15 PM
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Oooh, long time ago but I believe my 502 w/Canfield heads was very close to out of the box jetting with 850 DP Sea Demon. I had tuned it on the dyno w/wet exhaust. I'll see if I can find my notes. May take me a bit. ie hrs or days.
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Old 07-23-2021, 03:17 PM
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Ordering the Innovate dual gauge that you mentioned the other day.

I had the 4.5 pv that TomR sent with the carbs so I used them. What are your thoughts? Bring them in earlier?
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Old 07-23-2021, 03:29 PM
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Under load I think you are going to lean out at the top of the primaries. I would start with the 6.5 or even higher. Don't want a lean condition under load.
Did you get two of those gauge kits? Don't forget to use 90* spacers/heat shields, every try without them cost another $80-$160 for new sensors.
I got mine on ebay for about $20 a se\t. I'll try to find the link.
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Old 07-23-2021, 03:56 PM
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I believe these are the ones, it does not say stainless but I think they are, call them to verify.

2x Oxygen O2 Sensor Spacer Extender Adapter M18x1.5 90 Degree Bung Pipe CEL Fix | eBay
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Old 07-23-2021, 04:04 PM
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I donít have the room to use those so theyíll be direct into the tailpipes right after the water bridges. My tailpipes are TRS to Iím really far from the water outlet as it goes into the tips.
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Old 07-23-2021, 04:06 PM
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Originally Posted by SB View Post
Oooh, long time ago but I believe my 502 w/Canfield heads was very close to out of the box jetting with 850 DP Sea Demon. I had tuned it on the dyno w/wet exhaust. I'll see if I can find my notes. May take me a bit. ie hrs or days.
Thanks!

I read the Might Demon liked a lot more jet across the board. Iíve got enough jets to play with everything. Maybe Iíll just put it back together all stock and see what we get?
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Old 07-23-2021, 04:27 PM
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Possibly, i dunno. Most 850 everything are in the 80ís for hetting
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Old 07-23-2021, 04:40 PM
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How can those 90* things can be accurate when they aren't directly in the exhaust flow ? seems they would be getting stagnant uncirculated fumes
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