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-   -   Baseline Jetting - HP500 Carb on 454 Magnums (https://www.offshoreonly.com/forums/general-q/353262-baseline-jetting-hp500-carb-454-magnums.html)

distantthunder 04-08-2018 04:59 PM
Baseline Jetting - HP500 Carb on 454 Magnums
 
Hello Friends,

Just finished updating the Twin 454 Mags in my Formula.

454 Mags
Aftermarket Mild Cams (Isky) Do not have details...
GIL Offshore Exhaust asystems
Edelbrock Performer RPM Air Gap Dual Plane Intakes
HP500 Carbs (stock staggered jetting)

Spinning Bravo 1 26P to 5100 RPM.

The HP500 carbs have the stock jetting (staggered, etc.), does anyone have any baseline jetting recommendations for my set up before I fire up the engines this weekend? Should I roll with the stock jetting or is it too fat since it’s for an HP500??

Griff 04-09-2018 12:48 AM
I would get rid of the staggered jetting and use 78 primary and 93 secondary jets.
Pretty sure that's what I used in the HP500 carb I used on a bumped up 454mag that had a long time ago.
I used the stock magnum intake with a 1" open spacer also.

picklenjim 04-09-2018 03:05 PM
I can tell ya the Merc 420-440-460 used the 850 holley with 84's all the way around.

distantthunder 04-09-2018 03:14 PM
Originally Posted by picklenjim (Post 4620676)
I can tell ya the Merc 420-440-460 used the 850 holley with 84's all the way around.
What intake did they use on those?

MILD THUNDER 04-09-2018 05:36 PM
Get an o2 sensor bung welded in your exhaust, and a portable wideband. Will pay for itself vs the time spent running back and forth up and down the lake guessing at the jetting, burning fuel.

Lee 04-09-2018 08:24 PM
Originally Posted by distantthunder (Post 4620677)


What intake did they use on those?
Weiand #7513 single plane with brass lined water crossover.

Baja Rooster 04-09-2018 10:17 PM
Originally Posted by MILD THUNDER (Post 4620693)
Get an o2 sensor bung welded in your exhaust, and a portable wideband. Will pay for itself vs the time spent running back and forth up and down the lake guessing at the jetting, burning fuel.
He speaks the truth. My tune ran great then the wideband showed a 500rpm lean spot right at maximum torque load.

Griff 04-10-2018 01:06 AM
Originally Posted by picklenjim (Post 4620676)
I can tell ya the Merc 420-440-460 used the 850 holley with 84's all the way around.
But did the secondaries have a powervalve??? HP500 carbs don't have a PV so the secondaries need to be jetted at least 10 sizes bigger.
Pretty sure they were Holley 800cfm also.

picklenjim 04-10-2018 04:17 AM
Originally Posted by Griff (Post 4620764)
But did the secondaries have a powervalve??? HP500 carbs don't have a PV so the secondaries need to be jetted at least 10 sizes bigger.
Pretty sure they were Holley 800cfm also.
I'm talking about the Cyclones back in the 80's. They were 850's with no power valve in the secondaries. Used 84's all the way around. Aluminum high rise dual planes. Their in Manual #9.

picklenjim 04-10-2018 04:22 AM
Originally Posted by distantthunder (Post 4620677)


What intake did they use on those?
Aluminum high rise dual plane. Staggered jetting is more of a single plane thing.


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