AFR and jetting for dual Holley 800/509ci SC/825HP?

[Tomas Wallin]

I just splashed the Pantera after redoing the interior and getting the new DPM XR with oil cooler, filter and circulation pump and some other stuff.

I boost referenced the carbs and installed an AEM wideband lambda and I got 10:1 in a/f ratio from low to 3500-4000rpm... at idle it's at 12:1 but it drops very quickly to 10:1.

I started out with jetted at 86/96 and primary 6.5PV's that's boost referenced. I have 10-15mmHg (since it loopes from 800-1200rpm) in vacuum at idle in neutral and 5-10mmHg in gear.

I changed the jets for 78/89 (from 86/96) and got to 10.9-11.1:1 from "high" idle to 4000rpm on a 20minutes ride.

I went out for a couple of hours and a total of 70miles and I never saw the AFR above 10.6 from 1500-4000rpm. The EGT is at 1200F at 4000rpm in the SM Gen III end pipes.

The transom gets real black so I need to raise the AFR quite a bit.


The engine specs:
825HP@5800rpm / 815Ft-lbs@4000rpm
Block: Merlin II 509ci
Crank: Callies
Rods: Manley H-Beam
Pistons: JE blower pistons (757F)
Piston rings: Total seal TNT gapless top
Leak down (COLD): 1-3% on all 8
Cam: Marine Kinetics Hyd roller
242º/250º @ .050" / .646"/646" lift valve / 112LSA
Wot RPM: 5800rpm
Heads: GM 188 mildly ported chambers
Head gaskets: FelPro MLS
C/R: 8.3
Valves: 1.88/2.25
Valve springs: ISKY 8205 PLUS
Rockers: Jesel Comp Series 1.7
Intake: BDS Blower intake
Intercooler: Superchiller
Blower: BDS 8-71
Boost: 8PSI
Carbs: 2x Holley 9022 800cfm
Jetting: 78/89 2.5PV Boost referenced (NOW: 74/80)
Ignition: MSD 6AL (6k rpm chip)
Distributor: MSD 84891 Crab cap
Exhaust: Stainless Marine Gen III with 4.5" dry tails
Fuel: 93octane; The pickup tube has an id of 5/8", then AN-10 from there to the A-1000 pump and AN-8 to the fuel regulator. AN-8 to the fuel block and hard tubes to the carbs, AN-8 return with fuel cooler to the fuel filter. It should be steady through the rpms.
Fuel pressure: 8-9PSI

[Tomas Wallin]

The vacuum drops to 5mmHg quite early in the rpm range, like 1500+ rpm so the 6.5 PV's opens very early.

I put in a couple 2.5's and went for a 60+ miles ride yesterday. At 4000rpm with 160F oil temp it's now at 11:1 and at 5000-5100 at 11.3 with 180-200F oil temp. Still during drive break in so staying out of too much boost. about 2-3PSI at 5000, and 5-6PSI at just above 5100rpm.


What a/f ratio should I jet for through the rpms to get the most out of it but still not taking any chances? And what jets should I try to get there?

[Mrhorsepower1]

Didn't we go through this already ??

[Tomas Wallin]

Yep, but wanted to start a tech thread here as well :)

I still don't know the answer to this though:
What a/f ratio should I jet for through the rpms to get the most out of it but still not taking any chances?

[Blue Thunder]

I believe 11.5-11.75 WOT and 12.5-12.75 cruise is a good target for safety and resonable performance. Works for me anyway.

[RumRunner]

When dealing with A/F there is not a magic number that always works on everything.

It's pretty simple leaner is better, but not safer. As far as what's safe that will depend on you. How often do you do your maintenance, how good is the fuel you're using, how diligent are you (or do you want to be) on checking everything.

The last things to consider is what do you want... If every last bit of performance is important (to the tune of having to re-jet when the weather changes) or is just keeping the transom clean and not really concerned with MPH.

I prefer to keep an engine leaner at idle, now the cam profile will tell you what it needs, but I'd rather be at least the upper 12's or so range if not leaner. This is great if you have time to fully warm your engine up before leaving the dock or cove, but be prepared it's not going to run great until she has some heat in it. Usually low to mid 12's for light cruise (no boost) and then mid to upper 11's for WOT on most engines.

Better fuels, better engines, and better maintenance routines I'll run a lot leaner.

[Tomas Wallin]

I always run on 93octane and some occasional 94 when filling up on land.

I'll shoot for around 12.5-12.75 at cruise and 11.5-11.75 at WOT and see what she thinks of it.


Thanks guys!



How much can the two banks differ in A/F ratio if everything is healthy?! Are we talking .1 or .5 or what?

[RumRunner]

How much can the two banks differ in A/F ratio if everything is healthy?! Are we talking .1 or .5 or what?

Without everything optimized I would expect to see that kind of difference from cylinder to cylinder let alone side to side.

[Blue Thunder]

I always run on 93octane and some occasional 94 when filling up on land.

I'll shoot for around 12.5-12.75 at cruise and 11.5-11.75 at WOT and see what she thinks of it.


Thanks guys!



How much can the two banks differ in A/F ratio if everything is healthy?! Are we talking .1 or .5 or what?

Depending on the AF meter you run the resolution can be worse than .5. What unit do you have? It is important to have some faith is what you are seeing for readings. Watch oil temps for lean and transom soot for rich. You don't fly jetliners with just instruments, they have windshields too so you can sanity check the instrument readings.

BT :sifone:

[Tomas Wallin]

The A/F gauge is a wideband from AEM with a calibrated Bosch sensor.

I do of course not run only on the gauges, but for the moment I'm all too rich on the AFR, too cold on the EGT and too black on the transom...

I will make one more test ride with a WOT run as well before I call it this year.

[Tomas Wallin]

I changed the jets to 74/89 and kept the 2.5 PV and got the A/F ratio to above 12@5000rpm (I think 12.2-3 but not sure) I'm getting there and this is good enough as something to start with when spring comes.

I couldn't rev more than 5100 for some reason but I'm looking into it and right now I'm thinking of the 6k rpm MSD rpm chip. It behaved exactly the same last time I went WOT (about a year ago...). I thought that problem should be fixed but ofcourse it wasn't... I will measure the resistance in the chip and check the connectors for corrosion and hopefully it's an easy fix!

[Tomas Wallin]

The reason for the engine not revving more than 5100rpm was the 6k rpm chip, without it it revved like it should...

I measured the chip and it was spot on what it should be according to MSD, so I guess it's the box that's faulty.

[Ratickle]

The reason for the engine not revving more than 5100rpm was the 6k rpm chip, without it it revved like it should...

I measured the chip and it was spot on what it should be according to MSD, so I guess it's the box that's faulty.

So how's it runnin?????

[Tomas Wallin]

So how's it runnin?????

Now it runs well, didn't get to use it as much as I would have wanted this year though.

Went for a test ride a couple days ago after some distributor/spark plug trouble and it ran flawless in the first snow for the season, 35F. :ack2:

Cleaned the dist, had ALOT of rust in it, and regapped the plugs and everything was fine.

We'll see what next year brings.