Jetting with primary PV vs no PVs

[dave brode]

Hello All,

I've been launch and WOT testing jetted square w/o power valves. [test and tune events this month]

When calculating for a combo with a PV, I'm using smaller jets for nice cruise afr and adding the rest via PVRCs. I'm using total area of PMJ and PVRC with area equal to 1.15x the area of the jet it likes for wot w/o a PV. Example: A # 76 works well for cruise. A #76 and a .073" pvrc = area of a #88-90 x 1.15.

Although they like different jetting, this is happening with a 4781 and an 80514 annular 1000. Both have similar emulsion setups in the blocks and .025" MAB, .070" IAB and .035" IFRs. The [4781] runs well WOT at 88 square, no pv. Not so with the #76/.073", even with a 10.5" pv. It stumbles badly on a 1/2 throttle launch, worse with a wide open launch. I've done ABA tests. Throttle response is great with 88 square no pv. The sound of the engine is markedly different.

What the heck am I doing wrong?
Dave

[Right hand drive]

I’m thinking of a few things to consider. The response time of the power valve to react to pressure drop? The speed and volume of flow of fuel through the power valves valve? Potential velocity therefore response time through the larger jet v each smaller jet/pvcr?

I’m not sure on the validity of or the strong correlation of calculating area for change of jet and bleed sizes. These orifice sizes do affect flow but also provide pressure differential ie between fuel bowl and main well, then subsequent to that main well and air well differential. Effects of pressure differential changes may not coincide with calculated flow changes expected.

[rgalajda]

Agin you are all over the place. What was the point of this test?

[GTO Geoff]

I think RHD is on the right path. And I am assuming the match is correct for equalising the jetting.

One of the failings of the 4150 design. With no PV, & the MJ increased in area to compensate, the fuel is sitting 'there' in the main well. Ready to drawn into the booster as soon as the right foot hits the pedal. If look at a diagram of fuel flow for the PV, from the float into the main well, it is quite tortuous. Many bends & turns, & then has to fill half of the circular cavity in the met block until the fuel level reaches the PVCRs. Then....it finally gets into the main well. This all takes time....a flat spot.

It always amazed me that H did not have a single entry hole at the BOTTOM of the PVCR cavity [ branching out to a separate orifice for each main well ], rather than half way up. It would have sped up throttle response.