IFR Size

[MD383]

Hello. I have a Q-750 on a 400 sbc with a 244@50 cam with 292 duration. Carb has .031 IFR’s, .067 TSR’s, 71/81 jets, 8.5 PV, 73 IAB, and 28 HSAB. .054 PVCR’s. IFR’s relocated to bottom and two hole .028 emulsions being used. 9.92:1 compression with a edelbrock dual plane rpm air gap manifold. 4 speed with 3.31 gears and if I don’t slip the clutch enough it will buck off idle and when at lower rpm like 1500-2000. Cruise and WOT is fine. Increasing IFR size to .033 but I didn’t see a difference so I put the .031’s back. I heard that with cams duration over 233@50 should utilize at least a .033 IFR. Is this true? I also have a 327 with a 250@50 cam and the carb has .028 IFR’s if that’s the case then I should increase those as well. My spark plugs look good with the .031 IFR’s and I have .020 transfer slot exposure on the primaries and none showing on the secondaries. Timing is 20 initial 36 total and 10 vacuum advance. Is there a range for IFR sizing depending on cam size/cubic inches? Thanks.

[kgb]

I run a Mighty Demon 750 on a 440 Dodge with a cam very similar to yours, 242@.050", the carb came with .038" IFR in all 4 holes. I've got an A/F gauge and am able to record RPM, vacuum and A/F. Cruising around I get a lot of 11-12:1 and have had comments about the fumes the car puts out.

I did not relocate the IFR's down low, but have recently tried a set of .035" then a set of .032" in place of those .038" on the primary side. The idle has always gotten very fat when the mixture screws are more than 1/2 turn open from seated, with the leaner restrictors I turned them out an additional 1/4 turn for a hot idle in the 14:1 area.

I recorded trips around a 1-mile square circuit from home, each 1-mile leg separately, for an A-B comparison on roads with speed limits of 45mph and 55mph. The results show a definite leaning of the A/F, get a good amount of 15:1 at steady, level speed driving, not sure I've read just how lean you can go in those specific conditions, but at .032" IFR for my 440 with a Racer Brown SS-H-44 .510 lift hydraulic as described it seems a safe zone.

I'm about to put .032" IFR in the secondaries, I don't know how important that's supposed to be when the secondaries get so little use the way I drive it unless it's on the strip, but for continuity I'll set it up square like that.

[cat herder]

The idle mix screws control closed-throttle idle fuel. The IFR controls part throttle fuel, the range between idle and when the mains start. If you need a leaner idle just close the mixture screws (mostly - since the IFR feeds both the mix screws and the t-slots, if the IFR is way too big and the throttle blades are open too far thus exposing too much t-slot, the mix screws may not be able to cut enough fuel).

IFR should be sized to make the t-slots work right, and then the mixture screws end up where they end up to get a clean idle.