View Full Version : stagger jetting the B-block single-plane M1 intake

07-20-2015, 02:59 PM
Hi All,

Just want to post some results on stagger jetting the Mopar M1 intake on my street/strip 383/4-speed '69 Roadrunner. We had been running a 440 up until last year at the Sonoma drag strip when we lost the engine to a broken exhaust valve. When the car was running the 440 (with the Mopar '509' cam) would turn 1/4 mi in the high 12's. Broke cam, cracked block, big mess, mostly a total loss. Anyway, we built back the old '69 383 with stock rods, stock forged crank, 0.030" over KB flat top pistons (with huge valve reliefs), rebuilt 440 source heads, with Comp Cams '484' Nostalgia grind, full engine balance. Valve train is the old stock heavy duty rockers with ancient 3/8" Erson pushrods and new Comp 925 double springs (no valve seals !) on the 440 Source heads. With the rebuilt 383 we are now running in the low to mid 13's at around 103~104 MPH.

The best drag strip jetting on the B-motor M1 with our Holley 750 DP/HP carb is 73's up front and 85's in the back, no power valve in the back. I always felt this was "rich" so I recently tried some "stagger jetting". The Mopar Engine book ("Big Block B-RB Engines", Fig. 2-109, pages 247-249) suggests adding fuel distribution fixes using wooden Popsicle sticks and epoxy. NO WAY am I going to do this ! The last thing we want is chunks of epoxy and wood jamming up the valve seats !

You'll notice from the Fig (attached) that cylinders 5 & 7 run rich requiring fuel dams. Also cylinders 1 and possible 8. If we drop the jetting on the right rear corner (5 &7) (looking from the front) this might smooth out the distribution. But 5 & 7 are also short manifold runners compared to 1 & 3, and so are 2 & 4 compared to 6 & 8. So I made a wild guess that the left front (2 & 4) should also go lean. I dropped the jetting to 83's on the right rear and 71's on the left front. Pictures attached.

Back at the drag strip, with the staggered jetting the Roadrunner didn't go any faster (ET or MPH) but the motor idled better, less exhaust smell, and ran smoother over it's operating range 2500 ~ 6000 RPM. On the freeway at cruising speeds, between 3000 and 3500 RPM, the throttle respond was much improved, and I could swear that the fuel mileage improved.

Anyway, not a scientific test, but something all of you with B-motors and M1's might want to try,

Mark Henesian