Wayne Cook
May 11, 2010

Size Your Jets
Jet size changes can be called for if the engine is determined to be running too rich or too lean. An air/fuel meter used during a dyno session concluded that our engine was running in a too lean condition at over 14:1 air/fuel ratio at wide open throttle. We knew that besides being a dangerous condition, more power could result with a jet change on the secondary side of the carburetor.

Photo Gallery

View Photo Gallery

Carb upgrade? Don't forget the timing
Once you've got your engine running right with any carburetor issues ironed out, is it possible to wring more performance out of your engine by going to a larger carb size? We wanted to find out so we took our car, which was running fine with a 600cfm Holley four-barrel on it and decided to step the carburetor size up a bit. The 351W engine has AFR aluminum cylinder heads on it, headers, and a reasonably rowdy roller cam. We felt fairly confident that the engine had enough oomph to absorb and successfully utilize some extra induction capacity. We wanted to see what differences might emerge when comparing a Holley 4160 600cfm single-feed carburetor with vacuum secondaries to a 4150 650cfm double-pumper with a mechanical secondary.

Photo Gallery

View Photo Gallery

Here are the dyno results with the new carb and the same timing. A simple carburetor swap to the slightly larger 650 got us to well over 300 horsepower with a solid 304.3 horsepower at the tire at 5,300 rpm. Although the difference in peak-to-peak horsepower is only 5.6 hp, the curve is better overall. For example, at 4,000 rpm the difference is 7.6 more horsepower with the 650cfm carb. The bigger carb made a nice improvement on torque. The peak improvement in torque output ended up being 9.1 lb-ft, or 344.2 lb-ft with the 650 carb as opposed to the 335.1 lb-ft reading we got with the 600cfm carb.

BaselineCarbTiming Change
RPMHPTQHPTQHPTQ
3,000182.0318.6175.1306.6186.4326.3
3,200196.0321.7202.0331.5210.4345.4
3,400207.3320.2219.5339.0227.6351.6
3,600226.2330.1235.9344.2240.1350.3
3,800242.5335.1247.3341.8247.9352.0
4,000250.7329.1258.3339.2253.1349.9
4,200261.1326.5266.6333.4263.1345.4
4,400270.9323.3273.5326.4273.9342.5
4,600277.1316.3277.8317.2282.3337.0
4,800284.7311.6282.1308.7287.8328.6
5,000294.4309.2293.3308.0296.9324.8
5,200298.7301.7302.4305.5309.1324.7
5,300304.3301.5314.2317.4
5,400302.7294.4320.2310.7