We learned several things during our staggered jetting test, and some might surprise you. We also came to a couple of conclusions. 4-Corner Jetting is not for everyone, and ignorance is bliss. And then there are the questions. But before we get into those, let’s get into the test. Our goal was to determine whether we could effectively tune a single lean cylinder by adding jet size to only one corner of a Holley carburetor. Trick race guys do it. Would it prove to be a tuning tool that was worthwhile on our 530 horse street motor?
To find out, we strapped our venerable Dart SHP 372 on the dyno at Westech Performance Group and made a couple of pulls to establish our baseline. As expected, the little beast made 536 horsepower and 481 pound feet of torque. To truly understand what was going on in each cylinder, we used Daytona Sensor’s Wego2 which allows us to monitor each cylinder with a seperate oxygen sensor. To tell you the truth, years of using a standard single or dual oxygen sensor setup do not prepare you for actually monitoring 8 of them. Individual cylinders do strange things sometimes, and the numbers can certainly make you nervous.
Our test engine is based on one of Dart’s SHP 372 Short Blocks with all the good internals. With AFR L98 195 CC CNC aluminum heads it has 10.5 to 1 compression. A Comp 242/248 Hydraulic roller cam and associated valvetrain lets it rev to 6800 without issue, a TCI Rattler harmonic dampner keeps it from shaking, and a Milodon oil pan keeps the Lucas oil contained. For our tests we ran Holley’s Strip Dominator intake, and the proven Holley 850 Street HP Carb. You can see our test engine below.
With our baseline set, our little small block had 36 degrees of timing and the Holley Street HP 850 had 82 jets and a power valve up front with 92 jets in the rear. For this test we have taken the Air Fuel Ratio (AFR) average for each cylinder during our 3000-6500 rpm pulls. We have also average those to get an overall AFR average for the engine. For our Baseline, we saw an average of 12.3 for all cylinders, but as you can see below we had 13.9 and 14.1 average AFRs in the front two cylinders. That is a scary number when compared to the mid to high 12 AFR we look for to achieve peak power. At this lean a level, most people will be nervous about lean condition detonation, piston damage, etc. See Corner Jet 13 below for each cylinder’s average.
For our first shot at fixing the two front cylinders, and especially number 2, we added two jet sizes in the passenger side primary, making our jetting 84 in the front and 92 in the rear. We expected to see some improvement and were rewarded with a total of 4 tenths of a drop in AFR in cylinder number 1 and 4 tenths in cylinder number 2. AFRs in almost all the cylinders were affected, but we were surprised not too see more improvement in number 2 rather than number 1. Our overall AFR average changed from 12.3 to 12.1. See Corner Jet 14 below for each cylinders average.
For our final test we threw even more jet at the passenger side primary. We increased jetting from 84 to 88 so that our overall jetting was 88RF/82LF and 92 Rear. This was our most interesting test yet. We found that adding that much jet really made a difference in the number 2 cylinder and lowered the AFR to 13.5 rather than the previous tests 13.8. We were now at 13.5 on both front cylinders. In comparison, our overall average changed only 1 tenth from 12.1 to 12.0. Clearly even two jet sizes is not enough to move the AFR around much, and it takes significant change to make a difference. How did all of this affect power levels. No surprise, it did make more power. In fact we saw peak improvements of almost 5 horsepower and 4 pound feet of torque. Check out the graphs below.
So what did we learn here? Well first off we learned that it takes a much more dramatic jet size change than you would expec,t in order to change the AFR in a particular cylinder. We also learned that this dramatic change in one jet only affects overall AFRs slightly. Our most dramatic realization however was that the AFR for any given cylinder can be dramatically different than that of the overall engine. The scary part is actually looking at those numbers and seeing just how bad one cylinder, or group of cylinders, can be. It proves that while using an O2 sensor to determine overall AFR is a great tuning aid, and one that all of us should have, you still want to check your spark plugs to see how they look for each cylinder. Keep in mind, we are testing with a fairly healthy small block, but certainly not a race engine. On a higher horsepower, multi carb, engine we expect that we can change AFRs even more. Is this the right solution for your street car? We’re not sure. We were testing on the engine dyno, and don’t now how this jetting would affect drivability. We’ll test it out in a car when we get the chance, and see if we can come to a good conclusion and let you know. Until then, race guys should be hitting the dyno shop, testing all cylinders, and making some adjustments. It could be the difference between winning and losing.