Tuning in a Pinch

http://www.rchobbies.org/images/pinchtest.jpg

The pinch test.

Many racers find it difficult to properly tune the low speed needle. We've developed a method that makes it easy, even for beginners, to adjust the low speed needle to within 5% of peak performance and it's done right on the bench. We call it the pinch test and here's how it works.

Step 1. Start your engine and warm it up for about 30 seconds by revving the engine from idle to mid-throttle while holding the wheels off the ground.

Step 2. Allow the engine to idle for 10 seconds. If the low speed needle tuning is so far off that it won't idle, simply bump the idle position higher until the engine will reliably run at a higher idle.

Step 3. With the engine at idle, pinch and hold the fuel line near the carburetor, cutting off the fuel flow and carefully listen to the engine rpm.

If the low speed needle is set correctly, the engine should increase rpm only slightly and then die.

If the engine increases several hundred rpm before dying, then the low speed needle is too rich and needs to be turned in.

If the rpm doesn't increase at all and the engine simply dies, the low speed needle is set too lean and should be richened or turned out.

After doing this a few times you'll get the hang of it, and tuning the low speed needle will become easy. The only way to get that last 5% performance on the low speed tune is through careful track testing. By evaluating the engine as it accelerates from a dead stop or out of slow corners during actual race conditions is the only way to get those last few percent of power from your engine. Variables like the clutch setting, gearing, traction, atmospheric conditions and even the car's weight will have an effect on this last 5% of tuning performance, and the optimum setting can only be reached by careful on-track tuning.

Fine Tuning in a Punch
On high traction tracks instant throttle response and acceleration are the keys to being competitive. To get that last 5% of performance thru tuning, many top drivers and engine tuners use the punch test. Here's how it works.

With the high speed needle properly set and the low speed needle set per the pinch test above, run several laps at race pace to bring the engine up to temperature. Now stop the car at the beginning of the main straight and allow it to idle while you count to 15. At the count of 15, hit the throttle instantly full punch while carefully listening to the engine and note the rate of acceleration. If you did a good job with the pinch test above and the high-speed needle is properly set as described below, the car should rocket off the line with a powerful, crisp consistent engine sound. The trick here is to tune the low speed needle for even greater acceleration and an even crisper throttle response. Do this punch test several times until your very familiar with the acceleration rate (it helps to judge the amount of wheel spin) and the sound of the engine as it accelerates to full throttle. Now lean the low speed needle 1 hour (or 1/12th of a turn) and do the punch test again. Is the engine's acceleration rate and the crisp consistent sound better than in the previous test? Remember judging the wheel spin (the amount the wheels slip during heavy acceleration) is a good way to compare. If it's better or there's no change, lean the low speed needle 1 hour more and try the punch test again. At some point the engine will lean bog (hesitate) off the line during acceleration indicating that you've gone too far.

When you finally find this point through repeated punch tests then leaning the low speed needle one hour each time, richen the low speed needle 2 hours. This will be your engines peak setting giving maximum acceleration and throttle response for the specific conditions of the day. Just remember if you change anything, like gear ratios, clutch settings, exhaust systems, fuel, glow plugs and even changing atmospheric conditions like temperature and barometric will cause this optimum setting to change. Repeat the punch test often and especially just before a heat race to ensure maximum performance during the existing conditions.