7.90 slowing at night

Home Forums Engine 7.90 slowing at night

Viewing 2 posts - 1 through 2 (of 2 total)
  • Author
  • #1096
    Eddie Scott

    Got a craws 3 1/2″ motor with a 42mm carb and a shockwabe clutch. Car runs pretty consistent until we go from day to night racing or hot to cold. Go from running 7.93 to a 8.03 on a single pass. But once it gets there it stays. Any ideas on what maybe happening?


    Matt Parlett

    What size jet are you running and do you have any graphs that you can post so we can see what is going on. Preferably the graph before the night change, the graph after the night change and then both of the graphs laid over top of each other. Without seeing the graphs or knowing the weather, I would say you are either hitting the dew point and the humidity grains are shooting up or the weather is changing enough that you are hitting your tuning wall.

    When you live in a humid area there is a lot of moisture in the day but the sunlight is constantly eating it up. At night time without the sun that moisture, or humidity grains, will jump up and cause there to be a lot more moisture in the air than when the sun was up. This causes the engine to loose power and go slower. The only way to compensate for this is to have a weather station like the RaceAir Cloud that will be able to monitor the weather and then a program like our RaceBase software that will automatically tell you how much you will pick up or slow down based on that additional moisture.

    If you are not seeing the humidity grains go up but the performance is still dropping, then it may be that you have hit your tune up wall. Now most people that race juniors and even many engine builders will say that you never need to change your jet but I personally believe that to be out of ignorance or laziness. The reality is that as the seasons change, so does the quality of the air, and if you are not changing your jets to compensate for that, then you air / fuel mixture is changing and thus how much horsepower you can create is changing.

    Think of your tune up like the scales of justice. In one hand you have air and the other hand you have fuel. If you drop your air hand down because the summer kicked in and the weather is worse and you don’t change your jet, then you’re air fuel mixture has changed whether you like it or not. And thus how much horsepower you create has changed. If you go to far lean, you get lean spikes and pops down track. If you go to rich you get bogs and inconsistencies at shift. Either is not ideal and if you think of your tune up like a room where rich is one wall and lean is the other, then we want to stay as close to the center of that room as possible. That way if a big weather change does happen during race day, we may move within that room but we won’t hit a wall where we really see a performance problem.

    At Computech we have been running the @RowansRacing junior dragster for a couple years and have built the RaceAir Cloud with RaceBase software with junior dragster racing in mind. That’s why we have an easy to customize et prediction formula to help you predict those day to night changes better. We have a mikuni tuning program to keep you tuned the same from week to week and we even have an index weight prediction to help you dial the 7.90, 8.90 or 11.90 index. If you run a Jr. Dragster, having the RaceBase software gives you a competitive advantage over your competition who doesn’t.


Viewing 2 posts - 1 through 2 (of 2 total)
  • You must be logged in to reply to this topic.