I will let you Cat guys in on some under hood temp testing I have done. Last year I used a wireless temp probe and a digital output monitor on my sled. I wanted to know what the air temp was going into my carbs. I too was using under hood air filters at the time but my issue was that after the sled was run for a while if I held it wide open throttle (WOT) it would start to nose over at 700 feet or so. If I turned around and came back it would start nosing over at 600 feet and it I turned around again and hit it down again it would nose over at 500 feet. I couldn't figure out what the heck was going on. I let the sled sit overnight and the next day I put the temp probe under the hood and hung it right in front of the back of the carb. It was 19 degrees outside air temp when I did this: In the first 5 minutes or so the temp measured was nearly what the outside temp was but after about 10 minutes it got warm...really warm. I was just riding normal around an open 40 acre field…just on and off the throttle like you would be out trail riding. It was up to over 75 degrees. After riding about 15 minutes the under hood temp was over 90 degrees...I did a few WOT blasts and it got even warmer and it began to lay down like it did before. So, then I put the air box on it...I have a 2000 chassis and removed the dividing plastic wall that is in it that separates the intake area from the back of the carbs. I again hung my air temp gauge in the air box in front of the carb and did the same testing. Of course in the first 5-10 minutes the air temp was closer to the outside air temp. After riding for 15-20 minutes the air temp in the air box was anywhere from 48-54 degrees and when I did WOT blasts; it did not nose over any more after 500, 600 or 700 feet like it did before. I also figured out another easy way to measure the air temp: Every time I checked the air temp gauge reading (I had the gauge with me on the sled so I could see the air temp readings as I rode around) it was within 1-2 degrees of the ID of the rear bell on the carb when I shot it with a laser temp gun. If I shot the laser temp gun at the inside edge of the carb it would give me within 1-2 degrees the same temp reading as the air temp gauge. So if you have a laser temp gun you can do the same test if you want to by just shooting the back of the carb on the Inside Diameter where the air enters. Long story short: On a TRAIL sled you are losing HP using air filters or running open carbs. Put a modified air box on it and jet it for the air now going in it. If you are switching from no filters or filters to an air box, make sure to watch the plugs because more than likely you will now be a little lean. It was interesting to me that we aren't actually jetting for 10, 20 or 30 degree weather: We are actually jetting for closer to 50 degrees...at least on my test day it with 19 degrees F air temp that was the temp that was going in the carbs. Also, don't bother putting a D&D power breather on the air box...it draws air from under the hood.
Cold air means the air is more dense and the result of more air molecules. Hot air is less dense so you will get less air. More air = more fuel. Less air = less fuel
There is a general rule that says that for every 10 degrees in intake air temp changes you lose or gain 1% hp. So if you’re 10 degrees warmer you lose 1% of your motors hp. If you’re 10 degrees cooler you gain 1% hp. So if your big bore triple makes 200hp and you're using 100 degree intake air temps instead of 50 degrees you are losing 10hp and maybe even slightly more. I didn’t test the air temp under the hood when I had the air filters on it after doing back to back to back WOT blasts…but I believe that the under hood temp would have been 100 degrees or more. The effects of the outside air also may be compounded. What I mean by that is my testing was done with 19 degree outside air and the under hood temps were over 90 degrees and the intake air temp using the air box was roughly 50 degree. If the outside air temp were raised to let’s say 32 degrees F the under hood temps may have been much higher and the intake air temps may have only risen slightly so there may have been an even bigger range/spread between the under hood temps and the air box temps. Of course the opposite could also happen if the temps were lower…the under hood temps could have only dropped a little and the air box temps may drop 5-10 degrees. Only using a laser temp gun testing in these different conditions can give the real answers.
I also imagine that some of the stabilization of the temp in the air box/just outside the carb is also due to the fuel mist that blows out of it a bit when run up. So mixing that with the outside air coming in it just eventually stabilizes to a temp. As before mentioned: the general rule is that every 10 degrees is worth 1% total hp so you can see where air temp can add up in a hurry to start effecting motor performance...especially on a modded motor which wants more air. Also, when using smaller carbs it can compound the problem….or at least make it rear its ugly head even sooner because you're already restricting the engine with a smaller carb and then you start taking away the cold air too...it can start falling on its face in no time. Now you can see why they work so hard at keeping the incoming air from turbos as cold as they can because they heat up big time in a hurry.
Cold air means the air is more dense and the result of more air molecules. Hot air is less dense so you will get less air. More air = more fuel. Less air = less fuel
There is a general rule that says that for every 10 degrees in intake air temp changes you lose or gain 1% hp. So if you’re 10 degrees warmer you lose 1% of your motors hp. If you’re 10 degrees cooler you gain 1% hp. So if your big bore triple makes 200hp and you're using 100 degree intake air temps instead of 50 degrees you are losing 10hp and maybe even slightly more. I didn’t test the air temp under the hood when I had the air filters on it after doing back to back to back WOT blasts…but I believe that the under hood temp would have been 100 degrees or more. The effects of the outside air also may be compounded. What I mean by that is my testing was done with 19 degree outside air and the under hood temps were over 90 degrees and the intake air temp using the air box was roughly 50 degree. If the outside air temp were raised to let’s say 32 degrees F the under hood temps may have been much higher and the intake air temps may have only risen slightly so there may have been an even bigger range/spread between the under hood temps and the air box temps. Of course the opposite could also happen if the temps were lower…the under hood temps could have only dropped a little and the air box temps may drop 5-10 degrees. Only using a laser temp gun testing in these different conditions can give the real answers.
I also imagine that some of the stabilization of the temp in the air box/just outside the carb is also due to the fuel mist that blows out of it a bit when run up. So mixing that with the outside air coming in it just eventually stabilizes to a temp. As before mentioned: the general rule is that every 10 degrees is worth 1% total hp so you can see where air temp can add up in a hurry to start effecting motor performance...especially on a modded motor which wants more air. Also, when using smaller carbs it can compound the problem….or at least make it rear its ugly head even sooner because you're already restricting the engine with a smaller carb and then you start taking away the cold air too...it can start falling on its face in no time. Now you can see why they work so hard at keeping the incoming air from turbos as cold as they can because they heat up big time in a hurry.