I haven't pounded you guys with one of my long-winded posts in a while, so yank those straps tight...
A key difference in this discussion is Predictability vs. Consistency.
Depending on your total program, either way can be made to work very well. (Talking strictly door cars here, and especially street-strip cars, or those with engine-driven water pumps and/or fans).
With over 10 years of racing experience using under-hood air, and more than 10 using fresh air induction, and having my share of success both ways, here are MY observations - (your results may vary!).
1. Fresh (outside) air will virtually ALWAYS make more power, maybe very little difference, maybe a lot, depending on difference in temperature of ambient (outside) air vs under-hood air temp. Simple physics - air density. But in bracket racing, that won't win you many rounds.
2. Fresh air is less Consistent
across varying atmospheric conditions, but it is more Predictable
, (IF you have a lot of good data on how your combination responds in all types of weather conditions). This can be via weather station, log book entries, or a combination of both.
3. Under-hood air is basically the opposite - it is GENERALLY more Consistent
, but less Predictable
over a wide range of air conditions. (This includes changes in Humidity, Barometric Pressure, and Temperature).
Couple things to consider:
1. None of this matters a whole lot in very stable weather conditions, where the Density Altitude hardly changes during the race day/night. Those days are usually rare, and that is when your driving ability (and luck) are the major influences.
2. Keep in mind the Fresh air vs Under-hood air question only deals with one of the 3 primary variables in air quality, (Temperature). A big swing in Humidity or Barometer will absolutely affect your car's performance, even if temp remains constant.
3. There are several things you can do to help your ability to dial the car, regardless of which air intake route you choose: Under-hood air? You need to do what you can to regulate the air temp in that engine bay. Think about this: Say you have a stock (un-modified) 180* thermostat. Ambient temp is 65*. Round 1, you heed the call quickly, so you are at the head of the lanes. You do your best to warm the car, your coolant temp is 160* at the tree. Your thermostat stays closed the entire pass, so that wall of air passing through your grill, radiator, and to your carb is roughly 65*, (some heating of intake air from your headers will likely occur, how much depends a lot on your car's configuration, grill opening, wheel wells, etc).
Round 2, ambient air temp almost the same, barometer and humidity. But maybe your pit neighbor has his generator and car running, so you missed the first call to the lanes, so now you are at the back of the lanes this time. Which means you have to start the car 10-15 times as you move forward, so you are building a bit more heat in the engine. You hit the water box at 170* this time. After your burnout, you stage, your coolant is now at 175* and before half track, your thermostat opens, now all that air passing through your radiator is heated to well over 100* before it enters your carb. Not gonna run the same number this pass. This is just one example of a ton of scenarios you might face. I am not a fan of removing the thermostat, unless you have an electric pump and fan you can control. But lets say you drill 4 small holes, (.080-.120"), evenly spaced around the flange of your thermostat. It will still work to build heat when you need to, by restricting most of the coolant flow. But it will never stop the flow completely. The result is that the temperature of the air flowing through your radiator to your carb will vary much less, (more consistent air temps = more consistent ET's, under a given set of air conditions). When I used under-hood air, (which I recommend if you don't have a lot of run data on your exact combination, or borrow/rent a car, etc), I always tried to get the coolant temp as hot as it might possibly get all day, and duplicate that for every round. Because you can generally always build heat, but you can't always get things as cool in late rounds, as you were Rd 1 or 2.
After I went to a fresh air intake, sealed with an air pan, it took many runs, (a LOT), under very wide ranging weather conditions, at multiple tracks, with NO modifications to the car, to build enough data to where I could predict the car's ET within a couple hundredths. Obviously that curve lessens over time, to where I pretty-well know (after years) what it will run off the trailer to .01 or better, even if I change carbs or fuel.
(Note: It also took a lot of runs to learn what the car would do breathing under-hood air, as well).
Bottom line, for me, is if I were going to an unfamiliar track (with limited time runs, like 1 or 2 max available before eliminations), or racing a car I didn't have a lot of passes with, or new to racing, or running a street-strip car, I would recommend using under-hood air. Maybe not quite as quick, but easier to get a handle on dialing, under most weather conditions. If I am taking my long-time piece to the local track, for a typical Summer race day, give me my fresh air hood, my trusty old weather station, and a full cooler.
Dan "Jim" Moore
Much too young to feel this damn old!!