I was watching Horsepower TV this weekend. They took a trip to Roy Hills Drag school. During the show, Hill talked about "closing the carb off" with a carb plate and foam around it, allowing only the air that comes in from the scoop to enter the carb....said that without it, there would/could be turbulance created in the engine bay (at speeds) and it could actually pull fuel from the carb. Which makes since to me :16suspect I have seen this on many cars at the track. My question is this: Is this something that needs to be done if you are having a lean condition or is there actually a performance advantage? I do not believe this is causing me any trouble at all with my set-up, but I was just wondeirng if anyone as experienced any performance gains by doing this?
Back when I was racing my original Maverick I had made such an isolation plate by using the base of an old aftermarket air cleaner and glueing enough foam rings on the base so that it would make a tight seal against the underside of the hood directly under the hole for the carb. I always used it until one day I forgot to take it to the track. I had to race without it and my et's were about 2 tenths slower without the it. So yes, it will make a performance difference.
the scientific answer is that for each degree drop in incoming air temp, you will get 1% hp increase. if you are already lean, a scoop will lean you out more. best results will be gotten by running the car slightly rich so there is fuel available for the denser incoming air to help burn. also makes car's et more predictable by being able to track the air temp on each run and record et changes for predictions. its very difficult to predict what the under hood air temp will be for the next run, and tricky to actually measure it on the previous runs.
I use that exact setup in your attachment on my Mustang. I cant say that I have ever run it without it, but I do like to contribute it, and other simplicities like it, to my cars very consistent runs. Like I said, I dont have any numbers with or without it.
I would think a lot would depend on the style of the scoop. In the case of the Nova on Hot Rod they had a cowl scoop which acts on the low pressure area at the base of the windshield. In your case John the intake is on the front and has more of a ram air effect. I think it is definitely worth a try in your case.
John in a lot of situations it's a definite performance advantage, at launch your carb is using mostly superheated engine air, as you move down the track this air is displaced by outside air that the scoop is gathering, isolating the carb allows only the cooler outside air in. If your using a fiberglass hood with pins you have to be careful, the first time I ran an isolator tray it scooped so much air that it looked as if it was gonna pull my hood off, scared the crap out of me, when I got back to the pits I trimmed some metal off the backend of the tray so whatever the carb couldn't intake it would bypass back into the engine compartment, mine is homemade and kinda looks like a surfboard sittin on top of the carb..not pretty but very functional, the first time I ever used it was in the dead of summer,scorching hot at the track and had been running flat 6's...put the tray on and ran a 5.97 and have never ran another 6 unless it was intentional (index), also I dont use it when its below 70 degrees. It's not really necessary IMO.
Another thought for you racers with open carbs. Back when I worked with the Pro Stock car we always made sure either someone was standing in front of the scoop or the scoop was plugged while the car in front of us was doing their burnout. This was to prevent accidental debris from entering the carbs.
i found a way to mount a k&n filter inside the cold air box. then you dont need someone to block the scoop.
Good points I am going to make one myself. Once I get to the track, I will make a couple runs without it, then put it on and see what happens Thanks!
I used to run one back before the carb got so tall that it protruded up into the hood scoop, then It wouldn't fit anymore and I haven't messed with it since then. I think on your car it would be beneficial because the scoop is so much taller than a comet scoop, yours can actually grab fresh cool air. IMO the comet scoop isn't tall enough to really get a "ram air" effect, just tall enough to let in a little fresh air.
I should have the stuff to make one this weekend. With my carb spacer on, the front of my carb is flush with the hood. But the back is about an 1" or so. I will build it up with foam and see what it does. I hope to get to a T&T the first part of next month.
