Has anyone tried the Wilson tapered spacer on a dual plane manifold? My concern is how it might affect idle quality/low end vs high end.

 

I have used spacers for decades and I can give you this IMHO...….
for street use, a 4 hole spacer is what you really want as it in essence makes the intake runners longer and mixes the fuel just a little bit better giving better off idle throttle response, better idle and better response all the way through the midrange...….whereas an open spacer provides increases at the top end ….5k +
Now with regards to the Wilson tapered spacer...….ok, they used a cnc to create this great spacer...and yes, I too have always aligned the ports with my carbs and intakes but I also have aligned everything to work as one unit....let me explain..... if you very so slightly decrease the port diameter progressively, it will increase the velocity of the air flow...kinda like a turbocharge effect very slightly injesting more air (and fuel) faster and sooner which makes for improved throttle response. What they are doing is nothing new, and no, it should have no effect of the duel plane intake, but, for the price, you can easily buy a 4 hole spacer and with a basic file and sand paper achieve the exact same thing...without the powdercoating of course! IMHO, don't expect a "performance gain" so to speak as just doing a bit of googling, HRM did a couple of runs at a drag strip (1/4 mile) and with a prepped car, they gained 1 mph......now in racing conditions that would be important but in a street condition, heck, you wouldn't even begin to see that......which is why I say, I am a proponent of spacers, buy a standard one and modify it yourself.....

 

Thanks, I see your points and am running a 1" 4 hole now. Always on the look out for a couple more ponies. What worrries me about this space is it seems to open up the two planes under the carb. This should have a detrimental effec t on idle and low rpm.

 

All these fancy spacers are just marketing BS.

You will see a change in performance because, like you said, it is essentially giving you somewhat of an "open plenum." This may be bad or good depending on your situation, but you can make this happen for a lot less $$$ than what they are charging you for this wilson guy.