I bought one of those "wolf whistles" and hooked it up to my wiper vacuum line. It was very disappointing. So disappointing in fact that I immediately took it off and resold it on ebay. Here's the problem with them:
Number 1 - They are not a whistle, they are an itty bitty siren. They do not change pitch as a human produced wolf whistle does, they are only one pitch. Varying the vacuum only varies the volume, not the pitch.
Number 2 - They are pitifully quiet. Two reasons for that: 1. The mechanism that creates the sound is very small. 2. Instead of the sound being projected out of the horn (what a joke) as it would if the air was
blown through it, it is
sucked into the engine!
Before doing any permanent modifications like drilling a vacuum port into your manifold you really ought to hook it up to some vacuum some way and see if it's what you were really expecting.
I actully had a handheld version of such a "whistle" for many years that I could hang on a string around my neck. But it operated by blowing into it and I could get a lot more pressure blowing into it than 15 lbs of vacuum sucking on one. It was really loud and effective for calling my 10 kids together before we installed intercom in the house. But, sucking with 15 lbs of vacuum does not cut it in my opinion.
