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Discussion Starter · #1 ·
Hello all,

The sound is not of my wallet emptying,, I have been reading dozens of post and no answers.

After Full throttle, to ~4500 rpm, when I let off the gas completely i get a sucking sound almost like a kid and a straw changing the pitch using their fingers to pinch it.

Both motors,, one AMC 360, one ford 400
over 11-1 compression,
mild cams [email protected] and 224 @ .050
aluminum heads and intakes.
Basic 3 inch air cleaners with breathers to the valve covers.

nothing crazy.

Does it with a old holley and a new QF 600 just the same. Motors run perfect now, over 15 inch vaccum, idle perfect, no leaks.

Any ideas??? I might try to film it and post.

Never had this on any other build over the years, does not do it on my SBC 406 with an AED
 

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210 Posts
Discussion Starter · #4 ·
With or without an air cleaner mounted?

Bogie
Either,,, I tried to film in the ford last night but it does not pick it up in the video.

I wonder if it is blow by pushing out somewhere? I do run 5/8 hose from the breathers to the underside of the air cleaner on both builds.
 

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Race it, Don't rice it!
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8,541 Posts
Either,,, I tried to film in the ford last night but it does not pick it up in the video.

I wonder if it is blow by pushing out somewhere? I do run 5/8 hose from the breathers to the underside of the air cleaner on both builds.
There isn't blow by on deceleration when the cylinder is 25+ lbs of vacuum.
Just an idea, perhaps the brake booster is venting excess.
 

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Hates: Liver. Loves: Diesel
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6,169 Posts
You probably have a vacuum leak under your dash. When you do a full throttle and then close the throttles at 4500 rpm, the manifold vacuum shoots sky high. Maximum suckage from the engine with a throttle plate blocking the flow. It's possible it was always there and you're just hearing it now because of a different air cleaner setup. My 96 Impala SS with a cold air intake made a HUGE sucking sound when you shut it off. The throttles were closed, and turning off the switch closes the IAC, so there was about a half-second sucking sound as the air rushed past the throttle to equalize the pressure.
 
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