View Full Version : Engine noise when accelerating - hissing
Looking for some help trouble shooting a noise coming from the engine while accelerating hard. Build is a 427 (351w) from blueprint w/ external fuel pump.
When I accelerate hard I hear a hissing noise (sorry for the bad description). If I accelerate slowly I don’t hear anything. Doesn’t make the noise after I let off the throttle and still at high rpms. Doesn’t sound like a exhaust leak or at least not one I’m use to.
Any ideas? Could it be the intake? External fuel pump?
I’ll try to take a video next time.
65 Cobra Dude
09-12-2021, 07:52 PM
Vacuum leak?
That’s what I thought at first. There aren’t many places a vacuum leak can occur on an engine like this, right? Any thoughts on how to test/identity where the leak would be coming from?
Post a video with audio of what you hear.
You do know that vacuum is highest at idle and lowest at WOT & low RPM (hard acceleration). And it's common to hear intake noise when accelerating hard, so is that your hiss?
I hope it’s just normal intake noise. I’ll record a video tomorrow and share.
Otee453
09-12-2021, 10:49 PM
427 fuel injected from BluePrint?? That is likely the Holley Sniper “fuel injection”. The sniper is well known to whistle a bit under heavy throttle. It’s the IAC sucking air. Very common. The fix is a 4 hole carb spacer. Even as little as a ¼” thick spacer will eliminate the whistle. An open spacer won’t fix it, it must be a 4 hole spacer.
FF33rod
09-13-2021, 10:50 AM
This thread has links to youtube videos demonstrating the noise and also has links to spacers that helped. Note that despite the wording in the link, the noise happens to a lot of people but it isn't necessarily "normal", you can get rid of it
https://forums.holley.com/showthread.php?20273-Sniper-loud-whistle-hiss-normal
Steve
AtlantaCobra
09-13-2021, 11:30 AM
I love the title of this post! "Engine Noise when Accelerating" For some reason that's just an awesome title.
FF33rod - Thanks! The noise in my build is exactly like those videos.
The fix seems simple enough. Thanks again FF33rod and others.
toadster
09-13-2021, 04:55 PM
well - it IS a cobra... hissing included LOL j/k
no clue just thought that was funny - I'll show myself out now :rolleyes: