Noise only at high rpm and high boost

sajds0430

Registered User
Hi all -

I'm getting a noise (not a squeal) that sounds similar to a blowoff valve on a turbo car. It's kind of similar to the howl of spinning performance tires.
It happens only at 4300 rpm or higher, AND only at full boost (11-12psi)
10% OD blower pulley
10% OD JS pulley - all done with proper belt lengths.
Ported m90 on a 1989 engine installed in a 98 Mustang.
New belts and tensioner springs last year, best quality I could find.

When it happens, all gages remain steady (no drop in boost). I don't have the Wideband on it, so I'm not sure what happens with the AFR. But, the car continues to run smooth and pull good power. No hiccups or performance issues, just the noise.

Weird thing is, I've been running with this combination for 4yrs with no issues until recently.

I can hold 12psi no problem until 4300rpm, then the noise.

Any ideas?

I was thinking either belt slip or something with the bypass valve. I was going to try Comet powder on the belt to see if that makes any difference (good or bad). At least I would know what to go after.

Any help is appreciated.

Thanks!
Scott
 
I think you are on the right track. It could also be a boost leak from the piping after the blower.
 
Hard to say without hearing it myself but definitely not the bypass valve. Does it do it when you rev it that high at idle or only when driving? If its under a heavy load like that I might suspect supercharger bearings or something of that nature. To help single it out, you could remove the blower belt and drive, see what happens then.

chris
 
Thanks to both of you for your input!

The boost leak after the blower is possible. After this many years, the couplers are getting old and the outlet gasket may be weeping.

It only does it under load and high rpm. I can rev to the moon in neutral, no issue. I can floor it while driving from idle to 4300rpm , no issue.

2 years ago, I rebuilt the supercharger. I even tracked down that special Dow Corning grease for the rear needle bearings. But, it could still be bad bearings.

Thanks for helping me rule out the bypass valve.

I'll check it out after work today.
 
Well, I put a dabble of belt dressing on the 2 supercharger belts.

I wound it out past 5000rpm full throttle with no problems. I guess it was definitely belt slip. Interesting how it howls more than squeals.

I'll keep testing it this week, don't want any cops around today.

Thanks for your input!
 
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