Engine tone changes at 5k rpm

[bandit]

Hoping to get some insight on one. Last week I swapped out my header gaskets for some copper ones. Have no idea if changing those is related but yesterday at right about 5k rpm (first time I pushed it up there since gasket change)the sound tone of the engine all of a sudden noticeably dropped down and it felt like the engine just smoothed out from it's normal rumble/vibration. Happened all of a sudden at 5k and not a gradual change. Did it twice yesterday when I pushed it up there.

Some things to note:
-It never did this before.
-I also checked and tightened manifold and carb bolts last week
-had spark plugs out at same time and they looked good.
-not sure if it matters but had to take off valve covers to get to header gaskets
-I get coolant loss through header bolts when I have to remove them so had to drain and refill some coolant when I did the gaskets.


Other than that, all is the same. Been doing some reading and wondering if this sounds anything like the start of valve floating? Just taking a stab here. It is raining today otherwise I'd have it back out listening to it again. I let off the gas a second or so after it happened so not sure if I am losing power at the same time. One thing I want to get back out there and see.

 

[67 coronet]

valve float can start at about 5000 rpm on hydrolic cams,so that could be your culprit.what kind of valve train are you running and how old is it?

 

[bandit]

Not sure but I believe it is all original to the engine(1970, 383) I know it has had a rebuild in it's lifetime but not sure when. It was all fine before the header gasket change. Could higher back-pressure from better sealing exhaust gaskets trigger valve floating somehow? Never had this issue before.

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Is there any way to verify or eliminate if this is valve float?

 

[67 coronet]

not unless the leak was massive.are you sure you just never heard it with the exhaust leak?does it have a noticeable loss of power?

 

[Cranky]

Higher back pressure shouldn't have anything to do with valve float....at least not at 5k. What's the rest of the exhaust system like down stream? Had a car once that would do something to the muffler/s at high rpm that made a sound change but it wasn't long after I noticed it that a muffler blew out. What kind of mufflers are on it? Glass packs?

 

[Dennis H]

Chassis Dyno say my 440 does not pull after 4850 so I don't go upstairs. No need.

 

[bandit]

Going to try and take it out tomorrow to verify if it is losing power. Supposed to be sunny. The exhaust leak wasn't that loud before. This is a very noticeable change. I'll post if it is losing power up there once I get it out again.


I put the exhaust system (headers back) on last year myself. Jegs exhaust system with no name turbo mufflers that came with it. Not a super high end system but still looks and sounds good. Checked mufflers and they show no signs of failure. I have also had it up over 5k many times since putting it on with no issue.

 

[Cranky]

Chassis Dyno say my 440 does not pull after 4850 so I don't go upstairs. No need.

What's your build? Even tho a lot of mild/stock engines start falling off at that point, it doesn't mean they are not pulling anymore. Does yours go dead flat at 4850? Also, a 383 with it's shorter stroke will usually spool up higher than a 440 and still pull well. A buddy and I played with a teen some years back and used a mild cam and it pulled hard all the way up to 6200 where it went dead flat and just wouldn't go anymore. The engine didn't change it's tone at all so we suspected that the cam gave it's all at that point....

 

[bandit]

I want to add that I took the heads off and had them milled two years ago and they checked the valves then. Not sure if that included the spring tension though.

 

[Cranky]

I want to add that I took the heads off and had them milled two years ago and they checked the valves then. Not sure if that included the spring tension though.

Any shop worth their salt should have checked the springs when they tore the heads apart......

 

[Dennis H]

Won't argue with Cranky, but thought that's why we drive Big Blocks. In the basement. Anything over 5, I consider excessive wear and unnecessary risk. Grab another gear. Or, get a 340.

 

[Cranky]

Won't argue with Cranky, but thought that's why we drive Big Blocks. In the basement. Anything over 5, I consider excessive wear and unnecessary risk. Grab another gear. Or, get a 340.

Had two 440's that were spun 7300. The teen went to 6200 on a stock bottom end

 

[fratzog lover]

An RB at 7300? I can hear the sweet shriek now.

 

[mopar 3 B]

A properly balanced and built 383 will hold power to 7200 so why stop at 4800?

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Did you get the sparkplug wires on right or damage a plug wire? I have never had to remove a valve cover to change header gaskets before what type covers you got?

 

[cryplydog]

I ran my 440 powered flat bottom Raysoncraft over 7 grand many times but they need to be setup right to live. I shift our 64 Polara about 5400+ with 4spd and 4:10s. Don't see any reason to go any further.

 

[bandit]

I'll check wires tomorrow for any damage and make sure the connections are good.

I have to remove valve covers because the headers have to move up and over to get enough room to get at the gasket. Hits cover lip on both sides.

 

[miller]

What kinda dizzy? If points, could have some point bounce going on, at those R's.

 

[bandit]

It is electronic dist.

I just took it out and it is not losing power over 5k. I took it to 6k. I have to say the sound is a little different today though. The pitch sounded like it went higher today instead of lower like yesterday. Before I took it out I rerouted a couple of plug wires, wiped down the distributor(looked ok but did it anyways), and backed of timing a sliver. I will say the abrupt change of tone at 5k seems to have faded though. It came on more gradual today(even though the tone was higher and not lower today). Yesterday it was like a switch when it hit 5k. Have no idea what is going on.

Broke the water pump belt while it was up at 6k so going to deal with that right now. Barely made it home.

 

[bandit]

The power steering pump belt came off at the same time the waterpump belt broke and just noticed the reservoir is pretty low. Wondering if my noise was coming from that. Getting some new belts and going to top off this reservoir and see if that helps.

 

[67 coronet]

could have belt slippage or a bad ps pump.let us know how it goes with new belts.