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AKA Lee in KC
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Discussion Starter #1
Just got my new T-bucket running and find that there is a shudder as the RPMs pass through about 1800-1820. Happens increasing or decreasing RPMs, and in all gear settings P-R-N-3-2-1, moving or not moving. Whatever it is it is definitely a function of RPM. Here are the specs:

- Reputable hot-rod shop built 350 SBC long block (010 casting, 4-bolt mains, 2-pc rear seal), decked, align-honed mains and cams, torque-plate .030 over, all new parts, quality rotating assembly for mild blower application, Comp hyd roller 224/228/112, Dart Sportsman II heads, 8.5 static comp
- BDS 6-71 blower @ 1:1, 6# boost, dual Edel 600s set up for blower
- MSD mag trigger distro & 6AL
- TCI Streetfighter converter 2800-3000 stall (new)
- TCI Streetfighter TH350 (new)

I've checked the flexplate bolts & converter bolts, all properly torqued. Tranny fluid is fresh and topped off. This motor doesn't have a harmonic balancer, only a crank hub for the blower drive pulley.

The shudder only occurs in the very narrow RPM band of 1800-1820.

Any guesses?
 

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AKA Lee in KC
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527 Posts
Discussion Starter #4
35WINDOW, yes the bloweris new.

Derkyb, BDS said I don't need a harmonic balancer. They said the crank hub, drive pulley and blower drive belt serve the same function.
 

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I assume it was Balanced (and I think even if not the Shudder would be at a higher RPM-Motor Mounts are tight-you do have Rubber Motor and Tranny Mounts, right?
 

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I guess I want to make certain that I understand "Shudder"-not hesitation, this is something you can feel through the Steering (hands), Seat (Butt), or Throttle, right?

Is it only between 1800-1820? (pretty narrow range)-is it like a balance problem or worse?
 

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AKA Lee in KC
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527 Posts
Discussion Starter #8
It is definitely a shuddercoming up from the engine/drivetrain. The RPM band could a little wider than 1800-1820... Could be as much as 1750-1850... Don't know how precise the tach is. But the affected band is certainly very narrow. I'm thinking converter.
 

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AKA Lee in KC
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527 Posts
Discussion Starter #9
It is definitely a shuddercoming up from the engine/drivetrain. The RPM band could a little wider than 1800-1820... Could be as much as 1750-1850... Don't know how precise the tach is. But the affected band is certainly very narrow. I'm thinking converter.
 

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ihello, xntrci t,,i had the very,,exact,, problem,,with a new rebuilt,,327. {found out with help from the guys,, on this site that it was a bent water pump shaft...[[BRAND NEW ]] u could barely see it wobble ,but if i loosened alternator,,and pushed it forward vibration went away,, carefully put tension on it with it running and vibe.. came back ,,,hope this helps,, watch your fingers :pain: R C
 

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T-Bucket, Corvette, Mustang
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My two cents. I have a T-Bucket that does the same thing between 2000 and 2500 rpm. While sitting still in park or neutral, you can slowly bring the engine into this range and you will feel a steady shudder-like vibration, not much but noticeable. If you are on the road it is noticeable as the same shudder but it is a rolling vibration through the car - there for a second or two, then gone for the same length of time, then back again and so on. Most noticeable in high gear at a steady speed - seems to lessen if you accelerate or decelerate in this rpm range. You can also feel it in the lower gears if you are really looking for it but normally you go through the lower gears fast enough that you don't feel it. I had this engine and trans in another T-Bucket and it did the same thing. I have replaced the water pump recently with unfortunately no change. This engine is a mildly modified 350 SBC with a TH350, 3.73 gears and 31" tall tires. At 50 mph I'm right in the middle of this range so I notice it but most of my passengers don't. If I throw it into neutral at about 55 mph and let off the gas, the vibration goes away immediately. Put it back in gear at 50 and it resumes. At 65 mph, it has pretty much gone away. New rubber mounts. Balancer looks good but I'm considering changing it. Feel it has to be in the engine or perhaps the torque converter. It has been suggested to me that this may also be a natural harmonic with the frame and the mass of the car - suppose that can't be discounted either. Car is well-behaved in all other respects.

I was speaking to a friend at a cruise this summer and he said his T-Bucket does the same thing at about 2100 RPM or so. We're beginning to treat it as more of an annoyance - I'm sure one day one of us will stumble across something. Good luck to you and let us know what you may find.
 

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Doc here, :pimp:

Just a thought..(and nothing more..not being able to "Feel" or "Hear" it..)

Is this like a vibration (less of a shuttering) , at that range?

And if so, are you running Flowmasters?

Flowmaster 40/50 Series have a major Sympathetic Vibration between 1500 and 2000 RPM , that depending how close to the floor pan can put a tickle in your toes and also a buzz in your ears.. :pain:

All the sets I have had did this..AND they have an added bennie..They will light off cheap alarms as you slow cruise down the main in the right lane.. :D :pimp: They also vibrate my neighbor's Kitchen windows on cold start warm ups in the mornings..

just another place to look , when places to look run out.. :thumbup:

Doc :pimp:
 

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How about the blower belt? Have someone watch it while you rev it to the affected range, and see if it starts vibrating back and forth at one of the long spans. If its a little loose, it could cause this vibration, and since you have a light vehicle, it would be more noticeable (less overall mass to dampen the harmonics).
 

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AKA Lee in KC
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Discussion Starter #16
Did a little more diagnosis this morning. Borrowed a friend's digital tach to confirm the RPMs. Sitting in the driveway in park, I increased RPMs slowly from idle to 3000. Starting at 1780 I could just start to feel the shudder coming in, starting at a relatively low frequency and intesity then building as I passed through 1800, peaking in frequency and intensity at 1810, then dropping off rapidly as I passed through 1830. No unusual vibration above that on up to 3000. Coming back down to idle, the same sequence occured... vibration building as I came down through 1820, peaking at 1800 and fading out by 1780. If I held the RPMs at 1810, the vibration held steady also.

The frequency of the vibration is well below one per revolution. Probably a harmonic, as cucumber1949 has suggested. I had the friend observe the blower belt and it did indeed shake in the same range as the shudder, so I had him hold a plastic screw driver handle against it (the idler is at its tighest setting) as I revved through the shudder range, and it did not affect the shudder. I guess the belt was vibrating in sympathy with the shudder.

I will try topfuel's suggestion of unbolting the converter sometime this weekend.

Doc, my exhaust system consists of roadster-style headers with sidepipes and turnouts. The mufflers are inserts in the turnouts. I checked the header bolts and all are properly torqued. The inserts fit snugly in the sidepipes, so they can't vibrate, and I don't detect any resonance in the exhaust sound.

As cucmber1949 says, this is really just a minor annoyance as long as something in the motor or drivetrain is not shaking itself apart.
 
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