SBC 350 vibration cause?

Ken47

Ken47

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Supporting Member
The saga of trying to get my 47 running well continues.... When driving the car there is a vibration starting at about 90 km/h. I thought it was a drive shaft angles problem, but I’m not sure now. When I rev the engine in neutral to 1800 rpm there is a noticeable vibration. 1800 is about where the engine runs at 90 km/h.
So far I have run the engine with the water pump, alternator etc disconnected and the vibration is still there. The engine was balanced (maybe not done correctly?) when rebuilt. So my question is; where should I look for the vibration cause before hauling the engine out and getting it rebalanced. I’m hoping it’s something else that is more easily addressed. Any ideas would be greatly appreciated.
 
Just a guess, but I would think a (different) race car engine builder would have the technology to analyze and diagnose the running engine.
 
Up to 1985? 350's were all internally balanced. After that they went to one piece rear main seal and had to externally balance. Have you got the proper flex plate / damper ?
 
Hi Jethro. The engine is a 1984 350 2-bolt main engine. Flat tappet H268 hi energy cam. It was built by a local “notable” engine builder here in Calgary so I assume it is the right components in it but I will check. I’m not sure how to know it has the right parts on it.
 
I’m wondering if my harmonic balancer might be bad. I assumed it was new when the engine was built but I don’t know for sure. The engine was built about 18 years ago but only ran for the first time a couple of years ago. The car so far only has 300 km on it but that may be misleading for the balancer.
 
Ken47 said:
Hi Jethro. The engine is a 1984 350 2-bolt main engine. Flat tappet H268 hi energy cam. It was built by a local “notable” engine builder here in Calgary so I assume it is the right components in it but I will check. I’m not sure how to know it has the right parts on it.
Click to expand...

Ken47: have you contacted that local "notable" engine builder there in Calgary to
get his opinion on what might be the problem?

Bgbkwndo.
 
Ken47, you might want to pull the dust cover off the tranny and check to see if there is any torque converter balancing weights missing. I’ve had one come off a 727 tourqe before that created a good vibration.
 
Hi taggart29. I will do that. I think while I’m in there I’ll weigh each torque converter bolt to be sure they are all the same. I’m very sure they are but just want to eliminate another potential cause.
 
Progress of sorts I think. I took the belts off again and removed the crank pulleys. The balancer is “home” up to the timing chain cover. There is about 1” of balancer hub extending out past the end of the crank. Is this correct? I would have thought the hub would go on farther. The balancer has 2222H as a part number on it. I looked it up. It says it’s for a 350 small block chev. I did notice the elastomeric isolator shows some minor cracking in the circumferencial direction. It is still soft to press on it. I suppose since I’m this far into it I should change the balancer anyway. Any recommendations on what one to buy?
 
More info; on the harmonic balancer I measured the run out both axially and radially. It’s an 8” balancer. Axially it’s .031” TIR (total indicated runout). Radially it’s .014” TIR. This seems like a lot to me. Any comments? I can’t find any specs for TIR on the internet.
 
Hey there Ken; that runout seems too high for me. I just picked up a new one for Karen’s A (SBC 350), as the old one had lot more cracking in the rubber than I was comfortable with. I will get a look at it Thursday night and can let you know what make we picked up.
The balancer needs to be “well engaged” on the snout. I believe they bottom out. I will also check the amount of free space from the face to the front of the crank when I get home... unless this has already been taken care of.
Your car is looking great.
Cheers.
Greg
 
Thanks Greg. I appreciate your help. I am a little discouraged that it has these problems. But I learn something new every time I tackle a problem. The used 3.70 gears I put in the Camaro diff are great (quiet) on the drive side but whine a bit on the coast side. I can live with it and maybe that will be a winter project to get new gears in the diff. 😄
 
Update: I changed the harmonic balancer today. Got a new one locally. The vibration is still there at 1800-2000 rpm. It’s is definitely better than before. So the last thing to check is the torque converter. If that isn’t it then I have to assume it’s a bad balancing job when the engine was built. The engine only has about 300 km on it. Can an engine vibration improve (lessen) as the engine breaks in?
I was able to contact the original engine builder. He’s leaning toward a bad balancing job. He subcontracted that activity out at the time the engine was built
 
Hey Ken, if the engine is vibrating now, something, as you know, is wrong. If the vibration goes away then something would have changed but the initial problem would still be there, possibly worse. A new motor, or old motor should not vibrate that bad, specially if it was “balanced”. Even my model A doesn’t vibrate that bad, and the engine, at the rear, is directly bolted to the frame. I used to do vibration analysis on all the equipment out here at my mill, bad vibes can wreck stuff real quick, the saving grace in a engine is the plain bearings, they take vibes much better then roller or ball bearings.
One question i have also is, what’s your oil pressure at when the engine starts vibrating? One other cause could be, however rare, I’ve only ever seen it in large recip compressors is oil whirl or oil wipe. This usually only happens though when the bearings are worn, the crank will essentially push a wave of oil around until it rides up over it, then starts the cycle over again and your crank journal will run a kind of triangular path through the bearing bore and create a bad vibe. I real doubt this is your problem though.
Let’s us know what you find on the tourqe conv.

Cheers
Brian.
 
Hey Ken, was your motor internally balanced or externally balanced? It should say on your balance report. If your motor was internally balance but has a externally balanced flex plate she’ll rock bad.
 
Hi. I assume it was internally balanced but not sure. I’m beginning to think this was a Mickey Mouse build. The harmonic balancer I took off was used when the engine was built. It had the oil seal surface repair collar on it.
Here is the only info I have from the “build sheet” regarding balancing.
Big end x 2. 420 - 840
Small end. 184
Piston. 735
Rings 59
BRG x2. 49 - 98
Oil est. 2
Total bob wt 1918

How can you tell if a flex plate is for internal or externally balanced applications? Mine is new an you can see where material has been removed which I assumed would be for balancing the flex plate.
 
Sounds like it is matched, an externally balanced flex plate will usually have weights, I’m no expert by any means though.
One thing I did notice was your wrist pins aren’t listed on you balance sheet.
And I do believe there are two different cranks for inter or external balancing.
 
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