Effect of a small harmonic damper vs large on a 350 SBC

DorianL

DorianL

solid fixture here in the forum
Staff member
Hey all,

In my effort to get my 350 to run smoothly I am hunting for sources of vibration, one I am eyeballing is the balancer/harmonic damper.

Mine seems to fall about an inch short of the timing tab. So I am figuring this is not the standard 8-inch balancer that was on a 350 but something significantly smaller. I'll measure it this evening once I get home.

What effect would a small balancer have on the motor?

MTIA,

Dodo.
 
A follow up question would be, is there anything I can check at the other end of the crank as a potential source of vibration? Flywheel, bellhousing, clutch? Can any of these lead to a vibration? Is there a way to check without dropping the M21 trans?

And, while I am at it, my motor mounts look in good shape... but is there a way to tell for sure?
 
Found this, source interchange manual:

6 ¾ x 1 and 3/16 350
6 ¾ x 1 and 11/16 267, 305
8 x 1 ½ 350

The 305 "thick" balancers have the timing mark located differently on the balancer, so the timing cover/tab must match to use that balancer.

Also, the 305 balancer is slightly hollow
 
If the engine is not running smooth and hitting clean on all 8 cylinders Dorian it will feel rough and create vibrations too.
Even Full drag race cammed its important.

The single plane intake you have is not the best choice.
Never was a good design new.
Vintage Offy Race single plane works better.
Modern Edelbrock Victor better yet.

Since its a Corvette and a street cam is in an Edelbrock Performer is good choice.
Holley 4-bbl will make it easy for you to Tune & all can releate to & help you when needed on the Holley 4-bbl.

True Race SFI harmonic balancers are expensive.
ATI & TCI Rattler is the best.
Grumpy likes ATI.
I like TCI Rattler, The Choice of many Engine Master Winners in the past.

Often stock GM was best for street.
Pioneer makes nice stock replacement Harmonic balancers that will not kill the budget.

To check the clutch you must pull the Muncie 4-speed out.
 
Bigger diameter balancers are better most of the time.

They can be bad detrimental too.
At High RPMs drag race the crankshaft snout will often snap off on SBC.
Why many option for a BBC Crank snout on new Forged and Billet Cranks.

Since a street engine its not a huge concern.
Find the right balancer meant for your Corvette and use it.
 
You can Judge How Good an intake manifold is by reading all 8 spark plugs.
Crappy intakes give very poor fuel distribution to all 8 cylinders.
You will see it on each spark plug reading.

Best intakes for the street all 8 spark plugs will read exact same.
 
DorianL, Refresh my memory, just curious what the block and head casting numbers on that sbc engine are if it has a 305 style damper?
 
DorianL said:
I was afraid you'd say that. If all else fails, I will go there.
Click to expand...
I dont think its there.
But I could be wrong.
Clutch disc may be falling apart.
Imbalance then.
Front clutch ball bearing on the muncie 4 failed.
But would have trouble shifting gears.
 
It is indeed a 6.75 damper.

I just came back from a test drive.

I pulled the muffs as far as possible away from the body and that has made a good difference.

I think part of if is that I am used to (spoiled by) my 200-4R’s overdrive and lock up converter. 1800 rpm gave me 60 MPH.


That and I wanted my muffs as quiet as possible on the TT (and these are loud Flowmasters).

I still have to check the motor mounts.

Once it has cooled off a bit, I might check the plugs.

Considering putting the fan back on. There seems to be a tiny electric fan on there, but it seems inadequate.

At this point, I find 70 MPH quite smooth, if noisy. (Lots of shakes and rattles to address.)

I think an 8-inch damper is probably what this needs
 
Plugs are all good and they all look like this one.

I might get a colder plug, no ?

CFF63693-9F70-49F0-B20E-2AE27924B00A.jpeg
 
Hi Gents,

This evening I am going to get a closer look at the motor mounts. They seem to be in good nick but I will give them a much closer eyeball. Replacing the trans mount made a significant difference.

One thing I have learned from true mechanics vs what I see in shops today: don't replace a part unless you know it is the problem or there is no choice in order to solve or get to the next level of diagnosis.

I know that smaller dampers came on (lower performance) 350s that didn't see much mid-to-higher RPM which is where the problem seems to be.

I know that prior to Vizard's research, it was thought that lighter dampers allowed motors to spin up faster. Short of all out drag racing, that seems to have been debunked.

Can I get you, Gents, assessment on the likelihood of a larger damper clearing things up? Is it likely this could be the issue?

Again, the idea is to avoid replacing something that was not the problem in the first place.

I will also be exploring replacing the mechanical fan with a quality electric puller. The mechanical fan seems so far away from the radiator and the shroud seems less than convincing. My goal here would be to improve cooling, clean up the engine bay and avoid parasitical vibration from the mechanical fan.
 
I just called a buddy of mine - he may have a used 8-inch damper. If he does, we will try swapping that on to see if it changes anything.
 
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