Engine vibration question
 

Lately, I've been noticing a vibration in the engine right at 1700 RPM. Under 1600 RPM, it goes away. Over 1800 RPM, it goes away. Vibrates the car right at 1700 though.

Short block was built by a professional shop and balanced. Crank is stock, rods are preped stock, pistons are KB. Balancer is SFI from FMS, flywheel is billet steel also from FMS. Clutch is a Centerforce unit.

If there was an out of balance condition in the engine, the vibration would get worse as the RPM went up. Thoughts? Suggestions? Anything to worry about?

i just posted pretty much same promblem with my engine,but mine does it at idle,if it had a balance problem i agree with you it should get worse not better like mine does.lets see what kind of ideas we get back.Good Luck

Usually the vibration at that rpm is from an out of balance engine. It will normally vibrate at 3 points, 1800rpm, 3600 rpm and 5200 rpm. It may not be out of balance much but the sidepipes carry this vibration through the car unless you have the rear mounting point isolated from the frame through a rubber mount.
Don

Don is right. The vibration does not get worse as the RPM goes up. It appears at specific points. Another key thing to look at is the transmission tailshaft mount. If that is going and you are starting to touch metal to metal, any tiny vibration will be amplified greatly and appear much worse that normal. If that mount is going, that could explain why a balanced engine is now becoming "unbalanced"

Ed

Also check the front and rear oil seals of the engine. If it's out of balance, those little seals get the stuffings kicked out of them and begin leaking (don't ask).

Thanks fellas. Haven't noticed the other harmonics (3600 - 5200) yet. Trans, yoke, and mount are new so they should still be tight. So far, it's just noticable. If it gets worse, my 393 stroker upgrade may come sooner than expected. Using the FMS billet SFI front damper with the bolt in insert. Wondering if that might have something to do with it? Any experience with this?

Jack, I've used the FMS balancer before with no problem. You have to make sure the balancer and the flywheel are both 28oz for the 351w, if not you will have major problems with vibration and engine damage. I'm sure you know this already but it doesn't hurt to check. I've been shipped the wrong parts before. I always send the balancer and flywheel to the balancing shop when I get the engine balanced just to be safe (lot of balancing in that sentence).
Don

Vib
 

Jack:
You have some excellent comments.
Since I was in this business of finding noise/vibs on subs; thought I would add a little.
You implied that the noise recently started? That would imply that there is nothing wrong with the balance. If it was a professional built that low probability.
Dscott is on the right track. You probably have a "sound short" ie, something has moved or you mounts are loosening up and your touching metal to metal when you engine going thru the imbalance harmonics. Side note, V8's can't be balanced perfectly; it's dynamically impossible with the V8 configuration.
Go under the car and look at everthing that within . 5 in of the frame. Headers, cluth slave, bell housing, trannie bumps and bosses. Look you any thing shiny or worn and you'll have found your "sound short" They can produce vibs and noises that you can't imagine.
gn

Jack

If the trans, Yoke and mount are new, then they might have loosened up since being installed. Might be a good idea just to go back and re-check.

If I do a rebuild of anything on my Gardner Douglas, as a matter of course I go back round and check after a couple of hundred miles.

Best Regards

Robert

The UK Cobra Replica Club - http://www.cobraclub.com

Don,

The crank was turned 10/10 and reinstalled. Rods were the originals out of the engine, magged, reconditioned, sides ground, shot peened, new ARP bolts, and reinstalled. Pistons were new KB's. They needed a touch of valve clearancing. Flywheel is FMS 157 tooth billet 28 in/oz.

The iffy piece, so far, is the FMS SFI balancer. It is a two piece unit with the small counterweight that bolts on from the rear before installing. I recall the instructions that came with it saying not to install the counterweight if the engine was internally balanced. Have you ever had any balancing problems with these? How does one determine if one has the 50 in/oz, or 28 in/oz counterweight.

Jack, Ford sells 2 balancers, the 50 oz and the 28 oz. Just to look at them there isn't much difference. I would check the number. The 50 oz is a M-6316-A50, The 28 oz is a M-6316-C351. Both have the removable weight. I've never had a problem with the balance but I have been shipped the wrong one. You can also call the Hot Line and I'm sure they can tell you how to determine which one you have. The number is 1-586-468-1356, 10am to 6 pm eastern time.
Don

Jack,
A couple of dumb questions. The bolt on weight on the back of the balancer is still there isn't it? Also the outside ring can shift on the rubber and may cause a slight problem. Does this vibration occur whether you're moving or sitting still or both?
By the way, where you been?
h dog

If an engine is INTERNALLY balanced by a shop, then shouldn't the flywheel AND the damper be zero balanced? I have seen zero balance flywheels, but never have seen a zero balance damper for a Ford. Where would you get one of these??

Ed

Ed, Ford sells a number of neutral balanced harmonic dampers for several different engines. I'm sure most of the aftermarket companies do as well.
Don

Thanks everyone. Engine had a standard warm street motor rebuild balance. It was not internally balanced. The balancer left the builder the option of leaving the counterweight off if the engine was internally balanced. Of course, if that were the case, the flywheel also would need rebalancing.

Anyway, I'll check the part number which should be stamped on the unit. Think I'll pay a visit to the shop that did the engine and get their opinion. (Which is why I like locally built engines) It's just enough to be annoying.

There is no way for the counterweight to come out. If it came loose, it would make a hell of a racket.

Jack,

Just checking up on you :) Did you find the problem? Nosey people want to know!

DV - 300 Cobras is NOT enough -- Neither is 4 Hotels!

The continuing saga. Spoke to the machine shop that did the engine. "Bring it in and we'll have a listen - feel, see what it might be."

Meanwhile, I'm juggling mufflers. Reinstalling the 30" set so I can send the 36" set to coating shop. Notice a fast idle stumble in process of cranking initial advance up a couple of degrees. Out with the vacuum gauge for lean best fast idle. Open the screws about a half turn, vacuum comes up, stumble goes away. (Per the Holley instructions on leaning out rich idle mixtures, I'm adjusting the idle air with the secondaries, while leaving the primaries below the transfer slot. As a last resort, I'll drill air holes in the primary butterflys, but we'll try this first.)

Next day. Idle has a little more lope to it, but vibration at 1700 is gone, - completely! This, to me, eliminates any engine imbalance problems. My major concern, hauling the engine out and back to the machine shop (and possibly missing DVSFII) is gone. That's a relief. But I may be twiddling with this Holley (700DP) getting the idle, and off idle mixture dialed in for awhile. (but, my gawd, the acceleration!)

Fuel economy is running 14.5 to 16 MPG back & forth to work, inching along in traffic backups, and around town, and plugs look good, so nothing is grossly off. Will need an exhaust gas analyzer to dial it in any tighter than this.

Car is coming up on 5000 miles, and front suspension that I thought was overly firm, is settling in. Would still like to covert front shocks from Carrera to Koni, but not an immediate priority. Getting upper control arms to stay planted after an alignment is my next priority.

As much as I bash CR for it's shi**y engineering on some things, they did well on weatherproofing this car. I expected a myriad of water leak problems driving in the rain. Passenger compartment stays dry, even at 60 - 70 MPH. I expected water on the distributor to stop me dead with the first heavy rain. Nope, dry. I expected water in the air filter to be a major headache. It's the Edelbrock oval with K&N filter. No problems. Defroster & Lucas wipers work fine. Falken tires work as advertized, plenty sticky on wet roads. One problem, wet brakes, no backing plates. I'll start another thread on that one.