Hi Guys, I rebuillt my Ford 351 engine with a complete set of iron eagle conrods that were lighter than the original conrods. Now I have vibration at idle, (in neutral) gets worse in the 1400 to 2000 RPM range, and is smooth after that. Could this be due to the lighter conrods?? I have read that the damper reduces crankshaft torsional vibration and does not do anything for an out of balance engine. Do I have out of balance with 8 new rods being lighter than original? No balancing performed during rebuild, just threw in the new rods. I have checked everything else and can't find anything wrong. Appreciate your thoughts, don't want to be chasing a red herring. Thanks Greg.

 

Absolutely any time you replace a component of the rotating assembly, it should be balanced, especially with "offshore" parts. Now, I'm not saying Eagle rods are bad- I've used their stuff, definitely better than Scat IMO- but I don't ever do an assembly without at least having it checked. I know they advertise +/- 2 grams, but I wouldn't bet on it.


Pete

 

petes79f150 is right. When balancing a V-8 the weight of the pistons, rings, pins, clips, and top AND bottom ends of the rods are all factors in the balance of the crank. Change any of these and you need a fresh balance job on the crank.

For the record, 4- and 6- cylinder engines are not as fussy about this. If the original balance of the crank is good, then any of the above items can be replaced without rebalancing, so long as the items are all of equal weight.

 

Remember to check and replace your bearings. An out of balance condition will beat the hell out of them. Check with the 351 forum for more help.

 

Thanks Guys, I did not mention that this 351 is in a ski boat, looks like I will be pulling the engine again.

If the balance is out wouldn't it get worse at high RPM??, I had it up to 40 MPH at 4700 RPM and it is smooth. It's just in that 1400 to 2000 range it vibrates, you can feel it through the floor and deck.

Cheers for now.
Greg

 

It's all about harmonics and the total package. It's vibrating all the time, it's just when it's at a certain range (or ranges) of rpm's that you actually feel it. Valve springs do the same thing. At a certain rpm there will be "surge" where the waves of stored energy collide. When you're at higher rpm's, the waves are canceling each other out and temporarily giving you a somewhat balanced condition. Like Eric said, it will beat the crap out of the bearings. When you take it to be balanced, be sure you take it all- crank, rods, pistons (with pins and clips), bearings, flexplate (or flywheel) with bolts, and harmonic damper.
About your damper: I personally witnessed one coming apart on the dyno at 7500 rpm. It contained enough energy to tear off a radiator hose, fly across the room, partway up the wall, and back across the room, bending or breaking anything in the way. Luckily, it didn't take out a fuel line , and it was in a concrete enclosure. It was one of those Chinese knockoffs with the bonded elastomer rings- just a word of warning. I doubt you'll be turning those rpm's, but it wouldn't have had a problem punching a hole in a fiberglass hull.


Pete

 

Hi Pete, you're scaring me now!! I live in Dubai where this kind of technical knowledge does not exist. I have made enquiries about crankshaft balancing and so far I am getting very blank looks...a mate of mine who is rated no.1 at the Umm Al Qaiwan drag strip V8 class is checking with his tech foreman for anyone that does crankshaft balancing. When I asked my mechanic/machine shop he said they usually change the damper to compensate for lighter rods. Will that work??? From what i have researched it will not! Appreciate your replies, Cheers Greg.

 

The harmonic balancer (aka: vibration damper) had a different job. It soaks up energy waves that travel back & forth from one end of the crank to the other and turns them to heat. If you are doing sustained high rpm, a better balancer is a good idea, especially if you buy a quality one that will not explode.

Your engine is externally balanced - this means that some of the balance weights required would not fit on the crankshaft inside the crankcase, so the harmonic bancer and/or the flywheel (both in your case) have a weight that is eccentric. Together, with the crank, this assembly achieves balance in the operating engine. Any time you change either the harmonic balancer or flywheel you need to be certain that you have the correct one for your engine.

Back to your rod problem: If you cannot have the crankshaft properly balanced (this means the entire rotating assembly), you could put the original rods back in or you could add weight to your rods. (Both the big end and the small end weights must be the same as the originals.) The second is not that easy. You can add weight at the small end by means of a weight pressed into the piston pin, but about all you can do at the bottom end is to weld more material onto the balance boss (which I would NOT recommend) OR drill holes in the boss (if it is big enough) and fill them with a metal denser than steel. Sad as it is, I think your only practical way to get back to a smooth engine is to go back to the old rods. (If you do not have them, original type replacements will be closer than what you have now.)

Did you change the weight of your pistons as well?

I hope somebody has a better idea than the above.

 

He can have the new rotating assy balanced and they can use Mallory metal inserts for the crank or drill it as required etc. Not a difficult problem for a professional shop. They will also be able to tell if he has the right flywheel and balancer.

 

Eric,

Of course you are right, but it doesn't look like he has access to a professional engine balancing service. Getting back to the factory balance may be the only viable option.

 

Thanks Guys, I do not have the oroginal rods, two were bent and when not available in Dubai I purchased the Eagle rods from Summitt not realising that this would upset the balance when the rods arrived lighter than original. Professional shops do not exist here. I will try to obtain factory rods and replace. Cheers Greg.

 

OK Guys, this is what I did, I got a real good deal from a shop called Precision Engines in Houston, got myself a new crankshaft, rods, pistons, rings just finished the install and ran the motor. I still have the same vibration at 1500 -2000 rpm, however I am now thinking this is a misfire rather than vibration. I can hear a random knock towards the rear of the motor, if I unplug #8 spark plug random knock stops (its a knock rather than a ping sound). Also viewing the timing marks with gun on #1 spark I can see the timing advance 2 -3 degrees with each knock. Viewing with gun on #8 appeared stable with each knock, retarding static timing from 5 to 10 also appeared to stop the knock sound. Plugging the knock sensor made no difference. Appreciate any ideas on how to troubleshoot this. Engine shakes in the 1500 - 2000 rpm range. Cheers Greg