I just converted my 67 F-100 over to a 460 /C6 combination. I had the engine rebuilt at a perfomance machine shop with 9.3:1 compression, a stock intake, comp. cams xtreme 4X4 cam , C8 ported 429 heads and headers. All the parts worked well together and ran very good on the dyno without any noticeable vibration. The block and crank are the old style 460 (internally balanced) and the rods were the football style type. The rods were balanced to the pistons but I did not want to fork out the extra $350 for the crank balance as I new the motor wasn't going to ever be spun higher than 5000 r.p.m. Tons of torque and no vibration on the dyno was looking good. I got a 77 C6 rebulit to mostly stock specs. with a factory stall converter. The two piece drive-shaft was completly rebuilt with new u-joints and balanced after.

All the parts were either new or just rebuilt and tested (dyno). The problem is that the truck vibrates like a demon at apprx. 2500 r.p.m. The engine seemed fine on the dyno and the tranny is just rebuilt. The vibration is at approx. 2500 in every gear and therefore I do not believe it to be drive shaft related. All of the tires/rotors/shocks and front end are new and doubtfully the problem. The flexplate I used is a pioneer (Canadaian brand I think as I am in Canada) FRA-207 flexplate for internally balanced motors. I have already done what the tranny shop suggested and loosened the torqure converter bolts to make sure of the correct play between the flexplate and converter. All was good there as there was no binding or squishing of the flexplate. I am at a loss as to what might be the issue. The crank was from an automatic truck before rebuild and there should be no issue with pilot bushings etc. The questions I have are; bad converter? rods and pistons so far out of balance with the crank as to cause the problem (did not show up in dyno session) or something like incorrect flywheel.( I cannot remember if I used it on the dyno. I am pretty sure I did as they would not have been able to start it otherwise.)

Any suggestions would be appreciated as with over $6000 in motor and trans I do not want it destroyed in 50 miles.

Andrew

 

I would suspect a bad converter by what you are descibing. I bought a new one to go with a tranny swap and had this problem. I however needed one right now, so I put the old one back in, that was 3 years ago I know it's not the easiest thing to check, but thats where I would bet my money. Let us know what you find and good luck !!

 

One some engines the flex plate can go on more then one way. Yours may be on wrong causing the vibration.
Robert

 

What are the symptoms of your vibration?
Is it a metallic buzzing, or some sort of shakimg?
Do you feel it through the floor,steering wheel, or gas pedal?
Does it also happen in park or neutral?
It's not a misfire caused by a change in your ignition circuit? -- different than that for the dyno runs?

It is possible there is some mechanical interference that won't let the rubber motor mounts do their job?

Or could you have a faulty motor or tranny mount?

I'm sure there are more questions to get all of us thinking about a solution.

--Matt

 

I'd go back and double check the flex plate. It is likely the dyno shop had their own flywheel. Does this vibration occur in neutral at 2500,rpm?
William in Atlanta

 

So far gentlemen this is what I can tell you. Yes, the dyno did have there own flexplate and therefore that item still could be the problem. The motor and tranny mounts are good and no interference with motor/frame.(now, minor header clearance issue but that was solved easily and no difference.) The vibration starts at around 1400 and is worst at 2400 in neutral. The flexplate I originally used was a Pioneer FRA-207 with NO extra weights added to it. In talking to the machine shop and a few others in the know they all suggested I switch to the FRA-230 flexplate with the extra weights added. I was also given the small bob-weight that goes behind the harmonic balancer if all else fails. I have almost completed the flexplate swap that I started last night and will be able to try it again this afternoon.

Let me know if you have any other suggestions as any extra info is helpful.

Andrew