I recently completed the rebuild of my big block. It is now a 496 with an internally balanced crankshaft. I re-used the vibration damper and flywheel (re-surfaced) from the original 427. I also installed a new centerforce clutch.

I am experiencing a very noticeable vibration that appears at 3500 RPM. The vibration starts at this RPM and does not go away, nor does it get any worse as the RPMs increase. It will always appear at 3500, regardless of the gear, and it is also present, although not quite as harsh, when reaching this RPM in neutral (clutch in or out).

The vibration is felt mostly in the seat of the pants and the shifter. It is not what I would call violent, but it is strong enough to not feel right and makes a complete blur of my rear view mirror.

Any ideas on what may be wrong? I am beginning to think that maybe the Centerforce clutch may be out of balance.

As I am typing this I am thinking of the installation of the clutch. My flywheel didn’t have any alignment dowels on it when I got it. I was reading the Centerforce website and they referenced alignment dowel or shouldered bolts. Are these flywheels supposed to have alignment dowels. Could this be causing my vibration?

Thanks for any guidance you guys can provide.

Larry

Who balanced your kit? I'd be on someones butt. My understanding that even if you order the BI kit you still need to balance it. So I hope you had it balanced.

Who balanced your kit? I'd be on someones butt. My understanding that even if you order the BI kit you still need to balance it. So I hope you had it balanced.

What is a BI kit? I had my rotating assy balanced by a rebuilder here in Chicago, but I am not sure the vibration is internal to the engine yet. That is why I am looking for some advice from the folks here to help me find the source of this.

I just had a 468 built and took out my 427. Because the 427 crank was internally balanced and the 454 is external, I could not use the 427 flywheel or harmonic balancer. I found a used balancer and bought a new flywheel I am sure others will chime in.

Edit.
Sorry, I just re-read what I had posted and I had 454 flywheel where I should have said 427 flywheel.
The more I think about it, the more I think that this is your problem. I know a friend of mine has stroked 400 engines he oval track races and even though he has them balanced, he has to use the balancer and flywheel with the external weights. The 400 small block is also externaly balanced. I would talk to you machinist but I bet this is your problem

Larry,
I have had several customers that had problems with CF clutches being out of balance. Unfortunately, the only way to find out for sure is to replace it, or take the trans and clutch out of the car, support the back of the engine and start it and see what happens.

Chevy clutches do not use alignment dowels. A lot of the Fords do and I dont iknow about the LS engines, but traditional GM V-8s do not use dowels.

One other thing that I have to mention ..... are you sure that it is really a "vibration: and not maybe just a characteristic of the camshaft, especially if it is a radical one ? The reason I ask is that an actual mechanical out of balance condition generally gets worse as the rpms increase. Sort of like an out of balance tire willl be worse at 80 mph compared to 35mph.

Larry,
I have had several customers that had problems with CF clutches being out of balance. Unfortunately, the only way to find out for sure is to replace it, or take the trans and clutch out of the car, support the back of the engine and start it and see what happens.

Chevy clutches do not use alignment dowels. A lot of the Fords do and I dont iknow about the LS engines, but traditional GM V-8s do not use dowels.

One other thing that I have to mention ..... are you sure that it is really a "vibration: and not maybe just a characteristic of the camshaft, especially if it is a radical one ? The reason I ask is that an actual mechanical out of balance condition generally gets worse as the rpms increase. Sort of like an out of balance tire willl be worse at 80 mph compared to 35mph.


Thanks for the reply Bill. No, I am not sure. This is the first agressive engine that I have built/driven so I have no real frame of reference. The cam is quite agressive (it is one of Harold's - 255 / 263 @.05), but I didn't realize that it could cause this type of condition. As I recall, the real power band was 3500 - 6500 for this cam.

In experimenting, I have lessened/worsened (only slightly) the vibration with changes to timing, however, it is always there and always shows up at 3500. It is a fairly noticeable vibration and one that causes a uneasy feeling that something is not right inside. I guess I would call it a vibration, as you can feel it in the seat of your pants and on the stick. It is a very rapid, high frequency feeling but not one where you see the stick moving. However, it is enough to completely blur the rear view mirror.

However, you raise an interesting point. It definately does not get worse as the RPM's progress. That I am confident. Is it common for cams in this category to to create this effect?

Are many vibration problems, such as those from out of balance clutches, the type where the vibration intensity increases with the RPM?

Thanks for your assistance. I really appreciate it.

Larry

Larry,
Some engine combinations will have a "bad spot" in the rpm band, for lack of a better word, where they just do not like to run smooth. My 67 Camaro drag car would sound like every piece of sheet metal in the interior was going to come loose at about 2500 rpm coming down the return road. But it usually gets better once you are though that rpm range.

If you are not real sure of it, I would see if you can find someone close to you to get a second opinion before you start tearing things apart.

One of the guys at the AERA www.aera.org is a racer and his dad owns an engine shop near Indy. The AERA is in Crystal Lake but I dont know exactly where he lives. I can ask him if he knows someone that can listen to it. If you are interested, send me a PM and I will ask him on Wednesday. Include your address so I can let him know where you are.

Bill, thanks for your generous support and offer to connect me. I will tinker with it a bit before I burden someone else. I'll PM you if I start pulling my hair out on this. I am beginning to think you are right and it could just be the build.

I did some additional research earlier today and went out and tested all the plug wires. Most were 10K ohms, but two were close to 50K. I replaced those wires and the vibration is much better. Still there a bit, but much less intense. Unfortunately it was raining, so only took it around the block once.

I also noticed that one booster starts flowing before the other in the primary side and it also flows more fuel than the other at the same RPM (it is a Holley 850 DP). I took the carb apart tonight and we'll see if this impoves it further. has anyone ever seen this before? Any ideas or this to look at while I have it apart.

Thanks again for the help.

Larry

what kind of shape are the motor and trans mounts in? any header tubes touching the steering shaft or frame? Oil pan hitting the crossmember? I'd be looking closely at this stuff before yanking the clutch.

try putting a jack under the oil pan, use a block of wood or something to keep from trashing the pan. run the engine at 3500 to feel the vibration, then have somebody trustworthy put some pressure on the jack, just enough to lift the engine off the mounts a hair, see if the vibration changes or disappears.

are you sure it doesn't get worse with RPM? As Bill noted, most balance vibrations seem to connected directly to the tach. with a point source of light in the mirror, it'll get fuzzier and fuzzier as RPM increases.

what kind of shape are the motor and trans mounts in? any header tubes touching the steering shaft or frame? Oil pan hitting the crossmember? I'd be looking closely at this stuff before yanking the clutch.

try putting a jack under the oil pan, use a block of wood or something to keep from trashing the pan. run the engine at 3500 to feel the vibration, then have somebody trustworthy put some pressure on the jack, just enough to lift the engine off the mounts a hair, see if the vibration changes or disappears.

are you sure it doesn't get worse with RPM? As Bill noted, most balance vibrations seem to connected directly to the tach. with a point source of light in the mirror, it'll get fuzzier and fuzzier as RPM increases.

Tom, Thanks for the tips. I am pretty certain it does not get worse with the increasing RPMs. If anything, it becomes a bit less past 4000 or so. I didn't examine the engine mounts too closely when the engine was out. I did look for cracks and separation, but did not notice anything. The trans mount looked solid and appeared in good shape.

I am fairly certain the headers are not touching. When I got the car there were two problem areas that I re-dimpled when I reassembled. I have at least a 1/2" everywhere. The oil pan is clear as well (using stock).

I will try what you suggested with raising the engine slightly. Should I feel less vibration or more when the weight is held by the jack?

Thanks,

Larry

Maybe I am completely wrong, but the engine I am having built is going from a 454 externally balanced to a 500+ internally balanced version.

My builder had me buy a neutral balance flywheel and damper for an internally balanced motor as the ones for a 454 have additional weights.

Wouldn't the extra weights on the damper/flywheel for an externally balanced 454 cause the problem? :confused:

Maybe I am completely wrong, but the engine I am having built is going from a 454 externally balanced to a 500+ internally balanced version.

My builder had me buy a neutral balance flywheel and damper for an internally balanced motor as the ones for a 454 have additional weights.

Wouldn't the extra weights on the damper/flywheel for an externally balanced 454 cause the problem? :confused:

I would expected a weighted flywheel to cause a problem on an internally balanced crank. However, my components came from a 427, which uses an internally balanced crank.

It sounds like a harmonic type of vibration except that it doesn't stop when RPM changes. I have had a couple of engines that would go through a shudder type of vibration at a given RPM then smooth back out. An engineer friend said it was just a harmonic disturbance peculiar to that engine(s) and should not cause a problem. They never did. I did have a 283 in a truck once though that would start vibrating at anything over about 2000 RPM. Turns out the crank was broken right in the center main.

I have a couple of updates and request for a little more assistance.

I found two plug wires with 50K resistance. Replacing these helped some to reduce the vibration. I also swapped the PCV valve for a different one and that helped a LOT. It let a lot of air through and it must have been going lean eventhough I couldn't see it on the Wideband. It now rev's much looser and with less vibration. I also milled the choke horn on my 850DP to make sure I was getting good air flow (it is a drop base with only a 3" filter).

Based upon your advice, I pulled the headers and re-dimpled, as some areas were pretty tight.

I am still getting vibration, but it is much less. If I run it in neutral, I pick up a slight vibration still at 3500, but it is not bothersome and is minor. The mirror doesn't vibrate and most serious symptoms are gone. The vibration is the same with the clutch depressed. Slight vibration still starts at 3500 and doesn't progress at all beyond this.

However, at highway speeds, the vibration is sometimes as bad as it was before, but not always. I also now notice that the vibration at 3500 in lower gears is less than in fourth gear. I am beginning to think that their may be a drivetrain issue here.

When I rebuilt the trans last year (B/W T-10), I notice that the front bearing had a little slop to it. I didn't have a new bearing at the time, so I re-assembled without replacing it. Could this be causing it? with the clutch engaged and trans in neutral, the vibration is slight, only under load is it bad. Or should I be looking at the driveshaft and u-joints. I did replace the bushing (where the yolk slides in) in the tail cone, so I don't think that is it.

Thanks for any additional guidance you can provide. By the way, this is in a Corvette.

Larry

Are your exhaust hangers solid mount or are thay dampered. I chased a vibration from front to back only to find that it was the exhaust hangers causing the vibration.

Larry,
Some engine combinations will have a "bad spot" in the rpm band, for lack of a better word, where they just do not like to run smooth. My 67 Camaro drag car would sound like every piece of sheet metal in the interior was going to come loose at about 2500 rpm coming down the return road. But it usually gets better once you are though that rpm range.

I like that spot. My car tries to shake the dashboard out. That's when I knew how big the cam really was.

Are your exhaust hangers solid mount or are thay dampered. I chased a vibration from front to back only to find that it was the exhaust hangers causing the vibration.

I have a 3" exhaust and it is solid mounted to the frame. In the Corvette, there are 3 1/2" holes through the cross-beam by the trans. There are fixed hangers there that you put a clamp around to hold the pipe in place.

I will try removing the clamps and see if that helps.

Thanks for the idea.

Larry

I just removed the front u-joint from the trans yoke to swap it 180* I noticed a bit of slop in the yoke. I put a dial gauge on it and it deflects about .025" at the ujoint. I would guess maybe that represents around .005 gap between the yoke the busing.

Could this cause vibration or is this pretty typical?

Thanks,

Larry

there shouldn't be any looseness in the caps and cross at all. now, the slip yoke is usually a little sloppy in the tranny bushing but that's not normally a source of problems. if there's looseness in cap/coss assembly you need to replace it ASAP. if it drops the front of the drive while driving something very unfortunate can happen.

lifting the motor a little takes the weight of the motormounts and can help isolate a vibration if it's transferring through the mounts.

the other thing you should try is dropping the belts off. if the 3500 vibration goes away re-install the belts one at a time, try to isolate the issue.

there shouldn't be any looseness in the caps and cross at all. now, the slip yoke is usually a little sloppy in the tranny bushing but that's not normally a source of problems. if there's looseness in cap/coss assembly you need to replace it ASAP. if it drops the front of the drive while driving something very unfortunate can happen.

lifting the motor a little takes the weight of the motormounts and can help isolate a vibration if it's transferring through the mounts.

the other thing you should try is dropping the belts off. if the 3500 vibration goes away re-install the belts one at a time, try to isolate the issue.

Thanks Tom. The looseness is where the yoke slides into the trans. The ujoint is snug into the yoke. The ujoints don't rotate as smootly in the caps that are pressed into the driveshaft yoke. It is not major binding, just feel like dried up grease. I don't suspect that could be cause my problems though.

I'll try lifiting the engine and pulling the belts. I guess it could be the flex fan as well, but I'll figure that out when I pull the belt.

I think I found the problem. I pulled the trans and bell housing, as I planned to send the P-Plate to Centerforce to have it tested. As soon as I pulled the trans, I could see it. TheCenterforce Dual friction P-Plate has a band of weights around the fingers to ease the engagement effort. These were cocked all the way to one side and it was obvious, from lack of wear, that it has never moved from this position. It is cocked about a full inch from center. I am figuring that was enough to throw off the balance.

Now I am not sure if I should keep the clutch in or replace it. I prefer not to repeat the enjoyable effort of pulling the trans every month.

I'll post back if this fixes it. THanks for your gracious assistance.

Larry

>>> pulling the trans every month.

Yeah, but think of all the character building you're getting. :)

Its not the Centerforce clutch... I just took a motor in to get it ballanced and the I asked the machinist if he needed the pressure plate. He asked if it was a Centerforce and I told him yes. He said no need, they have the best off the shelf ballancing of any clutch on the market, and he has never had to do one. If you internally ballanced it without the ballancer & flywheel/flexplate, that is a problem.

>>> pulling the trans every month.

Yeah, but think of all the character building you're getting. :)

Exactly. And if I do it enough times, it probably wont that long any more.

Its not the Centerforce clutch... I just took a motor in to get it ballanced and the I asked the machinist if he needed the pressure plate. He asked if it was a Centerforce and I told him yes. He said no need, they have the best off the shelf ballancing of any clutch on the market, and he has never had to do one. If you internally ballanced it without the ballancer & flywheel/flexplate, that is a problem.
=
If the weights were off center as you posted your machinist is incorrect
This is more than enough to cause a vibration
Take the pressure plate to him so he can see what you saw :thumbsup:

Finally fixed the vibration and thought I would post details to help others. I pulled the P/Plate and sent it to Centerforce to check it (they were good to work with and didn't charge me). The plate was slightly out of spec, but not enough to cause a problem they told me.

So, I pulled the flywheel to take it to someone to be balanced. I carefully examined it and found what I thought were cob-webs in one of the locator pin holes (not used in this configuration). Turns out some knuckle-head (not sure if it was P/O or the shop that cut the flywheel) stuck a locating pin deep in the hole. I tapped it out and assumed it was the source of imbalance. Put everything back together and the vibration is gone. A completely different engine. It revs very smooth now and it is amazing how much faster it runs through the RPM range.

I don't suppose I could have damaged bearings from just a few hundred milee, could I?

No,i dont think you hurt the bottom end/bearings in only 100 miles esp if oil psi is still good/the same as it was when you 1st fired it up and there is also NO bearing material seen in the oil too.

I installed a new clutch kit in my 69 396 couple yrs back with same flywheel and it too vibrated at approx 3,200-3,400 on up when old clutch was smooth as far a engine vibes went prior to replacing it usinfg same flywheel.

So i had to pull the new clutch and had a local machine shop chk ballance and it was out for sure,was a LUK clutch .

Post reballance and reinstall its now smooth as glass.

I have since sworn to have all new clutches i am installing checked for ballance prior to intsall so i dont ge burned again like that.

I have installed 2 new clutches(stock GM & Luk) for other people since then and both clutches were out of ballance enough to vibrate per machinest when i had them checked at the shop prior to installing them,glad i chk them out 1st.(PHEW!!!)

The Machinest also said it was a sign of the times if you get the drift and he finds on avg 6-7 of 10 cluches he chks for ballance weather low perf stock and even high quality perf stuff are out of ballance enough to cause vibes when 5-6yrs ago that wanst the case.

So if i were anyone installing a new clutch i feel its well worth the $ to have the ballance of new clutch to flywheel checked prior to install it to avoid having to pull it back out due to vibration issues caused by an out of ballance NEW clutch like i had to deal with.

At 53 yrs old that really sucked having to do that clutch job a 2nd time on my back on cement under my car on jackstands in my garage just because the mfg (LUK) didnt ballance it properly/good enough the 1st time arround. (GURRRRRR!!!!!!)

But the stock luk replacment clutch holds my mild 396 with approx 375hp/440lbs trq just fine when ripping thru the gears once in a while,but it see's NO strip use/abuse at all.

Scott

Larry,


Chevy clutches do not use alignment dowels. A lot of the Fords do and I dont iknow about the LS engines, but traditional GM V-8s do not use dowels.




Bill, I agree in principal with you but I would point out that most all GM V-8's pressure plates did use shouldered bolts to align the PP. They worked out pretty well in the absence of alignment dowels.

Bill