I've got an 88 Formula with a 3800SC swap. It has poly engine mounts and auto trans. When in gear and stopped at a light the car vibrates a LOT and makes a lot of rattling noises (it's a t-top). It feels kind like a cheap vibrating seat. I am strongly considering putting in rubber engine mounts because the vibrating drives me nuts. Do you 3800 folks with poly feel a lot of vibration? I don't now if it's "normal" or not. I increased the idle speed and it helped a bit, but it's still strong. If I put it into neutral it gets a bit better, but not much. When I leaned against the outside of the car while it was running, you could also feel the vibration just touching the car. The car itself runs great. Hoe much vibration do you poly folks feel?

I put poly dog bone bushings in for about 5 minutes. After a few of my tooth fillings fell out (kidding) I switched back to rubber.

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84 NB, 3800SC, E-85, VS Cam, 3.0 Pulley, 4T65E-HD, HP Tuners, AEM Wideband, Regal GS Gauges, S-10 Brake Booster. 1/4 mile -12.05 at 111.7mph

I have a 3800 SC III with a 4T65E-HD tranny in it. And I have four polyurethane truck engine mounts on the cradle so I have no dog bone. And when I pull up to a stop light I do get moderate shaking when if gear. It goes away pretty good if I put it in neutral. I had the idle speed increased to about 925 rpm at idle to decrease this effect. The lower in idle speed you go the worse it will get with the 3800 SC, since the firing is not equal due to the way the crank is made and the 90 degree of the cylinders.

So you can increase the idle some, but you will always have the lumpy idle with the 3800.

Every swap I have done using poly turns out a little different in the vibration category. Some have been mind to ones that have a pretty good vibraton to them. One swap I did started out with a prettg good vibration but after coming back for MODs years later I noticed the vibration wasnt near as bad.

3800 turbo solid mount with solid dogbone and its solid !

Granted I havent driven mine yet, but at idle I have some pretty good vibrations going on. On top of poly engine/trans mounts I also have aluminum cradle bushings (86 GT) so they could contribute as well.

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Project L67 Fiero

quote Originally posted by MadMark: The lower in idle speed you go the worse it will get with the 3800 SC, since the firing is not equal due to the way the crank is made and the 90 degree of the cylinders. So you can increase the idle some, but you will always have the lumpy idle with the 3800.

Sorry, but that's not true.. Some of the old 3.8 buick motors were odd-fire, but for the last 30 years they've been even fire.

Both the 3800 series II's and the 60 degree V6's fire every 120 degrees of rotation. In the 60 degree motors, the crank pins are separated by 60 degrees, and in the 90 degree motors, the crank pins are splayed 30 degrees.
From a smoothness point of view, the 90 degree motors actually have 2 advantages in that they have a balance shaft, and the 90 degree banks help cancel secondary harmonics.

My 3800 NA is on an 88 cradle (solid mounted), poly engine mounts, and with a poly dog bone. With the idle set to 940ish, it's got a bit of vibration at idle, but not bad, and once you're over 1300 RPM it's smooth as butter. Perhaps the vibration is somewhat due to the fact that you're running an auto and there is more load on the engine at idle.. Try upping the idle speed a bit.

Edited to add pics:
60 degree crank (notice the separation between 1-2, 3-4, 5-6 pairs)


3800 crank:
Notice that the crank is shorter and the 1-2 pairs have a 30 degree difference but no space b/t them. This in my opinion is why the 3800 bottom ends are so strong.

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Build thread for my 88 + 3800NA swap

[This message has been edited by aaronkoch (edited 08-22-2013).]

quote Originally posted by aaronkoch: Both the 3800 series II's and the 60 degree V6's fire every 120 degrees of rotation. In the 60 degree motors, the crank pins are separated by 60 degrees, and in the 90 degree motors, the crank pins are splayed 30 degrees. From a smoothness point of view, the 90 degree motors actually have 2 advantages in that they have a balance shaft, and the 90 degree banks help cancel secondary harmonics.

Even with the offset rod journals, you have timed ignition pulses happening every 60 degrees with a 90 degree separation between the banks. This is an inherently imbalanced configuration and will have noticeable vibration. The counter balance shafts are not a benefit of the design; they are a band-aid to lessen the vibrations to tolerable levels with rubber mounts. Add poly and you will feel them more. My poly mounted 4.3 vibrated more at idle than my solid mounted SBC.

60 degree V6's and 90 degree V8’s are inherently smooth by design.

[This message has been edited by fieroguru (edited 08-22-2013).]

My 84 with the 3.8 sc has a pretty bad vibration at idle, in gear. If I'm going to be sitting at the light for a while, I usually shift to neutral. It idles at 800 in gear. Once off idle it is fine. It has a poly dog bone with WC Fiero mounts, but the cradle has new rubber mounts, not poly. The vibration really detracts from an otherwise very nice swap by the PO.
Guru is right, my SBC with Dickman poly trans. and a solid Archie front mount has little to no objectionable vibration.

[This message has been edited by bjc 350 (edited 08-22-2013).]