Pennock's Fiero Forum
  Technical Discussion & Questions
  3800sc folks with poly mounts- How much vibration do you feel?

Post New Topic  Post A Reply
Email This Page to Someone! | Printable Version


next newest topic | next oldest topic
3800sc folks with poly mounts- How much vibration do you feel? by smartaxel
Started on: 08-21-2013 10:48 PM
Replies: 8 (1237 views)
Last post by: bjc 350 on 08-22-2013 03:35 PM
smartaxel
Member
Posts: 2826
From: Michigan
Registered: Feb 2002


Feedback score:    (15)
Leave feedback





Total ratings: 83
Rate this member

Report this Post08-21-2013 10:48 PM Click Here to See the Profile for smartaxelClick Here to visit smartaxel's HomePageSend a Private Message to smartaxelEdit/Delete MessageReply w/QuoteDirect Link to This Post
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?
IP: Logged
PFF
System Bot
DimeMachine
Member
Posts: 957
From: Eastern Metro, Minnesota, USA
Registered: Sep 2011


Feedback score: (1)
Leave feedback

Rate this member

Report this Post08-21-2013 10:52 PM Click Here to See the Profile for DimeMachineSend a Private Message to DimeMachineEdit/Delete MessageReply w/QuoteDirect Link to This Post
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.

------------------
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

IP: Logged
MadMark
Member
Posts: 2935
From: Owosso, Michigan, USA
Registered: Feb 2010


Feedback score: (4)
Leave feedback

Rate this member

Report this Post08-21-2013 11:22 PM Click Here to See the Profile for MadMarkSend a Private Message to MadMarkEdit/Delete MessageReply w/QuoteDirect Link to This Post
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.
IP: Logged
MstangsBware
Member
Posts: 11509
From: TEXAS
Registered: Mar 2002


Feedback score:    (108)
Leave feedback





Total ratings: 459
Rate this member

Report this Post08-22-2013 01:26 AM Click Here to See the Profile for MstangsBwareSend a Private Message to MstangsBwareEdit/Delete MessageReply w/QuoteDirect Link to This Post
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.
IP: Logged
Bozzie
Member
Posts: 1188
From: Plainville,Ct. U.S.A
Registered: Oct 2007


Feedback score: (1)
Leave feedback

Rate this member

Report this Post08-22-2013 01:06 PM Click Here to See the Profile for BozzieSend a Private Message to BozzieEdit/Delete MessageReply w/QuoteDirect Link to This Post
3800 turbo solid mount with solid dogbone and its solid !
IP: Logged
Jfrost
Member
Posts: 1092
From: Brookfield, IL
Registered: Jul 2009


Feedback score:    (29)
Leave feedback

Rate this member

Report this Post08-22-2013 01:57 PM Click Here to See the Profile for JfrostSend a Private Message to JfrostEdit/Delete MessageReply w/QuoteDirect Link to This Post
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.

------------------
Project L67 Fiero

IP: Logged
aaronkoch
Member
Posts: 1643
From: Spokane, WA
Registered: Aug 2003


Feedback score: N/A
Leave feedback

Rate this member

Report this Post08-22-2013 02:05 PM Click Here to See the Profile for aaronkochSend a Private Message to aaronkochEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
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.

------------------


Build thread for my 88 + 3800NA swap

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

IP: Logged
fieroguru
Member
Posts: 12496
From: Champaign, IL
Registered: Aug 2003


Feedback score:    (45)
Leave feedback





Total ratings: 260
Rate this member

Report this Post08-22-2013 03:32 PM Click Here to See the Profile for fieroguruSend a Private Message to fieroguruEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
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).]

IP: Logged
bjc 350
Member
Posts: 967
From: Astoria
Registered: Oct 2005


Feedback score: (1)
Leave feedback

Rate this member

Report this Post08-22-2013 03:35 PM Click Here to See the Profile for bjc 350Send a Private Message to bjc 350Edit/Delete MessageReply w/QuoteDirect Link to This Post
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).]

IP: Logged

next newest topic | next oldest topic

All times are ET (US)

Post New Topic  Post A Reply
Hop to:

Contact Us | Back To Main Page

Advertizing on PFF | Fiero Parts Vendors
PFF Merchandise | Fiero Gallery
Real-Time Chat | Fiero Related Auctions on eBay



Copyright (c) 1999, C. Pennock