Cummins 4BT & Diesel Conversions Forums banner

1 - 14 of 14 Posts

·
Registered
Joined
·
75 Posts
Discussion Starter · #1 ·
After lots and lots of insulation of various types I've dampened about all the engine noise from the 4BT I can in my 71 chevy. Next was to lessen the vibrations that resonate in the cab. I have the bread truck motor mounts which are supposed to be fluid mounts and very soft. So next place I looked was the support where the transfer case mounts to the crossmember. 2 peices of rubber sandwich the crossmember and yes after 36 years they were both very hard. The bolts were also very tight. I cut two 3" rubber boat trailer rollers in half and used them as big pieces of softer rubber sandwiching the crossmember. I bolted it back in, but not as tight as I could (tighter means harder means more vibes to the frame). This cut my vibrations resonating in the cab by about 50%. Well worthwhile. I have not changed the sidemount on the transfer case yet because it is more complicated, but I will do it this weekend. You do need longer bolts to accomidate the thicker rubber. I hear lots of guys worrying about their motor mounts, but no one talking about their rear tranny mount. It does make a difference!
I have thought about replacing the old hard rubber body mounts with some softer new ones, but I don't think I can jack the body up high enough to replace the mounts without pulling wires and hoses loose.
 

·
Registered
Joined
·
645 Posts
Great idea, I'm going to try something like that. I think if you used two rubber stock type trans mounts you'd be better off because the rubber is bonded to the steel so there is no thru bolt to transmit the vibes directly from the t- case to the frame. Might require more work depending on what mounts you get.
 

·
Registered
Joined
·
116 Posts
Hello,
I have changed body mount bushings on several 67-72 chevy trucks without stretching any wiring or hoses. You only have to jack it up high enough to clear the new mounts. Just do it one side at a time and it will be alright.
 

·
Registered
Joined
·
75 Posts
Discussion Starter · #4 ·
I have no doubt that stock type mounts are more secure and stronger than what I've got however, Even if new mounts were used they would transmit more vibes to the frame. The holes in the crossmember are about1.25" accross and are shaped like a square with rounded corners(it's quite a bit larger than the bolt going through it). My boat roller mounts are tapered and positioned so the pointed end is in the hole. Hopefully the squeezeing force is strong enough to squeeze the rubber into the hole so the bolt never contacts the edges of the hole. I will try and fix the side mount on the Tcase with more rubber tonight. If this was a real hard core 4wheeling rig I would keep the stock mount, as it is stronger. For my daily driving I'm more concerned with vibration noise.
Thanks for the advice on the body mounts. I used the same boat trailer rollers as a body mount/lift on a suzuki samuri once and it made a big difference in road noise and smoothness. I may give it a try this weekend.
 

·
Registered
Joined
·
3,391 Posts
Good & useful info, Rob, thanks! I had a 360 in my truck before the 4BT. I got in a tight and had to do some long & heavy full power stuff in low gears with a heavy trailer tagged on. Long story short: rippped out all 4 engine mounts at once! What may matter to us is that this wasn't a problem. The engine/trans stayed right where it should, held in place by a solid dual exhaust system.

The biggie on mounts is rotational forces, applied torque. Other than that if engine/trans is level in chassis it pretty much wants to stay where it is. I'm not talking about the angled mounts, but flat. Angled mounts have shear forces on bolt body, so that's different. Because of this the flat mount bolts aren't as critical as a guy would think, at least in normal use. My combined engine/trans is 1/2 ton of steel, so quite a nudge is needed to move it!

I used Nylock insert [self-locking] type nuts to allow some float or play while maintaining nut/bolt clamping force. I need to go back and loosen them some, and also put in some newer & softer rubber. I did find that a large part of my vibration comes from the rear trans mount as mentioned above. Also found that a lot of harmonics were transmitted where my exhaust pipe touched the frame, so installed a better hanger.
 

·
Registered
Joined
·
228 Posts
vib fix

Have any of ya'll bumped up the timing at all. It really helped with the vibration on mine (prob. about 25-35%) made a big difference in power too, expecially if you are running injectors. Also lowered EGTs.
 

·
Registered
Joined
·
4,089 Posts
One thing to add is need to turn up the rmp like 25 to 50 rmp takes out of the vib as well , adjusting the timing will make a big difference on how the engine will run ( adavance timing ) most are set at 12.5 to 13.5 * bump that to 15 to 16 *


Scott
 

·
Registered
Joined
·
34 Posts
I would like to try a transmission mount idea on my 'burb. I want to build a box around the rear of the trans, and suspend it with two more oil filled mounts like the engine mounts. This way I can use the existing crossmember and keep the trans at the same height even though the mounts are tall.
 

·
Registered
Joined
·
696 Posts
One thing to add is need to turn up the rmp like 25 to 50 rmp takes out of the vib as well , adjusting the timing will make a big difference on how the engine will run ( adavance timing ) most are set at 12.5 to 13.5 * bump that to 15 to 16 *Scott
Hi Scott, have you bumped the timing on a commercial 6BT? I heard that the commercial ones have different pump settings and don't take as well to the timing at 16* aswell as the dodge ones do.

Gaza
 

·
Registered
Joined
·
4,089 Posts
I adajust the timing on all my engines that I use for conversions and have yet to find one that never liked it.

As long as it's a on road engine than it will be ok to adajust the timing.

Scott
 

·
Registered
Joined
·
220 Posts
Hi Scott, have you bumped the timing on a commercial 6BT? I heard that the commercial ones have different pump settings and don't take as well to the timing at 16* aswell as the dodge ones do.

Gaza
Hi Gaza.
The main thing of concern here is not really the pump setting. The pump output pressure on the commercial engine is similar to an automotive engine. but you do have to have the correct injector spray angle into the fuel bowl for efficient combustion. An example: PODS were designed to run in a commercial engine.(Boat) The Boat engine with PODS will not smoke like an automotive engine with PODS. The spray angle is not correct for the Dodge Cummins engines, or our 4BT StepVan engines. That is one of the reasons for the excessive smoke from an automotive engine with PODS. The 4BT boat engine has different pistons with a different combustion bowl shape to work with the spray angle of the PODS.
Timing will also determine wether or not the sprayed fuel will enter the fuel bowl cumbustion area of the piston correctly. Too much timing, and some the fuel will spray outside of the fuel bowl. Thats because the piston is farther down in the bore and farther from the injector when fuel is injected, and the angle of spray makes some of the charge fall outside of the fuel bowl in the piston.
You can run thinner washers on the injectors, so the injector will protrude farther into the head, and then you can run greater advance, and the fuel will still end up where it is suppoased to.
My 4bt has New Era 370s that are matched to my pistons, so I can run 18degs advance.
 

·
Registered
Joined
·
854 Posts
HToo much timing, and some the fuel will spray outside of the fuel bowl. Thats because the piston is farther down in the bore and farther from the injector when fuel is injected, and the angle of spray makes some of the charge fall outside of the fuel bowl in the piston.... I can run 18degs advance.
Take any engine with the head off of it, and rotate it 18* before TDC. The piston isn't down the hole very far at all. I don't think you'll get any more spray on the cyl walls going from 15* to 18*
 

·
Registered
Joined
·
220 Posts
Take any engine with the head off of it, and rotate it 18* before TDC. The piston isn't down the hole very far at all. I don't think you'll get any more spray on the cyl walls going from 15* to 18*
It does make a difference whether you think it does or not. Not trying to be a smart a__ here, but it is critical! I'm also talking about running injectors in an automotive 4bt or 6bt that were designed for an industrial engine--like PODS. Also, if you have a 3500rpm governer spring, like mine, you need more advance if you want to put out more power at higher rpms.
But, I didn't say anything about spraying the fuel on the cyl walls. I did say that you will not be spraying fuel in the combustion bowl on the piston in the optimum position. The thinner washers to move the injector down farther in the bore are enough to compensate for more timing advance. (within reason) Look at the thinner washers--they're not much thinner, but they DO make a big difference.
Bob B.
 
1 - 14 of 14 Posts
Top