Cummins swap excessive frame twist in neutral

[Krwright]

Fired my cummins up for the first time yesterday. The motor is very healthy but The cab shakes at minimum 3" while reving in park. It's a 1 1/4 cucv frame and springs. Anyone else Notice excessive shaking on their swaps? I have a video of it rocking while reving but don't know how to post it. It is quite extreme everybody that see's it say the same thing "holy ****". It just makes me think what will happen when pulling on something
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[broey]

I have heard from a few people who have done diesel swaps that they boxed their frames and added cross support. I was looking into doing one on my truck a while ago and decided not to because of those same reasons.

[Pyrotechnic]

An I6 is a decently balanced motor, are you sure there isn't something about the way the motor is running that's causing it to shake? What did you end up doing for the motor mounts?

Now the 4BT, or any I4 without balance shafts for that matter is a different story.

[ryanroo]

i would be curious to see the mounting system. i have done swaps in a few different trucks in the past and none had that violent of a shake. even the XJ that we swapped a 4bt into didnt shake that bad.

i boxed my entire frame front to back and have upgraded the crossmembers all the way back as well. but i am still early in the build so i have no idea what it will do when it runs.

ryan

[Firebirdjones]

C-channel frame vs boxed frame sounds like the issue to me.

[Car'n'Guitars]

I can tell you that our Dodge Cummins with the 5.9 12v will shake pretty bad if the motor is not warmed up and you are running an accessory like our air conditioner. We usually just hold the brake and give it a little rpm boost to help it. And it will shake to the point where you can feel and see it.

[Krwright]

It runs smooth it's not shaking from idling it's when you throttle it. When I rev it just a few hundred rpm it will twist right then settle back when you let off. If I bump it say 500 rpm and let off to a rhythm it will easily shake 6" side to side. I used mounts from patc.
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[Krwright]

The motor itself doesn't move but maybe 3/8" at most. The entire truck is twisting from bumper to bumper
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[Axle]

Did you box the frame?

Alex.

[Krwright]

No I didn't. Wished I had now. I'm thinking of building a truss under it out of tubing
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[CentipedePerformance]

That Cummins has a ton of rotating mass, and tons of torque. Revving it throws that weight around pretty quick, which is what is causing the rocking. A cucv frame should be able to handle it. My old Dodge was still just a c-channel frame and it made 398/1011 and I never had problems with the frame. Axles and transmissions yes, never the frame lol.