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Nice job!

OK ok, end of thread jack!

TJ
 

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shorttrack
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i have broken my fair share of parts at the track. i have a strong feeling that if you do not make all the motor/trans mounts stiffer before doing the rear diff, it will break the bolt due to everything else being able to move so much, especially on properly sized slicks.<<<<<INSERT FOOT IN MOUTH HERE >:D

but for what its worth, i launched my car at 7800rpm on slicks and pulled off 1.5 60's in my last toy (crx) so i know how to launch;)
For our application (GEN COUPE) you are 100% WRONG johnzm.My 11.87 1/4 run and other hard launch runs were all done on drag radials with oem trans mount and oem engine mounts .I only use a custom engine torque damper I made and the sleeve and 4th c-bushing mod .
 

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shorttrack
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i have broken my fair share of parts at the track. i have a strong feeling that if you do not make all the motor/trans mounts stiffer before doing the rear diff, it will break the bolt due to everything else being able to move so much, especially on properly sized slicks.

but for what its worth, i launched my car at 7800rpm on slicks and pulled off 1.5 60's in my last toy (crx) so i know how to launch;)
Solid mounts ,yes you would break $hit .The OEM 4th bushing is easy on the OEM mounts as it still allows for a little movement in the drivetrain .
 

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Got the 2 7/8" hole saw and drilled out the hole. It was rather easy other than the tough steel. At first I left the 1/4" drill bit in for the pilot hole but that tended to enlarge the pilot hole. I took it out and put in a 1/4" punch instead.

Now I'm finishing up the enlargement of the hole with a die grinder.

This would be a lot easier to do if the subframe was out of the car. :) I gave up on the lip idea as the steel is so hard.



The 2 7/8" hole saw fit inside the depressed area on the subframe and the 3" would have rode on the transition from flat to pushed in depression.
So one can use 3" hole saw and will have less to grind afterwords?

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shorttrack
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Anyone doing this mod be aware that if you don't have true high horsepower #s , no traction or sticky tires on a prepped track = no problems.If your car has decent traction and run good numbers with high HP then you would have problems .Making a car truly run and hook up it will generate a different story.Member named ''The_Engineer'' has a 500 rwhp 3.8 2013 Gen Coupe.He tracks his car .When he and I looked at his 4th diff C-bushing install without the steel sleeve being welded in place the first viewing it was apparant his mechanical grip (traction)and high HP (500) was causing severe loading of the bushing to the subframe sheet metal causing the bushing to show itself being cut in the loaded area(the bottom side) by the thin sheet metal of the subframe .I gave him a spare steel sleeve I machined ,he said he will weld it in place .I was at 460 rwhp on nitrous with drag radials at the dragstrip ,I would not think of running the 4th bushing without a steel sleeve welded in place.Low horse powered cars without traction will not generate much loading of the diff to the bushing but should do the 4th c-bushing diff mod with a steel sleeve welded in place as it can only help and if you decide to increase HP and traction you will not have any problems later.
 

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I'd rather do it myself
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Well if you are doubling the HP on a car then you certainty need to beef up the mounting points. But what of all the Gen Coupes running around with just the factory three bushings? Are they in danger of destroying their car? Does adding the fourth bushing help distribute the load and lessen the total load on the rest of the bushings?
 

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In Charge of Snacks
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Well if you are doubling the HP on a car then you certainty need to beef up the mounting points. But what of all the Gen Coupes running around with just the factory three bushings? Are they in danger of destroying their car? Does adding the fourth bushing help distribute the load and lessen the total load on the rest of the bushings?
It absolutely would distribute the load and decrease that load on the original three bushings. BUT, it is possible that without a sleeve to spread the approx. 1/4 of the load on the fourth location that the sheet metal(unreinforced in the longitudinal direction by the collar) could deform longitudinally or "roll open" and vertically elongate the hole. The contours of the stamping in that location already give the material a head start. IF/WHEN that deformation happens you'll begin to get a reciprocating hammer effect under accel and decel. This will only widen the hole, furthering damage in a feedback loop until some critical failure.

As you mentioned and as your direct evidence backs up, with a relatively low horsepower setup, stock and maybe bolt-ons and tune and even with big sticky rubber you may never have an issue. Probably isn't worth screwing with what works for you. But normal practice is installing a sleeve of some sort to support the bushing. Case and point, the Sedan subframe with the same 3.8 has a sleeve. Additionally, nobody builds any subframe or bushing carrier anywhere that doesn't have the bush component supported outside along its axis to resist the deflection-caused seesaw motions inherent in single shear supports.

I'm not saying you need it, just saying its standard operating procedure, it wouldn't hurt, and it's the way I would recommend doing it.

Also, I'm not ignorant to the hassle, down time, and inflated costs associated with machining a sleeve and welding it on in addition to getting up under the car and getting nasty with the diff. I'm sympathetic to the no welds mod scene. Maybe I'll go out to my spare subframe and design up a bolt on sleeve system for all you guys. Of course it would still require lathe turned parts and welding and this crowd would just call my DIY too expensive when I was done. Oh well.
 

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I'd rather do it myself
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So assuming the bushing deforms the not so soft steel, it's rather a hard alloy I would guess, then what? Take it out and you would be back to what you had? And it is not a clean knife edge either on the whole.

Again this is for the 99.9% who aren't wanting to build to 400+HP.

What's the stock HP on a 3.8L 2015?
 

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In Charge of Snacks
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I had to take a hammer to my subframe in a couple places. The steel is actually pretty soft. But then "soft" in steel isn't really a worry unless we're talking about springs or cutting tools. Suffice to say the subframe steel is "Strong enough" in that it has sufficient resistance to plastic deformation for it's intended purpose and is malleable enough not to crack/fracture under the standard use reciprocating loads. Being technically "hard" in this instance would probably be a bad thing.

As for damage, I'm not worried about your subframe. If it bends too far, pull the bushing and hammer it back to shape: Experiment over.

I am worried about your diff cover. Like you, I thought that simply adding a fourth pickup to distribute loads would generally lower stress on all points. Unfortunately my first attempt had too much play in it which resulted in a broken diff cover. Frankly, I knew better but was lazy. With the welded in sleeve I've eliminated the possibility of the same failure. In essence, by installing the fourth point carelessly I had introduced a recurring impact load to the system. I added a deformation dynamic to my subframe that exploited a failure point for the diff cover.

Now, those suckers are particularly weak anyway, and I've seen more than one in stock configuration fail because of worn(loose) bushings. In general, aluminum work hardens more than steel and getting more flex from the threaded bolt hole twisting back and forth from an unsecured bushing(or worse, being pounded by a reciprocating hammer loose bushing) is plenty more than enough to eventually split open the threaded hole. It could, as in the case of my buddy Anthony, snap open a hole in the diff cover and puke gear oil all over the place on interstate 15 at 2am. He drove a relatively stock 2.0.

Beating a dead horse here, but seriously, as long as there's no play it should be fine, but as soon as you hear any weird clunking or anything from the rear end please check it out. Maybe inspect it working on a lift once a year or so. ...From a guy who cares and is driving on his third diff cover ;)

As for the '13+ 3.8 supposedly the stock HP is 348 SAE. I don't think the 400hp theoretical limit is a real thing, but it's definitely worth being cautious.
 

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In Charge of Snacks
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Hey, Unlike some people that want you to do what they want and belittle those that don't conform, I don't want to tell you what to do. Just here to share my dumbassery in the hopes that my successes and embarrassing failures can serve a purpose. Honestly, I'd like you to get away with it as the benefits to the 4th mount are indisputable. It's a way better car with the fourth bushing, and the easier/cheaper the mod is the more people will get to experience it. My point is simply that this mod like many others can cause problems if done wrong. I advise caution and vigilance. Better to be aware, right?
 

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In Charge of Snacks
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I may draw you up a bolt on solution, btw. You could totally hack it together yourself if there's room. I'll check it out on my spare subframe and get back to you.
 

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I may draw you up a bolt on solution, btw. You could totally hack it together yourself if there's room. I'll check it out on my spare subframe and get back to you.
Yes, bolton retainer is what I have been planning as well. It does not have to completely encapsulate bushing.

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