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Mechanical Engineers? Modifying JTR Mounts for More Offset/Math Calc.


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where does the engine move place the transmission shifter?

 

Here's a picture of my LT1 powered Z, also with a T56, mounted in the standard JTR position...

 

Dash.jpg

 

 

Its a modified Datsun shift lever, oriented so the offset positions the knob rearward. While its not etched in stone, its looking like the trans will be another 3" farther back. Flipping the shift lever around, I should end up with a knob an inch or two father back than the above picture, which I would welcome.

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To exaggerate, put 20,000 lbs of weight on top of the engine (stock mounted). The suspension will be on its stops and the four vertical bolts through the crossmember will try to tear out (vertically). The lower rail is seeing a lot of focused stress (in tension), but its being held to great degree by the upper rail due to the strut tower. A fairly robust load path.

 

On the other hand, assume the engine is mounted to the lower rail, roughly in the middle. Again, putting the same 20K lbs of weight on the engine. The suspension is still bottomed, but now the lower rail is bowed downward because the only thing attaching it to the upper rail is the inner fender and its not efficiently built for that load. Also, there is no longer ANY stress on the 4 crossmember bolts. In fact, you could remove them and move the crossmember up and down by hand. The load path changed.

 

OK, now I get your point and yes, you are correct in that assesment. Another way to picture it is from above the car.

 

Engine mass loads, when mounted to the crossmember, pass directly to the suspension. The same would be essentially true if the engine mounts were on the frame rail directly above where the crossmember mounts to the frame rail. Moving the engine mounting back on the frame rail makes the frame rail more of a beam connecting the front suspension with the engine mounts.

 

What I've learned by doing a few of these kinds of things (in Miatas, FD RX7s, 240Zs, and E36 BMWs) is that, with proper reinforcement, its really not an issue. For the 240Z, welding the OEM front crossmember in place and reinforcing the frame rails between the TC box and the welded in crossmember adds a lot of support in that area. I also, in the front wheel well, weld in plates that connect the reinforcement where the crossmember mounts to the reinforced area on the wheel well for the strut towers.

 

I did the SR20DET swap on Amir's 260Z about 4 years ago using the reinforcemnt I mentioned above. I saw the car towards the end of last year and the mounting looks good with no bending, cracked paint, etc.

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Engine mass loads, when mounted to the crossmember, pass directly to the suspension. The same would be essentially true if the engine mounts were on the frame rail directly above where the crossmember mounts to the frame rail. Moving the engine mounting back on the frame rail makes the frame rail more of a beam connecting the front suspension with the engine mounts.

 

 

Perfect! That's exactly it. I need to learn how to write.

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What I've learned by doing a few of these kinds of things (in Miatas, FD RX7s, 240Zs, and E36 BMWs) is that, with proper reinforcement, its really not an issue. For the 240Z, welding the OEM front crossmember in place and reinforcing the frame rails between the TC box and the welded in crossmember adds a lot of support in that area. I also, in the front wheel well, weld in plates that connect the reinforcement where the crossmember mounts to the reinforced area on the wheel well for the strut towers.

 

I agree. If an engine is frame mounted with suitable reinforcement, I can think of no significant drawback.

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