Stock subframe strength
 

Hi,
The stock front subframe is OK to be used in a street driven car up to what HP or torque level?
Engine weighing about 350 lbs.

what motor do you want to put in it?

I am trying to evaluate the cost of properly rebuilding my engine vs installing a Lfx engine with a mv5 transmission. Was thinking of trying to use stock subframe, modify the PPF to attach to the MV5 from the RX8 diff. I know I should find a 3.9 gear and they are rare. Or go with a different diff. I'd like to use the stock one though if I can find a gear.
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I know the High pressure pump on the rear of the lfx might require some mod to the firewall and also moving the ABS unit.

​​​​​Will the stability control still work with an engine swap?
​​​​I believe the traction control won't work since it controls the throttle body.

This guys started out LSX and changed gears to LFX...

https://www.rx8club.com/non-rotary-s...6-swap-256928/

I'll check it out. Thanks!

The only thing I would be weary of as far as modifying the PPF to fit the Aisin AY6 is that the AY6 (MV5) was never designed to be stressed at the back of the transmission like the RX8 trans was.

I see your in Montreal, I am in Wasaga Beach (North of Toronto) and have a 3.90 gear set I have been on the fence about using, as a 8.8 is likely required for my power goals, and I already have a 8.8 diff.

I believe others have fit the LFX with no firewall modification, it depends on whether the engine position matters to you. I pretty extensively modified my firewall for HPFP/Thermostat clearance, and because I wanted all cylinders behind the axle center line (which I did manage to achieve)

On the note of that, because of my tunnel mounted vacuum chamber, a thermostat replacement is an engine out procedure LOL. Oh well, the car will be lucky to see 10,000km next summer before its completely disassembled again.

As far as DSC goes, its not really that simple of a system to manipulate, as it is intended to be more than just a commanded throttle close during wheel slip.

When it comes to the subframe, it depends on how you modify it. I initially notched the stock unit, but became nervous I had removed too much rigidity from it, so I basically started from scratch with it and fit DOM tube across it with a square tube to fit my steering rack mounts to.

Attachment 286355

Can see in this picture how I modified the subframe.

Be sure its very well braced if your removing the center section and watch how its heated when welded as they deform rather quickly!

Thanks for your input! Really appreciate it. I really wish I have your skill and speed to achieve my project. Still am debating which way to go. If I go ahead I'd like to finish it by next fall but it will be hard between work, family and the fact that the car is 30 min from home.
Good point on the strength of the transport casting. You are right, better not stress it if it wasn't built for this.
Yes, I'd like to move the engine as far back as I van but OTOH i'd like to keep the heating fonction.
I'd like at least to regain ABS. I thought that DSC only worked with the ABS not the throttle either. i thought the TCS player with the throttle only.
Your pics give very good instructions.
i'll probably think it during the holidays before deciding
Thanks

Your welcome! Happy to help! I have done everything wrong at least twice, and learned a shit ton about these cars in the multiple thousands of hours I have invested into mine over the past 4 years lol. It helps being single, no kids and only 27 to have this much time to invest LOL :rollingla

The differential mounting is not the end of the world to sort out, but its definitely something that requires some good thought, or a kit, as that was the first major part failure I experienced in mine, where I broke all the mounting and the diff basically fell downwards, ripping the CV axles apart.

I have seen at least a couple swaps with the LFX/LLT N/A motors where there does not appear to be any firewall reconstruction, which would mean no need to alter any of the HVAC system. On mine, given it is merely a street legal track car, I designed and built my own heating system, and the original HVAC box is completely eliminated.

The ABS system from the non-DSC cars can operate as a stand alone unit, and will drive your speedometer even if nothing else on the gauges work

I'll check more of your pics on the other thread to have some inspiration.

If I had the money I wold just buy the subframes and whatever kits are available. But right now it's not possible and I don't have the patience to wait 2 years. I'd rather do it myself.

That's a freaking lot of hours, it's almost your baby now for the energy you put into it. Anyways I hope you'll enjoy it soon on the track again.

Cheers!

Nice job. They use high strength steel to get the weight down, but thin metal definitely want to warp and flow with the heat of shop type welding. That’s why they pre and post heat it in jigs for the production process.

If i were to notch and weld it, the fact that it might warp is what I am afraid of mostly considering my skills. This is one of the biggest details I would need to consider in my case.
Thanks Team

I definitely learned that the hard way!

I initially notched pretty deep in to the subframe and plated it, not thinking about the bracing of it while I welded, and sure enough, it had changed shape pretty drastically. That was part of my decision to re visit it, along with fear I had removed too much rigidity from it. I'm happy I did!

I also revisited my notch and added a shit ton more bracing in to it to be sure I haven't turned the chassis in to a noodle. Sometimes its a bit of a balance of strengthening the chassis, and trying to avoid adding too much weight.

Never feels like your adding that much with welding, but doesn't take long to go through 10lb of welding wire!