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@softopz ended up making 300whp and torque was at 280, that was at 6500rpm. I was limited due to my fuel system, it'll be one of the next things upgraded. @Ereschkigal @seattlejester thanks for the feedback, shortly after taking those pictures I ended up pulling the fenders further away from the body of the car in order to lengthen them a bit. I like how they sit a little better, i'll need to see if they hit or pass that body line a bit later today. The non circular flares were bugging me they just didn't look right, ended up cutting them up going to be attempting to make them more circular.

4130 easier to machine than mild steel? Maybe close to the same if annealed which it typically is not. Usually comes normalized which is going to be harder than mild steel, not terrible but definitely harder. If you go to a thinner section it would likely be easier than the large thickness parts. Welding is certainly doable but you need to be more careful with heat zone, ideally post treat to normalize. Choose filler carefully, fit properly, etc. I plan on using pretty much all 4130 on my car on what I add in the cage, floors, etc. It is stronger if treated correctly and can be lighter at the same strength if you go thinner wall. For example I'll use 1-1/2" .095 tubing in the cage where you would normally use .120 wall to get a similar or maybe a little better strength and a lighter part ~20%. But you have to be careful not to make it brittle or soften things up to the point of mild steel strength with the thinner section which is then weaker. And it is MUCH more expensive. Not insane but substantially more for material. 4130 is good for high performance thin walled structure type applications. Any of the energy transfer parts like hubs/axles look at 4340 or even 300M is best unless you get into some of the new "wonder alloys" and proper heat treat. 300M isn't stronger than 4340 natively, it just allows you to heat treat to a higher level without getting brittle which is how you get a much stronger part. 4130 might be a good option but cold rolled would likely be fine for most applications. The thicknesses you are showing here are plenty IMO. I'd look at bracing and structure a bit rather than throwing mass at the parts. I'll bet that it could be done thinner and still keep plenty of strength even in mild steel. That saves money and weight. 4130 would be much thinner. Generally you can get more strength three ways: mass stronger materials better design/engineering The second two are much better for performance. Design is free other than the time to do it which in the business world means money. But only costs on the initial setup, free on everything after that, maybe even cheaper if done right. Materials - cost is higher raw cost and often higher fabrication/heat treat/procedures. You just have to weigh out what it's worth to you or whoever is going to be buying it. Not trying to offend here, some of that is my opinion but based on facts and how things are done in higher performance applications. Every build on here is done differently and one of the things I really appreciate and enjoy on this forum is the tact that is generally afforded. Everyone's vision and budget is different and there is room for all of that here.