Time to build that LS3 Miata
 

I've enjoyed playing/racing my under powered miatas. Now I want a Corvette killing sleeper! My goal is to sell my 04 MSM and my 08 PRHT to fund the project. I have a 99 Sport Package with ABS that I plan to use for the swap. My goals are 500hp LS3 and a 200 shot of nitros. I've already got the suspension parts, wheels and tires. Looking for direction on which front subframe and rear diff to use. Pros and cons. This car will be autocrossed a lot, and run mountain roads primarily.

Sounds like you have a good plan and know a lot about Miata's already.

There was a discussion over on the regular Miata forum a few months ago about the "best" Miata to utilize for a V8 conversion.
Keith from Flyin Miata tossed in the opinion that the NC works best if you are going to go with a higher amount of HP such as you have mentioned.
It is also an easier swap because you don't have to remove the dash and cut up the transmission tunnel. The chassis is a bit heavier but also stiffer and more suited to higher horsepower builds.

On the rear differential, an NC build thread by a guy who has his running mid 9's (last I heard) went with a Ford 8.8 differential with limited slip and an electric locker.
That seems like the best path to follow because you can get a full locker with just the flip of a switch whenever you need that sort of thing. At the level you are talking about it sure seems like a necessary item. The differentials come in some big SUV's but I don't know the actual part number you would be looking for.

Hope you do a build thread because I am curious to see how you go about getting your build done.

Thank you for the info. I sat and read your complete build thread last night. I really like your clean and OEM look you were going for. I can only hope for my wiring to come together as well as yours. The NC option sounds inviting. I like the NC but I feel cramped in it compared to my NB cars. I will agree they handle in stock form much better. I prefer the body style of the NB over all other styles.

Thanks for the kind words.
When it is all said and done it really comes down to what you prefer and what you will be happiest spending your money and time building.

Based on your initial post it sounds like you intend to go with the 525 HP crate engine. Is that a correct presumption?
You didn't mention your projected budget or what transmission you expect to utilize.
If you haven't already done so you should take a look at Turbo Tom's Youtube video where he summarizes the cost to do his swap utilizing similar components.
From memory I believe his build cost was somewhere around $26k but he also bought a 2004 on top of that.
He utilized the Flyin Miata recommended parts for everything except the exhaust system.

I did a similar concept on my build except for utilizing used drivetrain components with a cost of $17k without the cost of the car.
Utilizing used stuff is a bit of a crap shoot.
In my case I got an outstanding engine but the transmission required a rebuild and the used rear differential was junk.
If not for those two issues I could have probably gotten by with a $14k budget.

You asked about which subframe to go with.
It is really three things rather than just one because several items need to play nice in the front subframe area.
You have the subframe, oil pan, and headers and then you have the additional issue of ground clearance.
Personally I feel that the V8 Roadster subframe is the only choice because it works without interference with the Flyin Miata oil pan and either the Flyin Miata or V8 Roadster headers.
Space is real tight in the lower front area.
The Monster Miata subframe is a fabricated subframe that is heavier as well as fatter than the V8 Roadster tubular subframe.
When you are working on stuff in that area you will really appreciate the extra working room that you get with the tubular subframe.
It is hard to quantify but a real advantage.

If, on the other hand, you intend to go the cheapest route possible then you will be cutting and patching the factory Miata subframe , cutting and welding the Camaro oil pan, and utilizing a set of Pontiac G8 exhaust manifolds.
For that route you should check out Taylor Ray on youtube and look at his build.
His budget was $10k but I am sure he went well over that after replacing the engine and transmission a couple of times.

Hope this helps you decide on a path forward.

The current monster subframe for the LS motors in the NA-NB is a fabricated piece, somewhat more professional looking than the previous modded stock subframe he used on the Ford swaps. Taylor Ray used the new fabricated subframe from Monster. Not sure what Martin (Monster) offers for the NC, if anything. Actually, an LS can be swapped into the NC using the stock subframe, and some pretty simple mounts or an RX-8 subframe for a little more room. Another good reason to start with an NC.

appreciate all the good info.
my budget is approximately $17k on top of what parts I already have.
4k motor used
3k trans t6060
2k for subframe
2.5k for rear axles and diff
1k for radiator and Oil pan
1.5k for headers and exhaust
1k for fluids and wiring.

i know from reading others threads many other items will come into play. I expect another 2k in miscellaneous small items as the build progresses.