Opinions on oddball LS build idea
 

So I have been thinking about an LS swap for my wifes miata since the day we picked it up but I'm not looking for a 500hp monster, just a slightly more powerful car that gets good mpg and has the V8 rumble.

I have a iron block 4.8 currently but ran across a smoking deal on a aluminum block LS4, I know the LS4's arent all that popular due the bolt pattern and odd crank but I would plan on using the 4.8 crank and rods in the LS4 block.

I would be perfectly happy with the 255-275 hp I have heard the 4.8 should put out with all stock AFM internals and was considering trying to adapt the engine to the stock trans to simplify the rest of swap and because I already have 2 stock spares. Plan includes stock (corvette c6?) exhaust manifolds and 2.25 or 2.5 duals, stock intake etc.

In short my build would be.

LS4 destroked to 4.8
modified stock subframe
Adapter plate to stock transmission

Thoughts? I have seen 250-300hp turbo 4cyl miatas with the stock trans so I dont think it would be too much power for the stock drivetrain maybe just throw a LSD in the rear and stiffen chassis up. This car will probably never see a auto cross or road course or even drag strip, its just a weekend cruiser and commuter.

Making life easier for me is that our miata has no PS/AC or cruise. I have also been looking into clutches avail for both 4cyl miatas and those meant for v8 swaps and it seem flying miata rates their 4cyl clutch at 300ft.lb so maybe theres hope for the stock size clutch.

It seems fiero guys are using the ls4's due to shorter crank and are swapping them into manual cars using a 10" or so clutch vs the stock miata 8" unit.

Come on guys over 100 views and 0 opinions? I thought everyone had one of those these days lol. I'm not going to be doing this swap for awhile so I have time to gather info and parts at my leisure.

It sounds like you may be creating a mess that you could potentialy avoid in the first place..
Also nothing wrong with using a 4.8 or a 5.3 IMO any all aluminum Ls is a good choice. However a 6.0 block with the 4.8 rotating assembly would be lots of fun in a Miata

LS4 and T-56? - LS1TECH

Only big mess with the LS4 is the 60 degree bolt pattern makes it hard to attach to a standard v8 pattern trans, but T-5's were avail with the 60 degree bellhousing so that would bolt right up and I'm thinking my stock trans should support the Hp and Tq that a 4.8 puts out.

My guess on the number of views to repose rate is because there is so much text in the first post and it's kinda hard to follow with all the detail that has little to do with what you are asking.

I think you are making a simple swap way more complicated than it needs to be. For the purpose you are building for there is no reason to have an aluminum block and even less reason to try to use the stock trans. There is just no benefit to doing either for the intent of the car and you absolutely can not use the stock diff because of the gear ratios.

I'd find a crashed van or pick up and you'll have everything you need but the diff. And for that you can still get Tbird diffs for 100 bucks easy.

Hmmm maybe I'll have to trim up first post, I guess it didnt make as much sense to others as it did for me.

I admit that I dont NEED an aluminum block, like the idea of the lighter weight on the nose. I just dont understand the aversion to doing a swap using an adapter to stock trans vs all new drivetrain and buying thousands of dollars worth of custom parts for a car that is just going to be a driver. I completely understand you 400hp+ guys not using the stock drivetrain but feel its within the limits to use it on a 255-275hp motor.

I had the fall back idea of the aforementioned V6 T-5 from a 3rd gen camaro. Most trucks and vans are autos that I can find (took me few months to find the 5spd 4x4 trans) so while they are good source for motor not sure about rest.

So in your opinion I should just jam the 4.8 iron block I have in the car with maybe a T5 and tbird diff? I would also need custom CV's with that diff I would guess and they seem pretty pricey.

My thought is who cares about 75 lbs on the nose in a cruiser. Not worth the price of gaskets to tear the motor down IMO. It's no different that putting weight on the other end so put 100 lbs of whatever in your trunk and see if you can even tell it's there when you are just going to pick up a loaf of bread...

No you just run Martins monster rear end set up and it's a slam dunk. It depends on the value you put on it working the first time with no issues is all. Horsing around with a custom bell housing to use the stock stuff would probably cost more than just doing the swap right. LOL and IMO for a cruiser nothing better than an auto! Sure is handy when you want an ice cream cone for the road!

I could swear I have seen a photo of a 5.3 aluminum block on scales next to a 5.3 iron block and the weight difference was 125lbs or so but I guess its still within reason. I guess I am more worried about the weight affecting the overall feel of driving the car than I need to, heck at 300lbs I probably ruin the handling more. I probably shouldnt even mention I have a whole 2001 SLS with the 4.6 northstar but after doing some measuring I think it will be hitting the classifieds soon for a fiero guy to swap lol

I have a extreme dislike of auto's, even with having knee surgery on my left knee a week ago I still don't plan to ever own another automatic car.

LOL ask anyone who as swapped a Miata and they well tell you the car feels lighter when it is done even though the scale says 200 to 400 lbs more. Why? Because you add 15% more weight but it is so grossly overwhelmed by 150% more power that the car feels lighter. I love the 6 speed in my track rat but love the auto in my cruiser just as much!

Maybe the photos showed a 6.0 iron block at 209.6 lbs, a 5.7 al block at 108.0 and a Ford 302 iron block at 127.3. So the iron block is a little over 100 lbs heavier than the al one, and 80 lbs heavier than the 302. 302's have been used successfully in Monster Miatas since the beginning, I would think the plastic intake of the LS vesus the aluminum one of the 302 would even it out even more.

That could very well of been it. I was just thinking keeping weight gain to minimum since I'm also keeping hp basically to the minimum a LS can put out. I need to find a line drawing of the miata's stock bellhousing bolt pattern to overlay on the 60 and 90 degree GM patterns to see what fits or doesnt, though now that charchri4 got me thinking about it a T5 from 3rd gen camaro might not be a bad way to go. I just love the way the stock trans feels in the miata as well as the way the car rides and handles, just needs bit more power and needs the sound of a v8.