This is my first post on v8 Miata Forums, so hello all. My name is Garret and I am in the planning/parts buying stage for a carbed 4.8LS miata swap, I will start building once my co-op job term here in Indiana is over and I am home in Ohio. I will try to document everything as best as I can. After helping a friend do a similar swap in an 85 rx-7 this past summer, I HAD to do a v8 swap for myself!

I know I have a long way to go, but I have a wide range of fab skills, and the car will be put together at my friend's shop (everything I can't do they will help me with ...which might be a lot haha). I do a fair amount of road course stuff with NASA and some autocross, after I get used to the car this will be its primary duty!


things currently in my stock pile:

-'90 shell with tokico blues/racing beat springs and an LS motor subframe (rough condition) great find though!

-'90ish miata, generic miata that has full miata drive train (VLSD) and wiring

-Edelbrock Intake Manifold / MSD box

-T5

All advice and help is appreciated, thanks and welcome!

 

Welcome to the community, garret. I hope you find the resources here as valuable as I have. Keep us posted on your progress!

 

Welcome from another Ohioan.

 

Hey Garrett welcome to !

It sounds like you are an adventurous kind of guy and this will be an adventure for sure. So I have to ask why a carb and if you are going to do an iron block why a 4.8 vs a 5.3? Seems to me you leave a lot of easy power on the table with both those options.

 

Thanks! I like the 4.8 for several reasons. It often gets sold short. Realisticlly it only puts down 10-20hp (for the older, cheaper versions of the 5.3) less than the 5.3, it's a good bit cheaper, parts are dirt cheap, and a quick heads & cam swap in the future could open up an extra-large can of whoop-***. Both are good options really, I debated it at first, but $ per result I had more pro's checked off for the 4.8. ..also, going with the smaller motor allows me to use the much lighter t5.

For the carb vs. fuel injection:
I like the mechanical simplicity and cost of the carb. I know it robs some power and efficiency, however knowing how racing goes and how easy things can break, the carb strikes me as a good option for that reason. Less wiring means less time spent tracing problems if any occur.
Also, I like the availability of tuning the motor on the fly with a screwdriver and a wideband.


In short, max power isn't my biggest concern just yet, I want to get comfortable with driving such a crazy car on track without it trying to murder me, and keeping this from-scratch project car from breaking down under constant abuse haha. But the great think about CSB's is how easy it is to bolt on extra power!

 

Especially when comparing basic, stock 4.8L versus 5.3 motors, the little 4.8 really isn't a bad choice for the lightweight Miata. It naturally makes less torque with less displacement and otherwise similar accoutrements, but its same-size bore allows the use of the same, good-flowing heads with large valves, and its shorter stroke allows revving it higher with the same piston speed to generate HP numbers, as you suggested, not far off from the bigger engine. Like the old 327 versus the 350 Chevy small block.

If I were going to plan future, expensive upgrades to try to make really big power, I would personally start with the larger displacement, to avoid the engine being too peaky and herky-jerky for everyday driving. But for a mild cam, maybe a bit of headwork, and some headers, I agree the 4.8 has more going for it than first meets the eye. I mean, well over 300 crank horsepower DOES represent something of an upgrade from the stock Miata, right?

Good, sound thinking. I'd still recommend no less than a T-5Z equivalent transmission for the 4.8L; even stock it will make torque right at the ceiling of a good T-5 box. Should work fine, though.

The lower torque output also helps better put the power to the ground with available tire choices, without sacrificing much in the HP department (revving higher).

You should not have to give up anything in peak HP with a good carb and manifold setup. The sacrifice comes more in the breadth of the torque curve, especially low end smoothness and general sophistication of everyday, part throttle drivability. Open the throttle, let 'er rip, and the playing field is leveled.

 

LOL didn't anyone tell you half the thrill of driving the car is knowing it could just as easily kill you as get you to work?

Sounds like you have a good plan and have put a lot of thought into it. I'm am interested in your thought process on the carb. Seems to me when you buy your engine it would be easier to just buy it complete so it has everything you need to run just as it did in the donor vehicle. I'm not following where the advantage is to set up the motor differently to run a carb. Wouldn't the motor run just as good in the Miata as it did in the vehicle you pulled it out of?

I get the sense $$ is a factor for your build and I am super intrigued by budget builds like Jacks epic Ford build. Do you have a target cost for the build?

 

Correct, with the edelbrock mani and a 600cfm carb, drops in hp should be at a minimum if not better than original (also going to throw in a used ls1 cam for dirt cheap). As for daily drivability I have no issues with making it un-street drivable, and a high sprung low displacement v8 sounds amazing.

carb v injection on price, buying the motor with full wiring still demands that you get aftermarket ignition/timing and control, which is expensive. Also, there is a high possibility that later on down the road this car could be entered into an endurance race with Chump Car. In that sort of environment heat and wiring can be a huge enemy, even in shorter races.

In the swap we did this summer on my friend's rx7, the total bill was $3,350 including the cost of the car so I am ball parking around $4,000 including the cost of stiffening the chassis, brakes, suspension, and cast off Rcomp Hoosiers ...I think that’s where the real payoff will be. even a bone stock 4.8 mounted to a well-balanced miata should be enough to crack out crazy lap times (good driving considered)

 

Also this is the rx7 build I am referencing ... 'Murica

 

Cool. Didn't know that on the timing deal so it makes sense. Gonna be a great build for sure!

 

Yea, cost-no-mind, fuel injection is clearly the better option, but for this it should work well. P.S. I like your blog page!

 

Not trying to talk you out of the carb, but if you WERE going to do fuel injection, couldn't you just pull the engine with complete wiring harness from a good donor and include the factory ECU? The engine should perform just like it did in the donor vehicle; don't see why aftermarket engine control would be required. But then I have not done an LSx swap, either.

Although I understand it fairly well in principle, I'm still intimidated by the thought of hands-on assembling an EFI set-up. On the other hand, I would be confident in my ability to put together a good carb-based fuel and electrical system, without the stack of manuals and hours of swearing great oaths to the gods of mechanical engineering.

Seriously, if you complete the project with less than 50% overrun from your target budget, you will be my hero! I love to see what can be accomplished with thoughtful ingenuity, good horse-trading, and expert fabricating skills. Anybody can build a car with cubic money; a hot rod on a workin' man's budget is as American as ice cream and apple pie. Love it!

 

after getting both shells, suspension (off one of the shells), the edlelbrock intake/msd box, and the t5 tranny I am currently invested $1,000 in the project. Of coarse my estimate excludes fire suppression and roll cage, which is another 1k

 

the EFI system and fuel injection intake would have been WAY more expensive to do properly than the carb. also, est. assembly and fab time for this once all the parts are gathered should be about a month if everything goes correctly (that's including screw up and start over with stuff time)

 

That was my thought too and the way it worked with my car. Seems to me the system doesn't care or know if it is in a truck or a Miata. As I understand things it's a simple matter to tell the computer what the rear end ratio is and delete the VATS if it has it and off you go. Is that not right with the 4.8?

+1 but I'm rooting for you to come in under budget all the way!

 

This is one to watch...love the concept of a car built to run WOT and screw the parking lot. Compromises are for the timid! (me).

 

Also, the truck intake is really really tall, switching to the ls1 intake + fuel rails + injectors is were it would really start to add up budget-wise for the 4.8

 

Hey garret, I am also garret, and I am about to do a 4.8 ls swap aswell. I just joined the forum, this is quite the coincidence lol.

 

Just one friendly word of advice. The two of you should never form a company that specializes V-8 Miata conversions. Everyone will think "Garrett and Garrett" is a law firm.

 

^^ that's a good one!

 

Hahaha noted lol

 

Hey Garrett I saw your post looking for a stock diff and was wondering what you are planning for a diff set up. I have not heard of a Kaaz or OS Giken so can you explain what your approach is here?

 

Kaaz and OS Giken are both big manufactures of aftermarket race diffs. Both make clutch type diffs for miatas that from as far as I can tell are nearly bulletproof. Turbo miata guys are big fans of both and I have seen lots of guys running well over 450whp with no issues to report, even over long periods of time.
So, after researching this, I talked to my parts guys and learned that they would cut me a pretty good deal on the Kaaz diff for 800 (msrp 1100ish). However, I know that the smaller 1.6 final drive gear is weaker than the 7" and that's what all the turbo and track guys say is the bees' knees. So as a result, my idea is get the Kaaz and swap the diff housing and final drive to the stronger gear. With the 4.8, I should be well below the threshold of braking things as well.

 

It is not the horsepower that you need to worry about. The torque will probably destroy it. You could get a bulletproof Ford 7.5 set-up for the price of the fancy Kaaz differential alone if you look in the right places.

 

torque was considered as well.. They are lasting, just google "breaking a kaaz diff". As far as it being cheaper than the 7.5, yes. Yes you can get a 7.5 diff for cheaper, and yes the 7.5 is stronger, those facts are 100% correct.

However, after axles, hubs, and everything else as far as I can tell, it's not cheaper, and I don't think it's really necessary either. BUT, that's why I'm weighing my options.
(I will be under 300ft/lb wtrq.)
Also, a clutch diff would be preferable for racing purposes as it doesn't unlock up on decel or when a rear wheel is off the ground.


Where are you finding full 7.5 setups for that price? I would not be opposed to the 7.5 if it were actually that cheap