Assist with Miata LSX conversion

[JayJohnson600]

My recent head gasket issue has prompted me to consider putting an LSX V8 into my 1990 Miata.
This is my daily, and gas saver, but I also have a full size truck that can take up the role for a while.

Also, I think next years inspection will be without the emmissions test for 1990 model years and before. (right?)

I've got 95% of the conversion parts in hand. I've been an LSX guy since 1998 and have lots of LSX stuff.
I can choose EFI or Carb'd, I've got the parts for both. Anyone know what PSI the stock Miata fuel pressure runs at?
I have a carb, but it needs to be inspected, and may need a rebuild.

I'm thinking of putting a Glide behind the motor with a high stall(I think I have a TH-350 for an option), that will be somewhat merciful on the rearend, and quite interesting on the street.
I used to street my race camaro with a glide and 4200rpm stall, running weight was 3300 lbs. I expect to enjoy streeting the Miata like this with 2300 lbs
I don't have a good option for a stronger rearend, so it will have to remain stock for now. What is a stronger option for rear end?
And I don't have any suspension upgrades, apart from a good amount of weight reduction in the Miata. It's already a touch high.

Anyone care to assist with a home-built Miata LSX conversion?






Jay Johnson
Flower Mound, TX

[Titus]

Also, I think next years inspection will be without the emmissions test for 1990 model years and before. (right?)

The law basically says that all cars 2-24 years old must be tested for emissions, and it defines a 24 year old car as a car with a model year that is equal to the current year minus 24. That means that in 2014, a 1990 model year Miata will be considered 24 years old and require an emissions test. In 2015, all 1990 Miatas become exempt.

With that said, a modern LS engine with a modern ECU should put out significantly cleaner emissions than a stock 1.6 Miata engine.

[JayJohnson600]

hmmm, yes but then it would fail citing original emmissions equiptment has been altered.
With a big-azz V8

I'll go in to get a new inspection sticker on it so I can proceed with out adding other hassles.

Jay Johnson

[goofygrin]

You go to the wrong place to get inspected if they'd fail you for "non original emissions equipment" lol.

I've passed a number of times with no cats...

[trickyrix]

You sure you wanna street that thing with a Powerglide? Are you thinking of sticking with the stock rear end? I can tell you right now that will end in tears in about 8 seconds...

[POS Racing]

You sure you wanna street that thing with a Powerglide? Are you thinking of sticking with the stock rear end? I can tell you right now that will end in tears in about 8 seconds...

That is what I was thinking heck the early Miatas (90-93) were fully capable of breaking the rear end with the stock motor.....

[OZMDD]

Yep, stock rearend is a non-starter. Most folks go with the rearend from a CTS or something along those lines. I'm not sure if a TH350 will even fit in the trans tunnel. Put a nice Getrag in there.

[Titus]

hmmm, yes but then it would fail citing original emmissions equiptment has been altered.
With a big-azz V8

I'll go in to get a new inspection sticker on it so I can proceed with out adding other hassles.

Jay Johnson

I have never had an inspector say a word about altering stuff on my Miata and I have a supercharger, intercooler, full ECU, headers, etc etc etc.

[trickyrix]

hmmm, yes but then it would fail citing original emmissions equiptment has been altered.
With a big-azz V8

Not a problem here in Texas. As long as it passes the sniffer, you're golden. Perhaps you're thinking of Kalifornia?

[effse7en]

If you have the car in the shop for something else along with the State, they might say something, but if you just come in for the state, they probably won't say anything. I wouldn't, and I'm an inspector.

[EricJ]

Not a problem here in Texas. As long as it passes the sniffer, you're golden. Perhaps you're thinking of Kalifornia?

If it doesn't pass the sniffer, you can get a special vehicle exemption that limits the car to 10k miles a year.

[JayJohnson600]

So I have some issues with the car.

The head gasket is leaking coolant into cyl#3, and that's causing a noticeable misfire at idle(near clean at driving rpm's), loss of mpg(from 355miles to 290miles on a tank), and making the exhaust gassy.

So, I need to run the car low on coolant to help the combustion hit, and clean up the emmissions for the sniffer test.
Doing this will require taking in the car during cold weather.

Jay Johnson

[trickyrix]

If it doesn't pass the sniffer, you can get a special vehicle exemption that limits the car to 10k miles a year.

...or you can find a friendly inspector and leave a sixer of Budweiser in the passenger seat.

So I have some issues with the car.


The head gasket is leaking coolant into cyl#3, and that's causing a noticeable misfire at idle(near clean at driving rpm's), loss of mpg(from 355miles to 290miles on a tank), and making the exhaust gassy.

So, I need to run the car low on coolant to help the combustion hit, and clean up the emmissions for the sniffer test.
Doing this will require taking in the car during cold weather.

Jay Johnson

If you can lay the car up for a little while, go ahead and swap the engine out and have it ready to go for spring...