Am I missing anything and opinions
 

Stay with the smaller turbo. It will make more power than those tires can hold anyway. It makes more than enough power to destroy that transmission eventually and will give you better transition in to boost.

You will want a better compound of brake pad for fade resistance if you decide to drive the car on the track. Drilled and slotted rotors are not generally of value on our cars. They create weak spots that propagate fractures when used hard on the track.

You will have great difficulty keeping the car cool at those power levels on the track. It will be fine for short blasts on the street.

Stock ignition coils will not be sufficient.

Solid build plan overall.

What compression ratio pistons? You want 8.6 pistons if using premium pump gas.

I vote for efr6258. I'm running it with a kraken setup and it's awesome sauce. I made 309hp at 17psi. Pump gas.

greg made 350hp pump gas and 415hp on e85 with a 6258, so really no bigger turbo needed.

forget 2860, it's a pig. You want the car to get exciting at 3500rpm or 4500rpm? That's the difference between the two.

your 6 speed should be fine to about 300-325hp. The closer you get to 400hp the more chance to grenade it.

I'm still on a measy 5 speed and typically have boost set to 15psi (280hp) street and 10-11psi track (250hp). Just to have some mechanical empathy for my 5 speed. Which btw is holding up like a champ.

as for wheels/tires. I have 15x9 with 225 rs4 and the grip is great. Next time I get tires, I'll probably opt for 245 rs4.

You are missing radiator ducting, copper plugs, suspension bushings, motor and diff mounts, fuel pump hardwired off battery/relay, new brake lines with centric plain rotors and good pads. Heavier flywheel like 13/14 lbs so you actually enjoy driving on the street. Welded not tapped oil drain return into pan higher than oil level in pan. Overall your car will be solid though.

With regard to Transmissions you really only got three reasonable choices. A Miata 5-speed which is good for around 250 horsepower, a Miata 6-speed which can get you into 300 horsepower range, and the BMW trans swap at $4k.

No, I found my 10 pound flywheel just as easy to drive as stock. That was a Flyin Miata Happy Meal with 10.3 lb flywheel.

I have a 8lb flywheel with a SuperMiata 4 puck and dont mind it one bit....

I'd look for 1000cc injectors with that power goal in mind. The 750s should get you there, but if you go E85 or if you end up wanting to turn it up anymore you'll just be swapping them out.

Every 3.63 rearend I've seen for sale has been around double the cost of 3.90 since they're so rare by comparison. Check out FM's gear calculator and take note of how close those 2 ratios really are. I'd definitely prefer the 3.63's, but the difference in cost just isn't justified IMO vs a 3.90. If you do look at 3.63's, take a look at doing a getrag swap too. It seems like it's not a ton more expensive, and near bulletproof in a miata. Once it's done you get tons of cheap gear ratios to choose from.

On any street class autocross tire you'll be fine traction wise in the low 300s. 1st will spin some, but it's so short that you're not there for long anyway. 2nd generally hooks up, at least in good conditions (the early part of the tire's life, warm weather, dry roads). 3rd and above have no issues at all.

I hated the shudder from take off and chatter during de-acceleration with my lightweight Aluminum Fidanza Flywheel with exedy clutch street driving(years ago). I just like more mass for smoothness.

Kiki - I see you are in the UK so there's a few things you prob need to bear in mind :)

E85 - I don't think there's any plans for us to get it over here, which sucks :( If you are speccing new injectors though, I'd still say get the ID1000's / ID1050Xs, no reason not to :) You should be able to get north of 300whp on 98/99 RON fuel though (so Shell / Tesco). Our Super Unleaded works out to be something like equivalent to 92/93 in US numbers.

BHP - everyone on here is talking power at the wheels. Everyone in the UK talks at the flywheel, so there's a significant difference :)

Diffs - 3.63 final drive diffs are a lot more common here (standard fitment on some mk2.5s), but 99% of them are Super Fuji, not Torsen cores, which are known to break. You can swap over a Torsen core from another ratio diff reasonably easy though.

Fuel Pump - The DW200 is supposed to be much quieter than the Walbro 255lph equivalent. There's also a lot of fake Warlbro's about so buy from a reputable vendor (not ebay etc!)

Head - if you are sourcing a head, look out for a mk2.5 (NBB) VVT one as the megasquirt can control it. mk2.5 EUDM cars get the square top manifold as well which is another thing ticked off the list.

Garrett vs. Borg Warner - there's lots of threads on here. I think the pricing in the UK is skewed differently though, BW work out more expensive compared to Garrett here. A GT2860R is about £700, EFR 6258 about £1500. Not sure what Borka has been smoking when he says that the GT2860 doesn't get exciting until 4500RPM - maybe if it's a shitty one from a 200SX, but the GT series ball bearing ones commonly used by Begi / FM spin up way quicker than that. Not as quick as an EFR though.

The new generation Garrett GTX / GTX II turbos spool up much better than the old GT series ones, but are more expensive, but pricing is closer to a BW at about £1200. Again lots of threads on here comparing - both are within the same 'ballpark' performance wise. I've got a GTX2860, but it came as part of a package - having a quick skim through Borka's dyno thread I'd say the boost vs RPM is actually very close, but it's not apples to apples as I've got a tubular manifold whereas he's got a cast one (albeit a very good one), and I'm running a lower compression ratio than him.

There's a few different options for us in the UK that the chaps in the US don't see much:

G19 : https://www.g19engineering.com/index.php?cat=41
Davefab : https://davefab.com/shop/mx5-t25-turbo-manifold-1800cc/

If I was starting again from scratch, I'd probably get a Kraken kit with the BW turbo - Kraken is in the EU so it makes life a lot easier import wise (at least for a few more months!), and it's known to work very well.

Tuning - Don't bother calling MTech. They will refuse to map your megasquirt. I think the others will be OK with it though, or dependant where you are there's plenty of other MS tuners..

Whereabouts are you in the UK?

The US gets a much better selection of tyres than we do - for 15s once you go over 205 width the choice gets pretty limiting - mainly 225/45s - R888s and some of the trackday special brands like Dmack. I'm running 205/50R15 Federal 595RSRs and they grip pretty well in all conditions and aren't crazy expensive..

Has there ever been anyone dissatisfied with an EFR? Should make decision easier.

watch this video closely, ideally pause it and move the time slider little by little. this is a 2nd gear pull. (i know i know, not ideal to build boost) but you can still see the turbo is very late to the party.
This is a BEGI S5 turbo setup, a gt2860r with a T3 exhaust housing, with a begi cast manifold with Tial EWG set to 8-9 psi spring only pressure.

I am sure a GTX on a good tubular manifold and good flowing exhaust will behave much better. but this is my personal experience with a crappy BEGI setup.


here is a pic of this setup:
https://cimg6.ibsrv.net/gimg/www.mia...70084589a1.jpg

My previous setup was a Begi cast manifold, IWG GT2860RS (0.64A/R), FM Elbow and a 3" exhaust - it used to get going way before your setup did. It didn't spool as well as my current setup, but it was on boost by low 2000s RPM and making a decent level of boost by 3000. The current owner of the kit has just had it dyno'd so if I remember I'll ask him for stats..

But anyway - sorry for the thread hijack :) Moral of the story - don't get a BEGI kit. Just buy the EFR :)