BP05 Limits
 

Hi all

car is a 94 1.8 with 33k miles on engine and chassis, car has been boosted for the last 12k with an old t25 journal putting down 202/200 at the wheels. Long story short, oil pan leak under RMS so pulled the motor and going full forged bottom end, efr6258, e85, 6-speed, MS3, and all the little supporting things hoping for 300-400whp, the bottom end is at a reputable engine shop currently. I'm on internship and don't make a ton of money at the moment, but I am trying to do the car right as I've had 5+ before (some turbo, some NA) and because of how clean/low miles I plan to keep this for awhile. Because of cost and how immaculate the head looked coming off, I was 100% planning on keeping the BP05 and recognizing it as the bottle neck to my whole setup, but being okay with that because this is 100% a street/weekend fun car and I don't *need* to rev much beyond 7200

Until yesterday when my friend with a 2000 had bottom end issues offered me the BP4W head for a stupid price. It is coming out of a 100k+ car and would need to be completely gone through with new parts ($$$), especially considering he isn't totally sure of what happened to his engine -- just that its his only car and is just swapping full engines as fast as possible.

So of course that put me down the rabbit hole of "what if" and I searched through the two big miata forums for any real comparisons between the BP05 and BP4W, even took a solid read through the bench flow testing/cross-sectional thread and the differences are enough to theoretically at least consider the swap.

SO we all definitely agree that the BP4W is better for numerous reasons, I am not here to argue that at all, but all I could really find was that the 4W flows ~20% better on a homemade flow bench and can't seem to find any real-world type of dyno data between the two or anything. I also couldn't seem to find any "higher" 300-400whp builds using a BP05, are there any here? This being a 100% street car, I would prefer a really fat midrange to a high rpm/hp type of build, so I don't *need* the higher rpm capabilities of the 4W if the BP05 can manage that power below 7200

so where are the limits of the BP05 head? At what point does the delta between the two become significant, are we talking from 6000+ rpm or are we talking from 2500+ rpm? And, if I am not going for all out numbers - does the cost and time delay of saving for that on top of everything else justify any of those gains for a fun street car? I recognize that it will need higher boost to make the same power, but the efr can likely handle that; force its way down the higher angle air pathways of the BP05 no problem (theoretically)

HELP!

TL/DR: 1.8 full forged e85 efr6258 BP05 ~350whp possible or take out a second mortgage and refurbish a BP4W without a question?

The single most important aspect of any build is airflow!
You have gone down the rabbit hole of high horsepower Miata builds. You should go all the way.
The difference in head flow between the two choices is significant enough to demand the higher flowing part.
Boost is a measure of restriction of the engine. A less restrictive engine will always make more HP with the same pressure of boost. The total airflow will be greater, the boost level will be the same.
A better flowing head will ALWAYS make more power per pound of boost.
I would not consider a serious build without a 99 and up head and a 99-00 manifold or a square top.
Forged bottom end should be paired with best head available.
Doing a stock (or mildly modified) head is NOT big bucks.
A 100K head might not need valves or guides.
There are threads on this forum where members have made 400 hp plus on basically stock BP4W heads.
The condition of the available head is the question at this point...
Tear it apart and post pictures. There are very competent folks on this forum that will steer you in the right direction.

If you're building a motor and have access to a head that isn't a BP05, you use the head that isn't a BP05.

If you don't have access to a head that isn't a BP05, the BP05 will not keep it from making 350.

Technicalninja, I would agree with everything you said, but there are many BP05 motors making 400 hp and more. I know of a drag racer who made over 800 with a very large turbo. He did go up to a BP4W, and is now close to 1000 hp. It all depends on what you are willing to put into the motor, budget, and what Motorsport you participate in.

That said, I would never go back to a BP05 as long as I had a BP4W or BP6D available.
https://cimg6.ibsrv.net/gimg/www.mia...28ebf092bd.png

I bet someone, somewhere has gotten huge numbers out of a 1.6.
I'm not going to try it...

The original poster has already stated forged lower end, E85, and EFR turbo. This makes it serious.

From my point of view the BP4W and the intake in "core" shape should not cost more than 250-350.
Rebuilding that head might be as cheap as a set of springs and valve stem seals which I would also replace on the original head if I was forced to use it.
You can spend big bucks on a head but his power expectations do not require that.
The increased flow from the later head WILL have a fatter midrange and spool the turbo a tiny bit faster.
It will also hit the required power level with less boost which makes it less stressed. the IAT should be lower at a lower boost level and this makes it a tiny bit less likely to detonate.

For less than a 500 investment he will hit his power level easier with a less stressed engine with lower IAT temps. The engine will be more "robust" and dependable.
This is CHEAP horsepower in my book.

Some of the earlier heads had HLAs which I don't like. The BP4W is shims

Miata2fast: Your set up is way cool. I am a ITB enthusiast and yours is one of the few that appears to be set up right.
My ultimate NA (naturally aspirated) Miata would be 13.5/1, Giant cams, E85, EFI ITBs with 45mm throttles, big stepped exhaust.
I'd be faster than you but not faster than a properly set up Kswap which would probably be cheaper to build and more robust.

It is also interesting that your friend appeared to gain 25% with the later head. This is astronomical if that was the only change he did.
It never is however...

I'd expect maybe 10% gain over the early head with torque up across the entire range.

Okay, so I guess I will report back with pictures of both heads when the BP4W is in my possession and see what everyone thinks; in the meantime I'm going to start window shopping building a head -- so heres this; I was planning on a skunk2 intake

would you:
1. build the BP4W and use the stock 94 intake for now
OR
2. Skunk intake with BP05

The intakes do not interchange between the BP05 and BP4W. the ports overlap/aren't in the same place - the ports are off by about 10mm.

edit - https://www.flyinmiata.com/default/s...-manifold.html

looks like the Skunk has a flange with 2 sets of holes to shift it up and down. If you are set on that plan, then skunk and BP05 ans swap head later, cos the BP4W and NA8 manifold won't play nice with each other

I have a BP4W head and other parts if u wanna buy them

Buy my BP4W head and 10:1 compression rods and pistons lol, cheep

what’s cheap exactly? My dogs name is Cheep! Anyways, forged JE pistons and manley rods are being assembled at the machine shop already, so don’t need those but what’s a “cheap” BP4W these days - treasure coast seems to have them at 950 fully rebuilt, which honestly I don’t want to have even that much into a head when I’m done IF I can avoid it

Heck I dunno what these things are going for these days. Send me a PM and I'll show you what I got for sale. I have a bunch of other stuff you might be interested in.

Well, turns out he is just going to give me the BP4W in exchange for helping him pull the motor -- cant beat that price! So im going to go down the head rebuild route, any suggestions? I need to go budget with the head to afford the rest of what I need, so I was thinking mostly OEM stuff minus the springs? (I believe I read on here that the springs are a little light for lots of boost or RPM) I dont want to spin it to the moon, but a safe 7500-8000 would be neat

Check cam journals, valve guides, valves and seats for wear. If they are OK, replace stem seals; I would use OEM, but people had luck with Supermiata before. Cam seals and VC gasket, use OEM. Springs, you already know the answer. I cannot think of anything else that would reasonably need replacing on a decent head.

Because you chose the 6258, it would seem the faster spool of the BP05 would be less apparent over the BP4W.

I don’t follow, shouldn’t the BP4W be better throughout the range including spool? It’s all connected, right, so it would make sense to me that more flow through the engine would increase spool

Ive been wrong before though 🤷🏽‍♂️

I was theorizing that with a fast spooling turbo like you chose, the VVT advantage of the BP05 would be minimal. In your case I would choose the BP4W.

BP05 doesn't have VVT. *boggle intensifies*

No mystery. My bad. My brain was mixing up the head designations. I shouldn't try to multi-task with the 4 brain cells I have left.
Clearly, the better choice is the BP4W. Clean up the casting's with 80 grit, valve job, you're good to go at the budget you're at.