Which BP into NA6

[DanZig]

Hi all,

After a bit of advice from those who knows more than me when it comes to MX5 engines, specifically of the NA variety. I've been doing a fair bit of reading and searching but some things are still a bit unclear to me, specifically which engine to go with.

My current setup is a 1.6 with T25 and it's been pretty fun up until it met it's demise after a good innings. Now I feel it's time to go down a different route and try something a bit different. All of my performance cars up until now (besides my NA6 when stock) have been turbo so I'd like to go down the NA path and besides, my dedicated track car is an Evo 6 so I want this thing to be as different as possible. The aim of the build is to be as fun as possible on the street (this car is not a daily) and be completely reliable should I do a prac day or sprint while I'm saving for Evo tyres and E85. Not concerned with going the fastest or getting the best bang for my bucks, I know turbo's are great.

Budget always helps, I'm not on a strict one but I don't want to spend needlessly as this isn't a competition car I need to push to the nth degree but I'm realistic about what things cost and don't want to skimp. All work will be done by myself (excluding machine work).

Now, the main dilemma I face is which engine to go with. I will be going 1.8, that's my only real certainty. I know the later model's are better but I'm having a big of difficulty finding one for a reasonable price and currently I do have an NA8 engine in good condition ready for rebuild I bought with the intention of building a tougher turbo motor a while back. This leaves me with the following choices:

1. Rebuild my NA8 engine, give it some fruit (get a headjob etc.)
2. As I have a complete (in pieces) NA8 engine, I can grab the Astina BP from the wreckers and chuck in (quoted $450). Leaves a decent amount left over in my purse.
3. Wait until a 99+ shows up for a fair price.
4. Search for a VVT which would require me upgrading my MS to control VVT which would push the price up a fair bit.

I'm pretty aware of the differences between the motors. The 99+ is certainly more appealing with it's higher comp, solid lifters etc. and would be the best for the future (cams and ITB are the dream) but given the price of Astina engines, the fact I already have one and given I'd like to get this car back on the road (I miss it already), it is tempting.

What do those in the know recommend I do? The price different would more than leave enough money to grab some cams straight away and I can shave down my spare head if I go with the older engine. I could also look for a later head should one come up down the track.

Is the later engine really worth me waiting and the extra price or can I make up for it with a bit of work?

[Magpie]

If going naturally aspirated I would suggest a BP bottom with a NA8 head. The NA8 head allows better porting.

[DanZig]

Really? I was under the impression the 99+ and VVT heads were considerably better than the NA8 (BP-05 which I have).

[Magpie]

It depends if you want to do a custom port job. The NA8 head has more 'meat' on it and allows more shaping options whereas the 99+ and VVT heads have been shaped by Mazda for economy reasons and not performance. Horses for courses I guess.

[gslender]

Really? I was under the impression the 99+ and VVT heads were considerably better than the NA8 (BP-05 which I have).

Correct, in stock form, the 99+ (BP4W) heads are the better free flowing head. General consensus (posted globally) is that you are better off starting with the BP4W head, but I don't doubt you could get same or better gains with a BP-05 head if you had the $$$ to spend.

G

[madjak]

The VVT is probably worth using over a BP05 if you are just keeping it stock. You need to do a lot to a BP05 head to get it to flow but when they have been ported with oversized valves and big cams etc they flow just as well as a developed VVT head. The later model head can get some gains from the VVT itself which helps reduce that middle RPM torque drop especially with big cams.

Why not build and install the NA8 engine you have (or the Astina engine) and get the car running first and enjoy driving it whilst you do your own DIY porting on a spare head. When the better head is ready, drop it onto the block you have. The only issue with this path is the ITB's won't interchange between the BP05 and the VVT heads as they have different stud positions on the intake.

[hks_kansei]

Since you already have a BP block from an NA8 i'd just rebuild that, it's a good opportunity to add some nice things too, like higher compression pistons (thus removing the benefit an NB engine has in stock form)

if it were me, and the budget allowed, i'd rebuild the engine and have the machinist enlarge the bores (you'd need to talk to them about the maximum, but from memory it's about 85mm or so, which gives about a 1929cc engine)

I'd then have the head refreshed and put in a set of largish cams (tell the cam grinder what you want and they should be able to grind cams to suit the power/rpm)

[Magpie]

85.5mm is the largest however need to do sonic testing of the block. If going 85mm you should still do sonic testing.

on Madjack's, gslender's and hks_kansei's posts.

[DanZig]

Great advice. For the price, I'm pretty happy to throw the 1.8 Astina in and get to work on my spare motor. Perhaps jazzy up my head a bit as you suggest madjack then eventually ITB’s purely for sound as Grant’s sounds great!

I’ll reiterate that this build is by no means to win certain classes, but to make something that’s great fun to drive on and off track. I know in saying that people will mention that just the stock motor will be fun, I agree and enjoyed mine as a stock NA6 for quite a while but who can resist?

Any initial suggestions on head work or links? I’ve got some pretty good books in the shed I’ll have to dust off re 4 cylinder performance.

I have been looking at previous threads on cams and some say mild while others say wild. It’s not going to be a race car but I do want something I can enjoy and will live in the upper RPM in which case I’m happy to lose some low down, drivability and would probably be happy if it idled angrily. At which point will HLA’s become an issue or should I just go straight for solids and buy cams accordingly?

I’ve seen many different claims as to safe head shaving points from 30 thou to 60. If anyone can give me a nudge in the right direction it’d be great. Same deal with porting, I’m used to adding boost. Haha.

I'll have to chase up with the SA club who's the go to for BP knowledge in SA if there is someone.

[bruce]

An SE motor into an NA has always seemed attractive to me as a street car. But it won't suit your plans.

[Magpie]

Look at Madjack's build thread and go for a Honda intake manifold etc, easier than an ITB build.

With a Naturally Aspirated build the cams should be matched to the flow of the head and for where you want to make the power. I have very good mid range in mine.

[gslender]

With a Naturally Aspirated build the cams should be matched to the flow of the head and for where you want to make the power. I have very good mid range in mine.

Yeah, and whilst that is true, I just randomly picked a set of used wild cams in mine and it is still very drivable with standard ITBs trumpets. I don't doubt that Magpie has got a well engineered and great performing engine, but bang for $$$ it didn't cost me 1/10th to get something that is still very drivable and fast. I have no real midrange issues, but yeah, it doesn't pull as strong at 3K as it does from 5K, but I'm not finding that a problem when it has a redline of 8K.

My point is this - don't be put off by doing something and having a go yourself. No doubt the "pros" will tell you how it won't work or you'll be leaving HP on the table, but you'll still have something that is better than what you started with - and if you did it all yourself, you'll be 10x more proud of the outcome every time it roars down that track!

[speed]

I'm not a pro but am familiar with doing things on a budget.

Decide now if going itb's, so you can then work out what you are doing with head and cams.

If the Na8 works with the skunk manifold, then I'd be doing that.

Itb's do sound amazing. Guess it just depends on your budget and desires

[DanZig]

My point is this - don't be put off by doing something and having a go yourself. No doubt the "pros" will tell you how it won't work or you'll be leaving HP on the table, but you'll still have something that is better than what you started with - and if you did it all yourself, you'll be 10x more proud of the outcome every time it roars down that track!

Grant, exactly what I was saying above and goes a long way towards my "fun". I have no doubt pros will do it better than I will. If I wanted the best or needed the performance advantage I would be coughing up the dough. I don't need that however and at least in this car it hold more value in doing something myself if it's not that great. I do however want to learn as much as I can from the people who know these cars so I don't make a complete POS haha!

Speed, sound does play a big part in it for me. Maybe it is worth getting on the ITB wagon before I make the other choices. I imagine my biggest hurdle will be getting them running right should I go down that road.

I'll be certainly looking over madjak's thread tonight.

[Magpie]

Yes Grant mine was not a cheap build and I'm not trying to convince the OP to duplicate my build. Rather I'm trying to impart some of the knowledge I learnt along the way, as you are.

Doing a Natural Aspirated build needs more planning as you just can't wind the boost up to make up for a questionable decision. Madjack has done a great build with very little 'pro' input whereas G has as well. I'm in the group that has paid for all the engine work as I neither have the time or the knowledge. Just because I have 'paid' for mine does not imply that I did not learn anything. I could elaborate on the trumpet lengths and its effect on where the power is made but again if you are not trying to squeeze out the little bit of power it is irrelevant, but it should be part of you thinking. Even if you go the Honda manifold the runner length still impacts the build.

Just remember that Madjack is not building a road registered car and whilst most of what he is doing translates into a road car some of it does not. For Danny to come on and post that he is impressed with Madjack's work is testament to his skill. Madjack and I have talked more than once on the phone about our respective builds.

I'm not a 'pro' engine builder so take what I post with a grain of salt.

If you like the sound of ITB's enjoy the video below, it was taken at Noosa Hill Climb and sent to me. It is one of the few videos I have from the outside.