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Discussion Starter · #1 ·
Hi all, long time lurker, infrequent poster looking for some advice from those in the know.

I've had my V-spec 1994 for a long time now and have spent the last 2 years doing a nut and bolt restoration on the car to get the chassis, exterior, rust protection and interior all sorted. A few years ago my engine went kaput so I swapped in a 35k 2004 NBFL VVT motor to give myself a bit more power. After that I went about buying some of the bits for a M42 supercharger setup, TR lane mounting hardware, teflon supercharger etc in order to fit that.

Since then I've sort of mellowed on the idea of a supercharger and never fitted the kit I bought or got the last few components to complete it, plus life got in the way. Now that I'm back on the horse however, I've started to revisit the idea of getting a bit more oomph out of the engine without going FI.
After the rebuild a couple of months ago I got the car mapped at DanST engineering near me and the car made 153bhp at the crank with only minor mods -silenced decat, 4-2-1 manifold, cobalt exhaust, pipercross panel filter and a tune on the ME221 gen2 I'm running.

Now this car seems to have a pretty healthy engine that's making good power, and my car is strictly for the road and won't be tracked, so I'm not looking for insane power, but I want purity and driveability. With that in mind I'm considering scrapping the FI route and going for NA power instead. I know these engines are crap for high bhp figures NA so I'm being realistic and looking to get about 170-180 at the crank, whilst keeping the car relatively civilised for weekend drives, occasional commuting in summer and the odd road trip here and there.

Dan makes throttle body kits which I'm sure you're all aware of, so I was considering fitting one of those and then purchasing a second head with the intention of porting and polishing, a slight head skim, fitting a set of cat cams 278/278, vernier pulley on the exhaust side, supertech +1mm valves and springs and a harmonic balancer. I want to keep all the work in the head rather than in the block so future engine repairs will (hopefully) be kept fairly straightforward and simple. I've never built an engine before so this would be done over a period of a few years in all likelihood, with the ITB's being fitted to the engine as-is and then a head being built, learning as I go after doing lots and lots of learning and reading!

My question is, before I go spending any money and wasting lots of time is would it be worth the hassle? Bearing in mind the fact I'm not chasing numbers but am aiming for around 180bhp on the upper end, and that I'm going for the driving experience on country roads where I can wring it out to 7500rpm without losing my license or my life, would I be stupid to attempt all this whilst keeping the car relatively streetable?

My alternative option is to save circa 5k for a rotrex and be done with it, making 200-220 without any issues, but I'm a sucker for a screamer of an engine and the instant throttle response and crispness of an NA engine. The Mx5 is such a well balanced car that I would only want to enhance that sense of balance, rather than go chasing big power figures.
Given what I've said, am I being as realistic as I think I am, or am I just going to end up down a rabbit hole of costs and dissapointment?!

Pictures of my build so far attached for your perusal, and all your advice is much appreciated!
Cheers,
Alex
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NA builds are cool but they're so expensive for the least amount of gain, they must be a hoot to drive though! With a supercharger the torque curve will remain the same so its essentially intense NA with more torque. Only fitting a turbo would make the car 'feel' different as the torque curve changes. Totally up to you though! Not many people regret doing NA builds

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Look up the garage quinn itb car. Ignore the ITB side and just pay attention to what he did to the motor.

Combination of block and head gets you the power you want, no special aftermarket bits really needed.
 

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Discussion Starter · #6 ·
NA builds are cool but they're so expensive for the least amount of gain, they must be a hoot to drive though! With a supercharger the torque curve will remain the same so its essentially intense NA with more torque. Only fitting a turbo would make the car 'feel' different as the torque curve changes. Totally up to you though! Not many people regret doing NA builds

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Yeah, the idea of a supercharger still excites me, but if I were to do one it would be a rotrex to avoid the supercharger whine. It's not that I dislike whine per-se, but I prefer the sound of a high strung NA and I feel I'd get more enjoyment out of being at WOT more of the time, which I wouldn't be able to do as much with a charger due to grip restrictions. The Rotrex is by far the most appealing option for a supercharger for me personally, but it's around £5k so I'm just weighing up whether £5k in NA expenditure would be as good an investment from a smiles-per mile perspective, even though it's obviously going to be worse bhp-wise.
 

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Yeah, the idea of a supercharger still excites me, but if I were to do one it would be a rotrex to avoid the supercharger whine. It's not that I dislike whine per-se, but I prefer the sound of a high strung NA and I feel I'd get more enjoyment out of being at WOT more of the time, which I wouldn't be able to do as much with a charger due to grip restrictions. The Rotrex is by far the most appealing option for a supercharger for me personally, but it's around £5k so I'm just weighing up whether £5k in NA expenditure would be as good an investment from a smiles-per mile perspective, even though it's obviously going to be worse bhp-wise.
Just be aware that rotrex chargers are centrifugal which means that they 'kick in' at high rpm which essentially makes them a belt driven turbo; this changes the torque curve. I'd stick with positive displacement superchargers if you want the NA feel (m45 is common but m62 gives more power)

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Discussion Starter · #8 ·
Look up the garage quinn itb car. Ignore the ITB side and just pay attention to what he did to the motor.

Combination of block and head gets you the power you want, no special aftermarket bits really needed.
The garage Quinn car actually was what made me change my mind! He's done things a similar way to how I'm envisioning it, all the work he's done is in the head and he's left the block untouched. His skim job was ridiculous (0.100") and I'd probably only go half or less than that, and his cams are much more aggressive, but other than that I'm thinking his philosophy is a good one. Keep the work in the head and then if ever you need to rebuild it's much more straightforward. I also would put the rev limit a bit lower. Think he's at 7800 on stock rods and I'd max at 7500 probably.

There's a good build list of his car here actually: Quinn's Screaming All-Motor ITB Miata – RUBADUB MEDIA
 

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Discussion Starter · #9 ·
Just be aware that rotrex chargers are centrifugal which means that they 'kick in' at high rpm which essentially makes them a belt driven turbo; this changes the torque curve. I'd stick with positive displacement superchargers if you want the NA feel (m45 is common but m62 gives more power)

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Is that the case? I knew they are centrifugal, but I thought the torque curve was still linear as the turbine speed was proportional to belt speed, so rather than wait for exhaust pressure to build spool, it would just spool as revs increase? I'm perfectly happy to stand corrected on this as I've never driven one, and if that is true, it means going the ITB NA route is even more appealing! 😄
 

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I think what to need to decide is if you will be happy with 170hp?
If there is any doubt you need to go the FI route.

I'm building an NA screamer at the moment with a focus on trying to increase the rev limit (safely!)
I'm in too deep to change my mind now, but had I counted up all the things I'm now buying "while I'm in there" I should have probably just gone FI 🤣
(Similar to Emilios Garage Quinn build)

So far just an expensive pile of parts 🤣🤣
 

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Is that the case? I knew they are centrifugal, but I thought the torque curve was still linear as the turbine speed was proportional to belt speed, so rather than wait for exhaust pressure to build spool, it would just spool as revs increase? I'm perfectly happy to stand corrected on this as I've never driven one, and if that is true, it means going the ITB NA route is even more appealing! 😄
I used to think the same haha! Yeah for what you're after centrifugal supercharger should be bottom of the list (along with turbo) I'd definitely go NA build or positive displacement
 

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Discussion Starter · #12 ·
I think what to need to decide is if you will be happy with 170hp?
If there is any doubt you need to go the FI route.

I'm building an NA screamer at the moment with a focus on trying to increase the rev limit (safely!)
I'm in too deep to change my mind now, but had I counted up all the things I'm now buying "while I'm in there" I should have probably just gone FI 🤣
(Similar to Emilios Garage Quinn build)

So far just an expensive pile of parts 🤣🤣
This is sort of the problem... I have no idea! 😂
I'm lucky enough to live on the edge of the Yorkshire Dales, so 99% of my fun driving is on country lanes. In my mind, I think anything more than 220bhp on those sorts of roads would end up with me launching myself into a hedge/cyclist/rambler (delete as appropriate) so would 170bhp be enough? I don't know, but what I do know is I enjoy the sensation of feeling fast over actually being fast. The noise of the engine bouncing off walls and hedges is as much fun as the actual driving, so there's that side that really appeals.

Without a point of reference, my 153bhp feels adequate, and not at all slow on those sorts of roads, but I'd enjoy a bit more pep without going silly. Would a 20-30bhp bump be enough on the butt-dyno to do it? I don't know, which is why I'm here I suppose! My mate has an MK Indy with a 2.5l duratec and makes about 230bhp in a 650kg car, so I've been looking at him and chasing his sorts of numbers; but then he tracks it and I have no intention of doing that without a roll-bar and I don't much fancy fitting one either as it'd ruin the looks of the car, so I suppose I'm having to recalibrate my expectations.

Do you have any experience with driving a high horsepower NA? What made you decide to go the route you did? 😎

I used to think the same haha! Yeah for what you're after centrifugal supercharger should be bottom of the list (along with turbo) I'd definitely go NA build or positive displacement
Well that's a bummer! Still, better to know that now than after having dropped £5k in parts on it! The other option would have been a FFS coldside M62, so I suppose I could look at that as well. If I'm going to do it, I want to do it properly so a good + displacement charger would be a must. Cheers for the info! 😄
 

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153bp is about 120whp on an mx5 and seems very low to be getting from an M45. An M45 set up should see you reach 180whp. I'd just keep the M45 with a suitable reduction pulley if I were you. Going NA is really hard, you generally sacrifice low end and mid range for peak power and really need to know what you are doing with matching parts together or you'll likely see very minimal gains. There's a build on here where being 2mm off on the manifold primary diameters cost 20hp. You'll also need a lot of dyno time and a tuner who is very knowledgeable. To put it into perspective the aforementioned build made over 200hp NA but spent over £3000 on dyno tuning alone IIRC.
The M45 will add low down torque and be much more fun and usable on the road rather than a 5k rpm only screamer. It'll also idle much better and be easier to live with. I say just turn the M45 up. M62s are very similar, they make a touch more power but do so more efficiently which is the real benefit. Same for the rotrex, they are a lot of money and are more for those who want turbo levels of power but for what ever reason dont want a turbo. As you already have an M45 there's little reason to upgrade to either IMO, no when you've not got all you can out of it yet.

BTW, the coldside M62 kits use an MP62 which has an internal bypass, are a lot of money, and very hard to find in the UK. Before you consider one you're best off looking at the fitment and intake routing issues as they are very tight and not something I'd be happy to live with.
 
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Discussion Starter · #15 ·
I think you misunderstood me there Shadowraven; I have an M45, but haven't fitted it yet, so I'm making 153 naturally aspirated at the moment.👍😁

The M45 was a project but I'm considering sacking that off and going for 170-180bhp with headwork, cams and ITBs. I know it's hard to do high NA power but I feel that number isn't exactly pushing the limits of possibility like going for a 200+ bhp build would be. I'm not aiming for a balls to the wall fully forged and high stressed engine but I would like to take the engine up to say 7/10ths of its potential as I think it would be as much fun as a charger, but as I mentioned, I have no experience of driving either so have no point of reference!

If I could do this work for circa £4-5k it would be comparable to a high end charger install, so I'm interested to know if that's feasible or pie in the sky. Hopefully I won't need 3k in Dyno tuning, in which case I'm buggered from the get go! 😅
 

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Ah sorry.

180whp is hard to get to NA (I know you said bhp), but it's easily enough with a charger, and 180bhp won't be much higher than you are now. As I said, you really need to know what you are doing, cam pulses need to be matched with exhaust sizing and lengths, everything needs to aim for about the same power band, there's too much in it for me to bother, and even if you do you are still giving up the low end and drivability you'll use most on the street. To get an idea what is really involved have a look at some of the NA threads on here. Just throwing parts on without understanding how they need to match together will likely net little but a lighter wallet.

Cost wise an M62 can be done with some bells and whistles for under £2000, mine sits at £1500 but I also have things like a new exhaust, racingbeat style intake, engine mounts etc that you like don't need. You are better off going M45, a fresh build can be done for around £1000 as you can keep the stock clutch in most cases, and as you have most of the main stuff already by the sound of it it'd be silly not to. I'd not recommend anyone pursue a supercharger or NA build if they had a budget of £5000, supporting mods like suspension and brakes aside that budget would net you a very very fast forged turbo build. The gt2860rs build I'm starting shortly is set to cost £3000 excluding the mapping, and that's with a new mishimoto oil cooler, 600cc injectors, new garrett turbo, new mid pipe, kraken manifold and 3" DP, you could certainly cut some of the cost out of it if you wanted.
 

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Discussion Starter · #18 ·
Ah sorry.

180whp is hard to get to NA (I know you said bhp), but it's easily enough with a charger, and 180bhp won't be much higher than you are now. As I said, you really need to know what you are doing, cam pulses need to be matched with exhaust sizing and lengths, everything needs to aim for about the same power band, there's too much in it for me to bother, and even if you do you are still giving up the low end and drivability you'll use most on the street. To get an idea what is really involved have a look at some of the NA threads on here. Just throwing parts on without understanding how they need to match together will likely net little but a lighter wallet.

Cost wise an M62 can be done with some bells and whistles for under £2000, mine sits at £1500 but I also have things like a new exhaust, racingbeat style intake, engine mounts etc that you like don't need. You are better off going M45, a fresh build can be done for around £1000 as you can keep the stock clutch in most cases, and as you have most of the main stuff already by the sound of it it'd be silly not to. I'd not recommend anyone pursue a supercharger or NA build if they had a budget of £5000, supporting mods like suspension and brakes aside that budget would net you a very very fast forged turbo build. The gt2860rs build I'm starting shortly is set to cost £3000 excluding the mapping, and that's with a new mishimoto oil cooler, 600cc injectors, new garrett turbo, new mid pipe, kraken manifold and 3" DP, you could certainly cut some of the cost out of it if you wanted.
No worries, and thanks for the advice!

As you say, NA is hard, which is why everyone goes FI.... call me a masochist but there's an appeal in that I suppose! How could an M62 build be done sub £2000? I'm not a fabricator so I'd have to buy mounting hardware, then I'm presuming I'd need an intercooler and hosing, the charger itself, a bypass valve, injectors, pulley, belts, probably a new radiator and maybe a new clutch depending on if my exedy gives up at the 200-220bhp mark.

I've already got Meister's and brembo disks with Roddisons pads, plus a skidnation butterfly brace on the cards so I think I can stop and steer ok.

M62 appeals more than M45 just because I can run it at lower rpm to keep heat and noise down while making the same power as the '45 screaming it's tits off. Still very much open to ideas, provided I can keep the twin cam, lotus-esque feel of the original car! (y):)
 

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If you want to go down the m62 route find Edd Kellett in the facebook groups (mx5 supercharged is the name of the one im in) and drop him a message. He does a full mounting kit and tensioner for about half a grand. Shadowraven has already done the m62 route so he knows more than me since Ive barely even begun the parts accrual lmao
 

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As above, Ed Kellett/Garage42 kit DIY welded can be had for £150. M62s can be had or £150, DIY EFI ECU is £270, intercooler and piping is about £200, rx8 injectors £60-80, bypass £40, wideband £170, reduced pulley is expensive at £200, and a clutch is £300 but that's things mostly there at £1560 in parts. There's about £500 left for boost gauge (£30), IAT (£20), filter (£40), second TB (£15), idler pulleys (£20 tops), BOV (£90) if you want one, clamps and other little things you might need. It's perfectly do able. With an M45 the reduction pulleys are under half the price and you shouldnt need a clutch so you can save £400 ish there.
 
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