Upgrades to prep for NC 2.5 swap

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Upgrades to prep for NC 2.5 swap

Postby ToddW187 » Sun Mar 06, 2022 1:41 pm

Hey everyone,

I'm hoping to 2.5 swap my 2006 NC sometime, so I'm looking to do some upgrades this summer to make that better when it happens. I'm envisioning a mild-cam naturally aspirated 2.5L at this point. This is my summer daily driver, so I'd like to keep it nicely streetable while still making it as capable at autocross as I can. So far, I've swapped in a set of Bilstein B6 shocks and 58 White/58 Red GWR bumpstops, a Hotchkis front sway bar, and a Roastersport Street Single muffler. I'd like to work with those existing mods and add the following mods to prep for the 2.5L:

  1. Improve total exhaust system flow
  2. LSD
  3. Wider Tires

Improving the exhaust flow should be easy. I'm thinking the Roadstersport MAX Power 1.8 header and Roadstersport Helmholtz Midpipe will nicely improve the exhaust system's flow, knock down the drone from my Street Single, and maintain a catalytic converter (since I'm just not crazy about totally removing them). Does that seem reasonable?

An LSD should also be relatively straight-forward. I'm leaning towards one of the Tomei options at this point since they are cheaper, and I don't plan on going bonkers with the 2.5L power. Again, does that seem reasonable? Would you recommend the 2-way or 1.5-way?

Tires get a bit more complicated. As I understand it, to fit the widest tires I can without flares (245), I need to lower the car some to improve camber to ~2 degrees. I'd like to stick with the B6 shocks if that's reasonable as they only have about 5k miles on them. To that end, I'm looking at lowering springs. The options I can find available today (unfortunately not all on GWR) are:

Progress Technology - 126 lb/in front 116 lbs/in rear - expect 1/2" drop
Racing Beat - 145 lbs/in front 112 lbs/in rear - expect 1/2" drop
Flyin' Miata - 300 in/lbs front 196 lbs/in rear - expect ~1" drop
H&R - 235 lbs/in front 155 lbs/in rear - expect ~1 1/3" drop
Eibach - 140 lbs/in front 70-120 lbs/in rear - expect ~1 1/2" drop

This leads me to several questions.

  1. Would you expect 1/2" drop on an 06 to be enough to get the requisite camber to fit a 245 tire?
  2. The rates on the FM springs seem very high. Would you expect these to work with B6s? I'm guessing they would be stiffer than I want for daily use...
  3. Would 1 1/2" of drop be too much on stock length B6 shocks?
  4. Which of these options would you recommend for my application (as much grip as possible for autocross while still being a great daily driver)?

Thanks so much for your help and advice!
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Joined: Sun Mar 17, 2019 9:27 pm

Re: Upgrades to prep for NC 2.5 swap

Postby ToddW187 » Mon Mar 14, 2022 3:39 pm

Maybe I'm asking too much at once here. I've spent some more time digging through forums, and I'm just not quite piecing together the info I'm trying to find.

I'd like to fit a 245 width tire for autocross if possible. How much negative camber is typically needed to fit a 245 width tire on a 17x9 45et wheel?

What ride height is typically needed to get that amount of camber? I have found a few forum posts that help with this, but I'm not sure if they are the final word for this question: https://forum.miata.net/vb/showthread.php?t=649828

Would running the H&R or Flyin' Miata Springs on B6 shocks be a terrible decision (if I need that much drop)?

If I can't make a 245 on 17x9 et45 work with shock/springs and alignment, I'll stick to a 225 on 17x8 et45, but that still prompts the same questions:

How much negative camber is typically needed to fit a 225 width tire on a 17x8 45et wheel?
What ride height is typically needed to get that amount of camber?

Thanks to anyone who can lend some advice.
Posts: 11
Joined: Sun Mar 17, 2019 9:27 pm

Re: Upgrades to prep for NC 2.5 swap

Postby Brian » Wed Mar 16, 2022 7:03 am

Coilovers is the answer for the plans you have, do this right the first time. Crazy amount of energy already invested here in trying to make a nonadjustable shock work with your plans and that's just the wrong direction and not going to work to get you a fast and happy result.

The 'Bilstein B6 shocks and 58 White/58 Red GWR bumpstops, a Hotchkis front sway bar, ' is a mess for autocross with 17x9, not within miles of enough roll resistance, no control of damping means no control of balance, etc. If you want 17x9, plan coilovers to get the spring rates and damping control you need with that amount of grip. Otherwise the tire grabs just long enough to hurl you onto the bumpstops at which point the car will suffer massive and terminal understeer from having just a front bar. I do not recommend trying to solve the issue with just a set of springs, because you lack control over the results you really need to get it all fitting and working right. Coilovers is the right answer, do it right or be doomed to be unhappy with the results of 17x9. On the low end of costs my suggestion is MeisterR ClubRace, that's an amazingly fast setup for the money and I have done Top Time of Day at events with over 100 competitors with that setup. You will never be within miles of the top time of day in that level of competition with the FM or H&R springs. What the Ohlins adds is far better control over rough pavement (on race days and in the daily driving when you hit a pot hole, etc). So, first thing to understand is COILOVERS is the answer for 17x9, anything else is a massive waste of your time and money down the rabbit hole and the result will never really be right. If your goal is frustration and SLOW car, stick some springs on those Bilsteins. Heights we usually start coilover customers with are 13.25 front and 13.5 rear (measured center of wheel up to lip).

Roadstersport MAX Power 1.8 header and Roadstersport Helmholtz Midpipe and Street Single is far too loud for most folks. You may not be 'most folks' but just be aware that the header change adds massive volume and most folks decide they need the SuperQ at that point, which is why the website says SUPERQ over and over and over and over for anyone going catless header.

The Tomei is a fine choice, 'good enough' but if serious about autocross days the OSGIKEN is better. Nonetheless, I have won with the Tomei back there, it works for normally aspirated 2.5 just fine. Note they both start really tight and need some break in time. With both we suggest the standard 2 way setting, which is how they ship because that is the best choice in my experience. People not willing to put in the time to get the break in done with either choice often jump to 1.5 way and that is the wrong choice in my experience, once broken in fully the 2 way is the fast way for both choices.
Brian Goodwin
Good-Win Racing
Site Admin
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Re: Upgrades to prep for NC 2.5 swap

Postby ToddW187 » Fri Mar 18, 2022 8:45 pm

Thanks so much for your thorough reply, Brian. When I purchased my Miata, I didn't have intentions for it to become an autocross machine, so when I needed to replace a blown shock on a car with 115k miles on it, the small upgrade to the Bilsteins seemed like the right choice. I've been thinking I'd like to keep them on the car since it seems a shame to take them off after only a few thousand miles, and I'm trying not to break the bank with mods, but I definitely understand what your saying about doing things the right way (which certainly isn't always the cheap way).

I really appreciate your explanation for why coilovers are the right path forward. I'm clearly no suspension guru, so I had no idea that damping the Bilsteins provide would be so insufficient as to lead to catastrophic understeer, but your description of the problem certainly makes a lot of sense. I already suffer from pretty bad understeer on 205 RE71Rs (the more I read, I'm thinking this is due to insufficient negative camber up front, insufficient damping up front, and the rear stock sway bar being out of 'balance' with the stiffer front sway), so the goal of handling mods would be to fix this understeer, not make it worse. I clearly have some thinking to do about what my goals are for my Miata; I either need to keep things inexpensive by sticking to stock class, moderate my asperations for tire width and stick to a 225 or somthing like that, or I need to alter my plans to be able to fit in a set of coilovers.

I hear you loud and clear on the exhaust volume. I have access to a RSII that I can try should the RS Street Single be too loud, and I understand that that may still be too loud. I've been really enjoying my current exhaust note, but would like to knock out the bit of highway drone with the HH midpipe, but I definitely understand that the volume increase of the catless header will more than likely outdo any help the HH offers.

Your explanation of the LSD makes. I'd been able to find what the physical differences between a 1.5 way and a 2 way are, but hadn't found anything about the practical implications of those differences. Faster certainly = better :D

Thanks so much for taking the time to reply to my long post and offering your expertise. I very much appreciate your help.
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Joined: Sun Mar 17, 2019 9:27 pm

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