turbo on the 1.6, 1.8 conversion, or 94+? - Miata Turbo Forum -Boost cars, acquire cats.

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Old 02-18-2008, 12:08 AM   #1
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Default turbo on the 1.6, 1.8 conversion, or 94+?

Been thinking about this for a bit, and trying to decide the best way to go, looking for some feedback and opinions. Spent too many hours reading the forums, faq's, etc., maybe there's something I've missed in the search, but haven't seen a good thread yet.

Recently acquired a '91 BRG, mostly stock, with a matching hardtop. No power adders, KYB GR2 shocks, few things here and there. Unfortunately, it is a bit "tired", both suspension and engine, and I want to pep it up a bit. Obviously, boost would be an effective method.

My goals are 200-250whp, but that may change in the future (increased). Want to run it on the track, but probably not in a competitive class. (Again, this may change, but not expecting high HP to work well for types of racing suited to a Miata.)

My focus is on the BEGI's current S kit, probably with FMU for CARB approval (sigh). But, before I start spending money on a prefabbed kit for a 1.6, is the 1.6 really the right platform? Should I consider a 1.8 conversion first? More displacement will always benefit the turbo later...

And if I'm looking at a 1.8 conversion, does it make more sense to just go with a 94+ car (probably NA), when the current engine seems to have a reasonable amount of life left in it? (100k on the engine, some concern about the trans, but engine "sounds" good.) The 94+ also offers the advantages of rear diff, brakes, etc. Any compelling reasons to stick with the '91, aside from personal preferences and "good deals"?

Posting this in the general area, mods, feel free to move if it's better suited to the prefabbed or DIY area. Starting here, rather than m.net, since a turbo is "when", not "if".

For reference, I also have an '04 MSM; I love the handling, but not satisfied with the power output. For now, leaving the "new" one factory stock, and the '91 is the "toy"... unless I sell/trade it to go 94+.



EDIT: To clarify, this will be a daily driver, that I can afford to have "down time" on. Crank up the boost for the track on the weekend, around town or into the twisties during the week. I.e. not a dedicated track car.

Last edited by macker; 02-18-2008 at 12:11 AM. Reason: clarification as to purpose/goal
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Old 02-18-2008, 01:51 AM   #2
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boost the 1.6
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Old 02-18-2008, 01:59 AM   #3
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Originally Posted by disturbedfan121 View Post
boost the 1.6

Then when it gives up the ghost, throw in a 1.8, and turbo that too!
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Old 02-18-2008, 03:27 AM   #4
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(The sky must have fallen. A new member with rational goals and proper grammar who has searched the forums and done some homework? Blasphemous.)

OP, you bring up an excellent point in that the '94 car is simply a better platform. Better brakes, better diff, and more displacement is hard to beat. You can install those things in the '91 if you want to, but if you aren't attached/invested in the current car, I would have to say that swapping to a '94 car now is a really good idea.

As far as your power goals, I would try your hardest to get a ride in a 200whp car; it may end up being perfect for your tastes, or it may end up being an absolute handful. That much power really affects the way the car feels and the way the car drives; it still maintains that lightweight feel, but you can't be as belligerent with the throttle as you can with a stock car. The stock car is very much a momentum car; add 100whp to it, and it's still a momentum car, but at the same time you have to be focused not only on corner exit speeds, but getting the wheel straight and getting the power hooked up. It can be a struggle on street tires, even in third gear exiting a corner. The power is 100% a personal preference thing; I've had at least one person tell me they wouldn't want 220whp every day; at the same time, though, after driving and living with this much power for 7 or 8 months now, I want more. I've gotten accustomed to being polite with the throttle in 1st and 2nd gear. As a comparison, my car at 220whp is comparable in straightline speed to an STI with a chip and an exhaust (about 15% more power than stock). I will pull E46 M3s really, really hard, and I will still creep on E39 M5s, and getting to the end of 4th gear is probably a little bit too easy.

As far as track use goes, you'll want to do a few things differently than some members here have done. You mentioned going with a BEGi-S kit and an FMU for smog reasons, but I cannot recommend a standalone strongly enough if you are building the car for track use. A big radiator is a requirement, as is (IMO) an oil cooler. A coolant reroute is also strongly suggested; a quick forum search on that topic (both here and at M.net) will turn up lots of reading material for you. You may also want to consider a vented hood. With a big radiator, a vented hood, a good-sized oil cooler, and a coolant reroute, you should never have any overheating issues.

Regarding more power than 250whp: It's going to get pretty involved and expensive if you expect to maintain reliability. I've been doing the research on that myself, and there are a few options for each problem.

-First, the motor itself. The rods and the rings both start to go weak around 250whp; the best solution IMO for the 300whp range is a set of OEM pistons with some sort of ceramic or heat treating, a set of CAT/Belfab rods, some quality rings, and ARP hardware. To push beyond 300whp, you'll want to use a forged piston. The issue with the forged pistons is motor longevity; the clearances have to be increased so much with a forged piston that the motor, instead of lasting 150-200k miles with a cast piston, is due for a rebuild in 40-50k. You also should let it sit and warm up for 10-15 minutes every morning before driving it.
-Second, the drivetrain itself. At 250whp (and even a bit lower, down to 220), the 5-speed transmission starts to show a little weakness; I'm on my second 5-speed (lost the first one to a hard 2-3 upshift). You can baby a 5-speed and make it last, or you can look into the Quaife gearset or a 6-speed. The Quaife gearset is prohibitively expensive ($3k plus synchros plus labor), and the 6-speed has extremely short gear ratios that necessitate a lower rear end, such as a 3.636:1 ring&pinion. Lots of reading on that subject as well; I'm personally going to go with a 6-speed, a 3.636 (or mabye even a 3.9) and an elevated redline.

Hope this helped a little bit, rather than just confused you more. You sound like your head is screwed on straight, and your goals are very, very obtainable. Feel free to shoot me a PM or hit me up on AIM with questions.
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Old 02-18-2008, 10:40 AM   #5
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there are many ways to look at this, here is my take...
if you will be happy and never want to exceed 250whp, get a 94+car and boost away...
if you want to start with lower power, sub 200whp, and then move up and increase gradually, id say stick with the 91 you have...reason being you will have to upgrade so many things even if you started with a 94+ platform to stay reliable...
go in this order
1 boost you car as it is
2 upgrade clutch-turn up boost
3 upgrade the rear end (shaft, axles, your choice of type of diff. and gear)-turn up boost
4 upgrade brakes and suspension to your liking...turn up boost
5 source 99-00 motor, build the **** out of it
6 source 6 speed tranny, mate to built motor and install both-turn up boost and change your panties
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Old 02-18-2008, 12:56 PM   #6
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Thank you! Really couldn't ask for a better answer than what you offered, for technical guidance and a checklist.


Sounds like a reasonable approach. My main thought here is, in avoiding pissing money down the drain, is whether I'm actually going to save much starting with the '91. Having a matching hardtop, and knowing the PO, is compelling. But upgrading it, then stripping it back to stock and selling it to go with a '94 later... how many parts will I re-use, how much time and blood will I have invested, and/or money on labor (e.g. machinework).

Opinions still welcome, my thoughts have been everywhere from "what are you thinking? just get a camaro, and get it out of your system" to "it's not _that_ hard to play musical chairs with street parking..."
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