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I am looking to buy the donor car and currently have two solid options.

1. 91 Rx7 reasonably rust free about 180,000 miles Manual Transmission $1650

2. 9x Miata 120,000 Automatic transmission $800

I want to be able to have 13x10 wheels on the car for the track is that possible with the Rx7 spindles, the Rx7 was a "higher performance" car it its day but are there any significant advantage over the Miata? I think I will use a different engine in either case and would get a Manual transmission for the Miata if I were to use that engine. Both engines are very tired and I would like a stock 200HP+ motor.

My starting budget it $2500 and would like to keep the whole car in the $5k-$7k range. I have most automotive tools but will need a welder + welding gear and a grinder.

Which car do you think would make the better donor?

Miata

I love RX-7's , but the rotary can fail without warning. My 79 with the 12A siezed on the drive home from work one night. Plenty of oil, no overheat, no smoke, no nothing. All it did was screech for a split second and lock up SOLID. I couldn't even beat the housing apart when I tried to tear it down.

Horsepower is a function of cubic dollars.

I built my Locost with a '91 RX-7 N/A donor. That being said, I think I'd recommend the Miata.

I chose the RX to learn about the engines as I have read about the Wankel since the mid 60's and never experienced one. It was sort of a bucket list item for me. I think generally, most will say that a 180,000 mile rotary is about shot unless it has already been refreshed. The housings will most likely be flaking badly and the seals definitely need replacing by then. Figure ~ $700-$1000 for a refresh of gaskets and all the seals not including any hard parts if required. Seals are cheap enough but refreshing a rotary with housing damage can be an expensive undertaking unless you use used housings. Finding good used housings are getting tougher and tougher. If you plan on rebuilding anyway, there are a lot of $500-$750 RX-7's out there. $1650 for a worn out RX seems high.

The Miata will make a respectable donor at a reasonable cost. at 120000 miles, it still has a lot of life left in it. But with that car, you'll have to source a manual trans setup and most likely (I don't know) the ECU, in addition to the donor.

I don't think you'd get 13" wheels on a '91 RX7 upright. The brake/calipers will not fit inside the wheel. If you are planning on getting a 200HP stock engine, and the wheels won't fit. The same will most likely apply toward the Miata as a donor. So if you are looking at a new engine and 13" wheels, why are you even considering a donor car? It would seem that your wants are inconsistant with the donor car path. Perhaps getting the parts individually would serve you better. $5-7K is can be accomplished if you do 95% of the work and keep watch of your pennies along the way.

What year of Miata? The earlier 1.6 liter NA engines have lower compression pistons and less duration camshafts, netting about 16 hp less. The later 1.8 liter engines are mechanically the same for manual and automatic transmissions. The ECU's and wiring harnesses are different.

Stock, the automatic 1.6 was rated 100 bhp. 116 for the manual 1.6. 128 bhp for the 1.8. That's a long ways away from 200 bhp. Getting there with a normally aspirated 1.8 will probably consume most of your $5-7k budget.

I bought a 86 RX7 for $100 for the same reasons as Rx7locost though I was not as sure I would use the rotary as the engine was toast.
As a doner the miata is slightly better then the Rx7 IMHO.

200 hp is not really in a $5K-$7K budget. Maybe 10K. As for what donor to use, I think it's always best to use a donor you're familiar with.

I don't know if you have ever ridden in one of these cars or not but it may pay to go for a ride in one with the lesser amount of horsepower to see what they go like. These are light weight and it doesn't take much HP to push them along. Mine is about 160 hp and weighs 1320 lbs which equals about 8.5lbs per hp. It will do 0-60 in 4.7 secs. It is not the fastest thing around but for 1600cc it goes pretty well. Don't think a stock Miata motor won't impress you, it will. Build one with a Miata donor car with a manual trans and then once you get it going and get used to it, then turbo or do what you want. Just my 2 cents worth.

Thanks for all the comments guys, My Rx8 is a well set up Autocross car with coilovers, sway bars engine tune, race tires etc and I have won lots of trophies this year but it is also my daily driver so I want a dedicated track car for obvious reasons.......I have driven the Brunton Super stalker which is about 260WHP and very fast indeed, I was instructing the driver and loved the car but my times were quicker in my Rx8 as the stalker did not transition well in the slaloms and felt like it was too stiff, I want to build a car that is much faster than my Rx8.

I want to keep the weight down as it will primarily be built for Autocross. Initially I will use which ever wheels I get with the donor car but ultimately want a set of 13x10 wheel with race rubber. I have a spare set of race tires+wheels for my Rx8 I can use but they are 17x8 and a bit big for a locost.

A 100 HP Miata engine is going to give me similar HP performance to my Rx8 which is a well set up car. (I realize the locost should handle better as its lighter but I want more power too) The reason I want to build a car is to take it to the next level. I am thinking about getting the Miata to start the build and keep a look out for a bigger engine maybe 3-6 months down the line, like the S2000 engine or similar.

Also I think the donor car is definitely the way to go because I can recuperate a lot of the cost of the car by selling the parts I do not need for the build. Hood, Trunk, lights, roof, doors etc all add up.

The 1.6L ones were 115 hp from the factory and the 1.8L ones were a little over 130 hp. It sounds like getting a few more horsepower from them isn't that difficult with a little tuning and cams.

If you are autocrossing, I would decide on DM or EM early and go from there. The DM cars have a lower minimum weight but are also limited to 2.0L or smaller engines.

There is nothing wrong with having a power goal but making it the #1 item on the build list is stacking the cards against you completing the build. I don't know what building experience you have but keep the goals reasonable and within your capabilities. There is nothing wrong with making power upgrades a project for a future winter.

Having considerable experience in both IRS (Miata) and live axle (Corolla) Locosts, I'd give the Miata the nod over the Rx7 for rear suspension alone. The unsprung/sprung weight ratio goes up as you lighten the car, and on anything but the smoothest tracks, you're going to feel that rear axle.

Thanks again it seems pretty unanimous that the Miata is the way to go, I will make some calls.

you can turbo a miata, cheap. the fuel savings over a rotary would pay for it in a tank full.

Hi Guys,
Over the pond in the UK several people have used the Mazda V6 2.5 with Miata gearbox, Triumph motorcycle throttle bodies and meggasquirt ecu to achieve the 200 hp mark.

The best example on LCB is Clairetoo who has a Fisher Fury V6 and now building an MX5 ( Miata ) with all new front suspension to her own design and a V6.

I believe she is going to supply a complete kit to fit the V6 to the MX5 gearbox and the manifolds for the Tb's.


Happy Christmas to all.

Mike.

I would prefer to stick with NA because I want the car for autocross and the turbo makes it much more difficult to get the power down than NA. I would really like the S2000 engine or similar, If I can get a 4 cylinder engine that makes more than 180HP I will be happy.

Excellent thinking. It's popular to think of turbos as cheap power, but for a lot of aftermarket turbo owners, and I know a bunch of them, boost creates a whole new set of problems and expenses.