RossD
RossD MegaDork
9/7/18 1:20 p.m.

I spotted an ad for a '99 miata that some guy bought as a roller and tried to install a 94-97 engine. The ad says: "Bought the car with no engine as a roller, installed a 94-97 Miata engine but the sensors on the head do not plug and play therefore the car does not run."

What would it take to get something switched over to make everything work?
 

codrus
codrus UltraDork
9/7/18 3:57 p.m.

94-97 are driven by a high-res cam angle sensor on the back of the exhaust cam, 99 has a crank angle sensor and a low-res cam sensor on the front of the intake cam.  (96-97 had a crank sensor too, but it's different and not compatible).  You can't easily put the low-res sensor on the 94-97 head (you could swap the cam pulley for one with the nubs, but there's no way to secure the sensor on the valve cover, and the 99 valve cover doesn't fit). 

 

So, you either change out the ECU for one that understands a 94-97 cam angle sensor (different pinout, so getting a stock one to work is hard.  Megasquirt or similar probably best choice there), or you swap on a 99 head, intake manifold, and crank sensor.

 

(Or, theoretically, you could build an electronic circuit to simulate the 99 low res sensor using data from the 94-97 sensor, but I'm not aware of any off-the-shelf solutions for that, so it'll take some electronics hacking skills)

 

BoxheadTim
BoxheadTim MegaDork
9/7/18 6:09 p.m.

Isn't the '99 engine somewhat higher compression, too?

My impression is that a complete and running 99 engine isn't much more expensive than the head itself, so it might be worth just dropping a complete used engine in there.

Keith Tanner
Keith Tanner MegaDork
9/7/18 6:18 p.m.

The 1999-00 engine is one of the most desirable, has the shortest production run and suffered from high mortality due to some thrust bearing problems. So they can be expensive. But it might still be cheaper than making the current thing work.

Compression is (I think) 1994-95 8.8, 1996-97 9.0, 1999-00 9.5, 2001-05 10.0 (non-MSM).

I'll bet you could get that low rez cam sensor into the NA valve cover with a bit of drilling and tapping.

codrus
codrus UltraDork
9/7/18 7:39 p.m.

Fuel, exhaust, alternator, AC, and power steering are all going to be slightly different too.  Nothing you can't fix by sourcing parts from the right year, but if they aren't in the car already then it's going to be more money.

If you bought this car, I suspect you'd be far better off finding a 99-00 motor for it and selling the 94-97 one.

Interestingly, while it's true that the NB1 is the least-common basic motor config (assuming you don't count the OBD1 and OBD2 NA8s as being different), it's not as much as you'd think.  52K US NB1s and 60K NB2s.  The 99 model year was big.

Keith Tanner
Keith Tanner MegaDork
9/7/18 7:44 p.m.

Putting the NA alternator on the NB shouldn't be bad. Fuel is solvable - feed the rail on the feed end and put a damper in place of the regulator. Exhaust depends on how much of the original 99 parts remain, but the EGR system might have a fit. PS shouldn't be a problem (the 99 pump is probably still on the car) and that may also be true for the AC.

Definitely more work than necessary for a downgrade. The car would have to be pretty darn cheap.

RossD
RossD MegaDork
9/7/18 9:01 p.m.

It is a "firm" $1000 and it has rust that he was going to fix to save it. I think I dont need another non running project car. If it was just a swap of some sensors...

 

Thanks guys. I am saving my project car workload for the Renault.

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