Hello miataturbo! Long time reader, first time poster here. I've gathered vast knowledge from these forums and over the last few years, and managed to semi-restore my 91 miata from rotting in a field. It's had all the basic maintenance done to it, including refreshing the cooling system, replacing almost all the engine seals, suspension work, and of course, a manual swap. Now that it's at a healthy baseline, I would of course like to turbo it. Aside from the discussion of pre-turbo prep (have this covered already), I have some concerns about the health of the old 1.6. I've done a compression test twice now, and the results are as follows:

When I first bought the car(cold bc wouldn't start, 184k miles): 135, 120, 135, 140

Now (warm, 186k miles): 115, 120, 115, 121

Same Harbor Freight tester used both times. I know similar results is good, but these numbers are quite a bit lower than optimal. Is this something I should look into fixing, or should I just accept that it's an old car and hope that the turbo is kind to it? If so, what steps should I take? Thanks for reading and helping out a dumb kid with a cool car.

 

You have an auto 1.6? Start by putting manual cams in it and retesting, they’re only 105hp vs manual’s 116. I dunno how that’s going to affect your comp results, but regardless you’ll want them.

if it doesn’t help much, which again I don’t think it will, rip it apart, have a shop refresh the head, replace all seals, ball hone the block, and install new rings/bearings. If bore doesn’t look good, bore for either +1 OE pistons or some aftermarket.

all this costs a decent chunk of change, and is better spent on a 1.8 of course, but the 1.6 is a bit more pnp and you already have it.

 

Not the answer I was hoping for, but I appreciate the input! It actually already has manual cams, as I had an issue with the cam alignment pin causing timing problems. Long story. Also, I have been reading and found out that supposedly the automatic 1.6 cars had lower a compression ratio than the manual ones. Could this have to do with anything? Would a wet test tell me anything useful? I would really like to diagnose any possible problems as cheaply as possible. Thank you for the help!

 

If you are going to stay with the 1.6 I would just send it. Since it runs already I would go through the learning curve of tuning and make all of your mistakes on the old engine. Once you have low boost tuning figured out and the rest of the car gets sorted, you can build a forged rod and low compression forged piston block to get to the next level.

 

Only problem is he'd be re-buying a manifold if he goes 1.8, but not terrible advice.

I'll also say, that if this is a "loose" engine, you'll be having more smoking/blowby/oiling issues than you may have with an otherwise healthy engine.

 

Thank you both for the replies, I appreciate any help you can provide. Today I re-did the compression test with intent to also try a wet test, along with taking measurements of the "first crank turn" pressure. I have to say, the results were pretty confusing. I pulled the fuel pump fuse, CAS connector, and injector plug. With the engine warm and all spark plugs out, along with the gas pedal depressed, I could not get a reading above 3-5 psi in each cylinder. I was tearing my hair out wondering what possibly changed?? After tweaking with it and seemingly doing nothing differently, it started giving me higher pressures than yesterday. Here are my results from today's test:

First crank psi: 20 20 20 30
Max psi: 150 150 140 150
Wet test (cylinder 1): 175 psi

I have no idea why it initially did not build pressure, but I have drawn a few conclusions from today's test. I hope you can confirm my theories.
1. The HF compression tester may be slightly defective/could not seat well (even though I tried several times)
2. It seems to be minimally within a healthy range of compression. If it manages to hit 150 at all then it should mean something, right?
3. The wet test in cyl 1 proves that the rings are not overly worn (to an unsalvagable amount)
4. The first crankover reading points to my valves being more of an issue than my rings
5. My car is sentient and continues to play mind games with me

 

I have had issues with the Schrader valves in my Harbor Freight Compression tester. Had to replace them with new Schrader valves.

 

First off, congrats on bringing the ’91 back to life, it sounds like you’ve done an awesome job so far, and a manual swap is always a great touch. About those compression numbers: while the consistency across cylinders is a good sign, the overall readings are pretty low, especially for a warm test. This could indicate some wear in the rings or valves, which isn’t surprising for a high-mileage 1.6. The drop compared to your initial cold test could also mean the Harbor Freight gauge might not be giving perfectly accurate results, they’re known to vary a bit, but it’s worth taking the numbers seriously.

Before boosting, it might be a good idea to dig a little deeper to understand the engine’s health. A leak-down test would be your best bet to figure out if the low compression is due to worn rings, leaky valves, or something else. Also, checking your timing and valve clearances could help rule out simpler issues that might affect compression. If the leak-down test shows significant problems, you might want to consider a refresh or rebuild before adding the extra stress of a turbo.

That said, if the engine seems to run fine, doesn’t burn oil excessively, and the leak-down test comes back reasonable, you could probably run a conservative low-boost setup. Just make sure you’ve got a solid tune and don’t push it too hard. The old 1.6s are tough little engines, but boosting a tired one can be a gamble. Best of luck, and keep us posted on how the project goes, it sounds like you’re well on your way to building something great!

 

I'd send it on those compression results. HF testers are notoriously not super accurate, plus things like altitude and baro pressure can greatly affect comp results. The fact that they are pretty consistent tells me it's either healthy, or evenly worn (no major issues), and is worth boosting. Of course if you find the engine is loose like Curly mentioned you may run into blowby and smoking issues, but if that happens then you'll know to go for a rebuild!