Diagnosing low power on Miata - just checked the compression

Hi all,

I have a 1990 Miata that has been gradually dropping in power over the last few months. It is most noticeable when leaving from a stop. Also, the fuel economy is gradually getting worse. I just checked the compression and here are the numbers:

Cylinder 1: 140 psi

Cylinder 2: 145 psi

Cylinder 3: 150 psi

Cylinder 4: 150 psi

Can someone tell me if these numbers seem low? I also changed the plugs, and that didn't seem to make too much of a difference. Is it worth changing the O2 sensor? I'm also wondering if the cat might be clogged. It looks like it may be the original one. Does anyone have experience with the Good-Win-Racing Magnaflow high flow cat?

Did you change the wires with the plugs? And what kind of plugs did you put in?

Although it sounds more like the cat than anything else.

GRM did a big miata highflow cat comparison and tested them all.

The magnaflow metallic spun cat won out, 3 or 4hp on topend vs stock. You can order it all by it self for around a 100 but you will need a to have the flanges welded on. An easy and cheap way to do this is to purchase a 'test pipe' for around $40. Install it and take it to a muffler shop, they will cut the center out and weld in the cat without even having to remove the pipe.

I think they also make a full bolt in cat but it is more expensive.

Just so you know the magnaflow spun cat is VERY small, about the size of a coconut. It is extremely highflow... which means you can smell the exhaust at times and it is also only slightly quieter than a test pipe. Depending on your exhaust setup that can get pretty loud.

It is loud on my RB PP with Resonator. Though it does sound great!

That's from another forum circa 2012. I wasn't around then but someone may have been who could point you at it. I just went with a standard CARB legal replacement when I replaced mine last year because it was the cheapest option I could find.

About those metallic core cats - they're very short lived. Free flowing, but we've moved away from them in our applications due to a very high mortality rate.

On this car, I'm thinking the O2 sensor is a likely culprit. It's certainly worth changing regardless. Have you tried pulling codes?

Short lived in what sense? Losing effectiveness or self-destructing? I've always understood that it's harder to damage a metallic core cat in a performance application (high EGTs, afterfires, etc.) than a ceramic core one.

Self destructing and low effectiveness. The fact that you can smell exhaust means they're not working properly at actually being cats. I'm sure a backfire in the cat would be better absorbed in a metal core than a ceramic, but that's about it.

I had one come apart on my car - it shook loose of the housing and started to ricochet around. Every time it bumped the housing, it would get dented. So the core got smaller and smaller and denser and denser as it bounced around more and more. I've got it on my desk, I'll have to get a picture of it.

We got tired of warranty replacements.

Thanks for the feedback. I replaced the plugs with the same model of plugs that came out. NGK platinum.

To check if the cat is clogged, I read online that I can remove the O2 sensor and put a pressure gauge in its place to check the exhaust back pressure. Apparently, over 3 psi is too much and could be the result of a clogged cat.

Any other recommendations for aftermarket cats? Rock auto has a few that are not too expensive.

I just put an exact fit magnaflow 23692 cat in my '92, and the fit was less than exact. YMMV.

cmcgregor wrote: I just put an exact fit magnaflow 23692 cat in my '92, and the fit was less than exact. YMMV.

Was there a noticeable difference after you installed it?

The FM cats are a perfect fit

We tell people not to expect much power increase with a cat on a naturally aspirated car. The muffler's a bigger restriction.

Keith Tanner wrote: The FM cats are a perfect fit We tell people not to expect much power increase with a cat on a naturally aspirated car. The muffler's a bigger restriction.

Does that mean there would be a power increase with an aftermarket muffler?

I seem to remember reading on one of the Miata forums that there wasn't much power gain to be had in a Miata from either an aftermarket intake OR muffler.

There are not a lot of naturally aspirated power gains to be had, honestly. Certainly not easy ones.

But when addressing the exhaust, you start with the worst restriction. You'll get the biggest gain from a muffler, then a header (depending on the year of the car) and a cat. Doing the whole exhaust system from exhaust ports to exhaust tip will gain you maybe 10-12 hp on most Miatas, with the 1999-00 California cars gaining a bit more due to the removal of their first cat.

In reply to mainlandboy:

It was replacing a hollowed out OEM cat on a car I hadn't driven in 6 months, so I had nothing to compare to.

mainlandboy wrote:

cmcgregor wrote: I just put an exact fit magnaflow 23692 cat in my '92, and the fit was less than exact. YMMV.

Was there a noticeable difference after you installed it?

I put a Racing Beat header, FM cat and Good-Win Racing cat-back on my '90 in fairly short succession (Had to do the cat after the header murdered the OEM converter in a week's time) and never noticed any power difference. The 1.6L is kind of a pig, especially when it comes to naturally aspirated power.