Compression test pre-turbo time
 

Hello all,

It's cold up here in Canada, which means no miata and time to buy parts for next summer!
I plan to put down for a Flying Miata no electronics kit this winter. I've already installed an MS2e from Reverent, who's been incredibly helpful in helping me make the most of it N/A.
I've always wanted to turbo the darn thing, and i've got the possibility of doing it now!

I know I did a compression test two years ago and it went well, around 160/cyl IIRC.
I want to know what's scarier, having them all at 150?, or having three at 170 and one at 150.

I know I should be doing a test before I commit to a purchase, but I don't have access to the car until April.
I'm going to look through my stuff at home to find the compression test results I had.

The car is a 1.8L, 1997.

What should they be at? I searched the forum and found a 180psi brand new spec. What's an acceptable range?

At what point should I not turbo it?



help?

As long as they aren't crazy low and all within ~10psi max of each other and the leakdown test looks good. You should have no worries.

And if going new, might I suggest the TSE kit...

FM is a full kit, where I don't have to get anything else from anywhere else.
brokerage fees, USD to CAD exchange, Shipping........

The above make it so that I don't want to have to order parts from multiple places.....

Having one read at 170 and one read at 150 would be scarier. They shouldn't deviate more than 5% IIRC (someone correct me if I'm wrong).

5% is a good engine. 10% is still acceptable by most, and even 15% is okayish. 20% is definitely not good, assuming you did the test right.

But really, who cares. These engines are dirt cheap, boost it no matter what and build it if it dies. Or start saving for build now. Whatever

I don't compression test before turboing miata's. Motors are cheap and I know I want a turbo.

If you compression test and get low or inconsistent numbers, then what? Do you swap the motor? Do you not turbo the car and keep it N/A? If the car runs good, I assume the motor is good and I turbo it. If it continues to run good I keep driving it. If it runs bad I look for the problem and fix it or swap in another cheap motor.

If a used motor was $5000+ I'd certainly change my strategy, but when you can pick up BP's for a couple hundred bucks I just don't see where it's worth stressing over.


*Edit - 18psi beat me to it.

Thank you for the great replies!
I am reassured.

Unless your gauge cost you $$$ and was calibrated, the raw numbers don't matter. The split is what you want. No more than 15psi between cylinders, preferably 10psi max deviation from max to min. Then add some oil to each cylinder and try again. Should be no more than a 10psi jump in any one cylinder.

A friend has a good gauge and I checked mine recently. Will likely retest again to do the wet test.

I was under the impression that you could do this to monitor engine health and not be caught with your pants down when it's time for a rebuild. Is this incorrect?

Leakdown tests are far more accurate and tell you far more. I own a very nice aircraft-grade leakdown tester, but you won't find a compression tester in my shop.