Advice Needed on NB1 Engine with extremely low compression
 

Hi All,

I have a 99 naturally aspirated miata (ms and i/h/e). It has been my hpde car for about two years now. For the most part i have taken pretty good care of it - fresh fluids before and after each trackday etc. It has about 140k on the engine and 20 or so trackdays and a lot of autox-es. Lately I have been noticing significant loss of power and rough idling as well as what can only be described as truck like noises coming from my engine.

I am by no means a talented wrencher so please excuse the terms i am using.

A local reputable bay area shop did a compression test and leak down and the results were:

110/120/110/70 and 30%/30%/30%/60%

They think that my head is toast and that the bottom is salvageable. Do i want to is the real question here.

I'd like everyones opinions of where to go from here. I have a budget of about $2000 to get this sorted. Would there be a budget engine build to consider here or would buying a salvage engine be my best bet?

Where do people usually source engines from?

Thanks in advance for any advice you are able to offer mt.net

Since you already have a megasquirt get a low miles VVT engine, squaretop, coolant re-route and refresh all the seals and belts. That will run to close to 2k.

Not enough info. Was the compression test hot or cold? Wat was dry compression vs wet? You have leakdown numbers, but don't mention where the leakdown is suspected.

Take the first chunk of your budget and buy compression and leakdown testers. Get a cheap compressor, too, if you don't have one already.

Have you ever money shifted the engine?

My 'truck like noises' turned out to be a spun bearing. I had the engine rebuilt.
It may have been cheaper to buy a rebuilt engine. Running NASA TT, I wanted to be able to use all Mazda stock parts, so no classing issues.

Thanks for the insight. I spoke with the mechanic briefly and he says he suspects the exhaust valves have sealing issues as air was coming out of the tail pipe when he was conducting the leakdown. I would be lying if I said that I haven't overreved but nothing too crazy like a 5-2.

I'd buy a stock VVT longblock and have your mechanic swap it in. Sell the BP4W as-is with full disclosure - it's still worth several hundred dollars even with 70psi in one cylinder.

The bottom sure has fell out of the VVT motor market.

https://www.lkqonline.com/?ref=g-ppc...gine-Assembly/

Weren't these like 1k-1.2k 3-4 years ago?

11:1 pistons on this block with some forged rods and a head refresh... or my rebuilt head in the FS section ;)

Just thought I would jump in here, as reading your post reminded me of my own experience...

I got similar compression readings (around 100 on all 4) as a hot test, and adding oil to cylinders made no improvement.
I didnt have access to a leakdown tester, and as I already knew the engine was knackered I decided to strip it apart (Best case result would be a head rebuild, worst case would be an engine swap/full rebuild).

When I pulled off the head at least 10 valves were visibly not seating correctly and covered in debris, some by up to 1-2mm!!!
Also looking at the bores they are completely smooth (no honing visible whatsoever) and the piston face is coated in carbon/other crap.
There also appears to be a buildup of carbon on the exhaust outlet of the head (where it meets the manifold) of 1-2mm.

This is as far as I have got at the moment, but as for options I would say that neither you or me know if the engines we have are salvageable without some measurements or a trip to the machine shop...
So this is when you need to decide if you want to rebuild the engine? (I do) as the second hand engine fitting will DEFINATELY be cheaper, but not without its own risks.

Just try to test the compression on the new engine if you can, all the breakers in the UK just state that the engine is from a "Running" car - this doesnt give me any confidence as my engine was "running" with 100PSI compression.... :vash:

(trip to machine shop for me next week)

I'll second the low compression bad valves on one of my stock motors. While 140k miles is no spring chicken I bet a valve job would get you 90% of the way back to expected performance.