When does one need pistons?
 

I have a '94 1.8 BP that I'm planning to pull apart for a lite rebuild and build up for a light turbo build. This is going into an exocet, so I'm not going crazy. When I say "only 8 psi", that's truly going to be it.

So here's what I have in mind.

-pump gas (probably 93, but I'd prefer 87 if I can get away with it)
-stock intake
-stock cams
-forged rods
-stock flywheel
-cheap air-to-air intercooler off ebay
-VF22 turbo (like a GT2860RS size turbo)
-homemade exhaust manifold
-performance cats

So my question is really, when does one need to upgrade to forged pistons? Should I be trying to drop the compression ratio? I've read the BP is knock limited, but I don't really know how much I should be planning around that? Does it run on 87 in stock form or does it need 93?

You need to run 91/93 or E85 if turbo.

Stock NA8 pistons will be fine for your power goals.

If 200 WHP or less, I would suggest the CXRacing IC0049 as a pretty good ebay FMIC. I am using it for about 235WHP street, and it can get up to 40F rise on a long, up-hill pull. For less HP, it should do well for you, even at the track. < $125.00.

Not sure how well that will fit in the Exocet and such.

I would use stock pistons in your case OP.

Good Point.

Oh, one other question. Is there a book anyone can recommend for rebuilding the BP? Something with all the various measurements I need.

No E85, I just don't want to deal with hard to obtain fuel. Ok, so stock pistons, any extra ring gap? Pulling timing to avoid knock means extra heat rejected into the oil and coolant, so is there anything that's recommended to make the engine handle that better?

I think I have that exact intercooler already, I bought it many years ago for my saab and never installed it. Maybe it won't fit the exocet, but since I have it I'll start there.

OEM ring gap is probably fine for your power levels, I personally would open it a small amount but I'm sure most people would think it is fine as-is. Make sure you leave the OEM oil jet squirters alone, plugging them will bump up oil pressure and lower oil temp a tad, but will cook the stock pistons to the point of possible thermal failure. There's thermal coatings you can apply to the inside of the head, or just to the tops of pistons which is pretty easy to do. I've seen very good results in other motors, and I've seen them used successfully in Miata endurance builds for keeping temperature under control.

However, for the power levels you are making, a decent oil cooler and radiator should keep the temperature down all by themselves. I would HIGHLY recommend a coolant reroute (if you haven't done one yet) since you will already having everything apart anyways.

My tuner has tuned miatas over 500whp on stock pistons. I’m running them in my otherwise built bottom end with a 400whp end goal

OK, so open it up a hair, but nothing crazy. Any rebuild books I should get? Or is there an online resource that has all the measurements I need to check?

I'm definitely leaving the oil squirters in, and I'm going to upgrade to a good capacity aftermarket oil pump. Do people actually plug the oil squirters to improve their oil pressure? That seems so assbackwards. I'm planning on doing the coolant re-route, but I'll probably pass on thermal coatings. Gotta keep costs in check somewhere. :P I'm also an electronics nerd, so this engine is going to have sensors for everything and I plan to implement knock control schemes in the ECU. Probably a Speeduino, but I'm not set on that just yet.

Oh I should probably state what the intended purpose of this car is going to be so you guys have an idea of how this engine is going to be used. As I've stated this is going into an exocet, but this isn't going to be a race car by any means. The idea behind this build will be 4 wheeled motorcycle. Something for my spouse and I to hop in on a weekend when the weather is nice and cruise the scenic back roads of PA. Like 6/10th ish most of the time, 8/10th max, on a regular-ish all season performance tire. Power will be mostly demanded in short squirts from 30-60 most of the time. Definitely no track work where I'll be truly heat soaking the cooling system, it just needs to survive 100F summer days and the occasional autox. Most days will be cooler than that, but that's like worst case scenario here in the Pittsburgh area. While the car will be emissions exempt here, I do plan to build it with some sort of aftermarket high flow cat as I dislike pollution.

OK, so now that I've fully fleshed out the use case here, what exactly is a "decent oil cooler and radiator"? Like is a new stock replacement rad that isn't gunked up sufficient or do I need something more? I was also thinking of plumbing in the factory AT fluid cooler in the rad as the engine oil cooler so I wouldn't have to run a thermostat for the oil cooling. Would something like that be sufficient or do I really need a dedicated oil radiator?

The squirters aren't really needed once you put forged pistons. Some have stated that it actually weakens certain forged pistons. Plus the oil pressure helps with trying to achieve a higher RPM without shims.


After reading the intended use of the build, you probably could get away with a stock rad w/ a reroute if on a budget. However its pretty common for them to overheat when FI is introduced during spirited driving. But I wouldn't use a stock rad if the car will see any sort of autox.
I'd just get either a mishimoto, or koyo radiator w/ a reroute and call it a day.

As for oil cooler, I don't think you'd really need one. I don't run one and I'm currently pushing 19lb's @ 300whp. Car primary use is drifting, so it gets abused. My oil temps get pretty high but I run T6 so as long as I'm not reaching 280~ I'm not too worried.
It wouldn't hurt to install one, but unless you're doing some serious track time with your build you should be fine without one.
However I do still use the stock oil cooler/warmer. Idk if that really keeps the temps down or not.

But for short bursts of abuse, you shouldn't be cooking your oil if running good oil.

I would look up the factory engine manual (or whatever it is called?) for the Mazda 1.8L in the Miata, has the ring gaps...or I remember whatever Mazda resource I read having the ring gap spec. I read it at some point. Good place to start, sorry if I can't help more than that. I used to have links/bookmarks/downloads of all those things, but my previous laptop got wiped. Boo.

Supermiata Crossflow radiator with a coolant reroute is all you need then. No aftermarket oil cooler is necessary. A tiny bit more expensive, but their radiators are very lightweight compared to other aftermarkets I've seen. Extra lightweight bonus points since it is going in an exocet. You won't be able to cook the oil with your use. There's sufficient documentation on how to run the crossflow radiator with a coolant reroute. Think there's a couple off the shelf hoses that fit if you chop off a couple inches. That's just from memory though.

Best engine shop in the PIttsburgh area is Domhoff racing engines in Harmony

Hard to have a running motor without pistons tbh.

Pretty good, Doc. Why is your car not in the FS on this forum?

I posted it in the Facebook group.

Let me post it in the forum as well.

Edit: done!

Thanks, I’ll keep that in mind when I get to the engine assembly stage if I need machining.

Just a general question out of curiosity, what's the lowest compression piston you can get for a BP?

8.6:1 is the recommended compression for pump gas turbo miata pistons.

Ok, but question there was more what options are typically available? Like can I even get 8:1 for example?

You can get pistons custom-made in whatever spec you desire.

In terms of OTS options, 8.6 is what you've got to work with.