Cost vs. performance on pistons
 

I'm tentatively gathering data for a tear down and rebuild of my motor which will be inevitable and so I want to have all the info researched and know the plan before that day comes.
This car is the uber-budget build, fast and reliable, things done "right" but nothing but the necessities - if the coolant isn't getting too hot at races, then I don't need a reroute sort of thing. So, consider that context in your responses, if it were absolutely necessary to have everything on the bottom end forged, I'd do it but it's not..
Anyways, I know I will be doing forged H-beam rods. Now to the pistons.. First off, I've heard mention of using automatic pistons and I can't find in any searches the actual compression ratio specs of one - can anyone help me out there?
I want to drop the compression some. Builds like Hustler's have convinced me that a marginal compression drop allows much more power from boost than is lost from lower compression. So, if auto pistons are a lower compression, sweet deal.
Now, on to the cost issue - assuming auto pistons are a lower compression and would work well (and yes I would get nice rings for them), I would have the engine guy do a slightly loose bore and go with those pistons and new rings with the H-beam rods, but are would the cost of the pistons + new rings + tri-coating (is this necessary?) add up to almost the cost of forged pistons which come with rings? Because if so, no sense in not getting the forged ones, but if it is a more affordable option, why not go with the coated auto pistons?

Anyways, discuss...
Thanks
-Ryan

Interested in the responses as I have a 96 automatic motor at home. IIRC, compression is 9:1 instead of 9.5:1 for similar 5 speed motors.

I don't believe there was a difference in compression in auto vs 5-sp in a 1.8L.

94-95 have been rated at 8.8:1
96-97 9.0:1

I would get the higher 10:1 pistons. Better mid range. If you know how to tune you can be perfectly safe with a slightly higher compression.

I wouldnt bother with all that voodoo with coating pistons and stuff.

I considered tri-coating 9.0:1 pistons and I was basically $100 shy of forged slugs. Think about the headroom and pizza mind.

I firmly believe that even on budget builds, "do it once and do it right" with the fundamentals or engine internals. Audi, mitsu, subaru, and toyota go with forged guts for a reason on their sportscars.

he probably wants to make some torque. Do you consider 4000rpm mid range? I'm making 252wtq there, that's roughly 275wtq on a dynojet. I consider that midrange. I've auto-x'd my car a few times too, and the powerband is fine. I wouldn't want boost to come on any higher, but never had a complaint yet.

while sam doesn't subscribe to voodoo, I doo.

see a piston will die from detonation whether it's forged or cast. the coating provides a little thermal protection and reduces (some of the swain coatings) liklihood of detonation. here's your voodoo:

but swain is MOTHER EFFING PRICEY last time I looked.

still, it is good insurance.

but if you're going to be under 250 rwhp, you might not need forged or coated. what power level are you looking at?

Brain, to counter your quote, here is a quote from the solomiata page so I don't know which one is correct:

"In 1996 the peak HP rating of the Miata 1.8 motor was raised to 133 hp from 128. This comes soley from the new for 1996 ODBII software having the ability to lean out the above 6000 rpm fuel curve. There have been rumors that there was a running change to the pistons to up the compression ratio to 9.5:1 during the 3/95 cutover, but I have not seen any documentation to prove it. "

My reference to the auto being lower compression may be rumor as well.

If my research on pistons was correct, 1.6L Miatas were the only ones to have a different piston for automatic or manual configurations. I could find no evidence of this happening on the 1.8L.
I will also say that the SoloMiata quote, for once, is actually wrong about the compression ratio. My '95 R had the same pistons as the ones I pulled from a '96 (they have a slightly raised dome), however the '94s I pulled were completely flat topped and would be the ones rated by Mazda at 8.8:1. Also had a set of MSM pistons (same as '99-'00 - 9.5:1) and they had an even higher dome than the '95/'96 pistons (9.0:1).

Cmon man, seriously. Coating pistons on a street driven gas engine? Come on.

I can think of much better things to spend 200 bucks on.

Interesting. Thanks for that. I need to check, as I would have sworn today that my 96 auto pistons are actually dished. I'll check tonight. The head is off of it.

If that's the case, something interesting there for sure. Please post back with what you find.

You're just focusing on the top coat, most people go with a tri-coat option. If you're using OEM pistons, it can't be a bad idea to help them out in any way that you can. The top coat helps as y8s described, but the side skirts are coated in a dry-film lubricant, so less friction = less heat, and the undersides are coated to help shed the oil off faster, which helps keep as much oil in circulation as possible. I paid $140 ($35/piston) for the tri-coating with Swain, a justifiable cost in my book.

If you figure $440 for a set of Supertech forged pistons w/ rings from Belfab and then the machine shop charges you more to overbore the cylinders than just to hone them (3 of the ones I got prices from quoted $75-$150 more for an overbore). So say $515 at the least, possibly more depending on piston and labor.

So even if you're buying a new set of OEM pistons (about $120 for all 4), having the cylinders honed ($50-$75 depending on shop), you still come out at least $180 under the cost of going with oversized forged pistons, which is great for a budget like what the OP is aiming for.

I personally went with the tri-coated option as I didn't have to buy the pistons and I'm not searching for insane power numbers, just something to support a nice bump in power. Oh, and not to mention I was tired of hearing piston slap during warmup on the cars around here running forged pistons, even though everything is properly clearanced inside the engine. That wear and tear has to add up sooner or later.

I would think a street driven motor with all those miles would be a BETTER reason to put the coatings in your motor.

If this is a cost vs. preformance issue i dont see much preformance for the dollar. So i would spend the money somewhere else. Where? that depends on what u have and what else you need. The tuning of the ECU may net more power and torque. Is the injector duty cycle over 80%? then maybe bigger injectors. Boost toping out at 18psi @ 6400RPM and not making any more power past 6500RPM? Then maybe a bigger turbo. These are all if's and but's to where your cars at and where you expect it to be (HP level's).

Well, I checked and I was all wet. My 96 auto pistons have a slight (maybe .020-.030") high dome in the center with about a 1/2" band of lower material around the periphery, so likely the same as stock.

As to my goals, right now I'm probably in the 200 whp/11 psi area which is right on the edge of what is generally accepted as the limit for preserving engine longevity.. I will probably not go above 250 whp but in the spirit of doing things properly the first time (with budget in mind), I am aiming to make everything good for 300 whp - the intercooler, the injectors/ecu, etc. all could do 300 if I chose to swap to a 19T down the road so when I crack open the motor I would like to close it back up with the ability to do 300, even if I may not actually push it that high.

The recommendation to go with higher compression pistons is one I will have to decline, I just don't feel comfortable with that - yes, I know y8s has pulled it off beautifully, but I want to create a car that is my daily and race rat e.g. something that has the highest probability of lasting for as long as possible and imo running high comp pistons on a moderately boosted motor, while possible with a good tune, leaves very little wiggle room for a slightly imperfect tune.

So.. you guys were saying the 1.8 pistons did not differ from manual to auto but that the 1.6 did... I have a 1.6 which is exactly why I am researching this - can anyone actually confirm or bring forward info about the 1.6 pistons from the auto? I'd love to find out that they are 8.8 or so...

Rotornut - thanks for that break down on what you found you could save by going with coated stockers instead of forged. If I can save close to $200 by doing a factory piston and the tri-coat over a forged, perhaps that w/ H-beam rods would be perfect for a budget 300 whp ceiling motor..

-Ryan

edited

You know why Chuck Norris named his cat after you? Because its a pussy.


;) j/k

I'm excited to see 1.6 guys going with the low comp / high-torque option. It works for subaru and mitsu pretty well, lol.

:naughty:

you live in Dallas...where is your discussion on high compression, superchargers, and avoiding stand alone?

Does anyone have any "proof" of the tri-coating actually being so grand? I could justify the price if it worked. I've never understood how those coatings stick to the pistons so well and don't wear off in a few thousand miles. It's destiny that I will be pulling my engine back apart, or dropping in a new one since I decided to run stock rods. When the time comes, I might run stock pistons with coating. Or might go Belfab and run forged, I will have to see what money does.

Bwahahahahahah!!
I sent you pictures of my mom. Did you get them? ;)

the skirt coatings usually diffuse into the outer surface of the aluminum. I imagine swain uses a similar "bonding base" for the other coatings to make them more adherent. call them and ask.

i cant remember where i saw it but there was an article about a 2jz motor that they were using for a standing mile car and they were trying out ceramic coatings.they coated the pistons, combustion chamber, faces of the valves, the back of the exhaust valves and the exhaust port in the head. they said the result was less piston slap during warm up, turbo spool 600rpms sooner! and 104fl lbs of tq gain! seems like a lot but i guess the difference of 1200hp and 600 ft lbs and 1200hp and 700ft lbs is not really that intense. either way i would either go with coated stock pistons or forged pistons, i would not consider completely stock pistons an option on a daily driven 300hp 1.6. you might get away with it with a good tune but for 150 bucks its worth getting them coated so they dont break a ring land if your tune isnt dead on.

thanks for the replies guys. Can anyone dig up specs on the automatic 1.6 pistons? I'm still not finding anything and I really would rather go with 8.x:1 rather than the stock manual 9:1's. If the auto ones aren't lower (but I keep finding hints that they are) I may have to go with the forged just to get the lower compression.
-Ryan

I was under the impression that stock 1.6L pistons were 9.0:1 - from what I've been reading it seems that they are in fact 9.4:1 while the automatic pistons for the 90-93 are the 9.0:1. If this is so, that is a generous drop in compression and they could be a great choice for a budget turbo motor build, no?

I've been looking up tri-coatings and Performance Coatings will do the pistons and pins for $120. Nice price. That is for the thermal top coat for dispersing heat evenly, the friction reducing side coat, and the oil shedding underside coat.

What do you guys think?

On another note, the auto 90-93 also has cams that have less duration, is that a good or bad thing for turbos?
I was considering when I take everything apart getting stronger valve springs (only about $350) so that I could bring the redline to 7800 or 8000, but would I have to get different valves themselves/different retainers or could I use the stock stuff? I just want to avoid valve float while bumping the redline to safely (but affordably) get more top end.. what do you guys think?

Thanks
-Ryan

You are correct on the 1.6 piston ratios. It's great info to have for a budget build!
If you're looking to pick up & coat some used OE pistons, I'm guessing you've already checked the block to assure that it only needs a hone and doesn't need the bores opened up, correct? If so, sure, give those coatings a try.
Another source for coatings that I found on here (forgot who posted the company, sorry) is Synergy Coatings.

I'm no expert in valvetrain bits, so I'll definitely leave that for someone else. My guess, however, would be that you should be able to reuse the valves and retainers for those higher revs if the parts are still in reasonable condition.

Good luck with finding any real info on cams. It takes big money to get cams done right.

$450 to regrind cams at www.elgincams.com - you can provide the spec, or you can tell him what characteristics want.

nevermind, forget cams for now that's an area I will need to do a lot more research on before having any idea what I want to do in that regard, and I can do them at any time, no need to add that to the cost of the rebuild.

juxt3r - I have not opened the motor up yet, as I said I am gathering info and developing a plan so that I can begin to accumulate the parts I will use in the build for once I do need to do it. What would make it necessary for me to bore the cylinders if I was just going to go with oem pistons? Excessive wear? I will probably end up opening the motor up for this before it blows (it only has 117k miles right now) so I am hoping it will only need a hone.

I would probably not re-use my old valvetrain bits but would save the $ on the big buck valves, etc. and just go with fresh stock bits with Supertech or similar springs for 8k rpms... Looking at most of the dyno plots of 1.6's with my turbo it does look like HP is still increasing or at least staying strong when they hit redline at 7k so I'm thinking this would give me a nice 1000 more rpms at perhaps not peak hp but a very good amount of hp, which would be pretty nice...