I have spent a good deal of time reading posts here and over at the other miata.net forum. It seems no one has a really direct answer so far.
So here is the question:

While rebuilding my 1.8l block is there any reason to opt to purchase forged pistons and rods?

Are there any advantages or disadvantages?

I am going to rebuild the bottom end either way I just want to know if there is any good reason to purchase the rods and pistons

My power goals have changed, I want 200-250 rwhp. I also want to maintain daily driver reliability.

My sig does not have a car listed since I don't own the car yet. I have the engine and am spending my time working on it. I have a 1994 Miata body that I am working on getting...but I need to do some more negotiating on it before I buy it.

 

Uh... Really? You looked?....

Wow...


Stock motor's good for 200-250. Want more, and you will, you want to upgrade. Have it apart anyway it's by far the cheapest time to do it.

Good luck.

Really? you searched? Hmmm....

 

I did search...
What I read was confounding. Some say that you should go ahead and do it while you are in there. The logic being that stronger internals make a stronger engine. Better forged internals are great for high power engines under high pressures but will not last as long. Which makes some sense in that forged internals are strong....
However, others say that the expansion of forged pistons and their subsequent looser tolerances to accommodate the piston expansion under heat means that the cylinder walls suffer damage and will ultimately fail in less time than stock internals. Also, there seems to be some sort of slapping of the piston into the wall of the cylinder which can't be good.
Finally, there is a third group who indicated that changing just the rods is sufficient since cast pistons are very good and the added strength of forged rods does help the motor without the negative slapping of pistons against the cylinder wall.

My curiosity is the validity of any of these statements. Does the forged piston really "slap" into the cylinder wall? Are the stock pistons indeed strong enough for 200-250 rwhp without any modifications? Will forged internals not last as long as stock internals under the same conditions? Is just replacing the stock rods with forged rods the best of the two?

That is what I am trying to figure out. I want the best strength and reliability for the money without wasting any.

I realize that I am a newbie and sometimes newbs don't really "search." However, this case I did search but the information that was provided by that searching seems to be in many cases confounding....

 

I'm in Austin until saturday and staying in the arboretum area. Let me know if you want a ride so you can see what 250whp feels like...preferably early in the evening before I get shitty drunk and offer my perfect body to your girlfriend because the minute she sees it she's going to leave you for something better, most likely a guy with a turbo miata.

Lots of keep saying "you'll want more." I've put 5k miles on this motor...I'm fairly content.

You can get rods and pistons through belfab for $700...why not run them? I know a great, affordable machinist in Dallas, and a great dyno in Austin. I'm also happy to teach you how to tune it too. I have Supertech 8.5:1 pistons with the 4816 allow (not the typical 2618 that supposedly expands more). I've cranked my car when 25* outside and never heard the piston slap supposedly because of the asymmetrical wrist pin location. I assume the "loose bore" helps that too. I wanted longevity more than anything out of this car...I hate working on cars. I love the debate on forged vs. cast piston longevity. What part of the piston wears again? Exactly...what rings are you using?

 

Supposedly the piston heats and cools and as this happens it expands inside the cylinder. This expansion causes the rings to press against the cylinder wall with pressures exceeding what is acceptable and apparently in some fashion gouges the cylinder wall. This slapping also seems strain the wristpins.
These are the pistons that were mentioned most often and as such I was thinking of using them with their included rings:
Weisco: Mazda Miata 1.8L BP - 83.5mm Bore, .020 Oversize, -4cc Dish, 8.6-9 Comp Ratio

Also, it was stated that these forged pistons, especially the Weisco's, crack and fail with some frequency.
I find this all a little hard to believe since forged internals are designed to be capable of handling higher temperatures, tolerances, and physical forces than stock internals.

Hustler: I may take you up on that ride.
But you can't ride my girlfriend until after I get off yours! And yes she does like to yell me name "Oh GOD, oh GOD!!!" yep-that's me!

 

My K20 rebuild - Honda-Tech
looks like we get lots of expansion, lol.

 

show me a piston that cracked from old age. Let me repeat that I've incurred no piston slap, and I'm not worried about expansion because I went with a loose bore at the discretion of my engine builder who's built MANY reliable FI miata engines.

 

I believe you Hustler. I looked over your whole build on this forum and have been reading that other link you posted. I think given your recent success with your build that I can rely on the pattern you created. If it worked for your build then I think it will work for mine.

Now I just hope it doesn't take me as long to build mine as it did to build yours. OUCH!

Your car is awesome....from all the pics I have seen.

 

oh, and I never compared my wrist pins to stock, but they were big and thick like this:

Supertech is hailed as god among the Subaru crowd, even more than the almighty Mahle and JE products.

 

I have a Subaru now and Supertech is definitely the best for that application. I guess what works well for one engine may very well work well for another.
I will be talking to belfab shortly to see about some rods and pistons...

I guess the most important thing here though is to get a piston made from 4816 aluminum alloy.

 

As a matter of being correct....

According to Belfab they use a 4032 alloy not a 4816 alloy....unless I missed something on their site.
BELFAB RACING PRODUCTS

Hustler, did you get a specially made rod and piston combo or did you use their off the shelf part? If it is off the shelf then I suspect it is the 4032 alloy.

 

I may have the #'s wrong.
edi: I probably have the #'s wrong, but the supertech's are supposed to expand and contract less.

I bought off the shelf from belfab.

 

Then for those who are really interested in being 100% correct. We will say that the Supertech pistons are 4032 alloy.

As we already know....Hustler's ride is PIMP and he uses them.

 

quoted for truth.

 

Get a room.

 

What year is your 1.8 block?
What exactly is the rest of your build going to look like?

We'll assume 250whp for this post.
The generally accepted upper limit of stock internals with minimal sacrifice on longevity is 250whp... give or take some whp depending on how much torque you're making. Guys have made 250whp on a GT2554r, at around 210ftlbs... or 250whp on a GT2860 making 250ftlbs too. There's a big difference in the midrange. This also assumes a fantastic tune and 93octane.

Since you can make this kind of power on stock internals, it begs the questions whether forged internals are necessary. The Belfabs are $700... compare that against the cost of OEM parts. If you're looking to save money, you can opt for just the forged rods, and have a set of stock pistons tri-coated. That will eliminate any worry about fitment and slap. It's probably $100 difference in the end. Belfab rods and stock tri-coated pistons will get you well over 300whp with room for tuning mistakes.

Final answers:
No, forged internals are not necessary for 250whp.
Yes, this depends on keeping knock at bay and your torque levels.
Yes, if you've got the money, there's no debate.
Yes, the generally accepted internala weak link are the rods.
Yes, you can get the margin you're looking with forged rods/tri-coated stock pistons for less $$.
Yes, Hustler is the man.

 

Hustler: That post is funny...I especially like the quoted material. This is the forum form of "photoshopping" the post.

samnavy: Your answer is pretty much what I was looking for. After all there seems to be a bit of conflicting information about what the truth is. At 250rwhp I think that stock pistons with forged rods makes the most sense. Anything above that would definitely require the fully forged bits.

Do forged internals last for fewer miles overall? I know compression ratios, engine knock, turbo heat, detonation, and internal cylinder running temperatures can make a big difference. So for the sake of this question consider a good tune, knock sensor, and appropriate cooling. Given the engine is under the same running conditions do forged parts really last longer or do they just handle the power for a shorter period of time? Would forged internals run for 100,000 miles or will I be looking to rebuild every 25,000 miles?

Once this thing is built I want to drive it not work on it. I would rather do it right one time than do it half-assed twice.

The relatively low cost of forged pistons and rods from belfab seems to beg to be utilized. If it is only $100 above well made stock internals then it really is a no-brainer.

 

Oh...and the block is from a 1994 1.8l miata.

I was, and still am, thinking about an overbore on the block to 1995cc or there abouts depending on the pistons I end up buying. Since it is likely that I am getting the supertech pistons I may bore the engine..... However, many posts here are stating that the stock bore can handle the power I want and so I may not do the overbore. I have a TD04 turbo which at the high end can get close to the horsepower I want, a walbro 195lp fuel pump, a saard BOV, and some other bits. I am not to far along right now which is why I am asking this question.

 

That's funny, I don't see Crawford sporting Supertech, nor do I see Supertech in Ken Block's car. From what I've seen, Crawford Performance builds some of the highest performance engines in the Subaru industry, what do they run?

Also, there's a lot more to selecting a wrist pin than the length...Aside from diameter and length, there's thickness, hardness, material, and coating. How much side to side play does your rod have in the piston? That will explain why the pin is longer...(pin-piston engagement).

 

Crawford?

Google "Crawford bobblehead pistons"