Who here has done a rods only build and later regretted not putting forged pistons in at the same time? Or indeed who here is totally happy with their rods only block?

I currently have my 1.8 VVT torn down and considering re-using my 10:1 stock pistons for a 300whp power goal.

I understand stock pistons are safe and reliable at this power level in the absence of detonation. I know 10:1 pistons can't run as much timing as 8.6:1 pistons before becoming knock limited.

For context, it's a street car that might see a couple of track days each year.

 

I'll let ya know as soon as mine is on the road! Currently working out a fueling issue then doing remote tuning. Sounds like my goal and use case is about the same as yours.

Coming from a typical EFR 6258 225tq build that was pretty lively.

My unqualified feeling/hope is that sticking with the EFR and more boost will still be plenty of fun since fast spool instead of dialing in "allofit" timing wise.

 

I would happily tune a car to 300 on stock rods and pistons. In fact I made 299 on a bone stock bp4w, but we turned down the boost after that pull. I really think the pistons can take it, especially if you're not giving it constant heat like a track. I'd be slightly concerned hitting the track with that setup, but as long as the tune is good...

I would want to search around for some BP4W pistons at the very least, but it's not a very big decrease in compression. You could also put a thic headgasket in there to lose a couple tenths, looks like they go up to .080.

 

No regrets whatsoever, but I built mine with stock pistons specifically because it's a DD - and 20* cold starts on forged pistons aren't great. I've been tracking and road tripping my car since I rebuilt the motor ~3 years ago without issue. I'd do it again this way in a heartbeat. If I were shooting for much over 300whp I would definitely still go forged pistons, and then I probably wouldn't DD the car either.

 

Thank you gentlemen. I appreciate the response.

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I'm also running an EFR 6258 225wtq(ish) build on a stock VVT engine so your progress will be very interesting for me! Got a build thread?



I'm intrigued you made 300whp on a stock BP4W, curly! You wouldn't have a dyno plot by any chance? Curious how you managed torque on the stock rods.

In theory if we can hold 225wtq to 7200rpm (and the EFR certainly can) then we'd be making just over 300whp by redline. Seems like in practice the BP doesn't flow that well however. From what I've seen our engines need 250+wtq to hit 300whp.

I hear what you're saying about the BP4W pistons. 9.5:1 compression isn't much better but it's moving in the right direction. I think for track work I'd probably dial the boost back a little as a safety margin.

 

Thank you for the feedback! Yes in some ways I can see why stock pistons and forged rods are the 'sweet spot' for a daily driver. No cold piston slap issues to deal with and in theory, good longevity (although I believe the supertech pistons and their 4032 alloy are better at avoiding the thermal expansion issues of forged slugs).

There's a few reasons I'm aiming for 300whp and not much more. I'd like to run my EFR 6258 in its maximum 76% efficiency zone around 17-18psi, by which point I suspect I'll be limited by the quality of my pump gas anyway... perhaps a max of 320whp. I'd also prefer to keep my 6 speed in one piece and not make it a consumable, so I intend to keep torque below 300wtq.

All of which seem to point towards stock pistons being viable and even sensible for my power goals... am I doing this right?

 

I made 299 with uncontrollable boost creep on a ebay turbo. After that, the rubber cap for the 94-97 idle valve blew off the intercooler pipe, giving us a perfectly controlled boost leak and 200hp. Tuned it like that, customer was very happy.

 

You are basically building the same thing I was running for a while, except I was running the 8.whatever Supertechs. The 6258 will have no problem meeting your goals and spool will be nearly instant. For me, the limits of CA91 pump gas and the cost of 6 speeds kept my ambitions in check.

The only reason it came out of the car was my lack of skill. As in, I don't have enough seat time to be comfortable in the car on track at that power level. A 300hp Miata is rowdy as ****. If you are planning on doing trackday things there is no reason you can't have a 200-225hp tune for a bit less... drama. You don't have to run ALLOFIT all the time, boost control and table switching are actually things

Probably? Lots of people making power on stock pistons. You won't have the extra margin of error that a forged piston theoretically provides, but that's all just part of the experience
Was the customer also happy after the motor exploded from FOD through the hole in the pipe, or was it when the turbo overspun itself into oblivion?

 

Yes exactly. We get 95RON pump gas here in the UK which is ≈ CA91. We also have 98RON ≈ US 94. Then there's 99RON ≈ US 95... but at $7.73 per US gallon, it gets a bit painful.

I'm leaning towards the 8.6 supertechs to give myself a better shot at MBT with the higher octane fuels. Does anybody here actually achieve MBT? My understanding is 95% of pump gas turbo engines are knock limited.


Having different tunes for the street vs the track makes a lot of sense. I'm a novice track driver and judging from videos I've seen of 300whp miatas doing WOT pulls, I can easily imagine that being a handful on track!



Can one truly call oneself a miatatrubo enthusiast/autist if they haven't bent rods on multiple axes and cracked pistons in half?