Miata Turbo Forum - Boost cars, acquire cats. - Question about connecting rods

- Engine Performance (https://www.miataturbo.net/engine-performance-56/)

- - Question about connecting rods (https://www.miataturbo.net/engine-performance-56/question-about-connecting-rods-109245/)

is the damper necessary... yes... IMO more important than oil pump gears. BP crank flex is a problem.

Bearings? Let the machine shop buy them.

Pistons? Let the machine shop buy them after measuring and determining how much, if any, your bores need to be opened up by.

MOST pistons come with rings. Machine shop will gap the rings for your application.

Can I tell them what bearings I want, example from ACL? Same goes to pistons (if I do them at all), connecting rods, etc. If possible am I able to ask for certain gap specs?

Lower CR pistons will let you run more boost and/or more timing without detonation, and therefore make more power safely when in boost. If running only e85 it won't matter but on pump gas it does.

If you want to limit timing and therefore make less power and be forced to run less boost, then a higher CR piston is the way to go.

A forged piston is stronger and will tolerate the occasional mistake in tuning or fuel quality a little bit better. Either forged or cast can only take so much though.

Would you recommend 9.5 or 9 CR?

9 will allow more timing and boost if detonation limited by pump gas. That would be my choice, if all other things are equal.

Quote:

Originally Posted by Dokii (Post 1648381)

Can I tell them what bearings I want, example from ACL? Same goes to pistons (if I do them at all), connecting rods, etc. If possible am I able to ask for certain gap specs?

Have you ever walked into a machine shop and talked to them?

MOST machine shops have a preference for this or that brand of various parts. Im getting ready to have 2 LS motors done and have gone into two machine shops with a list of desires/questions to get their take. What I want is NOT set in stone, its simply gathering opinions and seeing who I want to work with. Both shops when I asked about King bearings said "they arent great right now, not sure what happened, we prefer xxxxxx brand"... both were different brands. I told them everybody offers a coated bearing and thats what I want, but beyond that I am not to picky as both brands were known to me to be very high quality and I would be happy with either.

As far as clearances, again machine shops will have their opinions based on what you are doing with your engine. If its 99% street car that would lead to different clearances vs. a drag car vs. a road course car. Does it have nitrous? different clearances... just a turbo...again different clearances.

Dont use a machine shop you dont trust and go in before hand to ask questions and let them know what you want.

The first engine I had done I didnt have a clue about the finer details and just dropped off a box of shit. While I dont think the machine shop did bad work at all, I wish I had known a bit more about the process, parts selection, etc... While they knew what I was using the car for, we didnt discuss much of anything outside "basic rebuild" as the BP motors in stock form tend to last pretty long on the race track.

Quote:

Originally Posted by Fireindc (Post 1648363)

I'll chime in here, just because I went through all of this about 5 years ago when I rebuilt my motor.

I ended up going with stock pistions (bp4w 99-00, IIRC 9.5:1), stock OEM VVT oil pump, ebay rods, NPR rings, and ARP everything. It was a budget build with the intent to not go much over 300whp on e85.

The car made 308whp on e85 (2860rs) and has been dead reliable, including lots of track use. I also DD'd it for a winter and didn't feel as bad about sub-freezing cold starts, short drives to/from work, etc. I think this is the benefit of a stock piston setup, you can still use it like a stock car.

If I were to do it again, now that I'm making more money, I'd probably just go fully forged. But like I said I have no regrets of going the way I did. IIRC I had less than $1500 (parts + labor, including having the machine shop actually assemble the bottom end and head) into this motor and it's been a beast. I don't remember the tolerances, but I told the shop (a well known spec miata shop in the area) that it would be a turbo motor targeting 300+whp, and told them to build it to the "loose end of OEM specs".

My reasoning was that I didn't want to go down the rabbit hole that comes with >300whp miatas. The whole drivetrain swap thing, the idea was to keep it on a 6 speed. I.E. ~250whp track, 300+ street type of car. And it has done exactly that for the last few years (and 30k+ miles) reliably.

I love hearing this actual experience. I struggled to find anecdotes like this when I was researching. I also think this is really sound reasoning. Going too much over 300 HP brings a lot of money and work once you start looking at swapping transmissions and/or diffs. If that doesn't seem like a path you want to go down (which it doesn't seem like OP does), then your stock pistons are probably going to be fine, especially if you are cognizant of that with your tuning.

Also to OP, get on facebook and look at some of the used Turbo Miata Parts pages. The Buy/Sell page on here has good deals sometimes as well.

While probably not the norm, I was able to pickup Flyin Miata pistons ($800), a BE stage 3 Oil Pump ($800), ARP Head+Main Studs (~$100), Cometic Head Gasket ($100), Main Bearings($~50) all brand new for $800 shipped. I also got a fluidampr + trigger wheel for ~$400, Flowforce LS Coils for $100 and a Skunk2 TB for $100.

Some of those have needed a bit of work (LS Coil wires are a little janky), but if you're trying to save money then that's not a terrible resource to have.