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I'm trying to map out a (potential) rebuild of my 195k mile BP6D VVT engine that's seen a fair amount of track time. The rebuild was triggered by persistent misfiring during warm-up, which after a bunch of diagnostic work drove me to stick a borescope down the cylinders. I found clear signs of water getting in the cylinders causing rust, but I have no idea how or when it would have got there in my 9 years of ownership. Anyways, it's out of the car now and needs a rebuild. I bought a used JDM engine as the easy button to replace it, but it has some of its own issues. I figure if a machine shop is going to get involved and the crank is coming out, I'd prefer to spend that $2k as a starting point for rebuilding my current engine.
I don't want to skip over upgrades that make sense, but also not fall for every "while I'm in there". This car has been completely reliable and I want to give it a refresh so I can target another 100k+ miles and/or 100hrs of track time. I'd like to run on a stock ECU at first before potentially moving to a Megasquirt or BMM ECU. Redline will stay at the stock 7,200 fuel cut. Boost is not in my plans.
For the block, I expect to have a local machine shop I've come to like overbore the cylinders, polish the crank, size the main and rod bearings, check/adjust ring gap, and hot tank everything along with cleaning the oil passages.
Pistons: That'll depend on the overbore and head skim required to clean everything up. 9.5:1 Supertech forged pistons and a skim to get to 10:1 CR seems most likely, but oversize 10:1 USDM or 10.5:1 JDM pistons could be an option. Being able to run 87 is neat, but I can hear the engine grumbling on low octane and will give that up after a rebuild. I don't want to increase my compression so much that I have to run 93 at all times. 91 minimum octane would be fine. If I install Supertech forged pistons, I expect to delete the oil squirters.
Rings - Wiseco for Supertech pistons, OEM for OEM pistons.
Oil pump - Debating my options here between a new OEM pump, Boundary gears in my current housing if wear is acceptable, or new OEM pump and Boundary gears in that. Some people seem to have issue with the relief valve sticking on the full replacement Boundary pump and it's not made with an OE casting, so I am leaning away from that. Once the engine is in the car and running well, I'll consider upgrading to a Fluidampr harmonic balancer for extra insurance.
Main and Rod bearings - I'm not sure on the choice of OEM, or ACL for long term durability on a car that sees most of it's starts commuting to work? I know the VVT engine can only use OEM for the thrust bearing.
Rods - Since I'm not planning on boost, I wasn't planning on forged rods. K1 forged rods can be had for only $370 though, and that's kind of tempting if I get forged pistons.
For the cylinder head, I'm hoping I can do most of the reconditioning at home and no valve guides will need replacement. I just want to replace the springs and seals, and lap the valves. That'd leave the machine shop only needing to check flatness and possibly skim the head, and clean it up in the hot tank and blow out any oil passages. I could also have them check the valve guides there. I plan to adjust valve clearances myself.
Valve springs - I know these should be replaced after so much track use. I have heard stiffer springs can create VVT control issues on the intake cam, and on the other hand that valve float is not far off either. BP6D-12-125 seems to be the intake, BP6D-12-114B is listed on parts fiches as the exhaust, which takes me to part BP2Y12125A. Per Supermiatas engine build guide, NA8 valve springs have a slightly higher seat pressure and appear to be identical on intake and exhaust. Would there be any issues running BP05-12-125 springs all around? Cat Cams PAC-S90015 also have come up in my research. Or is the price difference here too small to run anything other than Supertech singles? As long as they're new springs good to control valves to redline, and my VVT will be happy, I'll be happy.
Valve stem seals - Uncertain. I'm leaning towards Mahle off Rockauto, or OEM. Supertech seems to have had issues in the past.
Valve lash shims - OEM, as needed.
For basic supporting mods, I've already got oil pan baffles. I've never had a problem with coolant temps measured via OBD sensor at the back of the head running a well sealed Koyo 36mm radiator and the VVT head gasket, but since it's getting replaced I might consider going for a '94-'00 gasket and a coolant reroute.
While I know engines fairly well and have torn down several, I've never built one back up. I'm trying to gather as much info as I can as getting the details right on this has intimidated me from building an engine in the past. The info sources I've found most useful so far are below.
Since I know I'm not that special of a snowflake, I've also read over a few engine building threads, and experiences with some of the parts I've listed above, grouped by topic.
For background, here are some pictures of the potential victim. It's a 195k mile BP6D VVT engine that's original to my 2001. I've owned it for 9 years and put on 75k miles. It started occasionally misfiring if left idling during warmup a few years ago. Since then it has gotten worse and worse to the point it will always misfire unless you drive hard immediately after startup, which I don't really do. Changing plugs and coils did not fix it. Fuel injectors were cleaned and flow tested, no fix. When I compression tested last year, it showed 185 PSI across all 4 cylinders. Once hot, it ran well, but was starting to smell like it was burning oil. No one ever reported smoke behind me on track and the bumper stayed clean. I have an NA6 oil pressure gauge in it. The readings are a bit lower than they were 50,000 miles ago when I first installed it, but I don't know how much of that to attribute to the engine wear and loosening up, vs a 35 year old analog sensor and gauge.
This is the cylinder condition I found with the borescope that prompted me to pull the engine for repair or replacement. I'm a bit confused by the compression readings vs cylinder bore condition, but it is what it is.
Here's what it looks like under the valve cover. Based on the oil varnish being present on some cam lobe base circles and not others, I assume it needs a lash adjustment and that may have been a major factor in the engine running rough. I had planned to do that as a next step, until I saw the cylinder bore condition.
Here's the #3 main bearing half, cap side. It's the only one I've pulled. The crank could use a bit of a polish as it had a small line in it matching what you see in the bearing, but there's no terrible wear or copper showing through on the bearing. Nothing that caught a fingernail.
And a rod bearing, #2 or #3, I forget. Again, I just pulled the one.
I don't have much to add on top of the research you've done. I will say forged motors are fairly loud during warmup. 4 years in and I still think a rod is about to launch out the side in my Jeep's forged engine until it's warm. If you decide to go with a forged piston consider adding skirt coatings to help with the noise. I believe SuperMiata mentions that in their guides too. I've also seen GM does ignition advance modifications to reduce "piston slap" during warmup in the LS7, although someone else may have more in-depth info than me on that part and you would need an ECU with 4D/5D ignition tables.
Definitely interested in what you decide to do and learning from you on this. My vote is for JDM 10.5 pistons and 91/93 octane.
I don't have much to add on top of the research you've done. I will say forged motors are fairly loud during warmup. 4 years in and I still think a rod is about to launch out the side in my Jeep's forged engine until it's warm. If you decide to go with a forged piston consider adding skirt coatings to help with the noise. I believe SuperMiata mentions that in their guides too. I've also seen GM does ignition advance modifications to reduce "piston slap" during warmup in the LS7, although someone else may have more in-depth info than me on that part and you would need an ECU with 4D/5D ignition tables.
Definitely interested in what you decide to do and learning from you on this. My vote is for JDM 10.5 pistons and 91/93 octane.
This is probably a stupid question, but what is a 4d/5d ignition table? Couldn't you just have your base ignition table and then have it advance the timing during warmup based on temp? Isn't that pretty standard for most ecus? Is that actually what you mean by 4d/5d?
This is probably a stupid question, but what is a 4d/5d ignition table? Couldn't you just have your base ignition table and then have it advance the timing during warmup based on temp? Isn't that pretty standard for most ecus? Is that actually what you mean by 4d/5d?
It's a good question. Yes, warmup is standard across ECUs, but not what I mean. I'm speaking about an extra 3D correction table done on top of all other tables. Say if you want to change your timing based on barometric pressure, that's 4D, and if you further add one for oil temperature, that's 5D, add FlexFuel blending, that's 6D.
The "piston slap" table from my comment above is only enabled during warmup as well, so that just gets muddy, but I would still call it 4D for our current aftermarket lingo.
The OEM rods are forged steel and not bad, but the main issues is that they are heavy and the cost to rebuild them with new hp bolts starts to approach the cost of new after market rods. Forged pistons do have some short comings, but they will increase output and are worth the cost. The material offers thermal advantages, they are lighter and they use thinner rings. There is no Wiseco special rings. There is only one PN that NPR is supplying to all of the companies selling pistons, Wiseco, JE, ST/Malhe and CP etc. I would consider a skirt coating even though the ptf (xylon 10-10) coating is sacrificial but it does reduce some of the low speed, low temp noise. I would use the 10.5:1 ST pistons since the ratio is based on the head and block being surfaced. Remove the least amount and it will ok on 91+ oct.
From my experience the best option for an oil pump is the Boundary gears in a new OEM Mazda pump. It is also the most expensive option, but the heart of the engine. The crank should be ground, not just polished. The OEM factory bearings are better quality than the aftermarket. They are more expensive and oversized options might not be readily available. I have used the ACL and King bearings often. My preference of the two would be the ACL race.
The cylinder head should be completely gone through. Cams will make more power, but I find they are expensive and a lower value than other components. I recommend investing in performance valves which increase flow because of the undercut stem and more ideal head geometry. Increasing valve area will make more power. 1mm over is to restore the valve height with min area perf gains. A 2mm increase on the intake with mild porting is a good option. If you plan to run to 7.5krpm+ you should use the heavy double ST springs, under 7.5k the lighter doubles can work. Shim under bucket lifters weigh less and can improve upper rpm output. Use an OEM head gasket.
If you don’t have emission testing in your state I would use a 4-2-1 header and a mega squirt. More timing during warm up will help to put more heat into the piston to reduce the clearance. The risk is cold seizure. There is no substitute for an increased warm up interval.
High mileage N/A BP6D rebuild - street and track
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