NB8A, BP4W engine rebuild

wydra · Postby **wydra** » Thu Mar 26, 2015 11:13 pm

Hey guys,

I turbo'ed my standard engine a while ago now and have been running around without too many issues. Last year I blew a coolant hose and had the head gasket go so I had the head replaced/rebuilt, ARP head studs and a new MLS OEM gasket and it has been going great since. On the weekend at the track it got a bit warm after consecutive laps and am now down compression on Cylinder 2 (you can feel/hear it when starting the engine, rough idle and confirmed this with a compression test.)
I put a scope down the plug holes and it looks like I have had some serious detonation and the piston has started melting/pitted.

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So I have decided to pull the car off the road (after this weekend) and rebuild the engine. I enjoy doing as much as I can myself and best bang for your buck.
I have been scouring the forums trying to gather as much information as possible but have a few questions and would love some feedback from people in the know.

I am going to replace the rods and pistons with forged units and replace any other supporting hardware that is required.
I am going to drop the head back off to the machinist to give it a look over but this was only done a few thousand km's ago so it "should" be fine. All the timing gear, water pump, oil seals etc are also near new and wont need replacing.
So things I know I need to purchase are, pistons/rings, rods, main stud kit, full bearing kit.

Pistons: what do people recommend (that are best bang for buck)? Supertech seems to be a reasonably priced set but have a lot of variants to choose from. Is it best to try and keep the stock bore size of 83mm (if the block is within specs) and just re-hone or bite the bullet, get it re-bored to 84mm and matching pistons? Also what comp ratio would you recommend? Standard 9.5:1 or drop it slightly to 9.0:1? Is it best to have a ceramic coating on the top or not really needed?

Rods: The cheapest seem to be the Manley or Eagle H-beam rods. Does anyone have any info on the EBay rods? http://www.ebay.com.au/itm/For-Mazda-MX ... 3390e1cd4f
They are using ARP studs and "look" ok, or is it just worth getting Manley or Eagle? or something else?

Head gasket: I assume I cant reuse the OEM MLS head gasket I currently have even though it is very new? but I will be reusing the ARP head studs. I know the OEM gasket is great quality, is the Cometic a better option to go with?

Bearings: I'll get a full set or ACL bearings. My question here is, being the engine isn't knocking and I wont be doing any machining to the crank, do I go the H or HX in the standard size?

Main Studs: This should be straight forward, get a ARP main stud kit and torque them to ARP's recommendation.

Oil Pump: How are the standard oil pumps on the BP engines? Is it worth replacing or should the one in there be ok as long as there are no signs of wear?

Anything else I have missed or forgotten about?
My biggest decision is what I do with Piston size, comp ration and block boring or honing.

Postby **NitroDann** » Fri Mar 27, 2015 12:46 am

Email fab9tuning. Tell them what the go is, and give them your credit card number.

Honestly.

Dann

wydra · Postby **wydra** » Fri Mar 27, 2015 1:47 am

Cheers, I have emailed them and waiting for their reply.

Do you have any recommendation with the bores? Is it best to try and keep as much meat as possible on the walls with a hone (if possible) or bore out to 84mm?
And compression ratio? running a GT2560r @ 1bar+ on petrol 98ron.

And how are the oil pumps on the BP's with low RPM?

Magpie · Postby **Magpie** » Fri Mar 27, 2015 6:38 am

I would solve the detonation issue first as new parts will not fix it.

Postby **NitroDann** » Fri Mar 27, 2015 7:41 am

I assume he is blaming that on overheating while racing.

Leave the pump if you won't rev it. Sticking with 98 for certain? 8.6:1

Do not tune to a specific boost level, use your ecu to control boost and tune to a specific torque level.

Dann

sailaholic · Postby **sailaholic** » Fri Mar 27, 2015 7:42 am

Your going to need a full seal kit again. Never reuse seals / gaskets.

Also new crank nose bolt and key.

Sent from my iPhone using Tapatalk

hks_kansei · Postby **hks_kansei** » Fri Mar 27, 2015 8:31 am

You sure that's the piston being pitted and not just carbon deposits on the piston crown?

it's hard to tell on the pic, but if it's just deposits on the crown it should be fine.

zossy1 · Postby **zossy1** » Fri Mar 27, 2015 8:56 am

I appreciate your enthusiasm... But your questions don't fill me with confidence in your ability to do this. Still I wish you luck.

If the car has got hot again, have the head checked for hardness and for straightness. Do a leak down test before you pull the head to make sure the valves aren't leaking in any cylinder. This is important.

You will need to pull the bottom end down and mic the crank before you order bearings. Do not "guess" what bearing size you need.

Finding forged Pistons in stock 83mm bore size is not easy. You will need to go to 84mm+ in all likelihood.

Dann is right about 8.6:1. That's what I'd use.

Ebay rods are fine. There are one or two horror stories about failures but the for price they seem to be great value.

Buy a new full engine gasket kit. It will cost $250, that is not going to break the bank

Do not neglect the head. If you don't know what you're doing, take it to someone who does.

Magpie · Postby **Magpie** » Fri Mar 27, 2015 9:31 am

As I mentioned, find the cause of the piston damage, the OP mentioned melting and this would be caused by (as stated) pre-detonation. What type of fuel were you using, I assume an aftermarket ECU?

If you fix without finding the root cause then you will only destroy the new parts (rinse and repeat).

Pulling the head off will confirm the OP's thoughts, that is melting/pitting. If as zossy1 suggest it is carbon buildup then ignore my thoughts.

Another thought is that the loss of compression could also because by valve seats being worn or valve issues, The BP is a non interference motor so a valve that is not closing properly (or worse a snapped timing belt) will not result in the pistons impacting on the valves.

The loss of compression in my BP was caused by worn valve seats. Instead of just fixing the valve seats the root cause was looked at and a solution developed. Will know soon if the solution works, bonus is the car should make more torque in the mid-range thanks to the solution!

wydra · Postby **wydra** » Fri Mar 27, 2015 1:47 pm

Thanks for all the feedback.

Just to be clear, the head was rebuilt a few thousand km’s ago and when I take it off the engine, I’ll be dropping it back off to the machinist to inspect and clean up. If there is anything wrong with it since the last rebuild, I’ll get him to fix that up as well. So this discussion is only for the bottom end rebuild.

Nitrodann, I love the idea of E85 but over here in Perth, there are only a couple of places that sell it and nothing near my house. For now I have to be locked into 98.

Magpie, the damage was caused by detonation after running too long on the track on a warm day. I have a KnockSenseMS but it didn’t pick up the event correctly and didn’t pull any timing. I will be looking into why this didn’t work after the rebuild but for now, I can get the engine repaired/rebuilt and running without too much worry. I will 100% be making sure the issue that caused it to heat up is resolved before I get it back on the track for a long session.

Hks_kansei, it is possible and I also thought this might be the case at first. I gave the pistons a good clean last time the head was off so it is unlikely there is that much carbon build up since then.

Sailaholic, noted and added to the shopping list.

Zozzy1, thanks for the vote of confidence

. This is the first BP engine I have rebuilt but I have built a couple of SR20 engines in my past and they turned out well. I will have the crank measured before ordering bearings and will be aiming for 83.5mm pistons (and have the block honed out to this). I am running fresh 98 ron Petrol and a DIYPNP MS2. After I get the engine rebuilt and running again, I’ll be taking it back to the Dyno/tuner to go through the ignition and fuel maps again.

So far it looks like my shopping list will be:
Supertech 83.5mm pistons with Wiseco XX rings (yet to decide on the 9.5:1 or 8.6:1 ratio)
Manley H-beam rods
ARP main stud kit
ACL RACE bearings in HX size
Cometic head gasket .040
New woodruf and crank bolt
Seal kit

And I’ll be getting the machinist to confirm the head condition and clean it up ready for re-install. I’ll take the block in and make sure the bores are within spec and can be honed out to 83.5mm and have the crank measured up for the bearings.

davekmoore · Postby **davekmoore** » Sat Mar 28, 2015 7:26 am

For the avoidance of future financial pain, ensure you or your builder use the correct combination of OEM seals, sealants and gaskets. If not doing it yourself, check out the reputation of the builder on here including checking if they've done heaps of MX5s as there are things on these cars that have to be done differently from on others.

Magpie · Postby **Magpie** » Sat Mar 28, 2015 8:19 am

Off topic, but how was the car tuned? If an increase in air temp caused the knock then that would suggest it was already very owe to being too lean. Also what sort of fuel?

You should also consider using E85 as part of your build due to its resistance to knock.