So, what rods?

[sabretooth]

I'm expecting my Wiseco forged pistons this week ($725 - booya!) and so should really start with rod selection. So far, my short list is:

• Standard. Apparently capable of 20psi.
• PAR. Excellent quality, strong, but horrendously expensive.
• Crower. No idea about these, but they were suggested.
• Carillo. High quality, strong, and expensive. Especially the H-beams!
• B6T. Prices start at $150. Strong, but I don't know what sort of boost they'll cop.

I want this thing to be reliable at 13psi to start with, and then I probably won't push it past 16psi or so.

Can I get some experiences and recommendations?

B6T rods are cheap, but how much boost will they cop?

[ampz]

I've been told 2 weeks @ 27 pounds will 'bend' a rod... it was replaced with a standard rod again, and the engine is till going on less boost obviously in a 1600 mx5.

My experience has seen reliable 16 pounds on a standard tx3 92 engine (intercooled) and i still know the current owner, who runs the same 16 pounds of boost as sold to him 5 years ago.

I'm unsure if the rods are the same between the two blocks though...

[sabretooth]

Apparently B6T rods are 40g heavier than stock rods - but I don't even have a figure to compare that with as I haven't been able to weigh my stock rods (no scale, let alone one that's accurate enough). So percentage-wise, I have no idea if 40g is significant or not.


BPT and MX-5/Miata rods are the same. It's the BPT found in the GTR engine that has the beefier rods.

[sabretooth]

Yup, seen that pic a few times now, thanks.

On another forum I've also received recommendations for Eagle and Crower. Research time, methinks.

[The Pupat]

I reckon eagle are good enough, it's not like you are going to be pushing any boundaries at 16psi you want, hell you could probably run stocko's but I imagine like me you've got the whole if it's already apart I might as well just buy the better ones now and be done with it and it'll never fark up.

[sabretooth]

I've managed to find some rod weights:

• 323 GTX: 580g
• Miata: 562g
• Crower: 543g
• Pauter: 493g
• Oliver: 465g
• Carrillo (A-beam): 416g
• Carrillo (H-beam): 440g

That varies a bit from the "40g heavier' that I had heard the B6T rods were when compared to a stock MX-5 rod. Then again, that list says "323 GTX" and not specifically B6T - so I'll have to do some more reading - that could even be the now-superceded BPT rod, or the rare Familia GTR rod. Apparently standard rods are good for 20psi (see below for the reference) - so if B6T rods are beefier, then they're going to be more than ample for my needs. An xray, shotpeen and polish should get them at the spec I want. I'm not increasing the rev limiter, so my choice of rod isn't as critical here while chasing power figures involving <20psi.

I believe I've quoted this before, but for everyone's benefit, here it is again (from the archives):

Having tested B6 rods personally on a proper conrod axial loading equipment (at cyl pressures~20MPa), using MEASURED cylinder pressures, and measuring their cycle fatigue life, STD Miata rods are MORE than ample for 20lbs of boost, even at the standard comp ratio! If it's a boosted engine your building, you need to have ample BUCKLING STRENGTH (i-beam), to resist the high gas loads (compressive forces), whereas a high revving NatAsp engine requires relatively light but stiff rods (h-beam) rods, but with high beaming/bending strength, to resist the very high bending moments induced by the ineretia loads of high rpm operation. If you're not revving it much above stock, don't waste your money of $$$ rods...just use stockies. Carillos and the like are generally high-rpm rods, H BEAM, and offer not a large improvement in buckling strength over stock with big boost......they are VERY easy to buckle with high gas loads. Much better are 4340 billet H-Beam rods...we have used PAR rods from sydney, and they are AWESOME (and reasonably priced).....beuatifully & accurately machined....ask for Sam or Norrie and they can make with long rods more likely to buckle, they were perfect at 20lbs and beyond. They are used in WRX 2.0 litre engines with up 45lbs boost (and running 9secs!!). Cannot speak highly enough of their engineering prowess, and their helpful nature.

BPT (TX3) rods have been superceded by Miata rods, and have been for some time. Bolts are the same part. Bolts are the most important aspect of the rod for high revs...much more important than the rod design (except its more important to get a LIGHTER rod & PISTON for higher RPM...inertia loads go up with the CUBE of speed)...if you want to see great rods, pull down an M3 engine, virtually any Merc of recent times, and most particularly, 20V toyota (esp. black top AE111). S2000 is another i've seen with much more attention paid to redcuing mass rather than beefing it up. Boosted engines are the only ones that really can "bend" a rod...NatAsp engines at high rpm fail rods due to fatigue life of the beam/big end interface (tfatigue stress higher than endurance limit) or the bolts stretch (yield) and then its really all over!!

cont...

[sabretooth]

Unless you building some 9000-10000rpm monster (and be serious with yourself), and planning to run it at those revs ALL THE TIME (as in for long periods), carefully checked std rods are more than ample.

Regarding cast/forged...most people on the net seem not to know the difference between cast/forged carnks, rods, etc, even if you hit them on the head with one..........B-engine rods DEFINITELY forged, only other mainstream process is powder metallurgy (MUCH different to cast), such as many new euro/us engines (and MZR 2.3 & Escape V6!!). No modern mainstream i know of runs "cast" rods.....no mater how big the parting line on the side of the rod looks, they are FORGED, Baby! Endurance limit of the alloy required for castability would be hopeless for rods....many cranks are cast, but strong, being nodular iron. SG iron easy for mass prod, but not as good as NI (or steel).

Just a few comments on an immensely complicated subject that everyone is an EXPERT on!

A lot of good information there. I'm going to enquire about the B6T rods I've found, and see if they've been peened/polished/x-rayed. I'd be silly to pass them up if they were suitable for my requirements.

Now, apparently the rod bolts are extremely important here. I'll be sure to get something like ARP bolts. I probably will go ARP - they seem to be the status quo.

xtr-33m - I've found a guy already who is selling some, but if that falls through I'll get in touch. Thanks. As an aside, what sort of checks/cleanups have yours had?


Pupat, I can't seem to find Eagle rods for BP engines. I know they make them for Hondas - got any info?

[The Pupat]

Pupat, I can't seem to find Eagle rods for BP engines. I know they make them for Hondas - got any info?

Oops I just assumed they would make them since you made reference to them as well. But I looked in the catalogue and they aren't in it. An option could be the email them the dimensions of the B6 and see if they have something that matches otherwise it might be easier to just get another brand.

[Sean]

Pupat, I can't seem to find Eagle rods for BP engines. I know they make them for Hondas - got any info?

Oops I just assumed they would make them since you made reference to them as well. But I looked in the catalogue and they aren't in it. An option could be the email them the dimensions of the B6 and see if they have something that matches otherwise it might be easier to just get another brand.

Have you considered ARGO?

ARGO Marine are well known in the performance marine industry and produce rods capable of supporting over 40psi.

They are tooled to make MX5 rods already and sell them for around $1300 a set. As a bonus, they are about 15 minutes drive from my new place

[sabretooth]

Oooohhhhh yeah. Giggity.

More pics will be up tonight.

Oh, and after a bit of reading, I've found that B6T rods will be mode than sufficient for my needs and power goals. I've made an offer on the set I found for $150. I'll xray, shotpeen and polish them and they should serve me quite well. If I were going to upgrade the rev limiter (which I haven't got an intention of doing) then I'd get lighter rods due to the extra forces on the rods at higher RPMs. Other rods just appear to be too expensive for what they are and for what I'd be using them for. If I were to push over 21psi then sure, I'd be inclined to get Carillo H-beams or the like. But for my needs the B6T ones should suit nicely.

[sabretooth]

Piston pr0n, for those interested:







First thing that surprised me was the valve reliefs. They're rather big! I've also taken pics of the other parts of the piston for interest's sake - the size of the skirts, the various holes, that sort of thing. The piston kit included the wrist pin, the locks, oil rings, a compression ring and an oil ring separator.

[rhysk]

I would be inclined not to put them in the engine. They belong in some sort of display cabinet.