con rod choices

Hi everyone

I'm currently building an engine that will be capable of very high RPM (9500+) and I'm in two minds about conrods and would like some other opinions about them, the engine will be a drag only engine and naturally aspirated on E85 (maybe methanol later if it needs it) so high comp long rod build with an experimental high flowing cylinder head.

The first question I have is weather I use an alloy or steel rod, I have done research and can't come to a conclusion about which one to pick, I am not opposed to replacing the alloy rods as I know they have a life span but being a naturally aspirated engine is an alloy rod needed or would a steel rod be preferred in this type of application.

The second question is about rod journal size, I have 2 options in rods to choose, both are the same length but one uses a Honda k24 (48mm x 19.7mm) size journal and the other conrod uses a Mits 4G63 (45mm x 26.3mm) size journal, my question is would a smaller diameter but wider big end be better or worse than a bigger diameter but narrower big end.

Thank you in advance and I will be more than happy to answer more questions about the engine.

Kind regards

Anthony

I've put together Honda H22 engine with exactly the same intends. My choice was alloy rods since the engine is ment to rev very high. Torque in NA engine isn't that big so you don't need steel rods to hold it but something that is light and strong enough to withstand inertia. So alloy rods are perfect fit. The only downside with them is that they don't like Rev limiter as the bolts might come out of threads when constantly hitting the revs limiter so I disabled it in tune file.

I'm attaching the picture from D. Vizard book explaining the importants of weight reduction with hight RPM engine - please check it out.

I've put together Honda H22 engine with exactly the same intends. My choice was alloy rods since the engine is ment to rev very high. Torque in NA engine isn't that big so you don't need steel rods to hold it but something that is light and strong enough to withstand inertia. So alloy rods are perfect fit. The only downside with them is that they don't like Rev limiter as the bolts might come out of threads when constantly hitting the revs limiter so I disabled it in tune file.

I'm attaching the picture from D. Vizard book explaining the importants of weight reduction with hight RPM engine - please check it out.

Thanks for the reply, I don't really plan on holding the engine on the rev limiter for to long, just on the start in line for launch but even that will be kept to a minimum, so it sounds like an alloy rod might be the go to rod for this kind of engine unless someone else chimes in with other info, so now I just need to figure out the journal size dilemma.

thanks

Well, once again - the lightest that will withstand maximum RPM is the choice. If the weight of both rods is the same wider bearing is better.

What engine is it? Stroke, piston weight/mass and, to a lesser point, rod/stroke ratio, etc?

For some engines, 9k5 isn't "that" high, but others?

I'd suggest making a list of your options, such as piston weight and single/dual ring, Gudgeon pin options, such as lightweight taper, "short" rods put more tensile load on the 'rods, etc.

Oh, what is the cranl pin width/clearance, as you may need to figure out some means of keeping the rod centralised in the piston and on the journal?

The engine is a Holden 202, I didn't say it ealier because I always get the "why bother with that engine" comment but stroke is 82.5mm x 93mm bore, rod to stroke ratio is 1.82:1 with a 150mm length conrod, piston weight is 294g but not sure with rings and pin as I haven't measured it yet.

The pistons have 2 comp rigs and 1 oil ring as per normal but the comp height is 25mm so the pin hole is in oil ring.

The crank pin will need to get machined regardless for the rod width so I can accommodate either rod width and have the rod guided by the crank.

Cheers

That's an interesting choice of engine, is there a head and cam' combination that will actually make it worth winding that high? Oh, and crankshaft that will tolerate it, even with the best harmonic damper? Been around 40 years since I had one apart, I think it had seven mains, but if only 4 it further complicates things for balancing.

Unless you're focused on pure power, for class rules, etc, these more unusual engines can be rather cool to work with. I guess there's a lot of checking out swap meets, and the like, for things like YellowTerra (sp?) heads, rockers, and other top end components.

If you're not one already, might be a good idea to get friendly with a GOOD machinist, as I expect there'll be a lot of custom parts for it. I would also expect there are many performance parts from other engines that will be close enough to be adapted for use, as you're doing with the pistons and 'rods.

In the meantime, the more info you can find on the weak points, the better, because there may be something you just can't work around without a LOT of work?

The head will actually flow enough for more rpm than I am going to rev it with the correct cam, that being a solid roller.

They are a 7 main block and there is no aftermarket crank for them unless you buy a China one which I don't want to do but the factory late model cranks are fully counterweighted and are extremely strong for what they are, I have made a custom hub to accept an ATI damper off of an internally balanced SBC with a Barnes 6 stage dry sump running off the front.

I know all the weak points for the engine and have hopefully either resolved them or will move past the rpm range where they are a problem like the harmonics of the engine.

The only issue I'm having now is deciding on the conrods to choose with the alloy vs steel and journal size differences but aside from that I think I have everything else covered...........well I think I do at least.

Aside from the block itself I don't think there are any factory parts left on it to be honest.

The pistons were a custom job, there was no off the shelf combination that i was happy to run so it was either get a custom piston or custom conrod, I chose to get a custom piston to i could squeeze the longest conrod I could in the engine.

As to the why i am building this particular engine, I just have a soft spot for them, had one in my first car and have always wanted to build one to see how far I can take it with no limitations and this is my chance now, plus I'm a sucker for cranky naturally aspirated engines.

cheers

Oh, never got round to your actual discussion point.

I would go for the 48mm x 19mm pin option, because it will leave more material in the cheeks between the mains and the big ends, which should result in a significantly stronger, and stiffer, crankshaft.

Weight, or rather mass, is usually put forward as a reason to use them as well as they're usually cheaper.The alloy also has a 'dampening' affect that will help protect the crank and bearings from MILD detonation.

The disadvantages, though, are they need to be much bulkier, which may require the sides of the block and sump to be clearanced, but also the bottom of the bores and I don't know if that's a possible issue with coolant passages in the block - if you're grouting the block that won't be a problem, though.

The other disadvantage is fatigue life, as with any aluminium alloy it will work harden and eventually there will be a failure - on the positive side of that, the engine will see limited use and the rods can be "lifed" and changed regularly - the manufacturer(s) should be able to advise there.

There is a third option, titanium, which offers the strength and durability of steel, and the lightness of alloy - they are rather more expensive, but I think companies like SAENZ (sp?) offer K20 and K24 rods in all three materials.

All things considered, I would probably opt for a good set of steel rods, and the weight difference may be less than you expect, especially if you make sure the supplier knows they're for a N/A engine, as the design won't need to account for the compressive forces the forced induction/Nitrous engines will be subject to.

Before you make the call, I'd suggest contacting the 'rod manufacturer(s) with the piston assembly weight, rod/stroke ratio, and anything else you can think of that's relevant and see what they advise.

Bottom line, though, is it's your call - do you have a build channel, I'm rather intrigued and hopefully will be around to see it running. Had a quick check, and you don't seem to mention what it's going into?

[edit] Forgot, there may also be higher spec' material, but lighter, gudgeon pins available for the piston, to further reduce loads on the 'rods and crankshaft.

Thanks for the reply, I was more than happy to have to replace the alloy rods if that is what is deemed necessary for my engine, the clearancing does not bother me either, I don't think there will be much to do as I am machining the big end down to use a smaller big end and that should make things easier for me even though the alloy rods are bulkier, I am genuinely not opposed to using either conrod, I just want something that is best suited for the job.

I have never even thought of the titanium, I do not have any experience with ti rods at all but have worked with it on the fabrication side, in that case I have found it work hardens and hardens with heat, would this still happen with the conrods?, even though they are being subjected to much less heat they would still be subjected to a considerable amount of force ?.

I have sent emails to Carrillo, Oliver, Molnar, GRP, MGP and Saenz with the specs I have posted above and will now just wait for a reply, I will be especially interested to what Saenz reply as I did mention the titanium rod in the email if it should be an option.

Carrillo, Oliver and Molnar offer a light weight version of the conrods I need so we will see what the reply is from them too as I did mention the light weight vs the heavy duty rods, I think the heavy duty rods are more for power adder engines though.

GRP and MGP are both Alloy rod manufacturers so hopefully they shed some light on alloy vs steel or if alloy would be a better suited option.

I will be posting the reply here as soon as I hear back from them as I think the emails will be interesting to say the least.

I don't have a build channel but I'm happy to post the build on here when I start the assembly on it, I don't have any secrets to hide and if I can help other people make a decision I'm more than happy to answer questions, the engine is going in a little LJ Torana I have just finished restoring, it will be coupled to a 5 speed dog box and go through a Hilux diff so it will hopefully be pretty quick but once I get this N/A build off my chest I think a mild boosted engine will go in the car.

Kind regards

Anthony