What's the advantage, if any, of H beam con rods for a road engine? The ones I'm looking at weigh 490g each and are balanced end to end within 1g

Is it a 9000 rpm road motor?

If the answer to that is "No", then none.

_________________
Too many cars, and too little time.

Thanks for that - pretty definitive. Not worth wasting my money on then

If you want to spend money, then match, peen and polish the Cooper S rods and fit decent rod bolts.

It's not the rods that let go at high RPM in an A series - usually the bolts or the crank.

_________________
Too many cars, and too little time.

Yes, I would agree that most A series rods are quite reliable and strong. It is rare that a rod will let go, usually the bolts, however, strength aside, there could be an advantage to using these rods, in that they are lower (much lower) in weight than any standard rod or what a standard one could be modified to. Mind you the full weight reduction advangage won't be gained with the rods alone, you would also want to do the flywheel and backing plate and if the funds stretch, have the crank wedged.

hmmm,,, $1500 for a set of H beams v's $180 for a set of ARP big end nuts and bolts for the sake of a road motor that will probably be unlikely to see even 6500rpm once a month.... Sorry Mowog,, I know what I'd go for,,, just make sure that you find a decent machine shop to give them a close and hone and check the spot facings if your going to go for these nuts and bolts,,, so i guess its more like $300 v's $1500..

But I do agree with you, going for a lightened bottom end will increase the life expectancy of the motor,,, reducing the reciptrecating mass in you bottom end is probably one of the only mods you can do that will make your car drive better and last longer,, but you can lighten the standard rods enough for a good road car (i reckon anyway)... just another angle to think from I guess...

Lot's of good advice. Thanks guys. I will go for the match, peen and polish route and while I'm at it get them lightened a bit and forget the idea of the H beam rods. I have already got a set of ARP rod bolts for them.

Hey, no one mentioned $$, only will they be of benifit.

Me - I'd spend the $1500.00 for the rods - I've modified plenty of factory ones and I don't think I could look at doing another set - The labour bill to modify them and then have them finished in the normal way + a balance $1500 isn't too bad - but that's just me.

You know MOWOG that's a good point -I was thinking about this last night. I'm going to the UK some time in the next 6 months for work and can go to specialist-components.co.uk to pick up a set of their H beam rods. So with no shipping to pay and the VAT refund at the current exchange rate they would work out to be NZD$706 (GBP326)for the set incl ARP rod bolts that the set comes with.

I have to offset that against approx $300 to get my Cooper S rods lightened, balanced etc and then if I buy the H beam rods and sell my Cooper S rods and the ARP bolts I've already got for them, the difference in price might not be that great.

Yeh, I agree. If you add up the cost of getting 'S' rods + cost of lightening/balancing/shotpeening + arp bolts - the specialty components Rods are very good value - getting better value with the price of the AU dollar as well

any money spent on special connecting rods for a road engine would be spent better elsewhere in about 95% of road engines. You don't mention the spec of your road engine, but I'll bet your money would be better spent elsewhere. Even super lightweight connecting rods like the ones I use in my racer provide less benefit than a better cam, head, carb, exhaust, etc. Lightweight engine internals should be just about the very last place you spend money in a road engine. A lightweight flywheel/backplate unit is a different matter though. Both cheaper than rods and at least as effective in allowing the engine to accelerate faster.

Why H beams not I beams? I'm thinking purely in an ultimate power situation here...



This is me thinking out loud, but, halfway down the power stroke when the rods at say 20 degrees the I would be bending about the major axis and would therefore be stronger, whereas the H would be bending about the minor axis. The + as shown in the picture above is just inefficient as it doesn't optimise the material placement well at all.
This makes me think that the I beam rod is more effecient in terms of weight and that the ultimate rod would be tubular, although very difficult to manufacture.

Well at least thats the way I see it anyway, are there other reasons for using H rods that I am overlooking, or am I just completely wrong?

It can be hard to identify stress concentrations but the H does look like it may have the advantage at the point where the beam merges into the main cap, is this more likely to be the reason for there shape?

_________________
The 850 Man

albino - as you said in your posting "I'm thinking purely in an ultimate power situation here... ".

It's fine to think that way, but most of us with A-series engines are looking for 1) light weight rods and 2) failure proof rods when used in our "less than ultimate power" engines.

My vintage racing Mini is nowhere near the "ultimate power situation", but the rods I run weigh only 439 grams with the special ARP bolts. I also got them for a price that is cheaper than what lightened and prepared Rover rods cost. So, mine are: much lighter, stronger and cheaper. It works for me.

The rods in your photo(with the plus '+' sign) look like those made by a California firm named Pauter Machine. Here are some sample weights for their rods.
http://www.pauter.com/exotica.htm

you do have a good point there... 40+ hours of mind blowing boardom while I polish up a set of factory rods is not one of my favoraite past times either. Especially when you get that far with it only to have one of your dickhead mates walk into your workshop and go "uh whats this-oh fire truck I dropped it, it wasn't important was it?"..

But when you don't have a lot of spare cash to throw around, sometimes,, with somethings,, I reckon you can get a pretty good result working with what you've got.. Especially if it is not going in a race engine that will spend a good part of its working life at speeds of 7000rpm and higher...

But hey, its just some food for thought for those playing at home

Hey, just a quick question... Mowog mentioned wedging cranks earlier, I have done this to and it does make a difference,,, but has anyone tried "knife edging" them as well? I'm curious because some of my mates who are into V8's have mentioned it and it got me thinking,,, (oh crap watch out ),,,,,, but has anyone had any experiences? Just curious thats all.

Need to be pretty careful knife edging cranks, it can weaken them easily, but if done right can allow it all to spin up that little bit quicker and will reduce 'windage' in the crankcase. IMO it's one of those items that you will spend a lot on for small gains, but sometimes, that's all that's needed.

Be interested to hear of others opinions / experiences.