Shot peened vs Forged Rods

[andrew_mx83]

i am about to order pistons for my build and have been offered to have a set of rods made cheap. had previously organised to linish, polish & shot peen the standard ones and now not sure which way to go.
i would rather not spend the extra $500 if i can help it but will if its deemed "necessary." i have already gone much more overboard than i had planned on the motor, with the view of it being as close to bulletproof as possible.

the motor is a high revving, destroked 7m-ge race motor
basically will consist of:
85mm stroke from 5m crank & rods
85mm forged flat top pistons giving 12:1 compression
280 deg cams
head porting with big valves & double springs
metal head gasket
arp head studs, rod bolts, main studs & flywheel bolts
good oil system: dry sump, 7m-gte block (oil squirters under pistons), twin coolers, approx 15lt of oil all up in system.
super light flywheel & balancer.
properly balanced and clearanced.


expected redline is 8000-9000 rpm and it will spend most of its time above 5000.

also the motor will be coming out every 12months for a new set of main & big end shells and to inspect the clutch plate. i am imagining that lots of compression & timing will kill bearings pretty quickly and dont want to wait for one to fail.... that was expensive last time!



so the question is, how much better are forged H or I beam rods compared with cast shot-peened ones? and are standard peened ones likely to be a weak spot with the above combination?

[oldcorollas]

cheap forged rods will be no better than stock rods if they are not made properly, and could be worse quality.

polishing is to remove cracks and defects from the surface, to prevent cracks starting.
peening is to induce a layer of compressive stress in the surface to help prevent cracks starting, and slow their growth if they do start.

forged rods can be peened as well, but the main difference is the strength inside the rod.

are stock rods cast or forged? do they have a thick parting line down the middle of the edge?

which forged rods are you thinking of?

[andrew_mx83]

here is the rod in question (standard 5m-e):


i believe it is cast.

the rods (and pistons) are being made by these guys:
http://www.pureperformancemotorsport.com
apparently they supply the pistons and rods used in the ferrari XX

here is one of their h beam jobies:
http://www.pureperformancemotorsport...9e1ca30b1e8e8f


i would be particularly interested in your opinion as a metal man as to how much stronger shot peening really makes them.


wow forged rods then peened must be nigh unbreakable!

[andrew_mx83]

also should have mentioned in the first post that decreasing the stroke from 91mm to 85mm dramatically reduces the piston speed therefore putting less stress on the rods.


@8500rpm
7m (91mm stroke): 5075fpm
5m (85mm stroke): 4740fpm

piston speed at 9000rpm with 5m crank is still lower than the 7m at 8500.

[oldcorollas]

when a rod has a thick parting line like this one, then it was forged. casting will have nearly no parting line at all (no need to have it)
ie, the stock rods are forged also...

[oldcorollas]

i would be particularly interested in your opinion as a metal man as to how much stronger shot peening really makes them.

peening makes them weaker....

yes.. weaker....

rods almost always fail in tension, but peening puts compressive stress at the edge, which reduces the cross-section available for tensile force = they will fail at lower tensile load.

however.. rods also fail from fatigue.. shot peening can dramatically reduce susceptibility to fatigue.

then again, it is also said that rods often fail due the rod bolts, not the rods themselves...


forged and peened rods are just as breakable as any other.
basically, every rod has a lifetime due to fatigue. above a certain threshold rpm, every rpm of the motor reduces the available lifetime.
but it is hard to say, without extensive testing to failure (ie stats), what the average lifetime will be, and even then, the variation from rod to rod will make it impossible to predict lifetime.
suffice to say, rod lifetime is probably proportional to time x rpm^2

stronger, more fatigue resistant base material will help.
forging will help (with grain structure, but many/most stock rods are forged anyway)
polishing will help remove cracks.
peening will help cracks stay closed...

after that, it's just hope that holds them together


edit: looking around, it seems that in US (in days gone past) most rods were forged, but pontiac had some cst iron rods. Opel and Fiat in europe often had cast rods..
toyota started making powder forged rods in the early 1980's?

[ed]

the stock M series rods are GOOD rods

linish, balance and shot peen and whack some ARP 2000 rod bolts in there (rod bolts will cost you about $90). i concur with stewy re the role of peening, but would recommend it in light of the fact youre going to be cutting sick on the rod side faces etc when cutting/linishing. the peening will close/finish that surface

cheap forged rods are, cheap, and no better that what youve got. the only benefit is they usually come with ARP fasteners.

9000rpm? i think your 280deg cams wont really go there. which is good though, as i dont think the rest of what youre doing is quite 9k material either. if you get a power band from say 4.5 - 8.0 with cut at 8.5 i reckon youll be on the money

[oldcorollas]

but would recommend it in light of the fact youre going to be cutting sick on the rod side faces etc when cutting/linishing. the peening will close/finish that surface

ahh, thanks Ed
I forgot to mention that if you are going to deliberately put crack starters in the rod (gridning/polishing) then you really SHOULD get them peened

[andrew_mx83]

ed the original goal was 8000rpm but since going for forged slugs and a dry sump etc i figured it would be more like 9000.... wont really know til i get it together i guess!
i thought 280's would be good upto around 9? ill speak to clive tomoro when i pick them up.

the rods will have arp bolts either way, but yes the ppm rods already come with them. so not having to buy bolts and buy beer to exchange for peening puts them at about $500 more. seeing as the ones i have a forged anyway the only benefit i can really see would be the weight?

[3jcelica]

andrew_mx83

Not sure if the 7mge has floating pins or not?, but no doubt the forged pistons will, this may need to factored into the cost i.e bushing the old rods for floating pins.

[PeteH]

Hi,

Just a tip with peening, the shot has to strike the part surface as perpendicularly as possible to be effective........it appears easy but with some items it can get pretty complicated trying to get worthwhile increases in compressive stress.

Rods are fairly basic and really any peening will be a benefit if you are going to linish them.

Cheers,

Pete

[ed]

ive got an excellent and well priced guy in syd who does high quality automotive peening. not sure if other folks in this thread also offer that service? so ill let them put their hands up, but otherwsie, shoot me a PM and ill put you in touch. he does mail order, no problems

weight? doubt youll honestly see any weight saving

and yes, the M is a full floating pin/rod

[oldeskewltoy]

I know these are 4AG, and you are using 5M... but for general appearance after machining.....





Also... if you are building a high rpm engine... you might want to look at lighter wrist pins.... here is the difference I found.


89 grams on the left(stock pins), 63 grams on the right(aftermarket)

[andrew_mx83]

pretty sure the pistons come with pins.... will find out for sure otherwise make some

those 4ag rods look nice.... but there seems to be a couple missing


ed the peening is already sorted, found an old bloke who works at a huge metal treating plant (they harden all unbrako bolts & heaps of OEM automotive parts, something like 11 huge furnaces) who used to race corollas and likes looking after the boy racers like myself..... apparently james boag premium is the payment method of choice lol. he has also offered to heat treat anything i need done.... they treat millions of bolts a day there and he will just throw it in with a batch
will probably get the crank bolt from this thread (http://www.toymods.net/forums/showth...t=crank+pulley) hardened at the same time rather than try to machine a chunk of chromoly down

[jonathan yee]

i would go quality forged rods if your goin for big rpm.

pretty sure RPM is what puts the biggest strain on eng components.

also good quality forged rods will be significantly lighter than shotpeened standard rods, stronger, more aero dynamic etc. the less wight will allow higher rpm and relieve stress on the crank and bearings.

from my research into building my 3s motor,i found that that apparebtly the stock rods are strong as hell but due to their wieght will put too much strain on the bearings and crank if revved high(7500 constantly) hence killing my bearings and or cracking the crank.

eagles make cheap ($300US) rods. i'd use them at a minimum.