Cosie Convert

As title........

I have read on this forum that the pistons/rods become the weak link at elevated outputs.

My experience is that rods fail in tension not compression ie RPM related, not torque/power.

If the RPM is kept within design limits, is there any limitation to the power/torque output they will deal with ?
(say to 380lb/380bhp)

Andy

Caronte

The more power the more the temperature on the explosion chamber and therefore on the surface of the piston. The maximum speed of the piston is reached on the range 270-90 degrees (therefore considering a cirle divided into 4 90 degrees angles and starting from 0 degrees the piston travels fast (0-90 deg), slow (90-180), slow (180-270) fast (270-360).
I personally woouldn't reccomend output power higher than 280 bhp on original engine. This however is influenced by a large number of variables.

Cosie Convert

Still don't get it

If the RPM is not increased then only the compressive loads on the rod will change with torque. In fact the tension forces will actually be reduced at higher boost pressure due to the 'cushion' effect during tdc at exhaust/intake overlap phase.

Caronte

No , I don't think so. AFAIK Just the first 90 degrees of compression and the last 90 deg of inertia are the most stressful for piston rods.

I'll try to draw an engine scheme soon.

JamesS

Peak con rod / crank loading is exhaust stroke, when exhasut valve open at rev limit ie no compressive loads from combustion off-setting inertia loads.

You can virtually double torque (BMEP / in-cyclinder pressure) in which is still less than the inertial forces at 7200rpm.

David_Wallis

Ah at last something I know the answer to....












Try it If it breaks fix it, if it doesnt then . Seriously though it makes sense as in vauxhalls you used to put cosworth big end bolts in to get the rev limit up to 7750.

Oh Yeah forgot to say.... Your a Nutter

David

[Edited by David_Wallis - 5/14/2002 8:33:50 AM]

Cosie Convert

James

I agree, the increased exhaust pressure will, to a degree, offset the maximum tension on the rod at peak time (top of exhaust stroke)
So, back to my original question - If the RPM is not increased over standard, why would you fit uprated rods ?

R19KET

Andy,

If you ask Dave nicely, maybe he'd knock you up a set of rods out of the leftover ally block he had

I don't think there is a definitive answer. I know of a few cars where the rods have let go, running far less power than you, and others who are getting away with more.

I was running up to 1.7bar (not recommended), and my rods, and pistons held up, it was the head gasket that finally let go on mine.

What it boils down to, is "do you feel lucky" well do you ???

Mark.

steve McCulloch

I'd be very interested to here about this one!

When you say above std - you talking say above 6500 or 8250? - I rarely take mine above 6500, and especially not any more in 5th

Presumably your on about fairly short bursts? - not sustained performance at those levels on a modded car?

380lbs!! - will the gearbox cope, will the rear diff survive?

Doubt very much that a std turbo would produce this much power?, on a fairly conservative RR

Put me down for a test drive in the car once its been sorted...?? pls?///....

Cosie Convert

I have been called a lucky B in the past

I've ported the exhaust headers and fitted a 2" uppipe which I matched to the turbo. I chopped off that tight 47mm bore bend on the TD05 turbo inlet and replaced it with a 75mm straight pipe and nozzle under the manifold. I've also thermally insulated the intake manifold from the heads by ceramic spacers....must be worth a few more CFM at higher RPM ??

I don't intend using more than 7000 rpm. Currently it's making 349bhp @ 5600rpm so if I can just hold on to the torque a little bit later in the rev range ie more airflow, it should help improve power but without needing high RPM.

Head gaskets are going to be working hard at that torque level, not sure if I have a closed block or not ?

I guess that STi cams would help push the power up the rev range but that means pistons, probably rods too I assume ?

Further turbo mods may be the next option, bigger compressor and cut back exhaust Mark ?? Expensive ?

Adam M

Cosie,
if you are after a new turbo in the future, give me a bell,

I still have a very large standard fit unit going spare, straight off the 22B.

VF series are never as strong as the TD0s, but this will flow more air and spool up very well too.

ustolemyname??stevieturbo

iTrader: (1)

I would tend to agree with cossie. Surely horsepower does not break rods. Revs break them. Does anyone have any pics of "failed" rods. Where have they broken?. Have they broke in 2 along the main length of the rod, or at the big end journal. ie. more relating to bolt breakage. I have been turbocharging relatively crappy engines for a while now, in various cars, minis, CVH's, Pintos, Rover V8's, and have never seen a rod break, despite running up to 25psi in some cases, but more usually around the 20psi mark. These are all on standard bottom ends ( sometimes I would buy good pistons, but usually just machine them to lower the compression ) which were designed to take about 15 bhp. Surely an engine designed to take in the region of 250 bhp standard should have pretty strong rods.
Rods should be their strongest in compression where the horsepower will be seen most. Probably more likeley to bend rods??
For the money to change them, if it aint broke etc etc...works fine for me.

MorayMackenzie

Re "For the money to change them, if it aint broke etc etc...works fine for me."

If a rod snaps it can take out your engine block, the heads and the turbo. These are costs that would not be incurred if you were to uprate the internals before the standard ones fail.

I know this from expirience. Power Engineering rebuilt my STI engine using standard heavy cast pistons rather than the STI forged items they destroyed. Of course, they failed to tell me this, although it explains why they failed to hand me the original broken parts as I had requested, so the engine rev limit remained at the STI standard 8000RPM. When a rod finally snapped it took the block, the heads, and the turbo with it. Do the maths, that lot costs serious money.

Moray
bbs.22b.com

dowser

So what's the definitive answer then?! Thinking about it, the exhaust stroke sounds logical...but I've never been that comfortable just taking my foot off the gas at high revs either....

Moray - when did yours let go?

Richard

Cosie Convert

Thanks Adam, not sure about a vf though, I don't run a dump valve and this is quite hard on the turbo, the TD05 seem fine so far.

If the STi's run the same rods as WRX's then that extra 1250 rpm can make a BIG difference. The force increases as the square of the RPM. To go from 7000 to 8250 is an increase in rod tension loading of 39%

ustolemyname??stevieturbo

iTrader: (1)

True, rod breakage can be expensive. But there is always the chance it will not happen. As long as the revs are kept sensible they should be OK. How much are a set of uprated rods? From most subaru prices people ask, I wouldnt expect them to be cheap. Isnt 8000rpm limit plenty for anyone?? Why would you really want/need to rev any higher than this, especially on a road car. And I ask again, where is it the rods break?? Could it just be rod bolt failure?? I do know that the rods do break, its just I havent actually seen one yet and would like to know.
If someone built an engine with the wrong parts after originally destroying your first engine, surely they should be responsible for the damage caused??
When trying to build in reliability, how far do you go?? Rods break, gearboxes break, diffs, the list goes on. While there is the chance it might cost more money in the future, for most things, I'll still go with ... if it aint broke, dont fix it. And think of it this way. Ive saved myself £10000 on an all steel engine, full set of gears, uprated diffs, driveshafts.. etc etc. and start saving for when something breaks, if it happens

R19KET

I've seen two engines with broken rods, both were broken mid way down, and the bolts were intact. I've also seen pic's of Subaru rods bent like a banana.....

I've always been led to believe that it's torque, and increased rpm limits that will break rods. Generally speaking, when we start tuning for higher bhp figures, we also increase torque.

I know rods are expensive, but if you intend to really push the limits, £600~700 now, is cheaper than trashing your engine later.

You pays your money, and you take your chances.

Mark.

Cosie Convert

I have read but don't yet believe that a moderate increase in torque can break rods however I am waiting to be convinced !

I read, on 22b I think, that crankshaft acceleration/deceleration rates can be changing rapidly throughout a single revolution.
I think the discussion was on the effect of lightened flywheels.

Could it be that as combustion takes place and creates a high force on one piston, the opposite piston/rod gets a 'tug' from the acceleration pulse ?? I assume that increased torque would increase this torsional vibration ?

Caronte

The inertia forces are based on a piston movement which is called simple harmonic motion (SHM).The fig (a) shows the piston movement.
Consider a point P to be travelling at constant speed around a circle of diameter AB, and another point N moving in a straight path from A to B. The point N is said to move in simple harmonic motion (SHM) if it always keeps at the foot of the perpendicular NP.
Point N varies its velocity as it travels across AB and this is shown by the graph (b).
Comparing the movement of an engine piston with SHM shows that during the first 90 deg rotation of the crank from t.d.c. the piston covers a greater distance, whereas within the range 90-180 deg a smaller distance is covered in the given time (c). This produces the following effects:

1. The piston travels more than half the stroke when the crank is rotated from t.d.c. to the 90 deg position.
2. Starting at t.d.c. the relative piston velocity for each 90 ded of crank movement is: fast,slow,slow,fast.
3. The piston DWELL (the angular period where the piston movement is small in relation to crankshaft motion) at b.d.c. is far greater than at t.d.c.
4. The inertia force at t.d.c. is far greater that at b.d.c.

Caronte

sorry for the poor graph..

ustolemyname??stevieturbo

iTrader: (1)

On the subject of lightened flywheels. I dont understand why people say they can affect the reliability of engines, whether it be rods or big ends. In saying that however, A friend built 2 engines which lasted less than 2000 miles each before running a big end bearing. The only thing common to both engines was the billet flywheel. ( and clutch ) The 3rd engine which was really a threw together bit of junk, with bits from everywhere, is still running perfectly, with std flywheel and same clutch. The billet flywheel had been balanced with the crank before fitting, as the engine(s) were being rebuilt anyway at the time. The 3rd engine has had some serious abuse, and is running approx 1.4bar + motec +FMIC etc... and hopefully this weekend if it arrives, It will grow a new HKS Ball bearing turbo!!!! Hopefully some power to be seen there. Just hope the rods dont break, coz were gonna giv it as much boost as she will take. No point in havin a 3 bar map sensor if u dont try and use it

Cosie Convert

Caronte

I dont understand your calcs. Are you saying that when the crank is at 90 and 270degrees, the piston is not mid stroke

As the con rod is a fixed length, I can't see how the dwell at TDC would differ from dwell at BDC ? Using simple trig, where the con rod is the hyponenuse, it would appear to be identical.

Am I missing something ?

Adam M

cosie,

I would tend to agree with you.

would caronte possibly be correct if the gudgeon pin was off centre in the piston?

Adam M

actually, the total dwell time would be the same both, but the proportions contributed by before dc and after dc (top or bottom) would differ.

Cosie Convert

That's what I thought on dwell times Adam.

I'm not sure if the gudgeon pin offset is compensated for in the engine design ie with the bore c/l offset to the crank c/l ?

I thought a simple harmonic was a Sine wave ? Caronte's description does not comply with a Sine wave function

Interesting as it may be, this is all related to velocity and not torque as far as I can establish.

Andy

Adam M

andy, a simply harmonic wave doesnt have to be a sine wave.

There are other functions usualy exponential which exhibit simple harmonic motion.

JamesS

The piston is at max acceleration at zero speed ie tdc and bdc. This is when the maximum forces are exerted on the bearings / con due to the peak acceleration. The torque, or in-cyclinder pressure, acts almost in anti-phase with the acceleration forces, hence the inertia loads are offset by the compressive / expansion force. Hence increased boost, increased torque & power BUT with the same rev limit = not much more stress in the conrod.

The problem with running big boost is in cyclinder temperature, this could lead to detonation. If this happens you get HUGE in-cyclinder pressure spikes, outside the allready burning flame front, this in turn can lead to more det events, more HUGE pressure spikes - bent conrod........

Cosie Convert

Cheers Adam/James

I appreciate the increased dangers of Detonation Regardless of the rods fitted though, I would think the pistons/block and heads would be most unhappy with that situation James

Although I didn't hear it first hand, a tuner I respect was quoted as having said 310lb-ft torque was the max he would recommend on std rods..........and I still don't understand why

Andy

JamesS

Commercial tunners are all optimistic when quoting engine capability. They don`t want dead cars on their rolls.

Also, all IMHO, just because a cetain place has done a certain number of a certain type of car, it does not make them experts. It just means they know what works. Outside of these bounds is unknown. I have yet to find a garage enviroment with a fully qualified auotmotive engineer running it that REALLY understands fundamental engine design / operation from a first principal viewpoint rather than a `this worked on X`.

Adam M

agree with james although I think he means pessimistic.

I hate it when people say such things as:

its okay to run this boost, or this much power is ok so long as.......


there are so many variables that cause things, and identical engines can differ enormously in performance. More so between subaru engines, but for apparently no reason.


in cylinder temps, ignition timing, ron rating, boost pressure, compression act. charge stratification etc.

all these affect the conditions under which the torque is produced, and hence define a range of stress levels on the con rods, for the same output torque.

it makes it a foolish statement to generically "ok" standard rods to any torque.

Such statements have to be sweeping statements for the benefit of those who ned reassuring, rather than someone who understands the it is a waste of breath.