Katana

Here's a story that may or may not be relevent to engine discussions. A few years back there was a small bridge somewhere in this country (assume it is in reading area) that was rated to only be capable of handling around 10 tonnes (or something like that). My structures professor was given the chance to test the maximum load of that bridge as they were taking it down.

They attached the device that pulls the bridge down and slowly stressed it up to the maximum load specification. The bridge held. Loaded it up to 15 tonnes and it still held. In the end I think it buckled at around 40 tonnes.

Moral of the story is? You can't really predict when something would reach its max. Engineering is like an art and you can only predict it up to a certain extent. Having said that, if the bridge was built in an identical manner using the identical materials somewhere else, it could've broken at 11 or even 14 tonnes.

edit: just because the designer rates it at 300 Bhp maximum, doesn't mean thats the maximum you can go. Then again, it might break down at 240 bhp. Its all a gamble mate.

[Edited by Katana - 12/8/2002 5:53:09 PM]

Katana

Yup Andy over time, materials definately have an expiry date especially if its subjected to repeated tensile and compressive stresses. Did some calculation in class on something last year and found that by increasing the stress, you effectively shorten the lifespan of the object.


edit: you guys don't really want to know how much less the lifespan would be by just increasing the load by 10%. But no fear, its not like we're gonna be on full torque continously right?


[Edited by Katana - 12/8/2002 6:41:28 PM]

Pavlo

Andy,

A new (non-cast iron) conrod will be stronger in tension than compression, I was going to say I could guarantee it, but that's not the case. Anyway, the problem is you're allowing both ends to pivot, so it buckle. However, after Xmillion cycles fatigue may change that, leading to a tensile failure. I have seen a conrod bend, with perfectly good bearings, even after the event.

In pure tension I would be a little worried about the small end, but I guess due to acceleration, there is the mass of half the rod to decellerate as well (thinking of tension in the centre) which is why a rod in actual use may well fail (in the middle somewhere) in tension without being fatigued.

It's called fatigue.

Paul

[Edited by Pavlo - 12/9/2002 9:35:45 PM]

MrContro

Hi all,

Just a query for all you automotive engineers and engine builders out there.

Bare with me this could be a long post.

When tuning a stock engine, at what point do you come to the decision that its necessary to uprate the engine internals.

Take the EJ20 engine for instance, there seems to be alot of specualation on here as to how much power you can run the engine at on stock internals, but has anyone actually tuned one to destruction ?

There must be a tuning company that while in the process of developing its tuning package must have tried to work out how much working tolerance the engine has before arriving at the "safe" limit of tune with stock internals.

It'd be good to take a internally standard, good condition EJ20 engine and then tune to its power handling limit destruction, by "power handling limit destruction" I mean destruction from the power output itself of the engine, not by running excessively high intake temps/lean AFR, engine knock, pre-ignition etc.

When speaking to most people who are tuning engines they mostly seem to be trying to get the most out of the engine without producing knock, the limiting factors being intake temps, fuel octane, AFR etc. but what if you could keep all these variables at a safe and acceptable level, ie really efficient turbo, intercooler, correct injector size/fuel pressure and AFR, high octane fuel, water injection. It must be possible to acheive what I have mentioned and then actually trash the engine from the power but how does the tuner know when to stop tuning any further because of the need of uprated internals, does he just guess and say, right we are running 1.6 bar with cool charge temps, AFR bang on etc, it must be running xxx BHP so now you must get some uprated internals before we can extract any more power.

BOTTOM LINE - when people say you need uprated internals at an arbritrary boost pressure/power output is it usually to cope with the heat/knocking(in other words the inability to keep these parameters to an acceptable level) or is it to cope with the actual power.

Sorry for the long post

I would be very interested to hear some views.

Phew !!! lol cheers

R19KET

Brett,

I wish it were that easy !

I remember a while back, a Subaru feature in one of the well known motoring magazines. They asked the "tuning" advice of several of the "better known" Subaru "specialists".

The general opinion, was that the cars were fine up to circa 300bhp (iirc) but were time bombs, if you pushed them much further, and even now, I know of some tuners who avoid tuning STi's, because they fear the engines will blow.

Well, even back then, there were a few of us pushing 350bhp, and now days, these are pretty common.

However, I'd bet that if you asked any of the guys running this type of power, just what the engines are good for, they'd feel they were pretty much on the limits.

The problem we have, is that the stock pistons, and rods have been known to let go on even standard engines, so it would seem that there maybe a big disparity in the quality of the stock items.

Destruction testing an engine doesn't help. All it will tell you, is that a particular component let go, in a particular engine, at XXX bhp/torque.

You could repeat the test on another engine, and it would be fine, or, it could let go earlier, and, due to another component.

I think you'd have to destroy more like 10, or 20 engines, in a controlled enviroment, and the work to the lowest figures.

If you supply the engines, I'd be more than happy to destroy them for you

Part of the problem, is that a tuner can't afford to blow up a customers car. You'll find that most of the more powerful cars are owned by guys, who either tune their own, and, or, are prepared to accept the risks of pushing things further.

So, what it boils down to, is just how far are you prepared to push your engine, accepting that at some point, it's going to go BANG !!!!!

Mark.

Andy.F

Good question (and answers) I for one am keen to find where that line is but as Mark said that will only prove that MY engine can run at xxx bhp, anyone following my (probably bad) example may not get the same result.

High RPM kills rods/cranks/pistons/valves so I need to keep within original rev limits (7500 limit on my engine)

Excessive Torque (bmep) kills head gaskets, pistons (and possibly rods but I don't think so)

An undersize turbo can cause excessive heat and exhaust backpressure, this will overstress an engine at a lower output than a big turbo would.

My findings so far are that 350lb-ft torque/ 370 bhp is reliable (on my engine) having done 40+ drag races, a number of track days, a few dyno days and a fair amount of 'involving' road driving

My next task is to retain this 'safe' torque higher up the rpm range by using a turbo that is more efficient at higher flow rates.
Currently I'm making over 340 lb-ft from 4300 to 5700 peaking at 4800. Holding 350 lb-ft to 6k rpm would give 400 bhp retaining it to 6.5k rpm would give 433 bhp.

So that's the plan, on totally standard (non Sti) internals to top 400bhp/350lb-ft all under 7k rpm and still retain decent bottom end response.

ps
Better get those pistons and rods ready for me soon Mark

R19KET

Andy,

I can only guess at what my old STi engine was producing, before I popped a head gasket.

I was holding 1.5bar to the red line, on a relatively big hybrid turbo, and max'ing out 550cc inj's, running 3.5bar fuel pressure (so about 588cc's) about 11.5:1 AFR

I always accepted that "something" was going to let go, and like you, I suspect, was just waiting for an excuse to build another engine

I think one of the main reasons some of us get away with the higher figures, is because we are very careful to make sure that we have enough fuel, avoid det' like the plague, and make sure intake, and EGT temps are sensible.

However, I doubt either of us would "recommend" that others do what we do !!!!

Mark.

Pavlo

Rods can go due to high torque, high in-cylinder pressure at BDC (max acceleration load) could in theory collapse a rod due to buckling. By that I mean the sort of load that wouldn't snap it in tension.

EG, drinking straw is stronger in tension than compression due to bucklin.

Paul

MrContro

Thanks everyone, hope we can keep this discussion because I for one find it VERY interesting.

Following is a very long post and I do go on a bit, I wrote it in the ealy hours after a few beers but I thought I best read through it today before I send it as you to make sure I haven't made a **** of my self in it anyway here goes...


I have sobered up a bit and it this seems to go on a bit too long for Scoobynet, read it and laugh at me if you want lol, but I think I may have some valid arguments..

Thanks Mark,

I still don't know at what point people decide that uprated internals are required, does the tuner just sort of try and calculate a maximum power figure that he dare tune an engine with the stock internals based on what quoted standard output of the engine is. For example, the standard UK scoob is qutoed as having 208-212 bhp, the Japanese models have upto 280 and these are basically the same internally, then take into account that this would probably be a conservative power output for long-term reliability reasons due to potential lack of maintenance etc. and so add 10% and hey presto 280 + 10% = 308, as long as the tuner gets all the operating parameters safe ie the fueling, charge temps, and no knock then he could take this engine this far.

I mean from talking to most companies the basis they use for determining how far they can tune a car gets no more sophisticated than the above, some companies pride themselves on not blowing a single customers engine and I can't see them blowing 10 of their own engines like you said you'd probably need to do to get a decent idea of the robustness on the engine.

If you look at the difference in power output between say the <97 model UK and the <97 WRX(not sti as these do have different internals) models its basically down to one thing, fuel, in Japan they can gurantee that the cars will be run on by comparison to European stuff, very high octane fuel that is resistant to knock, that is the only difference between your 212bhp domestic and 280bhp import. If the UK type Impreza didn't have a Japanese counterpart and if we used the above simple calculation to work out safe tuning potential would we only be tuning the UK to 212 + 10% = 233bhp.

Subaru obviously think that the EJ20 can be run at a safe 280bhp and thats with an interwarmer lol, restrictive exhaust/airbox crap fuel pump, and the standard conservative power output because of possible lack of maintenace on the owners part, and also last for 100,000 miles plus, and Japan has a tropical climate.

Obviously some engines will have manufacturing faults etc. and so could fail even in standard tune for apparently no reason, but i'm talking about a to design, in tolerance free from manufacturing faults engine(might be being naive here as I'm not an expert on engine production).

So with a turbocharged engine, is the limit on tuning more down to the ability to control potentially engine damaging parameters(like using higher octane fuel in the example above)rather than the actual strength of the engine components and their ability to cope with high power outputs.

In Formula 1 turbo era of the 80's, they were getting upto 1500HP from a 1.5litre V6, I know this was only for short periods in qualifying but come on 1500hp from 1500cc 20 odd years ago. And I have read that one of the main contributing factors that enabled these outputs was that they could run the engines with crazy boost levels because of the fuel concoctions they developed and things like water injection, the limiting factor was not the engine internals but the safe operating parameters as discussed above.

The limiting factor on modern formula 1 cars seems to be RPM and the forces these put on engine internals, the Turbo F1 cars were running at about half the RPM's, they got their power from the boost.

Is it RPM's that are internal killers for say the rods and crank, and temperatures for the pistons.

Is it just that limiting factors on turbo cars aren't the same as normally aspirated cars.

Obviously you would have to uprate the internals at some point but are we estimating this a bit low.

BOTTOM LINE - It seems to me that the limit on the standard internals is more to do with the problem of keeping the engine within safe temperature and engine knock limits. But what I don't understand is that even with uprated internals you still have these problems or is it that the forged pistons etc. can just cope with these more extreme conditions that the standard internals can't. It seems that the people that are "getting away with" high power on standard scoob internals are just people who are using common sense and their tuning knowledge which unfortunately alot of people don't seem to be doing, they just jump in feet first with no research, they don't know that they could be running lean causing increased in-cylinder temperatures etc. or the fact that they are running on crap fuel.

Obviously though there will come a point where you can't keep the parameters to an acceptable level and so there is a need for stronger materials to be used internally but that doesn't mean the engine wouldn't have been able to handle the power if the parameters could have been kept under control does it ?

Anyway, I've gone on for ages and I think I've sort of gone in a bit deep but I've been out and had a few beers so I best go.


Cheers

Brett

Andy.F

Brett

I think most of your questions have been at least partially answered here >
Regardless of uprated internals or not, any 400+ bhp 2.0L engine is going to need to treated with a bit of care !
Lack of fuel, high EGT's or Det will destroy any engine, at 200bhp/litre it just does it a bit quicker !!


Paul, I've never known a rod to fail in compression, they will however bend due to a big end, small end or piston failure. Otherwise they are capable of higher forces in compression than tension. (unlike straws )
The maximum compressive forces are as you say at BDC but the pressure on the piston at this point is only exhaust backpressure, possibly plus or minus a little bit depending on shock wave tuning in the exhaust system.
As an aside, higher boost actually reduces the tension forces on the rod due to the cushion effect at overlap TDC.

Andy.F

Structures and materials is a massive subject, but totally relevant !
With engines you also need to consider fatigue failures, a rod may hold at 9000 rpm for X number of cycles then fail later at 8500 rpm depending on how far beyond it's elastic limit you've gone.
Drag racers typically measure rod/valve stretch to keep an eye on how much they have overstepped the elastic limit, they aim to change the item before it reaches the yield point, sometimes they are unucessfull !

Even on 6000 bhp nitro methane burners, rod tension is the problem, not compression.

MrContro

Thanks guys

Well at least I know what some you have run.

I'll find out on my engine, I was just trying to find out what is high power and what is crazy power on standard internals and then I can make an educated(RISKY) guess on how far I should go.

Cheers

Brett

Hoppy

Interesting debate. One day I might even understand some of it

There are obviously many variables in an engine, but isn't the stock Scooby clutch and gearbox the first to go? After 350ft/lbs aren't we beginning to pray?

Richard.

Katana

Well if thats the case Hoppy, care to pretend that you are me and sit for my exams in April?

Anyway, too many variables. Past 350 Bhp, anything can go.

Andy.F

Paul

Disagree re tension/compression strength. This is not so much material dependent as it is structure.
Con rods are H or I section for a good reason - to resist buckling ! The ends are free floating, this eliminates side load.

A top fuel drag engine - Combustion (compressive) forces on rod = 9000 kg, rod tension at 8000 rpm = 2100kg. Ratio of more than 4.2:1

A quick calc shows that even at 350lb-ft torque on a Subaru, the compressive forces due to combustion at mid stroke are only in the order of 1280kg. The tension forces at 8000 rpm are however 1750kg
Ratio of 0.73:1

We are NO WHERE NEAR causing bent rods due to excess torque.

Detonation can cause pressure spikes of more than 2x normal combustion, this relates to an instant 700lb-ft torque equivalent compresive force, yet still the piston will eventually fail or the head gasket blows before a rod bends.

Pavlo

Nice numbers Andy, did you remember to the compressive forces, the force required to negatively accelerate the piston and top half of the rod as it approaches BDC? These forces will be equal to those you mention in tension at TDC.

The standard subaru rod is quite narrow where it meets the BE, so i can imagine it bending. The Lateral Performance rod are noticable tapered towards the LE.

You can also snap a brittle rod in compression due to shear forces,

All that said, agreed rods are much more of a worry in tension, and that rotational speed is the biggest contributor to failure.

It's all semantics really, the rod is more likely to suffer tensile failure, though i think it could be actually stronger in tension.

Paul

Katana

Although true, your engine is still not gonna last long, especially if your car is oldish (like mine). Remember, manufacturers always try to make their product so it will fails after a few years. You're just bringing that time closer everytime you up the power, a risk that I know and am willing to take.

siwrx

ive seen 590 bhp on a jap one. pistons, crank and rods and cams were changed, rest of block was standard

nom

I always thought that the forces were much greater in tension than compression too... to such a degree that running ridiculously high boost makes no odds in itself on the rod, etc., only in the increase of liklihood of det. And then det over time (through shock loading - vastly accelerated fatigue) weakens the bigend so it goes; that is if the det doesn't lead to pre-ignition & it all go that way instead

R.B.5

Very interesting read, more threads like this should be seen more often. Nice one lads!

Andy.F

Yes. Fortunately, well before the maximum deceleration point of BDC, the Exhaust valve has opened and released the pressure.
The maximum combustion pressure force on the rod will be before 90 deg ATDC.
Up until 90 deg ATDC the piston is being accelerated by the rod (tension force), this to a degree offsets the combustion (compressive force)
As the piston travels further down past 90 deg, the deceleration starts (compression) but the combustion pressure has reduced by this point.

Pavlo

If you did add the forces, I fail to see how there can be a 4:1 ratio.

The highest tension will be see on overrun, where there is no pressure to act against the piston, exhaust stroke in paticular.

The highest compressive force will be somewhere before or at BDC on the combustion stroke depending on the exhaust timing and power being made.

At any given RPM, the 2 forces are likely to similar, certainly not a 4:1 ratio.

Paul

Andy.F

The 4:1 ratio I am referring to (on a fuel drag car) is compression force (4) tension (1)

On a Subaru we are nearer 1:1 ie we can stand much more torque before getting anywhere near the compression/tension ratio of a drag car.

On a Subaru the risk is more from high RPM as the forces do not increase lineary, the torque however does reduce with high rpm so the balance changes completely.

Katana

Keep it rolling guys. I learn more from here than when I'm sleeping, I mean studying in my lectures.

I'm gonna throw in another grenade here, how does heat effect the materials in the car (apart from expanding it slightly)?

rex11

I agree that fatigue of the rod material is the biggest factor in engine destruction that isn't fuel related. If you study materials science you learn that as these compressive and tensile forces are placed on a material (not just metals) small cracks and imperfections in the crystal structure that lead to a less tough (ie more brittle) material. When you look at high end racers that blow engines all the time you'll notice they are usually running high rpm's, which accelerate this enbrittlement do to the increase in the fatigue event. If you want an engine that will last forever you need to find a material that has very good fatigue properties, or just be prepared to rebuild when things wear out.

For most of us the fuel is the big issue. So turn up the boost, watch your AFR and have a blast. A rebuild is just a good excuse to go to a 2.5

Katana

Eek fracture mechanics is gonna be taught in our class after christmas. Any tips (apart from study hard or you'll fail)? But as the rpm increase, heat and pressure would also increase right? Whats the threshold before everything goes bang in terms of time and temp?

Pavlo

I still fail to see how you could see a 4:1 ratio between compressive and tensile load in any conventional reciprocating piston engine.

Any tensile loads will be from the accleration associated with the change of direction at TDC. It will be at a maximum when there is no pressure on the piston, which means either over run (throttle shut) or on the exhaust stroke under power.

The motion, and there for the acceleration is exactly the same at BDC as it is at TDC. The mass of the piston and rod doesn't change in between, so the force must be the equal and opposite, with the exception of the force on the piston crown from the pressure in the cylinder.

The only way the compressive loading on the rod could be less than the tensile loading, were if there was a vacuum greater at BDC, than the pressure during exhaust at TDC.

And at high speed, we're only like to see positive pressure at BDC due to either forced induction or ram effect during the induction stroke, or positive pressure from combustion on the combustion stroke.

So unless I've forgotten something of what happens pressure wise at BDC to reduce the overall compressive loading on the rod (which I might have done) I can't see this 4:1 ratio ever occuring even in a 5000hp drag engine.

But there is no getting away from the fact that most failed rods are snapped ones.

Paul

Andy.F

Paul, in the 4:1 ratio example, I'm saying rod compression forces can be 4x GREATER than tension, not less !!!

The point being that the rods still do not fail in compression even when it's 4x greater in force than the tension.

MrContro

Hi all,

In Chapter 1 - "AN ENGINEERING LOOK AT THE BASICS" in the book Maximum Boost by Corky Bell the author discusses the stresses imposed on an engine by turbocharging.

According to him, tensile stresses are what cause fatigue/failure of conrods/bolts and when an engine designer is looking at maximum loads they are going to handle he is more concerned with tensile loads, more specifically the tensile loads caused on the exhaust stroke at TDC(tensile loads at TDC on compression sroke are custioned by the compression/ignition of the mixture).

He also goes to say that stresses increase exponentially as the RPM increases, can't remember exactly but its something like if you double the rpm the loads increase four fold something like that.

He also gives an example of power loads on a turbocharged engine compared to a N/A one.
He says if you take a turbocharged engine and run it at 2 atmospheres of boost effectively doubling the engine output(not taking into account turbocharger/intercooler efficiency), the peak combustion chamber pressures may only increase 20% due to the fact that maximum pressures in a turbocharged engine occur at about 20 degrees ATDC yet at that point only about 20% of the mixture will have burned, even with high boost pressures the small amount burned will not result in large maximum pressure changes, as the burn nears completion, the greater mixture density can raise the pressure three-to-four fold at crank angles near 90 degrees such that torque input to the crank at that position can be twice as great, thats where the extra power is coming from, effectively a much longer push, not a massively harder one, thats how I interpreted it anyway.

Don't know how respected this guy is but I thought I'd post it anyway cos thats what I've read and it may be interesting to read.

Cheers

Andy.F

Yes, agree with that