Boost and Piston Damage

[Steve]

Hi, I am a bit confused here (nothing new, no cheap shots please unless you really feel you have to).

I have heard alot of 'if you are running stock pistons, you cant turn the boost up over xxxpsi (usually about 14psi), because you will put holes in the pistons'?

I was under the impression that damage to pistons was usually due to detonation which 'may' be caused by excessive boost.

How can boost pressure (eg 14 psi is safe, 15psi and you will have a hole in your piston) be true. It just doesnt seem to add up.

WHAT AM I MISSING?

I would be interested to hear comments for and against, anyone had experience running over 1 bar boost with stock pistons?

Thanks for any input.

Steve

[Warpspeed]

You are right Steve, detonation is what wrecks pistons. Usually it breaks the ring lands or sometimes even the top ring.

Detonation is when the combustion process turns into an instantaneous explosion. The idea is that the combustion process creates a steady increase in pressure which smoothly pushes down the piston.

There will be a critical combination of temperature and pressure where the mixture spontaneously explodes. This is the dreaded tink tink shock wave created by detonation.

High octane fuels can stand more heat and pressure before they spontaneously explode.

Excessive combustion chamber temperatures due to lean mixtures, or too high a compression ratio will do it as well as excessive boost pressures, or excessive ignition advance.

There is a combination of factors, not just one factor. You cannot say that a certain engine will detonate with 10:1 compression, and not detonate with 9.8:1. Or that 14psi boost is o/k and 15psi is not. Or a certain particular weak air fuel ratio will detonate at full load.

It is the combination that counts. If you have detonation you are going to have to change something to get rid of it. What is easiest or best to change depends on what you have.

Usually there is something wrong with the engine tuning, and a proper dyno tune by an expert can put things right. But the higher boost you run, the less margin for error there is.

Also remember that the fuel you buy may not always be up to the advertised octane standard it is supposed to be. So a bad load of fuel can destroy your engine if it is tuned right up to the detonation limit on best quality fuel.

[Sydneykid]

Good answer Warpspeed, I would only add that we have run up to 1.7 bar with standard pistons without issue on a properly tuned engine. I have seen improperly tuned engines crush the ring lands at 0.7 bar. It is more about tuning, that boost.

Hope that helps

[GTS-t VSPEC]

Yeah, I agree with what is being siad, high boost will not the piston damage. I think most people say high boost cause if your car does start to detonate, which is more likely at high boost, then you will wreck the pistons.

Full aftermarket management is the only way to go for boost higher than 14psi, cause then it can compensate for air temp etc, to prevent detonation in any situation.

See'ya:burnout:

[meggala]

the otehr issue is when people up the boost and still hav ethe stock fuel pump which leans it out andcauses the big bang nissan motors are tough its the qualityof your tune which is critical

meggala

[Steve]

Thanks for all the replies,

Clears things up nicely, and somewhat of a relief, as I was intending on going over 1 bar, with everything else sorted of course:)

Cheers

Steve

GTS-t VPEC, have you been watching a star wars vids of late?

'high boost will not the piston damage'...j/k

[GTS-t VSPEC]

Sorry Steve, But the thought was there. Sounds better that way:p

Anyone tried the Aries pistons and rods, very cheap, a set of pistons and rods for an SR20 with rings, pins etc, about $1600.

See'ya:burnout:

[Steve]

Do you know who supplies these, very interesting

[crooser]

Originally posted by Steve

How can boost pressure (eg 14 psi is safe, 15psi and you will have a hole in your piston) be true.  It just doesnt seem to add up.

Your sources might be confused with the max recommended

boost (14psi) with the standard turbo?

chinese whispering is rampant amongst the import world

[mr180]

Sounds good GTS-t VSPEC. Would be interested to hear some reports on how they perform.

[Xeron]

yer detonation has a million variables, u cant say all engines will detonate on a certain CR or boost level. Things like quench distance of the engine (if it has a quench head at all), the cr and boost levels, the quality of the casting (if u have say little spurs sticking out of ur combustion chamber, they tend to induce detonation) and a million other things all go towards it.

Things uve gotta watch out with stock pistons are rpm levels (depending on wether the stock RB pistons is cast, hyper or forgie?) and detonation. There the main killers.

[GTS-t VSPEC]

Steve/mr180,

I know that Steve at SST (Perth) has ordered an number of sets to build race engines with, he reckon's there very good, just wanted to know other people opinion if they had come across them before.

See'ya:burnout:

[Steve]

I have heard of them, but never heard any reports as to relaibility etc.

Crosser, I think you got it in one there.

[Guest MFX_R33]

Obviously you are not looking at boosting over 14psi with the standard turbo?

Jeff.

[Steve]

MFX_R33, no, I have a HKS 3037S56T going on soon.

Steve

[Sydneykid]

Hi GTS-t VSPEC, we have used Arias pistons a lot, good product no problems, although we use Total Seal rings not the ones supplied by Arias with the pistons. Haven't used their rods, don't know if they make them themselves or buy them in. There is a bit of this going on in the US at the moment, piston guys selling rods and rod guys selling pistons.

Competition makes the prices cheap.

[Guest Peewee]

.

[Warpspeed]

Peewee, what sort of peak cylinder pressures are you talking about that will break pistons ? Give me a pressure figure. I would also like to know the MEP of the engines that one of your good tunes can create that will break pistons.

[Steve]

I was under the impression that the main damage caused by pressure in a well tuned engine was at TDC and BDC and effects bearings and conrods???

Anybody shed further light?

Warpspeed, can you explain MEP?

Cheers

Steve

[Warpspeed]

Well Steve, I certainly do not claim to be an expert, but my beginners understanding of the stresses on engine parts are as follows.

Maximum pressure loading on the piston will occur at around 20 degrees ATDC on the power stroke. It should rise to a smooth hump peak around this point. Thermal loading depends on all sorts of things, but if the engine tuning is correct there should be no hot spots, and the piston crown temperature should not be high enough to cause pre ignition or lower the structural strength of the piston material. Detonation is really the only way you are ever likely to kill a piston, assuming it was installed and clearanced properly.

Conrods really only have two problems. Firstly the tensile (pulling apart) stress at TDC on the exhaust stroke is the highest stress the rod will see. This depends on piston weight and RPM. The cyclic tension and compression can eventually fatigue the rod to the point where fatigue cracks can begin. That is the reason they polish rods to help to eliminate surface imperfections where cracks can begin.

The second problem is that the same tensile stress can stretch the big end bolts and cap out of shape. This can lead to bolt failure, or lubrication problems where the big end bearing can spin or burn. As the big end stretches, the sides pull inward and squeegee all the oil off the journal. This is RPM related as well.

You only really require special rods if you are planning an increase of RPM well above stock. On a road car a piston speed of 4000 FPM should be the redline with steel rods. For drag racing, or where the engine will be regularly inspected and rebuilt, 4400 FPM might be as high as you would want to go.

A large power increase, using stock RPM levels does not require special rods unless the original stock rods are rubbish, or boost levels are truly extreme. Usually the rod bolts are the weakest part of stock rods.

Crankshaft bearings should not present any real problems, provided engine balance is reasonable. Beware of crankshaft torsional vibrations with six cylinder engines that have longish strokes. This can lead to crankshaft fatigue failure, and ring flutter problems causing blowby in the front cylinders.

I am sure Sydney Kid will be able to add to this list.

MEP is mean effective pressure. If you add up all the positive and negative pressures during the induction, compression, power, and exhaust strokes, you end up with a positive average pressure dominated by the power stroke.

This average pressure can be thought of as what pushes down on the piston and produces output torque from the engine. If you know MEP, bore, and stroke you can easily calculate flywheel output torque. It is a figure of merit for an engine.

There are two ways to gain more power, increase MEP or increase max power RPM.

The reason I was asking Peewee what MEP his supertuned engines run that easily break pistons. I wanted to know if he could give me a figure, for peak pressure, and average pressure (MEP). If he can so easily break pistons, I would not want him to tune my engine thats for sure.