Potential Piston Problem

[CrutyRammers]

Sardonicus said:

Pumaracing said:

Max, that was a very nice and well written explification of detonation and pre-ignition. I'm sure many on here will find that helpful.

+1 Thats probably the most comprehensive I have read on the subject anyway.

Yep, I expect that there are many more of us who are "readers" rather than contributers in this section. I'll add my appreciation

[Pumaracing]

the_stoat said:

The previous spec had standard rods and was running hydraulic lifters, so the standard valve train....

For info the previous compression ratio for the damaged pistons was 11.1:1.

There's part of your problem then. That's a lot of CR for the stock cam duration with ITBs and presumably a decent exhaust. The ignition advance would have needed pulling back a good bit from stock and I can now see that easily catching a mapper out. I think you might have to put your own hand up to this one. Ironically it was perhaps fitting the high comp pistons that errmm, well killed the high comp pistons. The stock pistons are already forged ones in those engines and don't need changing except when it's necessary to get very high CRs with race cams.

[the_stoat]

Sorry by stock valve train I meant everything bar the cam shafts. I would have to rummage in my records for the cam spec but it was aligned to the compression ratio and the specs were given to the all that mapped it.

[Pumaracing]

the_stoat said:

Sorry by stock valve train I meant everything bar the cam shafts.

Ah! Hmmm.

Anyhoo. The Kent AST1627 you've chosen, if I'm correct, is a sensible general purpose grind. Not too intractable. The CR suggested should be fine with it. I can't see this spec posing the slightest problem for any competent mapper.

You might want to try and get the coolant temp down a tad. Low 80s would give you a bit more leeway on mapping settings and reduce the risk of det.

[Pumaracing]

Max_Torque said:

I hope you'll let me add the following.....

Oi! Don't milk it son. You wrote a nice article and got a well deserved pat on the head. Don't start getting cocky now.

[the_stoat]

Pumaracing said:

Ah! Hmmm.

Anyhoo. The Kent AST1627 you've chosen, if I'm correct, is a sensible general purpose grind. Not too intractable. The CR suggested should be fine with it. I can't see this spec posing the slightest problem for any competent mapper.

You might want to try and get the coolant temp down a tad. Low 80s would give you a bit more leeway on mapping settings and reduce the risk of det.

Thanks for the advice, it is actually a QED 450V and was chosen for tractibility. Have a lower temp thermostat that I can test for water temperature reduction.

[Pumaracing]

the_stoat said:

Thanks for the advice, it is actually a QED 450V..

Suspiciously similar, well ok identical, lift, duration and timing ain't it though

I do get a chuckle from that phrase you see so often in engine supplier's blurbs "we have developed" when the bits are usually just someone else's off the shelf items with a custom part number stamped on them.

[Evoluzione]

Max_Torque said:

I hope you'll let me add the following, by way of some defense of the "idiot mapper".


For "aftermarket" mappers, they are generally never given enough time (because both their time and using a rolling road /engine dyno is expensive). Added to which is the pressure to make as much performance as possible for their client.



Without access to accurate knock determination, be that from proper in-cylinder measurement of combustion pressure, or via a secondary sensing system (eg engine block vibration sensors("knock" sensors) etc) it's difficult to know "where to stop".......




On turbo engines, what tends to kill them is not usually the nominal ignition angle, but failure to fully and correctly compensate for those "off nominal" events like high charge temps, or poor boost control, or insufficient injector headroom, and even barometric pressure effects.

I don't think that not investing in the correct and readily available equipment, charging so much that people can't afford your time and not understanding compensation values are a very good excuse for a shoddy job!

[PeterBurgess]

Spot on Monsieur Le Evoluzione.

Peter

[anonymous-user]

Evoluzione said:

I don't think that not investing in the correct and readily available equipment, charging so much that people can't afford your time and not understanding compensation values are a very good excuse for a shoddy job!

I'll have you know that that^^^ is pretty much the basis of most UK industries! We are world leader in doing as little as possible for the largest amount of money....... ;-)

[Huff]

CrutyRammers said:

Yep, I expect that there are many more of us who are "readers" rather than contributers in this section. I'll add my appreciation

+1 to that. Thanks all.

[PaulKemp]

It is clear that informative threads such as this take me a little way from "not knowing what I don't know" and a bit closer to " Knowing what I don't know"
And thus I have half a chance of asking reasonable questions of the mapper.
Still an expensive minefield

[Pumaracing]

PaulKemp said:

It is clear that informative threads such as this take me a little way from "not knowing what I don't know" and a bit closer to " Knowing what I don't know"

Wise words and one of my favourite quotes. It's always what you don't know you don't know that catches you out. What you do know you don't know can be always be looked into. Something that has cropped up in this thread is the relative importance of A/F ratio versus ignition mapping for detonation control. Engine temperature vs A/F ratio is a complex and very misunderstood topic. Flame temperature for most fuels maximises at stoichiometric which for petrol is 14.7 A/F ratio by mass. However this only a tiny part of the issue. I think we might need to get into this further. Maybe after I've walked the dog.

[Evoluzione]

And fed the chicken.

[jagracer]

OP, out of interest what do the big end bearing look like? I went through quite a few engines by running miles too much advance, recommended by a couple of so called very experienced engine builders and rolling road tuners, which resulted in bearing, and on two occasions crank failures. This went on until one day I managed to melt a head gasket, luckily before the pistons succumbed. It wasn't until a friend explained it as Max has that I realised what the problem had been.

[one eyed mick]

I have had thoughts very similar to Paul above ,this thread has cleared away some of the fog ,thanks to all involved

[Pumaracing]

jagracer said:

OP, out of interest what do the big end bearing look like? I went through quite a few engines by running miles too much advance, recommended by a couple of so called very experienced engine builders and rolling road tuners, which resulted in bearing, and on two occasions crank failures. This went on until one day I managed to melt a head gasket, luckily before the pistons succumbed. It wasn't until a friend explained it as Max has that I realised what the problem had been.

I very much doubt if det was your main problem, or only problem. It certainly increases loads on the bearings but it's almost impossible for it to kill those before it damages the piston noticeably. The OP's det is mild and has gone nowhere near pre-ignition. I suspect his crank and bearings are just fine.

Your issues could have been many things including bearing clearance, oil viscosity, oil pressure and yes perhaps mild det was some part of the problem but unlikely to be the only one. Head gaskets can die for many reasons and poor surface finish on head and block is a main one. Many engine machinists manage barely better than a ploughed field finish, which is fine, well for ploughed fields.

One of the first engines I ever built for a paying customer was an MGB race one. It had rarely finished more than a few races in a row before the head gasket gave up, this over many seasons of racing. Successive engine builders had tried stock gaskets, copper gaskets, various types of goop and sealant. I used a surface grinder to put the perfect engineering finish on the head and block, a stock Payen composite gasket, nipped the head studs up a tad over the OE torque setting and it never had an issue from then on. The devil is usually in the details.

[Pumaracing]

the_stoat said:

Thanks for the advice, it is actually a QED 450V

http://www.kentcams.com/product-details/75/Camshaft/Camshaft/AST1627-Race/

QED 450V duration 294, Kent AST1627 duration 294
QED 450V lift 0.458", Kent AST1627 lift 11.63mm (0.458")
QED inlet full lift 106 ATDC, Kent AST1627 inlet full lift 106 ATDC
QED exhaust full lift 104 BTDC, Kent AST1627 exhaust full lift 104 BTDC.

Hmmmm.

Call me cynical.

[anonymous-user]

Pumaracing said:

jagracer said:

OP, out of interest what do the big end bearing look like? I went through quite a few engines by running miles too much advance, recommended by a couple of so called very experienced engine builders and rolling road tuners, which resulted in bearing, and on two occasions crank failures. This went on until one day I managed to melt a head gasket, luckily before the pistons succumbed. It wasn't until a friend explained it as Max has that I realised what the problem had been.

I very much doubt if det was your main problem, or only problem. It certainly increases loads on the bearings but it's almost impossible for it to kill those before it damages the piston noticeably. The OP's det is mild and has gone nowhere near pre-ignition. I suspect his crank and bearings are just fine.

I'd imagine bearing failure was simple related to the mean ignition angle, rather than the detonation!

Detonation results in pressure waves, typically of around 10bar at a fundamental frequency of something like 7 to 10KHz. Because that frequency is high, the inertia of the piston and rod effectively prevents those loads getting to the bearings! However, the more you advance your ignition timing towards TDC, the higher you peak firing pressure is going to be (because you will be burning the fuel at a low average chamber volume). Fast burning engines, that run with advanced ignition angles (because retard = more time to burn!) have short sharp "tall" cylinder pressure profiles, whereas slow burning engines (such as heavily turbocharged ones, that have a LOT of fuel to burn per cycle) have long low smooth ones.

Typical traces look like these:



You can see that although the burn starts just fractionally earlier (look where the pressure traces first starts to rise above the "unfired" trace) the peak pressure value is hugely higher

As it is the peak cylinder loadings that define the oil film thickness and bearing material fatigue limits, by retarding your ignition angle to "avoid det" you probably just reduced the peak bearing loads enough to prevent another failure......

[stevesingo]

The problem Max described is seen often in the BMW S14 (E30 M3) community.

Many folks fit aftermarket chips to the ECU which have 6deg more advance between 3k rpm and 7k rpm.

Because the engines are not knock limited due to the relatively low compression (10.5:1) there is no audible knock, but the peak pressure is occurring too early leading to worn and sometimes spun big end bearings.