Boost and Piston Damage

[Steve]

Warpspeed, thanks for a really excellent reply - it has certainly helped my understanding a bit, and helps explain why some people can get much better hp figures than others with stock engines - it all relies on how you make the power, not how much power - is this on the right line?

Your reply also reminds me of where I have heard MEP - Corky Bell, I should have remebered, but glad I asked the question, alot of interesting stuff in your reply.

Raises another question, hope you dont mind, you mention 4000FPM when talking about piston speed? Is this for Feet Per Minute? Also, is there a simple formula (or a complex one) for converting this to RPM - I take it, this will be number of pistons and length of stoke dependant?

Thanks again

Steve

[Warpspeed]

Yes Steve, I think Corky Bell mentions MEP in both his books.

I find it interesting that it is the combination of MEP and RPM that give the final power figure. High RPM are going to put a lot more stress and wear on engine parts than high MEP.

So if you can get 400BHP at 6,000 RPM with a turbo, it is going to be a lot kinder on the engine parts than 400BHP at 9,500 RPM normally aspirated. The turbo engine will have a far higher MEP (and torque output) because of the boost pressure.

The formula for piston speed is: stroke in mm, multiplied by RPM, divided by 153. This gives you mean piston speed in feet per minute. This is the figure everyone uses. Maximum peak piston speed will be a lot higher.

The main reason Formula one engines can reliably rev so high, is that the stroke length is very short. They get the required capacity from bore size and number of cylinders. Motorcycle engines also rev easily because the cylinders are small and the stroke is short. Some of them can reach crazy RPM without huge conrod stresses.

I am not sure what the exact stroke length of the current V10 engines is, but I think it might be around 55mm. At 18,000 RPM the piston speed would be 6470 fpm. Well beyond a steel conrod, and getting pretty high even for an aluminium or titanium conrod.

[Guest Peewee]

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[Steve]

How is this poor piston design. It is used to enable oil to cool the pistons - further removing the chance of detonation.

I find it interesting that an engine design that has stood the test of time, been modified to high degrees, with great reliability, produces high output reliably in stock form, has an at the time unbeaten track record and you call it "such poor design".

Can you explain this please, as it flies in the face of all thought currently being expressed on GTRs? I am more than a bit confused also, you own a skyline, join a skyline forum and then start slagging off the cars? This forum is for use constructively, if you have an opinion, why cant you back it up with facts or personal experiences - instead of just making wild claims?

I am not having a go, but I do believe that perhaps you are missing the idea behind this forum, and the reason people come here - to hopefully be able to learn something and share knowledge - not provoke arguement and disrupt threads started by people trying to gain knowledge.

Have you had a look in the Wasteland? If you want to have a go at skylines, why not pop in there, I am sure you will find people willing to further discuss it.

[Warpspeed]

The GTR pistons have proved to be a very good design. I know nothing of MR2 pistons, I have only seen pictures. Also Mazda are now doing this as well on some turbo engines.

The circular oil cooling gallary that runs behind the top ring and the two diagonal oil holes in the GTR piston are very clever design. I have looked at this for a long time, and I am stuffed if I know how they manufacture it in a cast piston.

The oil squirter squirts oil straight up into one of these holes. The oil then runs around inside the hollow piston, and drains out of the opposite hole. A very ingenious and efficient way to remove heat from the piston crown.

But Steve is right. You would make yourself more welcome I you added to the knowledge rather than slinging off at people and thier cars.

If you piss off enough people the moderators might start to take an interest in you.

[Sydneykid]

Hi guys. very well covered topic, the only thing that seems to be missing from this thread is the stroke versus rod length discussion. You can have an engine configuration (rod length versus stroke) which causes excessive side loading on the conrod, ie; as in Warpseeds example, at 20 degrees ATDC on the power stroke, maximum pressure loading is exerted on the piston and rod. If the stroke/rod length ratio is excessive then this can rapidly accentuate the cyclic fatigue of the rod itself due to the angle of the pressure not just its maximum amount.

This (excessive rod angle) is quite common when you increase the stroke of engine and shorten the rod to accommodate this longer stroke within the block height. This is the reason why the RB26 3 litre kits from OS Giken and others actually increase the height of the block by longer cylinder liners and deck spacers. Thereby keeping the rod length long enough to avoid the excessive loadings.

This is also the reasoning behind longer rods sometimes appearing to be weaker because they are narrower in profile. And a shorter rod stronger because is has to be thicker in profile.

The other issue worth considering is the failure of big end bearings due to premature pressure loadings (eg; before 20 degrees ATDC). It is known to have engines that do not detonate sufficiently to cause piston damage, but have pressure loadings early enough in the cycle to over accelerate the piston and conrod thereby compressing the oil film and causing damage to the upper big end bearing shell.

This is something that I have seen a number of times in high torque turbo and methanol powered engines. No piston damage, no signs of detonation, no logged detonation ( in meaningful amounts), no discernible lower bearing shell wear but extreme (to the point of failure) upper bearing shell wear.

This is atypical of these engines having been tuned very close to there max.

Perhaps a couple of practical applications of the theory would be useful. Let's get the ball rolling, a standard RB26 is 3,850 FPM at 8,000 rpm, which is getting pretty close to the theoretical max for steel conrods. An RB30 is 4,170 FPM at 7,500 rpm, this is why we strongly recommend a conrod upgrade in RB30's when a DOHC conversion (RB25 or RB26) is done. This is also why an RB20 can see 8,750 rpm (3,990 FPM) without excessive conrod loadings.

Hope that stimulates some further thoughts.

[Warpspeed]

Hi again Sydneykid.

I knew you could add something really worthwhile, you always do.

Have you ever tried putting an RB26 crank with some verrry long rods into one of your RB30DET motors ? The rod ratio would end up well over 2.0, I did work it out once, but cannot remember the exact rod ratio figure.

I think it was Bill Sherwood on another thread said the Formula one guys are using a rod ratio of about 2.3 or something. The change in piston motion around TDC and BDC would be pretty dramatic.

From what I have read huge rod ratios give better high RPM cylinder filling for a given cylinder head flow, and port volume. Any ideas on this ?

[Xeron]

long rods are a key element of any good engine. Hell a standard mopar v8 uses 6.123" long rods, thats a BIG rod Longer rods cause the piston to 'dwell' at tdc longer which produces better combustion, and increases the engines volumetric efficiency. Higher vr = more air/fuel into the engine, so more power.

And saying a stroker engine will need shorter rods is a misconception, the added stroke is usually made at the bottom of the stroke, and shorter pistons (thus stroker pistons) make up any added length at the top of the stroke. Although stroker rods are different to normal rods, i think it has to do with the size of the big end tho, im not actually too sure.

Any of u guys ever had any experience with pistons that have been ceramic coated on the top?

edit: although strokers do have lower r/s ratios, due to the same length rod but a longer stroke obviosly.

[Steve]

Yet more interesting replies, thanks again.

Raises another question, how would you reduce to the point of insignificant the damage to big ends by over acceleration of the piston?

Xeron, I think I am missing something here, I thought it was mentioned that shortening the rod to increase stroke commonly causes excessive rod angle?

Can you explain how a longer rod causes the piston to dwell longer? And how stoke length is made up at the bottom of the stroke? Sorry for all the quesitons, but this really flies in the face of my really basic understanding of physics as it applies to the geometry of the internals. Sorry there are so many questions here, but understanding how things work helps me remeber:)

Cheers

Steve

[Xeron]

Shortening the rod dosn't change the stroke, the crank still pushes the rod up and down the same distance, it will just kill your compression ratio and like you said, it induces excessive rod angle.

Hrm, its kind of hard to explain... im ****ing sick as and drugged up atm, so if i confuse you im sorry

Okay since the rod is longer, it works on less of an angle right. Due to this its at TDC for more degrees of angle, thus a longer period of time.

Ill try and find a site that explains it better when i get home tommorrow.

Um with strokers the extra displacement is made by pulling the piston further down the bore, but it still onl raises to (roughly) the same tdc hight, the extra displacement comes from the bottom of the cycle. The main problems you have are the rods wont usually clear the block if you use the standard sized rods, so some griding needs to be done on the big end.

[Sydneykid]

Hi guys, I just knew this thread would get interesting. The problem I have with "stroker pistons" is they move the gudgeon further up the piston. This reduces the amount of piston skirt available for the rings ie: they may have to be closer together. This leads to a propensity for ring land damage as there is less metal to absorb the load on the rings.

A higher gudgeon pin also causes more bore wear as the thrust loading is no longer balanced on the piston skirt ie; it is higher up than previous so less surface area to absorb the sideways thrust.

Somebody asked about ceramic coating of the piston crowns. We have done a number of engines with full ceramic coating inside the combustion chamber, exhaust ports and exhaust manifold. The reduction in oil and water temperature is certainly noticeable. I can't say that there is more horsepower or less lag (due to higher exhaust speed) as it is more difficult to measure. We always do other things to the engine, so it is never a straight before and after comparison.

We have had one bad experience with an engine that had the ceramic coating come off, made a real mess of valves, turbo, exhaust, intercooler etc Coating was not done by our usual guy, the owner had it done himself. We have not a problem with our supplier.

Hope that adds to the discussion some more.

[Xeron]

From what ive heard the coating reduces detonation a fair bit due to the ceramic reflecting heat back better and not absorbing it and creating hot spots. Ive never actually heard of the combustion chamber being coated, most people just polish it to a mirror finish from what ive seen.

[Warpspeed]

Steve.

Engine geometry is a fascinating subject in itself. If you start off with just the cylinder block, the distance between the head face and the crank centerline will be some fixed dimension. When you decide to fit crankshafts with various strokes to this same block, there are a few side effects to the internal geometry.

The first thing is that at TDC the piston is always going to be at the top of the bore, level with the top of the block. At BDC it will be the stroke length down the bore. So what Xeron says about all the additional stoke length being at the bottom in a long stroke engine makes sense if you think about it.

Carrying this a bit further, a longer stroke engine will have the crankpin higher at TDC, and lower at BDC than a shorter stroke engine. So it follows that If you use identical pistons the long stroke engine must have conrods shorter by half the increase in stroke length. This all assumes an identical block is used for all these changes in stroke.

There will be a ratio of conrod length to crankshaft stroke, called the rod ratio. If the rods are 100mm long and the stroke is 50mm the rod ratio will be 2.0

If you increase the stroke, the new conrods might have to be 95mm, and the stroke 60mm, giving a new rod ratio of 1.58

As sydneykid says another way to do it might be to keep the same conrods in the long stroke engine, and move the piston pin 5mm higher up the piston. In this case there would be 100mm rods with the 60mm stroke giving a rod ratio of 1.66

The higher pison pin placement may not be physically possible, or it might require the ring package to be very high with close ring spacing. This will weaken the ring lands and make the piston more susceptible to detonaton damage. But it also might lead to a shorter and lighter piston as well.

So whats the big deal with rod ratio ?

Try to imagine an engine with the shortest possible conrods, at the 90 degree stroke points the conrods will be at an extreme angle to the bore. They might slope 45 degrees one way, and then slope 45 degrees the other way. (not actually possible, but you get the idea)

Now imagine some very long conrods, maybe two metres long. The conrods will always be almost vertical throughout the entire stroke.

Vertical downward pressure on the piston is going to try to wedge the piston against the side of the bore with the very short rod engine.

The long rod engine will push straight down onto the vertical conrod putting almost no sidethrust onto the piston at all. This is more mecanically efficient, but very long conrods are going to make the engine block tall and heavy. So there has to be a compromise in this.

Conrod ratio also has an effect on the motion of the piston in the bore. If the conrods are very long the piston moves almost in a perfect sinewave motion.

Short conrods distort this, the piston moves faster during the top half of the stroke than the bottom half of the stroke. This is difficult to visualise, and almost impossible to put into words.

But either get yourself an engine, or make a cardboard model, and plot piston position for each ten degrees of crank rotation. You will find that the piston almost stops dead for maybe thirty or forty degrees of crank rotation around BDC. The piston might move less than 1mm or 2mm. The bottom of the conrod swings in an arc on the piston pin, which corresponds to the arc of rotation of the crankpin. So the piston just stops for a while.

At the top of the stroke, the piston rapidly decelerates and changes direction very suddenly with a short conrod engine.

So the rod ratio has a dramatic effect on the piston motion in the bore. This has all sorts of side effects, and influences what happens on the induction stroke and power stroke, and also valve timing requirements.

[Steve]

Some more great replies, thanks all.

Warpspeed, for someone who describes something as difficult to put into words, you did a bloody good job - its all alot clearer.

Sort of explains why all the wear is being done at the top and bottom of the stroke, loads increase as the energy is trying to transfer with poor mechanical advantage. Have I got this right.

Also asks the question, how much more efficient would an engine be if there wasnt the loss of energy here, then again I suppose it also has some positive features too?

Cheers

Steve

[Guest Peewee]

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[Guest Peewee]

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[Warpspeed]

Peewee, I am sure all you say is true.

But why do you always just rubbish without offering anything constructive. People read these posts hoping to learn something new or interesting. So why not try and give facts, figures and explanations.

If you have the knowlege and experience that you claim to have, share it with us. What exactly is it about the RB engine that you do not like. If you were going to re design it how would you do it and why?

[Warpspeed]

I will have a go at answering your piston question.

The thing about any highly stressed part is not how much metal there is, but where the metal is, and where the stresses are. The aerospace people are very good at designing very strong parts that are also light. The trick is that you can safely remove material from places where there is little stress.

If you have ever examined a piston that has cracked due to detonation it is the ring lands between the rings that fractures. The ring grooves are rectangular in section, and the cracks start at the root of the ring groove, and goes through to the next ring groove. Usually a whole section of ring land breaks away and rattles around trapped between the rings.

The way to make the ring lands stronger is to put more meat into the ring land. In other words place the rings further apart so the ring lands are wider.

In most cases the crack does not go further into the body of the piston, but sometimes it will. So putting an oil cooling duct in the pison crown is probably not going to weaken the piston significantly.

I have never seen a detonation fractured GTR piston, but I am unaware that there is a design problem here. If pistons were cracking through to the cooling duct, and it was a common fault, we would all have heard about it by now.

I have actually broken a piston into two parts due to massive detonation (and my own stupidity), but I could not say that the piston design was in any way at fault.

Design problems always show up as the same recurring fault. A one off failure is not a design problem.

[Guest Peewee]

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[Steve]

If bigger were better, wouldnt pistons be 3"+ thick at the crown?

I was under the impression that to strengthen ringlands, you placed rings further apart, not increased the amount of metal beside them.

Anybody else heard of this?