severly stumped 2tg/13t hybrid build

[Born-Enthusiast]

I noticed the piston crown have not been blended at all. Still all very sharp edges.
The section which has broken away from the fly cut section is very thin and sharp. This area although right next to the fresh charge from the intake valve, was probly creating a hot spot in the chamber and causing pre ignitio/detonation. The Duralite ACL pistons (which is what they look like) are hopeless for detonation resistance so this is a very important factor to consider.

you know this explanation actually is quite plausable im surprised noone i'v taken the pistons too said that detonation was always brought up but cause there is no divet marks in the pistons has been dismissed by a few people shouldn't the machine shop have made sure this couldn't happen or is it something you specifically have to ask for.

would you be able to explain for me how the sharp edges causes hot spots and how blending helps cheers
oh lastly what pistons would you recomend to get this puppy running sweet as im going to need new ones obviously lastly if anyone has some they don't want any more for instance an unfinished project thanks for the help

[oldcorollas]

did you properly gap the rings before you installed the pistons?
if they were too tight, the first time you got their temp up a bit, they maye have expanded, kind of bound in the cylinder, and smashed the ring lands out....

i'd pull them off and check them in the bore now...

whaddayathink Jeffro?

[jeffro ra28]

Fair call their Stu, BUT 98% of the time the rings that come with the standard replacement style ACL pistons are correctly gapped from factory. It is only in performance applications that the rings need to be filed back for more end gap clearance, and the rings that are supplied are designed to do so. Coming out of the packet with very small clearance designed for you to file them back. These are called "file to fit rings".

If the rings did bind, there would be a single sharp deep score verrtical in the bore which would suggest the area where the rings ends clashed.

As stu said tho, remove the current rings from the pistons and place the top ring parrallel to the bore about 1" from the deck. This can be done with a ruler to measure that all sides are eqaul length down the bore, or you can simply leave the oil ring in the piston and place the piston in the bore upside down to push the top ring down parrallel in the bore.

The advice with blending of pistons. This really isnt something you would get from many engine builders at all. It is more of a race engine preperation type thing that is only ever done to race engines. I practice this on any engine i build because it helps leave a larger safety margin for detonation. Even a set of custom made JE or CP pistons which will cost around the $2000 mark for a set dont just get thrown in the engine, they are always prepared in this fashion. Because i beleive it is not satisfactory from the factory.
The machine shop or engine builder are not inclined to do this at all because time is money. They wont do it unless requested, thats if they even know what it is.

The other thing to consider with these style of pistons is that they are a dome type because of the hemisperical chamber the dome is needed to gain some compression. This dome interfeers with flame travel. So the blending of the crown and all the sharp edges helps promote flame travel aswell which in turn will increase burn rate and finally increase horsepower. Whilst mimimising potential "hot spots" that will cause pre ignition/detonation.

"Hot Spots" is a term we use to describe a place in the combustion chamber that could be hotter then it should be. A hot spot can be things such as, sharp eges, burrs on cylinder head and cylinder deck from machining that where not deburred, overlapping head gaskets, spark plugs with to high a heat range, valve with burrs in the edges and lastly, very thin sections of metal anywhere in the chamber or piston crown. This is the problem with your pistons. What happens is that the "hot spot" gets very hot and can cause the engine to pre ignite before the spark plug fires. Which can mean that your engine is pre igniting itself well before the set ignition timing, a similar affect to having your timing set way to advanced.
I suggest that you google, "pre ignition" and "detonation" for a better explanation.

In my mind, a chamber design to take a flat top piston is the ultimate design. Nascar are of the same principle. A very small wedge chamber to suit a flat top piston and this can result in 12.5:1 compression easily and whilst at the same time being very detonation free. Nscars in the nextel cup series are running from 12-13:1 compression and are using what they call "union 98" which is equivilant to our 98 RON fuel. This proves the design.
I just built a motorcross race engine with 13.5:1 compression and it will run on 98 RON premium unleaded with no drama's at all. However all these precautions where taken into mind and the chamber itself was also reshaped and modified.

A HEMI chamber however is the most detonation proof chamber of them all, however the piston dome is what creates the problems. Thats why it is so important to blend the domes.

As far as piston choice goes. I am building a similar engine to this one as we speak. 2/3tg with the 3t bottom end. Using ACL duralite pistons. I have made this engine with 11.2:1 compression. Im sure a regular 3tg combo has about the 9.5:1 mark. Which is still good. But i should get away with it as everything is prepped accordingly.

May i sugest you invest in a timing light if you dont already have one. It is a MUST if you are going to be accurate with setting the timing. At work we are all basically pro's when it comes to setting dizzies and magnetoes. However we dont always get it within 1 degree. Its usually 2-3 degrees. Which can be all the differance. So you NEED a timing light to check it, everytime no matter how good you are.

I hope that answers most of your questions

[merc-blue]

Its intresting that you didnt confirm the ring gaps?
I have never trusted a machist on bearing clearnences we always double check them, same i wont trust a piston manufactorer with rings gaps,
every one can make a mistake, so the more times things are checked the safer you can be,

We had a similar thing happen to one of our engines, but that was more a dodgee previous owner putting on different heads and hacking a set of JEs with a die grinder to suit

[Rinmax]

Well if nothing else comes of this thread I have learnt a lesson. Thanks for the write up Jeffro, it totaly explains what happened to my engine too.

[allencr]

Is that 2nd ring installed wrong?? Usually the combo compression-scraper 2nd ring has a notched lower edge to handle oil, and the pics show it on top.

I think it's detonation(detonation IS NOT pre-ignition), and a piece o'shit piston. It looks like the broken land may have been that way quite a while because it looks pretty carboned up & not too fresh.

[jeffro ra28]

Defonantly detonation, no doubt. Nothing else would do this sort of thing. Those ACL pistons cannot handle it one bit.

[Born-Enthusiast]

thanks heaps for the info guys now im not as pissed my engine is out of action i still have to do a bit of work again tho we did check the ring gap before we put them in my dad made sure i did it and we were all good with it we used hasting rings had adequet gap between the meeting points on the rings when we checked them in the chamber i think hot spots is my problem and seeing as tho i never asked for blending this is probably where my problem is however due to the fact i have very little money at the moment its going to still be a bit till its running

[jeffro ra28]

I may have mislead you a little bit there. This probly shouldnt happen even if the hotspots where not there, I think you may have had your timing way to advanced or to lean a fuel mixture to cause this.

[oldcorollas]

well spotted Allencr
http://hastingsmanufacturing.com/Ser...stallation.htm

Rings with a groove in the outside diameter and no "pip" mark or dot must be installed with the groove toward the bottom of the piston.

[Born-Enthusiast]

we set the timing off the dizzy by setting the engine to top dead center then slowly rotating it by spanner making sure that the spark was being produce right before top dead center was achieved we had to pull out the dizzy a few times and rotate the thing at the bottom of where it sits a few times to get it right my dad is really cluey when it comes to this stuff hes been working with engines for over 30 years and wouldn't let me do it wrong we spent three hours making sure the timing was correct before we even tried starting it and fidling with timing advancement and retardation

[Born-Enthusiast]

well spotted Allencr
http://hastingsmanufacturing.com/Ser...stallation.htm

the pistons rings were put in by my dad why i watched we followed the instructions on the box so unless we had it upside down we thought it was right

[andurils_sheath]

we set the timing off the dizzy by setting the engine to top dead center then slowly rotating it by spanner making sure that the spark was being produce right before top dead center was achieved we had to pull out the dizzy a few times and rotate the thing at the bottom of where it sits a few times to get it right my dad is really cluey when it comes to this stuff hes been working with engines for over 30 years and wouldn't let me do it wrong we spent three hours making sure the timing was correct before we even tried starting it and fidling with timing advancement and retardation

That sounds very wrong to me. The base timing on these cars is set at 1000ish rpm, and the spark should be firing 16 degrees before TDC. Timing lights are very inexpensive and all you do is get the car idling and have the timing light connected between the cyl 1 spark plug and the dizzy. Shine it on your crank pully and itll flassh at the time cyl 1 is firing. This should light up a small line that corresponds to the current timing, against a scale on the timing cover. As you rotate the dizzy you should be able to see the timing change against the scale. Sometimes a small amount of white paint is required on the line to make it easier to see.

EDIT: BTW if its idling this process should only take between 5 nd 10 minutes max.

Cheers hope it helps and hope you get it all going.

[Born-Enthusiast]

Sorry thats what I meant the timing was definately not the issue here. The idle was fine sorry if im missinforming you. Im not all up with the correct terminology, but like I said my dad did the fine tuning with the carbies and the timing while I watched. He showed me the spark as you rotated the crank and how to line it up right. The timing was correct, with general detonation it should have been present from word go and after 1000KM surely there would have been tell tale signs on the pistons. Right? I'm incline to agree with JEFFRO about the hot spots being the cause, opposed to the timing my dad is very good with the things and as I said we took a long time making sure it was right. Sorry if im coming across definsive its my fault i got the explanation wrong thanks for the input tho i do appreciate it

[thechuckster]

edit: obviously carbis.


Did anyone tighten down the dizzi clamp? you might have set it right, but vibration and a loose clamp might have let it shift