Squish/quench Boring?.....trying To Understand It All

[nsta]

Hey guys, i thought id start a new topic after having not much information on the rb26 head squish/quench areas(ive attached a picture to show what i mean). I just pulled off the head on my rb26 engine to find that no.6 chamber got the worst treatment from the cylinders, and from the oxidation - it looks like it chewed through the no.6 bore on head squish/quench area with the help of detonation most likely, the other cylinders were well of and had no real signs of the squish areas being taken off at all....

It was no.6 cylinder that had the most coolant in it when we pulled off the head so it kind of makes sense....

The car was rebuilt not long ago at all, and i suspect having seen the block had been bored out, it was fitted with oversized pistons, and it seems alot of people on the forums skim some of these quench or squish areas of the head to firther limit engine detonation and so forth?....ive even seen where its been removed altogether to stop it altogether, whilst others remove the exhaust side only, and skim the inlet size.

I need to know the pros, and cons of doing this when talking to my specialists, and whether or not its worthwhile to see if my pistons protrude or not while the block is open?...can i work out how much to remove from the squish/quench area depending on piston protrusion?

thanks in advice

Edited January 5, 2008 by nsta

[StockyMcStock]

hey look it's my old bedroom floor! glad someone else found a use for the pic. that's taken straight after i had my head reconditioned (RB25 head BTW)

to figure out what you should do, you need to understand squish/quench. basically the piston moves towards TDC, and as the edges of the piston get within 40 thou or less of the quench areas, the gas/fuel mixture gets pushed as massive velocity in from the sides, towards the crown of the chamber (spark plug). it swirls and mixes further, and the pressure changes induce a better, more complete burn of the charge. hence you can put more timing in, and the engine is more efficient (generates more power)

Now my understanding with race setups is limited, but you should aim somewhere around 30-40thou for quench if memory serves me right (clearance between piston edge and head) to allow for some rod stretch at high RPM.

with a pure race engine which will be operating with high boost pressures, i believe some people take off the squish areas in the 25/26 heads because at very high RPM, the squish effect starts to sap power rather than add it (logically the higher the piston speed gets, the less the swirl effect from squish will help combustion, because the charge will be so mixed anyway). i think someone mentioned something about the problem of the squish area sending sections of the gas supersonic which may harm things as well, but i'm not sure on that.

someone with a race 26 engine will know more hopefully.

[nsta]

Thanks makes alot more sense now, so is this an engine out job to do?, ie will i have to remove the block aswell, and calculate how much thou needs to be taken off the squish area???? or is there a baseline of say 30/40 thou take off the squish to accomodate for high rpm piston stretch?.....would it be a good idea to see if the pistons are already protruding over the block bore first?or will this only occur at extreem rpm's?

im just a bit stumpt by the fact that only no.6 took the hit with the squish and got the hammering whilst all the others were fine (no.6 cylinder was the culprit for the gasket blow though....)

Thanks in advice.

[DVS JEZ]

Thanks makes alot more sense now, so is this an engine out job to do?, ie will i have to remove the block aswell, and calculate how much thou needs to be taken off the squish area???? or is there a baseline of say 30/40 thou take off the squish to accomodate for high rpm piston stretch?.....would it be a good idea to see if the pistons are already protruding over the block bore first?or will this only occur at extreem rpm's?

im just a bit stumpt by the fact that only no.6 took the hit with the squish and got the hammering whilst all the others were fine (no.6 cylinder was the culprit for the gasket blow though....)

Thanks in advice.

might also be due to the lack of cooling in cyl.6.

so you pulled the rb26 head off in the car?

not a fun job aye?

[nsta]

Hehe - no not a fun job at all.....we manage to pull the head off without having to pull off the turbos luckily enough, we removed the exh manifold studs, raised the head upwards, removed the manifold to turbo end from their studs, and then pulled the manifold out - what a job i tell you, mind you 2nd time around itll be much easier:).....

Cooling in no.6...how can i overcome this?

[DVS JEZ]

Hehe - no not a fun job at all.....we manage to pull the head off without having to pull off the turbos luckily enough, we removed the exh manifold studs, raised the head upwards, removed the manifold to turbo end from their studs, and then pulled the manifold out - what a job i tell you, mind you 2nd time around itll be much easier:).....

Cooling in no.6...how can i overcome this?

not sure 100% but remember reading something about running a seperate header tank to give even flow of coolant at a even temperature or something like that.

seen it on a race setup.

i have set my inlet up to be removed alot easier, things like the IAC have been made easier to un bolt

[Adriano]

Do you have a photo of the damage in the head, it could be that a sppark plug tip melted off, and bounces around in the chamber for a while causing the damage.

[nsta]

Sorry i dont, its really wierd though that only no.6 hit the head itself, whilst others didnt, though it was noticeable as piston no6 had skimmed a certain part of the piston whilst the others had no sign of it even touching or melting away....ill post up some pics later on today .... could it be the no 6 piston is more vunerable to having this happen?or that the rods/pistons werent assembled correctly for something like this to occur in the first place....maybe revs were kept too high??

thanks.

[nsta]

I've attached a couple of pics as mentioned earlier.

If you have a look at the first picture, youll notice how no.6 piston got damaged whilst the others hadnt at all. Basically check out the circle i highlighted in red (the good piston), and compare it to the piston beside it, and in that squish area highlighted in blue....notice how theres no meat left in that squish area on the piston on the right (no.6)?, its the same on the head side of things (sorry the head is at a workshop so couldt take any photos), but basically the head looked fine apart from the squish area on the no.6 bore on the head which was geting hit at, hard to explain, but it seems odd that it was only no 6, thus the reason why i think the pistons arent actually touching the head like i 1st thought since the other 5 cylinder bores are fine opn the head itself.

The 2nd pic ive attached is the piston head, and was wondering if anyone can shed some more light on what type of pistons these are just by looking at them, and whether or not they can tell me how oversize they are etc....

thanks!

Edited January 5, 2008 by nsta

[nsta]

Just on a side note - this was not a built engine or nothing, it was just a stock motor with rebuilt internals (headwork and forged pistons etc...), and the gasket went and this is what occured, what worries me is this happened less than 15k after it had been rebuilt.

[Duncan]

yeah i thought those pistons looked to clean to have been there long. they are 0.5mm oversize (says so on the top) not sure what brand though. definately not factory.

its really messy in there, if you are leaving the motor in and putting the head back on you will need to really carefully clean it out first.

are you sure the gasket was blown? that soon after a rebuild it is either because the head or block are not flat (were they shaved/decked at the rebuild) or the head bolts weren't torque correctly.

[Duncan]

ahh actually they are ACL forged pistons. details here http://www.aclperformance.com.au/NissanRB2...rgedPistons.htm, thought i recognised them

[nsta]

Hey thanks alot mate:), one think i can tell you is the head bolts were torqued over 110ft/lbs, definitely not factory (could over torque settings also cause this?), thanks for clarifying the pistons mate:).If they are .5mm oversize, then the bore should be ~86.5mm, in which case an 87mm gasket will be fine right???

Yip the gasket was blown mate, all the signs were there, my car started to white smoke excessively, and the smell was acidic, checked the oil and it was all milky, no water in coolant..... car would start after a period of time once the gasket was bad, but i only ever cranked it over once then got it towed back home.I can see it being anything else....i was thinking it may have been bearing and oil pump, but my car still was starting and oil was still there.

- does the block need to be out for it to be checked to see if it's flat, as the heads already out, my concern was why no.6 piston was hitting the squish pads on the head, and the others werent, and whether or not its safe to weld the squish pads back for no.6 bore on the head and resurface and assemble back?, it doesnt seem like a major, and the pistons look all ok.

thanks in advice:)

Edited January 5, 2008 by nsta

[nsta]

Hey guys, one other thing id like to note when i pulled apart the head........whoever rebuilt the engine not long ago, didnt replace the headstuds, how i know?

1) they are still factory ones.....also note they were over-torqued!

2) in the receipts for the rebuild, it had no indication of cost for new headstuds....

i think this may have well been a big contributer to my gasket problem!

[nsta]

MORE PICS!!!

Hey guys, couple more pics have been added to further show headgasket damage, and piston clearence at the end of its stroke....

Picture one, shows highlighted in blue, the area of the gasket which got damaged, that little gasket shape thing i highlighted was not even part of the headgasket when i pulled open the head, and i just attached to show you which area it was in.....basically this i feel was most likely the cause of having the exhaust side of the squish area on no.6 bore on the head to have been badly damaged while the rest looked ok....due to this little gasket highlighted in blue leaking and causing amounts of fluid into the squish zone causing it to deteriorate on no6 on the head....does this make sense?

second picture shows the the protusion of the piston to block, with the 0.9mm stock headgasket, and .5mm oversize pistons.....should i be worried, or does this look all ok in terms of top of piston to head clearance....

Should the squish zone really need to be bored down a little?, or could i just get it repaired, and skim the head down and leave it at that?, as im going to be getting a thicker 1.2mm headgasket now....

[datsqik]

From the pics you've posted i can't see that the head gasket has blown. All i can see is heaps of detonation that has damaged the tops of the pistons and probably ring grooves/lands. As for getting you squish clearance right its simple a matter of checking how far the flats on the pistons sit compared to the top of the block at tdc of each piston. You measure with a feeler gauge and ruler and they will either sit just above the block or just below. You basically want no less than 35 thou between the head and the piston so if you were using a 1.2mm (48thou) gasket and the piston sat 8 thou proud of the block then you would have a 40 thou gap from the piston flats to the head which would be fine. If it were my motor i would be having a good look down the sides of the pistons with a borescoope to see the extent of the damage but from the pics youve posted i would probably end up fitting new pistons but it is hard to see how much damage has been caused by detonation. I definately wouldnt be running the same tune on the rebuilt motor.

[Sydneykid]

The white smoke was most likely burnt oil in the tubines due to lack of oil control caused by the detonation damaged pistons and rings. Unfortunately it's engine out and rebuild time. Who did the tune? Don't answer that, just make sure that they don't tune it again.

Cheers

Gary

[BOOSTD]

I've attached a couple of pics as mentioned earlier.

If you have a look at the first picture, youll notice how no.6 piston got damaged whilst the others hadnt at all. Basically check out the circle i highlighted in red (the good piston), and compare it to the piston beside it, and in that squish area highlighted in blue....notice how theres no meat left in that squish area on the piston on the right (no.6)?, its the same on the head side of things (sorry the head is at a workshop so couldt take any photos), but basically the head looked fine apart from the squish area on the no.6 bore on the head which was geting hit at, hard to explain, but it seems odd that it was only no 6, thus the reason why i think the pistons arent actually touching the head like i 1st thought since the other 5 cylinder bores are fine opn the head itself.

The 2nd pic ive attached is the piston head, and was wondering if anyone can shed some more light on what type of pistons these are just by looking at them, and whether or not they can tell me how oversize they are etc....

thanks!

Worn No 6 Rod bearing will do it.

[StockyMcStock]

as above.

worn rod bearing on 6 from detonation = rod flop around the crank = excessive vertical travel of the piston = things hitting.

if the holes in the piston top are only on one side of the piston, then i would think that some foreign body has got in there between the two surfaces and smashed around a few times before being ejected.

however the most likely option is that it detonated due to a hotspot on the piston top, and that's what those marks are.

unfortunately i would advice pulling it apart and getting it rebuilt

[GeeTR]

Det'ing cyl #6 isnt new and fairly well documented (not just in RB's)

The long block design causes some issues, that need careful planning to get around when running on the edge.

The plenum often design, means more air to the rear cyl in mass airflow situations (big boost / revs) This clearly causes it to go leaner then the other cyls.

Water jacket design most likely doesn’t help "hot spotting" around that cyld head chamber either, increasing chances of det.

I mention revs, as the stock water pump can cause cavitation, and air bubbles rise... to the head.

People use not just an extra coolant reservoir, but attach extra coolant relief lines to the heads water jackets to aid hot water removal from the head.

People often use after market plenums, like the Nismo or Greddy, not so much for mass flow, but for flow equality in each runner / cyl.

Flowing and fitting your highest flowing inj to cyl 6 can help, though using individual exhaust runner EGT thermo couples and trimming each inj in your ECU is ideal.

I agree with others in that the squish zone is to induce high speed movement of the charge towards the centre, increase tumble roll etc.

The zone being an issue due to the parts (outer eddy most likely) going super sonic does make sense. That even higher pressure rise from a mach phenomena being an issue makes sense. Instead of the squish sapping power, I would think it would simply cause detonation.

I would think that removing the zone completely for when tunning for very high cyl pressures (read - torque) would raise the given det threshold at the cost of low end charge mixing and resultant peak pressure (again read – torque)

The above is no surprise, as building or changing something to increase ability at a higher level almost always results in less a sacrifice at lower lowers.

The damage on the exhaust side of the piston could be due to the area not having cooler air always coming in, as per the intake side.

Most people done have this issue, as they simply tune to the highest threshold of the most weak cylinder. Trying to equalise cylinder conditions across each piston, means of course you can run them hard, so less fuel, more ignition, more boost etc. It prolly translates to more even bearing life, and reduces resonance through the crank / driveline.

I could of course be totally wrong, (how ever unlikely mauwaha) but I believe this until proven otherwise.

PS: Be sure to search, this has come up before, with more details on the companies that DO remove it, like JUN. Greg from ProEngines has had some input before as well.