higher compression requires better head sealing

[HeadSmess]

according to most pages on the interweb regards teflon gasketing materials, its capable of 3000 psi or so. even a high performance engine is lucky to peak at 1000psi at the instant of combustion. so thats plenty of safety margin.

as mentioned, surfaces have to be close to a good seal in the first place, teflon has no real strength, it cannot cope with large gaps. it just seals. it is NOT a gap filler! (like when i replaced my water heater the other day, no matter what, one of the fittings leaked...until i replaced it with a slightly tighter union. note how much tape plumbers use... they do not understand how it works. they just bung on half a roll of the stuff on each join. you only need three, four turns on the average fitting...)


a case of oversquish!




i replaced the piston in my nsr150. trimmed the head to a 0.6mm squish. then went on a long ride with E10(our ethanol fuel is the exact opposite of your american stuff...we dont get any more than 10%ethanol here. noone likes the stuff, or trusts it. strange. now i see why there is a big argument about what oil and fuels to use on this predominately american site...)

i blame the cheap, non genuine honda piston, rather than fuel or squish or anything... oh, maybe the lack of re-jetting also. that may have been a factor.

one thing i noticed. the stock head has about 1.6 mm squish. now i have a spare engine(mainly because i have had to tear the original back to the crank shaft to get bits of piston out, and am waiting on bits n pieces while its apart), so i have two heads, one stock, one trimmed. the trimmed head causes clutch slip at 8-9000 rpm, whereas the stock head does not... i prefer the trimmed head, but i may have to increase that squish just a touch before i trust it...and im only going to run 98octane from now on...

buts its so much more rideable with the reduced squish! power band starts at least 2000 rpm lower down( or at least, it isnt so gutless at 6-7000rpm?). more snappy, easier to start when cold, blah blah, yarda yarda...

time to invest in a tig welder again, i need to make a new exhaust...

 

[HeadSmess]

this was, once upon a time, an iridium plug...



at least i caught it as it seized, and pulled the clutch rather than having it suddenly happen through a corner at 130km/h like the last time...


and no, i dont use teflon for this particular head gasket as its a water cooled engine, and to make the required shape with teflon is more hassle than its worth.

 

[Deleted member 12676]

HS, there is no universal squish band clearance. A 150cc normally needs more than a 66cc does. The only universal value is the squish velocity.
So when you shaved down the head you increased the compression ratio and you also made the squish clearance too little which increased the squish velocity too much which contributed to the detonation.

 

[Steve Best]

I don't think you can blame squish for this one.
Bottom of the piston is barely blued, so it wasn't the result of general overheating.
This was a rapid detonation and overheating of the center of the piston.
Lean, advanced timing or not enough octane for CR or a combination.
It happened quick.

I'd start with backing the timing off and richen up the jet, see where it takes you.
An engine that will run OK on a low CR will self destruct as you increase the compression.
Maximum power often takes an overly rich mixture for several reasons.
While lowering the head will increase CR, squish gap is a separate effect.
I've seen auto engines that knocked at 0.130" squish and ran quiet at 0.040" even with the increased CR.
It all has to do with the turbulence velocity and chamber shape.

Larry Widmer was a guru on this topic. He ran huge CR on NASCAR and drag engines and Hondas.
His "soft head technology" was to pack all the burn into a ball under the exhaust valve.
I tried it, it works. It works on 2 strokes as well in the guise of the torroidal chamber engines.
The result is close to homogeneous charge compression ignition where you can back off the timing and the heat of the sparkplug will ignite the mixture. Start rich and lean out while reading the plug for max power. Larry is still kicking and willing to share knowledge with anyone with a brain and a wrench.

Steve

 

[Deleted member 12676]

It's commonly agreed that too high a squish velocity can cause detonation.

 

[HeadSmess]

500km later with the shaved head, running a genuine honda piston and 98 octane only... not a sign of detonation or any other issues yet. other than that clutch slip at 9000+

i'm going back to my former conclusion, that the cheap non-genuine piston was exactly that...cheap and nasty. i had my suspicions when i first received it... saving a few dollars always ends up costing more, dangnamit!

also gone back to the specified plug, b9eC, half a thread shorter with a recessed electrode.

dont have much control over ignition with this thing other than to file the pickup holes... pretty complex CDI unit that has two sets of plugs, controls the RC(exhaust) valve as well. there is meant to be an aftermarket unit but finding one for a decent price is relatively impossible.

CR is pretty low as stock, 6.8:1.

i love my nsr, its like riding a bmx with a chainsaw engine... except it gets to 180km/h. not bad for a squit-box 150.

 

[Aladdin91]

I have a 80cc china is bogging out on start up took head off it cleaned it and. It had a thick layer of substace and also sparm plug did to cleaned it and tryed again there was a loud sparkchecked carb evrytging is on tight

 

[Deleted member 12676]

you need to plane the head surface flat. stock ones aren't flat. also gt a new head gasket or better yet make your own out of high temp gasket material.
you heard a pop out of the bad head/cylinder mating surface, not a spark

 

[Aladdin91]

I replaced head gasket still same issue

 

[Frankenstein]

I replaced head gasket still same issue

Take spark check the plug with the head put to on the top turn the key. Don't put carb. Blow the holes with twist blow.

Carb level set level for best, about as level when upside down. Push the clip to highest. Clip it then read plug from ride.

If it rides better then good, if not take pic. Thkns.