Posts
403
Joined
5/21/2017
Location
Shelby, NC
US
Edited Date/Time
12/24/2020 12:33am
Pulled my head off because the owner before raised the compression which has cause me to buy a lot of race gas and I’m tired of buying it and so I am planning to get my squish set for pump gas. There are some pitting marks alongside the intake side of my head. I have added a pic of my spark plug and my piston head (37 hours) for help of diagnosis. I also added a picture of the cylinder head that is not pitted. 
Can you tell us your jetting, fuel, oil and ratio ?
I would have advised you to do a compression test before you pulled the head to see where your PSI is as well as a squish test to see how much space you are dealing with and advise you to remove material from the bowl as opposed to recessing the squish to get your octane with in spec for your needs..sometimes the squish is fine but the head needs less material in it to lower the comp ratio.
The main determining factor for detonation is oct vs comp ratio, squish plays a part in it, but your overall comp ratio is more responsible for deto than your squish clearance bc when your engine is under "load" this is where you can get into trouble fast...the YZ250 has (2) compression ratio's and depending on where you ride it in the powerband can determine how you should be dealing with the engine..trail ride vs mx so to speak.
A trapped volume test would have been able to tell you how many cc's you have when you piston is a tdc, this is the factor to use to determine how many cc's you want to change either way to get either lower or higher comp ratio.
I pushed the YZ250 to above 250 psi on VPC12 at about 1.2mm of squish and it ran great but many back and forth trips from the bench to the lathe to get the cc's right.
So the PO cut the head, you want to run pump, I say have the tuner lower the comp ratio of the head by removing material fro the bowl to get it right, leave the squish clearance alone. If this gets to difficult bc you need to know what you are at now vs what you want to be at, buy a new head, run pump gas, go ride. Just simply cutting into the squish band will make it worse.
Good luck- J
The Shop
Good luck and keep us updated.
What year by the way? And what other parts?
(I'm not much help, but always trying to learn)
I cut a lot of YZ and YZX heads and I've long been a proponent of lowering the compression on the YZ. You can actually make more power with a lower compression and a tighter squish while still running good pump gas. On top of that you get a more controlled power delivery and longer engine life.
If you are mainly into XC racing you really don't want more compression, you want less.
Mr. Robinson-"I think what Jbone meant with the "2 compression ratio" comment was based on the open and closed power valves. You have a higher ratio with the power valve closed and a lower one with them open. So depending on where you ride in the RPM range dictates which compression ratio you are running in." YUP!! I should have been more detailed and explain it but you did it for me lol. Also, based on your posts you definitely know your stuff.
Most of the mods I did on the YZ250 was to lower the cyl base by .020" to retard the port timing a bit and shift the hit a little lower..this also achieved a tighter squish clearance to about 1.1mm or so, around a 100 oct seemed good for this set up, a 50/50 cut of like vpc12 and 93 oct worked great and actually made better power with more pump gas in it.
For woods and lower rpm riding and to increase lower rpm riding, I would recommend doing to cyl base lowering with a stocker head and doing a port window clean up and then a small amount of epoxy work in the rear two transfer ports so the port windows appose each other and the fuel charge gets directed away from the exh port where it wants to go and across the piston crown where it cools the piston and gets used more efficiently for combustion. More Powah,,,(say it like you are from Boston).
Good luck!!
Pit Row
If heard, (sometimes it is a silent killer), it has been described by riders as a "pipe bang" or engine knock.
It is caused by the octane rating of the engine as well we the compression ratio being out of balance with each other..so if your engine was designed and spec'd for 95 oct and your comp ratio is 8.5:1 which gives you a safe running condition and you raise the comp ratio to 10.5:1 but have the same oct of 95, your engine will start to not be able to control the combustion properly and the engine will fire and combust the gases, sometimes at the correct degree, after TDC.
When this happens wrongfully, the engine is firing with too much ignition advance and it basically starts to eat the metal around the combustion area due to heat..thats why you see the pitting. It is eroding the metal due to insufficient combustion and the explosive nature of fuel under pressure.
Raising the octane to be in balance with your comp ratio usually fixes it. I will write more on this later, I gotta run but squish plays a part in this as well bc if the squish clearance is too excessive, that can contribute to it.
J
I used to think the same thing, however, detonation only occurs after the spark plug has fired, it needs an ignition source.
Detonation or pre-ignition is an uncontrolled explosion and that is why you hear the knock or ping and it damages engine parts.
I used to think the same thing, detonation and pre-ignition are two different things..
https://blog.edgeautosport.com/detonation-vs-pre-ignition
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