squishbands and modding cylinder heads

The squish band is the area in which most people dont take the time to set up correctly and this is where you will have problems. If your going to do a head mod on a sled, you need to know what the stock squish is set at. This can be done in a easy fashion of simply using a bit of lead solder (.080" or larger)bent into a 90 degree "L", its important to have the solder touch the sides of the bore and you can feel the smooth side when inserted thru the sparkplug hole. You want to check the squish at the left and right side of the cylinder, this prevents the piston from rocking back and forth on the wrist pin and will give you a false reading, just stay on top of the wrist pin and youll be fine. Each engine can be differnt, there is machine work tolerances when engines are built, so the reading you get from 1 will not mean its the same in the others, deck clearance,head relief , piston top will all play a role in the reading. In most Yamaha sled cases, you will want a good .052-.055" to be considered safe pump gas range. The 2000-02 srx usually have .060-.063" stock, the 98-99 srx were a bit wider. So what you want to do is take your stock reading and subtract the safe range from it, this will give you the correct amount to mill off the head. You can mill significantly more then that,**BUT**, you will have to machine the squishband angle and "rechamber the head" to do so, its not something the average guy is going to do, and really isnt neccessary unless its a special built purpose engine, like ice drags, grass,etc. The stock squishband is 10.5 degrees, and by changing to a steeper angle like 14-17degrees, you can run more compression with out having detonation, the reason is this, the stock angle is very flat and by milling the head , it sits even closer to the piston at TDC then before, the mixture becomes trapped in the squishband at TDC, when the plug fires, it will only burn the mixture in the center, and the trapped mixture out in the band becomes stagnent and builds heat, it then fires from heat(like a diesel), this is the detonation, and it makes little indents in the edges of the piston, like a small pick hammer pounding on it. By changing the angle to a steeper one, it helps to force the mixture to the center of the head and promote better flame travel on the top of the piston crown. This enables you to run higher compression without the risk of detonation, but again, this is not just a quick easy flat mill of the head, and not really much gain for the "trail sled", was just tryin to give you a idea of why and where. The overall meaning of this post is that more milled off is not always better unless you do the entire job correctly.
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good post man, there's been alot of questions on the topic lately.

Thanks for posting this in the Tech Section. Now I won't have to search through a bunch of posts to get the basics.

Is the "Squish band" you speak of, not called the quench?

If I get a CPR modded head is there any issues with them??
for my 02 Viper CPR pipes

thats what the head mod is made for, to even out the compression for running pipes to keep it safer with the higher loads

thats what the head mod is made for, to even out the compression for running pipes to keep it safer with the higher loads

so this would be a Good deal for me,
now running pipes stock head and no rear heat exchanger

thats what the head mod is made for, to even out the compression for running pipes to keep it safer with the higher loads

so this would be a Good deal for me,
now running pipes stock head and no rear heat exchanger

echo, echo!

Sorry

Thanks for the info...... but what's your opinion on my SRX440 the cylinder head shape is oval with offset sparkPlug hole and the squish is different if you measure from the front/back or the sides, Im running 110 octane race fuel with about 40and 45 for my squish............


(before and after pics)

Well done.

Way to go Mrviper. That as good a job of explaining squish as I've heard.

..SNAKEBIT.. said:

thats what the head mod is made for, to even out the compression for running pipes to keep it safer with the higher loads

so this would be a Good deal for me,
now running pipes stock head and no rear heat exchanger

Click to expand...

put the modded heads back on and run a rear heat exchanger cause if not your lookin for trouble.

what modded heads? I honly have one set done to AAen spec to match the pipes and porting. and the rear heat exchanger, i don't think so, i only drag race this sled 660 feet at a time, and the coolant barely gets up to temp, I even had to put a thermostat in there so it would heat up faster...

MAX, I think he was talking to Snakebite? anyway thats a sweeeet 440, whats that baddog run? Any snow in labrador? must be coming soon Eh? I see redsnake is from Labrador also. You guys know each other?

oh, ok, yeah we had snow already with 5 days of flurries in the forcast, was out on the srx already, hehe

yeah i know Mountain Max we race against each other in the same class. he had the best of me last year but this year will be a different story(hopefully).
rob i think you should give mrviper ap.m. im sure he would be willing to help you.