Need Help in Figuring Compression Ratio / Squish

WildMax

WildMax

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Joined
Aug 26, 2005
Messages
98
Age
68
Location
Black Hills
Hello,

I had a seized piston a couple of months ago and am reassembling the engine. Hauck did "Stage 2" trail porting work to the cylinders, pistons and heads prior to the engine failure. I was wondering if someone on the TY site is able to tell me what my compression ratio might be and if my squish looks to be OK for an aggressive trail set up at elevations of 5000' using 91 octane fuel?

OK, here's the set up:

VX800 (Vmax 4) 791 cc stock -
Standard piston and standard cylinder height (not decked) with 65.0mm bore and 59.6mm stroke,
Base Gasket is 0.031" (stock is .019"),
Head Gasket is .018" (stock compressed),
Deck Height (top of piston at the edge to top of cylinder) is 0.015",
Squish Band Width is 0.390" at 8 degree angle,
Piston is domed with about 5.14 degree slope at the squish band,
Squish Clearance at side of cylinder (top of piston at the edge to head) with gasket is 0.049,
Squish Clearance 1/4" in from side of cylinder with gasket is 0.062,
Head Volume is 17 cc.
Top of exhaust port to cylinder deck is 2.984 cm (1.175").

Stock compression was 6.5:1 as per Yamaha manual. With the above set up except for a head gasket of .009" instead of .018, I had 141 psi compression at an elevation of 3000'. I am looking to go to the thicker head gasket to lower the compression and raise the squish.

Thanks.
 
What is the distance form the top of the jug to the top of the exaust port? How high is the dome of the pistin?
 
mountainx said:
What is the distance form the top of the jug to the top of the exaust port? How high is the dome of the pistin?
Click to expand...

2.984 cm (1.175") top of cylinder to top of exhaust port.
0.090" dome height.
 
tedgoesfast said:
use soilder put it in plug hole to the side turn engine over slow remove and mic it
Click to expand...

Already have squish measurements. Question is, is it in a safe range for 91 octane at 5000'.
 
I came up with 7.2 to 1. Im not %100 on the piston dome volume but im guessing about 4.2cc and that should be the high side. If you were at sea level I would say you are on the edge but at 5000' you should be fine.

If some one wants to correct me, feel free to do so!!
Thanks, Justin
 
mountainx said:
I came up with 7.2 to 1. Im not %100 on the piston dome volume but im guessing about 4.2cc and that should be the high side. If you were at sea level I would say you are on the edge but at 5000' you should be fine.

If some one wants to correct me, feel free to do so!!
Thanks, Justin
Click to expand...

Thanks, Justin. I have 2.67 cc for the dome volume with remeasured calcs. The piston is 6.439 cm diameter, .203 cm rise over 2.604 cm run, with a 1.232 cm diameter flat spot in the center. I did not know if I should calc from the top of the ring or top of the piston. The top of ring to top edge of piston is .343 cm.

I was taking the volume above the top ring at the top of the exhaust port and dividing it by the volume above the top ring at TDC but seem to come up lower than what I should. Is this the correct method?
 
You are very close!! You use the top of the piston because that is what rilly closes the exhaust port. It dose not seals it but what you loose in that time is very very small.

If you want to no exacely you have for compression the best thing you can do is:
put some vasoline on the piston ring and put it to gether
run the piston down the cylinder 1mm from TDC and do a liquid measurment.

This is the way I do it because of the complex shape of the piston dome.

let me now how it goes.

Thanks, Justin
 
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