Wobble

You guys were spot on on the compression. I have been dealing with this guy so long I couldn't believe he made a mistake of that order. He is working with me.

I am trying to make sure I don't need to go down this road again. Too soon.

more pictures

Geo 100

Make sure your timing marks are CORRECT!!!
Had same problem 10.5-1 92 plus 2 gal 110 per tank.
now run 32 deg. It took twice to find that marks were not correct though. Had 2 1/2 yrs or 1200 hrs on Pfaff motor,
its fresh again this yr.
GEO

Wobble

Got with the cylinder head guy today, we can get 3cc's out of the head without getting too radical.

That gets us to this:

Bore 4.5 (inches)
Stroke 4 (inches)
Rod Length 6.135 (inches)
Cyl. Head Vol. 119 (cc)
Deck Height .005 (inches)
Head Gasket Bore 4.54 (inches)
Head Gasket Thickness 0.039 (inches)
Piston to Cyl. Wall Clearance .004 (inches)
Top Ring Land Height .310 (inches)
Piston Dome Vol. 17 (cc)

Compression Ratio:
10.165 : 1
Bore / Stroke Ratio:
1.125 : 1
Rod / Stroke Ratio:
1.534 : 1
Total volume:
1156.46 cc's
Compressed volume:
113.773 cc's
my previous calcs used 0" for a deck figure, it is actualy .005"

they are going to machine the pistons to get 5cc off them if that still leaves .200" thickness.

that should get me
Bore 4.5 (inches)
Stroke 4 (inches)
Rod Length 6.135 (inches)
Cyl. Head Vol. 119 (cc)
Deck Height .005 (inches)
Head Gasket Bore 4.54 (inches)
Head Gasket Thickness 0.039 (inches)
Piston to Cyl. Wall Clearance .004 (inches)
Top Ring Land Height .310 (inches)
Piston Dome Vol. 12 (cc)

Compression Ratio:
9.779 : 1
Bore / Stroke Ratio:
1.125 : 1
Rod / Stroke Ratio:
1.534 : 1
Total volume:
1161.459 cc's
Compressed volume:
118.772 cc's

everything will be cc'd prior to actual assembly to make sure we have got to around 9.8-1

only one chamber was finished the others wer roughed out.

PatriYacht

With that much compression, you're still in the danger zone. Try never to run less than 92 octane, don't run more than 34 degrees timing. Run the carb jets a size or two rich. Run a 140 degree thermostat max, if you run one at all. Good luck.

Wobble

PatriYacht,

thanks for the input, what would you consider a safe CR for 92 octane? We don't know exactly what cc's we can take off the pistons and still leave .200" thickness. We are going to try one and mock it up and cc everything before final assembly.

PatriYacht

With alum. heads 10.0 to 1. With iron heads 9.5 to 1. With care, retarded timing, rich mixture etc you can probably run another half point but one tank of bad gas and you're right back where you started.

BadDog

With where you are at now how would your CR look with a head gasket in the range of .057? Look into the thicker Cometics. I think you may make more power with a slightly lower CR and more advance.

cstraub

I would check with JE/SRP tech and see if that is a solid dome piston. If so I would remove the dome then coat the tops of the pistons. For better atomization of the the fuel and air I would rough up the chamber around the intake valve side only.

Max Compression Ratio:
Remember some guys on this board are very capable of working on their own stuff and have access to machine shops to get work done very quickly. If you are a pleasure boater and don't enjoy "wrenching" then I would sacrifice a point of compression and 50HP to have a reliable mill that you don't have to tweak all the time. I would run conservative on the high 8 range to a max of 9 to 1. If you don't mind the tweaking then run 9.5 and through some octane booster in for good measure.

Chris

JIMG

I am running the same pistons in a pair of 509s. With the chambers opened up it was still over 9.5. Decided to use cometic extra thick head gaskets to remedy the whole problem and have a superior gasket.

rmbuilder

Mark,
That piston has .350" deck thickness, so you will be OK with the material removal. As I mentioned stay away from the plug relief and radius the edges after the cut.
Bob