What's the biggest cc dome I can run on tri-z 310? Anyone running pump gas?

[MOTOMAX27]

Anyone running pump gas on a 310 tri-z?

[MOTOMAX27]

Anyone running a 310z??

[John Tice]

What’s the bore & stroke of the engine? A little simple math will give you the answer. You probably want around 150psi cranking compression; which is around 10 to 1 compression ratio. Some may chime in & talk about heat & higher compression, thermal expansion etc. Keep it simple & use corrected cranking compression; I’ll run the numbers when I’ve got the bore & stroke.


John Tice
503-593-2908 Alternate 541-508-3944
www.smallenginemachineworks.com & www.nwsleeve.com
Turning Custom Cylinder Sleeves Since 1971

[MTS]

There between 76-77mm bore x 68mm stroke John, if the motomax is running a sprock kit. If I remember right my dome is around 26cc, running 170 psi with a Guage. ..would have to go digging for notes.

[John Tice]

76mm x 68mm = 308.5cc close to; Most 2 strokes have about a 180 deg exhaust port open timing. This would give you a 154.25cc corrected cylinder volume. 154.25/26cc dome gives your theoretical = a 5.93 to 1 corrected compression ratio, BAD. If everything rings & such are sealed; 5.93 X 14.7psi (sea level pressure) your cranking compression should be about 87.17 psi?

For a recommended 10 – 1 compression; 154.25 / 10=15.4cc dome volume including deck height.

Since you’re cranking compression is 170psi / 14.7 = 11.56 compression ratio which is a little high.


I use a plastic horse syringe to aid in the calcs


Finding TDC using a degree wheel


Checking deck height on Rotax Kart engine


Before any modifications are done to your engine; you NEED TO KNOW the above calculations, including your piston’s DECK HEIGHT.


This book is a basic read for understanding your 2 stroke


John Tice
503-593-2908 Alternate 541-508-3944
www.smallenginemachineworks.com & www.nwsleeve.com
Turning Custom Cylinder Sleeves Since 1971

[Mosh]

John, the 310z jugs were cut down apprx. .050" if I recall correctly.
so that technically will bump the compression back up.
sprock used the js650 jet ski piston and the jugs had to be cut to get the deck height back for the piston.
With the largest cc dome on the kit I think they had around 185 kicking compression.
They would ping without a minimum of 50 50 110 race fuel and 94 octane.
I believe that is why Max is asking. If you are getting pinging or knocking go to race fuel. Straight 110 works well.

It has been a long time since I messed with one, so I forget a lot of the specs, but If I get time I will call sprock and get a refresher on the numbers.

[John Tice]

Mosh, good info; I’m not surprised at the detonation problems. If the compression is dropped to 10-1, 150 psi or so & the squish band is properly machined, the performance will still be good with the pistons lasting more than a couple of seasons.
Many installers of these kinds of big bore kits are a little shy of the proper installation equipment. On ANY of the rebuilds the cylinders must be round & straight. The proper deck height is only part of the solution to a reliable runner.

The cylinder must be machined square to the crankshaft; this 400 Maico has a Double DH Reed welded in place which warped the base gasket surface. Circa late 70’s; back in the days when mounting a reed valve was a journey into the Wild West.




After a bore for a new piston; your cylinder must also be honed to size with the best equipment possible.

The double stone Sunnen con rod mandrels hone the bore straight every time.

I’m sorry I’m so winded; the E2S folks know me well. After a lifetime in front of a lathe & boring bar; I’m finding that there are few of us left in the small cylinder repair business. There is plenty of room for more enthusiastic people to become involved.





John Tice
503-593-2908 Alternate 541-508-3944
www.smallenginemachineworks.com & www.nwsleeve.com
Turning Custom Cylinder Sleeves Since 1971

[Mosh]

The issue with the tri z engine is that in had .065" of squish at factory settings. The domes for the the coolheads were pushing mid 40s to mid 50s....the Z engine hates a tight squish. Personally I feel it is becuase the exhaust port was way to small, as well as the intake that was factory cast into the jugs. I am no expert in the realm of these tolerances and engine designs as some, but I had a big bore Z, was one of the first to have one. I got mine to run good, but it took some monkeying around. For whatever reasons, that bigbore configuration ran best at lower compression ratios. They were never a hi revving engine and everyone seems to want that, but will not understand the physics of that engine just won't allow pushing the envelope. It is almost the identical twin of the 83 through 86 YZ 250 engine with a much larger heavier flywheel, no powervalve, and a smaller intake configuration.

[MTS]

I must be thinking of the original dome I had to relive to drop the cr down. To get it under 200 psi, since the original kits where done there where a few changes done to make everything work nicely together. The "kits" where just sleeved with stock port arrangement, mine was ported from the get go in the 196 degree ex duration range. Have to find my book. A gram bells performance tuning is a great read also. No such thing as long winded!

[cr480r]

the Z engine hates a tight squish.

Maybe your dome designs just didn't work with the tight clearances. Squish clearance is only one of several variables determining squish velocity. It's very difficult to adjust or single out any of the variables without also changing the others at the same time. Just saying this because there is a chance that the effects of too much compression, or too much squish area are being blamed on tight squish clearance.

[John Tice]

I’m the “OLD DOG” at 67; as my buddy on the Hunter series says “Works for Me”. If it works for you, you’ve got a good one. I’m only a Machinist who works on hundreds of cylinders. Some things are always the same on a 2 stroke; many things are different. That’s the difference between winners & loosers.
Chow



John Tice
503-593-2908 Alternate 541-508-3944
www.smallenginemachineworks.com & www.nwsleeve.com
Turning Custom Cylinder Sleeves Since 1971

[cr480r]

76mm x 68mm = 308.5cc close to; Most 2 strokes have about a 180 deg exhaust port open timing. This would give you a 154.25cc corrected cylinder volume. 154.25/26cc dome gives your theoretical = a 5.93 to 1 corrected compression ratio, BAD. If everything rings & such are sealed; 5.93 X 14.7psi (sea level pressure) your cranking compression should be about 87.17 psi?

For a recommended 10 – 1 compression; 154.25 / 10=15.4cc dome volume including deck height.

154.25+15.4/ 15.4= 11:1 . It's also important to include piston dome displacement in calculations.. A pump gas head for an engine this size will likely flat plate in the mid twenties.

Before any modifications are done to your engine; you NEED TO KNOW the above calculations, including your piston’s DECK HEIGHT

I find the quickest way to find a useful baseline, especially if the engine is still in the bike is to measure the chamber volume at tdc with the engine assembled. You then know actual compression ratio, and can determine the necessary changes for the head. To me the flat plate volume of the head is merely a reference point to use during alteration.

[John Tice]

Good advice from knowledgeable people. It's nice to communicate with folks who have more than mashed potatoes between the ears.
JT