How High Compression is TOO HIGH???

[tkarvelis]

I am rebuilding a 951 DI, and from what I have read the optium compression before running (break-in) is around 120 - 130

Mine is 135 on the PTO cylinder, and 140 on the MAG cylinder.

The engine is bored 1.50 over with Pro-X pistons

The base gasket is .8MM

Head Gasket is stock

The head and cylinder were shaved to flatten, and the base gasket went from .5mm to .8mm.

The squish was measured at .076 average.

i have s feeling I should be running 93 octane, but should I purchase a candoo and retard the timing also?

I read on another forum that on high compression engines running on pump gas should retard the timing to prevent detonation.

 

[jhjesse]

You are good on the compression.
2000-2001 should run 91 octane
2002-up should run 87 to 89 octane as they are the redesigned LE models.

 

[racerxxx]

the DI is supposed to have a higher compression, the 130 is for a carb'd ski.

 

[Sportster-2001-951C-Stock]

Yes, the DI is close to 140psi, I think about 142, actually. You'll be fine.

I've been running 87 pump gas 10% ethanol in my carbed motor, I hear no detonation, sounds/runs exactly same as with high grade 10% ethanol.
And XPS-2 of course, I use it in all my hot 2-strokes now but not my antiques, they get mineral 16:1

 

[tkarvelis]

WOO HOO!!!!

SMOKE Test tonight after work!!!

Going to dunk it on the trailer Thursday night and ride it Friday, and get this broken in... Then to rebuild the 02 this fall

 

[AZ Sea Doo XP]

Did you shave the RAVE valves? I am going thru my 03 XP DI and was curious. thanks!

 

[tkarvelis]

The were shaved by Dan @ Full Bore. It is in the process of break-in... long process...

 

[jhjesse]

So...how does it run????

 

[AZ Sea Doo XP]

how much did it cost?

 

[tkarvelis]

I took it out today, and started to do the break-in that @Milidymac provided below...

First half tank of fuel ride normal up to about 1/3 throttle for 15 minutes at a time with thorough cooling in between. Varying the throttle the entire time.

Continue same pattern for the next half tank going up to 3/4 throttle.

For the next full tank same process but you can go wide open just never hold it there.
That's it.

I only made it through 3 gallons of gas in 2+ hours of run time. At this rate, I will be old and grey by the time I get past the 1/3 throttle...

It did run great overall.

 

[tkarvelis]

how much did it cost?

I think $20 or so. He bored by cylinders at the same time. There should be a youtube video on it.

 

[Sportster-2001-951C-Stock]

Nice, I hear the DI runs smoother than the carbed 951 so hope to hear one someday.

And yeah supposed to sip fuel in comparison which makes perfect sense considering the DI design means exhaust ports aren't open with charge in the cylinders spilling fuel out into the stinger.

Good job man!

 

[AZ Sea Doo XP]

the DI I had/have uses about 3 gallons per hour. break in may take a LONG time!

 

[jhjesse]

FWIW,
The wife and I will run down the Tennessee river and back cruising between 40 & 50mph. The trip is approx. 80 miles round trip, give or take.
She will be on one of the DI's and I will be on the RXT. When we make it back to the boat ramp the RXT will be almost out of fuel and the DI will still have 3 bars or just over a quarter tank of fuel left.
We have also swapped skis and the result is about the same fuel usuage wise. Both skis have the same size fuel tank.

 

[tkarvelis]

I might need to finish the break-in next year... I plan to ride tomorrow, and if I can get my daughter out Monday, I will ride again. Next week might be difficult with kids sports, and winter is coming. Anything I need to know to continue for next year's break-in?

 

[AZ Sea Doo XP]

I live in Arizona. ship it to me! I will gladly donate my time breaking in the engine for you!

 

[tkarvelis]

You want to enjoy my labor, and pay for shipping I would be on a plane too lol