Miller Dome Compression and Squish

[DAVID_HX_XP_RX-DI]

Bill,
What are the engineering Specs. for Compression and Squish of the 947 Miller PG Domes using a 5-hole BG:

44cc
47cc
49cc

Thanx, David

[Bill O'Neal]

There are no specific "specs" to answer your question. You must measure the squish for yourself. Clyinder heights and deck heights vary amoung different engines. Most models of MRC (Miller racing components) inserts (domes) are clearly marked as to motor, compression ratio and size. For instance, the marking of S95121-49 would be a S (Sea Doo) , 95 (951motor), 121 (12.1 to1 compression ratio), 49 (cc) insert.

Next, the octane of the fuel is important in setting up squish clearances, less octane, more clearance.

[ September 30, 2003, 09:22 AM: Message edited by: Bill O'Neal ]

[DAVID_HX_XP_RX-DI]

OK Bill,

Well, with the 47cc Domes I have 150 psi and .060+ Squish respectively.
Running pump premium 93 with no problems and occasionaly (in a pinch) have to run marina fuel @ 89 Octane. It seems not to run a smoothly, but still no signs of DET after inspection. I would like to tighten up the squish with decking to achieve approx. .045 and using a 6-Hole BG to keep the port height high for TE performance. Compression to remain approx. 150 psi.

Your recomendations ???

David

[Bill O'Neal]

I recommend that you do not tighten the squish to .050 unless you are willing to run leaded racing fuel full time. You are probably detonating now at .060 squish and pump gas and just do not know it. The proper squish clearance suggested by MRC is over .065 for his 951 heads. This is a tested method using high dollar deto sensors.

[SuperMAC]

David, I know of others running 47cc domed R&D heads and have excellent performance. I wouldnt run anthing smaller if you want to stay safe.

From what I understand 49cc is the stock head volume.. I would imagine with a stock head and at your location, you would be looking at 140psi at BEST. More likely to have 135 or so.

Right now I have my 98 XPL spinning 6830 with flame arrestors and a stock prop, no intake grate and rejet. However, it is only going 58mph tops on the GPS... Prop is in xcellent shape and it tears out of the hole. Save this for another thread..

Anyway, max squish velocity is limited by the octane of the fuel. I dont have the formula handy to calculate squish, but I would stay above 60 thousandths at all costs. I don't know of anyone running less than what you are currently running for pump gas.

I've noticed that depending on the thickness of the solder I use, my squish readings can vary 10 thou or so... so to get an accurate reading, don't use any solder that is too thick. I suppose you could get creative and check across both sides of the piston at once to get fancy, but I havent done that. :D

[Bill O'Neal]

OEM 951 head chamber volume is 51cc measured with a burrette.

Oem squsih clearances are usually around .080, plus or minus.

Oem pumping pressure is 140 psi at near sea level.

If a 47cc insert is producing 149 #'s, the lost pressure is getting past the rings.

[ October 04, 2003, 10:11 AM: Message edited by: Bill O'Neal ]

[DAVID_HX_XP_RX-DI]

Bill, Like I mentioned before (a couple times),, I have checked my Piston Crowns and pulled the RAVE Valves and looked into the slots.
My definition of DET. symptoms is erosion of the Piston Crowns and edges above the upper ring land, I see none. It can usually be heard as it occurs in more severe cases as you know. I hear no unusal sounds with the tray out running in Glass water in my cove and putting the machine under a loaded condition with tight turns @ WOT and Mid. throttle.
I have heard//seen DET before with other machines, I know what to listen/look for.
There is no sign of DET.
If anything, it is very rich fingers extending from the ports about the size of dimes.
I have been running like this half the summer, Dont you think it would have died by now from DET if I have been running 40+ hours in this condition while using Pump Premium 97 Octane, and occasionaly Mid Grade 89 when in a pinch far from my dock ?

The real question I have is, when using the MRC 951 47cc Domes I have .062 Squish. I have taken this same clearence check multiple times. Little to no difference except maybe a couple thou. error factor in my eyeball looking at the Caliper.

Now why do you think, with that clearence,,, with those 47cc Domes,,,, I only have 149 psi compression ? That dosnt make sense..... and is why I started this thread to get the Specs. Certainly these Domes have been put on many 947's before and you are aware of what the average Compression and Squish "should be", or at least the working range from other customers.

Please dont tell me its the gauge either, it isnt, I have gotten the same measurement with 2 MAC Digital, new SNAP-ON Analog, and another newly Calibrated MAC Digital Gauge.
Also had Brian Thomas use his "no name" analog gauge. 150 F//151 R
My measuring tools are accurate.
Something here isnt quite Kosher, trying to figure out what..........
BTW: The MRC 44cc Domes only yielded me 165 psi at slightly tighter Squish, is that correct ?
With those I was also running mostly Pump Premium 93 although I did *start* with a 60/40 of 93/110, and slowly weened it back. Still never heard or saw any signs of DET. but got tired of paying 3.49/gallon for 110 so installed the 47's.

Remember when I sent you back those 49's in early summer? Those didnt come up to OEM stock compression of 140/5.....
You graciously exhanged them for the 47's, and I have been running them since,
Thank You.

This is why I had the idea of shaving back the deck height, perhaps the MonoCylinder of this particular 947 its on the upper end of the manufacturing tolerance.
Just not getting the performance I should be when using Top Shelf parts on the Top End.
(These are OEM standard sized pistons)

Any insight of what to look for that would help figure out why I am getting these lower compression numbers would be helpful.

David

[96xper]

Dave,
I have a 98xpl now. You've seen my posts. So hopefully this will help you out. I have 46 cc dome R&D head on my ski with a 6 hole gasket. Running 93 octane, squish is .07 and compression is 168/163 psi. Don't know if that will help you out or not.
96xper.

[DAVID_HX_XP_RX-DI]

OK Bill,

Compression getting past the Rings is a remote possibility, but.................

I have previously honed // measured the Cylinder to .00018 "taper" using a Starrett Bore Gauge and used new OEM Pistons/Rings.
Then religiously went through the Dumonde Tech. break-In Oil process

Also did an individual Cylinder leak-down test @ TDC and it held pressure just fine albeit a "cold" engine.

Does the RAVE Valve postion affect the compression readings ? I dont think it matters that much.

Any other ideas ?

David

[Bill O'Neal]

David,
The rave position makes a big difference in the compression readings, as much as 10-20 psi or so...

[DAVID_HX_XP_RX-DI]

Wow !, That much huh ? Never would have thought.......

So,,,,,,,,,,, should the RValves be Fully UP or DOWN while Cranking it over while tking a reading then Bill ?

D

[Bill O'Neal]

Just leave them alone......... Down.

[DAVID_HX_XP_RX-DI]

Thats what I thought.
...... and am ASSuming that the OEM RAVE Pre-Load Springs keep them in that position during compression testing.

Thank You Bill

David