04 rmk cylinder base gasket thicknes ?

peaceofwork · Dec 3, 2016

does any one here have a cylinder base gasket they can measure for me?
I am working with a stage 5 slp system with 3000-6000 foot domes. usually I start out at less than 3000 feet so I think of lifting my cylinders a bit to gain cylinder head volume to ease off on race gas requirements. rarely do I get to 6000 feet high but I often start out at less than 3000'
50.5 cc chambers for 3000-6000 twin pipe
52cc chambers 0-3000 single pipe is what I see on the slp web site. so 85 mm bore add 3.5 thou gasket thickness could get me to 50.85 cc chamber volume. assuming 10 thou would add 1,4 cc volume, 5 thou could get me to 51.2cc volume.
what do you guys think of this calculation ? port timing would change minimally and compression would drop a bit, squish gap would be a bit more I want to be able to run pump 94 octane, possibly a touch of good gas when colder out or sustained low elevation running.

whoisthatguy · Dec 4, 2016

Tightening up the 4 corner bolts will cut the thickness in half. I cannot imagine in my wildest dreams, that stacking a couple of Polaris gaskets is going to add 50cc to an 800cc engine. Numbers measured in thousands, usually are in inches. (85 mm x 85 mm x 3.1416 /4) x (0.010"/25.4) x 2 cylinders = 4.468 cubic mm Since the stock polaris gasket thickness in unknown but not zero, then you are adding only a fraction of the 4.468 cubic mm number. 2 cc to and 800cc engine, cannot likely be a noticeable difference. Adding a SLP pipe and a slp header, adding a slp can, and removing the venturi tubes in the intake, will make a huge difference in the performance. Unfortunately, that performance will come at a cost of wearing the crank bearings down faster and ripping the front left engine mount more frequently.

Goinboardin · Dec 6, 2016

peaceofwork said:
does any one here have a cylinder base gasket they can measure for me?
I am working with a stage 5 slp system with 3000-6000 foot domes. usually I start out at less than 3000 feet so I think of lifting my cylinders a bit to gain cylinder head volume to ease off on race gas requirements. rarely do I get to 6000 feet high but I often start out at less than 3000'
50.5 cc chambers for 3000-6000 twin pipe
52cc chambers 0-3000 single pipe is what I see on the slp web site. so 85 mm bore add 3.5 thou gasket thickness could get me to 50.85 cc chamber volume. assuming 10 thou would add 1,4 cc volume, 5 thou could get me to 51.2cc volume.
what do you guys think of this calculation ? port timing would change minimally and compression would drop a bit, squish gap would be a bit more I want to be able to run pump 94 octane, possibly a touch of good gas when colder out or sustained low elevation running.

You're line of reasoning is sound. BRP sells different gasket thicknesses to fine tune the squish, its generally said there is roughly 5psi cranking compression change for each 0.1mm gasket change. I would call or email SLP and see what they think. Does this ECU have a knock sensor? I'm not sure which year you're working on.

peaceofwork · Dec 8, 2016

whoisthatguy- I think you misunderstand my post, maybe I did not word it the best. I am not looking to add 50cc to my 800 by adding a few gaskets, nor am I looking for swept volume of my cylinders.
I am simply looking to add a little volume to my compression chamber by shimming my cylinder a little higher.
.010 added to an 85mm bore would add 1.4 cc of chamber volume. my 800 would in fact still only be a 800 as swept cylinder area would remain identical.
my chambers are now at 50.5cc and .7cc gets 51.2

2 1/2 thou adds 3.5cc volume taking my my domes to 50.85 cc
I hope I said this in a positive way?

peaceofwork · Dec 8, 2016

the local polaris dealer says the cylinder base gaskets are 030 compressed, -he thinks polaris when they give a gasket size gives compressed height .
I had the children snip and nip me some base gaskets out of 3/64 gasket paper . they are greased and torqued down now getting compressed - I hope. I will pull one and check its thickness .
in the past I have had 140-145 lbs compression so knowing this I will try out a compression test with my custom gaskets and head.
I have also order stock polaris base gaskets to confirm things.

Goinboardin · Dec 8, 2016

I think you're wording is clear.

One concern of mine is going too wide on squish. I'm not as familiar with the specifics of a 2 stroke head design, but I know on my small block ford, it is said ideal squish is tight, around 0.040", to prevent detonation. Use head chamber volume to adjust CR. Then I get on SnoWest and see guys saying the exact opposite, tight squish causes detonation. I don't know their knowledge level.

The reading I did to build a SBF taught me that tight squish prevents hot spots or trapped gases from lingering at the perifery of the cylinder, and concentrates force to piston center instead. As an example, the engine I built for my truck has 0.042" squish and 9.2:1 CR, I run 15° base timing, no detonation and runs on 85 octane. The engine I removed had 0.120" squish, 8.3:1 CR, 10° base timing, and detonated until it broke a ring and melted the ring lands of a piston (whoever built this was an idiot). Correct squish, so that there is effective quench, goes further to prevent detonation than compression ratios. I don't know exactly how that translates to a 800cc 2 stroke, so I would definitely call SLP. Their domes are well designed.

peaceofwork · Dec 8, 2016

cool project I have had my eye on the 348 stroker for a long time but back to sleds.
my knowledge is up their but short of professional on 2 strokes. squish gap is a little flexable without serious loss of performance level. squish band width and is a dictating factor in perfered operating rpm. squish band angle should match your opposing angle on the piston.
then comes chamber volume, so consider mechanical compression ratio versus static compression ratio - then we are into what fuel will i have to run with any selection I make.
for me 50.5cc domes running 92 0ctane requires race gas blend at less than 3000 feet.
52cc domes run o-3000 feet no race gas.
I want to be some place in the middle ground.
comp chamber shape or hat is complex because of some thing called blowdown, blowdown can blow the new charge of air and fuel down the transfer ports or out the exhaust port. eficent shape of the hat is critical to avoid excesive blow down. their is a term its MSV maximum squish velocity.
as for contacting slp -done it and they advise against my base gasket idea, but I will try it cause I am me and I think its fine to do it lol. I'd like to point out I respect their opinions and run their stage 5 set up but im going to test my way. look at the fix kits shim plates? I really am not scared of what 3 or 4 thou will do to my performance I will admit if I was wrong.

peaceofwork · Dec 8, 2016

yes I have a knock sensor

Goinboardin · Dec 8, 2016

Thanks for the explanation. I'm interested to hear how it goes.

peaceofwork · Dec 8, 2016

thanks for your input, I suferd some detonation on my last build with a stock head so I needed a head and belive the slp product has better coolant flow to control detonation a bit better . also great a performance boosting dome shape and squish angle and width.

Goinboardin · Dec 8, 2016

I had race twins on an 04. Two years with a stock head. Lost PTO rod bearing to det on trail. It was ok off trail but must have gotten hot on trail down around 9k feet. Next season I ran SLP domes and never had det again. Those were 13.5:1 domes, though I forget the cc volume. This was at 9-12k feet, on 91 octane.

peaceofwork · Dec 8, 2016

8 years I ran on a stock head det light blinking and ignition hitting detonation retard mode here and their. I run scratchers and often drop off the trail for deep cooling powder but low elevations are a problem no useable jetting fixes , I flip up the choke purge lever at times to help out.
20% C12 race gas helped out a lot but its dam expensive

peaceofwork · Dec 9, 2016

3/64ths gasket paper has squished down to .035 so if ideed the stock gaskets are .030 my home made gaskets boost my cylinders by .005 or 5 thou and chamber volume increase of .007cc.
50.5 cc stock 3000/6000' slp head chambers for pump 91
.07cc more chamber volume.
51.2 cc total chamber volume now I will check compression on both cylinders and load up the tank with chevron pump 94 no ethanol and mix Lucas semisynthetic at 32/1 and give it a try.
If low elevation starts don't get my knock sensor going and beating on the throttle or cooler than average temps does not get it sensoring my fun I will step back to 50.85chamber volume , it would only be an additional two and a half thou cylinder lift .
As was pointed out by goinboarding maybe squish gap is an exact thing?

Goinboardin · Dec 13, 2016

I came across some photos, I had 12-533 domes, 45.3cc volume. I had another set that was smaller but never did try them. Man that sled pulled hard for an 800!

peaceofwork · Dec 13, 2016

It's assembled and in the sled ,I am at sea level it,s freezing out side . Doing my copression test through cylinders with compression releases. I have 120 lbs . It hasn't,t fired yet. The rings aren,t seated.
With an ,04 vertical escape 800, what is normal compression readings?

peaceofwork · Dec 14, 2016

going boardin
elevation sure changes the game, I would probably be into nitromethane fuel to run your chambers at my elevation.
to what I remember reading
52cc 0-3000
50.5cc 3000-6000
48cc 6000-9000.
with your old set up what do you think you would make for horse power at your elevation?
I think I would be near 160 hp and I am lightend up by 78 lbs

whoisthatguy · Dec 14, 2016

peaceofwork said:
It's assembled and in the sled ,I am at sea level it,s freezing out side . Doing my copression test through cylinders with compression releases. I have 120 lbs . It hasn't,t fired yet. The rings aren,t seated.
With an ,04 vertical escape 800, what is normal compression readings?

140 psi

peaceofwork · Dec 15, 2016

140 got it thanks , I will do another comp test now the rings are seated.

Goinboardin · Dec 15, 2016

Cold versus hot, seated rings versus fresh, oiled cylinder versus dry, high end gauge versus cheap one. All make a difference. 120 cold and fresh rings sounds fine to me. I watch the value over time to see if it drops, but typically the crank snaps long beforehand. My etec is 115psi cold at 7200 feet. 03' 800 polaris is 105psi cold. Both run fine. It's a cheap gauge.

I would estimate my old sled was around 160hp at sea level. Easily as strong as my 800 Doo, probably more top end. Twin pipes really wake these motors up. 8300-8400 with the race twins felt very good. I never ran a single SLP though, they have good reviews. A stock 800 polaris is kind of a dog in my experience...

BILTIT · Dec 16, 2016

a 910 with twins at 2200ft helps wake up these 800's!

https://www.youtube.com/watch?v=a7IhNLCeeYw

04 rmk cylinder base gasket thicknes ?

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