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Hi guys it's been a while I use to be known as vintage 30 but my acct got messed up when I got a new phone . I'm getting ready to start back up on my 06 motor build again . Anyone in Michigan running a13 to 1 compression head on pump gas I was going to try it but think I'm gonna have it machined to be bit safer maybe 12.5 to 1 .
I'm running crank shop trail ported cylinders And this year I'm going to add a summit air box off the 07 as well as the exhaust with DJs helix. I put in a new crank and got the snow x dual ring Pistons installed on the rods started thinking about the head which is a slp for 0 to 3000 feet elevation which is 13 to 1 they claim it's safe but I'm leaning toward it isn't . I really only bought it to help with cooling the motor . Just wondering if anyone is running 13 to 1 with no issues on our crappy pump gas...thanks

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No I did not contact anyone I played around with it and I don't know if it was the phone or tap talk but I just couldn't get on here.

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13 to 1 on premium pump gas is not advisable. 12.5 is about as high as I'd want to go on pump gas. With a splash of race gas I think you could get away with it. But your jetting and pipe selection would be critical.
 

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Discussion Starter · #7 ·
13 to 1 on premium pump gas is not advisable. 12.5 is about as high as I'd want to go on pump gas. With a splash of race gas I think you could get away with it. But your jetting and pipe selection would be critical.
Thanks I don't think I'm going to even attempt it . I'm gonna do a squish test then have head machined to get me around 12 to 1 to 12.5 to 1 ..

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If u don't mind me asking who are u gunna use to machine the domes? I'm also in the same boat and bought an slp head that has wrong elevation domes in it than what I was told when I bought it.
 

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Thanks I don't think I'm going to even attempt it . I'm gonna do a squish test then have head machined to get me around 12 to 1 to 12.5 to 1 ..

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Darren, something I have learned from working with 600, 800, and 1000 owners is that the squish test is best done mechanically with an accurate depth gauge or dial indicator to get the same measurement as Ski-Doo. Otherwise you can easily get a squish that is far too tight. I have noticed and remarked this problem throughout various builds on DooTalk and it worries me. I noticed that engine builders who do use a medium like solder to measure squish also have their own specs for these engines that are different from Ski-Doo. So if you use solder I recommend using the spec from the builder. Hope that helps.
 

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A quick calculation says you need to gain 1.65cc's of compressed volume to go from 13/1 to 12.42/1 on a 400 cc swept volume. That's .012" added to your squish, or 1.65 cc volume cut from the chamber. Have you actually measured your squish? You need to do a cc on the assembled engine to know your actual theoretical c/r, and know what you need and have for a squish height.Contact the head maker and get the spec. There can be differences in cylinder and case height that can vary it. It may be called a 13/1 dome when you buy it, but it can vary + or - some. Guessing can cost you dearly. A few thoughts.
 

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Geez, even knowing can cost you dearly. That pipe action is a wild card that can raise cylinder pressure higher than you account for.A nice cushion in octane level is necessary if you're going to stray to far from the stock 12/1.
 

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Geez, even knowing can cost you dearly. That pipe action is a wild card that can raise cylinder pressure higher than you account for. A nice cushion in octane level is necessary if you're going to stray to far from the stock 12/1.
Why are we only beginning to hear about this??? I have been waiting patiently for you to bring this up. Here is a excerpt from a 4 stroke topic, only because recently I was looking for runs from Marcel Corriveau, but there are more for two strokes. Pipe pressure is everything.

Even if we tried to add fuel afr dropped but Pyro and exhaust pressure went higher.

http://www.dootalk.com/forums/topic/439654-top-speed/#entry3251319
 

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Not the same thing at all,daag. I was talking about the increased efficiency the negative wave adds to the induction process, and the raise in cylinder pressure by the stuffer phase.Unlike the formula for theoretical or actual compression ratio,there is no mathematical way to calculate this amount,not that I know of,anyway.Perhaps an engineer can calculate it? I call it the wild card, because it can raise the 'squeeze' above the octane level you have calculated it needs.Which,of course is why you want the most efficient pipe you can buy/make/borrow/design,increased volumetric efficiency. 4 strokes use the same negative wave during overlap to aid in scavenging the chamber, and produce a low pressure in the chamber to aid cylinder filling and ram effect.I saw a pic of an F1 car a few years back that had what looked like an expansion chamber on it. Could they have got induction on a 4 stroke working well enough to be filling the pipe with unburned charge, and stuffing it back in? Anyone have any thoughts or knowledge on that? A pic of a KTM works motocrosser my son has what looks like a diffuser and convergent cone on it.I'm sure the motocross guys on here have seen that.Interesting stuff.
 

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Are you saying we can't use center pipe pressure as a measure to detect if more or less unburned charge is being stuffed back into the cylinder?
It wouldn't tell you anything about the charge quantity that's getting stuffed or vacuum signal.The wave effect happens all through the pipe,and also happens in a reverse direction because the diffuser has a taper that acts as a convergent cone on the returning wave that is farther down the pipe, albeit in a much smaller way, due to the small angle.I'm a 4 stroke guy with limited 2 stroke and expansion chamber knowledge.I would say its very complicated, and like port directional flow and port timing, much easier to get it wrong than right.
 

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Zxmodv, the following links will give you a better idea of what I was referring to. Monitoring the center pipe is something I first learned from the few MachZ guys that knew how to tune their sleds. Then I noticed Dynotech were doing the same thing with a write-up on it, then I found it on Gus Bohne's MachZ, the Arctic Cat stock 800, the Ski-Doo Summits and Backcountry, then I found LRD's post which led to pipe builders. That is the reason why some months ago I couldn't understand why monitoring center pipe was not useful. I've only monitored AF but heck few even believe this is possible and still choose to adjust a fuel controller using EGTs. How on earth can the EGT tell you if you're nearing lean or rich misfires??? First you make a baseline without a fuel controller, then the fuel controller is installed to make adjustments. The lower the numbers the higher the fuel flow. I just don't get why we are over a decade behind the Arctic Cat owners. But knowing how much you like two strokes I figure I'd bring it up.

The graph was provided by BRP to dealers some years ago to help educate the effects of aftermarket mufflers. I have read the complete article from BRP and copied it somewhere, but somehow misplaced it :headbang . Krm could probably get us a copy. It was worth the read.

http://www.dootalk.com/forums/topic/218137-xp-500ss-full-wet-450lbs/page-8

http://www.dootalk.com/forums/topic/992753-silencing-the-can/#entry10951553

http://www.omega.com/pptst/DPG3500.html

Digital Pressure Gauge from Omega - NEMA 4X Case with Min and Max readings

Measuring instrument Electronic device Technology Font Auto part


800R Muffler comparison for the REV

Rectangle Slope Font Parallel Pattern
 

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My 770 has an egt in the centre pipe,daag. I'm putting fittings on the other pipes I have to use on it and the 800r. The wave action and the fact that the exhaust gas from a firing stroke not exiting the pipe in one pulse make reading 02 in the exhaust stream inconsistent, unlike a 4 stroke. That's why I don't use it.And to me, egt's are an indication of a process, not a tuning device.IMO. I rarely look at the egt's when I'm riding, in fact I avoid looking anywhere except far ahead. If it blows, so what. If I make a mistake because I didn't read the terrain for 200ft and go into the forest,well, the egt's don't matter. It has a high memory, and I get it tuned before I even turn it on.TT670 advised me on that.I like getting it set up, then just riding it.And I use only the stock muffler and ear plugs.I couldn't care less about cans. How 20 years ago.
 

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Spending money on an a/f gauge for a 2 stroke is akin to spending $300 on a can, or $450 on reeds. It just doesn't fit my world. Plug readings and wash are it for me.
 
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