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Discussion Starter #1
Ok. I've been hanging around this place for a long time, trying to make sense out of all this tuning and performance stuff. Now I want to get an idea of what affects reliability of an engine the most. I know pipes can be a touchy subject, but I am more interested in the "sleeper" mods
 

(Keep in mind this would be dealing with a 670)

-- Performance gains vs. How soon will she blow?

1) Boring the carbs
2) Trail port/mild porting
3) Cutting head
4) Any other 'invisible' mods of this nature

At the very least it will give the snow-starved guys something to argue about.
 

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I'd have to say shaving the head affect reliability the most(you can get an easy jump with this, if you keep it reasonable, and run 92 fuel). If you think about the extra stresses it puts the engine under(higher compression = bigger bang), and if you run low octane fuel, its even worse.   Pipes will also effect engine reliability, more so when the engine RPM get in the high 8's.
When you port you are essentailly making the engine more efficient, and when you bore the carbs you are letting more air get into the engine.
 

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Some porting can hurt reliability in the sense that you might have a hard time keeping it running good but as far as blowing due to more wear the higher rpm of most pipes would probably be the worst. Also, higher compression like 1000-imp already said. If I were you I would just put in the thicker base gasket and then shave the head to correct compression. Then bore your carbs and all that would go nicely with the 501 that you already have. Then if you really wanted to scream, trail port it.
 

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What are the pro's and con's to  installing a thicker base gasket. I here you will gain more top end but loose slightly on low end?  
 

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I've heard that raising the port timing will really give you a big increase in top end. Then I think you have to re-clutch to engage higher in order to not feel the loss in bottom end.
 

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Rocketman are you looling for ideas?How about dropping in a 670HO,comes with DPM,larger carbs,better exhaust etc.Might be more economical.Did you toss the tripple idea?
 

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Discussion Starter #7
Nah...just throwing out some ideas...making conversation..the tripple idea is not scrapped..it is just waiting for me to get a summer job locked up and find the right parts
 

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Rocketman, I have a 670 with all the mods you mentioned, my carbs are bored to 46.5mm  and the cases opened up,cly ported, stock pipe rev it 79000rpm.
I have close to 4000k(2485miles) on the engine with out any problems what so ever. Get the same fuel milage as a stock MachZ, Lot's of fun racen the big boys. It's reliable I ride it every chance I get(just turned 10,000K). The higher you rev a engine the shorter the time between rebuilds, keep the rpm down to stock levels and I don't thing you would lose any relibility or durability. Modded or stock, they all can still break!
 

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If you throw in a thicker base gasket to raise the ports a
little and then mill the head back to STOCK measurements,
do you actually lose a little compression from the higher
ports?
 

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Discussion Starter #10
So what kind of HP numbers is yours throwing off mr670?
 

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Yeah I think it does take down compression since you would have a larger combustion chamber. Shaving the head would bring it back up to stock.
 

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</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (670HO @ Mar. 12, 2002, 10:42am)</td></tr><tr><td id="QUOTE">Yeah I think it does take down compression since you would have a larger combustion chamber. Shaving the head would bring it back up to stock.[/b][/quote]
With raising the port timing and then milling the head to
get the squish back to were it was, do you lose compresion
or does it stay the same.
 

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maluchm, with the raised port timing you are leaving the exhast port open longer, so you would have a lower static comp, ratio that's why you have to mod the head to get back the comp.you would have a lower comp if you brought back to stock spec.
 

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Discussion Starter #15
Shaving the head after raising the port timing decreases the size of the combustion chamber therefore bringing the compression back up to stock specifications.
 

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Yes, raise the jugs up and bring the head back down but my
dealer told me that you actually can lose just a little
compression from the ports being closer to the combustion
chamber. I've also had somebody tell me that running
synthetic oil and/or ER will lower compresion because the
rings won't seal off like they would with regular injection
oil. Just wondering if these people are smoking something
that I should be or if there is some truth to this?  
 

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Discussion Starter #17
Oh..sorry. I misunderstood
 
 

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Take a squish test with the stock engineparts and then put in the thicker base gasket and bolt everthing back together and take another squish test and then machine enough off the head to bring it back to your original stock squish reading.this will keep your clearance within stock specs.you might lose a little in bottem end but you will gain acceleration from 50 mph and up.from what i remember about my old 97 mxz 670,it had tons of power down low anyways.
 

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Rocketman: I would go to Split Second performance in Alta. He does supurb work on Rotax's.  If you need more info e-mail me.
 

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OK, here's what I'm asking! On a stock motor the compression
is lets say 140# Mag & 142# PTO. Squish is at....0.065".
Then take a thicker base gasket (2mm or so), reasembly and
bring the squish back down to 0.065". Should the compression
be exactly what it was before (140/142) or will it drop some
from where it was at stock?  
 
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