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Discussion Starter · #1 ·
I have a pair of cylinders ported by CPI racing and resleaved to 80mm coming in the mail right now. I did the math it should make my 617 a 683cc Anyone ever run one of these setups?

On a 583 bottom end itll make a 640cc.

I want to possibly bore my carbs to 44mm, maybe put in a 670 rotary valve.

Tell me what you guys think.
What should I expect as far as clutching changes?

Im really excited to try this out but I wanna hear people input, maybe you will come up with something that didnt cross my mind.

the cylinders come with a recut head for the larger bore, except it should make around 180-200psi I'm kinda leary on tryin to run pump gas through it, If its runnin that high of compression but the squish clearance is still .055 or bigger should i worry?

if i HAD to could i double up on base gaskets to lower my compression? or would that change my port timing too much.

Any input is apprieciated. Thank you.

~jim~
 

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with that much comp. you will have to run race fuel, and for the gaskets out the two in and check your comp. again and see where its atand go from there
 

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Discussion Starter · #3 ·
But would adding base gaskets rase my port timing too much?

Someone has gotta have a little info on if the port timing is affected to a noticable amount with another .020-.030 base gasket.

please anyone? teamNFG?
 

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Interesting project
I would stay away from stacking base gaskets, blow out easier.
As far as your port timing, raising them you will loose compression (going with a thicker base gasket) The biggest thing with port timing is dropping it to low, if the piston doesn't drop below the exhaust port you have a greater chance of burning up your piston.
The only way to really know is to get it together, check your squish and compression, if they didn't have the complete engine they are only guessing on alot of specs.
But if your compression is that high you will need some high octane juice.
What does the builder recommend?
What are you going to use for pipes??
You may want to play on a dyno.
I'm running a big bore 720, it has taken awhile to get dialed in (carbs, clutching, and compression) so don't expect it to be a drop in and go its been over a year (mainly cause the areas I ride are a min. 3hr drive)
Carbs I would jet to a 670 for a base line and go from there, don't bore unless done by a professional taper bore or more probs are coming your way

Good luck
 

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Discussion Starter · #5 ·
Do you mean that carbs have tapered bores? or that i should't bore them unless done by a proffesional because i may end up with a tapered bore.

Also what can i expect as far as where most clutching changes are goin to have to be made?

Im atleast for now going to run a stock single pipe :/

although im going to keep my eyes open for a set of twin's

Theres a set of 670 crank shop pipes with stingers on the bay right now, what have you heard about crank shop pipes, any good?



Thanks in advance, ~jim~
 

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- carbs not bored properly cause a bog, hard to tune and as I had gave me a lean spot in my upper mid range (running rich...so I thought) and equaled a blow up. If you want 44mm carbs find some stock ones, it will help in the long run.

-clutching will be ????? you may be ok with your 617 clutching, it will give you a base line, but at this point a dyno run will benefit you, what will your max HP RPM be???

-your single pipe may not work and the same with 670 twin pipes, it would be like putting a Volkswagens exhaust onto a Diesel truck. They are tuned for a 617 and a 670 not a 640, not saying they will not work, but it is another factor, a dyno will help

-Crank shop makes good pipes
 

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Discussion Starter · #7 ·
Im totally oblivious to how much, and where to set up a dyno run.

Any info on that would be greatly apprieciated, i am very interested in running it on the dyno when its all together.

I've also located a pair of 42mm carbs for a good deal that im probably going to end up running. For now I'm going to see how I can get it to run on my stock pipe, possibly for the rest of the season, just because i wanna get some freaking miles on this winter (which i havent got much of any so far) due to my custom suspension gettin those bugs worked out.

But yeah any more input is apprieciated, anyone else done a big bore? I will post my squish clearance and compression as soon as i have it all put together.
 

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http://www.bigthunderperformance.com/
http://www.lakesmotorsports.com/miscpage_001.asp
I don't know Michigan, but here are afew I googled
 

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Discussion Starter · #9 ·
Team NFG, got another ? for you.

Stock jetting for 583's and 617's the mag and pto are different, but do you run identical jetting when running twin pipes?

I dont entirely understand why the jetting is diff in mag and pto carbs. please just clear this stuff up for me.

I talked to the guy i got the jugs from and he says hes got a set of pipes custom made by cpi for the jugs!! so im gonna try HARD to get those off him.

But please clear up the jetting issue for me.

~jim~
 

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The staggared jetting is common in rotary valve rotax engines (usually 10 jet sizes difference, eg. 350pto 360mag)
With the single pipe (end dump) and with twin pipes the mag side is allways hotter then the PTO side. 1996 doo came out with a single pipe with a side dump and this changed the characteristics of the tuning and actually changed the hot side to the PTO, then you put on twin pipes and the hot side went back to the Mag.
Just keep in mind that the mag side will be hot, and don't get worried if you have a difference with jetting, just tune it to were you are safe.
 

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Discussion Starter · #12 ·
Also i talked to the guy, and he ran it on 91 octane pump gas for 1500 miles, (witch was how long he ran it period) without any problems.

But he was also between 8000-10000ft elevation and im around 1000ft, so I think ill be runnin really high compression.

I can locally get 100 octane avgas for 5 bucks a gallon though, i may just mix my gas 50/50 with avgas, or just 93 and ill keep a close eye on the topend.
 

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Becarful
what is he recomending for octane, at a high altitude you can get away with the octane, but you are low, and running that type of octane you may require more than Avgas
get it, check the squish and compression and it will be a start.
 

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Discussion Starter · #14 ·
I got the cylinders and head, But i didnt put it all together yet, I brought the cylinders to a automotive starter shop and they are beadblasting all the crappy paint off the head and cylinders. I should have some numbers up this weekend, and tell me what you think as far as what octane I should be running.
 

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lets just see what your squish and compression is first

You should look up and study octanes and engine performance so you can better understand the ressonings
 

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Discussion Starter · #16 ·
I have a fresh coat of paint on the head drying right now, so I will atleast have some squish clearance numbers later tonight.

Compression numbers after I get the rest of the mag side junk on it.
 

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Discussion Starter · #17 ·
Well the only solder i had to check my squish was .060 thick.

And it didnt squish that solder at all! so I have atleast .060 squish clearance, and from what I can remember that is pretty safe for pump gas, am I correct?

I'll just have to check the compression once I get the stator and flywheel and recoil on so I can pull it over. to see if I can run pump gas.


TeamNFG, am I correct that .060" is safe for pump gas? as long as your compression is under like 160-170 is it?


I used just one OEM base gasket, they seem to be almost twice as thick as aftermarket ones like winderosa, so I'm hoping just this one thick base gasket will bump my compression down enough, well see.
 

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60 thou is good, 160-170 psi is high but lets see what your compression is.

As far as base gaskets they come in 8 different thicknesses if you look at the part that doesn't bond between the cyl and the jug should bu punch holes 1 hole being the thinest and 8 holes being the thickest
When playing with port timing with base gasket thickness be sure your piston is not sticking above the exhaust port, or you will melt the piston.
 

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I have a pair of cylinders ported by CPI racing and resleaved to 80mm coming in the mail right now. I did the math it should make my 617 a 683cc Anyone ever run one of these setups?

On a 583 bottom end itll make a 640cc.

I want to possibly bore my carbs to 44mm, maybe put in a 670 rotary valve.

Tell me what you guys think.
What should I expect as far as clutching changes?

Im really excited to try this out but I wanna hear people input, maybe you will come up with something that didnt cross my mind.

the cylinders come with a recut head for the larger bore, except it should make around 180-200psi I'm kinda leary on tryin to run pump gas through it, If its runnin that high of compression but the squish clearance is still .055 or bigger should i worry?

if i HAD to could i double up on base gaskets to lower my compression? or would that change my port timing too much.

Any input is apprieciated. Thank you.

~jim~
===============================================================
What 80mm Piston did you use on this Big Bore?

Thanks
Rich
 

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it sounds like you built a race engine are you ready for that?

squish is not relevant to compression. you can make 17:1 heads on .100" squish or you can have .045" squish on pump gas trail heads.

when you get the engine home do a compression test. if its 180psi go buy a can of vp c-16. you have built a race motor not a trail motor so you are going to need race gas.

for clutching find out what rpms you are running from the pipes. I would stay with your stock secondary and adjust weights to get rpms to where they need to be.

like NFG said stick with stock carbs, bored carbs run but you don't want to many hassles getting this thing going.

for jetting put the sled on the track stand, adjust air screw so from idle you can hit the throttle and it responds quickly with no bogs or pops. then move the needle up and down till you get a good clean response through the mid range, same with main jets. to rich and when you open the carb the last bit you can get a bog, to lean and it screams like a champ. now on the track stand you can crack the throttle and it runs clean to the top.
now you can set it on the ice and start doing short 500 foot runs kill it and check plugs. do that a half dozen times and check plugs. grey specs are detonation from to lean or not enough octane.
 
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