ANYONE WITH A 720 Big Bore??need info

[woodsRIDER92]

Over the summer I'm planning to big bore my 670 to 720 or 740 which ever one is easier and more reliable.

This past season i dropped and rolled the chain case, put 9tooth extroverts, 151 track and a few other things.
I'm now looking for more power so a big bore is my plan since i don't want nitrous. I'm looking for 140-150hp i think this is achievable.

I'm thinking of pulling the whole motor apart and porting the motor since the 670 don't have the greatest airflow and big bore it. Now my consern is jetting and clutching for the people who have or have run this setup what did you guys have? I still have the HAC in and i don't plan on pulling it out and i have ufo's as well. Right now i have my clutch set up for the longer track but im not sure if it will have to change for the big bore.

Any info would be great soo i could start to gather parts for my project.
Thanks

 

[renegade x]

Its not hard to get 140-150hp out of a 670 why bigbore it?

 

[Polarblu]

had a crankshop 990 bases on that crank. VERY SMALL crankshaft!!!!!! CS built a magnesium case to hold it together, fast revs, 200+ horse. Rotary valve engines sit high in any chassis. Round slide carbs are hard on your hands. best mod is a worn out turbo. rotary valves really shine with durability under boost. i have personaly seen an old green summit pull a "HOLY CRAP DID YOU SEE THAT" on a pack of big buck yamahas.

Ok back on track lol, that bottom end dosnt pay you back for what you put in on big bore.
Boost works only because it comes on smooth and linear. None of the big power "spikes" that big bores can make. On weaker cranks the "lag" actually helps.

To have more fun with that model, in my opinion, dont mod it. They are worth more stock.

If you have to mod it, loose the hood and seat!! It will feel like 15 horse. Thrasher hoods rock and are cheap. Build your own freestyle seat, not much to 'em. I like WRP. NO PIPE!!!!!!! waste of time for 5 horse. Cheap silencer is all you need.

I got ahead myself............. First thing is your comfort and to specify, your lower back. Riding all day looking like a pitbull screwing a football will make you miserable!!!!!!
7inch riser, wider bars.

The list can go on, in the end you have a fancy heavy M7. And im not a Cat guy.

I have a beautiful poo mod and it still dosnt stack up to stock now, and easy 3 time more cash.

 

[woodsRIDER92]

Its not hard to get 140-150hp out of a 670 why bigbore it?

what would you suggest then?
I did the short boots, and gutted air box, im going to port it, im not doing 501 plate since when i did it killed my fuel milage

more info would be greatly appreciated

 

[woodsRIDER92]

had a crankshop 990 bases on that crank. VERY SMALL crankshaft!!!!!! CS built a magnesium case to hold it together, fast revs, 200+ horse. Rotary valve engines sit high in any chassis. Round slide carbs are hard on your hands. best mod is a worn out turbo. rotary valves really shine with durability under boost. i have personaly seen an old green summit pull a "HOLY CRAP DID YOU SEE THAT" on a pack of big buck yamahas.

Ok back on track lol, that bottom end dosnt pay you back for what you put in on big bore.
Boost works only because it comes on smooth and linear. None of the big power "spikes" that big bores can make. On weaker cranks the "lag" actually helps.

To have more fun with that model, in my opinion, dont mod it. They are worth more stock.

If you have to mod it, loose the hood and seat!! It will feel like 15 horse. Thrasher hoods rock and are cheap. Build your own freestyle seat, not much to 'em. I like WRP. NO PIPE!!!!!!! waste of time for 5 horse. Cheap silencer is all you need.

I got ahead myself............. First thing is your comfort and to specify, your lower back. Riding all day looking like a pitbull screwing a football will make you miserable!!!!!!
7inch riser, wider bars.

The list can go on, in the end you have a fancy heavy M7. And im not a Cat guy.

I have a beautiful poo mod and it still dosnt stack up to stock now, and easy 3 time more cash.

iv thought about a turbo but its even more expensive then big bore and you have to play with jetting way more, but if you can find me the resources for cheap id definatly think about it :d

i crashed my sled last season causing my hood to be destroyed, soo im in the process of making an aluminum mesh hood and im going to make a freestyle seat just to loose weight and im putting in bar risers too before the season starts and if i come across some cheap bars ill put them on but its not too important to me right now

im just a kid thats having fun working on my dads old sled trying to make it better and easier to keep up with my dads 07 rev, ill never me neck and neck with him unless i turbo but i dont need to be

but thanks for the advice

 

[Polarblu]

Right on young man! I love to see passion for our sport.

But still........ one of the most durable engines in stock form, play with the other parts.

Dropping weight in the chassis, and making the clutches work for your weight and aggressiveness will make the sled perform 3 times better than just throwing power at it.

 

[woodsRIDER92]

Right on young man! I love to see passion for our sport.

But still........ one of the most durable engines in stock form, play with the other parts.

Dropping weight in the chassis, and making the clutches work for your weight and aggressiveness will make the sled perform 3 times better than just throwing power at it.

I am tryin to drop weight but im running out of ideas, i have gutted my silencer, put a 2 wheel kit in it, and it already has lightweight trailing arms now im making a mesh hood and freestyle seat for this upcoming season.

any suggestions on the size of tubing for the seat? im going to be using aluminum

thanks alot

 

[Modman]

Call up KORE on here, he has an ad in the classifieds. He has some sleeves for the 740 for sale, which is an easy mod on the 670, you just need to have them machined in and the cases machined to accept the sleeves, Mongoose Machine could do it for you since they are in your backyard there. The crank will be fine IMO at the 720/740 displacement, no worries. I have had several 670 motors in various forms, one stocker with tweaks 670, one race ported race gas mod 670, and maybe something else....LOL. You can build them into 825 or 990, as Polarblu has stated, but once over the 740 cc displacement, I would have to agree that the crank is getting a little small for the displacement of that size. SeaDoo Clinic has a wide bearing mod for the 670 motor, which would be a must if you wanted to go bigger than 740, at least IMO.

I have never found the 670 to be peaky, unlike my big bore triple, the twin makes so much more torque down low and if you keep it clutched right, it should pull right from engagement at 4000 rpm. The strongest pull should be from 5500-7500. Don't rev them past 8000 or the crank will go on you for sure. Make sure the jetting is correct in both cylinders as well, this way one cylinder is not working harder than the other, which will put extra load on the crank and bearings, causing failure.

The 720 is just as hard and just as expensive to build as the 740, since you have to do the proper spec on the pistons (pretty sure they are Polaris watercraft or 700 RMK pistons) and still overbore the cylinders. The 720 does not leave you with a lot of wall thickness either if I remember right. There is a lot of monkeying with the piston deck height and piston pin height, and you need to make sure that wrist pins are the same size. Val Simmons built a lot of 720's back in the day, you could try calling him. But for the $$ and the time it takes, you're better off with the 740 IMO.

 

[Summitric]

Like modman said, contact kore.... He has the 740 kit for sale, last i saw. Easy and very reliable conversion, and you've already shorty boots..... Put cpi racing dual pipes on it with the 740 and you'll be around 165 - 180hp, depending on set-up.........very reliable conversion! And very torquey.......

 

[woodsRIDER92]

Call up KORE on here, he has an ad in the classifieds. He has some sleeves for the 740 for sale, which is an easy mod on the 670, you just need to have them machined in and the cases machined to accept the sleeves, Mongoose Machine could do it for you since they are in your backyard there. The crank will be fine IMO at the 720/740 displacement, no worries. I have had several 670 motors in various forms, one stocker with tweaks 670, one race ported race gas mod 670, and maybe something else....LOL. You can build them into 825 or 990, as Polarblu has stated, but once over the 740 cc displacement, I would have to agree that the crank is getting a little small for the displacement of that size. SeaDoo Clinic has a wide bearing mod for the 670 motor, which would be a must if you wanted to go bigger than 740, at least IMO.

I have never found the 670 to be peaky, unlike my big bore triple, the twin makes so much more torque down low and if you keep it clutched right, it should pull right from engagement at 4000 rpm. The strongest pull should be from 5500-7500. Don't rev them past 8000 or the crank will go on you for sure. Make sure the jetting is correct in both cylinders as well, this way one cylinder is not working harder than the other, which will put extra load on the crank and bearings, causing failure.

The 720 is just as hard and just as expensive to build as the 740, since you have to do the proper spec on the pistons (pretty sure they are Polaris watercraft or 700 RMK pistons) and still overbore the cylinders. The 720 does not leave you with a lot of wall thickness either if I remember right. There is a lot of monkeying with the piston deck height and piston pin height, and you need to make sure that wrist pins are the same size. Val Simmons built a lot of 720's back in the day, you could try calling him. But for the $$ and the time it takes, you're better off with the 740 IMO.

Like modman said, contact kore.... He has the 740 kit for sale, last i saw. Easy and very reliable conversion, and you've already shorty boots..... Put cpi racing dual pipes on it with the 740 and you'll be around 165 - 180hp, depending on set-up.........very reliable conversion! And very torquey.......

hahaha iv been talking with kore for a bit now, hopefully next week i head up to his place to pick up all that he has related to the S chassis that he has for sale, im going to use his sleeves that he has and do the 740, and throw in the 44mm carbs that he has and his pipes he had made for his rev740(im going to have to modify them a bit to make them fit), i should be fine with pump gas with this setup right?or would i have to change the "squish" clearance(i think its called) to run on pump gas(94-96oct), hopefully i dont need too

You think ill have 165-180hp??? i thought 150 would be it but more is better

 

[woodsRIDER92]

What helix do you think i should use, i have a 44 and a 57degree but do you think i should go even taller?, im only 150lbs

 

[Modman]

hahaha iv been talking with kore for a bit now, hopefully next week i head up to his place to pick up all that he has related to the S chassis that he has for sale, im going to use his sleeves that he has and do the 740, and throw in the 44mm carbs that he has and his pipes he had made for his rev740(im going to have to modify them a bit to make them fit), i should be fine with pump gas with this setup right?or would i have to change the "squish" clearance(i think its called) to run on pump gas(94-96oct), hopefully i dont need too

You think ill have 165-180hp??? i thought 150 would be it but more is better

Good on ya - I noticed that in his classified ad after I posted that, that you had touched base with him already. Whoever does the machining for the sleeves should be able to set your compression, there is more than squish to the equation though.

The squish band width, angle and tolerance (clearance) all play a part in the turbulence of the combustion in the cylinder head. Also, a wide squish band will make the motor detonate at high RPM, a really thin squish band width will make your motor sluggish on the bottom end. Generally the same rules apply for angle and clearance. Your options for machining the head are as follows:

a) You can tighten the squish band up but also machine the domes out to keep the compression ratio the same giving you tighter squish but same compression ratio

b) you can machine just the bottom of the head to get the squish tighter, leaving the domes the same size, which will give you more compression and a tighter squish

c) you can machine off the squish band area to reduce the volumes of the dome, leaving your squish the same width, angle and height in relation to the cylinder deck, which will give you more compression but keep the squish tolerances the same.

Whoever does the machining for the sleeve installs should be able to re-cut the head for you as well. You can mess with the squish angles, widths, etc, but the stock head angles and width works well I have found, just bump up the compression a little IMO and pin the throttle. Keep in mind that you are going to need the bore of the head opened up, so your head will only fit a 740 once it's machined.

Important question - What elevation are you riding at? Blending race gas is fun because it smells so nice but my race motor inhales the stuff, it burns almost $80-$100 worth of race gas daily on the average ride. I would keep the compression down to 12.5:1 at the highest IMO, then you probably won't have to blend fuel if you are riding over about 3000 ft. Anything lower than 3000 ft I would go easy on the throttle or mix 20-25% race fuel for safety. If you want to run race gas all the time, then make the compression as high as you want, but just a warning that running your sled probably won't be cheap (from a fuel perspective).

Ralph at Split Second has a head program in the computer for the 740, or you can call Tom at Mot's Machining also. I don't know about Mongoose but they probably have something.

The 740 will probably run in the 150-160 HP range once you are done. with pipes you might be up around 165, I wouldn't think it's much more than that with the stock cases. They will make more power if you do extensive case porting but unless you have the 501 RV, bigger carbs, twin pipes, some decent case and cylinder porting and pretty good compression (13:1 or better), I wouldn't put it much past the 160 mark. You might want to try and find another set of 44 mm carbs and have them bored out (I'm running 46 mm carbs right now). Keep in mind that the torque is what sets your HP (HP = torque*rpm/5252), so I'd rather have a torquey engine at 7000 RPM where you need the grunt and run it all day (and will calculate out to lower HP), rather than a peaky motor that only starts making good power at 8500 RPM, but it rarely revs that high and has no bottom end (but will calc out to a higher HP #). You also want to look at cumulative HP, that is, how much HP (torque) the motor makes over a certain RPM range. HP is just a calculated #, and is based on torque. Also sled clutching is more efficient at lower RPMs, so the farther down your RPM band you can keep the torque, the easier the sled will be to clutch and the cooler the clutches will stay. It ain't all about the power though - You are already on the right track to building a good sled, with all the lightweight mods you're doing. 160 HP motor (that runs strong all day long and is not finicky) in a 450 lb sled is a nice little rocket......or so I've been told

 

[woodsRIDER92]

anyone have a part number for the pistons used in the 740 big bore i cant seem so find it
im having my my cylinder head machined and my sleeves put in this week and i need the pistons now

 

[Modman]

Wiseco kit is part #: SK1310

They will need the pistons to do the machining? didn't they tell you that?

 

[KORE]

I used Pro-X pistons in my build i just cant recall a part number!

 

[woodsRIDER92]

Wiseco kit is part #: SK1310

They will need the pistons to do the machining? didn't they tell you that?

yes thats y i need the pistons

 

[woodsRIDER92]

Wiseco kit is part #: SK1310

where did you get that part number ?, my dealer says that i cant order those pistons since they're special order.

and i looked on the Wiseco site and they have a kit for a 740 but its only 82mm not 82.5mm like my sleeves say, Wiseco says that 82mm bore makes 739cc and advanced sleeves say 82.5mm makes 739cc, who's right?

 

[Modman]

If you have advanced sleeves then I would call advanced and see what they say to run in it for pistons.

i got that part # straight from the Wiseco catalogue.

 

[woodsRIDER92]

So here's the update soo far,
I found pistons that will work, they're sea doo 720 pistons from a late 90's they have a bore size of 82.5mm which will make my stock 670 into a 748cc (ill just call it a 750 :d)

My jugs and head have been sent out for machining and my crankshaft has been sent out also to get balanced. Hopefully by the end of the month ill have everything back and started on match porting and polishing the top and bottom end.

 

[tex78]

So here's the update soo far,
I found pistons that will work, they're sea doo 720 pistons from a late 90's they have a bore size of 82.5mm which will make my stock 670 into a 748cc (ill just call it a 750 :d)

My jugs and head have been sent out for machining and my crankshaft has been sent out also to get balanced. Hopefully by the end of the month ill have everything back and started on match porting and polishing the top and bottom end.

crap im too late... i have a buddy that has a hole 740 for sale....