550 porting specs?
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  1. #1
    Frequent Poster Ganggreen's Avatar
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    550 porting specs?

    Hey guys, I'm looking for some good porting specs for an 1988 JS550, nothing too radical, just something to run on 91 octane. My other mods are 44sbn carb, stock head milled 155psi, westcoast full pipe, jetinetics alum. flywheel.

    I heard common porting for these involved raising exhaust port 3mm, and lowering the intake port 2mm? Any truth to this?

  2. #2
    Resident Guru XXX Racing's Avatar
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    I don't like advancing the port timing if I don't have to, I'd rather get the port timing area I need from width... within reason.

    Raising port timing like that (advancing) does less for making good power than it does for raising the power peak - it other words, advancing the port timing is good for getting a peaky powerband. Widening the ports, however, doesn't advance the timing much - or at all - and keeps the stocker's linear delivery, but enough port area to allow all the same RPM gains.

    Are you familiar with two-stroke piston port modification? It's not that I'm shy or hidden about giving out porting specs, but reserved about giving out porting info on the internet to "just anyone". Just like the parable: I would rather teach you to fish, than to give you the fish.

    Stand Up Rider #37

  3. #3
    Frequent Poster Ganggreen's Avatar
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    Piston port engine are new to me. I've been big into reed valve 2stroke atv's and dirtbikes. Usually on those we mess more with the transfers and exhaust.

    I don't mind losing a little bottom end in exchange for a boost in mid range and top end. I heard the 87 on 550 cylinder has much better power than the 86 and earlier one. I really like the piston port setup for it simplicity and uniqueness, just not to big on sending my cylinder out and spending $400 or so on some porting that I know I could handle. Equally important is the proper pipe conjunction to the porting which makes the difference from a great ski to an excellent one!

    A groupk setup would be sweet, just don't have that kind of $$. I just figured that since there are like thousands of these around the hot setup has been discovered to take away all the guess work. And I know the performance shop surely isn't going to tell you over the phone.

    Widening the ports sounds good if it makes more broad power with top end. Come on XXX share more secrets on this!!

  4. #4
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    Hey XXX do you know any good sites that talk about porting and the effects of doing different things? I don't plan on porting anything now but would like to eventually and could use some good background reading. I would do searches, but would rather find a site that KNOWS what they are talking about. I figured you might know one.

    Let me know where to do some reading. Thanks

  5. #5
    Resident Guru XXX Racing's Avatar
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    I found a site a long time ago that was beneficial in explaining how he came up with his porting specs, but overall he still hid his "secrets" so I didn't learn much from him. And you had to pay for a simple computer software program that only gave you some numbers that HE could use when you sent your cylinder to him. Otherwise, it was quite good with pictures and explanations of his process. Hell if I remember the site name though.

    If you really wanted to become a two-stroke whiz, find yourself a copy of Gordon Jennings' Two Stroke Tuner's Handbook - the only drawback, besides being hard to find, is that it was written in the late 60's (when two-stroke development was rising) before fuel injection and modern fuels, oils, and whatnot - his technology is a tad dated, but all the principles are still what drive the two-stroke builder's market.

    I still don't want to send you out into the world with some specs and a pat on the back, for fear that I would make some of MY mentors angry, and that an engine failure would come back to haunt me.

    I'll refrain from actual specs, because it's all dependent upon where your engine will fail in the RPM spectrum, and how close you want to take it to that fail point. Ports work on a principle of square area and the time they are open at a specific RPM, called "Port Time Area"; certain formulas have been used for how much Port Time Area per RPM - your reading should help you discover that. Using those formulas, you are all but guaranteed to have all your ports working for the same common goal. So specific specs can only come about after evaluating YOUR goals and YOUR engine setup. Even still, some truths about porting that came about from my experimentation:

    Exhaust Ports: The stock shape isn't all bad. Both widening and raising it together seem to work well without changing the motor's characteristics too much. 550's can handle raising it a good deal, but they turn peaky quickly. The cylinder will breach the water jackets around 5mm above the exhaust port, and you'll run into problems with fresh charge escaping through the exhaust port if you widen them too close to the transfers (like 3mm wider each side, max). Hint hint. But that's like for the hugest exhaust port ever, so don't do that anyway.

    Intake Ports: Lowering them too much will actually have the negative effect of having the charge blow back out the carb, so I like to widen them as much as is safe to the piston, and lower the middle to get the rest of my Port Time Area, with a very round shape. One 550 builder (Kawabunga on jetski.com) told me not to lower them past 90 degree overlap (so they're not open at the same time). I wouldn't lower them much more than 4mm on my raddest skis, and I think I widen them the same as the exhaust.

    Transfer ports on the 550 are perfect for super high RPM, so I leave them alone. However, for different riding styles (Jennings' book covers this aspect very well) you may want to change the way they flow into the cylinder and the time at which they come open. In general, the rear (small) transfer port can be aimed upward and raised a teensy amount to smooth out and lower the powerband.

    Overall, if you want your motor to shift power upwards, put more port area in the upper 3/4ths of the port, where it'll be "seen" at high RPM (for example, squared-off port shapes), and do that for both intake and exhaust. The opposite is also true - if you don't want it to be peaky, keep the port window round and low on all your ports, except the transfers, which could be raised.

    Also, gangreen, you mentioned the pipe working in conjunction with the porting. After all this, you need to develop (or in our case "tune") your pipe to work together with your porting rather than work against it. Unfortunately, Gordon Jennings' research is on two-stroke pipes that don't change temperature like ours do, so his dimensions will be *askew* - fortunately for us, PWC pipes work on a broad range of RPM. So if you have a ported motor, at the very least, you should have an aftermarket pipe.

    Stand Up Rider #37

  6. #6
    PWCToday.com Is My Home Away From Home the WaTeRhAwK's Avatar
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    Quote Originally Posted by XXX Racing
    I would rather teach you to fish, than to give you the fish.
    .

    HUH? !!!?!? you are kidding right? I mean seriously man paLEEEEZE tell me you're kidding!! hahahahahahahahahahhahaha,
    the guy asks for porting specs and you come off like the dude from the tv show kung fu LOL. I'm sure all he wants is for his ski to scream without taking tai chi class to do it.lmao

    There are lots of places on the "internet" to find out this type of information and there are plenty of message boards with peeps who dont give a rats hairy butt to share the info over the internet.lol

    I would start here:
    www.macdizzy.com

    and I'm sure harry klemm over at www.groupk.com would be more than happy to give you the specs you're looking for, or if you feel like spending some money you can ship your cylinder to him and have it done as well.

    let me know if you need a sparring partner. lmao

    not baggin' on ya XXX ,I'm sure you know quite a bit about these things, I just thought that was funny


    CCC


  7. #7
    Resident Guru XXX Racing's Avatar
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    It's cool. I was only getting at the point that I'd rather spend the time to teach somebody and have them be educated, rather than give them the answer and let 'em figure out why it is the way it is. Plus, the specs I like may not always be what other guys like; I set up my guitars in such a way that some guys love it and others don't... I figure my skis are the same way.

    Stand Up Rider #37

  8. #8
    Resident Guru XXX Racing's Avatar
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    But you know what, Zion3 made a good point; I *should* just lend my experience to ya! That's why I'm here, right? Just FYI, I hate giving out specs for no good reason, and would rather have someone building an engine discover it on their own. But I should also remember that people come here looking for the info, not a lesson. So here's a set of specs that are pretty *****in':

    Raise the exhaust port centerline 4mm on a 550 pump-equipped ski, and 5mm on a 440 pump ski; widen the port by 2.5mm on each side; shape the port so the original corners come up 1mm, and follow that radius out to your 2.5mm wider sides. I don't like severe port chamfers, but grind a chamfer about .015" deep around the port, more on the roof and sides than the corners. Overall, your port will have a more round roof shape than stock.

    (Just for education) The ol' rule of thumb for maximum exhaust port width is 70% to 75% of the piston's diameter. After that, you determine how much higher you need to go to get the rest of your Port Time Area.

    Lower the intake port about 4mm, widen about 3mm each side, and make the shape the same "roundness" like you did the exhaust.

    Leave the transfers alone. Every time I mess with them, the hit in the power goes away, and it loads up in no-wake zones. However, I learned a trick for making the rears pull in more fuel (from Kawabunga): draw an imaginary dot 3mm up and 3mm back from the rear (away from exhaust) transfer port's rear/upper edge; draw a line from the front/top edge of the front transfer to the dot, and draw a line from the bottom rear of the back transfer to that dot - you should have a triangle that leans rearward - now draw a line following the vertical rear edge of the port; this is all you will cut out (it'll be tiny) , but follow that shape in the port runners. It's very hard to reach without proper tools, which is another reason I leave them alone most of the time.

    If you have the time to do the WHOLE port, knife-edge the ring gap land in the intake ports and point it to the carb - ie. straight back for twin carbs, or 45 degrees for a single carb. Polish the intake port no finer than 120 grit. The exhaust is relatively straightforward, but you can polish it as fine as your patience allows. The bases of the transfers (down by the case) should be smoothed and polished out - I like to knife-edge the "septum" between them, and match them to the case. All ports on the gasket surface should be gasket matched. Never forget to chamfer the ports!

    A word about porting: if you lack time and/or patience, or if this ski will be a recreational ski, spend most of your time working the area of the port that makes the most difference: the top of the exhaust, and the bottom of the intake - the rest is insignificant, and the extra 5% you gain by doing the rest of the port isn't consequential for a rec ski. Any time you port, you shouldn't just cut a huge bevel in the cylinder to get the port timing, but actually take the time to make the whole port the same size out to the gasket surface - irregular airflow is your enemy (this is where you'll find out if the "porter" is good or not). In a two-stroke, fluids and air will "stick" to a surface that falls away, so tapering in the direction of flow will actually allow more CFM of flow. The intakes are difficult to achieve that property, but the exhaust thankfully cuts down to a nice round opening.

    Along with porting your cylinder, you should have a high compression head. With this porting, you'll be hauling a**, but feeling weak and peaky unless you bump compression up to around 10:1 to 12:1. If you use an unshaved aftermarket head, or a stocker cut up to .060", you can use 91 octane safely. I hate AVgas, but it can be mixed in at 25% for a safety margin, and it's 1/4th the price of race gas.

    It goes without saying, that if you're interested in porting, you should already have an exhaust system and a modified carb and FA. It would also do you well to have a better impeller and intake grate at the very LEAST in your driveline.

    There you go, 440 and 550 guys. I didn't mean to sound stingy or anything earlier, but this'll at least get you guys some specs that a LOT of people spent a LOT of time and money to figure out. Hopefully this brings this forum one step closer to being the all-knowing vintage ski place it should be!

    If anyone uses these specs, tell me about your results!

    Stand Up Rider #37

  9. #9
    PWCToday.com Is My Home Away From Home the WaTeRhAwK's Avatar
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    I have a friend that did that to his 440 and he milled the head to .060 and it screams. he runs a single bn44 and it will keep up with a 550, so doing that to a 550 just makes it even more of a monster.

    CCC


  10. #10
    I dream skis 550$$$'s Avatar
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    Another great write up XXX. Thanks.

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