Porting the 787 - Who knows how to build a monster?

[IDoSeaDoo]

Hey guys, I'm planning on building a super XP. Last year, I bought up a bunch of parts, but haven't gotten aroudn to assembling yet. The more I talk to people about building this super 787, the more I hear about porting, polishing and even adjusting RV timing. I have a SpecI Factory Pipe with 41mm NOVI carbs and R&D F/A. I mentioned this to a professional engine builder and he warned that if I bolt that on, more of the fuel/air charge will rush OUT of my cylinder before it has the chance to be compressed. I would actually be losing efficiency. To do it right, he mentioned two types of porting, perfomrning a charge velocity measurement to properly cut a new RV valve plate, as well as adjusting a custom sized booster hole on the pistons (that little hole on the side). All of this makes sense to me, but how does anyone go about learning how to do this? Anyone ever gone this deep into a build? Is there any literature or writeups on this out there? I've scowered the web and come up with nothing How do people become "tuners"?

 

[Matt Braley]

Well this is a post after my own heart. I would find the people that know how to do it and pay them for their experience. Or you could do what a lot of other people have done and copy their work. You can easily go backwards when porting. There is a lot of areas that are obvious like where your intake area is.

If it is for competition then the porting automatically moves you to a more competitive class. If is for rec riding then too much of a monster 787 would not like pump gas anymore. I, like you, see much to gain from learning more about it and think there is a happy median available there for both styles of riding.

Mel Miller is the undisputed king of two-stroke tuning but anything from him is way more expensive. There are other tuner's names out there porting but it does not really seem to get outside the race community because it is time consuming (expensive) and does not offer big gains if not used in conjunction with lots of other modifications.

 

[Dr Honda]

Yep... what he said.

With 2-stroke porting... there is a lot of give and take. If you raise the ports... you change all of the scavenging, and bypass. So... to gain WAY up top.. you kill idle, and low RPM's. That's why they started putting exhaust valves in them. That way you can get both torque, and a high RPM hit.

One other thing to remember... raising the ports gives you effectively, less power stroke.

 

[Matt Braley]

I Do- You have plenty to gain by adding that Spec 1 and those Novi's. Heck add a performance head and you have a "Limited" race ski. That is opposed to the "Super Stock" race ski that is allowed the porting. These terms as well as "Runabout 800" are good ones for searching for fast XP's.

 

[Minnetonka4me]

I was kind of thinking the same thing as Matt...but your question really is "where do you learn this stuff?" and for that I have no answer. Most prob started very young, along side a master. Basically its learning all the theories of the 2 stroke motor, then trial and error.

Harry at Group K was known to be kind of the "master" of the recreational porting. He has pages of notes in his archives that many 787 owners consider the bible of the X-4. His prices were pretty reasonable compared to say miller.

Harry has slowly gone back to his roots, vintage flat track motorcycles...but he still does PWC work. Great guy!

 

[vettepilot_6]

I have hotted up quite a few 2/strokes in the past....including Victa 125cc to run in excess of 13000 rpm for a go kart...my father was king of 2/strokes he did a Mculloch 90b ran 3 carbies (1 in crankcase) to run in excess of 21000rpm it used to detonate the piston if it wasnt changed after every race...the easiest place to start is to see how much exhaust gasses can get out...check piston position on exhaust stroke...if any part of piston shows in the exh port...you can cut that away to give you better exhaust then increase carby flow (bigger carby) and same on carby side at full induction, piston can be cut so whole of port showing....if this makes sense? There is others thing too such as raising compression (shaving head) but then what you take of head has to be placed on bottom of cyl so piston doesnt smash into cyl head.... But if you dont know what your doing it can all go to hell.... dont know too many 2/stroke specialist anymore..

 

[ocod]

You seem to have your handful in 787.

You and Matt should get together and become team.
I'm sure your GF would love to go out and race too.

Come back anytime!

 

[IDoSeaDoo]

I'd love to OCOD, Miami was fun! Wish we had more time and warmer weather back then.

 

[IDoSeaDoo]

I've done a bunch of youtube research this weekend, saw some good videos on porting/chamfering two strokes and what to look for. I will def. attempt to do this, but not sure how far I'll get withi my bulky Dremel tool. All the guys in the videos used these tiny air powered (what almost looked like) dentist tools, none of which I have or have even seen in a hardware store. Amazon, here I come!

 

[chucked]

I have a nice snap on high speed rotary tool (dentist drill). I dont know much about porting but Ill always clean up casting marks and match port. Works good

 

[1995hx]

Personally I would contact bo duprist of Neptune racing. He has some of the best port work and will help you set it all up. I can tell you be careful on the exhaust porting as to wide and it will pull the rings out. The guy who ported my 787 did a awesome job on the cases but when he ported the exhaust ports on the jug the were too wide and I went through 4 top ends before I finally figured out what it was. The guy wouldn't stand behind his work so I put stock jugs on and now my 787 hx is a 62 mph beast and this setup in a X4 would be doing 64-65 mph. Im also running 200 psi head and a 50/50 mix of pump and av gas

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[IDoSeaDoo]

I'm guessing he made them wider horizontally, right? So my assumption is that if I don't make the ports any taller, I won't affect timing and can use the stock RV plate? Will porting/chamfering affect where the fuel/air charge is during the time of detination? That's really my biggest fear is that I'll screw up the scavenging dynamics and will lose more of my fuel/air charge out the exhaust as gasses rush faster through the less restrictive F/A, carbs, engine and exhaust. Do you guys know if there are any particular places in the motor that NEED to be smoothed/chamfered? I saw a video on a Blaster's jug where the factory consistently made a really bad transition from cast aluminum to cyl. sleeve. Would it be prudent to port/chamfer a stock ski with no other modifications? I have one apart right now, waiting on a piston to finish assembly...

 

[Spimothy Leary]

Personally I would contact bo duprist of Neptune racing. He has some of the best port work and will help you set it all up. I can tell you be careful on the exhaust porting as to wide and it will pull the rings out. The guy who ported my 787 did a awesome job on the cases but when he ported the exhaust ports on the jug the were too wide and I went through 4 top ends before I finally figured out what it was. The guy wouldn't stand behind his work so I put stock jugs on and now my 787 hx is a 62 mph beast and this setup in a X4 would be doing 64-65 mph. Im also running 200 psi head and a 50/50 mix of pump and av gas

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Good people at Neptune.

my first question to the OP and to anybody that has build a hot rod 787.... would be...

Is the porting really necessary ? Not discounting the opinion of a professional builder by any means, i'm just a rec rider. But I'm pretty sure that 95% of the hot rod 787's i've been around were not ported (at least i'm pretty dam sure they weren't), and they rode fast and pulled dam hard, and for the most part trouble free, duly noted that when you add novi's and such you'll be constantly tweeking the adjustment screws for weather conditions and such. But once you get the right combination of pump/prop/nozzle, spec pipe, ECWI, peformance head, etc, you'll have a dam powerful ski that will hole shot like a mother, and give a slap in the face for the first 1/4 mile to all but the most souped up 4 strokes, they will eventually chase you down but by the time they do, you'll already be slowing down and smiling because you know you won the race.

so again, circling back to the original question... and coming from a clown like me... is porting really really necessary ?

at what point does that HAVE to be done ?

 

[1995hx]

Good people at Neptune.

my first question to the OP and to anybody that has build a hot rod 787.... would be...

Is the porting really necessary ? Not discounting the opinion of a professional builder by any means, i'm just a rec rider. But I'm pretty sure that 95% of the hot rod 787's i've been around were not ported (at least i'm pretty dam sure they weren't), and they rode fast and pulled dam hard, and for the most part trouble free, duly noted that when you add novi's and such you'll be constantly tweeking the adjustment screws for weather conditions and such. But once you get the right combination of pump/prop/nozzle, spec pipe, ECWI, peformance head, etc, you'll have a dam powerful ski that will hole shot like a mother, and give a slap in the face for the first 1/4 mile to all but the most souped up 4 strokes, they will eventually chase you down but by the time they do, you'll already be slowing down and smiling because you know you won the race.

so again, circling back to the original question... and coming from a clown like me... is porting really really necessary ?

at what point does that HAVE to be done ?

Is port work needed on a rec boat hell no is it needed on a race boat (where the rules allow) hell yes the difference in ported and nonported engines is huge the pull of the engine just never stops my superstock when it was fully ported made 7700-7800 rpms my semi-superstock only pulls 7500 max so there is a huge difference. But with that your reliability goes way down.


And yes he made the ports too wide and the rings were just pulling out oh well I'll resleve some day but not anytime soon

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[Spimothy Leary]

Is port work needed on a rec boat hell no is it needed on a race boat (where the rules allow) hell yes the difference in ported and nonported engines is huge the pull of the engine just never stops my superstock when it was fully ported made 7700-7800 rpms my semi-superstock only pulls 7500 max so there is a huge difference. But with that your reliability goes way down.


And yes he made the ports too wide and the rings were just pulling out oh well I'll resleve some day but not anytime soon


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that's what I was implying... on a Rec Boat...

Like they say... "Speed is expensive... How Fast do you want to spend ?"

and just as important.

"Fast, Reliable, Cheap... pick Two"

 

[1995hx]

that's what I was implying... on a Rec Boat...

Like they say... "Speed is expensive... How Fast do you want to spend ?"

and just as important.

"Fast, Reliable, Cheap... Pick two"

You couldn't be more correct my friend.

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