Port Mapping the 03 Wombat

[Zyx]

Having recently acquired a standard bore, stock port 03 cylinder, I decided to map out the porting and compare it to the port map provided by Harry Taylor in the Pabatco HT-4 Super Combat hop up tech tip article from October 30, 1974. My guess is that while working on the hop up details for the SC, Hodaka and Pabatco were collaborating on the design of the 03, because the porting for the 03 is surprisingly close to the stock Super Combat, being mostly just a bit milder on the exhaust timing, and having a fully bridged victory port. The other porting is very close. This may be why, today, the stock 03 cylinder can be used for anything from a street bike to an endure/cross bike without changing anything.

At any rate, having the 03 cylinder in front of me, I took the time to do a trace map to lay out the porting for review. While the drawing is a bit cluttered and smudged, it is as accurate as I can make it from what I have to work with. The smudges are there because I used my less than clean finger to rub/trace the port windows, and the photo shows the original tracing, dirt and all.

I measured port top dimensions from the top of the cylinder, and you will notice that the distance from top of cylinder to bottom of the exhaust and transfers is 51mm, not 50mm. This is because the piston does not stroke all the way to the very top of the cylinder in its 50mm travel. Just so you know.

At a glance, it looks to me like the 03 could benefit from some of Harry's old changes for the Super Combat, such as widening the exhaust and flaring the transfers. But otherwise, the 03 in stock form appears to be fairly advanced in its design.

Here is the photo. Let me know if it posts upside down. Also, if anyone has the stock SC dimensions omitted from the tech drawing from Pabatco, feel free to mention so I can pen them in. Mostly, the tech drawing shows the amount of change, but doesn't mention what stock was.

[viclioce]

You don't by any chance have the bottom end that went with that cylinder, do you? ; D Victor

[Zyx]

No. Bought the cylinder outright. But the bottom end of most any 125 is going to be similar.

[bobwhitman]

Shorty-

I own the last "project" bike Harry built, an 03 that he ported, added a big flat slide Mikuni and an HT2 pipe. While it is fully street-legal (with "Harry Upgrades" like modern turn winkers & Honda forks) he also added lightness wherever possible. It still uses stock ignition. The bike retains original 03 tractability and smooth low end, but absolutely runs away from my other, stock 03 and flat flies with more revs. Harry was big on widening, not raising the exhaust. While I've never opened up this cylinder, I am pretty sure you are on the right track.
I also suspect Rich can provide some useful response.

Bob

[Zyx]

I suspect that the widen versus heighten debate would be a thread of its own. Arguments both ways. I would be interested to see how Harry ported his cylinder, so perhaps next time it is apart, you could do a tracing and post it. Always something to be learned by studying the work of others.

[thirdstone]

Port map ,love em ".l measured port top dimensions from the top of the cylinder, and you will notice that the distance from top of cylinder to bottom of the exhaust and transfers is 51mm, not 50mm" I have noticed that as well,I wonder if that is to take into account the base gadget thickness? 03 cylinder is good and you are correct,just raise the exhaust port to 26mm and you have a mx top end. I,like others widen the top of the exhaust port to 70% of bore ie 39mm chord wise.keeping a smooth arc on the top to ease the ring in. The HT port map is pretty close to what you need. Harry later added case boost ports as well. For calculations I can't decide how to treat the victory port as its not a clean feed as the gas has to pass through the piston holes. I have some pictures of a jug with the HT spec ports ,I'll try and post them .

[thirdstone]

[thirdstone]

[thirdstone]

T

[thirdstone]

I got these pictures from eBay a while back when someone was selling the cylinder. Whoever did the porting did a good job. Should have bought it.

[racerclam]

The way I do exaust port is the way Harry taught me . Widening the center of the port is not the way he did it later on. His old specs in his own words are out dated. As for as the case port that was an idea of mine that Harry Liked and it works. Harry used to feed additional air through 2 holes in the lower part of the cylinder spigot but volume was limited and was being covered and un covered by the piston skirt , it did have merit but my case port has a continuous connection to the intake port and crank case.

Rich

[thirdstone]

Hi Rich ,credit where credit is due then , I assumed Harry had the case port idea because I once saw the case of one of Harry's bikes. I agree with the new way of exhaust port shaping , the idea of posting the pictures is to show folks what the mods look like completed.
Cheers
Kev

[Zyx]

The part I can't figure is the finger in the middle of the victory port. On the SC, it is a peninsula of metal pointing down, as shown in one of the photos above. On the 03 stock, it is a continuous bridge through the middle of the V port. Watching a piston traverse this area, I can see absolutely no purpose to having this bridge cut off so that it stands free. Seems to me it would vibrate anyway, and the tiny amount of metal removed simply does not open up any breathing space.

[thirdstone]

Agree with you. The 03 bridged design is superior for piston support. The new wiseco pistons have the ring positioned lower @ BDC I think the ring pops out on a 97 cylinder although some argue that it didn't , anyway you don't have that problem with the 03

[thirdstone]

My 03

[bchappy]

The 03 is superior however the Combat Wombat was a piston port and the victory port was fed through the wide slot (hole) in the rear of the piston. Therefore the "finger" was there to give the opening from the piston slot and not block any of the flow. I assume the Super Combat had the finger for the same reason as it also had the slot in the top rear of the piston. The 03 didn't have the slot in the piston is it was a two ring piston and there wasn't room so they were able to run the bridge all the way down.

[thirdstone]

Great explanation Bill
I remembered the slot after posting but I didn't know of the piston difference, not having a real 03 engine.

[Zyx]

The finger in the V port blocks the top slot in the piston by dividing it into two ports. It does this whether or not the finger is freestanding or bridged, so I still see no purpose to cutting it loose to stand free. Even if it were in the middle of the only air passage available, the port bridge would have almost no effect on air passage. It isn't big enough to make a difference.

Piston rock front to back is the most detrimental motion to the life of a piston. Having a full bridge more fully supports the piston skirt versus a wide open V port. Cutting the bridge so it stands free offers some degree of support while reducing port blockage. But, for it to stand free, it has to be wider than it otherwise would be so it doesn't tend to break off. So it may be shorter, being cut free, but then it is wider and offered the same resistance to flow.

I don't see a benefit, or even a reason, to having a free finger in that port. A full bridge offers more support while blocking just about nothing. The piston holes straddle that bridge; only the oval port at the top of the piston is partly blocked by the bridge, and is blocked more by the free standing, wider finger than with the narrower 03 style full bridge, except for about 4mm of stroke at the very bottom of the cycle, where the oval port in the piston is unblocked. Because this happens only at the very bottom of the stroke, it happens when piston speed is slowest and air movement at its least.

[Bullfrog]

Arizona, you seem to missing how the center ridge/peninsula of the "Victory Port" came about. It was designed and built for a piston port engine. The piston had a single, small rectangular window just under the ring. This window aligned with the bottom, fully open part of the Victory Port at/near top dead center . . . and NONE of the window was blocked by a bridge due to the design of the Victory Port. So it was indeed a feature which optimized flow FOR THAT PARTICULAR piston port configuration from the factory. In THAT PARTICULAR piston port configuration, having a full length bridge would not have improved piston support appreciably, but would have been an obstruction to flow.

NOTE: While the improvement in performance was relatively small due to the Victory Port, there was an improvement. Perhaps as important (and perhaps even more important) was the cooling flow immediately under the piston crown which was induced/provided for by the Victory Port system.

Ed

[Zyx]

Ed,

Do you mean bottom dead center? If so, I understand, which is why I said the window in the piston would be unblocked at the bottom of the stroke but only for about 4mm of stroke. The circular holes in the piston skirt straddle the bridge so they are not affected either way.

As for the benefit, it would have to be hypothetical at best. I seriously doubt the presence or absence of a tiny bit of bridge would affect total flow or total power output in a measurable way, because the window in the piston is unblocked only at that point where the piston speed is slow to dead stop. In any event, the V ports in the 97 style cylinder were widened to compensate for the extra wide bridge finger. The purpose of that finger is to support the ring at the bottom of the stroke. It also supports the piston skirt, marginally. My only point is that any benefit of the incomplete bridge in the V port is minimal at best, and that I would rather see a complete, narrow bridge than an incomplete, wide bridge finger. I would find it hard to believe that anyone has tried to dyno the difference. Surely the Japanese engineers didn't spend time or money to prove the difference between the two configurations. This, to me, is something that someone would just do, hoping to gain an edge without quantifying the result.

[Bullfrog]

DOH! Yes Bottom Dead Center!

Still, the design was "right" for the PISTON PORT Combat Wombat cylinder. Debate can rage on whether or not it was right for later reeded designs.

Ed