BaddJet Archive Home

Tuning

Class rules

Vintage Ski against Vintage Ski Limited, the same ski and opposite answers

Two IJSBA classes run the same Kawasaki 550 cases in the same hull and disagree on twenty three components. It is the clearest demonstration on this site that a part is not legal or illegal on its own, only in a class.

What this rests on

Basis
From a rulebook
Source
IJSBA Vintage Ski and Vintage Ski Limited class documents, rule prefixes VSK and VSKL, compared part by part on 11 September 2026.
Difficulty
Engine out

Same cases, same hull, two rulebooks

Both classes require original Kawasaki 550 engine cases. Both require the hull and deck to be what the manufacturer furnished. The Limited class states its own purpose as a traditional limited performance level with access to performance parts historically available from the aftermarket. So the machines start identical and the two documents then pull in opposite directions on what may be done to them.

The engine is where the two classes really separate

In Vintage Ski the ports may be modified, with diameters and shapes changed, an aftermarket cylinder is permitted, and stroke and rod length may both be changed. In Limited the cylinder must stay OEM with original port timing and no internal modification of any kind, porting is reduced to chamfering, and the bottom end is pinned to one rod length and one stroke. Limited is also the only Vintage class carrying an explicit numeric chamfer limit. That is not a small difference of degree. It is the difference between a built motor and a rebuilt one.

Ignition is the single cleanest discriminator

Vintage Ski allows a modified or aftermarket ignition box, permits the timing to be changed, and expressly allows total loss systems. Limited requires the unit to remain OEM, with no aftermarket box, no timing change and no total loss. One component, two classes, and the answer flips completely. If you only check one thing before entering, check this one, because nothing about the part's appearance tells you which class you are in.

Induction and exhaust, where Limited earns its name

Vintage Ski takes an aftermarket carburettor and leaves the exhaust largely open, including relocating the exit. Limited permits one single carburettor and no more, allows only the manifold, head pipe and expansion chamber, refuses relocation outright, and caps the exit opening at two inches. The waterbox and the stinger insert are both absent from the Limited list, which under the closed list principle means they are not available rather than unmentioned.

Exhaust sections 1 to 3, from the engine A box at the left stands for the engine. Bolted to it, a short tapering casting numbered 1 leads into a narrow straight tube numbered 2, with a small arrow inside it pointing right. The tube then flares into a wide body that widens, runs straight and begins to narrow again, numbered 3. A dashed line at the right edge marks where the drawing continues in the next panel. engine 1 manifold 2 head pipe 3 expansion chamber Exhaust sections 4 to 6, out through the hull Carrying on from a dashed line at the left, the narrowing end of the wide body becomes a thin tube numbered 4, and a dashed water line rises into it from below. A shorter, slimmer hose joins it to a closed box numbered 5. From the far side of the box a tube runs right to an open end, an arrow carries on past it to the outside, and the number 6 sits under it. 4 stinger hose 5 waterbox out 6 outlet through the hull cooling water

In order from the engine: manifold, then head pipe, expansion chamber, stinger, a hose, waterbox, and the outlet through the hull.

Gas from the engine meets these sections one after another. A class document can open some of them and hold the rest to the original part, so each section carries its own name in the rules.

How we know: the order is the one IHRA's rule P.2.1 lists and US patent 6,113,446 describes in paragraphs 28 to 30; the stinger and its cooling line are IJSBA's Ski Lites rule SKL.3.6.

Not to scale.

The hull answers are the ones that cost money quietly

Vintage Ski permits up to four sponsons, two traditional bladed plus two convex, within a stated maximum drop. Limited permits no aftermarket sponsons at all and lets you remove the originals. Vintage Ski takes an aftermarket handlepole; Limited requires the pole itself to stay OEM and allows only the bracket to be aftermarket, with the attaching point reinforced. An aftermarket hood is named as permitted in Vintage Ski and appears nowhere in the Limited document, which on the closed list alone would make it a refusal. The next section explains why we do not treat that as settled.

Two questions we treat as unresolved

The pump housing, stator and shoe, and the aftermarket hood, are all permitted in Vintage Ski and simply absent from the Limited document, so the closed list alone makes them OEM. But VSKL.1.1 also allows every Stock Class provision in Limited unless otherwise noted, and whether that brings either part back in is the most commercially significant open question in the class. We treat both as open rather than settled, because each rests on silence. Separately, titanium driveshafts and housings read as permitted in Vintage Ski where the two other vintage classes refuse them, and that one is flagged in our data as worth verifying rather than relied on. All three are printed here as open because that is what they are.

Closed list

Vintage Ski (VSK) and Vintage Ski Limited (VSKL) each name the changes allowed; anything a document never names stays as the manufacturer supplied it.

  • A change the document lists

    goes through

    • Legal
    • Legal, with conditions where the list sets a limit

  • A change it never lists

    stops

    • Not permitted, not listed in the rules
    • The rules do not say where our data leaves it open

Set of limits

The class states what may not change, or how far it may go, and leaves the rest open.

  • A change inside every stated limit

    goes through

    • Legal, with conditions where a rule names it
    • No rule, this is an open class where none does

  • A change past a stated limit

    stops

    • Not legal

Other classes in the same rulebook written this way

  • Stated: no IJSBA class; even its Modified, Grand Prix, Open and Freestyle documents allow only the changes they name, though their lists are broad (SKGP.1.1, ROP.1.1, SPGP.1.1, FREST.1.1, SKM.1.1).

Vintage Ski and Vintage Ski Limited are each written as a closed list, so the first thing to ask of any change is whether the document lists it.

How we know: the IJSBA 2025 Vintage Ski document (VSK) and the IJSBA 2025 Vintage Ski Limited document (VSKL), as the class records hold them: the rule type each record carries, and the closed-list clause VSK.1.1 and VSKL.1.1. The endings are the answers the class pages print.

Why this matters more than any single part

A seller can tell you a component fits a 550. Whether it is legal depends entirely on which of these two classes you enter, and the two are close enough in name that the distinction is easy to miss and expensive to discover on the day. We do not stock anything for either class: the whole catalogue here is SuperJet and SX-R. This page exists because the comparison is worth publishing even where we have nothing to sell against it.

Symptoms this addresses: Same ski, two classes, different answers · Not sure which vintage class my build fits · Told my ignition is illegal in one class but not the other · Want to know why one class is called Limited. All symptoms.

Cite this page

“Vintage Ski against Vintage Ski Limited, the same ski and opposite answers”, BaddJet archive, https://baddjetarchive.com/tech/class-rules/vintage-ski-against-vintage-ski-limited/

Add the date you read it.

Class rules · Engine out

Start
BaddJet Tuning