This month in the garage, Dan puts the Polini 221cc reed-valve engine through its paces on the dyno…
One of the things I enjoy most about working with Sean Brady at the garage is that neither of us can ever seem to leave well enough alone. Most people build an engine, bolt it into a scooter, ride it for a while, and call it done. We don’t seem to be wired that way. Every engine becomes a rolling experiment. Every dyno session raises another question. Every answer inevitably leads to another modification. That’s probably why there’s nearly always something interesting happening in the workshop.
Last month, I wrote about the Polini 221cc Vespa engine that Sean had just built for a customer, as it sat freshly assembled on the bench, ready to go back into the frame. At that point, it was still just a collection of carefully chosen components and educated assumptions. We knew what we were aiming for, but until an engine is strapped onto the dyno, all the theory in the world is still only theory. This month, we finally found out whether we’d got it right.
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The engine spec…
The specification is fairly good without becoming ridiculous. Sean started with the trusty Polini 210 kit but combined it with a 60mm bell-shaped crankshaft, taking capacity out to 221cc. The engine was converted to reed-valve induction using the Polini reed kit, breathing through a Dellorto 30mm PHBH carburettor. The casings were ported to match both the cylinder and reed block; the cylinder itself received further port work, and a quality BGM clutch completed the package. Exhaust duties were handled by SIP’s Viper XL. None of those parts are especially exotic individually. The difference comes from making them work together properly.
Too many engine builds end up as nothing more than an expensive shopping list. Somebody buys every fashionable component in the catalogue, bolts them together and hopes for the best. Engine building doesn’t work like that. Components have to complement one another. Port timings have to suit the overall package. Compression, squish, ignition timing, gearing, and exhaust all have to pull in the same direction. Get that balance right, and the whole package becomes greater than the sum of its parts.
Sean’s final port timings came out at 186 degrees exhaust and 128 degrees transfer, giving around 29 degrees of blowdown. That’s a figure which immediately tells you what sort of engine you’re looking at. It’s sporty, certainly, but it isn’t an all-out screaming-race motor. The intention from day one was to build something with road manners… an engine you could tour and rally on, overtake confidently, and cruise all day without feeling like it was constantly trying to escape from underneath you, which is why it was matched (and tamed) with that particular pipe.
Dyno time…
Once everything had been set up and checked over, it was time to wheel it onto my dyno. There’s always a moment before the first proper power run where you wonder if you’ve forgotten something. Have you missed a jet? Is there an air leak? Will it pull cleanly? Or will it decide to spit the dummy halfway through the run?
Fortunately, none of those things happened. They rarely do.
The graph appeared exactly as we’d hoped. Peak output came in at 22bhp, accompanied by a healthy 20ft-lbs of torque, all at just 6,000rpm. Now, I can almost hear some readers muttering already… “Only 22bhp? Here’s the thing… peak horsepower figures have become almost meaningless on social media. Everyone wants the biggest number because it’s easy to boast about in a Facebook post. But horsepower alone tells you very little about how enjoyable an engine is to ride. Torque is what gets you moving. Torque is what pulls you up hills. Torque is what lets you overtake without frantically changing down the gears. And, perhaps most importantly, torque spread is what makes a scooter enjoyable during the other 99% of its life when it isn’t being held flat-out. This engine delivers exactly that.
Rather than chasing an extra few horsepower at 8,500rpm, Sean deliberately built an engine that starts pulling early, carries that power cleanly through the middle of the rev range, and keeps delivering all the way into cruising speeds. On the road, it’ll feel considerably quicker than many supposedly more powerful engines simply because there’s always something available when you open the throttle. Personally, I think that’s exactly how a fast touring Vespa should behave. But, of course, that wasn’t the end of the conversation…
More power, sir?
No sooner had the dyno run finished than we both started looking at the graph and asking exactly the same question… “What happens if we fit a proper expansion chamber?” The SIP Viper XL has done precisely what it was designed to do. It’s a superb touring exhaust, producing a broad, tractor-like spread of torque with smooth, predictable delivery. It’s difficult to fault. But if maximum horsepower becomes the objective rather than everyday usability, there’s probably more to come.
My instinct is that this engine has a few more horsepower waiting to be unlocked with the right exhaust. The obvious candidate would be something like SIP’s Performance 2.0 expansion chamber. On paper, it suits this exact type of engine: a reed-valve 221cc with sensible but sporty timings and a 30mm carburettor. I certainly wouldn’t promise miracles, but I wouldn’t be surprised to see this engine move from its current 22bhp into the 25-28bhp region with careful jetting and ignition optimisation. The trade-off, of course, would almost certainly be giving away some of that wonderfully broad torque curve that makes the current setup so pleasant to ride. And that’s always the compromise. Maximum numbers rarely make maximum enjoyment… unless you are sprinting or racing. We’ll probably find out before too long, because curiosity generally gets the better of us.
Finally, while we’re talking carburettors, I had another reminder this month that buying cheap often ends up costing more in the long run. A customer dropped off an aftermarket copy of a Dellorto PHBH with a badly worn throttle slide. Side-by-side with a genuine Dellorto slide, the difference was remarkable. The original still looked almost new, while the copy had developed heavy scoring after relatively little use.
Now, I can’t say with certainty exactly what alloy the manufacturer used, but the evidence sitting on my bench suggested it certainly wasn’t the same quality as the genuine article. The wear was excessive, the finish was poor, and it serves as a useful reminder that although some reproduction parts represent excellent value, others achieve their lower price by cutting corners where you can’t immediately see them. Sometimes, genuine really is cheaper in the long run.
As for the Polini Vespa engine? I suspect this story isn’t finished yet. The dyno graph says we’ve built an excellent fast-road touring engine. The two idiots standing beside the dyno are already wondering how to squeeze another five horsepower out of it. I’ll let you know who wins, stay tuned!
Article originally appeared in Scootering Mag August 2026 issue. To subscribe with a great offer and get your mag delivered visit HERE
