Modern Vespa

Hey all - random question & nerd alert.

Is there a good place to dive deeper into the tuning calculations behind our awesome beasts? I know there have been some epic threads about tuning but I'd love to start with the basics and most calculators seem to be not used / out of date by a few decades.

I am diving into the Gordon Jennings Tuner's Handbook and a few SAE whitepapers about tuning but this raises as many questions as it answers for the uninitiated (this guy).

For example, calculating Gmax (piston speed) for a 177cc setup seems to give answers that don't line up with suggested ranges to minimizing ring flutter by quite a bit. For a 2mm ring, the Jennings chart suggests ~65k ft / sec^2, I'm getting more than double that from the calc.

Are we using different charts? Is Gmax / piston speed not as relevant? It seems pretty key for longevity but I'm still learning the sensitivity on two-strokes so maybe the calibration is just in my head?

If you are stuck in the 1970s the max rpm for a 2mm iron ring is very important. 50 years on, we have 1mm trapezoidal chromoly rings, now nobody cares.

Using my 125 engine as a guinea pig, I get 46,468.1 ft/sec2. This is piston acceleration, not speed. I think you are inverting the ratio for A.

I was messing up the unit conversion using the constant. Once I fixed getting mm to inches I get reasonable numbers

At 8k rpm w/ a 57mm stroke I get about 50k ft /sec2, going long stroke (60mm) brings it to 54k ft /sec2.

It takes about 10k rpm w/ a 62mm stroke before we'd need to worry! This is good to know! It looks like Vespa engines are also well under the speeds where you really need to worry about it. Makes sense why I couldn't find anything in the archives on it

Reading Jennings definitely helps me understand your Wizard Sleeve port description btw!

If you wanna know what that kind of worry looks like, I have pictures.

Yes please! That sounds horrifyingly fascinating

Start here

Holy crap - I missed this part of your build and omg!!

After catching myself up on your build more completely, slightly off-topic question for you. If you were to avoid the Mazzu cranks, for a largeframe build is there one manufacturer that stands above? Not that I'll be getting anywhere near the upper limits you were, but still...

At a glance it looks like aside from Mazzu there are a few options, maybe?
* BGM has one with ill-described timing
* Maybe this Pollini
* This Pinasco looks mostly right
* SIP unknown manufacturer, but nice timing number potentially

Back on topic - I am diving deeper into the math and building an online excel sheet that might be helpful to others. Port-timings in Jennings is very interesting. I captured a few paragraphs (screenshot) that nicely line up with what a lot of you experienced wrenches have been guiding us noobs to.

Still working this out and will hopefully get measurements dialed in on the engine tomorrow to help put it into context.

Mazzuchelli cranks are generally fine. If used sub 11,000rpm they last a very long time

Metric system is the only logical way!


Ohh, I remember doing these calculations. So much time to waste, but damn it's fun.

I have never used long-stroke pistons, as worked out how much faster the piston travels and how much further it travels on a minute at 6500rpm... and i seize numerous times, so a slightly slower speed is good.
Plus, many years ago I learned that a Nissan RB25DETT was FASTER than a RB30DETT due to squarer bore/stroke ratio. (or something like that)

I'm now running a Quattrini crank in the Smallstate. For the P200 build I'm contemplating, I'll probably go with an Uncle Tom 62mm bell crank. Basically, high end cranks run $400-600, versus stock-grade cranks for ~$100, and more normal "performance" cranks at ~$200. You have to learn the brands, and can pick up a lot by just watching the components that other tuners are the level you're (trying to be) at. Then, look at the spec's for those parts and you can extrapolate from there. SoCalGuy was working on this exact topic over the weekend with CM2 and I helping(?) out via text message. While a lot of that info is, as Jack said, based on the limits of 1970's materials science, it still fundamentally applies in determining how a particular change will impact the shape of the power curve.

First off, there's the basic math of determining port timings from port measurements. I have a sheet which I found that does that, plus I built speed and power requirements. The power requirements need some tuning, but the gearing calculator is useful, at least.

You should be able to download a copy of it here. It uses LibreOffice for the more interesting (i.e. macro's) bits, but the basic calc's all work if you import into Excel, too.

I recently started adding some new functionality to it that maps port timings to expected peak power RPM's, but it's not ready for prime time. It requires port width, not just port height, which means generating a proper port map to measure from.

And if you want to get really serious, you can plop down the cash for actual software. Patrick (OopsClunkThud) uses EngMod2t, but I haven't ever quite pulled the trigger on that...yet.

I plan on staying well below this

Although - tuning question. I have seen several different "Peak RPM" goals for fast touring. For the older largeframe gearing, would a good peak rpm goal be 7500 - 8000?

More likely +-7,000 RPM's. Put the gearing into a calculator, see where that leaves you in 3rd and 4th, and then start to tweak from there.

Completely agree! Converting / remembering silly constants is a drag... Definitely not necessary because of the expert knowledge and free guidance on here! That said, I hate asking dumb questions multiple times in a row and doing the math will hopefully help reduce the number of times asking.

That way I can bother everyone with even more dumb questions

I've been innovating in the field of dumb questions (and actions!) for years.

Perfect - I've been using 7k as a baseline. The gearing looks good until about then on the stock VBA, especially with a few tweaks.

I'm also following the general guideline below to think about power throughout the rpms. Jennings refers to motocross vs race tuning - touring sounds like much more of a motocross style.

Aside from lower overall power, are there any other drawbacks with picking 7k (ish) for peak power?

If you build it right, it'll last basically forever, which is definitely a non-drawback. Things have gotten better in the past few years, but in general, the higher tuned motors' lifespans fall off geometrically with the increase in performance, even with high quality parts.

Plus, all the gearing is pretty well aligned to that number, so you won't be having to build a crazy custom gear stack, which adds up fast.

I think it's great that you're reading Jennings. Technology has advanced since 1973, but the physics hasn't. Tens if not hundreds of thousands of engines have been built using his guidelines.

In the Vespa world, tuners have pretty much narrowed down what works and what doesn't for a particular application. It's really just a question of what you're trying to achieve.

Not being particularly math oriented myself, things didn't start to fully click until I had a degree wheel on the engine and actually watched and measured where in the cycle the various events were occurring - inlet closing, exhaust opening, transfers opening, etc. - and how altering one changes and affects the others. You'll see you when you pull your engine apart.

It's magical and fascinating stuff, and endless fun.

And to answer your question, no, there's nothing wrong with peak power at 7000 rpm.