Modern Vespa

Hey guys, mid engine rebuild here. To be honest I want to skip the step painful step of figuring out my crankcase volume, its quite modified for reed valves. So I'd like to know if anyone out there knows the crankcase volume of a PX150 crankcase or anything close. Thanks in advance 🫶🏻

Out of curiosity, what are you planning to do with that knowledge?

And how accurate are you trying to get here?

Figure the crankcase area is ~7700 mm^2. Multiply that by depth of the crankcase, less the cranks' volume (volume of the cheeks * 2).

Then, add the area of the piston * stroke and that's probably pretty close.

Fluid Displacement -
Remove cylinder and piston from the engine.
Seal all ports/openings.
Position the crankshaft at Bottom Dead Center (BDC).
Fill with a light oil ensuring no air pockets.
Measure the total volume of oil used with a graduated cylinder.

It's just been on my mind lately and im trying every possible thing to maximise power, and last on my list is crankcase compression. I want to try different crankcase compression ratio's and hopefully see if theres any difference on the dyno. Would love to share my findings too.

Do you think I could just use gasoline for this?

You could use gasoline if you like playing with combustible liquids and can do it in a well ventilated area. Not recommended due to the safety risk

I also point out a couple of modifications you will also need ro make to the setup as the current setup is very restricted in terms of gas flow

See attached image

I was just thinking the only place I've seen work like that is Indonesia, then I saw your profile.
Crankcase volume is not such an issue. Interesting to measure but generally it is too small for a 187. Using a full circle crank will make more maximum power. Using a bell crank more mid range.
Whatever crank you decide, the biggest easy power gain I would do is cut the bar from the cylinder skirt.

No worries that was fresh from the welders, its all drawn out now. Going to do the trenching today

Ive been thinking about cutting the cylinder skirt, but is that risky at all? I have 2 of the same blocks, one with the durations changed a bit for drag and dyno but i still sometimes use it on the road for events or touring. Im worried about piston life. Piston lives matter 🤣

I think the piston life won't change much. The risk is the cylinder spigot cracking. I would still cut it. At least cut parallel from the boost window. It's really in the way.
Be really careful about detonation or running overly rich. Both put pressure on that area.

Some time ago I had a Polini 208 and found (when looking at another issue) that the skirt had cracked. I cut out the window where the crack was and next time I rode it it seized. The skirt had sprung slightly inwards. So many problems with that cylinder!

I wonder if you could just reduce the width of the bit in the way of the gas flow?

Yea i didnt have the balls to cut it but i ended up widening it, also ran into some issues port matching the crank case, my dumbass forgot about the studs and i blew straight through. Got it all fixed now thanks to the quick indonesian welders 🤣 npt the best work left n right side arent exactly the same but id say its ok for my first time.

Yesterday finally got it all together and running, will say its running like a beauty. Before all that we did get to fill the crankcase with diesel. My goal was for a 1:5 ratio to engine cc. Which would be 280.5cc using the 187cc block. Well we ended up with roughly 295cc, we couldnt get an accurate measurement due to my reed valve leaking and just being lazy to wait for new reeds. We ended up just blocking off the reed all together then trying our best to calculate the volume of the inside of the reed valve and subtracting it from the total measurement.

what is the result with the optimum volume? What are you improving? are you needing to increase or decrease the volume to achieve the desired goal?

I imagine it will turn out to be one of those compromises... maybe more volume favours higher revs at the expense of lower torque. Or vice versa.

Curious.. what is the benefit of having a such modified intake?

Well im going off of graham bells 2 stroke tuning, there he states some old school japanese methods of raising crankcase pressure for better fuel scavenging but with old technology if you go too far youll lose some of the fresh mixture. But after doing a bit of research i hear 1.5x the blocks CC is a sweet spot so thats what I'm targeting. Meaning the next crankcase i build ill try aim for that number. Currently im only 10cc over which i dont think will make much difference.

The whole principle of the intake is allowing fuel and air mixture into the engine at an calculated time and holding pressure in the crankcase. Technically a rotary valve system is more optimal as throughout all the revs the timing will never change but vespas design is flawed in that it often leaks and gets worse over time. A reed valve set up keeps a better seal, lasts longer, no need for calculating intake timing, but downside is at higher revs the reed petals sometimes dont have time to close completely but overall you will get a way more responsive engine throughout all revs compared to a standard set up.

Took her out for a test run today, ill say she scared the shit out of me. Getting prepared for a dyno event next week but sadly sheared my 3rd gears. Hope to get her fixed tomorrow and aim for that 37hp

Glad to hear it's going so well, did you try this cylinder on a less modified setup before?
I guess I'm asking what the new intake location adds to the power on it's own?

Looks like that gear gave its all!! I feel like when they're talking about reeds at higher revs, they don't mean Vespa high revs. Some of these bikes are only getting on the pipe at the point where a Vespa is into over rev.

Yea, normally people remove all the rotary pad thingy, enlarge and match the hole then put intake manifold on top of it with reed inside the manifold.

What I mean is, that modified reed intake you did which looks like typical GP or Sport bike, what are the benefits of it? correct me if I am wrong, judging by the reed size, it doesn't differ much with the normal manifold.

Youre able to use bigger reeds, for example compared to a malossi reed id say its almost twice the size. Also people say the closer to the block the better, it also gives u more room to play with how much fuel u divide into going towards the crankshaft and how much you direct to the engine. I honestly have no idea whats an ideal ratio i guess i just got lucky and it worked well.

Ive always had a reed valve set up, my old set up had a smaller pwk32 carb and less ideal porting in the crankcase. With the old set up it was definately quick, but mostly in medium revs, dropped off up top. Id say somewhere in the mid to high 20hp. Now im using a dellorto vhsb34 and sip's sport coil with variable timing set to 25° atdc so that when i reach peak rpm at around 9.8k the timing will move to 16/17°. Block is just standard no porting, but its a complete different beast. Alot smoother power band. Holds a wheelie through the whole rev range. But crono blocks arent very high revving. The next step is to add a spacer on the bottom, and just lower the ports as to not raise the blowdown timing as its already at 30° from factory, but only 120° transfer and 180° exhaust. The goal is to get it to 11k rpm by with 192° exhaust duration, 126° transfer. In my head it all makes sense but i guess ill just have to see

I think this is correct - all the charge from the carb must go into the crankcase first for primary compression, before being forced up through the transfers into the cylinder.
If it was possible to charge the cylinder directly without going into the crankcase first, there would be no need for transfer ports. Or case volume. Just a need for separate crank lubrication. Was the old one a similar modified case like this? Or the more common reed location where the rotary vale normally is? Just wondered exactly what effect your new reed location had on it's own?

I was reading something just yesterday about the intake distance and volume, saying that the lower the volume the greater the dependency on the exhaust to make the power. Which perhaps explains the Vespa and box exhaust relationship?

Previous crankcase was similar but didnt focus so much on how the fuel flows through the crankcase. And about exhaust and crankcase volume mostly has to do with scavenging. The old method is to use crankcase pressure to scavenge the engine by forcing the old fuel out but calculating angles, pressure and velocity to not lose fresh charge out of the exhaust is very difficult. So the new method is using exhaust to scavenge. So maybe i dont focus too much and crankcase volume. Maybe even lower it if i find lots of unburnt fuel. But im happy tp learn this as it could be helpfull on building vespas with a standard or big box exhaust, using crankcase pressure to scavenge

Interesting! I guess you could space your reed out pretty easily to see the effects of more and less volume. Cool project.