Modern Vespa

Hello all, looking to continue a bit more on my old school tuning of my P200. So far I've got the squish knocked down to a bit under 2 mm, with resulting compression of about 150PSI. I'm running a SIP road pipe and a 24.24 carb - DRT version with increased fuel flow passages.

At high RPM and low load I get a lot of trace knock that doesn't respond to timing (I've tried as low as 18 degrees) and can't be tuned out with idle jet or atomizer (running as rich as it can stomach with the atomizer with no holes in the high RPM portion). I'm pretty much concluding at this point that my port timings won't happily support 6000 RPM scavenging and to fix it that I need to get the port timings better done, probably most easily by raising the barrel and having some chopped off the top of the barrel.

What are reasonable values for either port timings or for the # of mm I can raise the barrel to get along better at high RPM?

Thanks - cheers.

150PSI is a lot of compression on a (mostly) stock P. Our kitted scoots don't have that. What head are you using?

A shaved head done by Hot Rod Al with o-ring. The squish is still a bit high but if I want to reduce the compression I'd have to get the combustion chamber opened up a bit.

I'm debating doing this or slapping on a Malossi 210. The benefit of a top and tail is it helps me learn about port timings and such, so it's interesting. The Euro is really cheap right now and the Malossi Sport 210 kit with head is only pricing out at about $350 from Hodgespeed. But then you've really got to pull it apart and do the porting to do it right.

Ha, chasing the power dragon... careful, it's addicting.

I'd go with the 210 rather than try to make the head work. You'll eventually want the extra oomph anyway. Might as well pull the trigger now.

As for porting, like Hodge says, it depends what you want. Are you looking for torque? Quickness? My feeling is you want to port to get the most of whatever pipe you're riding. The SIP 2.0 has been around for a while and has been dyno'd on lots of set ups. Take a look at the charts. It has a pretty wide power band but I think anything over 6500rpm might be wasted on it.

https://i1.wp.com/www.scootering.com/wp-content/uploads/sites/4/2016/02/078_sip-Road-2.jpg

Well it's just that the P200 stock has only really the one transfer port in the case, so you have to port the case to do any of the kits, with the exception of the Pinasco. I have one of those, but I've concluded by measurement that it really needs the long stroke crank or it will suck because the squish is something over 4mm with the head supplied and the stock 57mm crank. And if I'm going to take it all apart anyway, then all options go on the table!

Stock P200 barrel:


Malossi barrel:

FYI, if you go the Malossi route, get the New 2016 Sport Cylinder with the Vertex graphite piston. It has bigger ports (side and boost), lighter piston and piston pin. Basically it has the ports of the MHR cylinder with a smaller exhaust port so as to not "empty" easily on lower rpm. That is what I am running for 6k Kms so far, along with a longstroke crank and a Polini CP30 carb (PWK)...

thanks, this is what I'm eyeing....

Sounds like your pinging may be due to compression.

If you want to study old school barrel tuning, the old booklets by Dave Webster and Norrie Kerr would be a good go to.

Also, the scooter center koln catalogs has a good listing of port timings for a number of stock and aftermarket barrels you can compare.

The granddady of books to go to to learn how everything works together in a two stroke is the a. Graghm bell book; skip the earlier edition you can find online and buy the newest edition out thee (I think 5th?). This one will take many, many readings before you understand a fraction of what he is laying down.

Beyond that, it is worth noting that other cylinders do not require case modifications; there is a slight benefit to matching the ports but they will run well without it. In addition ,next time you crack the cases they will be stained and you will know exactly what to remove.

Thanks Rob! I have the Webster book, I need to dig it out. I don't see anyone stateside selling the Norrie Kerr book but I do see that you can get it for me .

It doesn't seem you can get the SCK catalog in printed or download form currently, but that does sound interesting. Does anyone out there have a PDF of it?

I also appreciate the input on running the cylinders with stock cases. This allows me to stage the build if I want to where I can run by swapping the top end and then do more when I have to go in for something like a cruciform (inevitable, right?)

I'm actually quite interested in what can be done with the stock cylinder and have a new one with a grandsport piston on the shelf for an experiment.

Many years back I did a packing plate and decked the cylinder top for a friend. There's quite a bit that can be done to open the bottom of the ports without getting into the case. That plus a re-profiled head, mild expansion pipe and a bell mouth are what I've been thinking.

Stock barrel can be bored to take a Polini 208 piston from what Hot Rod Al told me.

i don't suppose you remember how much you raised it? I'm going to get that Norrie Kerr book.

If I recall on the P cylinder the bottom of the ports sit 3mm lower than the piston at BDC, and that's what the young unlearned me used. Now I'd actually use the adjustment to set the top of the transfer port height.

But, no I don't know what that value should be, and not sure it's the same for all 200 cylinders.

yeah, but at a point, the question really becomes 'why?'

most of the improvement is in the porting, 8 cc isn't going to give you much of a boost. and look at the cost! $75-100 for a bore job, and $75 for the piston ( $115 if you go grandsport)... vs $210 for a complete polini 208.

in addition, the tops of alot of two stroke ports are angled (especially boost ports), and the more you bore them, the farther the top of them retreats towards the base gasket. this can even be an issue going to a 3rd over, let alone going out to beyond standard oversizes.

you can browse their lambretta catalog here: http://www.scooter-center.com/en/product/KATLAM

the cylinders have port timings listed, and a chart is on page 41.

their vespa catalog is at http://www.scooter-center.com/en/product/KATVES
i think the best chart i've seen listing different cylinders is in there somewhere, but i have a slow internet connection and don't have my paper copy handy.

Of course what you can do and what you should do economically are 2 different things . If you have more machining equipment, some of this stuff will still make sense to do, but from a financial standpoint it is easier to just buy stuff and put it on for the most part. That's why Al isn't doing it any more.

Good points on the Polini 208 piston and what the overbore means to the ports.

Thanks for the links on the catalogs - I found them but I didn't realize how to use them until now . The Lambretta side cylinders are much better documented, unfortunately.

I did find this awesome page in the SIP catalog relative to the cylinder base spacers.
http://www.sip-scootershop.com/files/catalogue/index.html#/141

Back to my original issue - if I buy a thicker base gasket like this:
http://www.sip-scootershop.com/en/products/gasket+cylinder+base+th+15mm+_90760000
Then I can probably drop the compression ratio but at the expense of proper squish, I suppose. This is basically undoing some of the work I've had done where I had 1mm pulled off the top of the cylinder to get to where I'm at. But the port timings would be longer than my original case, so there's that going for me.

here's a little page I put together to convert from crank angle to piston position:

http://oopsclunkthud.com/tuning/PistonPosition.php?bore=66.5&stroke=57&Lcr=110&theta0=0&thetan=180&thetastep=1&Submit=Calculate

If you put the cylinder on with no gasket and measure where the top of the transfers are then that will give you a baseline. From there figure out what you want the transfer timing to be (Bell has some advice here) and from that what position the top of the ports should be at.

The difference gives you the thickness of packing plate.

The SIP packing plates make this way easier than it use to be cutting the whole thing out from scratch. Pick the thickness that get's you where you want to be and there you have the hardest part done.

With the top edge of the transfers set you can then address the lower edge of the transfers (if needed), the exhaust port timing, and the squish. The squish can be done by decking the cylinder or by turning the head so a spigot slips down into the cylinder.

We bought some aluminum sheet samples and cut our own base packers.

Wise words from Jennings:

"I will warn you, now, that it may be unwise to follow the old-time tuner's habit of increasing an engine's compression ratio as an opening gambit in the quest for better performance. Indeed, before your work is done you may find it necessary to reduce your engine's compression ratio below the stock specification. You see, in the final analysis it is not so much compression ratio as combustion chamber pressure that determines the limit - and these are not at all the same things. Your stock engine, with a carburetor size and porting chosen to lend it a smooth idle and easy starting, does a much less effective job of cylinder-filling than will be the case after it has been modified. More important, it will probably have an exhaust system that has more to recommend it as a silencer than as a booster of horsepower. These factors, in combination, make a very great difference between the cylinder pressures at the time of ignition in the stock and modified engine. Even given a certain willingness on your part to use a fairly cold spark plug - changing it frequently - and a further willingness to replace pistons and bearings more often in payment for added power, it may still be necessary to stay with the stock specification for compression ratio. Or, as I have said, to lower the engine's compression ratio from the stock condition. This last will be particularly true if you succeed in creating a much better than stock exhaust system."

Quickie but.. No one mentions compression ratios and fuel types.differrnt compression ratios effect detonation, different fuels detonate at different pressures. Ethanol detonates at a lot higher compression ratio to 95 unleaded.. Flame temp varies with detonation values. Why is there no mention of this any Where? Just asking. (probably does mention it but I'm stupid)

FYI I am running premium.

Bell covers compression as well.

With a properly designed squish you can use a much higher compression, and the poorly executed squish (like on the P200) increases the risk of detonation. Most people don't talk about it because there's not much they can do to change it.

So, you take the head for what it is and adjust the squish to work with that and verify the compression is not over what Bell recommends.

What does Bell recommend?

Patrick,
Can you throw out a rough number, in psi, when one should start getting concerned about too much compression? Same for 125/150's and 200's and smallie motors?

Is 140-150psi running a modified 125 head on a 150 motor(stockish w/ pipe) too high if the squish is OK? Should one tweak the timing to slightly more retarded to help control potential detonation?

can't recall and don't have time to look but 10-12:1 was not too high, I think.

As for PSI, that's only telling you if things are sealed well (rings, head). It doesn't tell you much about anything else, but if you had to use it then the ideal gas law is your friend PV=nRT, but you'd have to work with the trapped volume (exhaust closed) because you will not have the exhaust pulse helping to recover things.

For reference I think my DR130 measures somewhere around 140PSI and never pings at all.

A stock P's compression ratio is about 10:1. Raising it - which I think john's done by installing the Hot Rod Al head and decking the cylinder - is I think what's causing the detonation. But if Bell says 12:1 CR is ok I suppose Bell knows best.

9.8 is what is on scooterhelp, but was that real? was that for the USA head?

Have to measure to know

It doesn't ping at full load, only part load, and only about 5500 plus rpm. It's probably a combination of port timing, compression, and the pipe.

I have a feeling it would stop with better breathing, but the compression isn't helping.

'compression ratio' in the format of 9:1 or 12:1 or 10:1 or such is really irrelevant in two strokes. it's a theoretical number mathematically derived from the bore, stroke, and head volume.

the problem is, the engine doesn't actually compress anything until the exhaust port closes. and at that point, the pressure may be lower or higher than atmospheric pressure due to the exhaust design and the crankcase volume. Different cylinders can have vastly different exhaust port heights depending on how they are set up.

so you have a variable starting pressure, and an variable point where compression actually starts. 10:1 compression on a P200 cylinder can be a totally different dynamic than 10:1 on a RZ350. if one has an exhaust port timing of 160 degrees and the other is 200, even the same mathematical compression ratio leads to a totally different set of circumstances when the piston hits TDC.

if that wasn't complicated enough, transfer port timings even play into it, as well.

Just going to leave this here...

I've got a USA center squish head that I can measure this weekend. From that we can see what it takes to get the squish velocity into range (15-20m/s) and where the uncorrected compression ratio ends up (just as a datapoint).

If you tell me what port timings you're aiming for, and give me your current timings, I'll work it out for you.

http://www.race-base.com/2t-tools/steuerzeit/

+1 in the vote for compression as the likely culprit for your pinging. You might recall I suggested that you may need to do something about it before you took some off the head.

I do remember! But I am too novice in this to have any idea what timings I should aim for... got any ideas?

I was pondering getting some base gaskets and/or head gaskets to lower the compression to check that idea. Doing it both ways might allow me to decide more about port timings at the same time.

When I broke the Polini 208 Freakmoped suggested I do up the stock barrel. He said to use 174/120 as he had done a large number of them at those timings and it seemed to produce a strong motor without getting into peakiness. If that's actually a word!

The calculation from that site (if you give it accurate numbers) will tell you exactly how much to take off the barrel, and how much to adjust the exhaust port so makes it easy in that rspect.

Thanks Ginch, that is exactly what I was after. I'm going to order 0.8mm and 1.5mm head gaskets to try reducing compression, along with a 1.5mm base gasket to try raising the barrel if the reduced compression stops the knocking.

At the same time I'll try my hand at measuring port timings for the first time and dip my toes into the water on this stuff.

Holy crap, I hadn't noticed this link at the smallie forum:
http://vespasmallframeforum.proboards.com/thread/9601/stroke-tuning

Took some head and piston measurements at lunch.

US center squish head volume (not counting the straight sides that add to squish clearance) is 22.3CC

With 3mm squish clearance that gives 9.85:1 (not sure what stock squish is but it's at least that big)

To make the squish work correctly it needs to be in the 1.5-1.7mm range and that would take it to 12-12.5:1

With a squish of 2mm you get the MSV above 15m/s at 7000rpm. and raise the CR to 11.5, so, not quite there on the squish but added compression.

MSV on that page is really high. Blair gives 15-20m/s

Yeah I think I was closer to 4mm when I started. I couldn't believe it. I can check my notes later. If I follow you, this head can't be made to have proper squish without creating too high MSV and/or compression ratio and inviting detonation. What have you run for a head in this type of scenario then? or did you just open it up volume-wise?