Re: Vespa intake timing: let's talk numbers

Pete Puczasaurus /

Maciek, just out if curiosity, how many mm are left on the sides of your intake patch? I've read somewhere in here you need 2mm to make it seal. Youve seemed to have opened that guy up a lot.

Re: Vespa intake timing: let's talk numbers

I left around 2mm on the outside and 1.5 on the inside against the wall, I probably could have left more on the 'top' (right side in the pic)... It seals ok, specially considering the lobe of the previous crank got a little friendly with the pad when it snapped, I was concerned it would not seal after that, but evidently it does not have to be perfect. But yeah, 2mm is a pretty safe starting point, You go too close to the outside and you start peeking around the side of the crank lobe. Due to manufacturing differences, it's a good idea to pop the crank in dry before cutting to see exactly how much room you've got with your combination of parts and make some scribe marks etc. (edited)

Re: Vespa intake timing: let's talk numbers

Maciek, I am still trying to learn these concepts. So do these number suggest the following: only 13/360 of the time the pumping action of the expansion pipe is able to directly pull fresh gas through the carb? The rest of the time the transfer is open the fresh gas flow is not available and all gasses that enter the cylinder is sucked out of the pressurized gasses in the case?

Re: Vespa intake timing: let's talk numbers

Back to the timing discussion; The rv allows us to close the intake earlier than pp, helping with blow-back, increasing the ratio of starting volume of mixture in the case vs pressurized volume when the transfers open, and the time it has to pressurize. Jennings mentions that "curiously, the best port-closing timing for a wide variety of disc-valve engines is about 65-degrees after top center".

It also allows us to open the intake earlier, even before the transfers close. I'm wondering what detriment, if any, there would be to opening it up even earlier, up to 180°, or whenever case pressure goes negative (and reeds would open) between there and the transfers closing. Jennings touches on this, in the rv section, and the dutch seem to push the overlap past the recommended 10°. Seems like it might help with the port-stalling. Of course the only real way to find out is to start cutting on my stock crank a bit at a time and trying it till it gets worse...some things to do before I can commit to that.

My cycle as it is:

*Ignition

Intake Closes - 57° ATDC

Exhaust Opens - 95° ATDC

*Blowdown - 24° (95° ATDC - 119° ATDC)

*Crank Pumping - 62° (57° - 119° ATDC)

Transfers Open - 119° ATDC

Intake Opens - 132° BTDC (228°ATDC)

*Intake-Transfer overlap - 13° (132-119 BTDC / 228-241 ATDC)

Transfers Close - 241° ATDC (119° BTDC)

Exhaust Closes - 265° ATDC (95° BTDC)

*Ignition (edited)

Re: Vespa intake timing: let's talk numbers

Excerpt from Jennings on rotary disc-valve intake opening:

"If there is a rule, it is that the point of opening for rotary valves is best established right at the point of transfer-closing if you want a very broad range of power. On the other hand, maximum power is obtained by opening the intake port somewhat earlier: from 130- to 145- degrees before top center, which means that the intake and transfer ports have overlapping open periods. This presupposes that the engine will have been fitted with a proper expansion chamber."

He expands on this, so don't just cut to the max and hope it works, but it seems one can get a bit more aggressive than the anticipato cranks, by about 10° on each side in a stock case with a maxed out intake, especially on a variated engine where You don't really care about the low end.

A bit less in the polini speed case, as it gets you 6-10° more duration that's about 5-8° advanced compared to a modified stock intake. It actually closes earlier (http://vespa-ciao.nl/page.php?id=180) so there'd be more room on that side, but the opening would already be in the mid 140s. (edited)

Re: Vespa intake timing: let's talk numbers

Thanks man. I am going to slowly process this !!

Re: Vespa intake timing: let's talk numbers

Hey, I actually didn't see Your post before I posted lol, You've got the idea though, on the rotary you can have some overlap that will allow the case pressure to equalize through the carb if it goes negative at the tail end of the main charge getting pumped/sucked from the case to the combustion chamber. I'm just trying to get this down myself lol.

Does anyone know about reed-valve actual intake duration? When about does the case pressure go negative and suck them open? Is it when the transfers close, or closer to 180° as the piston is accelerating back upwards? (edited)

Re: Vespa intake timing: let's talk numbers

Graham is on vacation I think:)

Re: Vespa intake timing: let's talk numbers

I figured something like that, haven't seen him posting during the day lol.

Re: Vespa intake timing: let's talk numbers

We work on snowmobiles at our shop and I have some numbers from 1995 Rotax engines to compare to. I don't know if they are related to Vespa figures but I thought they might be good for relative measurements. Rotax has a lot of rotary valve experience from family machines to race sleds.

-------------------------Open BTDC -----------Closing ATDC

454 MXZ ----------------- 145.6 -------------------- 64

467 engine models ------ 132 ---------------------- 52

582 engine models ------ 129.5 ------------------- 69.5

583HAC engines --------- 134 ---------------------- 65

Other 583 engines ------- 140 ---------------------- 71

670 mach 1 Model ------- 145 ---------------------- 76

Other 670 Engines-------- 144 ---------------------- 72

Re: Vespa intake timing: let's talk numbers

Thanks for the info Pete, looks like most of them are running a bit more on either side, the 467 being mildest.. I'm assuming this is for a pretty narrow power band, snowmobiles being mostly cvt, which is right up the vespa alley. It would be interesting to see the transfer timing as well.

I'm planning to start with a few more degrees on the open side the next time I have my engine apart, then a few on the other side see what that does. 140/60 or bigger sounds pretty good :).

Re: Vespa intake timing: let's talk numbers

bored, need some vespiration, anyone else got their numbers and results to share?

Re: Vespa intake timing: let's talk numbers

I've been thinking about these numbers for quite some time and trying to compare this rotory valve design to the supplanting technology ie reeds. Take for instance the open time between tdc and The 60-72 degrees after tdc. Isn't the piston descending but the crankcase side unpressurized due to the intake being open? Why isn't the flow at the carb reversed until valve closure. What's the point of it? What's the point of fifty degrees or so of valve closure at bdc until it reopens. Does this somehow increase the rate and volume of charge flowing up the open transfer interval. Wouldn't a reed valve close much closer to tdc and reopen much closer to bdc? Why does increasing the intake duration further past tdc as in an anticipato grind do any good. Wouldn't increasing the intake open to almost tdc and closing it earlier after tdc increase the molar volume of gas and air to flow via the transfers when they open and increase horsepower?? Wouldn't the engine want to start better because the vacuum at the carb is more efficient?

This not entirely abstract. I am still trying to figure out why my polini kit with anticipato is such a dog.

Re: Vespa intake timing: let's talk numbers

I have enclosed two pics of a motion pro timing wheel on top of the large half of a motzing modded large case half and a ten pin anticipato crank. I assume the con rod to cam lobe to be the place to measure the degrees resides plus that the motor spins clockwise. So these numbers don't match anything else so wtf am I doing wrong with these measurements.

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Re: Vespa intake timing: let's talk numbers

Wouldn't the intake need to be here for this crank grind to work? The here is where I placed the scissors

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Re: Vespa intake timing: let's talk numbers

Using the lower cam lobe as the lobe that determines whether the intake is open and clockwise motion the numbers are open 90 after tdc and close about 50 before tdc.

Next I will phone if in fact it is counterclockwise.

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Re: Vespa intake timing: let's talk numbers

If rotating counterclockwise and using the deep cam lobe that is not easily visible I also get a weird set of numbers. Open 60 or so past tdc and close 90 before tdc.

So no matter how I set this up it makes zero sense. I usually assume it's my i experience at work here.

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Re: Vespa intake timing: let's talk numbers

You realize that the "deep can lobe that's not easily visible" is not the one that you need to worry about right? You have your crank in backwards, it will make your numbers all screwy.

Re: Vespa intake timing: let's talk numbers

Backwards ?? Ok let me check that.

Re: Vespa intake timing: let's talk numbers

Opens 125 before tdc and closed 74 degrees after tdc

Still off so still something wrong with my measuring or the crank.

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Re: Vespa intake timing: let's talk numbers

And the crank rotates clockwise while running.

Re: Vespa intake timing: let's talk numbers

Tyler the latest picture is that I believe. I have a stock case and crank that I have to separate so I can post the two cranks side by side.

Re: Vespa intake timing: let's talk numbers

The degree wheel goes on the ignition side, tdc aligned to a reference point on the case, engine spins clockwise looking at that side.

Re: Vespa intake timing: let's talk numbers

you kids and your fancy degree wheels. i've just been using a grade school protractor with a big ol hole drilled in it.

Re: Vespa intake timing: let's talk numbers

has anyone gotten to play with the athena anticipato and/or know the numbers on it? im starting to run out of stock cranks to hack up.

Re: Vespa intake timing: let's talk numbers

I have an Athena anticipato in a couple of my engines. I may try to check once I figure out how to use a degree wheel.

Re: Vespa intake timing: let's talk numbers

If you take a stock intake and ream it out to 13 mm as in the wiki that alone is a degree change of significant magnitude. I have both cases open for comparison and will try to ascertain how many degrees that alone is.

Re: Vespa intake timing: let's talk numbers

It all depends on the case though. I've seen unmolested stock cases with different size inlet windows, perhaps another 20/25/30 mph think. I don't even remember what my pile of cases came off anymore

Re: Vespa intake timing: let's talk numbers

So maciek how many degrees has you intake mods if any generated.

Re: Vespa intake timing: let's talk numbers

My numbers are with the intake opened up and mazz anticipato. I never degreed it before opening up the intake, or with the stock crank, but You can kind of compare what different size intake patches and cranks get you on that vespa-ciao.nl page I linked, there's numbers for two different (stock?) cranks in three different stock cases and the speedcase. ArvidWs intake patch is probably closest to mine. It started out looking like Tonnis but shorter.

I broke the exhaust off My cylinder :(, so will probably be breaking it down this winter to cut the crank some more. (edited)

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