Two-stroke performance tuning?

[SS90]

Ummm, I'm so new that I haven't enrolled in a school yet... To call me a beginner at engine work is way too generous.
The ESE engine thread is what encouraged me to give it a go and taught me what boost and transfers are!
Don't ask me for my opinion yet, I'm very much reading and trying to comprehend it all.
So far I've decided (probably) on a non-squish head and about 188° exhaust.
The rest I'm still mulling over and researching what will go best with that for my engine.
On top of that I have the option of piston port and reed (as it is) or going full piston port or full reed. Hence the long discovery process. There are many things I need to check and measure before I make any decision; and that will be passed by a few people for their input too.
The worst thing for me is everything seems to look at maximum power rather than maximum duration of power (which is my aim).

I have plenty of time and no resources, so don't wait up!

Ha, no worries on that one.

I guess you have noticed lately that the more modern approach to porting is now towards a better spread of power, and sometimes finding resources on that can be difficult. (on the web any way)

I am guessing you have downloaded some of the PDF's made available...... the basics is always the best place to start (even though some times it can seem boring!)

[SS90]

Exhaust Duration of 183 = Ex Opens 88.5 ATDC and Transfer Duration of 128 = Trans Opens 116 ATDC.

"you could go a few degrees higher"

Ex Opens 86.5 and Trans Opens 116 ATDC.


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Yea, as I have posted in the ESE thread, my experience over the last few years has shown that with modern set ups (scavenge patterns,primary compression ignition timing) you can get some very good power engines with some quite "low" port timings and durations.

so yea, Opens 86.5 exhaust would be 187 Duration.

The cylinder I designed the last 2 years, is 188 Duration ex, and 131 Transfer, and that (seemed) to be the best compromise for power/torque and spread of power.

Of course those figures are nothing new (I think Teeezee pointed out old air cooled Rm125's where something like that), but it is quite amazing what "the modern approach" will achieve with the same port timing set ups!

It seems from the measurements of the M1L cylinder I made today, that the exhaust duration is indeed 182.1 deg. (secondaries 179.8) and the transfers 127.9 deg (with a 5mm base gasket)

I personally think that it could go (like you say) to 187 or indeed 188, it is quite surprising to me, from the dyno runs I have seen (with quite poor results) I made the assumption (with-out ever measuring a cylinder) that there was simply too much exhaust duration.

After measuring today, I caclulate the following thickness gaskets will give the following port heights

No gasket transfers 125 exhaust 179.9 secondary exhausts 177.6
0.2mm gasket transfers 126.2 exhaust 180.8 secondary exhausts 178.5
0.35mm gasket transfers 127 exhaust 181.4 secondary exhausts 179.2
0.5mm gasket transfers 127.9 exhaust 182.1 secondary exhausts 179.8

Quite a bit lower that I thought it would be.

With the 5mm gasket, you also have 1.25mm squish clearance.

Personally, I think it would be logical to build the engine to the specs planned, and new exhaust) do some base runs (see what the curve looks like) and perhaps look at increasing the timing of the exhaust and transfers.

The attached dyno run is of a "plug and Play" version of this cylinder (exhaust unknown to me)

[SS90]

Ok, in the interests of putting a value to the primary compression ratios applicable to modern porting and exhaust designs, I have modified a crankshaft (just using logic as to how much material to remove, making a cardboard template, and marking with a yellow spray bomb ), and, as I assemble the engine I will measure the crankcase volume at TBC and at BDC, so as to calculate the primary compression ratio).

I will then compare the dyno curves to 4 other known "high tuned" engines ( 3 other tuner have agreed to send me some dyno runs for comparison) with the same cylinder (I don't know what primary compression ratios other are running, and also the exhaust being used is a prototype, so it will be an interesting comparison.

I will also use a retarding ignition, and using different CDI/coil units establish which "retard" curve is best (for this cylinder/pipe/primary compression ratio combination.

I will also measure the exhaust gas temperature.

I have also found the the standard "Transfer stock" ( fully open point at BDC) is about 1mm too high.

I am certain this was not intentional by the designer and is more likely a manufacturing/casting fault.

As such, I have sent the crankcases to have 1mm removed from the cylinder base. I'll pick them up on the way to work tomorrow, it is a little dissapointing, as it involves more work.

I currently still am of the opinion that the standard exhaust port duration is too low, and I really am considering raising it to 188 Deg before assembling it.......

[Brian d marge]

I would be very interested in how this goes , if i knew the weights and could calculate the forces I could put it through a FEA program

Stephen

[bucketracer]

Broad power is my aim - I don't give a rats about peak power.

Hi Skunk this may be of interest "How to Choose a Power Band" by Eric Gorr.

http://www.whidbeyparts.com/info/How...apowerband.pdf

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[TZ350]

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Something else that you might find usefull.

MSV Mean Squish Velocity Calculator:- http://myweb.tiscali.co.uk/teamsparr...rheadsv2_3.xls

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[speedpro]

It'd be interesting to know what the balance factor is with the crank cut down like that. I would guess it's tuned for very low revs unless you are using iron pistons.

[Skunk]

Hi Skunk this may be of interest "How to Choose a Power Band" by Eric Gorr.

http://www.whidbeyparts.com/info/How...apowerband.pdf

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Something else that you might find usefull.

MSV Mean Squish Velocity Calculator:- http://myweb.tiscali.co.uk/teamsparr...rheadsv2_3.xls

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Now that's what I'm talking about! Cheers guys.

Now I have to learn what it is I want with MSV...

[SS90]

It'd be interesting to know what the balance factor is with the crank cut down like that. I would guess it's tuned for very low revs unless you are using iron pistons.

No (Iron pistons in two strokes have fallen out of fashion at the last ice age)

I am fairly confident it will be ok...........

Time will tell.

I assure you I will post any findings (good or bad)

At this stage I really am keen on finding a minimum value!

[SS90]

OK,

Today I picked up the cases from the Machine shop (after having 1mm removed from the top)

I then matched the original Piaggio Vespa cases to the new Quattrini cylinder, before dry assembling the crank in the cases (to measure the primary compression ratio...no point in explaining how to go about that, it has already been covered in the ESE thread)

It turns out that with the "modified" "racing" crank (pictures above) the primary compression ratio is 1.42:1

This is certainly the lowest I have (deliberately) gone!

So, I emailed a friend of mine (who last year worked for KTM, and now works in Italy), and asked him his opinion of crankcase volumes (and in particular minimum values)

The following is an excerpt from his reply.

I would also add that (like he mentions) some of the things mentioned in the article linked, are not 100% correct, however, it is a fresh prospective on quite a few aspects of two stroke tuning.

That's always nice!

hi (SS90),

the primary compression is often misunderstood. for low rpm, a high primary compression is benefitial (because the crankcase then works as a pump). for high rpm, a low primary compression is benefitial (because the crankcase can then store enough gases for a good filling). but where do the "low rpm" end, and where do the "high rpm" start? and what type of performance do you want to obtain?

even charlie edmonds got it wrong in his very interesting "scootering" interview.
http://www.lambretta.it/eddy/edmonds.htm

he makes it sound like low primary compression is a key to finding power. this is correct when related to a scooter engine (they have very high primary compression ratios of 1.5...1.6), but he makes it sound like there's no minimum value. also, he refers to motocross engines, and makes it sound like they reduced the primary compression purposefully. truth is that on motocross engines the conrod length was increased to reduce stress on the piston, and ports were increased for more power. but the reduced primary compression was only a side effect. truth is that at ktm we put a lot of efforts into increasing primary compression, because midrange is very important on an offroad engine.

the lowest primary compression i have seen is the aprilia gp engine with something in the 1.25 range.

with the modified crank (looks nice) like you say, you have 1.42:1. quite good for a vespa engine and its requirements. of course you can still argue about whether you'd be willing to trade 1nm at 6000 rpm for 1nm at 9000 rpm...

(He finishes later with)

bytheway, the best 50 tuners in the world are dutch!

So, with 1:42:1, it sounds like it is a good "middle ground"

This particular engine will rev to 10,000 RPM (as do my own cylinders), which run a primary compression ratio of 1:47:1

depending on the results of this engine, I will look at decreasing future engines (that rev to 10,000 RPM)

It will be nice to have hard data as to "how low you should go"

and it would seem with the constraints placed on some buckets (available gear ratios), it would seem logical (so far) that between 1:4 and 1:45 (for high revving 6 speed transmission) and for "low revving 4 (or maybe 5 speed transmissions) 1:47 is somewhere "in the ball park"

Due to the different maximum exhaust port time areas (and maximum RPM) suitable for each type.

Let's see!

One good thing about having to change the cylinder height is that now I have to change the combustion chamber....any opinions welcomed on that one!

Looking at some of the things Teezee and Chambers has posted links to gives a few options.

Like Skunk, I like non squish heads personally.

[bucketracer]

From page 66 of the ESE thread.

The effects of crankcase volume on the delivery ratio:-
http://www.edj.net/2stroke/jennings/...ase_volume.pdf

Section 1.5 of the Discussion section on page 14 is worth reading.

[TerraRoot]

Thanks for the info ss90! can't wait for the results, good or bad least we will know.

[Skunk]

Like Skunk, I like non squish heads personally.

Having read this:

http://www.whidbeyparts.com/info/How...apowerband.pdf

I'm in two minds now. I have a spare head so I may do a squish and a non-squish to see the difference. I'm aiming to set up the squish to make the MSV around the 30m/s figure. The current squish head seems to be around 25m/s.

[SS90]

Primary compression aside for a bit,

I was talking today to 2 people who own/ride these quattrini M1L cylinders (just "plug and play", no work.) (17PS) and they both said the same thing.

"it uses alot of petrol"

That seems odd to me, as they only have 17PS.

This (in my opinion) would indicate that something is upsetting the scavenge pattern.

So, I started looking hard at the scavenge patterns, and in particular, the boost port angle.


I measured the boost port at 55 DEG (that's what I expected)

Now, these cylinders have very large "blow down times" in the transfers, (which would indicate VERY high RPM "power bands")

So, after some careful measurements I have decided to change the boost port angle, to better match the transfer timing.

I now believe I can safely increase the exhaust port duration to where I want to.

I also took the crank to a machine shop today to calculate the balance factor... I believe that balance shafts are for FAGS, but it would be nice to know!

[bucketracer]

So, after some careful measurements I have decided to change the boost port angle, to better match the transfer timing.

I now believe I can safely increase the exhaust port duration to where I want to.

"I have decided to change the boost port angle, " change to what?

"increase the exhaust port duration" increase from where to what?

Its very interesting to be able to watch your progress. Thanks SS90.

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