ESE's works engine tuner

[Yow Ling]

Sounds like a dress rehersal for the next BOB !

The devil is in the details

[Buckets4Me]

Sounds like a dress rehersal for the next BOB !

The devil is in the details

yep and now we have the details sorted watch out !!

[Sketchy_Racer]

To the Two Stroke genius's

I was wondering if anyone would be able to enlighten me briefly the effect of reducing my crankcase volume? What sort of changes to engine characteristic could I expect from it?

Also, what effect will increasing my intake duration?

Cheers,

-Sketchy

[Sketchy_Racer]

To the Two Stroke genius's

I was wondering if anyone would be able to enlighten me briefly the effect of reducing my crankcase volume? What sort of changes to engine characteristic could I expect from it?

Also, what effect will increasing my intake duration?

Cheers,

-Sketchy

[speedpro]

There's been a bit of discussion about crankcase volume and compression. My view, as explained to me by Wobbly, is that the crankcase is just somewhere to store fuel and air before the transfers open and it is sucked out. The fact the mixture is trapped in the crankcase and partially compressed by the descending piston helps move it a bit as well.

If the volume is large then the crankcase pressure will drop at a lower rate and therefore flow will continue at a greater rate up the transfers after that initial burst.

With a lower volume there will be a greater rate of flow up the transfers due to crankcase compression initially but as the volume is lower the crankcase pressure will fall faster restricting transfer flow later in the cycle.

The higher crankcase volume motor relies on the pipe resonance more to achieve good transfer flow and may hit a bit harder coming on the pipe. However potentially it could work better over a greater rpm range.

Plenty of theories on this . . . . .

[speedpro]

Duration will have all sorts of effects depending on where it is now and what you are trying to achieve. It is fairly well documented and proven that there are optimal "time/area" values you should be aiming for. Depending on use and which port you are talking about there is a proven range that is best.

[bucketracer]

To the Two Stroke genius's

I was wondering if anyone would be able to enlighten me briefly the effect of reducing my crankcase volume? What sort of changes to engine characteristic could I expect from it?

Also, what effect will increasing my intake duration?

Cheers,

-Sketchy

Page 80 of this thread has a collection of links where you can download Bells and Jennings books on 2-Stroke Tuning.

"briefly" its easy to find out for yourself, and the relevant chapters of Jennings and Bells books are a good place to start..........

Another useful link collection is on page 90 and there are more interesting collections on pages 100, 110,120 etc

[TZ350]

I was wondering if anyone would be able to enlighten me on the effect of reducing my crankcase volume? What sort of changes to engine characteristic could I expect from it? Also, what effect will increasing my intake duration?

Speedpros posts 2225 and 2226 are worth reading and like Bucketracer said, if you want to find out more, there is plenty of links to other interesting items to read on pages 80-90-100-110-120-130 and 140.

[Yow Ling]

Could always pm SS90 hehe

[TZ350]

NedKelly having fun last Sunday at Mt Welly.........

[SS90]

Could always pm SS90 hehe

Sure, why not, plenty of people do.

[SS90]

To the Two Stroke genius's

I was wondering if anyone would be able to enlighten me briefly the effect of reducing my crankcase volume? What sort of changes to engine characteristic could I expect from it?

Also, what effect will increasing my intake duration?

Cheers,

-Sketchy

There is plenty of mis-information available on the net, and some of the written material available in books can be a little misleading as well (in the sense that no one really gives finite values).

When you say "reducing" your crankcase volume, I assume you mean that you want to increase the primary compression ratio. Is that correct?

If that is the case, your results will vary depending on your expansion chamber design.



The first thing to do is work out what you have already.

There is plenty of information (one of my first posts) that indicates that the maximum value for a primary compression ratio is 1.5:1, and this is considered to be true by most tuners.

However, with such a ratio, you would need to have an expansion chamber design that has the second baffle with a shorter, sharper taper (much like you would expect too see on something like a mid eighties Japanese two stroke).

The effect of such a primary compression ratio and pipe combo would be slightly more peak power, but at the cost of range (simply put a "narrow power band"

My work (following up on others) suggests that a minimum primary compression ratio would be something like 1.33:1, but to get any advantage form this set up, you would need an expansion chamber with a second baffle that has a taper that is long and slow.

This can be difficult, as such baffle tapers can (and do) cause resonance "holes" just before they "come on the pipe", but this (mostly) be avoided.

Like I say, the first thing is to find out what you have, and chamber design is critical in getting the rewards.

1.5:1 is the maximum (any more and the work required by the engine to pull the piston down will negate any gains from the higher primary compression "a pumping loss")

1.33:1 is the minimum, (losing crankcase scavenging efficiency) That said, I remember asking around and being told that KTM used below 1.33:1 (something like 1.25:1) on one of their 125GP machines, but I suspect that their infinite choices for gear ratios give them the skope to be able to "tune out" the disadvantages of such a set up.

The attachment on the left is with a primary compression ratio of 1.42:1, the right, considerably more, from memory about 1.55:1.

The set up on the right struggled to pull top gear, and required lower primary gearing to be useable, even though it made more power.

The expansion chamber used in this series of tests remained unchanged (short, sharp baffle angle), and I found the lower the primary compression , the longer and more gradual the baffle taper was required.

With the right exhaust and primary compression ratios this cylinder put out 23PS.

[F5 Dave]

He’s quite right it is about the ‘package’.

For a starting point Duration: if you are talking about your H100 (my old) engine; the reed on back of crankcase is connected fairly enthusiastically around the transfers so is potentially open 360 degrees, but of course the reeds don’t allow that (unless they are broken). Inlet duration totally reed dependant.

As for primary com SS is talking some pretty low numbers for KTM, that surprises me but it might be a compromise that suits disc valve engines with the right pipes (maybe of which they couldn’t use in the past due to transfer design).

Interestingly just yesterday I was reading about the RG500s (privateer race ones not road) development cycle. At one point in part due to handling woes they figured that a heavier bike would give the chassis more grief so to aid handling lighter weight was to be pursued at all costs, with the added advantage that it helps acceleration. They redeveloped the engine with tiny crankcases, smaller flywheels etc. They then found that the engine was somewhat lesser of a performer as the compression ratio was too low. At least in the author’s opinion.

My work (following up on others) suggests that a minimum primary compression ratio would be something like 1.33:1, but to get any advantage form this set up, you would need an expansion chamber with a second baffle that has a taper that is long and slow.

This can be difficult, as such baffle tapers can (and do) cause resonance "holes" just before they "come on the pipe", but this (mostly) be avoided.
.

On this bike when I originally had it I designed a pipe (the hideous looking one) for supercross power, hard hitting, figured it was the right thing for kart tracks. It had a very steep baffle & I got quite a dip before peak power. Being that this was a fun bike I wasn't about to change the pipe & instead investigated the boost bottle which is why there was an adjustable one fitted. This largely diminished the dip to level so I left it like that. A band aid for sure but it was more for the experiment, that old chassis wasn't right for more power.

[SS90]

I will admit too, I once got carried away with baffle tapers (this is quite an old run, but I learned alot from this chamber) I managed to get 24PS and 19NM with this set up, ( I was experimenting with several things with this series of runs,and it was not just the baffle I changed to get the power that high),but that hole at 5500 required a lower primary gear ratio to be able to be rideable with 4 gears.

I always considered using a boost bottle set up like the Suzuki RGV150 (simply a large diameter rubber hose running the length of the frame), but small changes in baffle angle and the correct primary compression ratio (oh, and stiffer top reeds, for obvious reasons) was the answer in the end.

Long slow baffles are great for that nice wide flat peak power, (as long as the engine is designed to work with such a baffle), but, as you can see, there are limits....

The best advise I can give is to make your chamber be able to have a "clip joint" header, so you can test a few different baffle angles before finishing the pipe.

[TZ350]

Diagrams of various bellmouths and their effect on air flow..........

4-Stroke but still an Interesting read on inlet and exhaust tuning http://bmwmotorcycletech.info/InExTuning.htm