Hey guys, I woke up this morning with an interesting idea.

I have an MVVS 26, (front mounted walbro, front induction engine), but the walbro ends up being very close to the prop, which can make cowling different. I did have a thought of running a a tube from the crankcase intake to the the carb which may be mounted facing aft on the cold side of the engine.

I know that in 4-stroke cirlces intake runner length has an effect on torques and airflows.

How about 2 strokes? Is intake runner length an issue? We tune exhausts, so why not intake lengths?

Am I totally nuts?

You get into the atomization problems of fuel droplets falling out of suspension and also with the greater total volume versus pressure change etc.

It gets more difficult to predict the result

You will most likely experience throttle response problems

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So in esscene, having a short as possible distance between the piston and the fuel jet is best? Hence why everyone goes for pistion port induction, notwithstanding the timing issues that arise from it?

That is correct

The GT80 has longer runners, maybe 3" from carb to port, mine runs fine. I suspect there's is an optimal length, I couldn't find any information when I searched some time ago. Tuned ports in automobile engines increase performance.

Zenoah did an excellent job on their GT-80 intake manifold to eliminate dead airflow spots and to keep flow rates up.

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Piston ported engine intake pipes can be very well tuned across carburettors. Best carb position still is as close as possible to the engine, though in straight pipes the carb on the far end or halfway also works.
Across rotary disks and reeds, the tuning gets difficult because the pressure waves are not as pronounced. That is why only short ducts are used in thos engines. The MVVS 26 has a rotary drum for intake timing.

Wonder how many out there remember the old VW intakes? Those were maybe 2 foot long from the carb to the head, I don't think tuning was considered back in those days, just made things fit where they may.
There was an article on IMAA web site from the early 80's that suggested 7" was the best length, they changed the site however and I can find it. Heck, I don't even know if was accurate. Good conversation anyway.
Pe,
I would expect the rotary disc would exhibit similar characteristics as the port timed engines, only differenting in that the timing can be assymetric.

Tuned intakes work well on 4 strokes, that's given, but on small 2-strokes, that was the original question, which has now been answered.


I guess the only way to have a long intake runner and still be effective would be fuel injection!

Oh no! This length had a very pronounced purpose.
At 9000 rpm you need about 40cm on hot valve timing, so I guess those 2' were about right for 4500 rpm and street timing.
4-bangers follow different sets of rules. My 2-stroke piston ported racer had about 10" induction tract length for 10,000 to 15,000 rpm power range. I won't reveal induction angle, but it shows how long these tubes need to be to be beneficial. (hey, I like the (to be)^2 !!

I have seen no answers so far. You either need a dyno, or very sensitibe pant seats to get an engine just right! Exit seat of pants for model flight. We are most prone to judge speed by the noise an engine makes, which serves as substitute for plane speed.
Fact:
1)yes for piston porting, intake duct can be tuned
2) hardly: intake duct tuning in membrane induction. Too much resistance in the track, unless the reeds fling wide open by inertia. Let the guy who has intricate knowledge of reed inertia and reed cage volume calculations stand up here.
3) ?? in rotary disk. Most racers keep the track as short as possible, though the sinus-shaped piston velocity may set up sufficient wave activity.

It would seem this model [link]http://www.bgsoflex.com/intakeln.html[/link] would predict 1/2 the two cycle RPM, i.e. 14,000 rpm for 4 cycle equals 7000 rpm for two cycle. 7" yields 8000 to 10000 rpm.

your reasoning is very basic, but wrong.
The 4-stroke duct length is highly depending on valve timing. There is no such input in the sample you provided. Tame engines need short ducts, hot engines need quite a bit of pipe length.
Then again, the two stroke timing is depending on intake timing, both in duration as in port area, which is called the time-area factor.
Just assuming that the time is half the time in a 4-stroke is true for cycle duration only, and has no bearing at all on gas exchange capacity for a single two stroke port.

I would suggest that if he has the "play time", that he make up a manifold and give it a try and see what happens. It might hurt power, but help response with a longer manifold, if instead of having the carb on the "cool side" of the engine, that he put it on the "warm side"....

AV8TOR