Filter box resonance, effect on speed & power

Discussion in 'Photos & Bicycle Builds' started by sbest, Jan 23, 2016.

  1. sbest

    sbest Active Member

    Hey, temps were up to -7c here in the Great White North and the roads were mostly clear so I decided to investigate filter box resonance and its effect on speed and power. All I have for a camera is my old flip-phone, so no picts unless maybe we get great sunlight tomorrow. Please excuse me if some of you have seen this already.

    Back-story is that my engine suffered some damage from dirt from poor filtering so I want to improve that, but also take advantage of improved resonance to get better power. Typically this was done with trumpet type intakes or resonant boxes or a combination of both. I decided to experiment with 300ml-500ml plastic drinking bottles because they come in all sorts of shapes, volumes and tapers:

    I found a large plastic cap that fit over the NT carb face, drilled bolt mounting holes and cut a large hole to epoxy a 3/4" copper pipe that matches the carb throat but still lets the choke plate swing by. The neck of the bottle was softened up in boiling water and slid over this pipe. Various bottles were tried, with a foam filter insert.

    First of all a ride with the stock filter and body.
    Temps were -7c and today's top speed was 57kph for whatever reasons.
    Took the filter off completely and managed 58kph.

    I thought a long gentle tapered Perrier bottle would work best:
    The bottle on the left is the one I tried, with a foam filter in the middle, the bottom still on with large "breathing slots" cut in the bulgy part of the bottom. Wow, I had diesel like torque but speed was clipped off at 35kph, even downhill was only about 43kph max. It would however climb up my steepest hill on its own power. Best torque I have had to date, but what a governor.

    So I cut the whole bottom off the bottle. It still had the diesel effect, gained a few more kph on the flat and didn't have the abrupt shut down of top speed on downhill, but top speed was still hampered, even with the filter foam removed. It was also very loud, louder than my exhaust.

    Next bottle, a bottle that looked like the Evian with the bottom cut off:

    Better top speed, 55kph, slightly better hill climbing torque than stock, but not as good a top speed, so I switched to a bottle similar to the Aquafina.

    The Aquafina bottle had no more low speed torque than stock. Top speed increased to almost 60kph and bike was much stronger on top end. Would get up to 57-59 kph quite fast and stay there on small hills. It was quite fun, but then disaster struck. The sudden noise of a blown headgasket. I stopped and checked the plug, it was light grey, running lean and the head was hot. I limped home, the engine would run at 1/2 throttle 30kph without blowing out the gasket.

    More power is more pressure and more heat. Cold temps have a leaning out effect on engines, and so does some resonant tuning. I removed the head, it blew fore and aft, same as last time. Although the speed was about the same as I have done in the past, the bike would maintain it on slight hills. It was definitely making more power.

    A quick check revealed only a very slight warp in the head alone, mainly at the sparkplug side. I took less than 0.003" off to clean it up. Head-nuts were still tight. I had re-used that headgasket several times and will use a new one next. I think lean mixture, heat, and more power may have been at cause. I suspect timing is now too far advanced as well, but cannot prove it. No detonation heard or signs on the piston or plug.

    In spite of it blowing my head-gasket, I know what filter housing I will be using from now on...


  2. sbest

    sbest Active Member

    So I did overheat the piston from the cold temps leaning out the carb. Stupid me, too lazy to richen it up a bit. I pulled the jug off and the piston welded a little slug to the cylinder. I ground that off and keep going with it. Since I had it apart I did the first grinding on the main ports (transfers are cleaned up slightly). I opened both intake and exhaust up to almost 90 degrees of the cylinder bore. Only off the top of the exhaust and the middle of the intake. Slapped her together with a new headgasket and went out riding in the sunny -2c day.
    Max speed today was only 58kph but I could stay 55-58 on much of my route even with the badly battle scarred cylinder. Have some photoes of the bottle filter housings tried;
    Which of course fits on here:
    Another view;

    Bike runs strong in the 50-58kph range with this intake and the widened ports have helped the low end some, climbing the hills better at 30kph and less.

    As for running lean it was totally temperature related. No change to jetting and I ran WOT for 3 hrs today in -2c , no problems.

  3. jaguar

    jaguar Well-Known Member

    these experiments were with or without an air filter? (I suppose a filter would dampen any resonance effect)
  4. sbest

    sbest Active Member

    I tried it both ways and it does dampen the noise for sure but top speeds did not change noticeably.
    You can see the foam filter inside the clear bottle. Just a press fit and no noticeable restriction.
    The green bottle was very loud. I experimented with just the spigot, blank carb and various lengths. The bottle pictured worked best.
    The diagonal cut seemed to help as well. I tried that on the other clear bottle, full length, shorter, diagonal and then short.
    It made a difference and the diagonal cut seemed to affect top end speed less while keeping great mid-range torque.

  5. jaguar

    jaguar Well-Known Member

    Where did you get info about intake resonance?
  6. sbest

    sbest Active Member

    This is my GO-TO book:
    First bought it in 1976 and still read it. Paid a day's wages for it back then and now it is free.
    There are hundreds of books printed since, but Gordon is still mostly Bang-On and explains things as simply, but as accurate, as anyone.

    These bottles are pretty much resonant pipes like a tuned exhaust pipe and the closed length is not right, and I don't really understand what it should be at the moment because I haven't consulted "Gordon" and wrapped my head around it. What I did find is half of the answer, 8000rpm likes an intake tract of about 8" long from the piston to a sharply angled divergent cone. By cutting off the bottom of the bottle, I negated the convergent cone. By cutting the body at an angle, I spread the "tune" of the tube section of the bottle over a range. I still don't know those answers yet.

    Shortening the intake tract, with other mods, might get me reving over 9000rpm. I don't know yet. Have to test it yet. Also not tested yet is a Helmholz chamber effect, a chamber in or off to the side of the intake to set up a resonant wave. All the fuel injected cars use them these days. Using plastic bottles and glue, we can get very creative with designs.

    I don't know anything for sure. 40 years of reading and tinkering confirms some things and debugs others.
    I am retrying everything. These are lovely engines to learn from.

    Last edited: Jan 25, 2016
  7. Arty

    Arty Active Member

    Wow! That is a really interesting post.
    From the looks of the pictures, you are running it in the snow as well. - Kudos.
  8. jaguar

    jaguar Well-Known Member

  9. sbest

    sbest Active Member

    It is part of the effect I am sure, but shape is part of it too.
    The long green bottle made awesome power from idle, more than length alone would explain. Unfortunately it fell flat at very low speed.
  10. mikedabomb

    mikedabomb Member

    Wow. Very captivating read. I love coming across stuff like this. You are a gentleman and a scholar, keep up the great work and experiments!