Filter box resonance and speed

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.

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...

Steve

16v4nrbrgr, I made the mistake of replacing the stock filterboots on my KZ1000 with K&N pod filters.

Wind or even waving my hand near them would change the tune of the engine.
Filters and their housing make a big difference, don't they?

Well, this is embarrassing.
I hauled the cylinder off last night and I did overheat the piston from running too lean in the cold. I had a 2mm x 4mm x 1mm piece of piston welded to the cylinder under the exhaust port. The piston was blue-burnt on the bottom and exhaust side. Oh well. My laziness for not riching up the carb before heading out.

Cleaned it up with a grinder, and while at it, opened up the intake and exhaust to almost 90 degrees of the cylinder bore. No change of timing (height). Cleaned it up, new headgasket and back together for a ride.

Sunny day and only -2c. Bike ran great with a very repeatable 58kph over and over again. Not dragging down on the hills much at all anymore. These 2 mods (filterhousing and ports widened) are working well together in spite of the mess the cylinder has become.

I put about 3 hours on it at mostly full throttle. Lots of steep hills. Did not change the carb and no hint of over lean today. Just warm enough to be out of trouble.

Steve

Yes Chaz, that was the goal, this is the ideal:


but with the small port size and a 1/4" cutter it comes out looking more like;


Wish I'd taken some pictures. I stretched the port window out as far as I could towards the cylinder bolts and blended into the port shape. The goal is to not increase port volume any more than minimum, yet open the top of the port window up to maximum. The bottom of the port has minimal effect so don't waste the cutting effort and volume. Minimal port volume keeps velocity and momentum up. These are the theories, anyway.

My expectation was to have the rpm go up slightly from widening the top of the port. I've seen it in the past but nothing here. What I got was slightly more torque thoughout the powerband, from idle to max. It surprised me actually. I didn't expect the low end improvement.

The only reason the ports need any radius at all is cutter and casting limitations and to keep the rings from snagging.
The ideal shape would be something like:


So the whole circumference would be a window into the cylinder.
Have a look at the European exotics, they are much like this.

This was a damaged cylinder, I spent less than 20 min hogging out the sides of the ports. No polishing done, just the HSS 1/4" cylindrical cutter. Smooth, but no polish. I've spent hours polishing in the past and couldn't see the result in the performance. Pretty to the eye, but insignificant to the butt.

I am having a ball, merrily testing theories and getting surprised by results or lack of results. This engine is a hard gainer. Compared to the Yamaha Blaster I last worked on, gains are small. On the Blaster a new head chamber, intake and exhaust porting could nearly double the hp with a new pipe cinching the deal. I think a pipe on this engine will likely multiply the small gains I am seeing. I am also new to piston port engines and was amazed by the effect of the intake. It is obviously much more important than on reed engines.

Steve