Choosing Power Enhancing Parts

jaguar

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People getting into motorized biking usually don’t know enough about 2 strokes to make informed choices concerning what to buy to get more power and speed out of their ride. The sellers should know and should make responsible decisions about what to promote and sell instead of taking everyone as fools and laughing all the way to the bank. This post is to inform readers and dealers about what is needed. I’ll start off with the two biggest headaches, cylinder heads and expansion chambers.

Expansion Chamber blues

These are the pros and cons of putting an aftermarket expansion chamber on your motored bike. (This list excludes my design of torque pipe which eliminates the majority of the cons)

Pros
-An expansion chamber gives more top RPM power throughout a range of around 2000 RPM.
-They can look cool with a professional installation.

Cons
-For about 1500 RPM before the last 2000 RPM the return wave from the baffle causes less power than if no expansion chamber was used.
-They are loud as bloody hell even with a silencer. This is bad for the public image of MB riders and puts their legal status at risk.
-You have to modify the header length till you have the peak speed you want. If the header is too long then you get less top speed than with the standard pipe.
-If they are routed upwards then great care has to be taken in positioning so that you won’t burn your leg.

A racer of the Maico 400 (w/piston port intake) motocross bike emailed me this: "Did you know the Maico motor runs really strong without an expansion chamber till about 5000 rpm. Maico even used a similar motor for enduro racing that had basically a straight pipe and muffler. When you add an expansion chamber you get much more torque above 5000 rpm and less below. Of course the expansion chamber robs some of that smooth torque from the low end (because of the pressure waves from the pipe being out of sync with the normal scavenging and trapping)".
Before I designed my torque pipe I used a standard pipe with a header extension welded in which gave it more low RPM power.


Cylinder Head with Squish Band blues

Pros
-They give more engine power due to increased compression ratio.
-It takes longer for them to reach maximum temperature because there is more metal to heat up.
-They look nice.

Cons
-They cause cylinder plating to flake off due to excessive combustion temperatures and increased combustion pressure.*
-The presence of the squish band (which normally does not give the necessary .6-.8mm piston to head clearance) actually lowers the power potential. Squish bands only work as intended if that correct clearance is present.
-They are much more expensive than buying a piece of glass and some sandpaper and applying some effort to grind down the stock head 1mm (more or less) for higher compression.
-Excessive compression causes the early demise of the upper connecting rod bearing which has to bear all the excess combustion pressure. This can be countered by using an aftermarket CDI which delays (retards) the spark more than the stock ignition does.

* one of the cylinder replating companies had this on their web site: “The finish in the early years was a simple chrome or hard chrome electro-plating. The plating was generally only a few thousands of an inch thick. If it was more than about ten thousands of an inch thick it was prone to chipping or flaking because of heat expansion and contraction.” US Chrome had on their site that the old style chrome plating can only withstand 400 degrees celsius (752F).
Also the excessive combustion pressure causes the rings to press harder against the cylinder wall which probably also contributes to the flaking. I had been recommending people increase the cranking compression up to 135psi but now I’m thinking that even that much may be too much given the common problem of the plating coming off. For high performance I recommend getting the cylinder replated at US Chrome and then you can use high compression safely.
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I’m not against the aftermarket parts market, I just want them to stop selling things that are inappropriate for motorized bicycles and start making and selling that which are appropriate. Here’s a list of things they should be selling:
1. an offset woodruff key that retards the stock ignition timing by 5 degrees for a boost in power throughout the RPM range.
2. A stock cylinder head lathed down a bit for increased compression.
3. Other designs of cylinder heads that don’t increase the compression more than 120psi.
4. Adaptors so people can easily put really good carburetors on such as the Dellorto PHBG 21mm and Mikuni VM18.
5. Good foam air filters for the Dellorto and Mikuni carbs.
6. Substitute motorcycle pistons/rings because the stock rings are too thick and the ring gap is too much which allows compression/power loss.
7. Stock or replated cylinders that have appropriate moderate porting increases for more power and RPM without losing low RPM power (which is needed for in town riding).
8. Stock crank assemblies that have two 3/8” holes drilled at the correct locations of the crank wheels for a correction of the vibration problem that all these engines experience.
9. Stock wrist pins that are drilled with larger holes for lighter weight to counteract the imbalance that causes vibration.
10. A stock exhaust pipe that has its baffle modified for better flow at high RPM.

If the existing dealers won’t offer these items then any of you readers are welcome to go into business. I can promote your products on my site (click my signature link). On my site are plans for making the torque pipe, how to drill the crank wheels, what dimensions the carb adaptors should have, how to modify the woodruff key, how to modify your head for more compression, substitute piston/rings, and how to port the cylinders.
 
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FurryOnTheInside

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I do wonder how/why the expansion chamber pipe ever got that name in the first place. It seems it's primary function is to act as an echo chamber and calling it that would have given more of a clue about how it works (or doesn't) at RPMs that sync up (or don't) with the returning pressure wave.

I would like to see an industry standard of cylinder head volume measurement that allows me to confidently make a direct comparison between the different manufacturers' offerings. I have no idea what method of measurement is being used by each manufacturer. Is it filled level? or is it with the most common domed piston type in place, and did they add the volume of the clearance or did they measure with the piston in contact and I have to calculate and add the extra volume?

I see the "walbro style" diaphragm carburetors still come with no thermal isolation spacer, yet I have continually read that this will cause problems. Apparently people who can are making their own. I really like the idea of external adjustment that I can do out on the road side but the lack of this spacer is really off putting.
Some information from the vendors about the different types of diaphragm carb would be a great help too. At the moment they are sold as "stage 1, stage 2 or stage 3" but no info at all as to what those numbers actually mean.

I seem to be repeating this lately (and it isn't exactly an engine part) but BOTH the top hat adapters for mounting the sprocket on a ISO disc rotor mount only work with the 3 or 9 hole pattern of mounting holes instead of any of the existing 4 and 5 hole bicycle chainring standards. This limits the user to the manufacturers own sprockets, which only come in even number tooth counts. It's also a bit of a middle finger to the international market, who have to pay double in some cases for the shipping every time they want to try a different size or just need a new sprocket. It certainly puts me off ever trying a single speed build.

Most of all I would like to see the manufacturers taking part in the forum. I hardly ever see any of them except for Jaguar. Why? Are they too busy 24/7, really? Or just not interested? I would think that if you have a great original product that benefits the MB community you would want to shout about it!
 
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jaguar

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Also about cylinder heads- their ability to dissipate heat depends on their total surface area. A head with many thin fins will cool better than one with a few fat fins.
Some of the aftermarket heads actually have less surface area than the stock head although they take longer to heat up due to having more aluminum.
 

FurryOnTheInside

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Can you identify them? The manufacturers won't tell us lol. I was thinking the RSE one (they might have more than one, idk) but
The BMW and Fred heads look okay.
 

jaguar

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I haven't measured them but just by looking at them you can get an idea if they have plenty of surface area or not.
Be sure to find out the head volume. I wouldn't buy one with any less than 7cc.
 

FurryOnTheInside

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LOL I can't possibly know the volume because I don't know how different people have measured the volume. The experiment and thread by Steve Best showed there's no standard of measurement being used by the manufacturers to express combustion chamber volume. I can choose one listed as 7cc but I don't know if that's the same 7cc. Fred heads weren't included in Steve's test as far as I recall.. I think he had RSE, BMW and Grubee heads.

((I will be choosing the 7cc Fred head when I get to that stage anyway, because that's the biggest deepest combustion chamber available AFAIK and I read that that is good for my intended application))
 

jaguar

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THE RIGHT WAY to measure it is put a spark plug in and fill the dome with a light weight oil, then pour it off into a measuring cup.
 

butre

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it doesn't have to be oil, just something that doesn't have much surface tension, won't evaporate off on you too quick, and won't eat a hole in your measuring cup. I use gasoline to measure head volume.
 

jaguar

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Here's a way to lower the compression of an aftermarket head, use the below mentioned motorcycle piston which is 1mm lower. (only for 66cc engines with reed valve)

US Chrome says that plain chrome plating (such as the Grubee cylinders have) has a temperature limit of 400 degrees Celsius so a higher cylinder temperature due to advanced ignition at high RPM in combination with high engine compression (I assume over 140psi) can be a causative factor with cylinder plating flaking. So it is imperative that you also buy a compression tester when you modify your head or when you buy a high compression head. Some of the high compression heads cause 185psi!!! Yikes. guaranteed bearing failure and cylinder plating flaking!

from the Wiseco piston site I read that chromed cylinders need non-chromed rings:
"Some older model engines use a chrome plated cylinder, and are becoming harder to find. A chrome cylinder can appear like a Nikasil cylinder, so it is always good to research your engine prior to installing a ring. Only some Wiseco rings are compatible with chrome plating. These ring types are normally non-coated, such as a cast iron ring face, to prevent the ring from scuffing the bore and not working properly."

Substitute piston for the 66cc/80cc engine if it has a reed valve:
Hoca Minarelli 47mm piston and rings $30 from PartsForScooters; With the piston for the Dax/Speed-demon engines (the most common, the "low wrist pin" engines) the wrist pin is 21mm from crown edge to upper pin edge, and the piston for the Grubee engines (higher wrist pin version) is 16mm from piston crown edge to wrist pin edge. The Hoca Minarelli Piston was tried in Grubee cylinders. It has a 46.8mm diameter and has dual 1mm rings. When tested in two cylinders the chrome plating of the cylinder got scraped off by (we assume) the chrome rings so non-chrome rings are needed. The Hoca piston from the skirt to crown is 48mm, ours is 47mm. The 10mm wrist pin on the Hoca Minarelli is 20mm from the crown, 1mm less than stock which would relieve excess compression boost from an aftermarket head. HOWEVER, the piston is 112 grams (5 grams heavier than our piston) and the wrist pin is 5g heavier than our wrist pins (20g), so the wrist pin needs to have its center hole enlarged to make it lighter.

At 1977mopeds there is a 47mm non-chrome 1mm thick ring for $15. It is the Motobecane Airsal Piston Ring. Buy two of them for use with the Hoca Minarelli piston.

Every China girl cylinder I have owned has lost some cylinder plating so I know that their coating process is not completely correct. But both of the two cylinders tested lost a large portion of chrome during the first tests so we have to suspect that a non-chrome ring needs to be used. If I had a 66cc engine I would definitely try the Minarelli piston and Motobecane rings. Also I recommend using a synthetic or semi-synthetic engine oil for better lubrication. Also I would keep the compression no higher than 120psi.
 
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