high compression head mod

Norman

Norman

LORD VADER Moderator
I've been playing around with machining the head shaving off up to 0.030 and sometimes more. I should of cc'd the combustion chamber before and then after but this is done for fun more than anything else.
I made a fixture out of a china spark plug to chuck the head in the lathe then using the diamond wheel I cut me a bit out of a carbide tool to work over the head.
I went after my stock rat bike first its the cough. cough, 80cc engine. I seem to have boosted the performance a little how much beats me but it now has more of a bark out of the exhaust than it did before. I have some pictures to follow. If anyone wants to do this give me a yell.
Norman
left is the machined head on the right is a stock head 0.045 is removed from head measured from the edge of the plug hole to the lip on the gasket surface. To measure this I bridge a flat plate across the heads gasket surface then measure it with the dial calipers to get a difference in the depth from the heads surface to the edge of the spark plugs hole.
headwork005.jpg

what it looks like chucked up in the lathe
headwork003.jpg

the holding tool made out of a spark plug. Some times I can get the camera to focus and some times I can't.
headwork002.jpg
 
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thats a nice skill Norman, not many people would have the equipment or the knowledge on doing something like this to there head. But i will be honest , Im pretty content with my *cough* 80 cc motor and the speeds I get out of it and the torque also. Any faster and Im thinking i will need to classify it as a motor cycle . lol
 
nice job. i remember doing about the same with some cloth on a pane of glass, he said it cut very easy and the power level was increased.
 
I machined the chamber and the sealing surface. I did the wet/dry paper thing on the glass along time ago. This is a lot more different.
Norman
 
Indeed the volumetric change would have been interesting. Strangely it looks larger, but you probably made it smaller by removing more meat at the sealing surface (ie lowering the dome). Cool.
 
Norman if I may lend an experienced eye you might have been better served if you had have stopped at reducing the head to jug mating surface so as to correct the squish band clearance to say .7mm - .8mm. Increasing the angle of the squish band itself a degree or 2 after correcting squish clearance can help sometimes too as it increases squish velocity and turbulence which always equals better low and mid rpm power in a 2 stroke.

It appears in the photo that you removed much of the squish band and enlarged the combustion chamber? Your actual compression might be the same as stock if you did. A smaller\narrower squish band is usually better suited for upper rpm power...GP race bikes for instance would make good use of narrow squish bands.

A good reference for a 2 stroke combustion chamber for our domed piston engines is what's pictured. Fat squish bands, tight squish clearance and as small a combustion chamber as you can get away before detonation starts blowing holes in your piston are the way to get "streetable" power out of a 2 stroke :)

Cheers...
 
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Thanks for the info. I'll have to check into welding it up and re machining it to more of what you have in the picture. The heads as cast are not what you could call precision made.
I really need to get a mill.
Norman
 
I wouldn't stress....heads are only 10-15 bucks :)

If you get a mill and a rotary table to go along with that lathe you'll be able to make 2 piece heads sets with removable domes...all our 40 and 50cc BZM's have a head setup like this.
 
ok, i have modified my 66cc motor to the max right now, and i have just finished milling the head. i took off .09", and stuck in a larger heavy duty racing car sparkplug. the displacement of the head went from 11cc's, to 7cc's , i figured the compression went from 6:1 to 10.5:1 roughly. with this huge boost in compression, should i run 91 octane?
 
a buddy of mine was willing to run his biek with no head gasket, so i took off his head and used some silicone gasket maker on it and we went for a ride. He said he did not notice much of a difference but shortly after starting up his engine, it conked out and would only fire about one third of the time. I took it apart and realized that the piston had hit the spark plug and bent the tip till it was almost touching the electrode. Im not sure why the engine was able to run fot a while without this happeneing but we put another washer on the plug and it runs fine since.

Just wondering if you had to compensate for this in any way, Norm?
 
I haven't had the spark plug smack the piston but I did have to allow for the piston trying to hit the head out by the side of the head machined off just a hair so it wouldn't on the head. You can use modeling clay on the head or piston to check for clearances before a test run cheap insurance.
I might weld up the combustion chamber on the head and then remachine it just for fun, my motto is "We don't stop until someone bleeds" I'll probably be the one that will bleed from the shrapnel. Ilikea goes and hides when I do things like that.
You should of seen us trying to bead a racing slick on a go kart wheel 10" wide rim on an 7" wide tire we use either and set it on fire was a hoot boom!!! T
The tire went on the bead. I now have a tool to set the tires into and pinch them to seat the bead the either scares me the tire tool is safer and none of us will loose their eyebrows in the flash fire!
Ilikea says I live on the edge too much..duh.
Norman
 
I already have (bled)...kinda.

Ive machined a stock head till it was 9:1 compression with a 50/50 squish band and it looked great. Fired it up and rode it for like 10 meters and the engine came to a hissing stop. Yep, the aluminium was so thin in one point the combustion pressure put a hole in it (it was like less than 0.1mm think haha). For this to happen I had to shim my spark plug because it would hit the head. Thing is, on a stock head you cannot machine it too much.

That is why Ive got a slant head to test today. Ive remachined it for 10:1 compression (55/45 squish) and Ill try that with and with out the head gasket (only difference is squish clearance + slightly more compression with no gasket) and then compare these results to my other slant head (came of a 2 stroke generator) and use the best one. Im experimenting with squsih band areas as well as "optimal" compression. To give you guys a idea of how much mods are done to this engine it requires a 0.80mm main jet in order for a tan coloured spark plug (more fuel burning = more power).

After this test I should conclude on whether a conical or hemispherical combustion chamber favours these HT engines.

Oh yea, I also upgraded the spark lead...to an arc welders earth cable hahah (cross sectional area of 33mm^2 of copper lol - nearly NO resistance)
 
Go get em Mark! Is is gettin close to fall there isn't it? That's what I like work on it till it runs like crazy or goes BOOM!dnut
Norman
 
Ha yeah Ive had one of my modded slant heads give me pinging on 91 octane with octane boost! Bit too much compression there lol.

Now I need a colder plug...to be on the safe side, now Im not quite pinging but the engine is not being revved for more than 3 secs at a time (only 30km done on this one) so I cant tell yet. With the poor cooling on these engines and after extensive mods Ill need a B*8 (where * can be resisitor/compact/protruded tip).

Oh yea my hemi and my tapered compustion chamber gave the same results. Ill stick to my hemi head so I can position the S.plug over the intake side to even out heat.
 
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