Cylinder head/Fred head

[barnett256]

I'm getting an 80cc 2stroke... I want to replace the ugly stock head with what would be the equivalent to stock Fred head but I don't know what the specs mean or what works with what.

 

[Deleted member 12676]

when increasing the compression you also need to retard the ignition with an aftermarket CDI
http://www.dragonfly75.com/motorbike/Compress.html

 

[butre]

the 6cc fred head offers an 11:1 compression ratio and in my opinion is the ideal head for a mildly modified engine running on pump gas. a fred head doesn't just increase compression, it also improves cooling, so if you throw in a colder than stock spark plug (NGK B7HS is what I use) then you may not need to retard your ignition.

 

[Deleted member 12676]

well everyone is certainly entitled to their own opinion, even if it's wrong.
Butre usually you are pretty right on but not in this case.
an 11:1 ratio will give over 160psi easily, depending on how much the head gasket leaks and how much blowby these cheap rings allow.
I advise against higher than 135psi compression for 3 reasons:
1) the upper conrod bearing is not very high quality and will wear fast with that excessive combustion pressure.
2) excessive cranking pressure causes excessive engine heat, and these engines are limited in that they are air cooled.
3) increasing the cranking pressure decreases the burn time and so for it to stay in harmony with crank position you have to retard the ignition, otherwise too much pressure is built up and the optimal push against the piston is lost.

yes, excessive pressure will give more power but that power will be lost as the engine overheats.

Here's a good head temperature gauge: http://www.jnmotorsbikes.com/product_p/jnm1183.htm
http://www.dragonfly75.com/motorbike/headtemp.html

 

[butre]

that guy had a major fuel leak and possibly a lean condition. that's a stock low compression straight plug head, compression ratio had nothing to do with that fire. I'd bet money that gasoline had been dripping on his cylinder head the whole time he was moving, and then when he stopped it started dripping on his hot exhaust pipe.

my old bike had a 6cc fred head with a decked jug and stock ignition and somewhere in the neighborhood of 175 PSI compression and at max operating temperature you could touch the fins on the head without burning yourself. my current bike has a CDI I put together myself that is essentially a ripoff of an HD Lightning, but I also have one of Manic Mechanic's adjustable magnets advancing the timing by 2 degrees. the 5.3cc Fred Head and a B8HS keeps it nice and under 350 degrees.

 

[Deleted member 12676]

like I said, an increase in compression requires ignition timing retard with a different CDI.
really a person needs to monitor his head temp to make sure he hasn't overdone it.

 

[barnett256]

Honesty I'm not educated enough to turn this into a science project. I figured there'd be a better looking head with same as stock specifications (cc's, whatever) that I could bolt on and rock n roll! ? I thought I saw a 5 & or 5.5 on Fred's site. I can't believe there isn't a head same as stock that looks as nice, bolt on without retarded complications! Sheesh

 

[butre]

The 7 cc head offers a 9.4:1 compression ratio which should serve you well. No fuss, just better performance and better cooling. Just ask Fabian, that's what he uses and nobody runs their bikes like he does. 400+ pound towing at low speeds and high RPMs. A jaguar CDI doesn't hurt either. HD lightning is good too, but if reliability is what you're after the Jag can't be beat.

I use a bronze top end bushing instead of the original caged roller bearing for improved reliability and because the failure mode isnt quite so catastrophic. I don't have to deal with that junky bearing exploding on me and ruining my expensive billet head (which I bought when they were still $90) and ported jug I put a lot of time into.

 

[Arty]

I am running a 6cc Fred Head on my bike, and as I expected, the increased fin area really helped the cooling on my rig. It's really noticeable right after a fairly long run when leaning hard on the throttle. - The engine is nowhere near as hot, and settles down into a proper idle way better. It even sounds different - almost as if you can hear the air circulation around the cooling fins. - - The thing looks great too.
Also - Fred at CR Machine is really nice to deal with, and provides prompt service.

 

[skyash]

At work got my stock slant head and took off 1 mm then re done the face like stock again just been for a ride and it picks up better and tops out better but I can't put a hand on it. So hot so put more oil 30 ml to 500 ml ish of petrol and b7hs ngk plug and the head gasket might have been 1 mm but off another motor it's like paper thin so I used that one may of drop it by 2 mm over all now. all I can say is it goes harder for now how long?? It will be 34deg tomorrow so I will ride it after work think that's a good hot day test lol