Slant cylinder head vs. strait cylinder head

Frogster · Jan 18, 2013

Hey guys, I am somewhat new to the motorized bicycling online community.

My question is, do slant plug cylinder heads really create a better compression ratio then strait plug cylinder heads? Ive heard from one person that strait cylinder heads are better then slant cylinder heads because there is no actual difference in compression ratio while strait plug cylinder heads distribute the spark better and create less chance of a hot spot developing.

Also, is the billet cylinder head sold by motovelo a good performance upgrade? It says it says its an actual high compression head.

thanks!

DIANY · Jan 19, 2013

i been riding a few years.. and also wonder the same thing.....a lil help????.... i love my center fire!!!.... wish i could find a 66....

obesepolice · Jan 19, 2013

stay away from motovelo. That being said look into the Puch hi hi compression head. It is four times the size and will provide vastly better cooling as well as raise compression.

https://www.treatland.tv/puch-70cc-head-p/puch-hi-comp-head-70cc-new.htm

KCvale · Jan 19, 2013

Basically the slant head for these china kits came about so a motor could fit in a smaller bike cavity, hence the spark plug didn't hit the top bar.

As for performance there is no real difference other than a slant head will allow for the use of a slightly longer plug for example the NGK BPR7HIX Iridium plug which for $9 will give you a performance boost you can actually instantly feel.

One more personal note I have is nothing new comes to the US with a straight head since the 2010 EPA regs kicked in so any 'new' kit you buy with one was built at least 4 years ago if not much longer. That doesn't make it a bad motor, it's not like it's a loaf of bread, but little improvements at the factory take place year after year is all.

Frogster · Jan 19, 2013

obesepolice said:
stay away from motovelo. That being said look into the Puch hi hi compression head. It is four times the size and will provide vastly better cooling as well as raise compression.

https://www.treatland.tv/puch-70cc-head-p/puch-hi-comp-head-70cc-new.htm

That is very interesting obesepolice, thank you. Will this head come with a head gasket and fit properly on a regular 66/80 cc bike engine conversion kit? Have you put it on your bike?

obesepolice · Jan 19, 2013

There are several heads available. I used a new stock head gasket and coated it with copper gasket spray. Keep in mind if you have 8mm head studs you will need to drill out the head to fit, as it is drilled for 6mm head studs. Don't worry about that though its quite easy. I also had to upgrade the clutch pads when I put the head on. But I have alot going on mods wise. Next is to balance the crank.

http://www.piratecycles1.com/picyhicohe.html
http://www.jnmotorsbikes.com/category_s/67.htm
http://mmbikeparts.com/cylinder_heads.htm

I went with the puch head from treatland, partly because im cheap! ha ha
With everything ive got going on with this bike it is quite fast.

obesepolice · Jan 19, 2013

check out this thread

http://motorbicycling.com/showthread.php?t=39382

DIANY · Jan 19, 2013

can i file the ring on the cylinder head off to raise compression??

Frogster · Jan 19, 2013

Okay, interesting. Thank you very much.
So seeing as you use the puch moped 70cc hi hi compression head. WHat are your experiences? Do you use two head gaskets? Does using two head gaskets decrease the the compression significantly? Do you find that you burn your leg using this head? Also, how hard do you find it to pedal start your bike? Im 185 lbs and 19 year old and pretty fit.

obesepolice · Jan 19, 2013

IT has made the bike so much more fun to ride. Much faster and the cooling is great. Im using one head gasket covered in copper gasket spray. and I upgraded the headstuds. You make have to use two head gaskets as one might be to much compression. The head is much larger so yes burning is an issue. also the fins are sharp. I ground those down alittle bit rounded off the corners. Im fat 220 lds starting is not a problem at all for me.

Frogster · Jan 19, 2013

DIANY said:
can i file the ring on the cylinder head off to raise compression??

how do you mean? Can you please explain that a bit more?

So obesepolice you are using one head gasket and the piston isnt making conact with the head or spark plug? I plan on buying and iridium spark plug from SBP.com because it will help with cold starting up here in Ontario. Also, i think ill just get a wide pedal crank kit to help avoid burning. Also, can you post some more pictures of your build at different angles? It really interests me and id like to build something similar. I want to build a motor that gives me good fun performance, but is also reliable and will keep starting and working for me time after time.

obesepolice · Jan 19, 2013

Yes im running one head gasket with an ngk b7hs. I just bent my one piece cranks out a bit to compensate. I really only peddle to take off so I cant see spending money on some three piece cranks. And yes im going to post some more pics of my build over here

http://motorbicycling.com/showthread.php?t=44787

DIANY · Jan 19, 2013

on the cylinder head. there is a ring... i believe this is to ensure an airtight seal....where the gasket is placed.. if i filed it down some.. and decreased the size of the chamber when the gas is comressed would that raise compression????

i wish i could find a picture...

Frogster · Jan 19, 2013

DIANY said:
on the cylinder head. there is a ring... i believe this is to ensure an airtight seal....where the gasket is placed.. if i filed it down some.. and decreased the size of the chamber when the gas is comressed would that raise compression????

i wish i could find a picture...

It probably would a little bit, but you would be risking not making a tight head gasket seal which would be counterproductive seeing as you would lose compression. Also, out of all the high compression cylinder heads that obesepolice linked to in this thread, this one seems like it has the smallest combustion chamber. Some people need to use two head gaskets to raise it up so that their piston doesnt contact the spark plug. Not worth bothering with imo.

obesepolice · Jan 19, 2013

DIANY said:
on the cylinder head. there is a ring... i believe this is to ensure an airtight seal....where the gasket is placed.. if i filed it down some.. and decreased the size of the chamber when the gas is compressed would that raise compression????

i wish i could find a picture...

Yes it can be done. Search for some threads there are several. Its not hard to do really. But remember raising compression also raises temperatures. That's why I went with the Puch head, lots of cooling fins.

I only need one gasket and everything is kosher.

bulljo · Jan 21, 2013

Im running the puch hi hi compression head also with ngk iridium plug gonna need at least a 8 for the heat range to prevent ping and 91 gas with a dose of octane booster since ur boosting compression
im also running a super thin puch head gasket with copper rtv. The head incresed mid and top end revs . Brot......brot........brot xct2

dodge dude94 · Feb 4, 2013

Yeah, kind of an older thread. But I've got a slant head 66cc and I was wondering how much material I could safely shave off the mating surface to give a compression bump without knocking the plug? I was also thinking about smoothing the combustion chamber a bit.

My plug is a B7HS, btw...

bulljo · Feb 5, 2013

Most people shave off between .025 and .030 but every head is different believe it or not. Some have extra threads some dont. Sometimes just mating the head nicely will give a compression bump cuz you got a better seal. Dont bother polishing a stock head if you really want high quality combustion chamber buy the puch hi hi head, drill the holes out two sizes and use the stock gasket with copper rtv and bamm your engine will just keep pulling. xct2

dodge dude94 · Feb 6, 2013

I was thinking about shaving the head primarily for the compression bump, but I noticed that on the front side it's not mated well, so I'm sure that will help. lol

I wasn't going to "polish" the combustion chamber, just smooth it out a bit to get rid of some of the turbulence.

And I've looked at the Puch head, eh, not my cup of tea. I would rather do "free" mods more then anything else.

bulljo · Feb 7, 2013

Yeah ive had a couple different types of engines with all with different looking combustion chambers.
But one runs the best and i know why when i look at the combustion chamber. With this head she has more of a powerband if you will. Like you said about the puch head my first setup was alot of bolt on mods but second bike was alot sturdier and olds with only internal mods and let me tell you she rips.

Slant cylinder head vs. strait cylinder head

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