Comparison of 2 heads- please take a look

[Fletch]

These are both slant heads. The first is Grubee Skyhawk and the second is an engine from Zoom (not sure the model).

I'd like to know if the zoom head is OK with those "imperfections" or if it will crack down the road?

My second question (still being a novice to engines) is which head is better designed (better performance)? IOW...which one would you rather use?

I had them side by side and I'll let the pics speak for themselves, but I will add a few things:

The markings that look like cracks in the zoom head you can't feel with the exception of the largest one, and it is very minimal (I sanded and polished).

The skyhawk head seems more pronouncedly circular shaped (both combustion area and spark plug indent), and even appears to me to be somewhat smaller in diameter?

Lastly because you cannot see this... The part that mates with the cylinder... the square "lip", is much shallower on the zoom head (about 1.25mm) when compared to the Grubee (about 2.75mm). I'm sure this allows for more compression as it is like shaving the head or using a really thin gasket. The grubee is warped though and the zoom is nice and flat.

Any opinions on which I should use or comments are greatly welcomed. I'm still learning, and I may try both heads and see which gets better performance too.

Also FWIW, the zoom head has taller cooling fins.

Thanks

 

[GearNut]

The "cracks" are casting flaws from manufacturing and poor quality control. They will do no harm except give carbon a good place to start building up.
I prefer the looks of the Grubee head due to the better quench area which will promote more turbulence into the combustion chamber resulting in a more compete burn.

 

[Fletch]

The "cracks" are casting flaws from manufacturing and poor quality control. They will do no harm except give carbon a good place to start building up.
I prefer the looks of the Grubee head due to the better quench area which will promote more turbulence into the combustion chamber resulting in a more compete burn.

Thanks GearNut I appreciate the info. The Grubee head has about 1/2 of two of the large fins cracked off. I'm assuming it's still OK to use? The trade off seems to be the cracked fins and warped head with better quench area vs. the head with casting flaws and less complete burn that is flat.

I shaved the skirt off the piston last night, port matched and opened up the intake and exhaust a bit. I moved the exhaust up and the intake down and out a little on the sides. All this by hand with a dremmel. Really inaccurate and sloppy, but I'm hoping it will help some. I've got an expansion chamber from SBP that I just installed and I switched from 44 to 41t sprocket. Stripped an intake threading and decided to just JB weld the ported billet intake on the cylinder. Will be dry tomorrow so I've got my fingers crossed. I haven't decided if I'm going to replace the base gasket yet or just use some Indian head gasket shellac. This is only my second build and I'm already messing around with a lot of stuff. I better quit while I'm ahead

 

[GearNut]

Any good news?

 

[Fletch]

Any good news?

I decided to go with the head from zoom because it was already running with it, it is flat and I didn't want to sand down the grubee, and the grubee sits farther away from the piston because that square lip is thicker. Maybe if it didn't have two fins cracked off I would have sanded it down, but I was concerned about cooling issues aside from the fact it looks bad.

I read contradictory things on here all the time. Oil ratio's and 'break in' are two big ones, but there are lots of others. Some say that rubber in between the fins is ok because these engines don't get too hot, and some say that the fins aren't large enough to do the job. Then you've got mag wheels vs spoked, porting or not, etc. etc.

Engines hauling though with 50 miles on it. I opened up the intake and exhaust ports and trimmed the piston skirt (redundant, I know). My top speed with a 41t sprocket as of today is 39mph with the expansion chamber and all the other mods. I'm guessing it will get a little faster, but that's a good speed for me. The power band on the exhaust is at about 4/5 wot.

 

[GearNut]

Thank you for posting an update. I am glad to hear that you are enjoying the little beast.
The engine should pick up a little speed and power as it breaks in. I am a bit concerned about the porting and piston skirt trim though. There is such a thing as too much of a good thing. Only time will tell though, and the most important aspect of it is that you are happy with the results.

 

[Fletch]

Thank you for posting an update. I am glad to hear that you are enjoying the little beast.
The engine should pick up a little speed and power as it breaks in. I am a bit concerned about the porting and piston skirt trim though. There is such a thing as too much of a good thing. Only time will tell though, and the most important aspect of it is that you are happy with the results.

Are you concerned because of the what I did specifically (pics in other thread) or just about porting or trimming the piston in general?

I'm definitely happy with the results, but you're right about time will tell. I've got my fingers crossed! The only thing about my builds is that they don't fire up right away (2 different engines). I think it might be because of the ported billet intakes? I have to give them gas and sometimes it takes 2-3 tries. I've read other people have the same issue. I don't believe I have any air leaks. I've read that you shouldn't need the choke at all to start. My plug is a good tan color. The only other thing I can think of is that I'm not getting a good spark, but I have an upgraded wire, and NGK plugs. The only thing I can think of is HOW I have it wired- white closed off, one kill wire in between the blues, and one that is grounded to the engine. My suspicion is that maybe it needs to be grounded in a different place? I don't know though because the kill switch works fine. My best guess would be the ported intake not being at the right angle, so the fuel isn't being sucked in right away on starting.

 

[GearNut]

I just think that your intake will be open too long and you will get excessive blow back through the carb and reduced primary compression. I have a ported cylinder that I have been slowly (very slowly) working on in my spare time. I intend to rebuild the whole engine with the quality bearings throughout before ever starting it. I am still stumped on locating quality big end bearings for the connecting rod though.

 

[Fletch]

I just think that your intake will be open too long and you will get excessive blow back through the carb and reduced primary compression. I have a ported cylinder that I have been slowly (very slowly) working on in my spare time. I intend to rebuild the whole engine with the quality bearings throughout before ever starting it. I am still stumped on locating quality big end bearings for the connecting rod though.

How will I determine if this is happening outside of measuring compression? Will there be some kind of gradual loss of power?

Are you saying this because of the pics of how much I opened the intake, because I both trimmed the skirt as well as lowered the intake?

I have an extra block I can use if I have reason to believe this one is going to cause major problems.

thanks

 

[GearNut]

Measuring compression will not tell you anything as far as primary compression is concerned. Primary compression happens in the flywheel compartment, below the piston, after the intake port closes and before the transfer ports open. All a compression test will show is the compression in the combustion chamber, above the piston.
There will not be any gradual loss of power, just an overall lack of power at a certain RPM. In extreme circumstances the loss of power could be across the entire RPM range that the engine is capable of operating under.

I am stating this because yes, you both trimmed the piston and altered the port timing in a random fashion.
Now, altering both areas of port timing control in itself is not necessarily a bad thing if the proper measurements and calculations were done to determine precisely how much altering should be done.
I am in no way suggesting that you swap out your modified cylinder and piston for another set if you are happy with your current results.
I am really only trying to say that modifying both at the same time without doing the proper calculations to determine the best modifications usually results in a lackluster performance gain at best and typically hurts overall performance. I am also trying to describe the hows and whys of it all as well to hopefully convey to you what exactly is going on. Someone here has a link in their signature to a book, "Performance Tuning by Graham Bell". If you can find it, download it and read it. You will gain a heck of alot more accurate knowledge on this subject than I can share with you as I am not really a good teacher. I try to share what I know as best as I can, but I know that I flounder with getting my point across and sometimes I confuse more than help.