First Predator Build, I have lots of questions.

S

scottygrape

New Member
#21
#21
Hmm.. would it be better for me to go with the stock valve size and aluminum retainers then for $100 less? 27mm intake and 24mm exhaust. Also will I have clearance or geometry issues if I use a 18cc head milled .050? I don't want to have to use cut to length pushrods
 
ole4

ole4

#22
#22
I am getting confused here. Are you saying a 14cc head with a couple of valves is $210? You should always check clearance and geometry. Milling .050 is not going to change geometry enough to require new pushrods. Changing rods, head gaskets, heads does require checking everything just for safety sake and it is good building practice. All the information to do that is here on OBM on various build threads.
 
ole4

ole4

#23
#23
I just reread your original post so I see you are using a black mamba jr. Your combo is good. You mentioned 27mm intake and 24 exhaust. What head is that going on? Clone heads are 25 intake and 24 exhaust. Some predator heads are 27mm and 25mm. You can't just put a 27mm intake in a head that has the seat cut for a 25MM valve. Is that what you were trying to do? What rocker arms do you plan on using?
 
ole4

ole4

#24
#24
Just thinking of other options. You hemi uses different pushrod lengths from the standard headed motors so you will need different pushrods. Why not use the hemi head? It flows well has 27, 25 valves and the only issue is the compression. you could just buy the 11 to one pistons and 26lb springs, check for coil bind and you are good to go.
Forged Piston Kit, Predator 212cc Hemi
 
ole4

ole4

#26
#26
Isky black mamba series have a compression release so stop worrying about pull starting it. The head in the link is also OK. Black mamba is a more radical cam than the jr. I have used both. What are your intentions for this bike? Head and cam are the heart of the engine. There is no use in using a radical cam with small valves. (actually there is a little but not like when you match the parts correctly). You will lose on the bottom end and the top end will not be improved much unless you have a head that will let the engine get air at hi rpm. The black mamba only needs 26lb springs and the stock hemi head will breath good enough to give you a good top end. It will also not require you to do all the checking for pushrod / rocker geometry ect that you will have to do when changing heads. I have a number of clone heads here that will fit the bill but will have to look thru what I have and let you know if you are interested, however I would stick with the hemi head. You are adding a longer rod and the hemi head does not have a squish area so head clearance is not so much an issue. I will check one with a piston to see how far piston can go into the hemi dome. Anyway let me know your thoughts on using the stock hemi head.
 
BWL

BWL

Active Member
#27
#27
Yeah, Ole is right. The hemi head is a very good head. If anything I'd get that hemi head ported or at least have the sharp edges rounded off inside the intake and exhaust ports. If you want to know the true difference the right head can make, the latest episode of Engine Masters shows it. They had an engine wit ha massive camshaft but a small head and it couldn't breath. They ported the head well and gained 92 hp. It's about the carburetor and the head. The camshaft only complements what the carburetor and camshaft are doing. If someone is making a pop top piston for a hemi, you just solved all your compression problems.

[video=youtube;aijyLKCvbU8]https://www.youtube.com/watch?v=aijyLKCvbU8[/video]
 
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