High compression head

Chris7420

Member
Jul 23, 2021
39
16
8
21
I installed a high compression/run cool head on my bike, I’ll add a link below to the head and also the engine I have. Anyways it didn’t do anything the bike runs the same also I did a compression test and the bike actually has 5 psi less with the high comp head. It was originally 105 now it’s 100. I thought it was going to raise the compression, i reused the original nuts/washer and tourque to 150 in/lb. I left it for a bit with the compression tester attached and it stayed at the same reading so I don’t think the head is leaking cause if it was I’m pretty sure I would see a drop in psi on the gauge. Also on a side note how fast should one of these go? Mine goes 40km/h. The ad says they come with a 36t sprocket but I counted the teeth and it’s actually a 44t I think it may need to be tuned or something too, if I hold the throttle steady at any setting between probably half to wot it bogs out/4 strokes I think, it will accelerate but then one it gets to a certain rpm just bogs and yea at wot it bogs/4strokes and just barely goes 40 km/h but yea it’ll also do it if I hold throttle steady at 1/2 or 3/4 quarter throttle and it quits bogging as soon as I accelerate. It’s all stock other then the head I just put on, I also adjusted the needle clip to one position up from stock cause it seems to run better that way and I run the exhaust with no baffle, ngk br6hs plug, 40:1 mix, i modified the air box for better air flow. The bike I have the engine on is 26” it’s pretty light, I’m 175lbs
 

Chris7420

Member
Jul 23, 2021
39
16
8
21
Measure the SQUISH
If I understand correctly to measure the squish you remove the cylinder head and put the piston just below tdc and put a piece of solder on top of the piston and reinstall the head and turn the engine over to squish the solder, do I have that right? Also how do you find the actual squish measurement? Do you measure the solder before/after squishing it and then the difference is your squish? What size solder should I use? I don’t have any at home so will have to go buy some and I don’t want to get too big/small lol.
 
  • Like
Reactions: Tyler6357

Larry Trotter

Active Member
Nov 26, 2017
339
151
43
73
Have you got the needle clip in the top notch of the needle? I found mine is still too rich even on that setting. Don’t think your issue is the head.
 

Chris7420

Member
Jul 23, 2021
39
16
8
21
Have you got the needle clip in the top notch of the needle? I found mine is still too rich even on that setting. Don’t think your issue is the head.
I’ve tried all needle clip positions. It seems to run best at the one before the top.
 

Chris7420

Member
Jul 23, 2021
39
16
8
21
And yeah the issue isn’t the head, it ran pretty much the same before as it does now. There must be an issue with the head though or else it’s just junk cause it was supposed to raise compression but it actually lowered it by 5psi, probably squish like the guy above said. But yeah even before the head it seemed like it should go faster. It only goes 40 kmh at wot. I read post from people saying there’s go 30-40 MPH stock. Or are they just exaggerating?
 
Last edited:

Larry Trotter

Active Member
Nov 26, 2017
339
151
43
73
And yeah the issue isn’t the head, it ran pretty much the same before as it does now. There must be an issue with the head though or else it’s just junk cause it was supposed to raise compression but it actually lowered it by 5psi
 

Larry Trotter

Active Member
Nov 26, 2017
339
151
43
73
To know for sure you need to measure the combustion chamber volume of each head. Easiest way I know is to lay each head upside down with the spark plug in. Fill the head level with water. Soak it all out in a paper towel and weight it. The heaviest has the most volume and the lowest compression.
 
  • Like
Reactions: Lone Wolf

Lone Wolf

Member
Jul 28, 2021
31
37
18
61
And yeah the issue isn’t the head, it ran pretty much the same before as it does now. There must be an issue with the head though or else it’s just junk cause it was supposed to raise compression but it actually lowered it by 5psi, probably squish like the guy above said. But yeah even before the head it seemed like it should go faster. It only goes 40 kmh at wot. I read post from people saying there’s go 30-40 MPH stock. Or are they just exaggerating?
Those people are probably reading a kilometer/ hour speedometer.
 

Lone Wolf

Member
Jul 28, 2021
31
37
18
61
I have a BBR stage 4 racing motor that is exactly the same as yours. Before I put a racing carb on it, I was getting the same top speed as yours. I got about 4-5 mph more with the racing carb. Another thing that has increased my top speed is using it. You have two actual break in periods. The second one is called well broke in. It is after you travel about 500 kilometers. When you get there, you should notice that it will go another 1-2 mph faster. Other than that, you will need to do like me, and buy more upgrades to intake, exhaust, and ignition. I forgot to say that mine came with the high compression head.
 

Tyler6357

Well-Known Member
Mar 15, 2012
1,274
278
83
Santa Barbara, CA
If I understand correctly to measure the squish you remove the cylinder head and put the piston just below tdc and put a piece of solder on top of the piston and reinstall the head and turn the engine over to squish the solder, do I have that right? Also how do you find the actual squish measurement? Do you measure the solder before/after squishing it and then the difference is your squish? What size solder should I use? I don’t have any at home so will have to go buy some and I don’t want to get too big/small lol.
That's all correct, you're going to need some soft leaded rosin or acid core solder: .050" Rosin Core Solder, .062" Rosin Core Solder, .093" Rosin Core Solder, or .125" Acid Core Solder are all good. Once the solder in in place you only need to crank it over just once to squish it down, measure the thickness of the compressed solder at its smallest point (usually the edge closest to the edge of the cylinder). This measurement is your squish. Squish has to be less than .060" on a 6cc head to get any kind of performance boost from a stock head. If the gap is bigger than 0.060" you might as well not even have a squish band in the head, and you are not going to get the main benefit of the optimized squish band. A squish of between .025" to .035" is optimal. The next step is to deck your cylinder by sanding it down. One method is to get sand paper and sand down the top of the cylinder using a flat surface like a piece of glass, you must sand it as flat as possible, slowly and evenly with different grid sandpapers using the heavy ones first than switching to light ones. After you have taken some off, put the cylinder back on and do another test with solder and measure until you get the squish you want. I do NOT recommend sanding the head to lower the squish, always adjust the cylinder. This process is actually called "decking the cylinder." Of course, if you have a machine shop with the right machines this is much easier.

However, you should know that even if you don't deck your cylinder and just run it like it is, it does still have some effects. Those large fins and extra steel are helping your engine run much cooler which really helps these little engines last much longer with much less chance of heat causing a part failure. Also, the head provides a superior head gasket seal over stock heads with less chance of air leaks at the head gasket.
 
  • Like
Reactions: Lone Wolf

Tyler6357

Well-Known Member
Mar 15, 2012
1,274
278
83
Santa Barbara, CA
I should add that there has been discussion and some disagreement over exactly what squish gap is optimum. Some might disagree with what I wrote as optimum squish gap but certainly if it is larger 0.060" then your high compression head won't show any measurable performance increases over a stock head.
 
  • Like
Reactions: Testname111

Chris7420

Member
Jul 23, 2021
39
16
8
21
I have a BBR stage 4 racing motor that is exactly the same as yours. Before I put a racing carb on it, I was getting the same top speed as yours. I got about 4-5 mph more with the racing carb. Another thing that has increased my top speed is using it. You have two actual break in periods. The second one is called well broke in. It is after you travel about 500 kilometers. When you get there, you should notice that it will go another 1-2 mph faster. Other than that, you will need to do like me, and buy more upgrades to intake, exhaust, and ignition. I forgot to say that mine came with the high compression head.
What kind carb do you have?
 

Chris7420

Member
Jul 23, 2021
39
16
8
21
So I tried to measure the squish using 1.25mm/0.062” rosin core solder and the solder didn’t even come in contact with the head. The first time I did it I forgot to put the head gasket on and it squashed the solder. I’m guessing this means my squish is too big?
 
  • Like
Reactions: Lone Wolf

Chris7420

Member
Jul 23, 2021
39
16
8
21
2281741E-E914-4CED-942F-E26B42E1FEDE.jpeg

That’s when I did it without the head gasket. It squished it about 0.45mm
 
Last edited: