Predator Head Work
- Thread starter Bonanza Bryan
- Start date
.050 off your head should get you around 18cc's and with the .020 longer rod/.010 gasket and a flat top I believe your compression should be around 11.5 I think ole4 can verify this or give you a better reading. good thing if your compression isn't any higher then that you should be able to run a 91octane ethanol free gas.
I personally built a motor similar to what you are building and had no problems with .050 off the head and runs great with 91 octane.
maybe just compromise and go with .040.....lol either way I think you will be very happy with your engine setup on a minibike. just make sure you got some gearing in it. so you can pull some good rpm's. that cam with the right carb and header...etc. should pull 7000 easy.......
maybe just compromise and go with .040.....lol either way I think you will be very happy with your engine setup on a minibike. just make sure you got some gearing in it. so you can pull some good rpm's. that cam with the right carb and header...etc. should pull 7000 easy.......
Insufficient piston to valve clearance is is a terrible way to end a nice build. Be sure to check this. Recommended piston to valve clearances are:
0.080" Intake
0.100" Exhaust
Add 0.030" for aluminum rod stretch...yes they elongate when they change direction at the top of the stroke, more so than steel. They also tend to compress more at the bottom of the stroke, but that's another issue altogether and tends to help us in our pursuit for horsepower, ever so slightly.
Since we are running aluminum rods both stock and aftermarket, perhaps we should heed that advice to some degree. These rules of thumb are based on automotive engine piston masses and rod cross-sections, but we can assume a linear relationship between piston mass and rod cross-section that will scale nicely for our application.
Let's see how my build shakes out:
Predator 212 Hemi
0.020" Arc aluminum rod resulting in the piston at 0.004" in the hole.
Head cut 0.035"
0.027" MLS head gasket
To save you from doing the math, that results in a 10.6:1 static compression ratio (yes, I cc'd the head)
Black Mamba Jr. cam (0.275" lift)
Measured piston to valve clearances:
0.090" Intake
0.105" Exhaust
My build seems to be spot on for a steel rod. Add the aluminum rod correction factor in there and I'm risking a piston to valve strike (if the correction factor is to be believed). Another variable is RPM. Rod elongation increases with RPM, but it is not a linear relationship. It's an exponential relationship. Calculating these things suddenly becomes more difficult. That said, my engine lives just below 7000 RPM. This may not be enough force to elongate the rod sufficiently to cause piston to valve contact. In fact, I'd have to say that it is not enough force since inspections of the piston crown through the spark plug hole show no piston to valve contact has occurred. I'll throw this in here, make sure you have the correct valve springs and set them to the correct heights for your cam. If you can't control the valves then you are just begging for P to V interference. Remember that your static P to V measurement does not account for the spring force or valve train inertia.
Anyway, since I got everyone's ear, or in this case eyes bleeding (okay brain bleeding) with my nerd stuff, I'd like to restate my point. Check your piston to valve clearances. Do not mess this up. It is most certainly important. We need more bad-ass hotrod mini bikes out there roaming the streets.
-Ray
0.080" Intake
0.100" Exhaust
Add 0.030" for aluminum rod stretch...yes they elongate when they change direction at the top of the stroke, more so than steel. They also tend to compress more at the bottom of the stroke, but that's another issue altogether and tends to help us in our pursuit for horsepower, ever so slightly.
Since we are running aluminum rods both stock and aftermarket, perhaps we should heed that advice to some degree. These rules of thumb are based on automotive engine piston masses and rod cross-sections, but we can assume a linear relationship between piston mass and rod cross-section that will scale nicely for our application.
Let's see how my build shakes out:
Predator 212 Hemi
0.020" Arc aluminum rod resulting in the piston at 0.004" in the hole.
Head cut 0.035"
0.027" MLS head gasket
To save you from doing the math, that results in a 10.6:1 static compression ratio (yes, I cc'd the head)
Black Mamba Jr. cam (0.275" lift)
Measured piston to valve clearances:
0.090" Intake
0.105" Exhaust
My build seems to be spot on for a steel rod. Add the aluminum rod correction factor in there and I'm risking a piston to valve strike (if the correction factor is to be believed). Another variable is RPM. Rod elongation increases with RPM, but it is not a linear relationship. It's an exponential relationship. Calculating these things suddenly becomes more difficult. That said, my engine lives just below 7000 RPM. This may not be enough force to elongate the rod sufficiently to cause piston to valve contact. In fact, I'd have to say that it is not enough force since inspections of the piston crown through the spark plug hole show no piston to valve contact has occurred. I'll throw this in here, make sure you have the correct valve springs and set them to the correct heights for your cam. If you can't control the valves then you are just begging for P to V interference. Remember that your static P to V measurement does not account for the spring force or valve train inertia.
Anyway, since I got everyone's ear, or in this case eyes bleeding (okay brain bleeding) with my nerd stuff, I'd like to restate my point. Check your piston to valve clearances. Do not mess this up. It is most certainly important. We need more bad-ass hotrod mini bikes out there roaming the streets.
-Ray