Predator 212 head, order now, only 2 left.

65ShelbyClone

Well-Known Member
#22
Maybe "rippin'" wasn't the right word....more like "scrutinizing."

This thread brings up a question I've wondered about for a long time: how much RPM can the bottom end of these engines truly handle? 10k seems to be the high water mark for engines that mere mortals can afford to build. Leveraging a big head on a small engine means spinning it higher.
 

delray

Well-Known Member
#24
I would say stress/rpm. things start to move. some people would say if you blow the jug off, your building power. but really at that point things start to expand and flex.
you might think you have good clearance. for example piston to head clearance with a .010 head gasket. start run it real hard at high rpm's.
crank,rod....etc. starts to expand/flex and bang. your all done.....
 

65ShelbyClone

Well-Known Member
#25
What in the end causes the highest stress in a motor? RPM? compression ratio? heat?
I would say mechanical forces, but a common way they are increased is by raising RPM, as in Ruins Peoples' Motors. The first step many take is to remove the governor for $0. Then that's not enough, so they might get some $12 valve springs. Then they're spending $120 on a new Predator because RPM ruined their motor.

There are a lot of reciprocating parts in a piston engine that have to change direction very quickly. The acceleration factor increases to the square of velocity, so going from 4000rpm to 8000rpm (a velocity factor of 2) means acceleration goes up 2² = 4 times. Acceleration doesn't happen without force and force = mass × acceleration, so when acceleration goes up 400%, so does force. When that force is higher than parts are capable of, the parts exit crankcase left.
 
#26
I would say stress/rpm. things start to move. some people would say if you blow the jug off, your building power. but really at that point things start to expand and flex.
you might think you have good clearance. for example piston to head clearance with a .010 head gasket. start run it real hard at high rpm's.
crank,rod....etc. starts to expand/flex and bang. your all done.....
Years before finding the OMB forum for info, I was having a ball off-road pounding the stock
Hisan engine on a coleman ct200. One day after several WOT launches I had stopped to pee,
leaving it idle....that's when the rod decided to try to exit crankcase left!.. It was done. Since
I was in the boondocks I started screaming cuss words at it for a good while, scared the dog,
bought a Honda.
 
#27
I would say mechanical forces, but a common way they are increased is by raising RPM, as in Ruins Peoples' Motors. The first step many take is to remove the governor for $0. Then that's not enough, so they might get some $12 valve springs. Then they're spending $120 on a new Predator because RPM ruined their motor.

There are a lot of reciprocating parts in a piston engine that have to change direction very quickly. The acceleration factor increases to the square of velocity, so going from 4000rpm to 8000rpm (a velocity factor of 2) means acceleration goes up 2² = 4 times. Acceleration doesn't happen without force and force = mass × acceleration, so when acceleration goes up 400%, so does force. When that force is higher than parts are capable of, the parts exit crankcase left.
I would say mechanical forces, but a common way they are increased is by raising RPM, as in Ruins Peoples' Motors. The first step many take is to remove the governor for $0. Then that's not enough, so they might get some $12 valve springs. Then they're spending $120 on a new Predator because RPM ruined their motor.

There are a lot of reciprocating parts in a piston engine that have to change direction very quickly. The acceleration factor increases to the square of velocity, so going from 4000rpm to 8000rpm (a velocity factor of 2) means acceleration goes up 2² = 4 times. Acceleration doesn't happen without force and force = mass × acceleration, so when acceleration goes up 400%, so does force. When that force is higher than parts are capable of, the parts exit crankcase left.
Thanks 65SC, the math behind a piston engine is serious food for thought, I've been digesting it this morning(pun intended)
I laughed out loud at what RPM stands for!
 

65ShelbyClone

Well-Known Member
#30
Grinding advance into the profile moves the power curve lower by roughly 100rpm per degree. Retarding it would do the opposite. The regular Dyno 275 is a 9000rpm+ profile.

Wouldn't their Tillotson 212R motor $200 out-do $100 spent on a Predator?
Not as a platform for a no-rules build IMO. I think gathering and installing the parts on any other clone would get more bang for the buck. The Tillotson engines are designed primarily for very limited kart classes.
 
#34
I would say stress/rpm. things start to move. some people would say if you blow the jug off, your building power. but really at that point things start to expand and flex.
you might think you have good clearance. for example piston to head clearance with a .010 head gasket. start run it real hard at high rpm's.
crank,rod....etc. starts to expand/flex and bang. your all done.....
so what does the .010 head gasket dooo???
 

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