thoughts on engine combo to make

[atvcrazy]

I have too many bikes and not enough time the past couple winters to service them to make sure they run when I want them to.  I have been repairing them the past couple weekends (spliting cases, do to blowups, tranny breakage, etc) and am finding in a couple of motors that the main bearings are loose in the case as is a few transmission bearings so I guess it's time for a set or two of ESR cases.

If this is the case, might as well build a complete motor and want some opinions since there's several options out there.  This motor will be purely a dune motor, and I want to concentrate on what kind of power it makes but want to focus more on what kind of torque it produces along with what kind of power it makes under the power curve.  I want something that makes it's power down lower in the curve like a CR 500 would with a good broad power curve.  mid to upper 60's for HP and 50+ ft lbs of torque.

I don't want to balance the crank, so I'm thinking no more than 80 mm on the bore, not apposed to a 4 mil stroke but would rather stay with standard stroke since they don't vibrate as much.  so I guess the options are:

standard stroke Sphinx
4 mil sphinx
Standard stroke Puma
4 mil Puma
something from ESR either standard stroke or 4 mil stroke

If I could get to my goals torque wise with a standard stroke, I would go this route.

I've had a couple puma's and I just really don't know about them for a real torque dune motor.  It would be the easiest to set up, I guess with the right pipe (I would go with an LED pipe) I could get there but I want a fun play motor that you can chugg down low, grab a hand full of throttle have it pick up and go.

I'm also a bit concerned with the larger ESR cylinders (past 78 mm) that water jacket over the exhaust port gets too thin for my liking.

give me your thoughts

[seanoktm]

Getting 50 ft lbs of torque will be tuff. Only puma's I have seen make 50+ ft lbs of torque are alky drag motors. With the motor options your looking at on gas I think you will be more around mid or low 40's on torque. You will be able to get over 60hp pretty easily with these setup's.

[hontrx265r]

Im looking to do similiar. Thinking sphynx 78mmx76mm with ktm rod. But I was going to do the rod/weld/balance. For reliability.  There are so many choices now its hard to decide when it comes to that much money on a motor.

[rsss396]

50lbs of torque is a tough number to reach in a smaller gas motor. You really should go with the +4 stroke and have it balanced if vibration is a concern. The 2 sphynx hotrod +4 crank builds I did, had no complaints of vibration being a problem. But I am sure it is more than a stock trx small bore. I dont believe the +4 stroke is causing the increase in vibration as much as the heavier piston of the big bore, so by going with a very large bore and stock stroke does not mean it will vibrate less than a stroked crank and a smaller bore. The closer the engine is to have the bore and stroke sizes the same normally the more efficient it is.

[etccb]

I dont believe the +4 stroke is causing the increase in vibration as much as the heavier piston of the big bore, so by going with a very large bore and stock stroke does not mean it will vibrate less than a stroked crank and a smaller bore. The closer the engine is to have the bore and stroke sizes the same normally the more efficient it is.

That is what I was thinking as well.

[Rupp250]

No complaints here with my +4 Sphinx with vibes