rsieminski

Would you really see a noticeable difference? I had a discussion with another flyer, who claimed 1 to 2K lower rpm, but more torque from lowering the sleeve a few thousanths, and the opposite from raising it by the same amt. Anyone have any ideas as to what this sm mod would really do?
Thanks,
--Rick

yard-dart

My Feedback: (35)

The only thing you're achieving by raising or lowering the sleeve is a change in compression ratio. You dont have to raise the sleeve in order to decrease the ratio, you only have to add a head shim or two. As for lowering the sleeve, you would be increasing the compression ratio, but how exactly do you lower the sleeve. Is this done by machining the top lip of the sleeve? For all who read this, am I right or wrong on this one?

John

Hobbsy

My Feedback: (102)

At first blush that sounds logical, but then after thinking it over, the lowered ports would reduce cylinder filling and exhaust flow. If the port timing were on wild side and the engine set up for high rpm then lowering the ports would increase torque if a larger prop were used to lower the rpm and allow more cylinder filling time.

ChuckAuger

My Feedback: (12)

Raising or lowering the sleeve changes the timing of the ports.

SamD

Generally speaking: Lowering the cylinder increases low-end torque and raising the cylinder increases high rpm power. This is a standard mod for two-stroke motorcycles; they differ somewhat since the entire cylinder is removable from the crankcase. If you choose to do this, keep in mind piston clearance and clearance between the crankshaft and bottom of the cylinder.

Sam

yard-dart

My Feedback: (35)

But how do you physically raise or lower the sleeve?

ChuckAuger

My Feedback: (12)

Raise it with a sleeve shim, lower it with a lathe.

SamD

Chuck,

Are there commercially available shims or is this strictly a DIY item? If they are commercially available, a person choosing to do a little lathe work may want to do it in increments of the shim thickness (or close to it).

ChuckAuger

My Feedback: (12)

I have some commercially made ones for Rossi engines, and I have made them from shim stock.. I'm not very scientific in my approach, I stick one in and see if it runs better or worse. I've never tried to lower a sleeve.

SamD

Chuck,

I hear ya'...

DMyer

My Feedback: (2)

Keep in mind that when you raise or lower the sleeve.... you must also change the head shims to keep the head clearence(compression) the same prior to test runing. Otherwise you have changed both port timing and compression and you will not know which change caused any increase/decrease in RPM/Torque. Good luck!

ChuckAuger

My Feedback: (12)

Good point, DMyer...I usually just take a head shim out (if I have one in to start with) since I figure the compression will go down when I raise the sleeve. Then if it seems like it needs one, I put one back. Like I said...I'm not very scientific in my approach.

Flypaper 2

Lowering the sleeve also shortens the intake port timing.Seems like you gain on one and loose on the other without some butchering.

maxtenet

Raising or lowering the sleeve changes the timing of all the ports and is not really very useful. Changing individual ports timing with a dremel is far more productive in terms of getting performance. The only drawback is that if you screw up you have to buy a new sleeve.

Max

ChuckAuger

My Feedback: (12)

Yeah, it's not a world changing mod, it's just something I try when I've tuned the pipe to the max and can't get anything else. The best gain I got from raising the sleeve was when I'd already increased the exhaust duration to as far as I felt practical...slapped a shim in there and got a few hundred more R's. More of just a tinkering tool than something that's gonna put you out ahead of the world.

Do a little search in the pylon forums..those guys routinely change their sleeve timing to tune for different props, atmosphere, etc. They are running close to maxed out, so just a little difference here and there helps them set their engines for race conditions.

SamD

Oh Boy.... If you're gonna' start grinding on the ports and such be certain you know what you're up to... I'd be hard pressed to recommend this sort of thing to a person that's never done it before.

Apart from slipping with the grinder and taking a monkey bite out of the cylinder, if you don't properly chamfer the ports (and you're dealing with a ringed engine) you run the risk of catching a ring. Or, you can chip the plating off a plated cylinder... Or..... Lots of things to go wrong.

Chuck's comments about pylon racers are right on; this isn't the sort of thing you do to gain 5k on the tach; it's much more subtle and more about refining the power characteristics of an engine. If you take a look at the current state of technology with regard to two stroke motorcycles, you'll see variable port timing as a standard feature- one that's been around for over ten years. It's all about power characteristics.....

maxtenet

Exactly my point! Experiment with tuned pipes and fuel. Changing the timing is best left to the pros.

Max

DMyer

My Feedback: (2)

You are right about the Pylon racers.... They(I am one) typically will set the deck height for the sleeve (difference between TDC of piston to top of the liner) to the factory value (.192 I think for a Nelson) and leave it. This measurement is rarely altered. What is adjusted often however, is the compression by adding/removing shims under the head to compensate for temperature/air density differences. That can make 500 or more RPM difference in a hurry on a good racing engine when the air sucks.... or... the difference between winning and losing.

The diffference in these engines is that racing engines only have one speed so it is easy(known) where to optimize the sleeve height and timing at Henry's skunk werks factory. Most other engines the timing is a compromise between good idle, midrange and WFO power. So you must also consider these factors when deciding if it is worth fooling with the port timing. Good luck... the world loves tinkerers.