Crankcase transfer port opening

[tlanders]

In the three engines I have rebuilt over the last two years, a 1975 175cc, a 1975 250cc and a 1977 365cc, when you turn the cylinder upside down and look at the transfer port holes on the bottom on both sides of the cylinder, it is a large opening ( 3" or 4" long by 1.5" to 2" wide(from memory)) with a web in the middle that gradually and smoothly necks down to 2 fairly small rectangular passages that then turns 90 degrees to enter and make a hole in the side of the cylinder wall. I am rebuilding my 1978 250cc now. The bottom of the cylinder looks completely different. Instead of a large 3" or 4" long hole on each side, it has two small rectangular holes on each side which are not much bigger than their eventual size in the cylinder wall. It would seem to me that the big hole would work a lot better since it is the same size as the top of the crankcase opening and does not have an abrupt size change as does the transition from the crankcase to the two small holes. What gives? Why would KTM move towards something worse? Was this a manufacturing cost reduction move? Do I take out my grinder and remove an incredible amount of metal to make it like the 1977 cylinder? How about just radiusing the smaller holes, right now they are an abrupt 90 degree corner entrance, no good for flow, lots of pressure drop and flow resistance. Would the number on the cylinder help solve this puyzzle? Does anyone know what I'm talking about?? HELP!!!!

Teddy

 

[john durrill]

Teddy,
 can you post a picture of the bottom of the cylinder? That would be the best way for us to help i think. need to see what the base gasket area looks like on the cylinder.
John D.

 

[tlanders]

I just talked to Mike Lenz and he confirmed that that is what they did starting in 1978. He says that he couldn't figure out why they did it either. I try to take a picture tonight and post it. I'll have to tear apart another engine if you want me to show the difference.

Teddy

 

[john durrill]

dont tear one down Teddy. just take a pic of the cylinder your asking about. I think you could blend the edges on the bottom of the transfers. its a trade off. you increase the crankcase volume when you take any metal off.
 but you gain flow buy doing the blending.
 i just want to be sure what we are talking about before i offer any ideas.
John

 

[hrbay]

Hi Ted, was the engine a torqer or a revver? The 78' 400gs was a brute down low and did not really need to be twisted too hard. I know the 250 has a different power band but KTM might have been trying to get a similiar flow in the scavenging with the siamesed  non-tapered ports. The abrupt 90 deg is not good but you may be able to roughen( best to sand blast w/ low pressure) this area and contour the abrupt angle with a suitable high temp. epoxy. Be sure to de-grease(grain alcohol works good) and roughen the aluminum for good bonding. I have used Hysol 934na to do this and it has worked so far. The epoxy  temp range is 350-400? degrees F. Good luck. GC

GC

[Mike Lenz]

I looked into this when I got home today. They did it to the 250 transfer ports in 77 and on. Never did it to the 400 transfer ports. I have an 80 250 cylinder and they were still doing it. Could it have to do with air flow speed by having the bottom of the inlet of the port about the same size as the outlet at the cyl? Im sure they meant to do this, not a money issue. Are any new bikes designed this way?

 

[Mike Lenz]

BY the way Teddy you were right again. The later ft backing plate has a larger axle hole. I checked my 74 400 and I used the old backing plate and just drilled the brake arm hole out for the new style cam shaft.

 

[Dave H.]

I think the reason for 2 port-ways in place  of the original one, is to increase the fuel/air charge velocity, which occurs because you have aproximately the same volume as before, now flowing through smaller pipelines...the Bernouli effect (which I am not qualified to adequately explain, but like my physics professor always told us..."it's in the book...look it up"

 

[tlanders]

John, I just uploaded a picture of the 1978 MC5 250cc jug ports to the http://groups.yahoo.com/group/PentonRestorationandTechGroup/ site.

Dave, We don't know what the designers were trying to do. Did they want more charge velocity so that it mixed better or was distributed to a different part of the combustion chamber or were they wanting to get more flow, a bigger charge, into the combustion chamber. I would like to think that getting a bigger charge would be the objective, and therefore, increasing flow would be the goal. The design looks to me to be reducing the flow. Just like electricity, the amount of flow is simply a function of the amount of driving potential divided by the amount of resistance to the flow. In electrical terms I=V/R or current flow equals voltage divided by the resistance. In gas terms, gas flow equals pressure potential divided by the resistance to flow. The pressure potential is the difference in the pressure in the crankcase minus the pressure inside the cylinder. The resistance to flow is determined by the size, length and layout of the passageways for the gas to travel from the crankcase to the cylinder. Assuming the pressure potential stays the same given the rest of the characteristics of the engine remain the same such as the expansion chamber design, port size and location, inlet (air cleaner, carb & manifold) restrictions, etc., the amount of flow of air/gas and therefore the size of the charge that gets transfered from the crankcase to the cylinder is then solely a function of the amount of the resistance of the passageways. Given this, you would want to taper the center bar between the entrance to the transfer ports to a knife edge and open them up to match the hole in the top of the crankcase. However, John brought up the point that removing metal will increase crankcase volumn and that will reduce the crankcase pressure (in essence destuffing the engine). So I guess that if I remove some metal, it is a trade off between losing crankcase pressure verses reducing resistance. If I lose more resistance than pressure, I should do it. We can hope that the KTM engineers did lots of dyno testing and what we have is the best compromise. But then why didn't they do it for the 400, 175, 125? Was the 1978 250cc KTM the strongest engine out there, or did the other manufacturers have an edge? Anyone have a dyno they want to loan me or I'll send you a couple engines to test for free (ha, ha)? It sure would be neat to know what the engineers were thinking!!!!!

Sorry for the long post, I don't know why this stuff fascinates me so much. I probably should have read my 2 cycle engine design book again before taking off on this tirade.

Teddy

 

[rd400pi]

POGers,  Do these bikes have reed valves?  would/could this have any influence?  I'm just makin' stuff up now.

  Mike Husted (rd400pi)
  72/74 Six Day & 76 250 MC5

 

[Larry Perkins]

Mike, Penton never came stock with a reed and KTM dodged it for longer than most.

Ted, you make my head hurt.  That was way too much physics and smelled almost like algebra.  They did it to try and make it faster.  Put it together and go ride it.  You know what they say about curiosity.  Seriously, I am glad there are curious minds out there.  It helps to balance my look on Geology.  Rocks are for throwing.  If there is water around-skipping.

 

[AndyL]

Teddy,
Dont forget the impact on velocity.  Assuming same volume moves into the cylinder over the same time period, a smaller opening would require the a/f mixture to move faster, opening all kinds of variables on the combustion process.   Such as turbulencse, mixing, flame front propogation which impacts the power stroke.

I'm with you, though.  Why would they have done it only on one model?  You have to assume that if it was succesful, it would show up across the model line the next year.

One other possiblity;  remember staff changes in the design/build group could also affect the direction cylinder design went.

Man, this is great stuff!


Andy

[tlanders]

I called Barry Higgins. He said that KTM was trying to make the 250 have better low end torque with the change in the transfer port design. He also said that it was not the greatest success and that a lot of guys would blue print the engine and radius those transfer ports out to the size of the crankcase opening and grind the center web to a knife edge and they did get better performance!!!!! I THOUGHT SO!!!!! He also said that by the late 1970s, because they were producing so many engines, they were not taking as much time with each one and doing less hand matching of components. Let's see, which burr shall I use?

Teddy

 

[Mark Annan]

Teddy,

Without getting into a long winded and complicated explanation of airflow and port timing in an engine I'll try and shed a little light on the situation.  Most likely the transfer passages and ports on the later engine were divided and made smaller to increase the VELOCITY of the charge moving through the port (in short, higher velocity is a good thing).  Most likely the bottom of the port was left with an abrupt transition (edge) due to manufacturing considerations (aka. cost).  You can feel relatively safe, and make a positive improvement in airflow by gently blending the transition into the bottom of the port.  Keep the work in the first 3/8" to 1/2" of the port.  All you really need to do is radius the parts of the port that have a square edge hanging out into space.

You can make some templates out of paper (card stock works well) to help you visualize where the work needs to be done.  Make one template of the bottom of the cylinder area, make the other of the top of the crankcase.  A base gasket can be used but they may not have material everywhere it is needed to get a good picture.  Stack the templates on top of each other to see what you have.  Not having all the pieces in front of me to look at I would guess that the outer side of the port/crankcase juncture is fairly close to being aligned.  The inner side just hangs out in space (nothing below it in the crank case).  The front and rear portions of the port probably have a fairly large gap until they mate up with the case.  If this is the situation try to match the outer walls together for a smooth transition.  Radius the other areas.

Mark

 

[tlanders]

Thanks Mark for your comments. For the last two engines I simply put blueing on the crankcase, dropped the cylinder on it, then took the cylinder off and ground out the cylinder to the blue outline that was transferred to it.

The velocity of the charge entering the cylinder in units of in/sec for example, is simply the flow rate of the air/gas, in3/sec, divided by the port area in the cylinder wall, in2. Adding more resistance upstream of the cylinder ports reduces the flow rate and therefore the velocity also. Making the port smaller for a given flow rate will increase velocity through the port, but the goal is more flow, bigger charge!!! Also, reducing the area of the channels leading to the port means that the average velocity through the channels will be higher and therefore there will be more pressure dropped in the channels which will again reduce the total flow. The best would be a large channel that reduced down smoothly just before the port in the wall. Imagine the difference of the flow and velocity of water coming out of 1/2" diameter nozzel at the end of a 100 foot long 1/2" diameter hose with that of water coming out of a 1 foot long 1" diameter hose with the same 1/2" diameter nozzel at it's end.

Unfortunately, I will be gone on work to New York for the next week and will not be able to post to the POG site. Talk to you when I get back.

Teddy