More Mac 45 Carb options?

[DaveMarles]

Ian, There is no way you can compare engines in the way that you say. For example the induction port on the A90 is relatively small and with the same timings the time area is much less than the K90. The short stroke of the A90 means that it will have better torque in the lower rpm range too. Short stroke on a small two stroke engine does not mean high power at high rpm. This is the opposite to 4 strokes.

For example: If you take a Picco 90 (close to 1:1 bore stroke ratio)and make all the port timings exactly the same as a CMB 90 ( short stroke) then the P 90 will want to rev but have less low end power.

Also. If you use higher transfer timings ( lower blowdown ) on the A90 it will be less peaky than the K90.

Dave

 

[Ian Inverarity]

Dave,

Actually I would argue the time area point. I make the port in my A-90's larger, but also while the K-90 drum opening looks huge, when you measure it at the angle at which the flow goes thru it, it is not as big as you may think. Also the drum is much smaller than the diameter of the A-90 disc, thus for the same timings the disc is fully open longer than the drum.

However I do think that the carburation problems I am having are related to the drum induction somehow. But Andy may be right in saying the problem relates to the transfer timings.

Are you suggesting to function well the K-90 should have lower timings than what I am currently using due to its longer straok and smaller bore?

The latest A-90 liners that I have seen have less blow-down than earlier ones.

Ian

 

[DaveMarles]

Ian,

Guess I should said the A90 probably has less time area than the K90. I have not looked at a K 90 for a while but the A90 sure has a small induction port for a .90.

In general terms if you want the K90 to perform the same as a motor with a shorter stroke then the K90 would need slightly lower timings.

Dave ( still a newbie)

 

[Anders_Martinelle]

The short stroke of the A90 means that it will have better torque in the lower rpm range too. Short stroke on a small two stroke engine does not mean high power at high rpm. This is the opposite to 4 strokes.

Says who??

I don`t agree with you here Dave..

What do you mean with "lower" rpm??

Anders

 

[DaveMarles]

Anders,

I don't just say it, its a fact. Oversquare in a two stroke has a port size disadvantage. On a four stroke, a short stroke or oversquare is better for high rpm power because valves can be bigger, piston speed is lower etc etc.

If short stroke was better for high rpm on a two stroke then all 3.5cc car engines would be short stoke and they are not. Some are called short stroke but actually they are not.

Dave

 

[Anders_Martinelle]

Oversquare in a two stroke has a port size disadvantage

I buy that, but a short stroke engine 2 or 4 stroke will always gain rpm if it`s short stroke.(piston speed)

My guess why you see most 2 stroke engines "close to square" is that it`s the most efective configuration.......

Play with your mind that a 90 engine had 10mm stroke. not good....

Think it had 40mm and 20mm dia...... not good either.

So that`s answer you question about 3,5cc engines also.

They want them to be "short stroke" but they will loose more then they will gain..

Simple as that or????

Anders

 

[Terry Keeley]

Andy,
Thanks for the tips!

But still I have more questions on this subject. :blink: I run A-90's as well and do not have the carburation problems that I get with the K-90. I run the same fuel (25%), same plugs (Mc8's or 59's), same pipes, carbs with same bore (12mm), same head bowl volume, shape and clearance and actually I use higher transfer timings on the A-90's. The A-90's have a larger bore, shorter stroke, same rod length, but have disc induction. Is there something unique or different in the porting or induction system of the K-90 that means it doesnt like low nitro and high timings? :huh:

Ian.

Could it have something to do with the fact that the drum motor should have a lower base (crankcase) C/R? When the drum closes the entire internal volume of it is still open to the case, this isn't true with the disk. May not make a difference at high power settings when the pipe is working well but might effect idle and part throttle operation?

 

[DaveMarles]

Anders, piston speed is a limiting factor for a bigger two stroke with rings but for us its not.

The advantage in the longer stroke for us is the gain in effective port areas.

There is also a disadvantage with oversquare in that the piston is heavier and for us any excess piston weight is a big disadvantage.

Dave Newbie

 

[AndyBrown]

The Transfer passages in the A-90 are very much smaller than the K-90 an here lies the major reason you can and need to run high transfer timing in the A-90.

It will not matter if the intake valve has a diameter of 1 inch or 100 ft. If the timing is the same they will both be open for the same amount of time.

I have not taken exact measurements but from visual inspection I would say the K-90 drum has more area for flow than the A-90 disc port. Yes that disc is big, but the window is very small.

 

[TimD]

Really interesting for a side tracked thread :lol: :lol: :lol: Sorry Scott but I think we are having too much fun with this B)

The intake window in the housing and the disk are small on the standard A90HP but Ian's motors have seen a lot of dremel work in this area and he does open them up a lot!

I have always been of the opinion that the case volume has always been an over rated aspect of a piped engine, but what Terry has mentioned sounds like it could be feasible....opinions please?

Andy,

Do you approach your timing numbers differently for the MAC84 to the MAC67 based on the bore/stroke ratio scenerio?

Tim.

 

[Scott Schneider]

I would say you are having a good off topic talk.

But Like I said, Only but a few are keen to what your talking about. Start a specific topic on these engines you might get further faster. Scott

PS. 832 hits on this thread? I think others are more interested in finding out what carb tricks they think we are talking about. But NOT !!!

 

[TimD]

OK Scott,

Here you go! http://www.intlwaters.com/index.php?showtopic=3934

Guys - please continue the 90 engine debate on that thread rather than this one!