disc vs drum

[EatMyShortsRacing]

For those who didnt go into the CMB 80 needle thread the topic sorta strayed to induction issues with the CMB especially relating to disc clearances. Now the way I was taught was that the disc engines SHOULD be better for lots of reasons.

Firstly the area inside the drum is added to the crankcase volume meaning the amount of vacuum (fuel/air draw) and pressure (fuel into cylinder) generated in the crankcase is decreased. This is supposed to mean a performance reduction.

Secondly I was under the impression you could get more duration out of a disc induction engine than a drum induction engine. (this could be wrong...)

Thirdly the drum would produce lots more drag (unless bearing supported and then this would also affect the integrity of the crankcase seal)

So I would be assuming that the recent switch to drum induction motors must be for one of two reasons. Either the drum seal is better than the disc significantly to offset the loss of pressure due to bigger effective crankcase volume - or the reliability of the drum is desired. This I am thinking from the older Picco disc days where shearing the disc pin meant goodbye engine.... or are they just easier to make???

 

[Roy Munoz]

You probely can get better performance from a disc rotor,but like you stated the Drum is more reliable..No pin shearing problems or the pin backing out during a race.. The drum can be open up to give you more duration too..The only disk that I know that is as good is a Zimmerman once you done some clearience work on them..Just my 2 cense

 

[AndyBrown]

I elected the drum on the MAC engine for several reasons, but the Major reason was to make the engines as maintinence free as possible.

CMB has never built a disc system that works well right out of the box, so that was out of the question.

Check Dub Jett's (World Champion Speed plane, Pylon racer, engine builder.) web site. He explores the crankcase volume issue on his site, while pointing out that the crrent and fastest Pylon racing engines are all crank induction (big volume) and the disc inductions are only a faint memory.

 

[Tim_Duggan]

Cool thread Craig!

Firstly the area inside the drum is added to the crankcase volume meaning the amount of vacuum (fuel/air draw) and pressure (fuel into cylinder) generated in the crankcase is decreased.  This is supposed to mean a performance reduction.

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Personally - I think case volume is over-rated on a piped engine. More relevant - The directional changes the intake charge has to do on a drum engine can be more complicated. Disc engines have a "straight shot" into the case.

Secondly I was under the impression you could get more duration out of a disc induction engine than a drum induction engine.  (this could be wrong...)

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Not duration - but port area.

Thirdly the drum would produce lots more drag (unless bearing supported and then this would also affect the integrity of the crankcase seal)

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Yes - a drum rotor has a lot more parasitic drag. The Kalistratov housings are ballraced to reduce drag, but there is still a sealing face there to induce some drag.

What I like about the drum rotor housing is the reliability. (That and the Mac / K engines use the "reversed drum" to place the carb up higher. )

Ranjit told me that he prefers a drum on high nitro engines, but on FAI fuel the disc is his preferred weapon.

Tim.

 

[EatMyShortsRacing]

I elected the drum on the MAC engine for several reasons, but the Major reason was to make the engines as maintinence free as possible.
CMB has never built a disc system that works well right out of the box, so that was out of the question.

Check Dub Jett's (World Champion Speed plane, Pylon racer, engine builder.) web site.  He explores the crankcase volume issue on his site, while pointing out that the crrent and fastest Pylon racing engines are all crank induction (big volume) and the disc inductions are only a faint memory.

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Well 2 points come to mind...... crank induction adds zero extra load to an engine. Disc and drum must add some load. Secondly putting the carb behind accelerated airflow = supercharging to a degree....

 

[EatMyShortsRacing]

Ranjit told me that he prefers a drum on high nitro engines, but on FAI fuel the disc is his preferred weapon.
Tim.

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Going out on a limb here.... I had a ranjit disc induction for a Rossi 45 a while ago. I would imagine the disc would eventually fail because of stresses on the disc pin. I am wondering whether the inherent rough idling of nitro motors would cause additional stresses????

With the later CMB floating disc situation I would have thought the clearance issue would not be that critical so long as the disc sealed on either face. The only issue I can see is more clearance means the disc may flutter more and eventually fatigue...

Hmmm does that mean its time to look at my A90 induction closely????

 

[Tim_Duggan]

I am wondering whether the inherent rough idling of nitro motors would cause additional stresses????

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Not just at idle B)

 

[EatMyShortsRacing]

I am wondering whether the inherent rough idling of nitro motors would cause additional stresses????

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Not just at idle B)

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That raises another interesting question.... does more nitro make motors run rougher at the top end????

Oh and if Nitrocrazed is watching this thread - have you started on a drum induction for the A100 yet??? LOL

 

[EatMyShortsRacing]

I elected the drum on the MAC engine for several reasons, but the Major reason was to make the engines as maintinence free as possible.
CMB has never built a disc system that works well right out of the box, so that was out of the question.

Check Dub Jett's (World Champion Speed plane, Pylon racer, engine builder.) web site.  He explores the crankcase volume issue on his site, while pointing out that the crrent and fastest Pylon racing engines are all crank induction (big volume) and the disc inductions are only a faint memory.

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Andy,

I couldnt find anything on the jett page relating to crank vs other induction.

got a link or any other useful discussions on that topic?

 

[Ian Inverarity]

That raises another interesting question.... does more nitro make motors run rougher at the top end???? 

Oh and if Nitrocrazed is watching this thread - have you started on a drum induction for the A100 yet???  LOL

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Craig,

I'm here. B) Nope, no drum induction for the A100. The A motors dont have a true zimmerman disc as it doesnt float, and seals on one face only like the OPS/Picco style drums. However it is large diameter like a zimmerman disc. The advantages of the A motor setup is the disc is aluminium and very light so the out of balance mass is much less than a zimmerman disc, it is not adjustable so the user cant stuff it up, and the disc only goes on one way, again so the user cant get it wrong! But if it wears, it isnt adjustable to compensate, and if you dont do that centre nut up properly it shears the drive pin. In an A100 crank you cant get the broken pin out like you can with the A-90.

A zimmerman style disc has a time-area advantage over drum induction or the smaller discs like OPS/Picco EXR style. That is the inlet port is fully open (un-obstructed by the disc) for a longer time for the same open angle.

The disc engines do have a nice straight shot into the case, I can see the conrod thru the open carb of my 101!

Ian.

 

[EatMyShortsRacing]

Ahhh yes you can adjust the disc clearance on the sealing face on the A engines.....

There is a washer that spaces the Disc out from the crankcase. just grind it down a little......

and before you ask yes the twin is back on the project bench!!!! still a few months away though.

 

[Ian Inverarity]

Craig,

Grinding that washer thinner will reduce the clearance, but surface grinding is generally not a user adjustment!! :blink: That said, the oldest A-90's I had, even after I sold them on, showed no significant wear on the disc.

Hmmm, twins! Going to Finley?

Ian.

 

[EatMyShortsRacing]

Craig,
Grinding that washer thinner will reduce the clearance, but surface grinding is generally not a user adjustment!!  :blink: That said, the oldest A-90's I had, even after I sold them on, showed no significant wear on the disc.

Hmmm, twins! Going to Finley?

Ian.

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yes going to finley.... better bring my 100 with you

Twin wont be ready for that race though... think I will only have about 4 boats with me.

 

[Ian Inverarity]

Craig,

I will have to put that engine back together, I cant remmember if I pinched a rod and piston out of it tho! :unsure:

I am cutting back, just one boat! X hydro.... B)

Nitrocrazed racing: saving fuel....

 

[Tim_Duggan]

That raises another interesting question.... does more nitro make motors run rougher at the top end???? 

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Nitro + compression ratio's.

There is also the load factor to consider at the engine - air prop is very different to marine application - particularly surface drive.

 

[EatMyShortsRacing]

I cant remmember if I pinched a rod and piston out of it tho!  :unsure:

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Again???? LOL