Case Volume

[Jerry Wyss]

Well Marty asked about your thoughts on "Rotor/Crank Timing", so how about case volume.

In what areas is "more" better and areas where "less" would be more desirable?

 

[Kim J]

Several years ago on a airplane sport front induction engine this was tested.

A tube was glued in the rear cover - 10 inches long. The center of the cover was machined off,

and a piston with a rod inserted.

With this setup one could change the volum of the case while the engine running.

The max RPM only dropped about 400 when the piston was moved from min volume - as stock - to max volume.

Remember this was on an engine with moderate portduration and no tuned pipe, yet the drop in RPMs was unexpected small.

Hope this helps.

Kim J

 

[NITROSNIFER]

From much reading I came to the understanding that smaller case volume increased the pumping effect of the engine more efficiently moving the fuel in the case to the ports. This made for better low end power. If the same sized piston/stroke with a larger case volume the piston could not move the fuel/air as efficiently at low rpm as it did at higher rpm. So the larger cased engine showed it's performance/power at higher rpm than the small case engine.

 

[Andy Brown]

I remember that test. It goes Waaaay back. I think it appeared in Model Airplane News around 1971. Harry Higley? did the test and wrote the article.

One other point about that test is that it was a control line engine running on the bench at steady state rpm before the plunger was moved in and out while the engine was running.

That test along with an accidental finding on my own led me to design and produce the flow through backplate for crank induction .21's called the "Powerplate" in 1984. Bench testing on a K&B 3.5 showed a 500 rpm increase @ 26,000 and even a 100 rpm increase @ 12,000 rpm which I did not expect to see, because the "guru's" of the day told me it wouldn't work and that I'd loose all of my bottom end torque. In the boat it launched the same as always and you could actually feel the extra punch out of the corner when doing back to back tests. It also required the needle to be leaned a couple of clicks when doing the back to back tests, which proved to me that induction flow was improved and therefore fuel draw was improved.

If the passage volume in the "Powerplate" was made too large there was a noticeable loss in bottom end.

In the crankcase, the Correct transfer passage volume is important. Too small or too big and power suffers.

That's the area to work on first and then adjust the other areas accordingly, looking at gas flow first and foremost and worry about packing volume last. Improved gas flow will go a long way toward making up bottom torque that is lost by removing a little metal here and there.

 

[Kim J]

I remember that test. It goes Waaaay back. I think it appeared in Model Airplane News around 1971. Harry Higley? did the test and wrote the article.

One other point about that test is that it was a control line engine running on the bench at steady state rpm before the plunger was moved in and out while the engine was running.

Good memory Andy - but I think John Aldrich - spelling ? - wrote and performed the test.

 

[dwilfong]

I remember that test. It goes Waaaay back. I think it appeared in Model Airplane News around 1971. Harry Higley? did the test and wrote the article.

One other point about that test is that it was a control line engine running on the bench at steady state rpm before the plunger was moved in and out while the engine was running.

That test along with an accidental finding on my own led me to design and produce the flow through backplate for crank induction .21's called the "Powerplate" in 1984. Bench testing on a K&B 3.5 showed a 500 rpm increase @ 26,000 and even a 100 rpm increase @ 12,000 rpm which I did not expect to see, because the "guru's" of the day told me it wouldn't work and that I'd loose all of my bottom end torque. In the boat it launched the same as always and you could actually feel the extra punch out of the corner when doing back to back tests. It also required the needle to be leaned a couple of clicks when doing the back to back tests, which proved to me that induction flow was improved and therefore fuel draw was improved.

If the passage volume in the "Powerplate" was made too large there was a noticeable loss in bottom end.

In the crankcase, the Correct transfer passage volume is important. Too small or too big and power suffers.

That's the area to work on first and then adjust the other areas accordingly, looking at gas flow first and foremost and worry about packing volume last. Improved gas flow will go a long way toward making up bottom torque that is lost by removing a little metal here and there.

Andy

What determines the best transfer passage volume?

This is a picture of what I do to reduce the tras and boost passages in my RS engines and unshroud the intake window. the pic is of my 45 I also do the same to my 91.

David

 

[tnrcboatracer]

Andy, What is your opinion of the late model MAC67 motors that were made on the common 67/84 case? The late model 67's seemed to lack bottom end torque in the 1/8 scale applications. The volume (depth) of the coom case's transfer and boost runners are larger in the 67 motor due to the smaller sleeve OD. One solutiion that worked to improve the torque was to fill the runners reducing their volume. Is there a way to calculate/estimate the optimum runner volume/velocity?

 

[Andy Brown]

Kim, I'm sure it was Harry B. Higley. Just wasn't sure about the spelling. Harry was big into the control line Carrier events. George Aldrich was a premier engine guy from the same period and may have mentioned the test in some publication, but I do know that Harry and his son conducted the test.

David,

Port velocity! Somewhere just under sonic is where maximum torque resides. Obviously there is no one "optimum" volume for all applications. It all depends on the desired rpm where the engine will work.

The type of fuel will have an effect too. Full size engine builders now use "wet" flow benches to find optimum port volumes. The type of fuel will also determine the required volume. Gas, Methanol and nitro will all required different volumes.

Jon,

There are two different 67 cases and the 84 has it's own case. The original MAC 67, had smaller transfer passages. The next 67 version had large transfers which was faster in the rigger. And yes I've gotten some reports from the scale racers that they prefer the original case. The MAC 84 has even bigger transfers. The internal mold of the 84 case is different than the internal mold of the 67. Only the outside case mold is common between the 67 & 84.

Runner volume/velocity can be calculated/estimated, but all goes out the window when you add liquid fuel to the dry air. When running high nitro nearly half of the mass flowing through the ports is liquid.

Full size engine builders rely on their flow benches....and of course the Track is the real acid test.

 

[Jorgen Andersson]

Andy

I think that Aldrich also was a stunt guru, and the designer of the absolute classic CL plane Nobler.

A lot of know-how comes from the CL world, many of us started with CL way back.

I used Epoxi Patch in one Valvola engine and made the case volume as small as possible.

Torque was clearly improved, but top end rpm was slightly less, not as willing to rev.

I had better results when I filled the booster and transfer ports in the case with Epoxi Patch, and then used the Dremel to make more narrow channels. The idea was to get the velocity speed up, and this worked well with better torque and unaffected top rpm.

The channels were narrow in the center, and wider in both ends.

Jorgen

 

[tnrcboatracer]

Thanks for the reply And, I wasn't aware of the different internal cores, I'll have to look at inside of my cases more closely.

 

[Jerry Wyss]

Thanks for the intresting input here fella's. I would just just like to point at another possible aspect of larger case volume, and that would be cooling. All facets of R/C nitro /methanol engine useage have differing issues with cooling. Even between I/B and O/B we see some differing heat/cooling requirements in the smaller engine sizes.

To get it down to a fine point, some of the 3.5cc I/B & O/B engine/ powerheads we're seeing nowdays are, in the beginning, Car/Buggy engines. Some of these engine cases are just smattered to the max with cooling fins every square mm,, and some do have a fair amount of case volume in them as well.

I understand since they are car/buggy engines originally they will have to deal with over heating , hence the smattering of fins everywhere. The other side of my query is, "does the larger case volume have a roll in cooling also?"

 

[dwilfong]

There was a discussion on RC boats.com in the tech forum on port shapes. The entrance into the port and the shape of the port were said to make a big difference in the velocity. The higher velocity I think would fill the cyl better. But at the same time you have to have enough volume of flow.

This is the best example of case volume and port configuration I have seen. It is a Russian eng IR 21. The case is part of the port and the runners are shaped like a cobra about to strike.

I was thinking of making a wax mold to put in my MAC 84 case runners and then insert a CMB 67 sleeve and fill the runners with liquid alum DEVCON. Then melt out the wax.

David

 

[Andy Brown]

Thanks for the intresting input here fella's. I would just just like to point at another possible aspect of larger case volume, and that would be cooling. All facets of R/C nitro /methanol engine useage have differing issues with cooling. Even between I/B and O/B we see some differing heat/cooling requirements in the smaller engine sizes.

To get it down to a fine point, some of the 3.5cc I/B & O/B engine/ powerheads we're seeing nowdays are, in the beginning, Car/Buggy engines. Some of these engine cases are just smattered to the max with cooling fins every square mm,, and some do have a fair amount of case volume in them as well.

I understand since they are car/buggy engines originally they will have to deal with over heating , hence the smattering of fins everywhere. The other side of my query is, "does the larger case volume have a roll in cooling also?"

The larger case volumes have been consistently growing over the years to meet the high rpm horsepower needs of on-road race cars with 2-speed tran. Novarossi has been the major supplier in that field from 1985 until current.

Better materials have allowed those engines to constantly work at 40,000+ rpm.

It was not until 2002 that Novarossi began to address the off-road buggy market and that was just a "quick fix" engine by putting a low exhaust port timing in the sleeve. An off-road/buggy specific crankcase has yet to be supplied by Novarossi as far as I know.

The MAC OB powerhead was made using the CMB inboard crankcase. It also had fairly large transfer channels. The three large ports in the MAC liner coupled with the pair of large piston ports makes it a very cool running engine. We supplied that engine with a 7.5mm carb, because that seemed to be a popular size for other OB engines. I later determined that a 6.5mm carb is a better choice for the MAC O.B. , both in terms of throttle response and top end speed.

 

[Kim J]

Kim, I'm sure it was Harry B. Higley. Just wasn't sure about the spelling. Harry was big into the control line Carrier events. George Aldrich was a premier engine guy from the same period and may have mentioned the test in some publication, but I do know that Harry and his son conducted the test.

You are correct Andy. Aldrich described this test in Model Airplane News in the late seventies in his regular column.

Another interesting test he did was to have a master mechanic modify / fill / machine the case so the ports in the liner would match perfect.

The result was a slight power loss !

To my knowledge it was George Aldrich that helped Cipolla to put a taper in the liner when they started to make ABC engines.

 

[Jerry Wyss]

The MAC OB powerhead was made using the CMB inboard crankcase. It also had fairly large transfer channels. The three large ports in the MAC liner coupled with the pair of large piston ports makes it a very cool running engine. We supplied that engine with a 7.5mm carb, because that seemed to be a popular size for other OB engines. I later determined that a 6.5mm carb is a better choice for the MAC O.B. , both in terms of throttle response and top end speed.

If I could just indulge further Andy, if the CMB case had somewhat smaller transfer channels, what changes of what you have up there would likely have to take place? More carb bore? Less crank timing?

 

[Andy Brown]

The MAC OB powerhead was made using the CMB inboard crankcase. It also had fairly large transfer channels. The three large ports in the MAC liner coupled with the pair of large piston ports makes it a very cool running engine. We supplied that engine with a 7.5mm carb, because that seemed to be a popular size for other OB engines. I later determined that a 6.5mm carb is a better choice for the MAC O.B. , both in terms of throttle response and top end speed.

If I could just indulge further Andy, if the CMB case had somewhat smaller transfer channels, what changes of what you have up there would likely have to take place? More carb bore? Less crank timing?

With smaller channels the 7.5mm carb would be better and the crank timing would be fine.

 

[Jerry Wyss]

My Thanks again for your knowledge and experiences poured out here. This is a subject area thats kinda "been in the closet" for some time. I have been wanting to do some kind of volume reducing method for a while now and my brain won't shut up about it. I know its been a while since I have "pissed on the electric fence" but its looking like it may happen again :lol: