What's wrong with front (crank) intake?

[Kez]

In the 70's when there were not too many marine engines to choose from, it was common to convert an airplane engine to marine use. Most of the engines were front (crank) intake. Then as the hobby evolved, we started seeing specially built marine engines with rear intake (disk and drum). My question is, do they really have a power advantage?

Disk induction system, while simple, has several disadvantages. Most of the disks I have seen are not balanced well. Unbalanced rotor causes vibration. The drive pin can wear; and on more than one occassion, the rotor pin backed out. The resulting rod strike caused me an engine.

Drum induction system has its own disadvantages. Unlike a thin disk, a drum rotor requires dynamic balance. It is virtually impossible to perfectly balance a drum. From personal experience, if the drum housing is not bushed, the unblanced drum will cause accelerated wear.

The way I see it, a crank induction system is really a crank with an integral drum. The crank is supported by ball bearings so we don't have to worry about wear. There is no rotor drive pin to wear, or disk clearance to set. It is a very simple and effective induction system and it has worked very well in .21 size engines. Why don't manfacturers offer .45 and larger size engines in front induction versions? Is it because we automatically associate rear induction engines to performance?

 

[tnrcboatracer]

Could it be strength of the hollow crankshaft with the window cut in it? Even though the crank runs in ball bearings, it must still seal itself to the housing. In a rear rotor motor this seal can be a failrly narrow zone. On a front crank, the width is larger. But I think that the biggest issue is crank strength. The output of today's motors is much greater than that of the 70's. High output DF motors are also rear disk/drum induction.

 

[Blackout]

It's hard to design a front exhaust engine with a front intake also. The carb will simply get in the way with the exhaust outlet. Most boaters want a front exhaust engine so there is a straight shot to the rear of the boat for the tuned pipe. That's my thought...

 

[Scott Schneider]

Many dynamics effect efficiencies within a design. Here JMO on some of them.

Hollow cranks are a weak link and add volume to crankcase. Increased diameter needed for increases in torsional rigidity add even more volume.

Induction threw a back door induction allows shorter and beefier crank design and saves crankcase from wear or damage from induction debris mishaps.

Drum rotor induction where carb end feeds the cup of the drum has the added attribute of being very low volume when closed further reducing case volume. This low volume design on induction allows greater transfer passage volume and port area to be added to cases for better overall breathing.

Lots more, just my take on this part of the equation

 

[Glenn Quarles]

I would say the only real disadvantage is the crank strength when you really open them up to try high flow or high timing. A GREAT advantage they have is fewer rotating parts...this is more important on the smaller motors.

Glenn

 

[Kez]

Could it be strength of the hollow crankshaft with the window cut in it? Even though the crank runs in ball bearings, it must still seal itself to the housing. In a rear rotor motor this seal can be a failrly narrow zone. On a front crank, the width is larger. But I think that the biggest issue is crank strength. The output of today's motors is much greater than that of the 70's. High output DF motors are also rear disk/drum induction.

I believe crank strength is a consideration. In the 80's when the K&B 3.5 was a popular choice for 1/8 scale car and marine use, I heard of the crank cracking although I have not experienced a broken crank. The failed cranks that I had seen had all been modified. The LE of the crank opening were often grind sharp as it was believed that doing so will enhance performance. I am not sure if it wll buy anything but the sharp corners sure create stress risers and crack often start from there.

I have seen some OB power heads (Nelson, Rossi etc) run pretty strongly and I have not heard of crank failure on the outboards.

If you take a outboard power head and turn it around for inboard use, it is now basically a drum intake engine with the flywheel mounted to the drum. Best of all, the drum is ball race supported.

 

[Kez]

It's hard to design a front exhaust engine with a front intake also. The carb will simply get in the way with the exhaust outlet. Most boaters want a front exhaust engine so there is a straight shot to the rear of the boat for the tuned pipe. That's my thought...

Yes I agree that front intake and exhaust are mutually exclusive. However, most if not all 21 size engines are rear exhaust and the new OS 46 VXM is rear exhaust as well. Front exhause sure is neat for hydros but I prefer rear or side exhaust (if they make one) for Vees and monos. I think it looks nicer with the pipe hidden under the deck and to the left side in a mono hull than have it sticking out the back. If nothing else, it is easier to access the raio box without the pipe in the way.

 

[Jeff Lutz]

In the 70's when there were not too many marine engines to choose from, it was common to convert an airplane engine to marine use. Most of the engines were front (crank) intake. Then as the hobby evolved, we started seeing specially built marine engines with rear intake (disk and drum). My question is, do they really have a power advantage?

Disk induction system, while simple, has several disadvantages. Most of the disks I have seen are not balanced well. Unbalanced rotor causes vibration. The drive pin can wear; and on more than one occassion, the rotor pin backed out. The resulting rod strike caused me an engine.

Drum induction system has its own disadvantages. Unlike a thin disk, a drum rotor requires dynamic balance. It is virtually impossible to perfectly balance a drum. From personal experience, if the drum housing is not bushed, the unblanced drum will cause accelerated wear.

The way I see it, a crank induction system is really a crank with an integral drum. The crank is supported by ball bearings so we don't have to worry about wear. There is no rotor drive pin to wear, or disk clearance to set. It is a very simple and effective induction system and it has worked very well in .21 size engines. Why don't manfacturers offer .45 and larger size engines in front induction versions? Is it because we automatically associate rear induction engines to performance?

Front intake has too long of an intake tract, causing several disadvantages. Before you Novi boys chime in,consider the whole engine when making comparisons.Consider if a Novarossi was rear intake,it may run better. All though I have seen an engine setup with both, front and rear-(Yes 2 carburetors!!) Man did I say that in public,I guess the secret is out! Jeff Lutz

 

[Andy Brown]

In the 70's when there were not too many marine engines to choose from, it was common to convert an airplane engine to marine use. Most of the engines were front (crank) intake. Then as the hobby evolved, we started seeing specially built marine engines with rear intake (disk and drum). My question is, do they really have a power advantage?

Disk induction system, while simple, has several disadvantages. Most of the disks I have seen are not balanced well. Unbalanced rotor causes vibration. The drive pin can wear; and on more than one occassion, the rotor pin backed out. The resulting rod strike caused me an engine.

Drum induction system has its own disadvantages. Unlike a thin disk, a drum rotor requires dynamic balance. It is virtually impossible to perfectly balance a drum. From personal experience, if the drum housing is not bushed, the unblanced drum will cause accelerated wear.

The way I see it, a crank induction system is really a crank with an integral drum. The crank is supported by ball bearings so we don't have to worry about wear. There is no rotor drive pin to wear, or disk clearance to set. It is a very simple and effective induction system and it has worked very well in .21 size engines. Why don't manfacturers offer .45 and larger size engines in front induction versions? Is it because we automatically associate rear induction engines to performance?

Front intake has too long of an intake tract, causing several disadvantages. Before you Novi boys chime in,consider the whole engine when making comparisons.Consider if a Novarossi was rear intake,it may run better. All though I have seen an engine setup with both, front and rear-(Yes 2 carburetors!!) Man did I say that in public,I guess the secret is out! Jeff Lutz

Yep! I've seen that done with the old K&B 3.5 Marine back in the late 70's. Just drop in an Outboard crank (the timing works out just right) and add a Supertigre 29 disc valve which was still available back in the 70's. (modification to the ST disc required)

P.S. The fastest .40 pylon racing engines in the world are crank induction. The disc/drum systems are long gone.

 

[Marty Davis]

In the 70's when there were not too many marine engines to choose from, it was common to convert an airplane engine to marine use. Most of the engines were front (crank) intake. Then as the hobby evolved, we started seeing specially built marine engines with rear intake (disk and drum). My question is, do they really have a power advantage?

Disk induction system, while simple, has several disadvantages. Most of the disks I have seen are not balanced well. Unbalanced rotor causes vibration. The drive pin can wear; and on more than one occassion, the rotor pin backed out. The resulting rod strike caused me an engine.

Drum induction system has its own disadvantages. Unlike a thin disk, a drum rotor requires dynamic balance. It is virtually impossible to perfectly balance a drum. From personal experience, if the drum housing is not bushed, the unblanced drum will cause accelerated wear.

The way I see it, a crank induction system is really a crank with an integral drum. The crank is supported by ball bearings so we don't have to worry about wear. There is no rotor drive pin to wear, or disk clearance to set. It is a very simple and effective induction system and it has worked very well in .21 size engines. Why don't manfacturers offer .45 and larger size engines in front induction versions? Is it because we automatically associate rear induction engines to performance?

Front intake has too long of an intake tract, causing several disadvantages. Before you Novi boys chime in,consider the whole engine when making comparisons.Consider if a Novarossi was rear intake,it may run better. All though I have seen an engine setup with both, front and rear-(Yes 2 carburetors!!) Man did I say that in public,I guess the secret is out! Jeff Lutz

Yep! I've seen that done with the old K&B 3.5 Marine back in the late 70's. Just drop in an Outboard crank (the timing works out just right) and add a Supertigre 29 disc valve which was still available back in the 70's. (modification to the ST disc required)

P.S. The fastest .40 pylon racing engines in the world are crank induction. The disc/drum systems are long gone.

Andy:

And the best and fastest 60 Teather Car Engines are Zimmerman Valve Engines running in ball bearings.

I have always thought that the Zimmerman Valve was probably the most efficient because is cuts such a thin slice of air on each rotation.

I have many times wondered why people like Nova Rossi didn't slant the crank induction engines so that you could have a rear exhausting engine and also a crank induction - much fewer and less massive moving parts.

 

[Andy Brown]

In the 70's when there were not too many marine engines to choose from, it was common to convert an airplane engine to marine use. Most of the engines were front (crank) intake. Then as the hobby evolved, we started seeing specially built marine engines with rear intake (disk and drum). My question is, do they really have a power advantage?

Disk induction system, while simple, has several disadvantages. Most of the disks I have seen are not balanced well. Unbalanced rotor causes vibration. The drive pin can wear; and on more than one occassion, the rotor pin backed out. The resulting rod strike caused me an engine.

Drum induction system has its own disadvantages. Unlike a thin disk, a drum rotor requires dynamic balance. It is virtually impossible to perfectly balance a drum. From personal experience, if the drum housing is not bushed, the unblanced drum will cause accelerated wear.

The way I see it, a crank induction system is really a crank with an integral drum. The crank is supported by ball bearings so we don't have to worry about wear. There is no rotor drive pin to wear, or disk clearance to set. It is a very simple and effective induction system and it has worked very well in .21 size engines. Why don't manfacturers offer .45 and larger size engines in front induction versions? Is it because we automatically associate rear induction engines to performance?

Front intake has too long of an intake tract, causing several disadvantages. Before you Novi boys chime in,consider the whole engine when making comparisons.Consider if a Novarossi was rear intake,it may run better. All though I have seen an engine setup with both, front and rear-(Yes 2 carburetors!!) Man did I say that in public,I guess the secret is out! Jeff Lutz

Yep! I've seen that done with the old K&B 3.5 Marine back in the late 70's. Just drop in an Outboard crank (the timing works out just right) and add a Supertigre 29 disc valve which was still available back in the 70's. (modification to the ST disc required)

P.S. The fastest .40 pylon racing engines in the world are crank induction. The disc/drum systems are long gone.

Andy:

And the best and fastest 60 Teather Car Engines are Zimmerman Valve Engines running in ball bearings.

I have always thought that the Zimmerman Valve was probably the most efficient because is cuts such a thin slice of air on each rotation.

I have many times wondered why people like Nova Rossi didn't slant the crank induction engines so that you could have a rear exhausting engine and also a crank induction - much fewer and less massive moving parts.

If I remember correctly I saw a Zimmerman valve .21 under the bench at the CMB factory a number of years ago. Guess it didn't work good enough for them to want to put it into production. But no doubt, Zimmermans work!

The late great Chuck Wiechard built a .61 control line speed engine with crank induction that worked well. And I'm sure it would be good in a tether car if they could make room for the configuration.

 

[Marty Davis]

In the 70's when there were not too many marine engines to choose from, it was common to convert an airplane engine to marine use. Most of the engines were front (crank) intake. Then as the hobby evolved, we started seeing specially built marine engines with rear intake (disk and drum). My question is, do they really have a power advantage?

Disk induction system, while simple, has several disadvantages. Most of the disks I have seen are not balanced well. Unbalanced rotor causes vibration. The drive pin can wear; and on more than one occassion, the rotor pin backed out. The resulting rod strike caused me an engine.

Drum induction system has its own disadvantages. Unlike a thin disk, a drum rotor requires dynamic balance. It is virtually impossible to perfectly balance a drum. From personal experience, if the drum housing is not bushed, the unblanced drum will cause accelerated wear.

The way I see it, a crank induction system is really a crank with an integral drum. The crank is supported by ball bearings so we don't have to worry about wear. There is no rotor drive pin to wear, or disk clearance to set. It is a very simple and effective induction system and it has worked very well in .21 size engines. Why don't manfacturers offer .45 and larger size engines in front induction versions? Is it because we automatically associate rear induction engines to performance?

Front intake has too long of an intake tract, causing several disadvantages. Before you Novi boys chime in,consider the whole engine when making comparisons.Consider if a Novarossi was rear intake,it may run better. All though I have seen an engine setup with both, front and rear-(Yes 2 carburetors!!) Man did I say that in public,I guess the secret is out! Jeff Lutz

Yep! I've seen that done with the old K&B 3.5 Marine back in the late 70's. Just drop in an Outboard crank (the timing works out just right) and add a Supertigre 29 disc valve which was still available back in the 70's. (modification to the ST disc required)

P.S. The fastest .40 pylon racing engines in the world are crank induction. The disc/drum systems are long gone.

Andy:

And the best and fastest 60 Teather Car Engines are Zimmerman Valve Engines running in ball bearings.

I have always thought that the Zimmerman Valve was probably the most efficient because is cuts such a thin slice of air on each rotation.

I have many times wondered why people like Nova Rossi didn't slant the crank induction engines so that you could have a rear exhausting engine and also a crank induction - much fewer and less massive moving parts.

If I remember correctly I saw a Zimmerman valve .21 under the bench at the CMB factory a number of years ago. Guess it didn't work good enough for them to want to put it into production. But no doubt, Zimmermans work!

The late great Chuck Wiechard built a .61 control line speed engine with crank induction that worked well. And I'm sure it would be good in a tether car if they could make room for the configuration.

I bet that Jack O'Donnell has a lot to saw about this topic.....

I am not at all sure about the advantages or disadvantages of front vs rear intake engines. I have had awesome Nova Rossi Engines and I now have probably the best .21 engine that I have ever had. It is a MAC/CMB combo with a Brad Christy Piston. Have never had anything close to it.

I just keep thinking that either configuration is great as long as the details are correct or either. I often have thought that the friction that the drum valve or disk valve engines would be much more, but it doesn't seem to take away from performance. Maybe time to do a test to see the difference in friction.

Brian Callahan designed a really neat run down virtual instrument for our dyno that measures friction. Might be time to quantify the difference.

There are some really neat tricks to reduce friction that we pay a lot of attention to. Maybe this helps to offset the potential disadvantages of the drum induction? WHO KNOWS....

I would LOVE to have a Zimmerman .21 engine though....

 

[lohring]

As does Jim Allen. Look at his drum valve design at https://www.intlwaters.com/gallery/thumbnails.php?album=352 Not many people could make a good one, though. It is way to fussy and expensive for mass production engines. Jim got the best power on his engines with one, though.

lohring Miller

 

[Marty Davis]

As does Jim Allen. Look at his drum valve design at https://www.intlwaters.com/gallery/thumbnails.php?album=352 Not many people could make a good one, though. It is way to fussy and expensive for mass production engines. Jim got the best power on his engines with one, though.

lohring Miller

Lohring:

I think that he calls it a Bell Valve and it is an awesome design.

I wish that you would have met a friend of mine who made a disk valve gas engine several years ago. He got scared by liability and didn't produce it. Was a really neat engine. His name is Kurt Tedford. That would have been a nice addition to your SAW quests....

 

[Kez]

Many dynamics effect efficiencies within a design. Here JMO on some of them.

Hollow cranks are a weak link and add volume to crankcase. Increased diameter needed for increases in torsional rigidity add even more volume.

Induction threw a back door induction allows shorter and beefier crank design and saves crankcase from wear or damage from induction debris mishaps.

Drum rotor induction where carb end feeds the cup of the drum has the added attribute of being very low volume when closed further reducing case volume. This low volume design on induction allows greater transfer passage volume and port area to be added to cases for better overall breathing.

Lots more, just my take on this part of the equation

I agree that given everything else the same, disk induction intake will have less case volume than a crank induction. I, too, try to minimize (pack) case volume whenever I can. Using the McCoy crank in the K&B was one way back then. However, I am not sure that everyone would agree small case volume equals performance.

I remember reading an article by the late Duke Fox where he ran a series of experiments where he fitted a piston in the back plate so he can vary the case volume. If memories served, he reported a slight performance gain when the case volume was increased.

I am no engine expert but from experience in RC (boats, cars and planes), I have seen some very strong running crank induction engines that are comparable to any drum or disk induction system. I would be happy to even trade a slight performance loss to gain reliabilty and longevity if they ever make a front induction 45 or 67.

I have a feeling that a front induction engine will be perceived as a sport engine and that will not sell very well.

 

[Marty Davis]

Many dynamics effect efficiencies within a design. Here JMO on some of them.

Hollow cranks are a weak link and add volume to crankcase. Increased diameter needed for increases in torsional rigidity add even more volume.

Induction threw a back door induction allows shorter and beefier crank design and saves crankcase from wear or damage from induction debris mishaps.

Drum rotor induction where carb end feeds the cup of the drum has the added attribute of being very low volume when closed further reducing case volume. This low volume design on induction allows greater transfer passage volume and port area to be added to cases for better overall breathing.

Lots more, just my take on this part of the equation

I agree that given everything else the same, disk induction intake will have less case volume than a crank induction. I, too, try to minimize (pack) case volume whenever I can. Using the McCoy crank in the K&B was one way back then. However, I am not sure that everyone would agree small case volume equals performance.

I remember reading an article by the late Duke Fox where he ran a series of experiments where he fitted a piston in the back plate so he can vary the case volume. If memories served, he reported a slight performance gain when the case volume was increased.

I am no engine expert but from experience in RC (boats, cars and planes), I have seen some very strong running crank induction engines that are comparable to any drum or disk induction system. I would be happy to even trade a slight performance loss to gain reliabilty and longevity if they ever make a front induction 45 or 67.

I have a feeling that a front induction engine will be perceived as a sport engine and that will not sell very well.

I think that type engine would sell very well if they rotated the induction housing so that a carb could be put on without restricting a rear facing pipe install.

I also would like to run an engine of this type.

 

[Kez]

Many dynamics effect efficiencies within a design. Here JMO on some of them.

Hollow cranks are a weak link and add volume to crankcase. Increased diameter needed for increases in torsional rigidity add even more volume.

Induction threw a back door induction allows shorter and beefier crank design and saves crankcase from wear or damage from induction debris mishaps.

Drum rotor induction where carb end feeds the cup of the drum has the added attribute of being very low volume when closed further reducing case volume. This low volume design on induction allows greater transfer passage volume and port area to be added to cases for better overall breathing.

Lots more, just my take on this part of the equation

I agree that given everything else the same, disk induction intake will have less case volume than a crank induction. I, too, try to minimize (pack) case volume whenever I can. Using the McCoy crank in the K&B was one way back then. However, I am not sure that everyone would agree small case volume equals performance.

I remember reading an article by the late Duke Fox where he ran a series of experiments where he fitted a piston in the back plate so he can vary the case volume. If memories served, he reported a slight performance gain when the case volume was increased.

I am no engine expert but from experience in RC (boats, cars and planes), I have seen some very strong running crank induction engines that are comparable to any drum or disk induction system. I would be happy to even trade a slight performance loss to gain reliabilty and longevity if they ever make a front induction 45 or 67.

I have a feeling that a front induction engine will be perceived as a sport engine and that will not sell very well.

I think that type engine would sell very well if they rotated the induction housing so that a carb could be put on without restricting a rear facing pipe install.

I also would like to run an engine of this type.

I have both monos and hydros. For a vee, I actually prefer a forward facing exhaust so that I can run a 180 degrees header and put the pipe under the deck. I think a pipe sticking up and out the back of a mono really looks ugly. Not to mention the drag penality of the pipe at speed.

For a hydro, it is best to have the exhaust pointing straight back. This would dictate a slanted front housing like your mentioned like the early K&B 3.5. Another possibility is to use an outboard powerhead and turn it 180 degrees. The PTO has the square drive built in so no need for cable collet. I think geardrive is popular in Europe. This would also point the exhaust to the back of the boat.

 

[dwilfong]

I think that a Zimmerman disk is the way to go. the strait shot with the disk open at BDC will let the pipe draw the charge with the transfers at wide open. If you can see the flow in your mind and take into account what is going on. The piston on the down stroke will get the high pressure blast to get things started in the right direction. Then the pipe will pull it along. the flow path is much more direct with a rear disk intake. Cut the piston and sleeve out of the way and let it flow.

David

 

[Chris Wood]

i myself have asked stu many times about getting a VAC 21 with a Zimmerman disc.. it can be done.. they did a motor like that with a single pc crank yrs ago. in a bigger version..

i think hands down it would be better than a drum.. ALOT less drag..

that's just my opinion though

chris

 

[lohring]

I think it's hard to tell what really drives engine design decisions. I think production cost followed by durability are the most important in nearly all cases. That would make crank induction followed by rear drum induction the winners in model engines, though reeds have also had some success. Even in racing two strokes production cost is important since the engines are often derived from production engines.

The breakthrough modern, small two stroke designed by Walter Kaaden for MZ put together for the first time a tuned exhaust system, Schnerle scavenging, and a Zimmerman intake valve. It was his understanding of resonant effects that was new. That remained the standard until designers learned to match the disk valve's performance with reed valves. Today there are still some people who think disk valves are better. Aprilia runs a side disk valve on some racing engines. See below. Though the trend to small case volume ended with modern tuned pipes that can draw mixture from a larger case, the crankshaft induction and Mac style rear drums may have a little more than the optimum case volume for all applications. The peak power is there, though with the right pipe. In fact, the pipe probably has more to do with the power than the intake valve design .

Lohring Miller