372/385 Hybrid

[smokey7]

Your a mad man you know..... id love to see this work somehow. Something i just thought of looking thru a bearing book is a wrist pin bearing that has bigger rollers to go from 13mm crank to your 12mm pin. Im just throwing it out plus does a 066 bearing fit this setup? because i have a high rpm 20000 plus bearing ill never use.

 

[drf255]

Matty,

If you need any aluminum Tig on that jug, send it down to me and I'll bring it back to you at the CT GTG.

 

[blsnelling]

I hate to be a nay sayer, but I don't understand the motivation for this conversion. You're trying to use a 460 cylinder, which is the same bore as a 372BB, and giving up quad transfers. IMHO, you're going backwards. Now, if you were going 385/390, that would be a different story.

 

[Westboastfaller]

Right..he mentioned that been of 54 mm, maybe means 066 ? IDK.
I had a 372 BB with a Stihl piston I converted about 8 years ago.
With that stuffed crank it had amazing power Unfortunately it probably didn't help with me trying to port it as it had a short skirt to begin with I believe and me not knowing WTF I'm doing (not going to lie to ya) It was not very responsive to say the least. I made a bracket and used a 262 spring. Smooth on the hands.

 

[blsnelling]

Maybe he's using a 460BB topend. I'm not a fan. The OEM 460 topend is likely to outperform it.

 

[Westboastfaller]

Right...must be. Yes I here you guys say that a lot. Why is that Brad. Why would a smaller worked, out perform a bigger one worked? Just wondering

 

[strtspdlx]

Where is the fun in that ?

I can just as soon bore out a stock 372 jug bigger and have it replated...not the point of the build. ..lots of things I could do for sure...gonna see if I can just do it with what I have available though

Okay I think I get the agenda for this build now. You could definitely do it with a riser. I really think you'd still have to weld on it however just for reinforcement. Or what about making an adapter that's roughly .080 thick that acts as a stuffer to help in case compression to force more air into that big jug.


Regards-Carlo

 

[drf255]

I think its great youre trying something different.

Steps you take here can help others in the future with other projects.

My next pipe dream project is gonna be trying to shoehorn an 044 or 038M jug onto an 034 Super.

 

[Mattyo]

wow, what banter! ok... um... some of you haven't read the whole thread. might I suggest doing that.

also, yes this IS an AM 460 54mm jug. the 385-390 jug would not fit ... the bolt holes hung off the end of the chassis, see the pic I posted earlier.

I might be going backwards... but who knows. if the oe 460 jug works better, then it'll be easy to switch to that in the future. right now i'm playing with something cheap

worst comes to worst I screw up a crappy 372 chassis...

 

[wcorey]

Maybe I’m just overlooking the obvious but…

With these overbore discussions someone inevitably mentions that at some point going larger results in diminishing returns due to lack of crank case volume.
I know there are plenty of examples out there that supposedly demonstrate the effects…

I can understand that it would affect how the motor behaves to some degree but it seems like the larger bore would provide the adequate extra volume needed to charge the cylinder.

So… With the larger bore comes a correspondingly larger amount of air/fuel being pumped through.
I can’t picture how the volume in excess of the piston displacement is so important…
When the piston is on the up stroke, causing the suction that fills the crank case, it obviously creates a space equal to the volume of the bore/stroke.
And when it goes back down and pushes the charge up through the transfers, it evacuates an area also equal to what it displaces.
That’s not enough?

Something to do with resonance/timing/delay?

Does it push the usable power band outside the realm of reasonable rpms?

Can adding a bit more blowdown compensate for this? (think 7900 numbers…)

So why does crank case volume matter?

 

[Frank Savage]

Because it directly influences IF you can get enought mix into combustion chamber (if you don´t have enought volume of mix in the crankcase, it is not going be any more voluminous above the piston), as well as how much "blow" will the transfers have to "wash" the exhaust gasses out.
Low crankcase volume (and too big transfer chanel volumes) results into poor distribution of the mix (too low pressure and speed of the mix upon exiting the transfers), incomplete and erratic intake flow and thus irregular ignition and erratic burn. This is why BB saws have slower throttle response (unles heavily modded).
There is a workaround-partially filling up the transfers with apropriate material and remodeling shape of the chanels (not the exit windows), to reduce transfer chanels volume. Thus the crankcase pressure is a bit higher, flow speed is higher and the mix can wash and fill up the combustion chamber better, even if maxed out. But hard to find apropriate material. Maybe some of the materials intended for use inside prechambers of big diesel engines may work. Belzona comes to mind, but I´m not much familiar with their product lines-just know they produce alike stuff.

Read the "enought mix" as enought for combustion plus exhaust wash-out. Once the volume of the crank is not bigger than the cylinder volume, serious troubles arise. In bike, boat and aircraft engines, tuned intakes are used, along with Reed´s valves. No way to stuff this into regular saw. Into hotsaw without covers, yes. But not into worksaw of usual proportions.

Edit:
You also need some pressure conditions throughout the stroke. They are very similar, regardless the engine size. With a too little crankcase, you run into uneven and erratic flow in the transfers-front and aft, as well as erratic flow in front and aft parts if single transfers are used-too much pressure can be the cause.
Or, not enought pressure, because instead of flow into transfers from the crankshaft area, most of the flow is done from the inner piston volume, which always means much more curved trajectory and much more resistance to the flow (remember position of the piston relative to the crankshaft counterweights on downstroke). It may easily add up as high as the pump action of the piston can not override all the resitance and loses in the intake/transfer trajectory of the mix, so poor and incomplete fill results. This is, as far as I know, the bigger trouble, esp. with non-windowed piston designs.

 

[Mattyo]

sorry, not sure I can follow all that. i'm just gonna settle for getting the thing running, let alone run well ...

I got a bridgeport mill, and a dental degree, no degree in 2 cycle theory. i'll let you guys optimize such a design if it works

 

[Mattyo]

Your a mad man you know..... id love to see this work somehow. Something i just thought of looking thru a bearing book is a wrist pin bearing that has bigger rollers to go from 13mm crank to your 12mm pin. Im just throwing it out plus does a 066 bearing fit this setup? because i have a high rpm 20000 plus bearing ill never use.

The issue was the opposite.... the crank only accepts 12mm wrist pin, but the 390 pistons were 13mm ...

so I'd need to swap the crank, or figure a way to make the crank fit a 13mm piston

 

[blsnelling]

I vote for fitting a 390 crank in the 372 case and fabbing an adapter to fit the 390 cylinder

 

[wcorey]

Because it directly influences IF you can get enought mix into combustion chamber (if you don´t have enought volume of mix in the crankcase, it is not going be any more voluminous above the piston), as well as how much "blow" will the transfers have to "wash" the exhaust gasses out.

Not sure I phrased my questions very clearly...
I still don't understand how this explains why the extra volume of the crankcase is needed to to accomplish this, the piston can't 'push' any more than it can 'pull'....
Is there some sort of 'vacuum lag' that prevents it from pulling in as much as it can push back out? Maybe more crankcase volume helps that?

Low crankcase volume (and too big transfer chanel volumes) results into poor distribution of the mix (too low pressure and speed of the mix upon exiting the transfers)...

This seems contradictory.
While yes, too large of a transfer volume could slow things down, it would seem to me that lower crankcase volume would then speed it up.
I further don't agree with this as it relates to bb kits as the problem there is typically that the over bore gets pushed out into the transfers and they don't bother to compensate by pushing the transfer tunnels out correspondingly (or can't because it wouldn't clear the case and/or flywheel).
This makes the transfer passages not only narrower but the 'loop' also more shallow, decreasing/compromising both volume and direction of flow.
I assume this to be a big performance limiting factor on many of these kits.

You also need some pressure conditions throughout the stroke.

This I believe... though I'm not really following your explanation of why.
Maybe the extra volume is needed to act as a 'spring' to store some of the compression energy so to release it with more expediency/better timing?

Not sure what the actual requirement is but say the crankcase volume needs to be the same as piston displacement and say the overbore increases displacement by 10cc.
Could a 10cc 'boost bottle' type arrangement be added off the crankcase? It could even be a cylinder with a piston that could adjust the volume for tuning...

 

[Frank Savage]

Remember-the volume change during the stroke BELOW the piston is smaller than the volume ABOVE the piston-skirts, bearings, wristpin, conrod-all this occupies volume. Also, turn slowly the crank, watch the port opening sequence and think where the volume increases/decreases while what is opened/closed. You will realize that there is already some level of vacuum right in the moment the intake port is opened. This results in the crankcase is not "flow filled", but "surge filled" (the tuned intake with reed´s valves works right with this). If the crankcase volume is too small, the surge wave bounces off the front side and spits back into carb before the intake closes. The volume below the piston in the cylinder has not a lot to do with this, as the surge wave runs in the direction of the intake-so it aims into crankcase. The air, not talking the mix, is quite far from the ideal gas, so it acts pretty different than we are being told in the highschool.
I think yes, you can visualize the crankcase volume as a "spring", to store the intake surge wave energy. "Boost bottle" will not work much, because the entrance into it will be very small (probably max. the size of the intake) and resulting pressure resistance upon the entrance in there would diminish any gain.

As to your second note-low crankcase volume does not speed things up, because volume pumped below the piston is lesser than volume above it-and pressure loses due to curved trajectory from the lower portion of the piston into transfers are just loses.
And yes, transfers with too little cross-section (read too big flow resistance) will compromise the combustion chamber wash-out and fill. This is directly related to "You also need some pressure conditions throughout the stroke." With restrictive transfers, the conditions are just not there and unwanted surges occurs.

 

[Mattyo]

I vote for fitting a 390 crank in the 372 case and fabbing an adapter to fit the 390 cylinder

This was homelite410 idea as well. I may end up there ...who knows. ..

 

[Marshy]

Maybe I’m just overlooking the obvious but…

With these overbore discussions someone inevitably mentions that at some point going larger results in diminishing returns due to lack of crank case volume.
I know there are plenty of examples out there that supposedly demonstrate the effects…

I can understand that it would affect how the motor behaves to some degree but it seems like the larger bore would provide the adequate extra volume needed to charge the cylinder.

So… With the larger bore comes a correspondingly larger amount of air/fuel being pumped through.
I can’t picture how the volume in excess of the piston displacement is so important…
When the piston is on the up stroke, causing the suction that fills the crank case, it obviously creates a space equal to the volume of the bore/stroke.
And when it goes back down and pushes the charge up through the transfers, it evacuates an area also equal to what it displaces.
That’s not enough?

Something to do with resonance/timing/delay?

Does it push the usable power band outside the realm of reasonable rpms?

Can adding a bit more blowdown compensate for this? (think 7900 numbers…)

So why does crank case volume matter?

I think you should do some research on how 2 strokes function and the learn some facts about primary and secondary compression and the volumes that feed each other.

 

[wcorey]

Remember-the volume change during the stroke BELOW the piston is smaller than the volume ABOVE the piston-skirts...

I appreciate your taking the time to explain. Some of it is starting to sink in.

I guess what I get stuck on is the idea that the piston does displace the same amount of volume above and below and also that any volume below the piston (including the crankcase) is effectively the same… But because of port openings and other dynamics this really doesn’t happen as I envision it, the amount of displacement that exists doesn’t equate to how much it actually changes.

 

[wcorey]

I think you should do some research on how 2 strokes function and the learn some facts about primary and secondary compression and the volumes that feed each other.

Yes, easy to say but…
I’ve done/attempted quite a bit of reading, Graham/Bell/Blair/Jennings… I find it difficult and time consuming to get a concise answer to seemingly simple questions, much of it goes over my head and I quickly glaze over. Guess I need ‘two stroke theory for dummies’.

And then there’s the retention factor, lol…