Super low RPM running/lugging mods?

[stillsearching]

What type of combustion chamber is most suited to low rpm's, something off the shelf? Lots of engines seem to idle fine - I haven't tried putting any under load. I wouldn't even know where to begin with different shapes though... maybe Vortec type ones with their swirling to help keep the mixture tumbling longer since piston speeds are slow?

Engine size may not always be a problem - one of my experiments would be for a stationary engine, low rpm lets me make an engine last much longer, but yes i'd like to also use it in a car/truck too. One engine i'd like to try this on is based off the olds 455 - pretty tall deck height, long stroke, long rods. Those engines were known for high efficiency (for what they were) already - 15-20mpg on the highway (driven american style, not ecomodder style), no overdrive, carbureted in the 70's in full size sedans was something no 454 chevy or 460 ford or 440 dodge could ever touch. One plan is an Olds 427 - the 4.25" stroke 455 crank in an early 400 4" bore block. Another is a cut down Olds 425 crank 3.975" stroke in an Olds 260 V8 diesel block 3.5" bore but thats more work for less benefit. A second reason for wanting the Olds is that ALL heads interchange - meaning I can put the tiny 260 small block heads (which are already small for low rpm on that) on the bigger 427 - this should match my desire to turn really slow, likely making the torque peak below 2000rpm.

Is there any way to alter the "lugging" issue of torsional vibrations, to change the resonant frequency at which that happens? Heavy flywheels or would that have no effect? Different crankshaft counterweighting? I don't understand the physics of what causes it, but I have felt it so I know what it is. :-/

[slowmover]

FL is pointing you at piston speed . . it's critical.

[stillsearching]

I'm sure it is critical, and it's probably the brick wall setting a lower limit below which things wont work anymore. What i'm interested in what things can take it as low as possible whether it gets me 10% lower or 20% lower than would normally be the limit.. and curious why some engines seem more resilent than others. People running diesels at 800rpm in another thread for instance. And Listeroid engines run 600rpm off standard diesel with pretty familiar bore/stroke of 4.5" and 5.5" respectively.

Long stroke short rod increases piston speed away from TDC at least during the critical first part of the stroke. Piston speed is not a constant afterall and expansion is most needed after you trigger ignition. Someone else said combustion chambers matter - I was hoping they'd expand how, or what would work better. Also said EGR would slow it down. My own suggestion that slower burning fuels like producer gas should hopefully tolerate slower speeds as well. I don't know how slow I can get it but rather to stretch it. 1200rpm at highway speeds in 6th already done by the dodge viper v10 i'm told. I had a 62 chevy that would idle at some 200rpm once but I never put a load on it.

The ugly harmonic vibration seems to be both the reason it's hard on the engine and an obvious sign something is not working properly... that to me seems the first barrier to tackle even if there are other potential problems. After that can look at what else pops up as a side effect of the slow flame fronts, needing lower compression and such maybe.

[Big Dave]

Everything has a natural resonant frequency. You can vary itr with mass or damping (damping usually involve some energy wastage).

You can move the resonant frequency around but you cannot eliminate it.

Engineers find the resonant frequency and do their best to operate the engine at other frequencies. Under the resonant or over it works equally well.

Problem is that the air/exhaust path has its own resonant. This is why some naturally aspirated engines are peaky. Sometimes the engineers use this resonance to increase volumetric efficiency.

Classic example is the Chevy small-block. The mechanical drive line has a resonant around 950 RPM and one learns to blast through that RPM fast as possible. But the air/exhaust gas "pipe organ" resonates around 4800 RPM. Engineers use this to get very high HP (due to high vol eff at the resonant) but also get decent MPG at lower RPMs as the frequency is so far off the air/gas resonant. Lower engine HP demands at low RPM makes the lower vol eff insignificant. But when you need the power, rev her up to the resonant and the small block makes extraordinary power.

Two-stroke chain saw and motorcycle engines do this even more dramatically.

Don't go trying to change the resonant frequency of an engine. You'll just break parts. Engineers generally do know what they're doing, and you do not have the access to data they have.

Find your torsional resonant frequency and stay off of it.

[stillsearching]

Ok its starting to make more sense to me now.

Is this particular resonant frequency entirely mechanical or does any amount of tuning (head changes, cam changes) affect things? I think thats crossing over a bit with a conversation on airflow based resonance (which is usually torque peaks, "sonic tuning", 105% volumetric efficiency type effects done right) since it's about seeing how slow can you go.

I'm wondering how I would go about systematically finding the resonant frequencies of different engines to help punch in those specs... this obviously creates a brick wall I have to design around since i'd be doing things outside of what the engineers normally do.

[csnyder]

I am working on a similar project on a much smaller scale. A single cyl engine of roughly 13 cu in displacement - approxemately 2.6X2.5 bore and stroke - puts out about 5HP at 3600 Rpm and I want it to run closer to from 600 RPM to 2200 maximum.
From my research valve timing will be critical - particularly closing the exhaust valve early to trap compression pressure. I am being told advancing the cam one tooth is too much - the engine will "fall on it's face" at about 2000 RPM. Guess what? That would not be a problem for me!!. Have not tried it yet but looking forward to trying it soon. If it doesn't work I will aquire a spare cam and try modifying it - - twisting? possibly to advance only the intake about 4 or 5 camshaft degrees (8 to 10 crank) without changing the exhaust - Narrow the lobe center angle - and hope the overlap foesn't kill it - - -

[freebeard]

If it doesn't work I will aquire a spare cam and try modifying it - - twisting? possibly to advance only the intake about 4 or 5 camshaft degrees (8 to 10 crank) without changing the exhaust

In the 1950s there were shops that would regrind the cam profile. Flathead or OHV?

[csnyder]

It is a Briggs 130252 flatty trying to imitate a d motor wheel. 6:1 final drive instead of 8:1. Lots of high rev cams available but no “chuffers”. The original was a 2hp 12 incher with atmospheric inlet valve running the wheel off a 4 lobe camshaft

[Piotrsko]

Still are Cam shops in Los Angeles that will weld and regrind your cam. Isky comes to mind. But since it's a one shot effort requiring much manual labor, expect it to be pricey. You could DIY it on a single cylinder motor, the go cart racers have done this with various sucesses. Might even be able to get one from a go cart racing shop, but "Chuffing" doesn't have a lot of application in racing.

Im not even aware of the specs for such a grind. Easier to huff it.

[JRMichler]

My old Canyon would pull from 600 RPM, which was about 21 MPH in 5th gear. There was not much power, but it pulled smoothly enough to accelerate (slowly) from that speed on a level road. The ignition timing was right at TDC, or even 2 degrees after TDC at that RPM.

I once had a 3 inch trash pump with a single cylinder Wisconsin engine. That engine had magneto ignition with a impulse coupler. The impulse coupler winds up a spring until after TDC, then spins the magneto to generate a hot spark after TDC. That engine easily started on the first pull either cold or hot.

If I was experimenting with an engine to develop power at less than 1000 RPM, I would experiment with ignition timing set after TDC. I would not be surprised if 10, 20, or even 30 degrees after TDC would be the sweet spot at low RPM. You want the hot gas to expand doing work before the cold cylinder walls cool it down.