Valve/port size vs displacement??

[Generatorgus]

Maybe a bit late for this question as I've already started making parts for a 1/3 scale Henry Fords engine.
Rough scaled down from my 1/2 scale, which runs pretty good, I'm now thinking maybe I got lucky the last time.

Because I've gathered that smaller is more difficult, before I start the with the intake/exhaust plumbing I really should have an idea what size would be appropriate.
By "rough" scale I've come up with a .406" (13/32") bore and 1.5" stroke, about .194 CI. Also by scale I've come up with 1/4" MPT fittings, which will probably allow about 3/16" OD valves max.

Am I anywhere near what would be recommended?
Also how about valve lift?

I'm already thinking I may have bitten off more than I can chew , but I'm going to forge ahead in my usual way of doing things. Although I'd rather not end up with a shelf pet.
Wish me luck!!

GUS

 

[stevehuckss396]

You might be too big. I have .625 bore and my runners are .1875. The valves are .250 and they lift .070. Maybe shoot for .156 so the air / fuel mixture has a little volocity when moving through the ports

 

[awJCKDup]

This is an area I have been looking at too. Steve when you say your valve lift is .070 is this actual valve lift or cam lift ( before rocker arm ratio )?
John

 

[Generatorgus]

Thanks Steve.
My scale calculations are compromise between what I used on my 1/2 scale and what I scaled from pictures of the full scale when I built it, I was too cheap to spring for the plans.
Pipe fittings and pipe do not scale so well, and I was on the fence between 1/4 and 3/16, and chose 1/4 because I am not so sure I can handle making smaller valves.

My 1/2 scale ended up with a 5/8 bore and 1/8 npt fittings with 1/4" valves. I didn't much concern myself with the valve lift and ended up with something over .10.
Somehow it runs pretty good, but I had to step away from Henry's valve and fuel supply setup. I'm going to try to make this engine look more like the original.

I've already ordered a 1/4" fitting tree and street el castings, as well as a tap and die, but can make the ID anything I want beyond to threads.
If the fittings look too big I'll bite the bullet and reorder in the 3/16 size.

GUS

 

[dman]

scaling engines gets tricky, the reason is it's if you scale linear dimensions the volumes and mass change by the cube and the cross sections scale by the square... so you can see a small scale engine will have a higher port to volume ratio that it's full sized version giving it an inherently higher RPM range.

i don't know if there is a rule of thumb for scaling but i could figure out a few things if i knew what the rpm range was. i mean do you want authentic sounds? i would think you'd have to play with flywheel weight for that,, i'm kinda rusty on that part of physics. do you maybe want a little more rpm as a compromise and to keep things closer to scale physically? as far as structural integrity goes the rpms should be safe to increase by the reciprocal of the scale factor and i believe that's also what cross section to volume ratio would allow so that's quite a lot and there is no reason to go that high... but going with a scale engine speed instead means quite a large change in port diameter, the square root of the scale factor time the ideal size for that scale. so for 1/3rd scale engine at the same rpm the ports would be around .19 to .195 of the full scale rather than .333 but you may go somewhere in between because the scale engine may not be as smooth as the full size in the ports and a little extra rpm isn't completely a bad thing... not that you can't get away with a big port and still keep rpms down it just might be hard to start or keep running.

as far as valve lift, 25% the valve diameter is enough so that the curtain has the same area as the valve diameter, the port will be smaller so it might seem that;s more than needed, but the air will have some bias, the air doesn't flow perpendicular to the curtain and the valve is only at full lift for an instant, so it's ok to lift the valves a bit more than 25%, but if the valves are significantly larger than the ports, you may also lift it less. really on a low speed engine i wouldn't consider lift as critical as timing.

this was all figured with mental models so i may have made a mistake, i don't have any engineering resources and didn't take the time to write it down to check myself, if anyone can correct me it is welcome.

 

[Generatorgus]

dman, thanks for the reply.
I learned a few of the hazards of scaling sizes when I built the 1/2 scale.
The volume of the fittings compounded by the distance from the valves gave me an almost nonexistent compression ratio. I think I rough figured it at about 0.4:1
I had to make fillers to reduce the volumes which at least allowed me to get it running, coupled with novice machine work it didn't run very well at all.
Fortunately after abandoning the original valve design and a year or so more machine experience I came up with a good running engine.
Also fortunate is the use of pipe fittings allows easy retro fitting.

The few full scale H Ford engines I've seen were not very good runners and were very difficult to start. So I consider myself pretty fortunate that my meager skills allowed me to come up with a decent running engine as well as one that will usually start on the first spin.

GUS